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

Hiscock, K. M. ORCID: https://orcid.org/0000-0003-4505-1496 (1993) The influence of pre-Devensian glacial deposits on the hydrogeochemistry of the Chalk aquifer system of north Norfolk, UK. Journal of Hydrology, 144 (1-4). pp. 335-369. ISSN 0022-1694

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Abstract

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.

Item Type: Article
Additional Information: Funding Information: studentship funded by the Natural Environment Research Council and the then Anglian Water (Norwich Division). Professor J.W. Lloyd and Dr. D.B. Burgess are thanked for supervising the work. Water analyses were performed by staff at the Whitlingham Laboratory of Anglian Water and by N.R. Pacey at the University of Birmingham. Carbon isotope measurements were made by R.E.G. Williams at the University of Birmingham. Anglian Water are thanked for financing the tritium measurements performed by the Low Level Measurements Laboratory at the Atomic Energy Research Establishment, Harwell. Dr. T.C. Atkinson at the University of East Anglia kindly commented on an earlier draft of this paper. The views expressed in this paper are entirely those of the author, and do not necessarily reflect those of either Anglian Water Services Limited or the National Rivers Authority.
Uncontrolled Keywords: water science and technology ,/dk/atira/pure/subjectarea/asjc/2300/2312
Faculty \ School: Faculty of Science > School of Environmental Sciences
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Depositing User: LivePure Connector
Date Deposited: 11 Oct 2022 11:33
Last Modified: 25 Oct 2022 00:16
URI: https://ueaeprints.uea.ac.uk/id/eprint/88973
DOI: 10.1016/0022-1694(93)90179-D

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