Streambed Sediment as a Barrier to Groundwater Pollution: Insights from Fieldwork and Modelling in the River Thames Basin

Integrated fieldwork and modelling has been undertaken to assess the vulnerability of riverside wells to pollution when river water is contaminated. Results highlight the importance of streambed sediments as a barrier to groundwater pollution by river water. The bed of the River Thames is lined with brown and grey organic-rich silts of low hydraulic conductivity (about 0.002 m/d). A 3-D model for solute transport in stream-aquifer systems was developed and applied to two riverside sites in the middle Thames valley. Model results indicate that well-water quality would only breach EC limits if the river was heavily polluted for a week or even a month (clearly an unlikely scenario). Sensitivity analyses showed the models to be far more sensitive to streambed parameters than to aquifer parameters. Thus it would be more sensible to focus fieldwork on the measurement of streambed properties than on the more usually measured aquifer parameters.