Chalk recharge beneath thick till deposits in East Anglia

Marks, R.J.; Lawrence, A.R.; Whitehead, E.J.; Cobbing, J.E.; Mansour, M.M.; Darling, W.G.; Hughes, A.G.. 2004 Chalk recharge beneath thick till deposits in East Anglia. Nottingham, UK, British Geological Survey, 82pp. (IR/04/179) (Unpublished)

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This report describes the results of a project to investigate the Chalk-till groundwater system in East Anglia and to estimate rates of recharge to the Chalk aquifer through thick Lowestoft Till (chalky boulder clay). The project has involved drilling two cored boreholes, monitoring groundwater levels, sampling Chalk and till fracture waters and porewaters, numerical modelling of groundwater levels and the development of a conceptual model of the Chalk-till groundwater system. The main findings of the report are that: • the till has a significant impact on recharge quantity and distribution to the underlying Chalk aquifer. Beneath the interfluves recharge appears to be lower than previous estimates of 20 – 40 mm/a (Klink et al., 1996; Soley and Heathcote, 1998), maybe as low as 5 mm/a; • the Chalk groundwater beneath the interfluves is old (probably a minimum of several hundreds of years) and has negligible nitrate concentrations. This groundwater makes only a relatively small contribution to the active circulation system in the valleys; • recharge rates to the Chalk aquifer at the edge of the till are greater than the effective rainfall (rainfall minus actual evapotranspiration) because of the contribution of large volumes of runoff from the till sheet. This water characterises the modern (post- 1960s), high-nitrate, groundwaters of the main Chalk valleys with potentially short travel times from recharge to discharge. The arable land on the till sheet has had field drains installed and these contribute to the bulk of the runoff; as a consequence nitrate concentrations in the runoff are high; • the Chalk-till groundwater system and the spatial distribution of recharge to the Chalk aquifer determine the shape and dimensions of the catchment areas of abstraction boreholes. This in turn controls the proportion of modern water pumped by abstraction boreholes, which has implications for the concentration of nitrate in pumped water. One consequence of the redistribution of recharge by the till is that boreholes close to the edge of the till sheet are likely to pump a greater proportion of modern recharge than previously believed and these are likely to produce water with higher nitrate concentrations; • the Chalk groundwaters at the edge of the till sheet are vulnerable to pollution because of the potentially high recharge rates (due to runoff recharge) and the relatively shallow depth to the water table. As a consequence, travel times through the unsaturated zone may be short.

Item Type: Publication - Report
Programmes: BGS Programmes > Groundwater Management
Funders/Sponsors: NERC, Anglian Water Services, Environment Agency
Additional Information. Not used in RCUK Gateway to Research.: In April 2000, the British Geological Survey (BGS) funded, as part of its core science programme, a 4 year research project to investigate and quantify recharge to the Chalk aquifer beneath thick till (chalky boulder clay) deposits in East Anglia. This project was understood to be of national/strategic value given (a) the importance of the Chalk aquifer to the UK water industry and other users, (b) the sizeable area of Chalk in East Anglia that is overlain by thick till deposits and (c) the considerable uncertainty whether any significant recharge does occur through the till and, if it does, understanding the recharge mechanisms. In addition to the funding under the BGS core programme, Anglian Water Services (AWS) and the Environment Agency (EA) contributed financially to the project. AWS and the EA were interested in this research for three principal reasons: • an understanding of recharge to the Chalk, through till deposits, should allow a better delineation of the catchment areas for abstraction boreholes where till cover is present. This, in turn, should help with modelling groundwater flow to abstraction boreholes, which is an essential first step when attempting to predict future groundwater nitrate concentrations; • the research may indicate whether a useful resource of low nitrate groundwater exists in the Chalk aquifer beneath the till (which might be suitable for blending or other uses); • the vulnerability (to pollution) of the Chalk aquifer reflects the spatial distribution and rate of recharge through the till. This item has been internally reviewed but not externally peer-reviewed
Additional Keywords: GroundwaterBGS, Groundwater, Aquifer characterisation, Catchment processes, Environmental tracers, Groundwater dating, Groundwater modelling, Groundwater monitoring, Groundwater quality, Groundwater resources, Hydrogeological data, Major aquifers, Nitrate pollution
NORA Subject Terms: Earth Sciences
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Date made live: 21 Oct 2010 10:52 +0 (UTC)

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