Age and quality stratification of groundwater in the Triassic Sherwood Sandstone aquifer of South Yorkshire and the East Midlands, UK

Abstract

Groundwater from the Triassic Sherwood Sandstone aquifer of the English East Midlands shows a well-documented evolution in chemistry and residence time as it flows downgradient from shallow unconfined conditions to greater depths, confined by Mercia Mudstone. The flow path covers a lateral distance of some 40 km and depths approaching 800 m. The presence of fresh groundwater (Cl = 15 mg/L, Na = 17 mg/L, SO4 = 144 mg/L, TDS = 500 mg/L) of likely Pleistocene age can be demonstrated down to about 500 m depth in the aquifer, some 10 km downgradient of its confined/unconfined interface. This is amongst the deepest fresh groundwater identified in the UK. Beyond that zone, salinity increases in response to dissolution of gypsum or anhydrite, giving Ca-SO4 groundwater. Further downgradient still, saline groundwater (TDS = 10 g/L) is encountered in the deepest part of the confined aquifer. Downgradient geochemical evolution and increasing residence time are also observed in the South Yorkshire section of the aquifer, to the north of the East Midlands flow path. Here, analogous sequential geochemical changes are observed, controlled by carbonate and sulphate mineral reactions, redox changes and silicate hydrolysis, with evidence of increased inputs of e.g. Na, Cl and SO4 from modern pollution at shallow depths in the unconfined aquifer. Redox conditions are more variable in the western part of the South Yorkshire aquifer, due to local confinement or semi-confinement by fine-grained Quaternary deposits overlying the Sherwood Sandstone. Ratios of δ13C in this section of aquifer increase from −14‰ at outcrop to −11‰ in Mercia-Mudstone-confined conditions as a result of gradual equilibration with aquifer carbonate under closed-system conditions. Ratios of δ18O and δ2H become more depleted, reaching −9‰ and −61‰ respectively, suggesting recharge under climatic conditions significantly cooler than the modern era. Increasing residence time is also indicated by increasing concentrations of a number of trace elements (e.g. Li, Rb, Sr, Mo). These transitions are also observed vertically within investigated boreholes in unconfined and semi-confined sections of the aquifer. Deep groundwater (160–170 m depth) from a borehole within the unconfined aquifer of South Yorkshire has δ18O and δ2H ratios as depleted as −9.2‰ and −67‰ respectively. Such depth stratification indicates that good-quality palaeowater, likely of Pleistocene age, can exist at depth even in the unconfined Sherwood Sandstone aquifer, and that vertical hydraulic conductivity is much lower than lateral. A paucity of boreholes in the Mercia-Mudstone-confined section of the South Yorkshire aquifer makes estimation of the lateral extent of fresh groundwater there more problematic, though brackish groundwater (Cl = 1300 mg/L, TDS = 5.5 g/L) has been identified 12 km east of the aquifer's confined margin.