The role of alluvial valley deposits in groundwater-surface water exchange in a Chalk river.

2 CSIRO, Land and Water, Adelaide, Australia Abstract To understand the processes of surface water-groundwater exchange in Chalk catchments, a detailed hydrogeochemical study was carried out in the Lambourn catchment in southeast England. Monthly monitoring of river flow and groundwater levels and water chemistry has highlighted a large degree of heterogeneity at the river-corridor scale. The data suggest an irregular connection between the river, the alluvial deposits, and the Chalk aquifer at the study site. The groundwaters in the alluvial gravels represent a mixture of river water and deeper (Chalk) groundwaters, but simple two-component mixing is inconsistent with the data, and additional processes are required to explain the hydrogeochemistry in the alluvial deposits. It is concluded that the alluvial gravels form an important control on surface water-groundwater interactions at the study site and that the alluvial gravel deposits provide an important lateral flow pathway for pollutant transport along the river valley corridor.