Catchment Scale Contamination In the Chalk Aquifer of SE England. A ‘Streamtube' Approach to Modelling Contaminant Transport In a Dual-Porosity, Karstic Aquifer

The Chalk aquifer in Hertfordshire (SE England) is affected by bromate contamination over an area of 40 km², the largest occurrence of point source groundwater contamination in the UK.

Groundwater flow and transport in the Hertfordshire Chalk is governed by both dual-porosity and karstic influences. The complexity and interaction of these two transport styles present particular difficulties for prediction of contaminant transport. In the field area, around the source zone, transport appears to be dominated by dual porosity characteristics and a relatively stable ‘plume' has developed. Down-gradient of the ‘plume' centre a significant karst network allows rapid transport of bromate over distances up to 20 km from the contaminant source.

Finite difference models replicate head and flow observations reasonably well, but do not adequately represent trends in contaminant distribution. The difficulties of integrating representation of dual-porosity diffusion and rapid flow along karst features are considerable.

An alternative approach is to describe the system as a set of ‘streamtubes' (‘pipe pathways'). Contaminant transport is modelled semi-analytically along a series of streamtubes from the ground surface to discharge features such as priority wells and springs. Streamtubes and their connections to surface zones and discharge features are based on flow pathways revealed by an independent flow model and include rapid flow pathways indicated by catchment scale (>15 km) tracer tests. A Laplace transform approach is used to provide analytical solutions to a wide range of transport processes, including ‘pipe-flow' and dual-porosity diffusive exchange. The resulting ‘multiple analytical pathway' approach, informed by tracer testing, incorporates the karstic influence on contaminant transport, along with dual-porosity diffusive exchange, allowing more realistic models for the prediction of long-term, large-scale transport of bromate in the aquifer.