BGS karst report series: C6. Karst in the Chalk of the North Downs

This report documents the evidence for karst and rapid groundwater flow in the Chalk of the North Downs area in Southern England. It is part of the BGS karst report series 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 “Karst knowledge exchange to improve protection of groundwater resources”. The term “karst” applies to rocks that are soluble. In classical karst there are extensive caves and 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. These reports provide data and information on karst in each area. Karst data are compiled from the British Geological Survey databases on karst, springs, and transmissivity; reports and peer reviewed papers; geological mapping; and through knowledge exchange with the Environment Agency, universities, water companies and consultants. The report shows that in the North Downs area of the Chalk, there is extensive evidence for karst with dry valleys and many stream sinks and springs recorded; and particular evidence for karst in the Farnham, River Mole, Faversham and Canterbury areas. Throughout the North Downs area, 21 short natural karst caves have been documented. Many stream sinks occur, particularly in association with the Chalk-Palaeogene boundary. Apart from the River Mole where significant karst swallow holes are prevalent, the contribution of major rivers to point recharge via river losses to the aquifer was not assessed in this report. Spring discharges are generally unknown, but some very large springs occur in the area. There has been very little tracer testing conducted, but tracer tests from a doline demonstrated very rapid groundwater flow of ~1000-2500 km/day over a distance of 3.2 km. There is also considerable evidence for karst and rapid groundwater flow at groundwater abstractions in the North Downs area. Consideration of karst and rapid groundwater flow will improve understanding of how the Chalk aquifer functions in this area, and this report provide a basis for further investigations of karst in this area to enable improved management and protection of groundwater resources