Maurice, L.; (2009) Investigations of rapid groundwater flow and karst in the Chalk. Doctoral thesis , UCL (University College London).

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Abstract

This study, with fieldwork undertaken in the Pang and Lambourn catchments in Southern England, investigates the occurrence and distribution of rapid groundwater flow in the Chalk and evaluates the degree of karstification. A survey of surface karst features revealed a clear spatial pattern in their distribution with three distinctive geomorphological zones. Stream sinks and dolines occur frequently in Zone 1 where there is extensive Palaeogene cover. Only dolines are present in Zone 2 where Clay-with-Flints deposits overlie areas of the Chalk and there is little surface karst, other than dry valleys, in Zone 3 where the Chalk outcrops. Tracer tests from three stream sinks in Zone 1 demonstrated connections to springs and rapid groundwater flow (1-6 km.d-1) indicating connected networks of conduits and large fissures over distances up to 5.1 km. Rapid flow was accompanied by variable tracer attenuation. Unsuccessful results at two other stream sinks were probably due to total attenuation. Further investigation at one site using four dyes and one bacteriophage tracer demonstrated the occurrence of diffusion (probably into the Chalk matrix), but indicated that diffusion is only a minor contributor to attenuation. Very high tracer losses (~75 % of dye and ~ 99% of bacteriophage) appear to be due to transport down multiple flowpaths, many comprising at least one section in which flow is through narrow fissures and fractures. The Single Borehole Dilution technique was developed to identify the distribution of flowing horizons in all three zones. Flow horizons decrease with depth below ground level but have an average spacing of ~ 9 m. Comparison with borehole imaging data suggested that solutional enlargement of fractures to form fissures, tubules and small conduits is common in all areas, but that these features may have limited lateral extent. Overall the Chalk appears to be mildly karstic with small-scale karst development resembling the early stages of speleogenesis.

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