Recharge to the chalk aquifer beneath thick till deposits in East Anglia

This report describes the results of a project to investigate the Chalk-till groundwater system in East Anglia and to estimate rates of recharge through thick till (boulder clay) deposits. The project has involved drilling two cored boreholes, monitoring groundwater levels, sampling Chalk and till groundwaters and porewaters, and development of a conceptual model of Chalk-till groundwater hydrogeology. The main findings of the report have been: • the till has a significant impact on recharge quantity and distribution. Beneath the interfluves recharge appears to be lower than previously estimated (Klink et al. 1996, Soley and Heathcote 1998), probably <20 mm/a and possibly as low as 5 mm/a. Recharge to the Chalk aquifer is enhanced at the edge of the till sheet because of runoff from the till; • 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, which is normally characterised by modern (post-1960s), high-nitrate waters; • 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 is that boreholes close to the edge of the till are likely to pump a greater proportion of modern recharge than previously believed, probably with higher nitrate concentrations.