Abstract
AbstractThe Chalk is an unusual karst aquifer with limited cave development, but extensive networks of smaller solutional conduits and fissures enabling rapid groundwater flow. Small-scale karst features (stream sinks, dolines, dissolution pipes, and springs) are common, with hundreds of stream sinks recorded. Tracer velocities from 27 connections between stream sinks and springs have median and mean velocities of 4700 and 4600 m d−1. Tests to abstraction boreholes also demonstrate very rapid velocities of thousands of metres per day. Natural gradient tests from observation boreholes have rapid velocities of hundreds of metres per day. There is strong geological control on karst with dissolution focused on stratigraphical inception horizons. Surface karst features are concentrated near the Paleogene boundary, or where thin superficial cover occurs, but rapid groundwater flow is also common in other areas. The Chalk has higher storage and contaminant attenuation than classical karst, but recharge, storage and flow are influenced by karst. Point recharge through stream sinks, dolines, losing rivers, vertical solutional fissures, and soakaways enables rapid unsaturated zone flow. Saturated zone networks of solutional fissures and conduits create vulnerability to subsurface activities, and enable long distance transport of point source and diffuse pollutants, which may be derived from outside modelled catchment areas and source protection zones.
Highlights
The Upper Cretaceous Chalk Group of NW Europe is a highly transmissive carbonate aquifer (Downing et al 2005)
J. The Chalk Aquifers of Studies have demonstrated the presence of surface karst and rapid groundwater flow in the Chalk (e.g. Atkinson and Smith 1974; Rodet 1985; Banks et al 1995; MacDonald et al 1998; Massei et al 2002; Maurice et al 2006; Edmonds 2008; Gombert et al 2010; El Janyani et al 2014), but there is limited understanding of the extent of karst and the implications for chalk hydrogeology
There is evidence from borehole studies that as in more classical karst aquifers, the Chalk aquifer can be multilayered, with flowpaths developed at different depths which may be isolated from one another (Karapanos et al 2020)
Summary
Recent data compilation and literature review are used to present evidence for karst and rapid groundwater flow in the Chalk in England. The geomorphological evidence for karst includes dolines, dissolution pipes, stream sinks, large springs, caves, and smaller conduits and fissures. Hydrogeological evidence includes rapid velocities proved by tracer tests, high transmissivity in pumping tests, and the presence of short residence time indicators in boreholes (i.e. substances which degrade within the subsurface within short timescales)
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