Major Accidents to the Environment: A Practical Guide to the Seveso II-Directive and COMAH Regulations

By Ivan Vince

• If our plant catches fire, when should it be allowed to burn down to prevent pollution?
• When does enforcement turn into prosecution, following an environmental accident?
• Will our environmental insurance cover the costs of remediation?

This book provides a thorough and practical guide to the environmental aspects of compliance with the Seveso II Directive and COMAH regulations and surrounding issues. It guides readers through the technical, legal and insurance related complexities unique to the environmental aspects of Seveso II/COMAH. Individual chapters and sections written by relevant experts explain the implications of the Directive/Regulations and other laws that relate to major accident hazards. Valuable case studies underpin and illuminate the arguments presented. The comprehensive appendices contain a wealth of further case studies as well as focused supporting information on environmental design, assessment and management of major hazard installations, for safety, prevention and environmental professionals, risk assessors, insurers, managers and their legal advisors. Dr Ivan Vince is Director of ASK Consultants and co-founder of one of the first industrial risk consultancies in Eastern Europe. He has investigated several environmental accidents. Related titles: Introduction to Emergency Management, 2e Haddow and Bullock 978-0-7506-7961-9 Introduction to International Disaster Management, Coppola 978-0-7506-7982-4 Learning from Accidents, 3e, Kletz 978-0-7506-4883-7

* This is the only guide to working with and implementing the Seveso II-Directive and COMAH regulations
* Written by leading risk management, scientific, legal, and engineering experts, this book provides all of the key elements an organization must manage in order to comply
* Accompanied by a comprehensive data handbook that enables managers and health & safety professionals to assess and apply the approaches required in the Directives

• Language: English
• Publisher: Elsevier Science
• Release date: Apr 8, 2011
• ISBN: 9780080556369

Ivan Vince

Ivan Vince is a Chartered Chemist and Chartered Chemical Engineer with 40 years' experience in fields related to process safety, beginning with postdoctoral research at Imperial College involving flammability limits. He has taught postgraduate modules on risk assessment at several universities in the UK and abroad, given expert evidence at nine Public Inquiries and participated in the investigation of several major accidents, including Buncefield. Publications include Vince I (ed) (2008) Major accidents to the environment – a practical guide to the Seveso II Directive and the COMAH regulations (Oxford: Elsevier) ISBN: 978-0-7506-8389-0.; Vince I (2013) Explosion at a hazardous waste site caused by contaminated nitric acid, Chemical Engineering Transactions 31, 535.; Vince I (2011) Societal risk in land use planning – the scale of ‘scale aversion’, Hazards XXII, Symposium Series No.156, 408-410 (Rugby: IChemE).

Book preview

Major Accidents to the Environment

A Practical Guide to the Seveso II Directive and COMAH Regulations

Ivan Vince

Table of Contents

Cover

Title page

Dedication

Copyright

Foreword

Preface

Author biographies

Part 1: Major Accidents to the Environment

Chapter 1: Technical aspects

INTRODUCTION

RISK ASSESSMENT

PREVENTION AND MITIGATION

FIRE OR FLOOD?

EMERGENCY PLANNING

Chapter 2: Legal aspects

INTRODUCTION

KEY DUTIES AND OBLIGATIONS UNDER THE COMAH REGIME

OVERVIEW OF THE TYPES OF LIABILITIES THAT MAY ARISE

INFORMATION DISCLOSURE OBLIGATIONS

CRIMINAL LIABILITY

CIVIL LIABILITY – THIRD PARTY CLAIMS

SOME FURTHER PRACTICAL CONSIDERATIONS RELATING TO THIRD PARTY CLAIMS AND ENFORCEMENT ACTION

TRANSBOUNDARY POLLUTION ARISING FROM MAJOR ACCIDENTS

CONCLUSION

NOTES

Chapter 3: Insurance aspects

INTRODUCTION

TYPES OF INSURANCE POLICIES

CONSTRUCTION OF INSURANCE POLICIES

PUBLIC LIABILITY POLICIES

PROPERTY POLICIES

DIRECTORS’ AND OFFICERS’ POLICIES

PROFESSIONAL INDEMNITY POLICIES

ENVIRONMENTAL INSURANCE POLICIES

FINANCIAL SECURITY FOR DAMAGE CAUSED BY MAJOR ACCIDENTS TO THE ENVIRONMENT

CASE STUDIES

Part 2: Appendices

Appendix 1: The COMAH Regulations 1999, amended 2005 (extracts)

Appendix 2: Substances dangerous for the environment (in the context of Seveso II/COMAH)

Appendix 3: Major hazard incidents with environmental consequences MHIDAS records, 1987–2007

Appendix 4: Guidance on the interpretation of MATTE for the purposes of the COMAH Regulations

Appendix 5: Environmental consequences of chemical accidents Report of OECD Special Session (extracts)

Appendix 6: Environmental risk assessment in support of COMAH and PPC, paper reproduced from Hazards XVIII: (Calder, J. and Capewell, J. (2004) IChemE Symposium Series No. 150, pp. 919–33.)

Appendix 7: Regional inventory of MATTE risk spots, Tisa catchment area (ICPDR report extracts)

Appendix 8: Lessons learned from some recent MATTEs, paper reproduced from HAZARDS XVI: (Whitfield, A., 2001. COMAH and the environment – lessons learned from major accidents 1999–2000. Hazards XVI, IChemE Symposium Series 148, pp. 799–809.)

Appendix 9: MATTE case studies European Environment Agency report (extracts)

Appendix 10: MATTE risks from water reactive materials. Paper reproduced from HAZARDS XVIII: (Fernie, L., Wright, P. and Kapias, T., 2004. Water reactive materials – incorporation into safety and environmental risk assessments. Hazards XVII, IChemE symposium Series No. 150, pp. 739–50.)

Appendix 11: Emergency response plan for full-surface tank fires

Appendix 12: Managing fire, water and major spillages – PPG18

Appendix 13: Penalties for conviction under HSWA (including COMAH)

Appendix 14: Penalties for conviction under the Environment Protection Act, 1990, the Environment Act, 1995 and the Water Resources Act, 1991

Index

Dedication

For my father, Dr Stephan Vince, in celebration of the minor environmental disaster we perpetrated fifty years ago and for Gina, as ever

Copyright

Butterworth-Heinemann is an imprint of Elsevier

Linacre House, Jordan Hill, Oxford OX2 8DP, UK

30 Corporate Drive, Suite 400, Burlington, MA 01803, USA

First edition 2008

Copyright © 2008 Elsevier Ltd. All rights reserved

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher

Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: [e-mail]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material

Notice

No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein.

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

Library of Congress Cataloguing in Publication Data

A catalogue record for this book is available from the Library of Congress

ISBN: 978-0-7506-8389-0

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Foreword

Over the past two decades, fatalities from natural disasters in Europe have exceeded those caused by technological accidents by 20 to 1. Nevertheless the statistics for technological accidents make sobering reading. The Seveso II Directive Major Accident Reporting System (MARS) holds data on approximately 600 major accidents that have been notified since 1984, with approximately 30 notifiable accidents being reported on an annual basis since 2000. According to the European Environment Agency’s Europe’s Environment: The Third Assessment (2003), between 1971 and 1992 there was, on average, one technological accident every year in Europe that resulted in 25 or more fatalities. This trend resumed in 1998, principally due to mining fatalities in the Ukraine. The financial toll is considerable – a report commissioned by the HSE in 1998 estimated that the indicated costs of the top twenty UK accidents post-Flixborough totalled approximately £430 million.

Although injuries or loss of life are understandably the immediate concern of major accidents, harm to the environment is increasingly taking centre stage following high profile accidents such as those at Sandoz, Switzerland in 1986 and Buncefield, UK in 2005. Neither involved fatalities but still raised serious concerns over their potential environmental legacy. For instance, in the Sandoz warehouse fire some 500 miles of the river Rhine were impacted by water used to fight the fire. In addition to direct losses of $12 million, liability claims were estimated at approximately $70 million, largely covered by the company’s liability insurance.

Europe’s Environment: The Second Assessment (1998) reports that between 1984 and 1996, 21 out of a total of 300 major accidents were notified to MARS as having caused ecological harm, with or without concomitant impacts on humans and/or material assets. If this proportion were replicated annually across Europe, an average of between two and three notified major accidents per year would be expected to have caused significant environmental damage. When other accidents falling outside the MARS reporting criteria are added in, it becomes clear that environmental damage caused by accidental releases is far from being a secondary issue.

Adverse impacts on the environment from accidents, whether ‘major’ or not, pose daunting interpretative challenges to the stakeholders involved, in that each party must acquire a mastery of three separate disciplines. First, the accident itself, with the circumstances leading up to the event, needs to be fully dissected and understood from a structural and process engineering standpoint, so that appropriate preventative safeguards can be put in place. Second, the damage to the environment needs to be assessed, in terms of the physicochemical and toxicological properties of the chemicals involved, the nature of the ecosystems affected, and the extent of damage and timescales for recovery. Third, and perhaps most difficult of all, a monetary value has to be assigned to the quantum of damage and its remediation, together with restitution of amenities and services. Sufficient financial provision to cover these eventualities needs to be in place before an incident occurs. The recent introduction of the Environmental Liability Directive (ELD) adds a further dimension of complexity, with considerations of compensatory and complementary remediation in addition to primary remediation.

Nor are these disciplines of equal maturity. Compared to human health and technological risk assessment, environmental and ecological risk assessment is a relatively new scientific endeavour. Concepts of ‘harm’ and ‘significance’ have still to be fully worked through in an ecological context and placed on a firm legal footing. New financial products may have to be designed by the insurance industry to meet the particular demands of the ELD.

The interests of all of the actors – operators, health and safety practitioners, environmental risk assessors, regulators and insurers – are intertwined when environmental damage occurs, and each must appreciate the technical, legal and financial intricacies, albeit from different perspectives. On this basis, it might seem obvious that the field needs an integrated treatment of environmental harm resulting from major accidents, contained in a single volume. However, it appears that no such text has been conceived to date.

The breadth of knowledge required to do the subject full justice is unlikely to reside in one individual. Enter Ivan Vince and his collaborators, experts in their chosen fields and therefore able to tackle each facet of a particularly thorny subject clearly and authoritatively. Of particular value are the case studies underpinning and illuminating the arguments, and the comprehensive appendices containing supporting information, in large part from the grey literature that the average reader would be hard-pressed to identify and source without specialist knowledge of the field.

This text is a timely contribution to the fields of plant safety and environmental protection. This is a subject that will only grow in importance as we learn to appreciate the true worth of a clean environment, and yet avail ourselves of and benefit from technological advances. Major Accidents to the Environment will serve as a reference text, delineating the contours of a far more informed debate on these issues.

Dr Gev Eduljee, Technical Director, SITA UK

Preface

To my knowledge, this is the first book devoted to the topic of Major Accidents to the Environment (MATTEs). The intention has been to provide a guide to issues peculiar to MATTEs – in the areas of civil, criminal and insurance liability as well as regulatory (COMAH) compliance.

The uncontrolled release of dioxins from a factory at Seveso in 1976 fortunately resulted in no immediate deaths, but wrought enduring devastation on the local environment. Incongruously, then, the ‘Seveso’ Directive (82/501/ EC, implemented in the UK as the CIMAH Regulations 1984) focused narrowly on public safety, with somewhat vague environmental exhortations tacked on. An amendment to the Directive followed the 1986 Sandoz warehouse fire and consequent severe pollution of the Rhine, but it was only in 1996 that ‘Seveso II’ (96/82/EC, cf UK COMAH Regulations 1999) made explicit most of the current requirements for environmental protection from major accidents. (It, too, has been amended following an environmental disaster: the failure of a tailings dam at Baia Mare, causing a massive fish kill.) Reflecting the shift in emphasis, the UK Health and Safety Executive now formally shares, together with the Environment Agency, the role of regulating Seveso II/COMAH.

The first chapter of this book (supported by most of the appendices) addresses potential problems in complying with the environmental aspects of COMAH. The operator of a hazardous installation governed by Seveso II/COMAH is legally obliged to consider potential MATTEs in the same way as major accidents affecting the safety of the public, in a degree of detail proportionate to the risk. This is very much more easily said than done. The environmental aspects of COMAH present clear challenges, in theory as well as in practice. At the most basic level, even to decide whether an installation is capable of causing a MATTE is not always straightforward. Conceptual problems in risk assessment are compounded by potential conflicts in applying control measures: what is good for safety (and/or property) is not always good for the environment. Notoriously, contaminated firefighting water has caused untold environmental damage, as in the above-mentioned Sandoz incident.

Both of the Directive-shaking incidents mentioned above affected major watercourses in several countries. So it is, perhaps, surprising that there is still no international protocol in force to settle issues of civil liability for environmental damage from transboundary pollution. The reasons for this are explored in the section on international law, which is the final subject treated in Chapter 2, following a comprehensive account, supported by case citations, of UK civil and criminal law applicable to MATTEs. It will be seen that legal issues surrounding MATTEs are generally more complex than for minor incidents or gradual pollution.

The opposite is sometimes the case when it comes to insurance, since MATTEs tend to belong to a class of event that is ‘written back’ into policies which exclude cover for gradual pollution. Nevertheless, the subject is fraught with quirks and caprices, which Chapter 3 dissects systematically for the lay reader – but, again, with comprehensive citation of cases. Much hangs on definitions and the wording of policies: for example, in a chain of events leading to a MATTE, is there cover for the one qualified as ‘the incident’? Is there cover for urgent remediation, begun before a claim is made against the insured?

I believe that the reader will find, as I have found, the discussion of several incidents by more than one author to be illuminating. In any event, the appendices contain a further considerable number of case studies, in varying depth, assembled from disparate sources. The appendices also feature selected readings around specialised issues in Chapter 1, as well general background material.

Acknowledgements

As nagger, I am grateful to my co-authors for their good humoured patience; as naggee (but it was the gentlest cajoling), to Jonathan Simpson, Jackie Holding and Lyndsey Dixon at Elsevier; the last very kindly took over one of my contractual duties. Dr Ludovic Lemaignen of ASK Consultants and Dr Cris Whetton of ility Engineering gave valuable help with information searches. Dr Gev Eduljee of SITA UK and my daughter Gaia Vince of Nature read drafts and gave crucial advice, all of which I have followed. My wife Gina mopped my brow and kept the show on the road.

Ivan Vince

Author biographies

Valerie Fogleman is a Consultant at Stevens & Bolton and Professor of Law at Cardiff and Ghent Universities. A US attorney as well as a solicitor, she is listed in all the major UK legal directories. Her books include Environmental Liabilities and Insurance in England and the United States (Witherbys, 2005).

Daniel Lawrence is Chair of the UK Environmental Law Association and of Counsel in the Environment, Planning and Regulatory practice group of Freshfields Bruckhaus Deringer. He has significant experience of advising the water and sewerage, chemicals, power, electricity, telecommunications, biotechnology and nuclear sectors. His dispute resolution experience includes environment-related defence work, and civil and administrative litigation.

Tony Moore is Associate Director of the Resilience and Security Group within Cranfield University’s Department of Security Management and Technology, where he specialises in the management of crises and disasters. He is the co-editor of Tolley’s Handbook of Disaster and Emergency Management: Principles and Practice (3rd edn, Butterworth-Heinemann, 2006).

Dr. Niall Ramsden is Director of Resource Protection International, which has been responsible for developing much of the current hydrocarbon and petrochemical storage industry guidance on fire protection and scenario-based emergency planning. He sits on several international committees on fire fighting standards for the oil industry. He was an on-scene advisor at the Buncefield Terminal fire.

Bob Sargent, the immediate past-president of the Chartered Institution of Water and Environmental Management, is UK Head of Hydrology at Hyder Consulting. His interests include hydrology, sustainable water resources management and hydro-ecology.

Dr Ivan Vince is Director of ASK Consultants and co-founder AGEL-CBI, one of the first industrial risk consultancies in Eastern Europe. His research interests include environmental risk assessment and smoke dispersion from fires. He has taught post-graduate courses in risk assessment techniques in the UK and abroad and has investigated several environmental accidents.

Caryl Walter, who has a BSc in Environmental Policy with Economics from the LSE, is a solicitor at Freshfields Bruckhaus Deringer in the Environment, Planning and Regulatory group.

Part 1

Major Accidents to the Environment

1

Technical aspects

Ivan Vince, Bob Sargent, Niall Ramsden and Tony Moore

INTRODUCTION

As discussed below, Major Accident To The Environment (MATTE) is a defined, if somewhat diffuse, concept under the COMAH (Control of Major Accident Hazards regulations, SI 1999 No.743) regime. As regards the latter, there is a large volume of documentation, much of it freely available on the Health and Safety Executive (HSE) web site, providing very clear and detailed guidance on all aspects relevant to human health and safety. For several reasons, corresponding publications on the environmental aspects of the COMAH are both fewer and less detailed.

Therefore, the purpose of this chapter is not to give an overview (for which see CA, 1999), but to focus in some depth on certain problem areas peculiar to MATTEs, in risk assessment, accident prevention and mitigation, and emergency response. Key sections of COMAH relevant to MATTEs are reproduced in Appendix 1.

While this book concentrates on the impact of chemical accidents on the natural environment (i.e. ecosystems), it should be recognised that environmental impact can also affect people, e.g. through the contamination of farm land and water supplies, overloading of sewage treatment works, damage to amenities, etc. For a detailed discussion of conflicts between safety and environmental considerations (not all major hazard related), see Crawley et al., 2000 and Beale, 2000.

The variety of circumstances surrounding MATTEs (chemicals, processes, immediate causes, outcomes) can best be appreciated by perusing collections of incident reports. Over one hundred records in the EU/OECD Major Accident Reporting System (MARS) database refer to major accidents that have caused environmental harm (not necessarily MATTEs). The records, in the form of short reports, are freely available (MAHB, 2007); full reports have been prepared by the national competent authorities for some incidents. A thoughtful selection of case studies appears in Christou, 2000 (see Appendix 2). A twice monthly roundup of industrial accidents, available at www.saunalahti.fi/ility/HInt1.htm, has a section devoted to the environment. Appendix 3 lists brief details of environmental accidents in the last twenty years extracted from the MHIDAS database, maintained by AEA Technology on behalf of HSE (AEA, 2007); owing to space constraints, the extract excludes hydrocarbon spills. There are many other sources of incident data.

RISK ASSESSMENT

Loss prevention begins with risk assessment. Operators of hazardous installations need to have some understanding of the hazards and associated risks created by their activities to decide rationally what, if any, additional control measures to implement, to prioritise any remedial actions and, specifically under COMAH, to demonstrate that they are adequately controlling the risks.

To judge whether a given degree of control is sufficient, it is necessary to consider both the severity and the likelihood/frequency of events (‘scenarios’) that might result from the hazards being realised. The final step in a COMAH risk assessment is to subject the so-called ‘residual risks’ (i.e. the risks remaining after taking into account the effects of all prevention), protection and mitigation measures in place or planned, to a triage: risks are compared against two threshold criteria and assigned into three categories, often shown diagrammatically as zones. A risk in the highest category, labelled ‘intolerable’, cannot be justified whatever the economic benefits of the activity giving rise to it and, in the last resort, the activity can be summarily prohibited by the authorities until the risk is lowered. Risks in the lowest, ‘broadly acceptable’, category, may need no further attention, beyond monitoring (unless several such risks converge/overlap so as to exceed the ‘broadly acceptable’ criterion, see e.g. HSE, 2003). Intermediate risks must be reduced to ALARP (As Low As Reasonably Practicable), based on some form of cost-benefit comparison. Approaching the upper extreme of ALARP, the operator would be expected to act to reduce risks unless the costs of doing so were shown to be wholly disproportionate to the reduction achievable; conversely, risks marginally above the lower extreme would be acceptable unless a large improvement were feasible for a relatively small investment.

What is an appropriate depth of analysis will depend on the size of the risk, the complexity of the operation and, especially in the case of environmental risks, the availability of data on the vulnerability of ‘receptors’. In practice, however, even on complex, ‘top-tier’ COMAH sites where certain risks may demand the most detailed analysis, it is always possible to simplify the analysis overall, beginning with a preliminary screening-out of risks of palpably low consequence and/or low frequency/probability of occurrence.

Screening can take various forms. In a sense, the COMAH designation is itself a form of pre-screening, based as it is on the presence of threshold quantities of dangerous substances. Scenario selection is also a form of screening, since only physically credible scenarios should be considered; in particular, there must be an unbroken pathway from source to environmental receptor, there must be the potential for sufficient harmful material to reach the receptor to cause a major accident to the environment (MATTE), etc. The reverse approach has also proved useful, whereby crude modelling is used to delineate a set of events of minimum severity that might give rise to a MATTE, allowing the assessor to ignore (for purposes of COMAH compliance) events falling below the minimum set. Similarly, scenarios can be screened out on the basis of likelihood/frequency; the threshold for catastrophic events generally being taken as 10–6 per year [NB the value applies to the MATTE itself, not to the release that might cause it if all protection and mitigation measures fail; thus, depending on the circumstances, it may be justifiable to screen out catastrophic releases of harmful substances at considerably higher frequencies than 10–6 per year].

The safety report required for a ‘top-tier’ COMAH installation must include a risk assessment focused on major accidents, as specified in Schedule 4 of COMAH (see Appendix 1):

Part 1 (purpose of safety reports)

The purposes referred to in regulation 7 [Safety report] are as follows-

… . 2. demonstrating that major accident hazards have been identified… .

Part 2 (minimum information to be included in safety report)

… . 4. Identification and accidental risks analysis… .

(a) detailed description of the possible major accident scenarios and their probability or the conditions under which they occur including a summary of the events which may play a role in triggering each of these scenarios, the causes being internal or external to the installation;

(b) assessment of the extent and severity of the consequences of identified major accidents;

However, even ‘lower-tier’ sites are required to develop a Safety Management System (SMS), which addresses, among other issues,

identification and evaluation of major hazards – adoption and implementation of procedures for systematically identifying major hazards arising from normal and abnormal operation and the assessment of their likelihood and severity (Schedule 2, para 4b).

Thus, a role for each of the elements of a risk assessment is specified even in the case of lower-tier sites. What distinguishes top-tier risk assessments is the emphasis on rigorous demonstration that the assessment is complete and in adequate depth.

Elements of a risk assessment

The practice of risk assessment is conveniently divided into tasks, commonly four, as follows:

• Hazard identification (What can go wrong?)

• Frequency/probability assessment (How often will it happen/how likely is it to happen?)

• Consequence analysis (What damage will it cause?)

• Comparison with risk criteria (How worried should we be?).

This section will skim lightly over the first two tasks, in the application of which there is very little to distinguish environmental from safety risk assessment, which is well-documented. Interesting difficulties arise with consequence analysis (in safety assessments, often the most straightforward of the four) and, especially, with risk criteria.

Environmental risk assessment is a relatively young subject and research has been under-resourced in comparison with safety risk assessment, with some promising projects discontinued, presumably due to funding difficulties. Research results are published in journals such as Journal of Hazardous Materials and Journal of Loss Prevention in the Process Industries; in conference proceedings, e.g. the IChemE Hazards series, and in scattered reports often commissioned by EU organs and competent authorities of member states.

For the purposes of COMAH, the environment includes features such as architectural and archaeological heritage, groundwater, farmland and sewage works. However, these features do not present unusual problems in consequence analysis. This section will therefore focus on ecosystems.

Hazard identification

The strategies and techniques of hazard identification in safety studies are just as useful in identifying major accident hazards to the environment, as long as the environmental dimension is consciously included in the scope. Thus, for example, HAZOP studies do not require any parameters, guidewords or deviations specific to potential environmental harm. However, the team must beware of dismissing potential releases with no human health or safety consequences too quickly (and particularly should not skim over the less glamorous nodes, such as drains and utilities). On the other hand, fires can have a major indirect environmental impact through loss of containment of firefighting water; we return to this in detail below. Explosions can rupture tanks or pipework carrying non-COMAH substances – such as milk or fruit juice – which can nevertheless devastate an aqueous ecosystem, and the release would qualify as a MATTE under COMAH.

Frequency/probability

Beyond the initial screening, and particularly in the case of top-tier sites where the initial estimates of severity and frequency call for a quantified risk assessment, the assessor should take into account any fluctuations in the vulnerability of environmental targets. For example, a toxic release into an estuary may have a very serious impact if it happens at a time of year when migrating birds are present, but little or no significant impact at other times; a species may be more exposed or more susceptible to a given toxic substance in one stage of its life cycle than in another; a release into a river will be more rapidly and effectively diluted when the river flow rate is high than when