Baseline groundwater chemistry : the Sherwood Sandstone aquifer of the East Midlands and South Yorkshire

Mallin Martin, D.; Smedley, P.L.. 2020 Baseline groundwater chemistry : the Sherwood Sandstone aquifer of the East Midlands and South Yorkshire. Nottingham, UK, British Geological Survey, 102pp. (OR/20/046) (Unpublished)

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This report details the hydrogeochemistry of a broad suite of inorganic and organic analytes in groundwater from the Sherwood Sandstone aquifer of the East Midlands and South Yorkshire. The study aims to establish the groundwater baseline chemical compositions, particularly of those analytes that are and could be associated with onshore oil and gas (OOG) activities, in order to facilitate distinction between current compositions and any new industrial contamination from such activities. Analytes of particular interest in this context include indicators of salinity, indicators of redox conditions, dissolved gases including CO2 and CH4, naturally-occurring radioactive materials and organic compounds including volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs). Groundwater from the Sherwood Sandstone aquifer of the region shows a range of chemical compositions resulting from inputs of modern atmospheric and surface pollution from varying sources, superimposed on natural water-rock interactions. Natural reactions are dominated by carbonate equilibrium, redox reactions, gypsum/anhydrite dissolution and time-dependent silicatemineral reaction. The Sherwood Sandstone crops out in the East Midlands but further northwards into Yorkshire, the aquifer in places becomes confined or semi-confined by overlying Quaternary superficial silts and clays. The Sherwood Sandstone dips gently eastwards and becomes confined by the poorly-permeable marls and mudstones of the Mercia Mudstone Group (MMG). At outcrop, the groundwater is young and oxic with evidence of inputs of pollutants including NO3, SO4 and Cl and possibly of Br, Cu, Pb and Zn from urban, industrial (including mine drainage) and agricultural sources. Small quantities of PAHs, pesticides and solvents are detected occasionally in the unconfined aquifer. Further north into Yorkshire, the aquifer is oxic in parts but becomes anoxic in places with superficial cover and with increasing depth. The reducing groundwaters in this zone have low NO3 concentrations and increased concentrations of Fe and Mn. Increased concentrations of Co, Sb and V may be associated with release of Fe and Mn into solution under mildly reducing conditions, either within the sandstone or from the superficial deposits. As the Sherwood Sandstone becomes confined eastwards by the MMG, downgradient chemical changes are controlled by maintained equilibrium with calcite and dolomite, dissolution of gypsum or anhydrite and development of reducing conditions. These controls see progressive increases downgradient in concentrations of SO4, and slight increases in concentrations of Fe, Mn, NH4 and Mo. High concentrations especially of SO4 and NH4 in the north-east area around Goole are speculatively associated with facies changes in the sandstone further north, the groundwater possibly interacting with a greater proportion of sulphate minerals and clays. In the deep confined aquifer, conditions are insufficiently reducing for SO4 reduction to be quantitatively important. Under the reducing conditions in the MMG-confined aquifer and to some extent in the areas covered by superficial deposits, small quantities of dissolved CH4 are detected (up to 120 µg/L in this study). Concentrations are low in the unconfined sections of the aquifer. Concentrations in the confined aquifer are relatively low because of a paucity of organic carbon in the aquifer for significant methanogenesis to take place. No other hydrocarbon or PAH, pesticide or solvent compounds were detected in the confined aquifer. Detection of a small quantity of chloroform at one location in the shallow confined aquifer is anomalous and difficult to explain. The groundwater shows a well-established downgradient increase in residence time as it passes beneath the MMG. Limited radiocarbon dating in this study supports previous conclusions that confined groundwater close to the western edge of the MMG has model ages of around 2000– 10,000 years, increasing to late Pleistocene (19,000 years) along the flow path at its eastern edge. The study reiterates that within the aquifer, fresh groundwater extends to around 20 km away from the outcrop and to depths of some 400–500 m below ground. The confined aquifer may be especially vulnerable to pollution from any future deep hydrocarbon exploration activities and would require careful monitoring.

Item Type: Publication - Report (Technical Report)
Funders/Sponsors: British Geological Survey, Environment Agency
Additional Information. Not used in RCUK Gateway to Research.: This item has been internally reviewed, but not externally peer-reviewed.
Additional Keywords: GroundwaterBGS, Groundwater
Date made live: 26 Feb 2021 14:10 +0 (UTC)

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