Groundwater in Cretaceous carbonates: KG@B field trip 21st June 2015

The Upper Cretaceous Chalk of southern England is the UK’s most important aquifer, providing more than 75% of the public supply for southeast England, including London. The aquifer also sustains rivers and wetlands, and their associated groundwater dependent ecosystems. However, the aquifer is facing a multitude of threats including over-abstraction, nitrate pollution, and climate change. The Chalk is a complex aquifer in which groundwater flow is through the matrix, fractures and karstic dissolutional voids. The Chalk matrix has a porosity of around 35% (Bloomfield et al., 1995). The matrix is thought to provide an important contribution to storage, although the size of the pore throats is very small, and therefore the permeability is very low (Price et al., 1993). The average permeability of 977 core samples was only 6.3 x 10-4 m/day (Allen et al., 1997). The matrix is particularly important in solute transport, because solutes move between the matrix and the more permeable parts of the aquifer via diffusion (Foster 1975). The unmodified fracture network provides an important contribution to storage and flow, and has a hydraulic conductivity of about 0.1 m/d, and a transmissivity of about 20 m2/day (Price, 1987). However, it is the dissolutionally enlarged fissures and conduits that make the Chalk such a good aquifer. The median transmissivity from 2100 pumping tests is 540 m2/day, and the 25th and 75th percentiles are 190 and 1500 m2/day respectively (MacDonald and Allen, 2001). Borehole packer testing, logging and imaging have shown that most of this transmissivity comes from a small number of dissolutional voids (e.g. Tate et al., 1970; Schurch and Buckley, 2002). Laterally extensive lithostratigraphical horizons including marl seams, bedding planes, sheet and tabular flint bands, and hard-grounds have an important influence on these groundwater flows. They are all horizons where downward percolation of water may be impeded. Dissolution often occurs where flow is concentrated along these horizons, creating conduits or fissures, especially where they are intersected by joint sets.