Stream-aquifer systems of the Thames Basin : hydrogeology, geochemistry and modelling
The vulnerability of riverside wells in the Thames Basin to pollution by contaminated river water has been assessed by a programme of field characterisation and modelling. The Chalk, Quaternary river gravels and the modern streambed sediments control groundwater flow and solute transport in these stream-aquifer systems. The Chalk is a fissured aquifer, In which matrix diffusion is an important cause of pollutant retardation. On the basis of new field evidence, it is proposed that the distribution of permeability within the Chalk reflects the configuration of Quaternary permafrost. Flow in the highly permeable Quaternary river gravels is intergranular, and adsorption by organic matter and hydroxides may cause retardation of reactive contaminants. The streambed sediments comprise lowly permeable carbonaceous muddy silts and peats. Slow advection and sorption of contaminants makes the sediments an effective barrier to pollution. A mathematical model for flow and solute transport in stream - aquifer systems has been developed. Groundwater velocities are obtained by the solution of coupled flow equations (written in finite difference form) for up to three superposed aquifer layers. Vertical velocities are approximated using an interpolation scheme based upon the transmlssivity of the constituent horizons of each layer. A 3-D particle tracking formulation (including a simple representation of matrix diffusion) is used for solute transport. Hypothetical river spills of various duration were modelled for two sites (Dorney and Gatehampton). It was predicted that no wells would experience pollutant concentrations In excess of EC limits after 20 - minute spills, although the Gatehampton wells would probably succumb after a 7—day event. Well water at both sites would breach EC limits after a 28—day event. Travel times to wells varied from 12 hours (chloride at Gatehampton) to many years (lindane at Dorney). Model performance was more sensitive to streambed paraieters (permeability and sorption coefficient) than to aquifer parameters.