Baseline Report Series: 17. The Ordovician and Silurian meta- sedimentary aquifers of central and south-west Wales Groundwater Systems and Water Quality Commissioned Report CR/05/034N Science Group: Air, Land & Water Technical Report NC/99/74/17 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/04/034N ENVIRONMENT AGENCY Science Group: Air, Land & Water Technical Report NC/99/74/17 This report is the result of a study jointly funded by the British Geological Survey’s National Baseline Report Series: Groundwater Survey and the Environment Agency Science Group. 17. The Ordovician and Silurian No part of this work may be reproduced or transmitted in any form meta-sedimentary aquifers of central or by any means, or stored in a retrieval system of any nature, without the prior and south-west Wales permission of the copyright proprietors. All rights are reserved by the copyright proprietors. P. Shand, C. Abesser, G. Farr, N. Wilton, D. J. Lapworth, Disclaimer D.C. Gooddy, A. Haria & R. Hargreaves The officers, servants or agents of both the British Geological Survey and the Environment Agency accept no liability whatsoever for loss or damage arising from the interpretation or use of the information, or reliance on the views contained herein. Environment Agency Dissemination status Internal: Release to Regions External: Public Domain ISBN: 978-1-84432-642-6 Pro duct code: SCHO0207BLYO-E-P ©Environment Agency, 2005 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 Aberystwyth Grits on the shore at Aberystwyth. Key words Baseline, Devonian Sandstone, Wales, water quality, hydrogeochemistry, UK aquifer. Environment Agency Project Manager: Bibliographic Reference Dr Rob Ward / Jonathan Smith/ Alwyn Hart Shand, P, Abesser, C., Farr, G., Wilton, Science Group: Air, Land & Water N., Lapworth, D.J., Gooddy, D.C., Haria, A. & Hargreaves, R., 2005. Baseline Report Series 17: The British Geological Survey Project Manager: Ordovician and Silurian meta- sedimentary aquifers of central and Dr Paul Shand south-west Wales. British Geological Groundwater Systems & Water Quality Programme Survey Commissioned Report No. CR/05/034N. ©Environment Agency 2005 Environment Agency Science Group, Solihull 2005 British Geological Survey, Keyworth, Nottingham 2005 ©NERC 2005 BRITISH GEOLOGICAL SURVEY ENVIRONMENT AGENCY The full range of Survey publications is available from BGS The Environment Agency is the leading public body protecting Sales Desk at the Survey headquarters, Keyworth, Nottingham. and improving the environment in England and Wales. 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EXECUTIVE SUMMARY 1 2. PERSPECTIVE 2 3. BACKGROUND TO UNDERSTANDING BASELINE QUALITY 6 3.1 Introduction 6 3.2 Regional Geology 6 3.3 The Plynlimon area 7 3.4 The Teifi Valley 13 3.5 The Rheidol area 21 3.6 Rainfall chemistry 26 4. DATA AND INTERPRETATION 28 4.1 Project sampling programme 28 4.2 Historical data 29 4.3 Interpretation of pumped groundwater samples 29 4.4 Data handling 30 5. HYDROCHEMICAL CHARACTERISTICS 31 5.1 Introduction 31 5.2 Plynlimon groundwaters 31 5.3 Teifi groundwaters 37 5.4 Rheidol groundwaters 44 5.5 Pollution indicators 54 6. GEOCHEMICAL CONTROLS AND REGIONAL CHARACTERISTICS 55 6.1 Introduction 55 6.2 Plynlimon groundwaters 55 6.3 Teifi groundwaters 65 6.4 Rheidol groundwaters 69 7. BASELINE CHEMISTRY OF THE AQUIFERS 75 8. SUMMARY AND CONCLUSIONS 77 9. REFERENCES 78 ACKNOWLEDGEMENTS 80 i List of Figures Figure 2.1 Rheidol Valley, central Wales showing typical scenery………………………………3 Figure 2.2 Upland farming forms an important part of the economy in much of central Wales. ..3 Figure 2.3 Simplified geology of Wales and the Marshes, highlighting the occurrence of Ordovician and Silurian rocks and the three areas selected for study...........................4 Figure 2.4 Glaciofluvial deposit sand and gravel pit, Penparc (from Walters et al, 1997). ...........5 Figure 3.1 The Plynlimon catchments of the upper Severn and Wye Valleys. Landuse is displayed only for the marked catchment boundaries. The upper Wye and its tributaries are moorland and the upper Severn is largely forested. Sample sites used for study shown in text..................................................................................................8 Figure 3.2 Geology of the Plynlimon catchments..........................................................................9 Figure 3.3 The Rhaeder Falls, in the forested Severn catchment cascades over the resistant Rhaeder Mudstone. .....................................................................................................10 Figure 3.4 Fractured bedrock in the Gwestyn Shales, beneath thin podzol soils in the upper reaches of the Wye catchment. ...................................................................................10 Figure 3.5 Geophysical log of a 50 m deep borehole in the Afon Hafren. The presence of fractures (horizontal blue lines) is indicated by the calliper log (left, yellow) and the temperature (red), SEC (blue) and flow meter readings (green) measured during pumping. .....................................................................................................................11 Figure 3.6 Water levels measured monthly in shallow boreholes for one year. Rainfall during the period is also plotted for comparison. The boreholes have been classed as upper slope (US), intermediate slope (IS) and lower slope (LS) based on their location in the catchment....................................................................................................................12 Figure 3.7 Water levels in piezometers situated close to the Hafren