BGS karst report series: C5. Karst in the Wessex Chalk (Hampshire and Wiltshire)

This report documents the evidence for karst and rapid groundwater flow in the Chalk of the Wessex Basin area in Southern England which comprises parts of Hampshire and Wiltshire. It is part of the BGS karst report series on those karst aquifers in England in which cave development is limited – principally the Upper Cretaceous 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 usually 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; from geological mapping, and through knowledge exchange with the Environment Agency, universities, water companies and consultants. There is clear evidence for karst in the Chalk of the Wessex basin area, with stream sinks, losing rivers, springs, dolines, dissolution pipes, dry valleys and conduits present. There are no documented enterable caves, but conduits are observed in borehole images and in road cuttings and quarries; and it is likely that they are quite common. There are high densities of stream sinks associated with the Chalk-Paleogene margin, and evidence of recharge from losing streams and rivers away from this margin. Dissolution pipes are common where there is a thin Paleogene or superficial cover over the Chalk, and although many of the numerous surface depressions in the area may be anthropogenic pits rather than karst dolines, some of them are likely to be dolines in this setting of thin cover. There are many springs in the area, with these spring sites representing the natural outlets for the karstic solutional networks of fissures and conduits. While there are very few data on spring discharge, it is likely that there are many large springs, as well as springs that were large prior to the development of chalk groundwater resources. High transmissivities of 1000s m2/day are indicative of extensive karstic networks in the saturated zone, which has also been demonstrated by the single tracer test reported in the C5 area. This tracer test from a monitoring borehole demonstrated very rapid groundwater flow of up to 4750 m/day to three abstractions over distances of many kilometres, with tracer recoveries of ~1-2% at each of the three outlets. There have been few studies of karst in this area, and karst data are fairly limited. Hence further work is recommended to develop karst datasets, and to conduct investigations to improve understanding of the karst to assist with groundwater protection and management.