Hydrogeochemistry of Millstone Grit and Coal Measures groundwaters, south Yorkshire and north Derbyshire, UK

Abstract

Abstract A pilot study has been performed to ascertain the baseline hydrochemistry of both shallow (springs) and deep (borehole) groundwaters of the Millstone Grit and Coal Measures. In terms of major ion composition the majority of the shallow groundwaters in the two formations are rather similar (typically Ca/Mg-SO 4 2- /HCO 3 - ). The deep groundwaters show more variability. In some boreholes, marked seasonal temperature fluctuations and chemical compositions similar to shallow groundwaters indicate short residence times. In contrast, several boreholes in the Coal Measures indicate highly reducing conditions, evidenced by sulphate reduction and spectacular methane exsolution at two sites in Sheffield. Some deep Coal Measures groundwaters show elevated chloride concentrations compared with shallow waters, which may indicate the influence of deep formation brines, while others yield sodium bicarbonate waters indicative of cation exchange. At the other extreme, highly oxidizing conditions, e.g. in broken ground at the Mam Tor landslip, promote pyrite oxidation and yield acidic, ferruginous waters with high contents of heavy metals including uranium and thorium. At three sites in the Coal Measures, sampling of radon was feasible, yielding values in the range 8–32 Bq/l.