Baseline groundwater chemistry: the Corallian of the Vale of Pickering, Yorkshire

This report describes the regional geochemistry of the groundwater from the Corallian aquifer in the Vale of Pickering, North Yorkshire. The study aims to assess the likely natural baseline chemistry of the groundwater. Data have been collected from strategic sampling of 24 groundwater sources, in conjunction with collation of existing groundwater, rainfall, and mineralogical data. The Corallian aquifer comprise Upper Jurassic shallow shelf marine sediments, with a variety of facies including muds to micritic limestone and oolites, to bioclastic limestones with interbeds of silts and sands. These strata represent an important groundwater source in north-east England. The main chemical properties of the groundwater are determined by the interaction of rainwater recharge reacting with the aquifer minerals. Within the Corallian aquifer, groundwater major ion composition is dominantly influenced by reaction with calcite, and little variation is seen within the major ion proportions. Human impacts on the water quality are clearly evident, mainly through the presence of NO3-N as an indicator contaminant. The widespread presence, commonly in excess of the drinking water limit and the increases over time suggest that the aquifer is heavily influenced by agricultural practices. Agricultural land dominates the area underlain by the unconfined aquifer. With the exception of NO3-N, the available data for groundwater within the Corallian aquifer of the Vale of Pickering indicate they are of good quality for the analytes tested, with few elevated concentrations of potentially harmful trace elements. For many analytes the 95th percentile serves as a first approximation of the upper limit of the baseline range of concentrations, as this serves to eliminate the most extreme outlier concentrations.