Baseline groundwater chemistry : the Magnesian Limestone of County Durham and North Yorkshire

This report describes the regional geochemistry of groundwater from the Magnesian Limestone aquifer of County Durham and North Yorkshire. The study aims to assess the likely natural baseline chemistry of the groundwater by identifying the dominant controlling processes. Data have been collected from strategic sampling of 36 groundwater sources, in conjunction with collation of existing groundwater, rainfall, mineralogical and geochemical data. The Magnesian Limestone aquifer comprises a series of marine limestones and dolomites, marls and evaporites which reflect cycles of transgression, regression and evaporation of a shallow tropical sea. The depositional basin was divided by an area of high ground. As a result of this, the geology and hence groundwater are chemically different in regions known as the Durham Province and the Yorkshire Province. The aquifer is regionally important for public and domestic supply, agriculture and industry. It provides significant baseflow to the River Skerne, and minor baseflow contributions to the Rivers Wear and Tees. The groundwaters have notable regional variation in chemistry. Groundwater in County Durham is mainly of Ca-Mg-HCO3 type with some mixing towards SO4, Na, and Cl compositions. In Yorkshire, groundwaters are most frequently of Ca-HCO3 type and show some mixing towards Ca-SO4 type. The differences in water type reflect differences in the strata in these two regions, most notably the presence of dissolving gypsum in Yorkshire. Groundwaters with high salinity (Na up to 4600 mg L–1, Cl up to 9000 mg L–1) have been found in the coastal areas, particularly around Hartlepool. These are associated with saline intrusion. Relatively saline waters are also present in the area around the Durham coalfield in the central part of the study area. These waters have high concentrations of Ca, Cl, Ba, Fe, and SO4, and likely result from minewater rebound following closure of the mines in the 1970s. Human impacts are mainly visible in the presence of indicator contaminants such as nitrate from agricultural or domestic sources or trace metals from urban or industrial sources. The widespread presence of nitrate and increasing concentrations indicates the extent to which the aquifer is influenced by modern farming practices. Concentrations of Cu, and Pb are relatively high around areas of industry and near major roads. Anomalous concentrations of a suite of trace metals (Fe, Co, Cr, Cu and Ni in particular) observed in groundwater from one site have been were attributed to contamination from the borehole metalwork. Most of the sampled groundwaters from Magnesian Limestone aquifer of the study area are of relatively good inorganic quality, although concentrations of some solutes approach or exceed current national drinking-water limits in some areas. The most frequent exceedances above drinking-water limits are for nitrate, Mn and Fe with occasional exceedances for SO4 and F.