BGS karst report series. C7, karst in the Chalk of the South Downs.

Maurice, L.; Bunting, S.; Farrant, A.; Mathewson, E.. 2022 BGS karst report series. C7, karst in the Chalk of the South Downs. Nottingham, UK, British Geological Survey, 120pp. (OR/21/057) (Unpublished)

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This report documents the evidence for karst and rapid groundwater flow in the Chalk of the South Downs area in Southern England. 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 classic karst there are extensive caves and large scale surface karst landforms such as dolines, shafts, stream/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. This report shows that in the C7 karst knowledge exchange area, the South Downs area of the Chalk, there is extensive evidence for karst development with dry valleys, caves, stream sinks, dolines, dissolution pipes, and springs present. Short caves have been recorded in the area, and the longest known Chalk cave in England (~350 m) is in this area at Beachy Head. Observations of coastal cliffs have been particularly important in demonstrating that caves and conduits can occur even beneath interfluves and in the absence of obvious surface karst. Coastal sections also provide evidence for pervasive stratigraphical influence on subsurface karst with distinct stratigraphical horizons (particularly sheet and semi-tabular flints, marl seams and hardgrounds) being important for conduit development. In the Beachy Head area, the Seven Sisters Flint, the Belle Tout Marls, the Shoreham Marls, the Navigation Marl, and the Hope Gap sheet flint all host cave and conduit systems. Locally, many stream sinks occur in association with the Chalk-Palaeogene margin, particularly in the west of the area. Stream sinks are less common or absent in the eastern parts of the area where the Palaeogene cover is absent, although major rivers cross the Chalk and their contribution to point recharge via losses to the aquifer was not established during this work. Soakaways and SUDs with high infiltration rates into the Chalk have also not been identified. Dolines also occur in the area, although many recorded surface depressions are likely to be pits of anthropogenic origin. Dissolution pipes are extremely common, especially where the Chalk is overlain by thin unconsolidated superficial deposits, and in some cases can be 10s of metres deep and/or wide. There are many springs in the area, which would have formed the natural outlets for the karstic solutional networks, although little is known about the discharge of most springs and how this has changed in response to groundwater abstraction. Five large springs are identified, the largest being the Bedhampton and Havant complex with a discharge of ~ 600-1900 l.s-1. Evidence from 22 tracer test connections demonstrate very rapid groundwater flow, with velocities ranging from 0.2 to 12.3 km/day over distances of up to 6.6 km; and tracer recoveries ranging from 0.1 to 100 %. Other evidence of karst comes from hydrogeological studies including investigations of transmissivity and pumping tests, water level data from observation boreholes, downhole imaging and borehole logs, groundwater quality, inflows during tunnel construction, saline intrusion, and groundwater flooding. There is considerable evidence for karst and rapid groundwater flow at groundwater abstractions throughout the South Downs area. Karst is clearly important in enabling rapid recharge and providing some rapid flowpaths through the unsaturated zone, especially via stream sinks but also via solutional fissures with no surface expression. However, there appears to be a higher degree of protection from surface pollutants than in highly karstic aquifers, perhaps due to fewer and smaller stream sinks and the potential for more attenuation in the unsaturated zone. Saturated zone networks of solutional fissures and conduits appear to be very common, and there is evidence to suggest that they extend over distances of several kilometres suggesting that karst specific approaches to Source Protection Zone delineation are likely to be useful. Consideration of the karstic processes that result in the enhanced permeability of the aquifer through the development of solutional fissures and conduits is key to understanding the hydrogeology of this area; and improved understanding of rapid flow in both the unsaturated and saturated zones is important for groundwater protection and management of water resources. This report presents an overview of the current conceptual understanding of karst in the Chalk in the South Downs and provides a basis for further investigations of karst in this area to enable improved management and protection of groundwater resources.

Item Type: Publication - Report
Funders/Sponsors: British Geological Survey
Additional Information. Not used in RCUK Gateway to Research.: This item has been internally reviewed but not externally peer-reviewed
Additional Keywords: GroundwaterBGS, Groundwater
Date made live: 29 Mar 2022 10:28 +0 (UTC)

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