Catchment Scale Contamination In the Chalk Aquifer of SE England: Investigations of Karst Control

The widespread occurrence of bromate contamination over an area of more than 40km² in the Chalk aquifer to the north of the London Basin, constitutes possibly the largest occurrence of point source groundwater contamination in the UK. Concentrations of bromate at several 100µg/l (against a regulatory limit of 10µg/l in drinking water) have already forced the closure of a large public supply source and present an ongoing threat to the regional water quality and to water supply abstractions of both groundwater and surface water.

Solute transport in the Chalk is dominantly dual porosity in character, but overlying arenaceous deposits in the area affected by bromate have led to the development of a highly heterogeneous karst porosity exploiting local stratigraphic and structural trends along the Cretaceous/Palaeocene feather edge. The contaminant source appears to be in a region of Chalk dominated by dual porosity; however, it is the karst flow system that appears to have caused the catchment scale distribution of bromate. An adequate understanding of both systems, and the transfers between them is required in order derive robust predictions of bromate transport to public supply wells and to derive remedial measures.

The karst nature of the aquifer has been investigated through the use of a large scale (>15km) groundwater tracer test using multiple species of bacteriophage. Results have indicated connectivity and rapid flows (up to 3.3km/day), including bypasses of major abstraction wells to distant discharges at springs and abstraction boreholes. The dual porosity system is being characterised through single borehole dilution tracer tests, depth sampling and geophysical surveys.

Tracer test results are being used to constrain predictive transport models using MODFLOW. Flow-paths from MODFLOW are being used to define multiple analytical transport pathways, as an alternative approach to modelling the combined dual porosity and karst systems.