Abstract

The Cretaceous Chalk in Northern Europe and other similar fractured rock aquifers frequently have very thick unsaturated (vadose) zones which control both their hydraulic response to rainfall and the extent to which pollutants are delayed or attenuated before reaching groundwater. Understanding their hydraulic responses is a prerequisite for prediction of future trends in groundwater recharge and quality. Accurate characterization of these responses remain elusive because of difficulties in both obtaining in situ measurements and in devising appropriate conceptual models of flow processes in unsaturated fractured rock. In this study we addressed both issues by simultaneously monitoring soil water dynamics through continuously logged matric potential and moisture content and measuring discharge into a subsurface tunnel at up to 45 m depth within the unsaturated zone of Cretaceous Chalk in northern England. Winter drainage fluxes from the base of the soil zone were estimated using the HYDRUS code for one-dimensional variably saturated media. Comparison of soil zone drainage representing the hydraulic input into the Chalk unsaturated zone with tunnel discharge provides insights into the flow dynamics of the unsaturated zone. The relative magnitudes of the soil drainage and deeper unsaturated zone discharge show that flow pathways converge resulting in increased flow focusing with depth in the unsaturated zone. The observed short lag times between the soil surface and the inflow sites in the subsurface tunnel suggest that contaminants from the surface could rapidly reach the water table through thick unsaturated zones within the Chalk.

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