Hydrogeological impact appraisal for dewatering abstractions Science Report – SC040020/SR1 SCHO0407BMAE-E-P The Environment Agency is the leading public body protecting and improving the environment in England and Wales. It’s our job to make sure that air, land and water are looked after by everyone in today’s society, so that tomorrow’s generations inherit a cleaner, healthier world. Our work includes tackling flooding and pollution incidents, reducing industry’s impacts on the environment, cleaning up rivers, coastal waters and contaminated land, and improving wildlife habitats. This report is the result of research commissioned and funded by the Environment Agency’s Science Programme. Published by: Author(s): Environment Agency, Rio House, Waterside Drive, Boak R, Bellis L, Low R, Mitchell R, Hayes P, Aztec West, Almondsbury, Bristol, BS32 4UD McKelvey P, Neale S Tel: 01454 624400 Fax: 01454 624409 www.environment-agency.gov.uk Dissemination Status: Publicly available ISBN: 978-1-84432-673-0 Keywords: © Environment Agency – April 2007 Groundwater, dewatering, abstraction, conceptual modelling, risk assessment, uncertainty, licensing, All rights reserved. This document may be reproduced karst with prior permission of the Environment Agency. Research Contractor: The views and statements expressed in this report are Water Management Consultants Ltd those of the author alone. The views or statements 23 Swan Hill, Shrewsbury, SY1 1NN expressed in this publication do not necessarily Tel: 01743 231793 represent the views of the Environment Agency and the Environment Agency cannot accept any responsibility for Environment Agency’s Project Manager: such views or statements. 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Steve Killeen Head of Science Science Report – Hydrogeological impact appraisal for dewatering abstractions iii Executive summary This report provides practical guidance on how to assess the hydrogeological impact of groundwater abstractions in connection with dewatering operations at quarries, mines and engineering works. It applies to those who are preparing applications to the Environment Agency for transfer and full licences. The methodology for hydrogeological impact appraisal (HIA) is designed to fit into the Environment Agency's abstraction licensing process, including the changes brought about by the Water Act 2003. It is also designed to operate within the Environment Agency's approach to environmental risk assessment, so that the effort involved in undertaking HIA in a given situation can be matched to the risk of environmental impact associated with the dewatering. The HIA methodology can be summarised in terms of the following 14 steps: Step 1: Establish the regional water resource status. Step 2: Develop a conceptual model for the abstraction and the surrounding area. Step 3: Identify all potential water features that are susceptible to flow impacts. Step 4: Apportion the likely flow impacts to the water features. Step 5: Allow for the mitigating effects of any discharges, to arrive at net flow impacts. Step 6: Assess the significance of the net flow impacts. Step 7: Define the search area for drawdown impacts. Step 8: Identify all features in the search area that could be impacted by drawdown. Step 9: For all these features, predict the likely drawdown impacts. Step 10: Allow for the effects of measures taken to mitigate the drawdown impacts. Step 11: Assess the significance of the net drawdown impacts. Step 12: Assess the water quality impacts. Step 13: If necessary, redesign the mitigation measures to minimise the impacts. Step 14: Develop a monitoring strategy. The steps are not intended to be prescriptive, and the level of effort expended on each step can be matched to the situation. Some steps will be a formality for many applications, but it is important that the same thought-process occurs every time, to ensure consistency. The methodology depends heavily on the development of a good conceptual model of the dewatering operation and the surrounding aquifer. The steps of the methodology are followed iteratively, within a structure with three tiers, and the procedure continues until the required level of confidence is achieved. Advice is also given on how to undertake HIA in karstic aquifers and fractured crystalline rocks. iv Science Report – Hydrogeological impact appraisal for dewatering abstractions Contents 1 Introduction 1 1.1 Purpose of this document 1 1.2 The regulatory context 1 1.3 Applications for transfer licences 2 1.4 Full licences 3 1.5 Case studies 5 2 Basic concepts 6 2.1 Introduction 6 2.2 Uncertainty and risk 6 2.3 Conceptual modelling 8 2.4 Common misconceptions 11 2.5 Hydrogeology of dewatering 15 3 Background to the methodology 17 3.1 Development criteria 17 3.2 Tiered approach 18 3.3 Prediction of likely level of effort 19 3.4 Tools and techniques 21 4 The HIA methodology 26 4.1 Overall HIA structure 26 4.2 The HIA methodology 27 4.3 After each tier 47 5 Monitoring and mitigation 50 5.1 Purpose of monitoring 50 5.2 Principles of monitoring 50 5.3 Practical design considerations 52 5.4 Interpretation of monitoring data 53 5.5 Mitigation measures 55 5.6 Links between monitoring and mitigation 57 References 60 List of abbreviations 64 Glossary 66 Appendix 1: Regulatory context 68 Introduction 68 Science Report – Hydrogeological impact appraisal for dewatering abstractions v Habitats Directive 68 Sites of Special Scientific Interest 70 Catchment Abstraction Management Strategies 70 Water Framework Directive 72 References 73 Appendix 2: Dewatering 74 Introduction 74 Background to dewatering 74 Hydrogeological settings 75 Dewatering methods and drawdown profiles 77 Long-term implications 79 References 80 Appendix 3: Karst 81 Introduction 81 What is karst? 81 Karst groundwater systems 82 Hydrogeological impacts in karst groundwater systems 83 Types of carbonate aquifers 84 Prediction of hydrogeological impacts in karst 86 Management of potential hydrogeological impacts in karst 88 Characterisation of karst groundwater systems 96 Numerical modelling of karst aquifers 108 References 113 Appendix 4: Crystalline rock 118 Introduction 118 Characteristics of crystalline rock aquifers 118 Estimating groundwater inflow 119 Estimating hydraulic conductivity of crystalline rocks 119 Specific transmissive features 120 Wetlands in crystalline rock terrain 121 HIA in fractured crystalline rocks 122 References 123 Appendix 5: Coal measures 124 Introduction 124 Hydrogeology of the Coal Measures 124 Assessing hydrogeological impacts in Coal Measures aquifers 124 An anthropogenically enhanced aquifer 125 Inflows to opencast coal mines 127 Water quality 128 Sources of data 129 vi Science Report – Hydrogeological impact appraisal for dewatering abstractions References 129 INWATCO 130 Appendix 6: Water balances 131 Introduction 131 Water balances and dewatering 132 Allowing for surface run-off and direct rainfall 135 Practical problems with water balances 136 Notes on conceptual modelling of dewatering operations 137 References 139 Appendix 7: Tier 2 tools 140 Introduction 140 Analytical models 140 IGARF 141 Radial flow models 142 Recharge assessment – spreadsheet method 143 Other useful techniques 143 References 144 Appendix 8: Tier 3 tools 145 Introduction 145 Representation of mining and quarrying operations 146 Model-specific comments 150 Evaluation criteria for bespoke dewatering models 153 References 153 Appendix 9: Case studies 155 Table 3.1 Scoring criteria, classes and weights 20 Table 3.2 Relationship between total weighted score and likely level of effort 21 Table 3.3 Summary of analytical equations in Tier 1 tools spreadsheet 23 Table 4.1 HIA in relation to CAMS and WFD 28 Figure 2.1 The development process