| Abstract |
This report documents the evidence for karst and rapid groundwater flow in the Chalk of the Chilterns and the Berkshire and Marlborough Downs in Southern England. It is part of the BGS karst report series which is focused on karst aquifers in England in which cave development is limited – The Chalk and the Jurassic and Permian Limestones. The series is the main output of the NERC funded Knowledge Exchange fellowship 'œKarst knowledge exchange to improve protection of groundwater resources' undertaken between 2015 and 2021. The term 'œkarst' applies to rocks that are soluble. In classical karst there are extensive caves; and there are large scale surface karst landforms such as dolines, shafts, river sinks, and springs. In the past the Chalk and the Jurassic and Permian Limestones of England were not considered karstic because they have limited cave development, and because karst features are small and have not been well documented. However, permeability in these aquifers is determined by their soluble nature and groundwater flow is predominantly through small-scale karstic solutional features comprising small conduits ~ 5-30 cm diameter and solutionally enlarged fractures (fissures) of ~0.5-15 cm aperture. There are some short caves in all three aquifers; they all have dolines, stream sinks and large springs; and rapid flow can occur over long distances. Karst is therefore an important feature of these aquifers. Karst data are compiled from the British Geological Survey databases on karst, springs, and transmissivity; peer reviewed papers and reports; and through knowledge exchange with the Environment Agency, universities, water companies and consultants. The reports provide an initial overview of the evidence for karst and demonstrate that surface karst features are much more widespread in these aquifers than previously thought, and that rapid groundwater flow is common. Consideration of karst and rapid groundwater flow in these aquifers will improve understanding of how these aquifers function, and these reports provide a basis for further investigations of karst to enable improved management and protection of groundwater resources. The reports are structured to provide an introduction to the area and geology, evidence of karst geomorphological features in the area (caves, conduits, stream sinks, dolines, springs); evidence of rapid flow from tracer testing, and other hydrogeological evidence of karst. Maps of the area show the distributions of karst features, and there is a quick reference bullet point summary. In this area of the Chalk, there is extensive evidence for karst with hundreds of karst stream sinks recorded, and tracer tests demonstrating rapid groundwater flows of several km/day over distances of more than 15 km. There are also short caves, large springs, and dolines present, and there is potential for rapid groundwater flow through solutional features throughout the area. |
