Baseline groundwater chemistry : the Lower Greensand aquifer of South East England

This report details the hydrogeochemistry of a broad suite of inorganic and organic analytes in groundwater from the Lower Greensand aquifer of south-east England. 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 carbon dioxide (CO2) and methane (CH4), naturally-occurring radioactive materials (NORM) and organic compounds including volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs). Much of the exposed unconfined aquifer is oxic in nature, with groundwater pH controlled by the sporadic presence of calcite within the aquifer matrix. Concentrations of a number of dissolved ions increase along the regional flow path, including Ca, HCO3, Mg, K, Sr, F, Al, As, Mn, Cu, Ni, Fe and Mn. The unconfined aquifer is susceptible to a number of anthropogenic impacts. These include diffuse pollution from agricultural activities (indicated by elevated concentrations of nitrate in groundwater across the northern half of the study area), and mobilisation of metals by acidic rainfall recharge in parts of the aquifer where acid-buffering carbonate minerals are absent. Dissolved organic carbon content of the Lower Greensand groundwater is typically low, with an upper baseline concentration of 4.6 mg/L. Anthropogenic organic chemicals detected as part of this study included chloroform, trichloroethene and chlorodibromomethane, but concentrations detected are orders of magnitude below the drinking-water standard for these compounds and not a cause for concern. Dissolved CH4 concentrations in the Lower Greensand aquifer are generally low; most samples in the investigation area contained <1 μg/L, except for two locations where concentrations of >300 μg/L were observed (up to 461 μg/L).