The influence of pre-Devensian glacial deposits on the hydrogeochemistry of the Chalk aquifer system of north Norfolk, UK

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The distribution of hydrochemical facies in the Chalk aquifer system of north Norfolk, UK, is interpreted to reveal the hydraulic and hydrogeochemical processes operating in the area. The occurrence of two distinct glacial tills covering the Chalk aquifer is shown to dominate aquifer conditions. Chalk groundwater in the interfluve regions covered by Chalky Boulder Clay is characteristically of the order of 10 000 years old and compares with Chalk groundwater of about 1000-2000 years old in areas covered by North Sea Drift. The difference in age reflects the different mechanical compositions of the two glacial tills, with the Chalky Boulder Clay containing a higher clay content and lower sand content than the North Sea Drift. Hydrochemically, Chalk groundwater from beneath Chalky Boulder Clay is distinguished from groundwater associated with North Sea Drift by a HCO3 concentration of 300 mgl-1, with higher values occurring beneath Chalky Boulder Clay. The reason for the difference is that recharge water entering the carbonatedepleted soils developed on North Sea Drift attains calcite saturation for low values of soil PCO2 (in the range 10-2.5-10-2.0), whereas recharge entering carbonate-rich soils developed on Chalky Boulder Clay achieves calcite saturation for higher values of soil PCO2 (in the range 10-2.1-10-2.0). Additionally, pyrite oxidation and SO4 reduction associated with the Chalky Boulder Clay interfluve regions evolve further HCO3 and, to maintain calcite equilibrium, increase the dissolved PCO2 of the Chalk groundwater. In the interfluve regions, Mg, Sr and SiO2 are able to increase because of the groundwater residence time which favours incongruent dissolution of the rock carbonate and breakdown of clay minerals contained in the glacial deposits and the Chalk. In contrast, modern valley zone Chalk groundwaters contain high concentrations of NO3 (up to 120 mgl-1), SO4 (up to 130 mgl-1) and Cl (up to 100 mgl-1) contributed by contamination, mainly from an agricultural source. The hydrochemical interpretation demonstrates that the varying glacial deposits are important in controlling recharge and groundwater flow within the Chalk aquifer. The Chalk aquifer in the interfluvial areas experiences minor recharge whereas the Chalk in the valley zone receives direct, modern recharge water. At the fluvial margin, glacial sands and gravels control the storage and release of water, transferred laterally through the glacial tills, to the high-transmissivity Chalk in the valley zone.

Original languageEnglish
Pages (from-to)335-369
Number of pages35
JournalJournal of Hydrology
Issue number1-4
Publication statusPublished - Apr 1993

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