Impact of karstic flow on Chalk aquifer function and water quality

The Cretaceous Chalk aquifer represents the most important groundwater resource in the UK and is also important ecologically for chalk stream ecosystems. Similar aquifers exist in France, Belgium, Netherlands, and Israel. The extent to which chalk aquifers show development of karstic features (widened fractures and conduit development due to dissolution by groundwater flow) is of interest because where karstic features connect sources of contaminants directly to borehole abstractions, water quality may be poor. Where flow is more distributed because karst features less developed and smaller, water quality is generally better. Karst is generally associated with distinctive and often spectacular landforms, including caves, and results in rapid groundwater flow in subsurface streams and rivers, as well as flow through smaller solutional voids, although the latter are less well understood due to their inaccessibility. The Chalk is often not considered a karst aquifer because caves are rare, and surface karst features are small and until recently not well documented. Recent work has highlighted the potential importance of karst in the Chalk enabling rapid groundwater flow over long distances, but the nature of chalk karst and how it impacts groundwater flow and contaminant transport is not well understood. Recent advances in tracer testing techniques offer the opportunity to study the smaller sized solutional voids in karst aquifers, both in classical karst aquifers and in the Chalk. The development of tracers such as bacteriophage (non-harmful virus particles small enough to pass through fractured aquifer systems) and in modelling the development of karst networks enable a new way forward via systematic investigation of the extent of karst development in key areas of the Cretaceous Chalk coupled with numerical simulations of karst network development. Using this approach will test hypotheses about the key factors controlling karst development, the interactions between different void types in karst aquifers, and the impact of these on water quality.