Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 1 of 97
CDOIF
Chemical and Downstream Oil Industries Forum
Guideline
Environmental Risk Tolerability for COMAH
Establishments
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 2 of 97
Foreword
In promoting and leading on key sector process safety initiatives, through its members CDOIF has
developed this guideline on environmental risk tolerability for COMAH establishments.
The intent of this document is to provide a reference for those organisations completing
environmental risk assessments.
This guidance (or equivalent) should be used from the date of publication to carry out
environmental risk assessments required by COMAH. The document will be periodically reviewed
to allow industry and the Competent Authority
1
(CA) to feedback on any significant issues that arise
from its implementation and determine if revision is necessary.
It is not the intention of this guidance to replace the existing DETR 1999 publication ‘Guidance on
the Interpretation of Major Accident to the Environment for the Purposes of the COMAH
Regulations’, but provide a framework and screening methodology by which regulators and
operators can apply it.
There are no limitations on further distribution of this guideline to other organisations outside of
CDOIF membership, provided that:
1. It is understood that this report represents CDOIF’s view of common guidelines as applied
to environmental risk assessment, and determining risk tolerability.
2. CDOIF accepts no responsibility in terms of the use or misuse of this document.
3. The report is distributed in a read only format, such that the name and content is not
changed and that it is consistently referred to as "CDOIF Guideline – Environmental Risk
Tolerability for COMAH Establishments".
4. It is understood that no warranty is given in relation to the accuracy or completeness of
information contained in the report except that it is believed to be substantially correct at the
time of publication.
This guidance is not intended to be an authoritative interpretation of the law; however CA
inspectors may refer to it in making judgements about an operator’s compliance with the law. This
will be done in accordance with the CA’s published enforcement policies (refer to
www.hse.gov.uk/pubns/hse41.pdf ) and it is anticipated that this document will facilitate a
consistent national approach. Reference should also be made to the CA’s ‘All Measures
Necessary Guidance’ to local inspection teams.
It should be understood however that this document does not explore all possible options for
determining environmental risk tolerability or environmental risk assessment, nor does it consider
individual establishment requirements – following the guidance is not compulsory and operators are
free to take other action.
1
The COMAH Competent Authority (CA) is responsible for enforcement of the Control of Major Accident
Hazards Regulations 2015. The CA for an establishment is the Health and Safety Executive (HSE) or the
Office of Nuclear Regulation (ONR) for nuclear establishments, working jointly with the appropriate
environment agency. In England the CA is HSE or ONR and the Environment Agency (EA); in Scotland it is
HSE or ONR and the Scottish Environment Protection Agency (SEPA); and in Wales it is HSE or ONR and
the Natural Resources Body for Wales (NRW).
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 3 of 97
Contents
FOREWORD ..................................................................................................................................... 2
CONTENTS ....................................................................................................................................... 3
1. EXECUTIVE SUMMARY........................................................................................................ 5
2. SCOPE .................................................................................................................................. 7
2.1 Competency requirements ..................................................................................................... 7
2.2 Proportionality in Risk Assessment ........................................................................................ 8
2.3 Using this guidance .............................................................................................................. 10
3. DEFINITION OF THE TYPES OF ENVIRONMENTAL HARM ............................................. 14
3.1 Environmental Receptors ..................................................................................................... 14
3.2 MATTE Thresholds .............................................................................................................. 15
3.2.1 Designated area ................................................................................................................... 16
3.2.2 Widespread habitat (land/Water) .......................................................................................... 17
3.2.3 Groundwater ........................................................................................................................ 17
3.2.4 Soil or Sediment (Land/Water) ............................................................................................. 21
3.2.5 Built environment (Land, man-made) ................................................................................... 22
3.2.6 Various receptors, as defined (Water) .................................................................................. 23
3.2.7 Particular species (Land, Water, Air) .................................................................................... 23
3.2.8 Marine (Water) ..................................................................................................................... 23
3.2.9 Freshwater and estuarine habitats (Water) .......................................................................... 24
4. RISK CRITERIA AND EVALUATING RISKS ........................................................................ 26
4.1 Assessing the risk of potential harm ..................................................................................... 26
4.1.1 Terms used in risk assessment ............................................................................................ 28
4.2 MATTE potential matrix ........................................................................................................ 30
4.2.1 Grouping and compartmentalisation ..................................................................................... 30
4.2.2 Tables to assess MATTE potential ....................................................................................... 30
4.3 Aggregating risk and risk frequencies................................................................................... 32
4.3.1 Aggregating risk option 1 - Summation of risks .................................................................... 32
4.3.2 Aggregating risk option 2 – Developing scenario based risk criteria ..................................... 32
4.3.3 Impacts from adjacent sites .................................................................................................. 33
4.3.4 Determining risk frequencies ................................................................................................ 34
4.3.5 Determining risk reduction of prevention and mitigation layers ............................................. 34
5. COST BENEFIT ANALYSIS ................................................................................................. 36
5.1 Disproportion Factor (DF)..................................................................................................... 36
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 4 of 97
5.2 Benefits ................................................................................................................................ 36
5.3 Costs 36
5.4 Discounting Rates ................................................................................................................ 36
5.5 Evaluation of Environmental Remediation ............................................................................ 37
6. COMPLETING THE RISK ASSESSMENT ........................................................................... 38
6.1 Part 1 - MATTE definition and thresholds ............................................................................. 38
6.1.1 Identifying the major accident scenarios ............................................................................... 39
6.1.2 Determining the level of severity .......................................................................................... 39
6.1.3 Assigning a duration/recovery category ................................................................................ 39
6.1.4 Determining tolerability boundaries ...................................................................................... 40
6.2 Part 2 calculating the establishment risk frequencies ........................................................... 40
6.2.1 Aggregating risk - Examples ................................................................................................ 41
6.2.1.1 Single substance stored in a single tank .............................................................................. 42
6.2.1.2 Tank farm or group of tanks containing similar substances .................................................. 42
6.2.1.3 Groups (e.g. tank farms) with dissimilar substances/incident consequences ........................ 43
6.2.1.4 Comparison with tolerability criteria ...................................................................................... 44
6.2.1.5 Interdependent scenarios ..................................................................................................... 46
7. ABBREVIATIONS ................................................................................................................ 47
REVISION HISTORY ....................................................................................................................... 48
APPENDIX 1 - KEY GUIDANCE...................................................................................................... 49
APPENDIX 2 – DETR 1999 TABLE REFERENCES ........................................................................ 53
APPENDIX 3 – INFORMATION SOURCES .................................................................................... 63
APPENDIX 4 – MATTE TOLERABILITY TABLES ........................................................................... 73
APPENDIX 5 – TABLES TO ASSESS MATTE POTENTIAL ........................................................... 85
APPENDIX 6 – PHASES OF ENVIRONMENTAL RISK ASSESSMENT .......................................... 96
CDOIF
Chemical and Downstream Oil
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CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 5 of 97
1. Executive Summary
COMAH requires all Upper Tier establishment operators to submit a safety report to the
Competent Authority (CA) that demonstrates the environmental risk for the whole
COMAH establishment has been reduced to a tolerable level. Lower Tier operators must
prepare risk assessments making a demonstration proportionate and appropriate to the
environmental risk, and whilst these are not required to be submitted to the CA these
need to be available during inspection and may be requested by the CA.
The purpose of this guidance is to provide a common methodology by which this risk
assessment can be carried out. The methodology can be used by both operators and
the CA when preparing or reviewing risk assessments.
It is not the intention of this guidance to provide a detailed assessment process, but to
provide a screening mechanism by which risks to environmental receptors can be
reviewed. Depending on the result of this screening, further more detailed analysis may
be required. It is recommended that discussions take place with the CA on the approach
to be adopted for detailed analysis.
In summary, this publication provides:
A clear definition of the types of harm that should be considered in an
environmental risk assessment, and how the harm should be characterised for
the assessment. In this context the level of environmental harm that would be
considered serious (i.e. a MATTE) has been defined for various different receptor
types in terms of the combination of the:
o extent (the area / distance)
o severity (the degree of harm within the area of impact), and
o duration (the recovery period)
2
For environmental harm to be considered serious then all parameters must
exceed the receptor thresholds as defined in this guideline. The thresholds reflect
expert opinion on levels of harm that would be considered serious, with
consideration to various receptor specific areas of legislation (including the Water
Framework, Habitats and Environmental Liability Directives).
A definition of the risk criteria to be used in assessing the tolerability of the
environmental risk from an establishment and, where appropriate, individual
scenarios
Guidance on how the risks may be evaluated
Guidance on how to include the cost of environmental harm in a COMAH cost
benefit analysis
Operators are encouraged to use this methodology themselves for the purposes of
screening (Phase 1 assessment, refer to section 2.2), engaging specialists as necessary
where further detailed analysis is required (Phase 2 assessment, refer to section 2.2).
2
If an un-mitigated assessment is carried out (as required to establish tolerability thresholds) the natural
recovery period should be used. If a mitigated assessment is carried out then credit can be claimed for
intervention.
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joint industry / trade union / regulator action aimed at delivering
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 6 of 97
Due to establishment complexity, some operators may wish to go directly to a detailed
Phase 2 assessment, without carrying out the Phase 1 screening.
Environmental Risk Assessment is often an iterative process involving initial screening
for potential impacts, based on conservative assumptions and moving to more detailed
assessment where potential serious consequences have been identified. Further
guidance on proportionality of assessments and the phases of a risk assessment can be
found in section 2.2 and Appendix 6.
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 7 of 97
2. Scope
This document provides a screening methodology to help operators and the CA in
identifying environmental risk tolerability from an establishment, targeting resources, and
determining All Measures Necessary with regard to COMAH.
In addition, the guidance has been identified as potentially useful when reviewing the
environmental aspects of major accident potential of dangerous substances for the
purposes of Article 4 of the Seveso Directive. Article 4 provides a legal mechanism
allowing member states to highlight to the European Commission that it is impossible in
practice for a dangerous substance to cause a major accident. It is important to note
that this not only requires consideration of MATTE potential (the subject of this guideline)
but also the potential for serious danger to human health. The Commission will consider
such information and legislate as appropriate. (Note: individual Member States cannot
act independently to exempt a substance from COMAH if it meets one or more of the
COMAH 2015 Schedule 1 qualifying criteria, irrespective of whether the substance has
major accident potential). A separate CDOIF working group has been established to
provide further guidance on the application of Seveso III, Article 4.
The CA seeks to avoid duplication of regulation between different regulatory regimes.
This guidance will help identify scenarios and areas of installations which the appropriate
agencies will regulate under COMAH vs. those that might be regulated under other
environmental legislation (e.g. for inspection planning purposes at establishments that
are both COMAH and EPR or PPC etc.). Where measures (physical or procedural) are
necessary for prevention and mitigation of MATTE, then COMAH will be used by the
appropriate agency to regulate those measures; conversely for potential environmental
impacts below MATTE thresholds COMAH will not be used by the appropriate agency
(EA, SEPA or NRW), but other environmental legislation might apply (e.g. EPR, PPC
etc.). If there is a potential for a Major Accident to people, but no MATTE potential, then
COMAH will apply to the measures, which might require measures related to
environmental protection (e.g. those required by COMAH regarding emergency
preparedness). In these circumstances HSE will carry out regulation of such activities
under COMAH, whilst the appropriate agencies will have limited involvement under
COMAH (e.g. as their role as statutory consultees to emergency planning) and the
appropriate agencies will carry out regulation as required by other environmental
legislation.
2.1 Competency requirements
When completing an environmental risk assessment there is a need to ensure that
relevant competent resources are used throughout the process. In the context of this
guidance, it is likely that environmental specialists will be involved with the identification
of potential Major Accidents to the Environment (MATTEs), and in determining the
thresholds that should apply to those receptors around the establishment. Similarly, it is
likely that the skills of process safety specialists will be needed to evaluate the un-
mitigated risk frequencies to these receptors, and to determine the mitigation and
prevention measures already in place to reduce the risk.
In some circumstances it might be necessary to consult experts outside of the operator’s
organisation. For example, where a designated site could be impacted then discussions
with the relevant conservation bodies might be required to ensure the assessment
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 8 of 97
includes current information on the designated site status and vulnerability. Similarly, the
appropriate agencies (NRW, SEPA and EA) hold much information on water resources.
Caution should be taken when completing the screening process to ensure that over-
simplification does not take place – there will often be a need for expert opinion and
professional judgment.
2.2 Proportionality in Risk Assessment
For COMAH, environmental risk can be assessed within the established “As low as
reasonably practicable” (ALARP) framework and evaluated to be either ‘Intolerable’,
‘Tolerable if ALARP (TifALARP)’ or ‘Broadly Acceptable’. These terms have broadly the
same meaning as used in relation to risks to people. Further guidance on their meaning
and application can be found in the CA guidance on All Measures Necessary for
environmental risk and other HSE ALARP guidance (see Appendix 1).
The level of environmental risk can be used to guide the type and depth of assessment
that would be expected by the CA. For screening purposes, a qualitative or semi-
quantitative approach (using this guidance), combined with conservative assumptions is
appropriate.
There are no specific rules regarding the depth and level of conservatism in analysis, but
generally it can be guided by level of risk, availability of data or purpose of assessment.
Risk: If risk is in the lower half of the TifALARP zone, then the semi-quantitative
methods described in this document should be appropriate. If risk falls in the
upper half of the TifALARP or in the intolerable zone then a greater depth of
demonstration may be necessary to demonstrate adequate risk control.
Data availability: If data is not available then a qualitative or semi-quantitative
approach may need to be adopted, but as with screening this should be
combined with conservative assumptions.
Purpose of assessment: A high level screening to determine if there is MATTE
potential could be qualitative, using conservative assumptions. Conversely a
detailed assessment in support of an ALARP demonstration or emergency
planning would involve more detail and less conservatism.
Thus there are different phases of risk assessment, each with differing depth of
assessment (as depicted in figure 1a below – see also Appendix 6):
Phase 1a – MATTE screen;
Phase 1b – risk screen;
Phase 2 – detailed assessment.
If a hazard/scenario is screened out due to no MATTE potential (Phase 1a) or the risks
are demonstrated to be broadly acceptable through a conservative, high level
assessment (Phase 1b), then Phase 2 detailed assessment is not required. It should be
noted that overly conservative assumptions (such as use of PNECs in Phase 2 detailed
assessments) can increase the resource required for risk assessment (screening too
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 9 of 97
many scenarios into scope), can un-necessarily increase the investment in risk reduction
measures, lead to investment in the wrong areas, and can miss-inform emergency
planning.
Phase 1a and 1b are screening, and only scenarios which are identified as
potential MATTE (phase 1a) with risks greater than broadly acceptable, in
TifALARP, (phase 1b) need detailed risk assessment (phase 2). Phase 1a & 1b
assumptions can be refined in Phase 2.
Figure 1a - Phases of MATTE risk assessment
Depth and approach to assessment should be discussed with the appropriate agency
prior to carrying out risk assessment. This is especially important for the more detailed
(phase 2) type of assessment.
For Phase 1a, screening based on Safety Data Sheet information, combined with historic
incident reviews and a qualitative analysis of substance quantities and Source-Pathway-
Receptor linkages can be acceptable to screen out scenarios. Moreover, if the
qualitative screen reveals a potential risk, it may still be possible to screen out scenarios
prior to Phase 1b through a more detailed consequence assessment (e.g. use of PNECs
or LC50/3).
PNECs are often derived from lowest value LC50, divided by an assessment factor,
typically of the order of 100 (but the range can be 1 – 1000) to allow extrapolation of
short term, single-species lab data to predict ecosystem effects. If the Predicted
Environmental Concentration (PEC) of a chemical (e.g. concentration at receptor due to
release and dispersion following an accident scenario) is lower than the PNEC, then it
Phase 1 a
MATTE screen
•Qualitative approaches with conservative assumptions
•Suitable for screening out dangerous substances from MATTE risk assesment
Phase 1 b
risk screen
•Less conservatism
•Qualitative or semi quantitative approach
•Suitable for identifying broadly acceptable risks vs. scenarios to assess in more
depth
Phase 2
detailed risk
assessment
•Least conservative
•Semi quantitative or full Quantitative Risk Assesment
•Best estimates of risk for ALARP demonstrations or emergency planning
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 10 of 97
can be concluded the chemical poses no environmental risk. By reference to the
examples in Green Leaves III it can be seen that use of
PNEC provides a conservative screening suited to an assessment where it is
required to conclude no critical effect to an ecosystem (see pp.30-31 Green
Leaves III). This assessment may be suited to a Phase 1a screen to rule out
substances from possibility of causing a Major Accident, whereas
LC50 (or LD50 as on p.26 Green Leaves III) provide a less conservative
approach to evaluating concentration of chemicals that cause serious harm to a
specific species. Use of LC50 focuses on concentrations / doses that result in
morbidity (as opposed to lesser effects and without conservatism) so give an
estimate of actual extent and severity (although data for all sensitive species
present needs consideration).
When screening, it is important to consider the full range of hazards posed to various
receptors. For example, Natural Gas does not have an environmental hazard
classification and is quoted in SDSs as “No ecological damage caused by this product”
so could be presumed (and is typically found in practice) to have no MATTE potential –
But, screening out in Phase 1a can only be confirmed after consideration of the
Extremely Flammable hazard (e.g. potential for explosion impact causing a MATTE to
the built environment). Similarly, Benzene is not classified as COMAH Environmental
Hazard (E1 or E2), though it could theoretically cause a MATTE due to impact on
drinking water or other water habitats and thus a screen based on PNEC / LC50 as
appropriate to the receptor may be necessary to screen it out at Phase 1a (N.B. Benzene
is classified in some cases as H412 and thus has potential to be harmful to the aquatic
environment with long lasting effects).
Further discussion of types and proportionality of assessment can be found in the
references in Appendix 1, in particular Annex 3 of AMEC Environment & Infrastructure
UK Limited 2014, paragraph 292 of HSG (190) and section 2.5 of Green Leaves III.
2.3 Using this Guidance
As discussed above, this guideline provides a screening methodology for carrying out a
COMAH Environmental Risk Assessment (ERA). It does not provide detailed guidance
on all aspects of ERA and for this reference should be made to Appendix 1, which
signposts other available key guidance.
The process of ERA involves:
Identification and evaluation of source – pathway – receptor linkages for different
credible accident scenarios. This includes demonstrating an understanding of the
hazards of the establishment, and the sensitivities of the environment.
Identification of tolerability criteria for relevant receptors, dependent on the
receptor type and potential level of consequence to the receptor.
Evaluation of risks to the receptor, through examination of accident scenarios
(their consequences and frequency) and comparing this to the tolerability criteria
derived above.
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joint industry / trade union / regulator action aimed at delivering
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 11 of 97
Following completion of the ERA, determine what (if any) additional measures are
required to demonstrate that the risk has been reduced to ALARP.
This guidance provides further information on specific elements of this process:
Section 3 – How to quantify consequence to different receptor types, in terms of
extent, severity and duration of harm. In particular to identify accident scenarios
where the level of consequence exceeds thresholds for MATTE.
Section 4 – Evaluating risk and making judgements against tolerability criteria.
This process includes screening out of further assessment any scenarios where it
can be demonstrated that the nature and quantity of material present do not have
MATTE potential. Sub-sections include discussion of domino group
establishments, failure rate data and the credit that can be claimed for mitigation.
Section 5 – How environmental matters can be dealt with in CBA, if this is
required
Section 6 – An outline of the assessment process, by reference to the concepts
introduced in previous sections, with examples.
The appendices provide links to a wealth of important information, much of which
will be necessary to support an assessment of environmental risk. However,
Appendix 4 is most important since it provides the agreed tolerability thresholds
for various differing consequence scales of MATTE.
Figure 1b below depicts how aspects of this approach are covered in the relevant
sections within this guidance.
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 12 of 97
Understand the
types of
Environmental
Receptors
(Section 3.1)
Determine the
MATTE Thresholds
that apply to
Environmental
Receptors
(Section 3.2)
Evaluating the risk
from the
establishment to
the Environmental
Receptors
(Section 4)
Completing a Cost
Benefit Analysis in
support of an ALARP
demonstration
(Section 5)
DETR (1999) MATTE
Table Definitions
(Appendix 2)
Key guidance
relating to
environmental risk
assessment
(Appendix 1)
MATTE Tolerability
Tables
(Appendix 4)
Guidance on
completing a risk
assessment
(Section 6)
Tables to assess
MATTE potential
(Appendix 5)
Information Sources
(Appendix 3)
Is CBA Required?
(Section 5)
Yes
No
Figure 1b – Using this guidance
When preparing environmental risk assessments, operators of both Upper and Lower
Tier establishments can usefully refer to Section 13 of the Safety Report Assessment
Manual. This provides the structured approach the CA uses to assess and inspect
environmental risk assessment for the purpose of demonstrating All Measures
Necessary. It thus provides a strong indication of CA expectations regarding risk based
demonstrations and how they should be presented.
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 13 of 97
Note that in circumstances where inventories and or quantities of substances are subject
to change, a business envelope approach is acceptable provided that the impact on a
representative set of scenarios is considered.
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 14 of 97
3. Definition of the Types of Environmental Harm
The definition of major accident in the COMAH regulations requires serious danger (to
people or the environment). Serious danger to the environment is considered to occur
where there is potential for a Major Accident to the Environment (MATTE). A MATTE
would be taken to require harm or damage to the environment above the described
thresholds (refer to section 3.2 for extent, severity and duration thresholds).
In preparing this guidance, the following key technical documents have been referenced:
DETR 1999, Guidance on the interpretation of Major Accident to the Environment
for the purposes of COMAH regulations
EA, 2004, Guidance identifying COMAH Major Accidents to the Environment
(MATTEs)
EA, 2010, Incidents and their classification: the Common Incident Classification
Scheme (CICS)
Reference should also be made to Appendix 1, Key Guidance.
Appendix 2 provides a reference to the relevant tables from the DETR 1999 guidance on
the interpretation of Major Accident to the Environment for the purposes of COMAH
regulations.
3.1 Environmental Receptors
The types of environmental receptors that should be considered are as follows:
Terrestrial habitats
Freshwater habitats
Marine habitats
Groundwater bodies
When reviewing habitats the following points should be considered:
Small areas within the larger overall area of a receptor may be significant,
depending on the flora / fauna that inhabits them, reference should be made to
the DETR 1999 guidance table 10 and Appendix 4 for further details.
Any review of receptors should include migratory species which could be
transient in the habitat
Individual species (where appropriate) should be considered in the assessment,
regardless of the pathway to the receptor.
Links to sources of information on environmental receptors are provided for each
receptor in Appendix 3.
When choosing the receptor type (3.2.1-3.2.9 below and the tables in Appendix 4), the
receptor type is not always self-evident, especially where the receptor is an individual
species which is dependent on both land and water ecosystems. In these cases,
thought should be given, and precautionary approach taken, to the ecosystem(s) that
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 15 of 97
supports the species for which the designation is given, and to the selection of the
severity/duration criteria . It is also advisable to seek agreement with the appropriate
agency at an early stage in the risk assessment process.
A simple example is mud flats associated with an estuary – on the one hand
these might be considered to be land (they are uncovered at certain times of day
and species can walk upon them), on the other they might be considered to be a
marine habitat (they are covered at certain times of day and support an
ecosystem which is dependent upon the water environment). In these cases, it is
necessary to think about the way in which the ecosystem works – in this case the
mud flats would be considered to be a water environment as it is primarily driven
by a water based ecosystem.
Similarly, where the species designated are waders or other species that rely on
the water based ecosystem the water criteria should be used. Thus even though
the waders might roost on land, they depend upon the water based system for
survival.
Another example is a grazing meadow that is also a flood plain – on the one hand
the meadow gains its fertility partially from the river flooding and deposition of
silts, a water environment. However, the ecosystem (agricultural) is clearly
dependent on land that is not normally flooded and thus land would be the more
appropriate receptor classification.
3.2 MATTE Thresholds
The following thresholds should be used when determining the potential for a MATTE to
each of the receptors described in section 3.1.
These thresholds have been developed with regard to the Major Accident EC reporting
thresholds in the Seveso Directive (Sch. 7 of the COMAH regulations) and the DETR
1999 Guidance on MATTE.
Thresholds are presented in two dimensions
(i) Extent and Severity; and
(ii) Duration of harm
The thresholds for both dimensions must be exceeded for the scenario to be considered
to be a potential MATTE.
The thresholds referring to extent and severity are presented below and should be read
in conjunction with Table 1 of Appendix 4. To avoid applying small MATTE thresholds
using the percentage criteria for small sites, the percentage criteria will not reduce the
threshold to lower than half the area/distance criteria.
With respect to Duration of Harm, impacts with short term natural recovery (other than
drinking water impact on people) would not be considered MATTE. Appendix 4, table 2
provides natural recovery times for differing receptors that would or would not be
considered MATTE. When considering recovery periods, both chemical quality and
ecological/conservation status need to be considered (adopting the longer duration
within the risk assessment).
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3.2.1 Designated area
NOTE: The DETR 1999 guidance refers to ‘Designated Land’. The CDOIF working
group has agreed to refer to ‘Designated Area’, as this also encompasses water.
Nationally important: SSSI and National Nature Reserves (NNR) [Refer to DETR 1999
table 1]:
The level of harm that would constitute a MATTE is defined as follows:
a) Greater than 0.5 ha or 10% of the area of the site adversely affected
(whichever is the lesser, subject to a lower limit of 0.25ha); or,
b) Greater than 10% of a designated linear feature of the site adversely affected;
or,
c) Greater than 10% of a particular habitat or population of individual species
adversely affected (Population refers to the known or estimated population at
the site, and individual species named in the designation, not the national
population. For other species refer to table 10 of the DETR guidance)
Internationally important: SACs, SPAs & Ramsar sites [Refer to DETR 1999 table 2]
The level of harm that would constitute a MATTE is defined as follows:
a) Greater than 0.5 ha or 5% of the area of the site adversely affected
(whichever is the lesser, subject to a lower limit of 0.25ha); or,
b) Greater than 5% of a designated linear feature of the site adversely affected;
or,
c) Greater than 5% of a particular habitat or population of individual species
adversely affected (Population refers to the known or estimated population at
the site not the national population and individual species named in the
designation, for other species refer to table 10 of the DETR guidance)
Other designated land (ESA’s, AONB’s LNRs, NSA’s etc [Refer to DETR 1999 table 3])
The level of harm that would constitute a MATTE is defined as follows:
a) Greater than 10% or 10 ha seriously damaged, whichever is the lesser
(seriously damaged is defined in ‘EA, 2004, Guidance identifying COMAH
Major Accidents to the Environment (MATTEs)’, table 3
Scarce habitat [Refer to DETR 1999 table 4]
The level of harm that would constitute a MATTE is defined as follows:
a) Damage to 10% of the area of the habitat or 2 ha, whichever is the lesser.
Refer to DETR 1999, table 4 for a definition of ‘scarce habitats’. Note that
10% refers to the site area.
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NOTE: Definition of ‘Adversely Affected’: Means that the part of the site affected loses at
least one of its reasons for designation, or favourable conservation status, and would not
naturally recover (i.e. regain its designated status) within 3 years for terrestrial habitats or
a single season for marine/freshwater. Marine implies everything below the high water
mark, for example mud flats, estuary.
Due consideration should be given to features such as estuaries and sea lochs. Further
information on the definition of ‘Adversely affected’ can be found in the DETR 1999
guidance, tables 1 – 4.
3.2.2 Widespread habitat (land/water)
Non-designated land [Refer to DETR 1999 table 5]
The level of harm that would constitute a MATTE is defined as follows:
a) Contamination of 10 ha or more of land which, for two growing seasons or
more, prevents growing of crops or the grazing of domestic animals or
renders the area inaccessible to the public because of possible skin contact
with dangerous substances;
NB. The health effect above covers the impact on amenity
or,
b) Contamination of 10 ha or more of vacant land for three years or more.
(Refer to Appendix 3, Table 1)
NOTE: Definition of ‘Non-Designated Land’: Land means all non-designated land, not
just agricultural land.
Non-designated water [Refer to DETR 1999 Table 5]
The level of harm that would constitute a MATTE is defined as follows:
a) Contamination of aquatic habitat (freshwater or marine) which prevents
fishing or aquaculture or renders it inaccessible to the public
Where there is no potential to contaminate an aquatic habitat, the non-designated water
will not have MATTE potential, and should therefore not be considered as part of the
screening process.
3.2.3 Groundwater
[Refer to DETR 1999 table 6]
This guidance sets out a MATTE definition, based on different areas/values dependent
on the type of groundwater.
Groundwater Source of Public or Private Drinking Water
The level of harm that would constitute a MATTE is defined as follows:
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a) For England and Wales only, 1 ha or more of an SPZ where drinking
water standards are breached; or,
b) For England, Wales and Scotland, Interruption of public or private drinking
water supplied from a ground or surface water source, where: (persons
affected x duration in hours {at least two hours}) > 1,000
c) Groundwater drinking water sources lie within wider aquifers. It should be
checked whether the groundwater non drinking water source thresholds
below result in more stringent risk tolerability criteria (if so the more
stringent criteria should be applied).
NOTE: in England and Wales, some abstractions (including private drinking water
supplies) are not featured on SPZ maps and have a minimum default 50m radius. The
total SPZ might be larger (depending on scale of abstraction) and operators should
consult the appropriate agency who will provide further details where this applies.
Groundwater Non Drinking Water Source
The level of harm that would constitute a MATTE is defined as follows:
a) 1 ha or more of a principal or secondary aquifer where a relevant
groundwater standard has been exceeded or in the case of groundwater
hazardous substances, where pollution is discernible (see note below)
NOTE: Discernibility and groundwater hazardous substances
GP3
3
provides current discussion and guidance on groundwater policy for England and
Wales, including the concept of discernibility. For guidance on identifying groundwater
hazardous substances refer to the work of the UKTAG and JAGDAG
4
. It is
recommended that discussions are held with the regulator concerning the latest position
on groundwater hazardous substances. If pre-existing pollution is to be considered then
this will require evidence, such as a Site Protection and Monitoring Plan [SPMP], site
condition report for IED installations, or equivalent. If such evidence does not exist then
existing dangerous substance environmental concentrations should be assumed to be
those of natural background levels.
Groundwater in unproductive strata
Not applicable. Where the groundwater does not meet the definition of an aquifer /
groundwater body it is considered as a pathway to another receptor, and assessment
should be against the criteria defined for that receptor (for example marine, fresh or
estuarine water habitats).
3
https://www.gov.uk/government/publications/groundwater-protection-principles-and-practice-gp3
4
see http://www.wfduk.org/stakeholders/mrv-work-area
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Supporting notes for the definition of groundwater receptors
Because of the diverse nature of groundwater, it is not possible to attribute a single
threshold to determine whether a MATTE has occurred. The following criteria provide
the basis against which a MATTE to groundwater can be determined:
1. Pollution to any groundwater (as defined by the Water Framework Directive) could
occur where the groundwater is a receptor; however, pollution to groundwater is not
necessarily a MATTE if the groundwater is acting as a pathway.
2. A MATTE involving groundwater can occur within an establishment’s boundary if the
consequences to groundwater exceed the MATTE thresholds defined below (N.B.
the definition of a major accident includes serious danger to the environment both
inside and outside the establishment). For England and Wales decisions relating to
groundwater policy is based on the EA Groundwater protection: Principles and
practice (GP3) (2012)
3. The EC reporting criterion for groundwater is 1ha or more of significant damage to an
aquifer or underground water. CDOIF proposes that damage is only considered to
be significant where the groundwater meets the definition of a Water Framework
Directive groundwater body. That is, the [aquifer] must be at least of sufficient scale
either to supply 10 m
3
/d as an average or 50 persons or to support the ecological
quality of a surface water body or groundwater dependent terrestrial ecosystem
5
.
4. In England and Wales this applies to principal and secondary aquifers (all types) in
both superficial deposits and bedrock meet this criteria and are therefore
environmental receptors. Groundwater in unproductive strata does not meet this
criterion and here pollution would not be considered a MATTE. Pollution of
unproductive strata might be a MATTE either by ground contamination threshold
exceedance or by pollution migrating to another receptor (i.e the groundwater is
acting as a pathway, not a receptor).
5. In Scotland this applies to groundwater within groundwater bodies or other more
localised aquifers. SEPA has mapped all bedrock aquifers and selected extensive
sand and gravel aquifers as groundwater bodies, and these underlie the whole
mainland of Scotland and many islands.
6. All groundwater bodies in England, Wales and Scotland have been designated
Drinking Water Protection Areas. Further detail on assessing groundwater is
available on the appropriate agencies’ website (for England, see the EA Groundwater
protection: Principles and practice (GP3) (2012) in particular).
7. For the purpose of application of this guidance, CDOIF has developed 3 categories
of groundwater:
(a) drinking water;
(b) non-drinking water; and
(c) groundwater in unproductive strata.
5
See
http://www.wfduk.org/sites/default/files/Media/Characterisation%20of%20the%20water%20environment/Defin
ing%20Reporting%20on%20Groundwater%20Bodies_Final_300312.pdf
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Consequence thresholds have been assigned based on the relative value of these 3
categories (note: “Groundwater in unproductive strata” is of least value and impact
here would be sub-MATTE with this category of groundwater being a potential
pathway only).
8. In England and Wales For screening purposes, principal and secondary aquifers are
identified by reference to aquifer maps for both superficial deposits and bedrock (see
Appendix 3). In accordance with Seveso reporting thresholds, the area threshold
strictly relates to the aquifer (rock type) and not the area of groundwater within it. In
England and Wales all Principal and Secondary aquifers are depicted as coloured
areas on mapping, whilst unproductive strata is shown as un-coloured areas.
Screening based on published aquifer maps will in most cases be suitable for both
Phase 1 and Phase 2 type assessments (see 2.2 and appendix 6 for discussion
phases of assessment). However, for higher risk or larger establishments a more
detailed assessment might be required if there is the possibility of any local
unproductive strata which is not apparent on aquifer maps. In order to move away
from the conservative, aquifer map based approach, a detailed assessment is
required involving an in depth evidential based analysis, this could include (but is not
limited to) analysis of:
a. productivity of aquifer as related to 10m
3
/d and 50 person supply thresholds;
b. role of aquifer with regard to support of ecological quality of a surface water
body or groundwater dependent terrestrial ecosystem;
c. conceptual model and location specific pathway-receptor analysis.
d. analysis of relevant historic incidents.
For England and Wales decisions relating to groundwater policy is based on the EA
Groundwater protection: Principles and practice (GP3) (2012), for Scotland this is
based on discussions with SEPA. Such analysis does not aim to redefine aquifer or
groundwater body boundaries, but is suitable for the purpose if identifying local
unproductive strata that might be appropriate for consideration as a pathway for
CDOIF risk assessment purposes, rather than a groundwater receptor. In these
cases, local regulator agreement (including groundwater specialists) will be required,
hence pre-assessment discussions are necessary. It should also be noted that,
irrespective of handling for CDOIF risk assessment, pollution of groundwater may
constitute an offence, whether or not that groundwater is in an aquifer.
9. In Scotland it should be assumed that all saturated materials below the site form part
of the groundwater body unless it can be demonstrated otherwise. SEPA guidance
WAT-PS-10-01 provides further details
6
.
6
Position Statement (WAT-PS-10-01), Assigning Groundwater Assessment Criteria for Pollutant Inputs
https://www.sepa.org.uk/media/152662/wat_ps_10.pdf
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3.2.4 Soil or sediment (land/water)
[Refer to DETR 1999 Table 7]
For sediment, the DETR guidance refers to a change in overlying water quality - thus
sediment should be considered a pathway and the MATTE threshold to consider is the
one for the relevant overlying water or particular species.
For Soil, the level of harm that would constitute a MATTE is defined as follows:
a) Contamination of 10 ha or more of land which, for two growing seasons
or more, prevents growing of crops or the grazing of domestic animals or
renders the area inaccessible to the public because of possible skin
contact with dangerous substances;
Note The health effect above covers the impact on amenity
or,
b) Contamination of 10 ha or more of land by substances, preparations,
organisms or micro-organisms that results in a significant risk of adverse
effects on human health.
Note This definition is taken from DEFRA publication "The Environmental
Damage (Prevention and Remediation) Regulations 2009 Guidance for
England and Wales" and this also covers the impact on amenity.
NOTE: Land that is already contaminated
Refer to figure 2 below.
Where soil is already contaminated, a site-specific analysis of the potential impact of a
MATTE scenario may be required as this could have the potential to cause additional
contamination or suspend or reverse any existing recovery.
When completing this analysis, the following factors should be considered;
The pollutant from the MATTE scenario may not have the same chemical
nature/characteristics as any pre-existing pollutants, which may aggravate the
current contamination effects (e.g. solubilisation).
The pollutant from the MATTE scenario may suspend or reverse any existing
recovery (Reference: Environmental Damage Regulations).
In concluding the analysis;
If the potential MATTE scenario could exceed the MATTE thresholds in the
absence of any existing contamination, the receptor would be deemed as having
MATTE potential.
If the potential MATTE scenario does not alter the existing contamination
management (i.e. the existing pollution management system would not need to
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be updated following further pollution of the soil or sediment), then credit can be
claimed in the risk assessment that the current remedial approach reduces the
risk to ALARP.
Identify MATTE Scenario(s)
with potential to impact Soil/
Sediment (DETR Table 7)
Does predicted
consequence exceed
MATTE threshold?
Is Soil/Sediment a
pathway?
Is land already
contaminated?
Will potential MATTE(s)
alter contamination
management?
Gather baseline data or
assume soil/sediment in not
contaminated
Contamination Management:
Consider current management
approach vs those measures
required for MATTE scenario
Screen out of assessment
Screen out of assessment for
soil/sediment, but assess and
demonstrate risk is ALARP for
end-point receptor
Assess and demonstrate risk is
ALARP for soil/sediment (and
other receptors)
Credit can be claimed in the
risk assessment for the current
remedial arrangements also
providing mitigation for the
potential new MATTE (i.e. low
probability of failure to
mitigate) – assess to
determine whether current
remedial approach (i.e.
current mitigated risk)
reduced risk to ALARP
Credit cannot be claimed for
current remedial
arrangements – asess to
determine whether current
risk is ALARP or whether
further prevention/mitigation
is required to reduce risk to
ALARP
Yes
Yes
Yes
No No
Yes
No
No
Figure 2 – Assessing contaminated land
3.2.5 Built environment (land, man-made)
[Refer to DETR 1999 Table 8]
The level of harm that would constitute a MATTE is defined as follows:
a) Damage to the built environment (e.g. Grade 1/Category A listed buildings,
scheduled ancient monuments, conservation areas) such that its
designation of importance is withdrawn.
For other built heritage types (e.g. Grade 2 listed buildings), the MATTE definitions for
widespread habitats (land, water) apply, refer to section 3.2.2.
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3.2.6 Various receptors, as defined (water)
[Refer to DETR 1999 Table 9]
Not applicable, the definition (based on standards applicable to continuous emissions
which fall under other EU legislation) is not used to identify and assess a MATTE.
3.2.7 Particular species (land, water, air)
[Refer to DETR 1999 Table 10]
The level of harm that would constitute a MATTE is defined as follows:
a) 1% or more of the population; or,
b) 5% or more of the plant ground cover
Note: the 1% and 5% above refer to national populations of England, Wales or Scotland.
Note that for particular high value or special protection species, consult the relevant
conservation organisation to determine the appropriate threshold.
3.2.8 Marine (water)
[Refer to DETR 1999 Table 11]
The level of harm that would constitute a MATTE is defined as follows:
a) 2 ha or more of contamination to the littoral or sub-littoral zone; or,
b) 100 ha or more of open sea benthic community; or,
c) 100 or more dead sea birds (500 or more gulls); or,
d) 5 or more dead/significantly impaired sea mammals
NOTE: Further definition of these areas is defined below and in Figure 3.
Supralittoral: area just above high water mark, only submerged during storms;
otherwise ocean spray
Benthic: benthic zone is the ecological region at the lowest level of a body of
water such as an ocean or a lake, including the sediment surface and some sub-
surface layers
Littoral: intertidal zone between low and high water marks (e.g. from the Mean
High Water Springs to the Mean Low Water Springs on the OS map)
Sublittoral: subtidal zone below low water mark (e.g. from the Mean Low Water
Springs on the OS map), permanently submerged; extends down to the
continental shelf break (~200 m)
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Figure 3 – Marine (Water) zones
3.2.9 Freshwater and estuarine habitats (water)
[Refer to DETR 1999 Table 12]
The level of harm that would constitute a MATTE is defined as follows:
a) The chemical or ecological status given by the Water Framework Directive
(WFD) has been lowered by one class for more than 2 km of a
watercourse; or,
b) 10% or greater of the area (for estuaries and ponds, reservoirs and lakes);
or,
c) 2 ha or more of the area for estuaries or ponds, reservoirs and lakes, or
d) Interruption of public or private drinking water supply, where: (persons
affected x duration in hours {at least two hours}) > 1,000
NOTE (criterion a): In DETR guidance, the minimum length of watercourse for MATTE is
stated as 10km or 10% of the length (whichever is lesser). In practice, for a large
number of watercourses the 10% threshold will dominate, and for many a very short
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distance would be derived. To avoid very short distances (<2 km, where a watercourse
is <20km), CDOIF have agreed the minimum length of watercourse where serious harm
could occur is taken here as a fixed value of 2km. This aligns to the EA Common
Incident Classification System (CICS) category 1.
NOTE (criterion d): interruption of public or private drinking supplies is included here to
take account of where abstraction points exist in rivers, reservoirs and lakes. Risk
thresholds based on potential severity and duration are the same as for interruption of
groundwater drinking water supplies (Refer to Appendix 4, MATTE Tolerability Tables,
Table 1 row 7).
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4. Risk Criteria and Evaluating Risks
4.1 Assessing the Risk of Potential Harm
Following the identification of possible environmental receptors around an establishment,
it is necessary to evaluate whether the substance stored at the establishment (or other
substance which could be present, such as firewater or reaction by-products) has the
potential to cause a MATTE to those receptors. Where this potential could be realised, a
risk assessment is necessary to determine if any further prevention or mitigation (or both)
techniques are required to reduce the risk to Broadly Acceptable or As Low as
Reasonably Practicable (ALARP). The depth of assessment required is discussed in
section 2.2.
To complete this assessment, it is necessary to understand the following:
For each receptor
o Is there a potential for a MATTE based on the quantities and types of
substance stored at the establishment? (Note: include substances that might
credibly be produced/introduced in an emergency, such as firewater). This
screening step can also be used to rule out from further assessment parts of
larger establishments where there is no MATTE potential if that part can have
no involvement in other areas that do have MATTE potential. A site plan may
be a useful tool to highlight those parts of the establishment that have or do
not have MATTE potential.
o When screening for MATTE potential it is not appropriate to assume
consequence will be below MATTE thresholds based on CLP classification
alone (e.g. a substance that is not formally classified as Hazardous to the
Aquatic Environment could none-the-less cause a water related MATTE by
impact on drinking water or by preventing fishing or aquaculture or rendering
the area inaccessible to the public). Substances may be screened out as part
of a consequence evaluation.
o At this stage, the potential quantity of substance lost from primary
containment can be used to screen out from further assessment, either the
whole inventory, or a subset of smaller loss of primary containment
scenarios. For example, if a high level consequence assessment shows that
a loss of >50 tonnes of dangerous substance direct to a river is required to
exceed MATTE thresholds, then scenarios involving much smaller releases
from primary containment (e.g. a spill <1 tonne) could be screened out from
further assessment of MATTE risk to the river (note, that other layers of
mitigation – e.g. secondary containment or emergency response should not
be credited here to justify no MATTE potential because if the unmitigated
release can cause a MATTE then mitigatory measures would be classed as
safety critical layers of protection that reduce risk of, but do not eliminate
potential MATTE scenarios). In line with the conservative nature of early
screening, scenarios involving releases less than, but approaching 50 tonnes
should be screened in, but could later be shown to be sub-MATTE with more
detailed risk assessment. Thus it is useful to determine for each receptor the
minimum quantity of dangerous substance required to cause a MATTE along
with a consideration of the sensitivity of this value in order to set a
conservative screening threshold (e.g. tonnage or volume) for loss of primary
containment scenarios.
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If there is potential for a MATTE:
o Determine unmitigated consequences from credible accident scenarios and
use this to establish the tolerability thresholds per receptor per establishment
per year (this is from the Appendix 4 risk matrix);
o Determine the unmitigated aggregated risk to the receptor from all credible
scenarios (i.e. risk with no mitigation measures in place);
o Determine the mitigated risk (with existing measures in place) from all
credible scenarios; and
o Determine if further measures are required to reduce the risk to Broadly
Acceptable or TifALARP (if mitigated risk remains in TifALARP then the CA
will require an ALARP justification to demonstrate why further risk reduction is
not reasonably practicable).
The methodology for assessing risk within this guidance begins with determining the
unmitigated consequence (see definitions below figure 4). The unmitigated consequence
could be sub-MATTE (enabling screening out from further assessment) or MATTE level
A-D. Each MATTE level A-D has associated tolerability thresholds - the greater the
consequence the lower the tolerable frequencies for a MATTE (Appendix 4).
The tolerability thresholds are then compared to the unmitigated risk to the receptor from
the establishment. This approach may well indicate an intolerable risk from the outset.
However once the total unmitigated risk has been calculated, the process then requires
the analysis of mitigated risk by inclusion of all existing mitigation layers – this includes
such elements as good design practices, inspection and maintenance, secondary and
tertiary containment and emergency response procedures. It is important to recognise
the risk gap between unmitigated and mitigated risk since this is an evaluation of the
amount of risk reduction provided by existing mitigatory measures and will illustrate the
importance of maintaining these safety critical measures.
An overview of the process is given in Figure 4.
Note that the risk assessment process should consider only credible scenarios.
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Figure 4 – Overview of the risk assessment process
4.1.1 Terms used in risk assessment
Sources
The sources of pollution that could give rise to a MATTE (for example tanks, pipework,
warehousing, process units, reactors, etc.), refer to section 4.2 ‘MATTE potential matrix’.
Note that pipelines outside of the establishment boundary are not covered by COMAH
when covered by the Pipeline Safety Regulations, and should therefore be excluded
from this analysis.
Credible scenarios
The scenarios under which a source could credibly pollute a receptor (for example spills,
fire, explosion), refer to section 4.2 ‘MATTE potential matrix’.
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Consequence
A combination of the following:
the extent, severity and duration of harm to the receptor.
Refer to section 3 ‘Definition of the types of Environmental Harm’
Risk
A combination of:
consequence; and
frequency of occurrence (per receptor per establishment per year).
Receptor
The receptor that could be polluted by the source, refer to section 3 ‘Definition of the
types of Environmental Harm’.
Protection Layers
Risk reduction measures - either preventive layers (i.e. reduce the frequency of a
hazardous event from occurring) or mitigatory layers (i.e. reduce the consequences of a
hazardous event after it has occurred). Preventive layers typically include the primary
containment (pipes, vessels and control systems) whilst mitigatory layers include
secondary and tertiary containment or fire suppression systems.
Unmitigated consequence
The potential consequence from credible scenarios before any mitigation measures are
employed, refer to section 4.3 ‘Aggregating Risk and Risk Frequencies’. This is
essentially the worst credible consequence associated with the credible scenario, (with
no protection layers in place) and is used to establish tolerability thresholds.
Unmitigated risk
The aggregated risk from credible scenarios, before any mitigation measures are
employed, refer to section 4.3 ‘Aggregating Risk and Risk Frequencies’. This is the risk
(consequence and frequency) associated with all credible scenarios given failure of
prevention layers, escalation and no mitigation.
Mitigated risk
The level of risk that remains from all credible scenarios once existing protection layers
(mitigation and/or prevention measures) are employed, refer to section 4.3 ‘Aggregating
Risk and Risk Frequencies’.
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Further risk reduction measures
Further risk reduction measures which could be employed to reduce the risk further to
TifALARP or Broadly Acceptable. An ALARP demonstration, which might include Cost
Benefit Analysis, may be required to further justify a claim of TifALARP.
4.2 MATTE potential matrix
The sources, or more importantly the substances which could give rise to a MATTE
should be screened for each relevant receptor to determine their potential.
In order to screen for potential credible MATTE scenarios, it is important to understand
the following:
The types or groups of substances present at the establishment that could cause a
MATTE;
The receptor itself, and how it could be polluted (or otherwise harmed) to the extent
of causing a MATTE;
The specific scenarios that could cause the receptor to be polluted (or otherwise
harmed) to the extent of causing a MATTE.
4.2.1 Grouping and compartmentalisation
To simplify the process of risk assessment, operators may consider grouping different
product categories (or substances with similar hazard categories) which have a similar
nature, and can damage the receptor in a similar way, for example:
Petroleum products;
Dense non-aqueous phase liquid.
Grouping of similar products can also be considered based on geographical location, for
example where all products stored in a tank farm(s) have similar properties and all have
the similar potential to pollute a nearby receptor(s).
On this basis it may not be necessary for operators to complete risk assessments for
individual tanks and individual products but instead to group similar substances and
‘compartments’ of tanks within the establishment boundary.
4.2.2 Tables to assess MATTE potential
The tables defined in Appendix 5 provide a methodology for how to begin to assess the
potential for substances (and cocktails of substances) to cause a MATTE if released to
the receptor unmitigated. The tables are provided as guidance on the information that
needs to be presented and provide a suggested format; however the information may be
presented in another format.
Table 1 – MATTE Potential Summary Matrix
Table 1 can be used to summarise which substances or groups of substances could give
rise to a MATTE if unmitigated (i.e. no prevention or mitigation measures are in place).
The table should be completed for each receptor that is relevant to the establishment.
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A tick () can be used in each box to indicate that a MATTE could occur if the credible
scenario (as defined in table 3) occurred.
A cross () can be used in each box to indicate that a MATTE could not be caused by
the substance.
Further footnotes could be referenced with each tick or cross to justify the prediction.
Note that it is important that this summary table is used to cover the potential
consequence of all dangerous substances, both single releases and multi-release (for
example from a tank farm or warehouse) and firewater – the substance groupings
defined can be used to achieve this.
Table 2 – Receptor Detail
Table 2 can be used to provide further definition of each relevant receptor and the
environmental vulnerabilities which they present. Reference should be made to section
3 of this guidance document for further definition of the receptor type, and to Appendix 2
for the original DETR 1999 tables.
Please note, for designated sites it is expected that information will be sought from the
conservation bodies.
Table 3 – MATTE Scenarios
Table 3 can be used to provide a description of the consequences to the receptor from
the credible scenarios under which a MATTE may occur to each of the receptors.
Typical scenarios may be:
Tank Overfill
Catastrophic Tank Failure
Leak from tank base
Pipework failure
Warehouse / Chemical plant fire.
Escalation of the above or any other incidents.
The majority of MATTEs seen across Europe have involved harm to surface waters from
direct releases or runoff from fires, but toxic gas and aerial deposition impacts (e.g.
Seveso) should not be discounted.
Further guidance on typical Major Accident scenarios can be found in the Safety Report
Assessment Guides
7
, and in H1 Environmental Risk Assessment
8
, Annex A.
Table 4 – Dangerous Substances with Environmental Risk
Table 4 can be used to provide further definition of the substances or groups of
substances which have the potential to cause environmental damage. The final column
can be used to include a reference to link to a fuller description (e.g. a section of the
Safety Report or MSDS reference).
7
See http://www.hse.gov.uk/comah/srag.htm
8
See https://www.gov.uk/government/collections/horizontal-guidance-environmental-permitting
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Where substances share similar properties, grouping can be performed on the basis of
risk phrases.
N.B. a group of chemicals could be “contents of warehouse A, loss of containment during
fire” or “chemicals in bund B (tanks 1-5) and firewater”.
4.3 Aggregating Risk and Risk Frequencies
When analysing the MATTE potential for each receptor from the establishment, several
potential credible scenarios may be identified which could cause harm to that receptor.
Moreover, if there are several tanks, warehouses, process units, etc., the frequency of a
MATTE occurring from the credible scenarios associated with each of these, above the
specified consequence level, needs to be summed (independent events only) since the
establishment risk to a receptor is from all credible MATTE scenarios from all sources
(multiple sources will increase the risk). In practice, assurance that the total risk is
reduced below a specified target can be done in a number of ways.
4.3.1 Aggregating risk option 1 - Summation of risks
Add all independent risks from all sources affecting a single receptor and compare these
(both unmitigated and mitigated risk) to the receptor’s establishment risk targets (e.g.
Appendix 4 tolerability criteria) – this approach may suit small establishments with a
smaller number of Major Accident Scenarios.
4.3.2 Aggregating risk option 2 – Developing scenario based risk criteria
Once the consequence and frequency of an identified major accident scenario have
been evaluated it is necessary to consider whether the risk from this scenario is
'Intolerable', 'TifALARP' or 'Broadly Acceptable'.
However the tolerability criteria are established for the frequency of ALL major accident
scenarios from the establishment impacting on an environmental receptor. For larger
establishments this requires the summation of frequencies from a number of scenarios -
which may be followed by identification of which scenario results in the 'Intolerable' or
'TifALARP' conclusion, and consequently requires risk reduction and/or ALARP
assessment.
This approach can make it difficult for individual plant management teams to judge the
tolerability of their own area scenarios and drive risk management processes. It is often
more convenient, simpler and more empowering for plant management teams to
'allocate' a proportion of the 'Intolerable' risk criteria to each scenario, or each part of the
establishment, against which the risks can be assessed.
The simplest way to achieve this is to estimate the total number of scenarios on the
establishment which could result in specific MATTE severity level consequence to a
receptor and divide the 'Intolerable' risk frequency criteria for this severity level by that
number to define a scenario based risk criteria. If the receptor chosen for this calculation
is the one most at risk from the establishment, the resultant criteria will be conservatively
low for all other receptors. Therefore a 'scenario based' tolerability of risk matrix can be
defined for use in scenario based risk assessments.
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At the conclusion of the establishment risk assessment, it is clearly necessary to check
the validity of the 'number of scenarios' assumption. If a specific scenario risk is found to
be 'Intolerable' against the scenario specific criteria, further consideration of the total
establishment risk to the scenario will be required - it may be that other risks to the
receptor are sufficiently low that a greater proportion of the establishment criteria can be
allocated to that scenario and that the overall risk remains 'TifALARP' i.e. the operator
may allocate different risk criteria to different scenarios within the overall establishment
risk.
4.3.3 Impacts from adjacent sites
If the establishment is not currently designated as a domino group establishment, then
the establishment should consider only its own source/pathway/receptor analysis, and
not that of other neighbours – the risk analysis will apply only to the one establishment.
For domino group establishments:
If the establishment is designated as part of a domino group, then the operator is
legally required to consult with neighbours (who will also be designated as an
upstream or downstream domino group). In these circumstances the increased
risk of a neighbouring domino group establishment creating an increased risk of a
MATTE from your establishment needs to be included in the establishment risk
aggregation and may increase the whole establishment risk to environmental
receptors.
For domino events risk can be increased in two ways:
1) The neighbouring domino group establishment could increase the
frequency at which a Major Accident could occur on your establishment –
i.e. be an additional off-site initiator;
2) The consequences of the domino event could increase as the scale of a
domino-type incident from both establishments could be greater.
Both possibilities need to be reflected in assessment.
Scenarios from a domino group that do not increase risk of a MATTE (scale of
consequence or frequency) at your establishment should not be included in the
aggregation of risk to a receptor for your establishment. i.e. even though events
at a neighbouring domino group establishment might be MATTEs in their own
right, if they do not affect your establishment these do not need to be included in
your aggregation.
Domino example
Two COMAH domino group establishments, fuel terminal A and chemical warehouse B.
Fuel terminal A – MATTE scenarios: Leaks, Fires (including running pool fires)
and Explosion.
Warehouse B – MATTE scenario: Fire.
The domino scenario is a fuel terminal running pool fire, which could initiate a warehouse
fire causing a combined consequence greater than any other scenarios. For the purpose
of this example, no other scenarios at establishment A or B would impact on each other.
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The MATTE risk for fuel terminal A is as follows: To aggregate the risk for fuel terminal A
on a receptor, take the scenarios for fuel terminal A which could affect the receptor.
Because the running fuel fire could also initiate a fire at the warehouse, the
consequences of both events happening at the same time needs to be included. Hence
the overall consequences could be greater than from the running pool fire alone.
However, frequency should not increase, as the frequency of pool fire initiating a
warehouse fire should not be greater than the frequency of the running pool fire. Indeed
the frequency of this domino scenario might be lower than the running pool fire
frequency, if the running pool fire does not always lead to a fire at the warehouse.
The MATTE risk at warehouse B is the risk of fire, and this risk would be increased by
the domino scenario. The risk would not include the scenarios of leaks at the fuel
terminal which cannot impact establishment B. Thus the implication for establishment B
being domino (as opposed to not domino) is a potential increase in consequence and
frequency of MATTE.
Note 1: This example is based on one domino scenario. This circumstance would need
review on an establishment by establishment, scenario by scenario basis. If there were
multiple potential domino scenarios then the aggregate establishment risk could increase
– either due to increased consequences or increased frequency of a specific
consequence level or a combination of both.
Note 2: The Habitats Directive requires the assessment to consider a combination of
risks from multiple sites. The view of the CA is that provided individual establishments
routinely review the condition of Habitat sites which they can potentially impact upon and
can demonstrate use of all measures necessary (i.e. ALARP) for their own risks, this
would be seen as being sufficient, and would not require consideration of risk of
simultaneous Major Accidents from other neighbouring COMAH establishments (except
for those domino establishments noted above). If a Major Accident to a Habitats
Directive site does occur, then other operators will be expected to review the implications
of that accident for their own establishments after the event has occurred.
4.3.4 Determining risk frequencies
Company specific failure rate data (for the identified credible scenarios) could be used
when completing environmental risk assessments. However the CA would require
justification (for example hours of operation, circumstance of failures etc.) as to the
figures used where they were significantly different to published industry figures. In the
majority of cases it is anticipated that failure rate data will be the same for safety and the
environment (i.e. the initiating event frequency should be the same).
Where company specific failure rate data is not available, operators can make reference
to the table of typical failure rates and the Environmental QRA data and MATTE case
studies available in the CA’s ‘All Measures Necessary Guidance’.
Note that when completing environmental risk assessments, consideration should be
given to escalation of a scenario, which could give rise to a greater consequence.
4.3.5 Determining risk reduction of prevention and mitigation layers
Reference should be made to the CA’s ‘All Measures Necessary Guidance’ for
information relating to the risk reduction provided by different prevention and mitigation
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layers. Other sources may also be of use, for example insurance company databases
may provide failure rate data for fire prevention systems.
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5. Cost Benefit Analysis
This section provides advice on how to include the cost of environmental harm in
COMAH Cost Benefit Analysis (CBA). Existing guidance on CBA within an ALARP
demonstration is relevant to environmental CBAs and the general framework for carrying
out the CBA is the same for risks to persons and risks to the environment. Relevant
guidance includes application of CBA for decisions within the TifALARP zone, as
outlined by HSE guidance on ALARP (including SPC/perm/37 & 39) and general
principles associated with CBA, as outlined in the wider HSE CBA principles and CBA
checklist.
If the risk to a receptor is intolerable then the operator is expected to reduce the risk to
an acceptable level almost irrespective of the costs (CBA as outlined here does not
apply). In this case the CA will expect implementation of further risk reduction measures
dependent on their cost and benefits. The challenge mechanism may be used to
escalate where there are disagreements at a local level (this expected to be limited to
exceptional circumstances). Where the risk is intolerable to two or more media –
multiple intolerable risks – the operator should agree with the regulator the priorities and
timescales for the establishment to reduce the risks to an acceptable level.
5.1 Disproportion Factor (DF)
Disproportion Factors should be used in environmental CBAs in the same way as for
Health and Safety CBAs, within the range 1 to 10, (10 at the intolerable border, and 1 at
the broadly acceptable border). The operator needs to justify why a specific DF has
been applied. A Major Accident Hazard (MAH) could possibly result in several
consequences to both persons and the environment and that each consequence could
have a different DF. The CBA summation would be the last task following the application
of each DF.
5.2 Benefits
Health, safety and environmental benefits should be included in the CBA where these
relate directly to a MAH. Business related benefits such as avoided loss of production,
higher insurance premiums, damage to an operators own assets, insurance costs etc.
should not be included as a benefit. These business related benefits may be considered
by the operator when considering investment, but this is not required to be included as
part of a CBA supporting an ALARP demonstration to the CA.
5.3 Costs
Only those costs incurred solely from the implementation of the measure should be
included.
5.4 Discounting Rates
It is recommended that the same discounting rate is used for costs and benefits for
health, safety and the environment
9
.
9
Refer to http://www.hse.gov.uk/risk/theory/alarpcba.htm for further information
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5.5 Evaluation of Environmental Remediation
Where available, company specific costs should be used as this will often provide the
most accurate information as it is based on the company’s own experience of dealing
with environmental incidents. If no company data is available, generic cost information
can be found from a number of sources, including:
i. Worldwide Analysis of Marine Oil Spill Clean-up Cost Factors
ii. Cost Analyses for Selected Groundwater Clean-up Projects
iii. Assessing the Value of Groundwater
iv. Assessing fish kills
Refer to Appendix 1 for further details regarding these sources of information.
The following checklist may be helpful when considering activities to be included in the
costing exercise:
i. Reference to pre‐accident baseline data set of the ecological condition of the
impacted area
ii. Establishment of post‐accident data set for ecological condition of the
impacted area, e.g. monitor, sample, test and analyse watercourses,
groundwater, soil etc.
iii. Identification of the scope of remedial work
iv. Establishment of temporary facilities and utilities
v. Excavation and removal / storage / treatment of contaminated material
vi. Import and consolidation of fill material
vii. Pump out and removal / treatment of contaminated groundwater
viii. Mitigation / clean-up of surface waters (river / estuarine / coastal)
ix. Restoring the natural environment e.g. fish stocking
x. Restoring the built environment
xi. Clean-up of pollution to third party property
xii. Civil liability claims e.g. loss of fisheries / impact on tourism / loss of
abstraction
xiii. Environmental fines
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6. Completing the risk assessment
Risk assessments can be completed in two parts:
Part 1 – MATTE Definition and Thresholds, refer to section 6.1
Part 2 – Risk assessment process, refer to section 6.2
For screening (as discussed in section 2.2) Part 1 applies to screening phase 1a
(MATTE screen) and Part 2 applies to screening phase 1b (risk screen). For those
scenarios or risks not screened out then Part 1 and 2 can both be repeated and refined
in a phase 2 detailed risk assessment.
When considering receptors with MATTE potential, note that the Safety Report
Assessment Manual (SRAM) indicates that it is reasonable to screen within 10km of the
establishment. However, for linear pathways (such as rivers) this distance may be
longer.
6.1 Part 1 - MATTE definition and thresholds
With reference to sections 3 and 4, the Source-Pathway-Receptor approach described in
the flowchart below can be used to identify those scenarios from the establishment which
could harm each environmental receptor:
These stages are
done assuming that
there are no controls
in place.
Identify scenarios which have the
potential to cause a MATTE
For each scenario assess
severity of impact with no
controls and compare to CDOIF
table to identify Severity Rating
Source, Pathway and Receptor all exist
For each S2, S3, S4 scenario
assess duration of harm with no
controls and compare to CDOIF
table to identify Duration Rating
S1, S2, S3, S4
D1, D2, D3, D4
Use the CDOIF tolerability
threshold table to identify the
relevant thresholds.
4 C D D
3 B C D
2 A B C
1
1 2 3 4
Severity of Harm
Harm Duration Category
A
Broadly Acceptable
Frequency per receptor per establishment
per year
1.0 E-02
1.0 E-03
1.0 E-05
Consequence Level
(less than)
(greater than)
1.0 E-05
B
C
1.0 E-06
1.0 E-04
D
Intolerable
1.0 E-04
1.0 E-07
Receptor Type Significant Severe Major Catastrophic
While this level of harm might
be significant pollution, it is not
considered a MATTE.
DETR Criteria - the lowest level
of harm that might be
considered MATTE.
Harm / Severity Level -> 1 2 3 4
Row
DETR
Table
Ref
Description
Short term Medium term Long term Very long term
Harm with such
short recovery is
not considered a
MATTE.
Harm Duration Category -> 1 2 3 4
LAND
≤ 3 years
> 3 years or > 2
growing seasons for
agricultural land
> 20 years > 50 years
SURFACE WATER ≤ 1 year > 1 year >10 years >20 years
GROUNDWATER BODY
Harm affecting non-
SPZ.
Harm affecting SPZ.
Use the CDOIF tolerability
threshold table to identify the
CDOIF consequence level.
A, B, C, D
Figure 5 – Summary of MATTE Tolerability Tables (Refer to Appendix 4)
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6.1.1 Identifying the major accident scenarios
When considering which credible major accident scenarios to consider as part of the risk
assessment, two options are available:
Evaluate all credible scenarios that could have a MATTE potential on the
identified receptor, or
Select a representative set of credible major accident scenarios, in line with the
HSE guidance ‘Risk analysis or ‘predictive’ aspects of COMAH safety reports
guidance for explosive sites’
10
.
Note that when using a representative set of credible major accident scenarios, it is likely
that aggregation of risk will be based on developing scenario based risk criteria as
described in section 4.3.2.
6.1.2 Determining the level of severity
For each credible major accident scenario (or representative set of credible major
accident scenarios) and receptor affected, assign the Level of Severity that would be
associated with the unmitigated consequences (see 4.1):
Table 1 (Severity/Harm criteria for consideration as a major accident) in
Appendix 4 contains consequence descriptions – the “severe” column
represents the lowest level MATTE descriptor (as taken from the DETR 1999
guidance). Consequences lower than this, although pollution incidents are
not regarded as MATTE or covered by COMAH. Consequences greater than
this level may trigger the higher threshold categories in the table.
Each column in the table has a number assigned to it: 1-4. This is the
harm/severity level.
6.1.3 Assigning a duration/recovery category
For each credible major accident scenario (or representative set of credible major
accident scenarios), assign a duration/recovery category that would be associated with
the unmitigated consequences.
It has been recognised that environmental incidents differ in ultimate consequence
depending on the (natural) recovery time of the environment. Longer term harm will
produce a less tolerable consequence than one of only short duration.
For many scenarios there will be opportunities for clean-up and remediation as a post-
incident measure which will reduce environmental harm. However, these should be
disregarded at this stage, but discussed as “mitigation” measures within the ALARP
demonstration.
To assign a duration/recovery category:
10
http://www.hse.gov.uk/comah/assessexplosives/index.htm
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Using Table 2 (Duration/Recovery criteria) in Appendix 4, select a duration
descriptor for the relevant receptor category. These should be unaided
recovery times, without restoration and clean-up activity (though natural
attenuation can be taken into account)
11
. These are broad-brush categories,
and as part of the screening process, estimates can be used.
Each duration column has a category level assigned to it: 1-4. This is the
harm/duration category.
6.1.4 Determining tolerability boundaries
Determine Tolerability boundaries from the Tolerability Assessment Matrix (Appendix 4
Table 3 - MATTE tolerability assessment matrix)
Using the harm/severity level (1-4) and the harm/duration category (1-4),
determine the overall unmitigated Consequence Level (A-D) from the matrix.
Each consequence level (A-D) has been assigned tolerability thresholds to
define the ALARP band. i.e. Intolerable and Broadly Acceptable frequencies
per receptor, per establishment, per year.
The level of risk posed by the establishment, to each receptor, is then compared with
these respective tolerability criteria, as explained in section 6.2 below.
6.2 Part 2 calculating the establishment risk frequencies
Part 1 of the risk assessment process has identified the ALARP band. Part 2 sets out
how to assess the risk from the establishment to the receptor:
Determine the risk from the establishment to a receptor
o Determine the frequency of occurrence of all scenarios based on
available failure rate and/or event data (which may include
preventative or mitigatory layers and if so these should be clearly
identified in the assessments).
o Total the frequency of all scenarios from the establishment that result
in each Consequence level (A-D) to the receptor.
o The total frequency of events which meet or exceed each
consequence level of harm should then be compared with the
tolerability thresholds established in Part 1 (section 6.1). When
comparing the establishment frequency of lower consequence levels
(e.g. B) with the assigned ALARP bands, note that the total frequency
to be considered is the total of that and higher consequence levels
(i.e. B + C + D). An example of how aggregation is completed can be
found in section 6.2.1.
11
Whilst natural un-aided recovery periods are used at this stage to determine tolerability thresholds, credit
can be claimed later in the assessment for intervention (refer to section 6.2).
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If the risk is still not Tolerable if ALARP (TifALARP) then assess other potential
control measures, accept/dismiss these within an ALARP demonstration and
integrate into establishment improvement plan as appropriate.
6.2.1 Aggregating risk - Examples
Completing the initial screening (as described in section 6.1) will have discounted
potential receptors from the risk assessment process as the screening will have
determined that a MATTE is not credible.
For those substances and scenarios that do have MATTE potential, their risks to the
relevant receptor must now be determined. As it is the total risk to the receptor that is
required, i.e. from all substances, and credible scenarios, these risks must be
aggregated. Examples of how this can be achieved for each receptor are provided in the
following sections.
o 6.2.1.1 – Single substance stored in a single tank
o 6.2.1.2 – Tank farm or group of tanks containing similar substances
o 6.2.1.3 – Groups (e.g. tank farms) with dissimilar substances/incident
consequences
In each of the examples below, the first step is to identify the credible scenarios that
could cause a MATTE to the receptor being assessed (note that this could be credible
scenarios from a single tank, multiple tank or facility based on the grouping of
substances and compartmentalisation).
Once the credible scenarios have been identified, these should then each be categorised
using the MATTE tolerability matrix (refer to Appendix 4) to give a consequence level of
either A, B, C or D - this in turn provides the frequency per receptor per establishment
per year and thus the thresholds for broadly acceptable and intolerable.
When aggregating the risk to a receptor from all credible scenarios, the following text can
be used as a guide:
Tolerability of risk to the receptor, from the establishment as a whole, will depend on the
aggregate predicted frequency of all independent accident scenarios which could impact
a given receptor at or above the respective consequence level. Thus to confirm
tolerability at level D then all independent level D predicted incident frequencies should
be aggregated. To confirm tolerability at level A, all independent level A, B, C and D
predicted incident frequencies should be aggregated.
Refer also to section 6.2.1.5 on interdependent scenarios.
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6.2.1.1 Single substance stored in a single tank
Tank 1
Receptor
If we assume that credible scenarios, consequence levels of those scenarios and event
frequencies are as follows:
Scenario (Tank Farm Tank 1)
Consequence Level*
Event frequency*
Catastrophic tank failure
B
F1, 1x10
-6
Large hole
A
F2, 1x10
-5
Small leak from tank base
A
F3, 1x10
-4
*Provided for illustrative purposes only, and at this stage does not include mitigation. For
event frequencies refer to section ‘Determining unmitigated risk frequencies’ which is a
sub-section of 6.2. For consequence level, refer to ‘MATTE thresholds’, section 3.2.
The aggregated risk to the receptor for all credible scenarios can be calculated as
follows:
Category B incident frequency = F1 = 1x10
-6
Category A incident frequency = F1 + F2 + F3 = 1x10
-6
+ 1x10
-5
+ 1x10
-4
= 1.11x10
-4
6.2.1.2 Tank farm or group of tanks containing similar substances
Tank 1
Receptor
Tank 2
Tank 3 Tank 4
If we assume that credible scenarios, consequence levels of those scenarios and event
frequencies for each of the tanks are the same (because of substance
grouping/compartmentalisation), and can be defined as follows:
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Scenario (Tank Farm
Tanks 1-4)
Consequence Level*
Event frequency*
Catastrophic tank failure
B
F1, 1x10
-6
Large hole
A
F2, 1x10
-5
Small leak from tank base
A
F3, 1x10
-4
* Provided for illustrative purposes only, and at this stage does not include mitigation.
For event frequencies refer to section ‘Determining unmitigated risk frequencies’ which is
a sub-section of 6.2. For consequence level, refer to ‘MATTE thresholds’, section 3.2
On the basis that there are now 4 tanks, the aggregated risk to the receptor for all
credible scenarios (i.e. the frequency of any one of the scenarios at or above the
relevant consequence level occurring from any one of the tanks) can be calculated as
follows:
Category B incident frequency = 4 * (F1) = 4x10
-6
Category A incident frequency = 4 * (F1 + F2 + F3) = 4 * (1x10
-6
+ 1x10
-5
+ 1x10
-4
) =
4.44x10
-4
6.2.1.3 Groups (e.g. tank farms) with dissimilar substances/incident consequences
It is likely, particularly on chemical establishments, that substances/scenarios will not be
sufficiently similar to group together. However, the Category A, B, C or D incidents can
be aggregated in the same way as indicated in the earlier examples.
Tank 1
Receptor
Warehouse
If we assume that credible scenarios, consequence levels of those scenarios and event
frequencies for each of the tanks are the same (because of substance
grouping/compartmentalisation), and can be defined as follows:
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Scenario (Tank Farm Tank 1)
Consequence Level*
Event frequency*
Catastrophic tank failure
B
F1, 1x10
-6
Large hole
A
F2, 1x10
-5
Small leak from tank base
A
F3, 1x10
-4
Scenario (Warehouse)
Consequence Level*
Event frequency*
Warehouse fire
B
F4, 1x10
-3
* Provided for illustrative purposes only, and at this stage does not include mitigation.
For event frequencies refer to section ‘Determining unmitigated risk frequencies’ which is
a sub-section of 6.2. For consequence level, refer to ‘MATTE thresholds’, section 3.2
The aggregated risk to the receptor for all credible scenarios can be calculated as
follows:
Category B incident frequency = F1 + F4 = 1x10
-6
+ 1x10
-3
= 1.001x10
-3
Category A incident frequency = F1 + F2 + F3 + F4 = 1x10
-6
+ 1x10
-5
+ 1x10
-4
+ 1x10
-3
=
1.111x10
-3
It can be seen that in this example that the warehouse fire is by far the biggest
contributor to the risk frequency, and hence this indicates where best to look at additional
control measures.
6.2.1.4 Comparison with tolerability criteria
For the single tank and warehouse example above it was determined
Category B incident frequency = 1.001x10
-3
Category A incident frequency = 1.111x10
-3
These can then be compared to the tolerability criteria as follows:
Frequency per establishment per receptor per year (unmitigated)
Frequency at which
CDOIF
Consequence Level
is equalled or
exceeded
10
-8
–10
-7
10
-7
–10
-6
10
-6
–10
-5
10
-5
–10
-4
10
-4
–10
-3
10
-3
–10
-2
>10
-2
D - MATTE
C - MATTE
B - MATTE
X
A - MATTE
X
Sub MATTE
Tolerability not considered by CDOIF
Intolerable
TifALARP
Broadly Acceptable
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The unmitigated risk is depicted above by X.
Up to this point in the assessment, no mitigation has been considered. It is now
necessary to consider what forms of mitigation are in place to further reduce risk. The
calculations above need to be repeated to include the Probability of Failure on Demand
(PFD) of any protection layers present (e.g. safety instrumented systems, secondary or
tertiary containment, emergency arrangements) to estimate the mitigated risk to each
receptor, for each consequence category and thus whether mitigated risk is tolerable.
So, for example, if the tank is bunded (PFD = 0.1) and the bunded tank and warehouse
surrounded by establishment-wide tertiary containment designed to contain fire runoff
(PFD = 0.1) then the mitigated risk to each receptor would be calculated by multiplying
the event frequency with the relevant mitigation layer PFD(s) as follows:
Scenario
(Tank Farm T1)
Consequence
Level
1
Event
frequency
1
Independent
mitigation
layers (PFD)
1
Outcome
frequency
(mitigated)
Catastrophic tank
failure
B
F1, 1x10
-6
0.1 * 0.1
F5, 1x10
-8
Large hole
A
F2, 1x10
-5
0.1 * 0.1
F6, 1x10
-7
Small leak from
tank base
A
F3, 1x10
-4
0.1 * 0.1
F7, 1x10
-6
Scenario
(Warehouse)
Consequence
Level
1
Event
frequency
1
Independent
mitigation
layer (PFD)
1
Outcome
frequency
(mitigated)
Warehouse fire
B
F4, 1x10
-3
0.1
F8, 1x10
-4
Note 1: Provided for illustrative purposes only. For event frequencies refer to section
4.3.4 ‘Determining risk frequencies’. For consequence level, refer to ‘MATTE
thresholds’, section 3.2.
The aggregated mitigated risk to the receptor for all credible scenarios can be calculated
as follows:
Category B mitigated frequency = F5 + F8 = 1x10
-8
+ 1x10
-4
= 1.0001x10
-4
Category A mitigated frequency = Category A frequencies + Category B frequencies
= (F6 + F7) + (F5 + F8)
= 1x10
-7
+ 1x10
-6
+ 1x10
-8
+ 1x10
-4
= 1.0111x10
-4
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 46 of 97
These can then be compared to the tolerability criteria as follows:
Frequency per establishment per receptor per year (mitigated)
Frequency at which
CDOIF
Consequence Level
is equalled or
exceeded
10
-8
–10
-7
10
-7
–10
-6
10
-6
–10
-5
10
-5
–10
-4
10
-4
–10
-3
10
-3
–10
-2
>10
-2
D - MATTE
C - MATTE
B - MATTE
X
A - MATTE
X
Sub MATTE
Tolerability not considered by CDOIF
The mitigated risk is depicted above by X.
It can now be seen that the mitigated risk is TifALARP. Further risk reduction needs to
be considered and implemented so far as is reasonably practicable (but an ALARP
demonstration may show the cost of further risk reduction is grossly disproportionate).
6.2.1.5 Interdependent scenarios
When summing frequencies it is important that this should only be done for independent
events.
For example, from the four tank example above (6.2.1.2), consider a further possible
level C scenario of a multi-tank fire arising from a spill followed by escalation. The
overall escalated scenario frequency would be made up from the chance of any of the
other events occurring (spills) and then escalating (ignition). The frequency of the
escalation scenario would need to be compared to the level C tolerability criteria.
However, when considering the frequencies for A and B tolerability (all events with
outcomes at or exceeding level A or B), the risk assessor would not in this case sum the
A and B spill frequencies with the escalated event (level C) frequency. This is because
the level C event is not independent from the level A and B initiating events. The
escalated scenario frequency is derived from the frequencies of the lesser events and
their probabilities of escalation (the spill frequency includes the frequency of both un-
ignited and ignited events). Summing the spill events and the escalated fire events
would result in double counting of the same initiating events.
Conversely, if the level C scenario was caused by an event independent to the level A
and B events (e.g. explosion from adjacent site) then the frequencies would be summed
when examining level A or B tolerability.
Consideration of bowtie diagrams often helps to avoid errors in logic.
Intolerable
TifALARP
Broadly Acceptable
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 47 of 97
7. Abbreviations
Abbreviation
Description
ALARP
As Low As Reasonably Practicable
AONB
Areas of Outstanding Natural Beauty
CA
Competent Authority
CBA
Cost Benefit Analysis
CDOIF
Chemical and Downstream Oil Industry Forum
CICS
Common Incident Classification Scheme
COMAH
Control of Major Accident Hazards
DETR
Department of the Environment, Transport and the Regions
DF
Disproportion Factor
EA
Environment Agency
EPR
Environmental Permitting Regulations
ESA
Environmentally Sensitive Areas
EU
European Union
LNR
Local Nature Reserves (may be referred to as Local Wildlife Site)
MAH
Major Accident Hazard
MATTE
Major Accident to the Environment
MNR
Marine Nature Reserves
NNR
National Nature Reserves
NSA
Nitrate Sensitive Areas
OS
Ordnance Survey
PFD
Probability of Failure on Demand
PPC
Pollution Prevention and Control (Regulations)
PNEC
Predicted No Effect Concentration
SAC
Special Areas of Conservation
SEPA
Scottish Environment Protection Agency
SPA
Special Protection Areas
SPZ
Source Protection Zone
SRAM
Safety Report Assessment Manual
SSSI
Site of Special Scientific Interest
TifALARP
Tolerable if As Low As Reasonably Practicable
WFD
Water Framework Directive
CDOIF
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joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 48 of 97
Revision History
Rev.
Section
Description
Date
Changed By
0.0
All
First Issue
23-Jan-2012
Peter Davidson
0.1
3
Updated with WP2 definitions
27-Jul-2012
Peter Davidson
0.2
3
Corrected WP2 definition
01-Aug-2012
Peter Davidson
0.3
3
Updated to include TA comments
08-Aug-2012
Peter Davidson
0.4
All
Updated following WP 3 Meeting 13/08/12
23-Aug-2012
Peter Davidson
0.5
All
Updated following road testing
24-Jan-2013
Peter Davidson
0.6
All
Updated to final draft – for stakeholder review
08-Feb-2013
Hugh Bray
Ian Brocklebank
Jackie Coates
Mike Nicholas
Peter Davidson
0.7
All
Stakeholder review comments incorporated
23-Jul-2013
Hugh Bray
Ian Brocklebank
Jackie Coates
Mike Nicholas
Peter Davidson
0.8
All
Final stakeholder review comments incorporated
19-Aug-2013
Hugh Bray
Ian Brocklebank
Jackie Coates
Mike Nicholas
Peter Davidson
1.0
All
First publication
18-Sep-2013
Hugh Bray
Ian Brocklebank
Jackie Coates
Mike Nicholas
Peter Davidson
1.1
All
Review following initial use
2-July-2015
Mike Nicholas
1.2
3&5
Following CA review
11-Aug-2015
Mike Nicholas
1.3
All
Updated following working group review
26-Oct-2015
Mike Nicholas
2.0
All
Updated following stakeholder review
14-Mar-2016
Mike Nicholas
CDOIF
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joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 49 of 97
Appendix 1 - Key Guidance
The following provides reference to the key guidance relating to environmental risk assessment,
and how that guidance inter-relates.
Reference should also be made to the table on the following page which provides links to access
both L111 and DETR 1999 and other related guidance and legislation.
Major Accident
COMAH Reg 2(1)
See L111 p.12
Loss of control
Series Danger
Dangerous Substance
Major Accident to
the Environment
(MATTE)
DETR (1999)
Major Accident
(persons/human
health)
Terrestrial habitats Freshwater habitats Marine Habitats Groundwater
DETR (1999)
4.1 & 4.2
Tables 1-5, 7, 8, 10
DETR (1999)
4.1 & 4.2
Tables 1, 2, 4, 5, 7, 9,
10, 12
DETR (1999)
4.1 & 4.2
Tables 1, 2, 4, 5, 7, 9,
10, 11, 12
DETR (1999)
4.1 & 4.2
Table 6
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health, safety and environmental improvements with cross-sector
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 50 of 97
Topic
Guidance
Web reference
General
COMAH
A guide to the Control of Major Accident
Hazards Regulations (COMAH) 2015
(L111 3
rd
edition) (HSE, 2015)
www.hse.gov.uk/pubns/books/l111.htm
Guidance on the Interpretation of Major
Accident to the Environment for the
Purposes of the COMAH Regulations
(DETR, 1999)
http://webarchive.nationalarchives.gov.uk/201
30402151656/http:/archive.defra.gov.uk/envir
onment/quality/chemicals/accident/documents
/comah.pdf
CA procedures and strategic topics
(signposting CA expectations on
necessary measures)
http://www.hse.gov.uk/comah/ca-guides.htm
HSE ALARP suite of guidance
http://www.hse.gov.uk/risk/expert.htm
Guidance Identifying COMAH Major
Accidents to the Environment (MATTE)
Table 3 EA, 2004
N/A
Risk
Assessment
for COMAH
(guidance
applicable to
Safety Reports
and LT risk
assessment)
AMEC Environment & Infrastructure UK
Limited 2014
Final report Annex 3: Methods for
assessing environmental consequences
(Task 3)
Development of an assessment
methodology under Article 4 of Directive
2012/18/EU on the control of major-
accident hazards involving dangerous
substances
(070307/2013/655473/ENV.C3)
https://circabc.europa.eu/webdav/CircaBC/env
/seveso_seg/Library/studies/2014%20assess
ment%20methodology%20(Article%204%20of
%20Seveso%20III)/Article%204%20methodol
ogy%20-%20Task%203%20-
%20Assessment%20environment.pdf
N.B. this annex is part of a larger report - for
summary report see
http://www.hse.gov.uk/seveso/overall-project-
report.pdf
Safety Report Assessment Manual –
Section 13 (remodelled for use with all
Safety Reports) (N.B a COMAH 2015
http://www.hse.gov.uk/comah/guidance/sram.
pdf
CDOIF
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joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 51 of 97
Topic
Guidance
Web reference
version of SRAM is to be published)
Guidance on the Environmental Risk
Assessment Aspects of COMAH Safety
Reports, COMAH CA, Dec 1999
N/A
HSG 190 Preparing Safety reports
(HSE, 1999)
http://www.hse.gov.uk/pubns/books/hsg190.ht
m
Historic
incident data
eMARS (European accident database)
https://emars.jrc.ec.europa.eu/
ARIA (includes French and EU data)
http://www.aria.developpement-
durable.gouv.fr/?lang=en
General Risk
Assessment
Guidelines for Environmental Risk
Assessment and Management – Green
Leaves III (DEFRA, 2011)
http://www.defra.gov.uk/publications/2011/11/
07/green-leaves-iii-pb13670/
Related
legislation (see
also regulator
and DEFRA
websites)
Water Framework Directive
http://ec.europa.eu/environment/water/water-
framework/index_en.html and
http://www.wfduk.org/
Habitats Directive
http://ec.europa.eu/environment/nature/legisla
tion/habitatsdirective/index_en.htm
Environmental Liability Directive
http://ec.europa.eu/environment/legal/liability/i
ndex.htm
General good
practice
EA Pollution Prevention Guidance
(PPGs)
https://www.gov.uk/government/collections/pol
lution-prevention-guidance-ppg
CDOIF
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Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 52 of 97
Topic
Guidance
Web reference
HSE Health and Safety Guidance
(HSGs)
http://www.hse.gov.uk/pubns/books/index-
hsg-ref.htm
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 53 of 97
Appendix 2 – DETR 1999 Table References
The following provides reference to the relevant definition tables in the DETR 1999 Guidance on
the interpretation of Major Accident to the Environment for the purposes of COMAH regulations.
CDOIF
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joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
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Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 54 of 97
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 55 of 97
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 56 of 97
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 57 of 97
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 58 of 97
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 59 of 97
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 60 of 97
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 61 of 97
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 62 of 97
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 63 of 97
Appendix 3 – Information Sources
Note: For Wales, please contact establishment officer – further guidance to be available after the formation of Natural Resources Wales. The following
web links were correct at the time of publication, but are subject to change.
DETR
Table
Receptor Type
How can I decide
which receptors I
have around my
establishment?
How do I use/interpret
the information on the
website?
What features are
most relevant and
where can I find detail
of them – for example
designation land,
categorisation for
water
Which agency or body
should I contact if I
need further
information on helping
me determine MATTE
potential?
What impact have
‘similar’ incidents
had, and where can I
find more information
about these?
How do I use the
information gathered
above to help me
work out
Consequence (Extent,
Severity and
Duration)?
1
Designated
Land/Water Sites
(National)
England
www.magic.gov.uk/
http://www.natureonthe
map.naturalengland.or
g.uk/
International sites tab
www.jncc.gov.uk/
As per item 2 below but
focus on sites of national
importance
SSSIs, National Nature
Reserves, Marine
Conservation Zones
Natural England
Environment Agency
See also guidance on
Environmental Damage
(http://www.defra.gov.uk
/environment/quality/env
ironmental-liability/) for
interpretational guidance
on Damage to species
and habitats.
see item 2 below
Wales
Scotland
Scotland's
Environmental Web
interactive mapping
page (SEWeb)
“Wildlife” tab
Scottish Natural
Heritage Website
“Protected Areas” tab
www.jncc.gov.uk/
On both websites
interactive maps can be
used to search for and
identify designated sites.
The Marine Atlas can
assist in identifying the
location and population of
some species which may
be of interest.
SSSIs, National Nature
Reserves etc.
The area of the site can
be found on the relevant
information sheet or
citation for the area this
can be accessed via the
Joint Nature
Conservation Committee
(JNCC) website.
In some cases the
qualifying population
may also be included.
Scottish Natural
Heritage (SNH)
http://www.snh.gov.uk/
Relevant Fishery Board
List of fisheries boards
See also guidance on
Environmental Damage
(http://www.scotland.gov
.uk/Resource/Doc/21119
9/0087791.doc) for
interpretational guidance
on Damage to species
and habitats.
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 64 of 97
DETR
Table
Receptor Type
How can I decide
which receptors I
have around my
establishment?
How do I use/interpret
the information on the
website?
What features are
most relevant and
where can I find detail
of them – for example
designation land,
categorisation for
water
Which agency or body
should I contact if I
need further
information on helping
me determine MATTE
potential?
What impact have
‘similar’ incidents
had, and where can I
find more information
about these?
How do I use the
information gathered
above to help me
work out
Consequence (Extent,
Severity and
Duration)?
2
Designated
Land/Water Sites
(International)
England
www.magic.gov.uk/
http://www.natureonthe
map.naturalengland.or
g.uk/
International sites tab
www.jncc.gov.uk/
In magic the interactive
map can be used to
search “Rural
Designations – Statutory”
(step 1) around a given
location (step 2). This
opens a new map with
further map tools such as
a radius / linear and
polygon searches or
identify features at specific
point.
Depending on site
location, other layers, such
as “Coastal and marine
resources atlas” might
also be relevant.
SAC, SPA, Ramsar sites
and their component
SSSIs
Use magic to find the
sites (e.g. radius search
or manually explore map
along length of a
stream/river) – then
follow links to data (e.g.
on the JNCC and
Natural England
websites)
Natural England,
http://www.naturalenglan
d.org.uk/
Environment Agency
See also guidance on
Environmental Damage
(http://www.defra.gov.uk
/environment/quality/env
ironmental-liability/) for
interpretational guidance
on Damage to species
and habitats
https://emars.jrc.ec.eur
opa.eu/
http://www.aria.develop
pement-
durable.gouv.fr/index_e
n.html
Accident databases,
like the two above can
be searched using
substance based
keywords / CAS /
industry type and the
impacts from the
shortlisted incidents
compared to those that
might be credible for
the installation under
assessment.
Once receptors have
been identified either
assume impact is
possible and screen
scenario in or gather
more detailed data on
the vulnerability of
those to impact from
the chemicals
concerned need to be
assessed.
e.g. data at
http://evidence.environ
ment-
agency.gov.uk/Chemic
alStandards/home.aspx
This to be considered
along with the site
conservation objectives
and status.
Wales
Scotland
Scotland's
Environmental Web
interactive mapping
page (SEWeb)
“Wildlife” tab
Scottish Natural
Heritage Website
“Protected Areas” tab
www.jncc.gov.uk/
On both websites
interactive maps can be
used to search for and
identify designated sites.
The Marine Atlas can
assist in identifying the
location and population of
some species which may
be of interest.
SAC, SPA, Ramsar sites
and their component
SSSIs
The area of the site can
be found on the relevant
information sheet or
citation for the area this
can be accessed via the
links on the SNH
website or via the Joint
Nature Conservation
Committee (JNCC)
website.
In some cases the
qualifying population
may also be included.
Scottish Natural
Heritage (SNH)
http://www.snh.gov.uk/
Relevant Fishery Board
List of fisheries boards
See also guidance on
Environmental Damage
(http://www.scotland.gov
.uk/Resource/Doc/21119
9/0087791.doc) for
interpretational guidance
on Damage to species
and habitats.
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 65 of 97
DETR
Table
Receptor Type
How can I decide
which receptors I
have around my
establishment?
How do I use/interpret
the information on the
website?
What features are
most relevant and
where can I find detail
of them – for example
designation land,
categorisation for
water
Which agency or body
should I contact if I
need further
information on helping
me determine MATTE
potential?
What impact have
‘similar’ incidents
had, and where can I
find more information
about these?
How do I use the
information gathered
above to help me
work out
Consequence (Extent,
Severity and
Duration)?
3
Other designated
land
England
As per Table 1 row 1
“International sites)
Local Wildlife Trusts
Local Authority
Local Records Centre
Wales
Scotland
Useful webpages
include:
Map of National Scenic
Areas
National Parks in
Scotland webpage
SNH Local nature
reserves webpage
Wildlife Trust Site
search
Scottish Natural
Heritage
Local Wildlife Trusts
Local Authority
Local Records Centre
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 66 of 97
DETR
Table
Receptor Type
How can I decide
which receptors I
have around my
establishment?
How do I use/interpret
the information on the
website?
What features are
most relevant and
where can I find detail
of them – for example
designation land,
categorisation for
water
Which agency or body
should I contact if I
need further
information on helping
me determine MATTE
potential?
What impact have
‘similar’ incidents
had, and where can I
find more information
about these?
How do I use the
information gathered
above to help me
work out
Consequence (Extent,
Severity and
Duration)?
4
Scarce Habitat
England
www.magic.gov.uk/
http://www.natureonthe
map.naturalengland.or
g.uk/
International sites tab
www.jncc.gov.uk/
In magic, lower
designations might also be
found in other datasets
such as “Rural
Designations – Other” and
“Rural Land-Based
Schemes”
Local Wildlife Trusts
Local Authority
Local Records Centre
Wales
Scotland
UK BAP species and
habitats webpage
SNH Bio-diversity
webpage
Local authority
biodiversity action
plans
SNH Geo-diversity
webpage
Local Wildlife Trusts
Local Authority
Local Records Centre
5
Widespread Habitat
– Non-designated
land
England
See table 1.2 of H1
Annex A for data
sources
e.g. OS mapping
Use data sources to
establish main types of
land use, and in particular
any agricultural or areas of
public access
Generally land use can
be determined by OS
mapping, and if not by
local field surveying
(walking / driving round
to see what land-use is
evident.)
For food safety – FSA
For risk to people, HSE
& HPA
Wales
Scotland
e.g. OS mapping
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 67 of 97
DETR
Table
Receptor Type
How can I decide
which receptors I
have around my
establishment?
How do I use/interpret
the information on the
website?
What features are
most relevant and
where can I find detail
of them – for example
designation land,
categorisation for
water
Which agency or body
should I contact if I
need further
information on helping
me determine MATTE
potential?
What impact have
‘similar’ incidents
had, and where can I
find more information
about these?
How do I use the
information gathered
above to help me
work out
Consequence (Extent,
Severity and
Duration)?
5
Widespread Habitat
– Non-designated
water
England
See table 1.2 of H1
Annex A for data
sources
e.g. OS mapping
Angling trust
http://www.anglingtrust.
net/
Use data sources to
establish main types of
land use, and in particular
angling trust to find the
local angling society, club
or fishery.
Generally land use can
be determined by OS
mapping, and if not by
local field surveying
(walking / driving round
to see what land-use is
evident.)
For food safety – FSA
For risk to people, HSE
& HPA
For fishing – local
angling society
Wales
Scotland
e.g. OS mapping
6
Groundwater Source
of Public or Private
Drinking Water
England
See What’s in your
Backyard -
http://www.environmen
t-agency.gov.uk/
For surface water
abstraction information
discuss with EA site
officer or contact
03708 506 506 or
enquiries@environmen
t-agency.gov.uk
In WIYBY, enter place or
postcode, select the
groundwater topic and
check the Groundwater
Source Protection Zone
box (in Map legend on Left
Hand side).
N.B. you may need zoom
in or out – this layer only
displays at certain map
scales.
Principal or secondary
aquifer, and SPZs are
depicted as a colour
overlay
Environment Agency
If drinking water is a
relevant receptor the
drinking water
standards will need to
be considered – see
http://evidence.environ
ment-
agency.gov.uk/Chemic
alStandards/home.aspx
Wales
Scotland
Contact SEPA and the
relevant local Authority
asking for the location
of Drinking Water
abstraction in the area
concerned.
SEPA
Private water supplies
are the responsibility of
owners and users and
are regulated by local
authorities.
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 68 of 97
DETR
Table
Receptor Type
How can I decide
which receptors I
have around my
establishment?
How do I use/interpret
the information on the
website?
What features are
most relevant and
where can I find detail
of them – for example
designation land,
categorisation for
water
Which agency or body
should I contact if I
need further
information on helping
me determine MATTE
potential?
What impact have
‘similar’ incidents
had, and where can I
find more information
about these?
How do I use the
information gathered
above to help me
work out
Consequence (Extent,
Severity and
Duration)?
6
Groundwater Non
Drinking Water
Source
England
See What’s in your
Backyard -
http://www.environmen
t-agency.gov.uk/
In WIYBY, enter place or
postcode, select the
groundwater topic and
check the Aquifer Maps
(either superficial or
bedrock or both).
Principal or secondary
aquifers, including those
not used for drinking water
will appear as coloured
areas. See also the topic
“River basin Management
Plans – Groundwater” for
current and predicted
status.
Groundwater bodies are
a distinct volume of
groundwater within an
aquifer or aquifers
Environment Agency
See also guidance on
Environmental Damage
(http://www.defra.gov.uk
/environment/quality/env
ironmental-liability/) for
interpretational guidance
on Damage to water
Wales
Scotland
SEPA has mapped all
bedrock aquifers and
selected extensive
sand and gravel
aquifers as
groundwater bodies,
and these underlie the
whole mainland of
Scotland and many
islands. These
groundwater bodies
can be seen on our
interactive map.
Open the map and click on
the double down arrow
next to table of contents.
From the menu click the
2008 Classification status
box. Groundwater bodies
will now be shown on the
map. Use the “identify”
icon from the menu at the
top of the map to identify
which groundwater body is
under the area being
assessed.
Other more localised sand
and gravel aquifers have
not been mapped as
groundwater bodies due to
their inherent variability
and a lack of information.
The presence of these
SEPA
See also guidance on
Environmental Damage
(http://www.scotland.gov
.uk/Resource/Doc/21119
9/0087791.doc) for
interpretational guidance
on damage to water.
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 69 of 97
DETR
Table
Receptor Type
How can I decide
which receptors I
have around my
establishment?
How do I use/interpret
the information on the
website?
What features are
most relevant and
where can I find detail
of them – for example
designation land,
categorisation for
water
Which agency or body
should I contact if I
need further
information on helping
me determine MATTE
potential?
What impact have
‘similar’ incidents
had, and where can I
find more information
about these?
How do I use the
information gathered
above to help me
work out
Consequence (Extent,
Severity and
Duration)?
more localised aquifers
can only be determined
using site specific data.
SEPA’s Position
Statement WAT-PS-10-01,
Assigning groundwater
assessment criteria for
pollutant inputs provides
more details on how to
make this determination.
6
Groundwater in
unproductive strata
(i.e. where
groundwater is a
pathway to surface
and terrestrial
ecosystems)
England
See What’s in your
Backyard -
http://www.environmen
t-agency.gov.uk/
Area outside of SPZs,
aquifers (groundwater
bodies) are unproductive
strata and would not
appear as coloured when
the layers are selected as
described above .
Wales
Scotland
See groundwater
bodies above.
Groundwater bodies
underlie the whole
mainland of Scotland and
many islands, and
therefore may be a
pathway.
SEPA
7
Soil or sediment (i.e.
as receptor rather
than purely a
pathway)
England
Further information on
Environmental
Damage Regulations
http://www.defra.gov.u
k/environment/quality/e
nvironmental-liability/
and see the in depth
guide in particular
The definitions of
Environmental damage to
conservation sites and
water is aligned to the
MATTE thresholds above
and thus covered by the
above rows, thus potential
environmental damage to
land should be the key
consideration for this
receptor.
Damage to land is:
...contamination of land
by substances,
preparations, organisms
or micro-organisms that
results in a significant
risk of adverse effects
on human health.
Environment Agency or
Local Authority
see
http://www.defra.gov.uk
/environment/quality/en
vironmental-liability/
which includes incident
returns detailing
previous Environmental
Damage cases
Wales
Scotland
Further information on
The Scottish Government
SEPA or Local Authority
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 70 of 97
DETR
Table
Receptor Type
How can I decide
which receptors I
have around my
establishment?
How do I use/interpret
the information on the
website?
What features are
most relevant and
where can I find detail
of them – for example
designation land,
categorisation for
water
Which agency or body
should I contact if I
need further
information on helping
me determine MATTE
potential?
What impact have
‘similar’ incidents
had, and where can I
find more information
about these?
How do I use the
information gathered
above to help me
work out
Consequence (Extent,
Severity and
Duration)?
the application of the
Environmental Liability
Regulations can be
found at the
SEPA Environmental
Liability Regulations
web-page and the
Scottish Government
Environmental Liability
Regulations web-page
ELR Technical Guidance
gives definitions and
examples of
“Environmental Damage”.
8
Built environment
England
http://www.english-
heritage.org.uk/
www.magic.gov.uk/
“Rural Designations –
statutory” for
scheduled monuments
and world heritage
sites
In the English heritage site
you can search The
National Heritage List for
England to search for
listed buildings in your
area and download copies
of individual entries. The
site also provides world
heritage information
Use English Heritage
site advanced search to
limit search to Grade I
listing in a given location
then from the search
results see list entry
summary for detail
English Heritage, Local
planning authority for
listed
buildings, Institute of
historic building
conservation
(www.ihbc.org.uk), The
National Trust, County
Archaeologist
(local county council)
Wales
Scotland
Scotland's
Environmental Web
interactive mapping
page
“Built Environment”
Tab
Marked on the map as:
Listed Buildings;
Conservation Areas;
Scheduled Monuments; or
World Heritage sites.
Historic Scotland
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 71 of 97
DETR
Table
Receptor Type
How can I decide
which receptors I
have around my
establishment?
How do I use/interpret
the information on the
website?
What features are
most relevant and
where can I find detail
of them – for example
designation land,
categorisation for
water
Which agency or body
should I contact if I
need further
information on helping
me determine MATTE
potential?
What impact have
‘similar’ incidents
had, and where can I
find more information
about these?
How do I use the
information gathered
above to help me
work out
Consequence (Extent,
Severity and
Duration)?
9
Various receptors
10
Particular species
England
In addition to magic
and JNCC (links
above), see the
National Biodiversity
Netwok’s Gateway
http://data.nbn.org.uk/
In NBN gateway search
for data by species or site
Explore species
distributions, whole
datasets, protected sites
and habitats using the
interactive map
See in particular note on
appendix 4 where such
species might be
associated with a
designated site (thus
proportion of the local
population harmed, not
national population is
used)
Natural England and
others species specific
bodies such as the
Amphibian and Reptile
Conservation trust and
the British Trust for
Ornithology
Wales
Scotland
11
Marine
England
www.magic.gov.uk/
Select “Coastal and
marine resources
atlas” (step 1 of
interactive map).
OS mapping
See also What’s in
your Backyard -
http://www.environmen
t-agency.gov.uk/
For the status of coastal
waters - In WIYBY, enter
place or postcode, select
the River Basin
Management Plan
(Coastal or estuarine)
topic
Water body status
depicted as a colour
overlay
Environment Agency
and Inshore Fishery and
Conservation
Authorities,
See also guidance on
Environmental Damage
(http://www.defra.gov.uk
/environment/quality/env
ironmental-liability/) for
interpretational guidance
on Damage to water
Wales
Scotland
Scotland's
Environmental Web
(SEWeb) interactive
For the status of coastal
waters - In SEWeb, enter
place or postcode, select
SEPA
See also guidance on
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 72 of 97
DETR
Table
Receptor Type
How can I decide
which receptors I
have around my
establishment?
How do I use/interpret
the information on the
website?
What features are
most relevant and
where can I find detail
of them – for example
designation land,
categorisation for
water
Which agency or body
should I contact if I
need further
information on helping
me determine MATTE
potential?
What impact have
‘similar’ incidents
had, and where can I
find more information
about these?
How do I use the
information gathered
above to help me
work out
Consequence (Extent,
Severity and
Duration)?
mapping webpage
The Marine Atlas and
the Interactive Marine
Planning Tool can
assist in identifying the
location and population
of some species which
may be of interest.
“Advanced Maps” then
“Water” and the relevant
classification requirements
(Coastal or estuarine).
OS Explorer series (1:25
000 scale) shows the
position of high and low
tide marks.
Environmental Damage
(http://www.scotland.gov
.uk/Resource/Doc/21119
9/0087791.doc) for
interpretational guidance
on damage to water.
12
Fresh and estuarine
water habitats
England
www.magic.gov.uk/
For estuaries select
“Coastal and marine
resources atlas” (step
1 of interactive map).
OS mapping
See also What’s in
your Backyard -
http://www.environmen
t-agency.gov.uk/
For the status of fresh and
estuarine waters - In
WIYBY, enter place or
postcode, select the River
Basin Management Plan
(Rivers, Lakes, Estuarine)
topics
Water body status
depicted as a colour
overlay
Environment Agency
See also guidance on
Environmental Damage
(http://www.defra.gov.uk
/environment/quality/env
ironmental-liability/) for
interpretational guidance
on Damage to water
Wales
Scotland
Scotland's
Environmental Web
(SEWeb) interactive
mapping webpage
For the status of fresh and
estuarine waters - In
SEWeb, enter place or
postcode, select “Water”
and the relevant
classification requirements
(Rivers, Lochs, Estuarine)
Water body status can
be selected
SEPA
See also guidance on
Environmental Damage
(http://www.scotland.gov
.uk/Resource/Doc/21119
9/0087791.doc) for
interpretational guidance
on damage to water.
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 73 of 97
Appendix 4 – MATTE tolerability tables
Table 4.1 - Severity/Harm criteria for consideration as a major accident (based on unmitigated consequence)
Row
DETR
Table Ref
Receptor
Type
Severity of Harm
Reference to
Table 2
Comments
Significant
Severe
Major
Catastrophic
Corresponding
Harm/Duration /
Recovery row in Table
2
The 'Severe' to
'Catastrophic' levels of
harm are considered to
be included as 'Serious'
with respect to the
COMAH definition of a
major accident.
Receptors include:
While this level of
harm might be
significant pollution, it
is not considered a
MATTE.
DETR Criteria - the
lowest level of harm
that might be
considered MATTE.
Severity Level →
1
2
3
4
1
1
Designated
Land/Water Sites
(Nationally
important)
<0.5ha or <10%
>0.5ha or
10-50% of site area,
associated linear
feature or population
>50% of site area,
associated linear
feature or population
N/A
Land or
Surface Water
NNR, SSSI, MNR
2
2
Designated
Land/Water Sites
(Internationally
important)
<0.5ha or <5% (<5%
LF/Pop)
>0.5ha or
5-25% of site area or
5-25% of associated
linear feature or
population
25-50% of site area,
associated linear
feature or population
>50% of site area,
associated linear
feature or population
Land or
Surface Water
SAC, SPA, RAMSAR
3
3
Other designated
Land
<10ha or <10%
10-100ha or
10-50% of land
>100ha or >50% of
land
N/A
Land
ESA, AONB, National
Park, etc.
4
4
Scarce Habitat
<2 ha or <10%
2-20ha or
10-50% of habitat
>20ha or >50% of
habitat
N/A
Land or
Surface Water
BAP habitats, geological
features
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 74 of 97
Row
DETR
Table Ref
Receptor
Type
Severity of Harm
Reference to
Table 2
Comments
Significant
Severe
Major
Catastrophic
Corresponding
Harm/Duration /
Recovery row in Table
2
The 'Severe' to
'Catastrophic' levels of
harm are considered to
be included as 'Serious'
with respect to the
COMAH definition of a
major accident.
Receptors include:
While this level of
harm might be
significant pollution, it
is not considered a
MATTE.
DETR Criteria - the
lowest level of harm
that might be
considered MATTE.
Severity Level →
1
2
3
4
5
5
Widespread
Habitat - Non-
designated Land
<10ha
Contamination of 10-
100ha of land,
preventing growing of
crops, grazing of
domestic animals or
renders the area
inaccessible to the
public because of
possible skin contact
with dangerous
substances.
Alternatively,
contamination of 10ha
or more of vacant
land.
100-1000ha
(applied as per text
under 'Severe')
>1000ha
(applied as per text
under 'Severe')
Land
Land/water used for
agriculture, forestry,
fishing or aquaculture
6
5
Widespread
Habitat - Non-
designated Water
Contamination of
aquatic habitat which
prevents fishing or
aquaculture or renders
is inaccessible to the
public.
N/A
N/A
Surface Water
Land/water used for
agriculture, forestry,
fishing or aquaculture
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 75 of 97
Row
DETR
Table Ref
Receptor
Type
Severity of Harm
Reference to
Table 2
Comments
Significant
Severe
Major
Catastrophic
Corresponding
Harm/Duration /
Recovery row in Table
2
The 'Severe' to
'Catastrophic' levels of
harm are considered to
be included as 'Serious'
with respect to the
COMAH definition of a
major accident.
Receptors include:
While this level of
harm might be
significant pollution, it
is not considered a
MATTE.
DETR Criteria - the
lowest level of harm
that might be
considered MATTE.
Severity Level →
1
2
3
4
7
6
Groundwater
Source of Drinking
Water
Interruption of drinking
water supply <1000
person-hours
or
For England & Wales
only <1ha SPZ
Interruption of
drinking water
supplied from a
ground or surface
source (where
persons affected x
duration in hours [at
least 2] > 1,000)
or
For England & Wales
only
1-10ha of SPZ where
drinking water
standards are
breached
>1 x 10
7
person-hours
interruption of drinking
water (a town of
~100,000 people
losing supply for
month)
or
For England & Wales
only 10-100ha SPZ
drinking water
standards breached
>1 x 10
9
person-hours
interruption of drinking
(~1 million people
losing supply for 1
month)
or
For England & Wales
only >100ha SPZ
drinking water
standards breached
Groundwater or
surface water drinking
water source (public or
private)
Drinking water sources
(SPZs in England and
Wales) - See 3.2.3 for
further guidance.
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 76 of 97
Row
DETR
Table Ref
Receptor
Type
Severity of Harm
Reference to
Table 2
Comments
Significant
Severe
Major
Catastrophic
Corresponding
Harm/Duration /
Recovery row in Table
2
The 'Severe' to
'Catastrophic' levels of
harm are considered to
be included as 'Serious'
with respect to the
COMAH definition of a
major accident.
Receptors include:
While this level of
harm might be
significant pollution, it
is not considered a
MATTE.
DETR Criteria - the
lowest level of harm
that might be
considered MATTE.
Severity Level →
1
2
3
4
8
6
Groundwater –
non Drinking
Water Source
<1ha
1-100ha of aquifer
where water quality
standards are
breached (or
hazardous substance
is discernible)
100-10,000ha
>10,000ha
Groundwater (except
drinking water
sources)
Aquifers (non-drinking
water sources) :
Principal and secondary
as depicted as coloured
areas on aquifer maps -
See 3.2.3 for further
guidance.
9
6
Groundwater in
unproductive
strata
Groundwater not a
pathway to another
receptor.
Where the groundwater is a pathway for another receptor assess against
relevant criteria for the receptor.
N/A
Uncoloured areas on
aquifer maps.
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Row
DETR
Table Ref
Receptor
Type
Severity of Harm
Reference to
Table 2
Comments
Significant
Severe
Major
Catastrophic
Corresponding
Harm/Duration /
Recovery row in Table
2
The 'Severe' to
'Catastrophic' levels of
harm are considered to
be included as 'Serious'
with respect to the
COMAH definition of a
major accident.
Receptors include:
While this level of
harm might be
significant pollution, it
is not considered a
MATTE.
DETR Criteria - the
lowest level of harm
that might be
considered MATTE.
Severity Level →
1
2
3
4
10
7
Soil or sediment
(i.e. as receptor
rather than purely
a pathway)
Contamination not
leading to
environmental
damage (as per ELD),
or not significantly
affecting overlying
water quality.
Contamination of 10-
100ha of land etc. as
per Widespread
Habitat;
Contamination
sufficient to be
deemed
environmental
damage
(Environmental
Liability Directive)
Contamination of
100-1000ha of land,
as per Widespread
Habitat;
Contamination
rendering the soil
immediately
hazardous to humans
(e.g. skin contact) or
the living environment,
but remediation
available.
Contamination of
>1000ha of land, as
per Widespread
Habitat;
Contamination
rendering the soil
immediately
hazardous to humans
(e.g. skin contact) or
the living environment
and remediation
difficult or impossible.
Land
11
8
Built environment
Damage below a level
at which designation
of importance would
be withdrawn.
Damage sufficient for
designation of
importance to be
withdrawn.
Feature of built
environment subject to
designation of
importance entirely
destroyed.
N/A
Built Environment
This is limited to Grade
1 / Cat A Listed
buildings, scheduled
ancient monuments,
conservation area, etc.
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Row
DETR
Table Ref
Receptor
Type
Severity of Harm
Reference to
Table 2
Comments
Significant
Severe
Major
Catastrophic
Corresponding
Harm/Duration /
Recovery row in Table
2
The 'Severe' to
'Catastrophic' levels of
harm are considered to
be included as 'Serious'
with respect to the
COMAH definition of a
major accident.
Receptors include:
While this level of
harm might be
significant pollution, it
is not considered a
MATTE.
DETR Criteria - the
lowest level of harm
that might be
considered MATTE.
Severity Level →
1
2
3
4
12
9
Various receptors.
Should not be
used to identify
and assess
MATTE.
N/A
N/A
N/A
N/A
N/A
Refer to DETR.
Standards relating to
continuous emissions,
contained in other EU
legislation.
13
10
Particular species
(Note - these
criteria apply
nationally - i.e.
England, Wales,
Scotland)
Loss of <1% of animal
or <5% of plant ground
cover in a habitat.
Loss of 1-10% of
animal or
5-50% of plant ground
cover.
Loss of 10-90% of
animal or 50-90% of
plant ground cover.
Total loss (>90%) of
animal or plant ground
cover.
Land
14
11
Marine
<2ha littoral or sub-
littoral zone, <100ha of
open sea benthic
community, <100 dead
sea birds (<500 gulls),
<5 dead/significantly
impaired sea
mammals
2-20ha littoral or sub-
littoral zone,
100-1000ha of open
sea benthic
community,
100-1000 dead sea
birds (500-5000 gulls),
5-50 dead/significantly
impaired sea
mammals
20-200ha littoral or
sub-littoral zone,
100-10,000ha of open
sea benthic
community,
1000-10,000 dead sea
birds
(5,000-50,000 gulls),
50-500
dead/significantly
impaired sea
mammals
>200ha littoral or sub-
littoral zone,
>10000ha of open sea
benthic community,
>10000 dead sea
birds (>50000 gulls),
>500
dead/significantly
impaired sea
mammals
Surface Water
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Row
DETR
Table Ref
Receptor
Type
Severity of Harm
Reference to
Table 2
Comments
Significant
Severe
Major
Catastrophic
Corresponding
Harm/Duration /
Recovery row in Table
2
The 'Severe' to
'Catastrophic' levels of
harm are considered to
be included as 'Serious'
with respect to the
COMAH definition of a
major accident.
Receptors include:
While this level of
harm might be
significant pollution, it
is not considered a
MATTE.
DETR Criteria - the
lowest level of harm
that might be
considered MATTE.
Severity Level →
1
2
3
4
15
12
Fresh and
estuarine water
habitats
Impact below that of
Severity level 2
WFD Chemical or
ecological status
lowered by one class
for 2-10km of
watercourse or 2-20ha
or 10-50% area of
estuaries or ponds.
Plus interruption of
drinking water
supplies, as per DETR
Table 6
WFD Chemical or
ecological status
lowered by one class
for 10-200km of
watercourse or 20-
200ha or 50-90% area
of estuaries and
ponds. Plus
interruption of drinking
water supplies, as per
DETR Table 6
WFD Chemical or
ecological status
lowered by one class
for >200km of
watercourse or
>200ha or >90% area
of estuaries and
ponds. Plus
interruption of drinking
water supplies, as per
DETR Table 6
Surface Water
Notes for Table 4.1:
In applying the criteria on this sheet, an estimate of the mean population of species will be required, subject to data available. Variability in population might
be relevant for later detailed scenario assessments, but a mean is more relevant to the initial selection criteria here.
When applying the criteria above, note that receptors are not mutually exclusive - for example some sites are both Ramsar and SSSI, while the 'widespread
habitat' rows might apply irrespective of any specific designations.
To avoid disproportionate application of percentage criteria on small receptors, for small sites, the percentage criteria will not reduce the threshold to lower
than half the area/distance criteria.
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Glossary of terms for Table 4.1
Littoral: pertaining to the shore of a lake, sea, or ocean.
Sub-littoral zone: from the low water line to the edge of the continental shelf
Benthic community: is made up of organisms that live in and on the bottom of the ocean floor.
WFD: Water Framework Directive
SAC: Special Area of Conservation
SPA: Special Protection Area
RAMSAR: Wetlands of international importance,
NNR: National Nature Reserve
MNR: Marine Nature Reserve
BAP habitat: Biodiversity Action Plan habitat
ESA: Environmentally Sensitive Area
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Table 4.2 – Duration/Recovery criteria (based on unmitigated consequence)
Description
Short term
Medium term
Long term
Very long term
Harm with such short
recovery is not
considered a MATTE.
Harm Duration Category →
1
2
3
4
Groundwater or surface water drinking water
source (public or private)
Harm affecting
drinking water
source or SPZ
< 6 years
Harm affecting
drinking water
source or SPZ
>6 years
Groundwater (except drinking water sources):
WFD Hazardous/Non Hazardous Substances
WFD hazardous
substances < 3
months
WFD hazardous
subs > 3 months
WFD hazardous
subs
> 6yrs
WFD hazardous
subs
>20 years
WFD non-hazardous
substances < 1yr
WFD non-
hazardous
substances > 1yr
WFD non-hazardous
substances
>10 years
WFD non-hazardous
substances
>20 Years
Surface water
(except drinking water sources – see above)
< 1year
>1 year
>10 years
>20 years
Land
< 3 years or < 2
growing seasons for
agricultural land
> 3 years or > 2
growing seasons for
agricultural land
>20 years
>50 years
Built environment
Can be repaired in < 3
years, such that its
designation can be
reinstated
Can be repaired in
> 3 years, such that
its designation can
be reinstated
Feature destroyed,
cannot be rebuilt, all
features except
world heritage site
Feature destroyed,
cannot be rebuilt,
world heritage site
N.B. New groundwater duration categories have been included in version 2 of this guideline (c.f. version 1) to set a duration threshold below which pollution of groundwater would not be
considered MATTE (irrespective of extent & severity) and to aid prioritisation of larger risk scenarios by further differentiating between different scales of MATTE to groundwater
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Notes for Table 4.2:
Separate criteria are provided in Table 2 depending on the nature of the site, be it land, surface water or groundwater -
these shall be applied in conjunction with the corresponding harm criteria in Table 4.1.
Durations have been derived through working group discussion, and expert judgement with reference to other legal
requirements. For example, the 6 yr threshold for drinking water duration cat. 3 vs 4 has been derived considering the WFD
European reporting cycle. The difference between groundwater hazardous substances and surface water is derived from
the WFD directive duty to prevent entry to groundwater (see http://ec.europa.eu/environment/water/water-
framework/info/intro_en.htm for discussion of the different approach to groundwater vs surface water). Land generally takes
longer to recover naturally than surface water environments, so has longer duration thresholds. Groundwater generally has
the longest recovery periods however due to the Water Framework Directive requirements to prevent pollution to
groundwater more stringent thresholds have been applied.
It is common for the chemical quality of receptors to recover more rapidly than ecological/conservation status. Both
chemical and ecological/conservation status should be considered and the duration category should be based on the
longest duration. Thus even if the chemical quality of a receptor can recover in the short term, ecological damage may have
been caused which involves a longer term recovery.
The criteria are based on estimating the likely time for the habitat (or species, etc.) to substantially recover (unaided) from
the damage caused. For ecological criteria, complete recovery is difficult to judge and hence it is suggested that this should
be clarified as >80% of the damage recovered. For chemical criteria (e.g. drinking water standards), recovery to below
standard concentration should be considered.
For harm affecting drinking water, duration is also covered by the severity calculation (person-hours) in table 1. For
guidance on identifying water framework directive groundwater hazardous substances see
(http://www.wfduk.org/stakeholders/mrv-work-area)
For harm to particular species, duration of recovery relates to the population as a whole. Further guidance on species
recovery can be found in Environmental Damage Regulations Guidance, DEFRA (2009) – e.g. pages 85 onwards illustrate
the issues using a Red Kite example.
The time specified for long and very long term harm durations are stated as guides to help assess potential recovery time if
the impact to the receptor was left to natural recovery alone. Consider the mechanisms that could influence this, such as
(weathering, natural bio-remediation or breakdown and replenishment through flushing, dilution, repopulation of species
from neighbouring areas etc.) and if these alone could achieve the natural recovery in this specified time. When
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demonstrating the tolerability of risk, credit can be claimed for intervention where this results in more rapid recovery.
Table 4.3 - Method and Matrix for Deriving Receptor Tolerability for MATTE (based on unmitigated consequence)
1
Identify scenario and receptor affected.
2
Select Harm Severity Level (Table 1)
3
Select Duration / Recovery Category (Table 2)
4
Apply to Tolerability Assessment Matrix to determine tolerability boundaries.
Severity of Harm
4
C
D
D
Frequency at
which the
CDOIF
consequence
level is reached
or exceeded
Frequency per receptor per
establishment per year
3
B
C
D
Intolerable
Broadly Acceptable
2
A
B
C
(greater than)
(less than)
1
Sub-MATTE Harm
A
1.0 E-02
1.0 E-04
1
2
3
4
B
1.0 E-03
1.0 E-05
Harm Duration Category
C
1.0 E-04
1.0 E-06
D
1.0 E-05
1.0 E-07
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NOTE: The tolerability thresholds above are derived from DETR (1999) and the DETR (1998) Harm Report [add reference] combined with a verification
exercised based on 10 years of major accident hazard data in the UK.
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Appendix 5 – Tables to assess MATTE potential
Table 5.1 – MATTE Potential Summary Matrix
Row
DETR
Table
Ref
Receptor Type
MATTE threshold
Substance / group of substances (see table 4 of
Appendix 5 for description of substances or
substance groups)
See Table 2 of
Appendix 5 for
receptor detail
See Table 3 of
Appendix 5 for
description of
identified MATTE
scenarios
1
2
3
4
5
6
7
Etc.
Etc.
1
1
Designated Land/Water Sites
(Nationally important)
>0.5ha or 10-50%
2
2
Designated Land/Water Sites
(Internationally important)
>0.5ha or 5-25% (5-25%
LF/Pop)
3
3
Other designated Land
10-100ha or 10-50%
4
4
Scarce Habitat
2-20 ha or 10-50%
5
5a
Widespread Habitat - Non-
designated Land
>10ha
6
5b
Widespread Habitat - Non-
designated Water
Contamination of aquatic
habitat which prevents
fishing or aquaculture or
renders is inaccessible to
the public.
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Row
DETR
Table
Ref
Receptor Type
MATTE threshold
Substance / group of substances (see table 4 of
Appendix 5 for description of substances or
substance groups)
See Table 2 of
Appendix 5 for
receptor detail
See Table 3 of
Appendix 5 for
description of
identified MATTE
scenarios
1
2
3
4
5
6
7
Etc.
Etc.
7
6
Groundwater Source of
Drinking Water
>1ha SPZ or >1000
person-hours interruption
8
6
Groundwater – non Drinking
Water Source
>1ha
9
6
Groundwater in unproductive
strata
Please indicate if non
groundwater body is a
pathway to another
receptor.
10
7
Soil or sediment
(i.e. as receptor rather than
purely a pathway)
>10ha Contamination
leading to environmental
damage (as per ELD), or
significantly affecting
overlying water quality.
11
8
Built environment
Damage above a level at
which designation of
importance would be
withdrawn.
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Row
DETR
Table
Ref
Receptor Type
MATTE threshold
Substance / group of substances (see table 4 of
Appendix 5 for description of substances or
substance groups)
See Table 2 of
Appendix 5 for
receptor detail
See Table 3 of
Appendix 5 for
description of
identified MATTE
scenarios
1
2
3
4
5
6
7
Etc.
Etc.
12
9
Various receptors.
Not used to identify and assess
MATTE.
13
10
Particular species
(Note - these criteria apply
nationally - i.e. England,
Wales, Scotland)
Loss of >1% of animal or
>5% of plant ground
cover in a habitat.
14
11
Marine
>2ha littoral or sub-littoral
zone, >100ha of open sea
benthic community, >100
dead sea birds (>500
gulls), >5
dead/significantly
impaired sea mammals
15
12
Fresh and estuarine water
habitats
WFD Chemical or
ecological status lowered
by one class for >2km of
watercourse or >10%
area (estuaries or ponds)
or >2 ha of estuaries and
>2ha of ponds. Plus
interruption of drinking
water supplies, as per
DETR Table 6
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Table 5.2 – Receptor Detail
Row
DETR
Table
Ref
Receptor Type
MATTE threshold
Receptor Detail
1
1
Designated Land/Water Sites
(Nationally important)
>0.5ha or 10-50%
2
2
Designated Land/Water Sites
(Internationally important)
>0.5ha or 5-25% (5-25%
LF/Pop)
3
3
Other designated Land
10-100ha or 10-50%
4
4
Scarce Habitat
2-20ha or 10-50%
5
5a
Widespread Habitat - Non-
designated Land
>10ha
6
5b
Widespread Habitat - Non-
designated Water
>10ha
7
6
Groundwater Source of
Drinking Water
>1ha SPZ or >1000
person-hours interruption
8
6
Groundwater – non Drinking
Water Source
>1ha
9
6
Groundwater in unproductive
strata
Please indicate if non
groundwater body is a
pathway to another
receptor.
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Row
DETR
Table
Ref
Receptor Type
MATTE threshold
Receptor Detail
10
7
Soil or sediment
(i.e. as receptor rather than
purely a pathway)
Contamination leading to
environmental damage
(as per ELD), or
significantly affecting
overlying water quality.
11
8
Built environment
Damage above a level at
which designation of
importance would be
withdrawn.
12
9
Various receptors.
Not used to identify and assess
MATTE.
13
10
Particular species
(Note - these criteria apply
nationally - i.e. England,
Wales, Scotland)
Loss of >1% of animal or
>5% of plant ground
cover in a habitat.
14
11
Marine
>2ha littoral or sub-littoral
zone, >100ha of open sea
benthic community, >100
dead sea birds (>500
gulls), >5
dead/significantly
impaired sea mammals
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Row
DETR
Table
Ref
Receptor Type
MATTE threshold
Receptor Detail
15
12
Fresh and estuarine water
habitats
WFD Chemical or
ecological status lowered
by one class for >2km of
watercourse or >10%
area (estuaries or ponds)
or >2 ha of estuaries and
>2ha of ponds. Plus
interruption of drinking
water supplies, as per
DETR Table 6
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Table 5.3 – MATTE Scenarios
Row
DETR
Table
Ref
Receptor Type
MATTE threshold
Credible MATTE Scenarios
1
1
Designated Land/Water Sites
(Nationally important)
>0.5ha or 10-50%
2
2
Designated Land/Water Sites
(Internationally important)
>0.5ha or 5-25% (5-25%
LF/Pop)
3
3
Other designated Land
10-100ha or 10-50%
4
4
Scarce Habitat
2-20 ha or 10-50%
5
5a
Widespread Habitat - Non-
designated Land
>10ha
6
5b
Widespread Habitat - Non-
designated Water
>10ha
7
6
Groundwater Source of
Drinking Water
>1ha SPZ or >1000
person-hours interruption
8
6
Groundwater – non Drinking
Water Source
>1ha
9
6
Groundwater in unproductive
strata
Please indicate if non
groundwater body is a
pathway to another
receptor.
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Row
DETR
Table
Ref
Receptor Type
MATTE threshold
Credible MATTE Scenarios
10
7
Soil or sediment
(i.e. as receptor rather than
purely a pathway)
Contamination leading to
environmental damage
(as per ELD), or
significantly affecting
overlying water quality.
11
8
Built environment
Damage above a level at
which designation of
importance would be
withdrawn.
12
9
Various receptors.
Not used to identify and assess
MATTE.
13
10
Particular species
(Note - these criteria apply
nationally - i.e. England,
Wales, Scotland)
Loss of >1% of animal or
>5% of plant ground
cover in a habitat.
14
11
Marine
>2ha littoral or sub-littoral
zone, >100ha of open sea
benthic community, >100
dead sea birds (>500
gulls), >5
dead/significantly
impaired sea mammals
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Row
DETR
Table
Ref
Receptor Type
MATTE threshold
Credible MATTE Scenarios
15
12
Fresh and estuarine water
habitats
WFD Chemical or
ecological status lowered
by one class for >2km of
watercourse or >10%
area (estuaries or ponds)
or >2 ha of estuaries and
>2ha of ponds. Plus
interruption of drinking
water supplies, as per
DETR Table 6
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Table 5.4 – Dangerous Substances with Environmental Risk
Part 1 - Substance List
Substance
Reference
Substance (or group of substances)
Maximum
Inventory
(tonnes)
Description
Physical
State
Quantity
Ref for
further
info (e.g.
SR
section...)
Common name
IUPAC
Name
CAS Number
CHIP Index
Risk Phases
1
2
3
4
5
6
7
Etc.
Etc.
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Part 2 - Chemical Hazards
Substance Reference
A
B
C
D
E
F
G
Etc.
Etc.
Explosion/Flammability Hazards
Fire
Deflagration/Detonation
Electrical Static
Reactivity/Stability Hazards
Immediate Health Hazards
Inhalation Toxicity
Other Toxicity
Irritant/Corrosive
Sensitizer
Long Term or Delayed Health Hazards
Chronic Health Hazards
Radiation
Nuisance
Odour
Environmental Hazards
Aqueous
Gaseous
Ground
Hazardous Breakdown Products
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 96 of 97
Appendix 6 – Phases of Environmental Risk Assessment
Risk assessment is an iterative process. The following guidance outlines how a high level (Phase 1) screening based on conservative assumptions
might differ from a more detailed (Phase 2) assessment of risk.
See section 2.2 for signposting to further information on proportionality in risk assessment. Safety Reports and Lower Tier SMS risk assessments would
be expected to include Phase 1 and Phase 2 if necessary.
Phase 1 (High level screening), subdivided into Phase 1a MATTE screen and Phase 1b risk screen
Phase 1 can be summarised as two stages
Phase 1a – Determine if you have MATTE potential. This to be based on the substances and volumes present (Appendix 5 can help to map this
out). Any sub-MATTE scenarios can be screen out of further assessment. For identified MATTEs, the scale of the unmitigated consequence
can now be determined, which tells you what your target frequencies are (i.e. what is Intolerable/TifALARP/Broadly Acceptable).
Phase 1b – Risk Screen. Now you have the target frequencies, based on unmitigated consequence, use section 6.2 to help aggregate the
failure frequencies (these frequencies may be either mitigated or unmitigated) to determine what further risk reduction mechanisms may be
required. The CA will, as necessary, query the origin of the claimed failure frequencies used, and any layers of protection that are claimed.
High level risk screening to identify consequence, frequency and risk of all credible Major Accident Scenarios at the establishment (not only
those scenarios related to storage tanks) and thus qualify/quantify establishment risk to each environmental receptor. (N.B. to determine
establishment risk, all scenarios with MATTE potential will need to be included, not just those involving containment policy in scope substances).
Methodology - CDOIF guidance (or equivalent). It is anticipated use of the CDOIF approach will simplify the assessment process and make it
more likely that the risk assessment will be acceptable to the CA.
Assessment of environmental consequences, drawing on readily available data in Safety Reports and published environmental information with
simple, conservative assumptions based on professional judgements (guided by case studies from previous incidents)
Unlikely to require detailed site investigations or modelling to characterise the environment.
CDOIF
Chemical and Downstream Oil
Industries Forum
CDOIF is a collaborative venture formed to agree strategic areas for
joint industry / trade union / regulator action aimed at delivering
health, safety and environmental improvements with cross-sector
benefits.
Guideline – Environmental Risk Tolerability for COMAH Establishments v2.0 Page 97 of 97
Qualitative / semi-quantitative approach to frequency assessment, using generic failure rate data, such as
http://www.hse.gov.uk/landuseplanning/failure-rates.pdf (but company specific failure data can be used with supporting justification).
LOPA’s are not a requirement for Phase 1 screening - qualitative assessments are sufficient. However, larger / higher risk establishments may
require QRA/Semi-Quantitative assessment at Phase 2
With regard to CBA, a high level (e.g. justified spend) approach would be expected at Phase 1, using the CDOIF guidance. This could screen
out the most costly of risk reduction measures or screen in low cost measures. The CA would then expect a measure specific CBA/ALARP
demonstration to be in Phase 2 for those measures that were initially found to be borderline grossly disproportionate.
Phase 2 – Detailed assessment: necessary where Phase 1a screening does not discount a scenario (i.e. no MATTE potential) or Phase 1b shows risks
are not already Broadly Acceptable.
Discussion with CA on scope and timing of this work is essential!
More detailed risk assessment focused on the higher risk scenarios.
Techniques proportionate to risk – e.g. LOPA for risks in low TifALARP, moving to full QRA (e.g. bow ties / fault and event trees) as risks
approach intolerable.
Detailed site investigations might be required where a better understanding of the establishment and its surroundings is required (e.g. operators
have used LIDAR data to enable computational flow modelling of surface spills & firewater or have carried out groundwater investigations to
demonstrate that an area of aquifer identified by the EA as a groundwater body does not in fact meet the Water Framework Definition)
Generic or site specific failure rate data (requires justification)
If risk is TifALARP and it is proposed not to implement a risk reduction measure, a detailed ALARP demonstration including CBA with case
specific costs and benefit data (here benefits include those to people and the environment).