Baseline Report Series: 6. The Chalk of the Colne and Lee River Catchments Groundwater Systems and Water Quality Commissioned Report CR/03/69N Science Group: Air, Land & Water Technical Report NC/99/74/6 The Natural Quality of Groundwater in England and Wales A joint programme of research by the British Geological Survey and the Environment Agency BRITISH GEOLOGICAL SURVEY Commissioned Report CR/03/69N ENVIRONMENT AGENCY Science Group: Air, Land & Water Technical Report NC/99/74/6 This report is the result of a study jointly funded by the British Baseline Report Series: Geological Survey’s National Groundwater Survey and the 6. The Chalk of the Colne and Lee Environment Agency Science Group. No part of this work may be River Catchments reproduced or transmitted in any form or by any means, or stored in a retrieval system of any nature, without the prior permission of the copyright proprietors. P Shand, R Tyler-Whittle, T Besien, D W Peach, All rights are reserved by the copyright A R Lawrence & H O Lewis proprietors. Disclaimer Contributors The officers, servants or agents of both the British Geological Survey and the Environment Agency accept no R Hargreaves (GIS), M Barron (Cross section), C J Milne liability whatsoever for loss or damage arising from the interpretation or use of (database) the information, or reliance on the views contained herein. Environment Agency Dissemination status Internal: Release to Regions External: Public Domain Project No. NC/99/74 ©Environment Agency, 2003 Statement of use This document forms one of a series of reports describing the baseline chemistry of selected reference aquifers in England and Wales. Cover illustration River Colne at Bushy Mill Lane. Key words Baseline, Chalk, York, water quality, hydrogeochemistry, UK aquifer. Bibliographic Reference Environment Agency Project Manager: Shand, P, Tyler-Whittle, Besien, T, , Peach, D W, Lawrence, A R & H O Dr Rob Ward / Jonathan Smith Lewis 2003 Baseline Report Series 6: Science Group: Air, Land & Water The Chalk of the Colne and Lee River Catchments. British Geological Survey British Geological Survey Project Manager: Commissioned Report No. CR/03/69N Dr Paul Shand Groundwater Systems & Water Quality Programme ©Environment Agency 2003 ©NERC 2003 Environment Agency Science Group, Solihull 2004 British Geological Survey, Keyworth, Nottingham 2004 BRITISH GEOLOGICAL SURVEY ENVIRONMENT AGENCY The full range of Survey publications is available from BGS The Environment Agency is a non-departmental public body with Sales Desk at the Survey headquarters, Keyworth, Nottingham. particular responsibilities for aspects of environmental regulation The more popular maps and books may be purchased from and management in England and Wales. 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EXECUTIVE SUMMARY 1 2. PERSPECTIVE 2 3. BACKGROUND TO UNDERSTANDING BASELINE QUALITY 5 3.1 Introduction 5 3.2 Geology 6 3.3 Hydrogeology 9 3.4 Aquifer mineralogy 12 3.5 Rainfall chemistry 14 3.6 Landuse in the area 15 4. DATA AND INTERPRETATION 17 4.1 Project sampling programme 17 4.2 Other data 17 4.3 Problems in obtaining representative samples 17 4.4 Data handling 18 5. HYDROCHEMICAL CHARACTERISTICS 19 5.1 Introduction 19 5.2 Water types and physicochemical characteristics 19 5.3 Major elements 22 5.4 Minor and trace elements 26 5.5 Pollution indicators 27 6. GEOCHEMICAL CONTROLS AND REGIONAL CHARACTERISTICS 29 6.1 Introduction 29 6.2 Chemical evolution along flowlines 29 6.3 Temporal variations 36 6.4 Depth variations 38 6.5 Age of the groundwater 43 6.6 Regional variations 44 7. BASELINE CHEMISTRY OF THE AQUIFER 49 8. SUMMARY AND CONCLUSIONS 52 9. REFERENCES 53 ACKNOWLEDGEMENTS 55 i List of Figures Figure 2.1 Map of the study area showing topography, main towns and cities (shaded). The outcrop area of the Chalk is shown in red.....................................................................2 Figure 2.2 The River Misbourne (Tributary of the Colne) north of Denham, Buckinghamshire. .3 Figure 3.1 Geological map of the study area. Black line represent line of cross section shown on Figure 3.3......................................................................................................................5 Figure 3.2 Geological map of the study area showing distribution of superficial deposits. Rivers are shown in blue and the outline of chalk outcrop in red. ...........................................7 Figure 3.3 Simplified geological cross section trending north to south through the study area. Line of section is shown on Figure 3.1.........................................................................8 Figure 3.4 Hydrogeological map showing groundwater level contours, sample sites considered in this report and a potential flowline where new samples were collected for this study. Rivers shown as blue lines..........................................................................................12 Figure 3.5 SEM photomicrograph of the Chalk showing round coccolithic fragments and calcite platelets (Field of view approximately 5 µm).............................................................13 Figure 3.6 Diagrammatic sketch of chalk showing important chemical interactions at the microscopic scale (stippled region indicates matrix and clear area represents a fracture). The solid phase comprises mainly coccolithic fragments (1), foraminifera and other microfossils, and pyrite (black, (1)). Congruent and incongruent reactions occur on fracture surfaces forming mineral coatings or dissolution which may locally enhance porosity (2). Exchange of solutes between the fracture and matrix waters may occur (3). Ion-exchange reactions occur on clay mineral surfaces (4). The precipitation of Fe and Mn oxyhydroxides on fracture surfaces may also modify diffusional