Jersey Report RR 98 05.Qxp
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The Jersey groundwater study The Jersey groundwater study N S Robins and P L Smedley This report was prepared for the States of Jersey Public Services Department BRITISH GEOLOGICAL SURVEY RESEARCH REPORT RR/98/5 The Jersey groundwater study This report was prepared for the States of Jersey Public Services Department Cover photograph The National Trust for Jersey Bibliographical reference Robins, N S, and Smedley, PL. 1998. The Jersey groundwater study. British Geological Survey Research Report RR/98/5. 48pp. © NERC copyright 1998 Keyworth, Nottingham British Geological Survey 1998 Acknowledgements The Jersey groundwater study encompasses the Services Department, and to colleagues within BGS and combined efforts of people from a number of different IH for direct contributions to the study as well as for dis- organisations and disciplines. Contributors include field cussion and periodic scientific review. In addition, the workers, analysts, modellers, agronomists, environmen- significant role of the Groundwater Review Group in talists, sociologists, economists, engineers and lawyers, guiding the study towards a sensible conclusion has been as well as hydrologists, hydrogeologists and hydrogeo- extremely valuable. But above all, the authors thank the chemists. The authors of this report are grateful to all of people of Jersey for providing access to wells, boreholes them, not least for the unstinting interest and encourage- and springs, for their hospitality and good-humoured ment provided by staff at all levels within the Public tolerance of our enquiries. ii Preface The work described in this report represents a seven year This report highlights the principal issues relating to programme of investigation led by the British Geological groundwater in Jersey. It is not intended as a stand alone Survey on behalf of the Jersey Public Services Department. report, and reference should also be made to earlier project The programme was instigated after a succession of rela- reports and publications for supporting detail. The recom- tively dry winters in the late 1980s and the realisation that mendations are, however, very striking: the groundwater many boreholes on the island at that time were failing. At resources are not managed and there is an urgent need to the same time it was also realised that much of the ground- implement the proposed Water Pollution Law, and then to water, including baseflow to surface waters, was polluted. consider the drafting of groundwater resource regulations. Although much of the work has been carried out by the It is intended that a definitive description of the hydro- BGS Hydrogeology Group, major contributions have also geology of Jersey will be produced in due course. This will been made by others, most notably the staff of the Public await sufficient time series data to support trend analysis, Services Department, the Institute of Hydrology and the the release of data from the St Helier storm drain investiga- School of Environmental Sciences at the University of East tion, and the completion of a number of other areas of Anglia. The work has been influenced and steered through- hydrogeological enquiry which are ongoing. out by the Groundwater Review Group under Dr John Sharp, and has always enjoyed the encouragement and enthusiasm of Roger Culverwell at the Public Services Department. April 1998 iii Executive summary Between 1988 and 1992 rainfall was below average in water by hardstanding, and these in any case tend to coun- Jersey and water resources were stressed. BGS was com- teract each other. missioned to review the groundwater resources of the Groundwater quality is characterised by oxidising waters island in 1990; it was apparent that groundwater is a finite of Na-Ca-HCO3 or Na-Ca-Cl type, although some samples resource under considerable demand, and that much of the have a high SO4 concentration. The overall inorganic com- water is polluted to some degree. The main island aquifer position of the groundwater has changed little over the is a shallow zone of weathering, generally only 25 m in seven-year period of monitoring. Around 80% of the thickness below the water table, which occurs within ancient groundwaters sampled are acidic (pH <7), and most are igneous and sedimentary rocks. There are thin coastal sand undersaturated with respect to calcite. Most groundwaters aquifers along the west and parts of the east coasts of the are oxidising, but reducing waters occur in the south and island. Average annual rainfall is 877 mm and annual south-east of the island and in parts of St Saviour. These potential transpiration ranges from 648 to 754 mm. The reflect upwelling of deeper and longer groundwater flow- island has an area of 117 km2 and a resident population of paths from the north of the island. 84 000; it supports intensive agriculture over 55% of the All the groundwater is susceptible to surface pollutants land area. Demand for groundwater abstracted from bore- because the aquifer is shallow and generally unconfined. holes and wells is about 3.6 Mm3 a-1. Nearly 70% of the groundwater samples collected over the The typical sustainable borehole yield is about 0.5 l s-1; seven-year investigation had nitrate concentrations in the highest known yield is 4 l s-1, but this is exceptional. excess of the European Community maximum admissible The groundwater is unconfined although it may occur under concentration for drinking water of 11.3 mg N l-1. Many a confining head within fractures. Groundwater flow is sources also periodically exceed the limit for K and NH4. principally from north to south with the main discharge Investigation of nitrate profiles in the unsaturated zone area along St Aubin’s Bay. Analysis of borehole hydro- suggests downward percolation of polluted water at a rate graphs suggests that annual recharge rates lie between 30 of 1 m in 2 to 3 years and leaching losses from agricultural and 300 mm a-1, but a variety of techniques enable refine- land of between 23 and 52 kg N ha-1 a-1. These losses are ment of this estimate. A catchment and modelling study of a similar order to those found under heavily cropped produced, in conjunction with historical data, an island- land in the UK. Nitrogen isotope analysis enabled the com- wide recharge estimate of 132 mm over a 28-year period ponent of N derived from fertiliser and sewage/animals to (1968 to 1995). During the dry periods of 1975/76 and be investigated. This showed that the majority of the nitrate 1989/91 there was no recharge and baseflow to surface derives from soil organic nitrogen and ultimately from fer- waters was greatly diminished. Other techniques support tiliser. Some natural denitrification occurs in the reducing these results. waters in the south of the island. Organic pollutants, notably About half the long-term available recharge, or renewable pesticides, are increasingly being detected in groundwater. resource, is currently used either as groundwater abstrac- As yet there is no Jersey law in place which enables the tion or as baseflow in surface water. In dry years, water effective management of the groundwater resource. The levels fall, baseflow declines and many boreholes go dry, physical and chemical evidence collected during this sur- particularly on higher ground. The overall water balance is vey indicate that management is necessary in order to safe- little affected by irrigation returns and interception of rain- guard the resource for future generations. iv Contents Preface iii APPENDICES Executive summary iv 1 Analytical methods for analysis of Jersey 1 Introduction 1 groundwaters 35 1.1 Background 1 2 Median values for major-element concentrations in 1.1.1 The issues 1 Jersey groundwaters 36 1.1.2 Previous work 1 3 Median values for trace-element concentrations in 1.1.3 Work programme 2 Jersey groundwaters 37 1.2 Jersey — the island 2 4 Median values for organic compounds analysed in 1.2.1 Physiography and climate 2 Jersey groundwaters, 1990 to 1996 38 1.2.2 Geology and soils 2 5 List of project reports 39 1.2.3 Land use 5 1.3 Demand for water 6 1.3.1 Demography 6 LIST OF TABLES 1.3.2 Industry, agriculture, finance and leisure 6 1 Estimated groundwater use 1989 to 1991 6 1.3.3 Demand and recent trends 6 2 Mean aquifer properties 9 3 Estimated island water balance (mm) 1968 to 1996 13 2 Hydrogeology 8 4 Estimates of groundwater recharge 13 2.1 Physical 8 5 Summary of linear-regression analysis (slope, mg l-1 a-1, 2.1.1 The aquifers 8 and coefficient of determination, r2) for monitored 2.1.2 Flow-paths 8 NO3-N concentrations in Jersey groundwaters 28 2.1.3 Storage 9 2.1.4 Baseflow 10 2.1.5 Water balance 10 LIST OF FIGURES 2.2 Recharge 11 2.2.1 Instrumented catchment 11 1 Topography and place names 3 2.2.2 Measurements from the catchment 2 Solid geology 4 study 11 3 Quaternary geology 5 2.2.3 Catchment modelling 11 4 Groundwater consumption (Mm3 a-1) for selected 2.2.4 Other recharge estimates 12 categories of use 7 2.2.5 Induced recharge and runoff 12 5 Island-wide groundwater levels and flow-paths 9 2.2.6 Resource potential 13 6 Borehole hydrographs 10 7 Soil moisture relative to field capacity at Trinity 3 Groundwater quality 15 Manor 12 3.1 Hydrogeochemistry 15 8 Modelled gross recharge over the 28-year period 1968 3.1.1 Regional chemical trends 15 to 1996 14 3.1.2 Groundwater residence time 16 9 Regional distribution of TDS concentrations in 3.2 Groundwater pollution 16 groundwater 15 3.2.1 Distribution of nitrate in the 10 Regional distribution of pH in shallow groundwater 16 groundwater 16 11