Chalk recharge 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 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.