The Chalk aquifer is often considered as a single, homogeneous, isotropic groundwater system in regional groundwater management studies, even though it has been subdivided into several different lithostratigraphic units. Low-permeability layers, including marl bands and hardgrounds, extend on a regional scale and define different layers within the Chalk. Four case studies in different locations of the Chalk aquifer of SE England are presented: (1) a multi-level observation borehole in the Upper Colne catchment in Hertfordshire; (2) different water levels in shallow and deep boreholes in the River Ver catchment; (3) artesian conditions and rises in the groundwater level during drilling in the Chiltern Hills; and (4) groundwater level separation in a public water supply borehole in Kent. The evidence clearly shows a layered system in the Chalk and vertical hydraulic discontinuity within the studied sequences. The current conceptual model of the Chalk aquifer should be enhanced to include this new understanding and to update the existing numerical groundwater models. This will, in turn, increase confidence in the current decision support tools for environmental sustainability and the management of water resources in the Chalk aquifer.
The Chalk is a principal aquifer which provides an important resource in Southeast England. For two centuries, it allowed the establishment of a thriving watercress-growing industry, indirectly through diverted stream flow and directly, through the drilling of flowing artesian boreholes. The distribution of artesian boreholes across different catchments, suggests a regional control on vertical groundwater flow within the New Pit and Lewes Chalk units. Interrogation of location-specific information points to the confining role of a few key marls within the New Pit Chalk Formation, which can be traced up-catchment to where they naturally outcrop or have been exposed by quarrying. Evidence is found in geophysical logging of a number of boreholes across catchments, confirming a consistent pattern of the spatial distribution of such key markers. When tectonic stress was applied to the various Chalk Formations, the marl bands would have reacted producing more plastic deformation and less fractures in comparison with rigid rock strata. Such scenario would have created the conditions for secondary aquifer units, giving the Chalk confining or semi-confining hydraulic characteristics on a regional scale. This conceptual understanding helps explain the reasons that the river flow response to reductions in groundwater abstraction varies across the flow duration curve.
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