Groundwater vulnerability in the Chalk is related to the presence of karst features.
The Chalk is a porous carbonate rock subject to dissolution by rainwater or surface water infiltrating and percolating through it. The dissolving power of infiltrating water works upon fractures and other planes of weakness within the rock, over time producing enlarged fissures and conduits, and in places certain landforms (e.g. stream sinks, dolines, caves) that reflect the dissolution of the rock. In general, soluble rocks that have been subject to this type of chemical weathering, and have developed to some extent the features mentioned above, are termed ‘Karstic’.
The Chalk is designated by the Environment Agency as a Principal Aquifer, meaning that it provides “…significant quantities of water for people and may also sustain rivers, lakes and wetlands.” Indeed, the Chalk Principal Aquifer is the source of the majority of water supply to southern and south-eastern England. However, due to the high groundwater velocities (up to several kilometres per day) that frequently occur within flowing fractures, fissures and conduits, karstic groundwater supplies are among those most vulnerable to pollution.
EGG Consultants Ltd were commissioned in December 2016 to review the karstic groundwater potential of a site located at Markwells Wood within the South Downs National Park and within the catchment of the major public supply springs at Bedhampton & Havant.
Findings indicated that:
I) Dry valleys and other surface karst (dolines) are present within and nearby Markwells Wood. As surface karst is an expression of subsurface karst, subsurface karst in the area is expected to be well developed;
II) Subsurface karst (flowing features) are identified in a number of local boreholes;
III) There is an almost complete absence of surface water within the district, with the exception of ‘Winterbournes’ flowing in normally dry valleys during periods of unusually high groundwater recharge, thus indicating that all flow is concentrated in the subsurface;
IV) There is a correlation between the presence of karst dolines and the boundary between the Chalk Principal Aquifer and the overlying Clay-with-Flints deposits. This corroborates the same general findings in this regard from across the wider aquifer;
V) The dry valley immediately adjacent to the proposed site is an upstream tributary of the dry valley system that passes through Rowlands Castle. These and other local dry valleys exhibit orientations mirroring known major NW/SE and NE/SW regional structural faulting within the Chalk;
VI) Tracer tests from Rowlands Castle prove groundwater velocities of up to 12.3 km/day and travel times to springs at Havant of approximately 9 hours, and;
VII) Drilling through the Chalk beneath the water table at this site confirms karstic fissures and/or conduits directly beneath the site.
Multiple lines of evidence suggest that karstic groundwater flow conditions, of potentially kilometres per hour, are present in the vicinity of the site.
Additional inspection of the existing Source Protection Zones delineated for the Chalk Principal Aquifer found that there is little argument to substantiate the boundaries of the current SPZ1 and SPZ2 divisions. As an alternative, some crude estimations of potential groundwater travel times in the wider catchment are made on the basis of a number of proven connections between stream sinks and the Bedhampton & Havant springs. These suggest travel times from the site (located at a distance of 8 km from the springs) may be on the order of ten days, and that a 50 day travel time (i.e. the boundary of SPZ1) would correspond with a distance of between 10 and 11 km from the springs. This estimation is only based on a small amount of data and is necessarily to be treated with caution. However, given this finding, and on the basis of the precautionary principal and other evidence, there is considerable justification for the designation of the area around the site as within SPZ1 (that is, with travel times from beneath the water table of less than 50 days).
Additional groundwater vulnerability assessment, on the basis of the methodology of Edmonds (2008) was used to determine an Aquifer Vulnerability Rating beneath the site of between Moderate to Very High vulnerability, with High vulnerability established as the most likely rating using this method.