Investigating rising nitrate concentrations in groundwater in the Permo-Triassic aquifer, Eden Valley, Cumbria, UK

Groundwater nitrate concentrations in the Permo-Triassic aquifer of the Eden Valley vary from less than 4 mg l(-1) to in excess of 100 mg l(-1) (as NO3). A significant number of boreholes exhibit rising trends in nitrate concentration that either approach or exceed the CEC Directive 80/778 Maximum Admissible Concentration (MAC) of 50 mg l(-1). The main source of the nitrate is believed to be the nitrogen applied to grassland, both as slurry and as inorganic fertilizers. The variability in groundwater nitrate concentrations is thought to be due in part to land use, particularly where low-yielding boreholes derive their water from a limited/localized area, and in part due to the variability in the travel times for water and solutes to migrate from the soil to the water table and then to the borehole. This variability in travel times is a function of surficial geology, depth to water table, depth of borehole and superficial deposit thickness, amongst other factors. It is surprising, given the considerable storage within the saturated zone of the aquifer and the slow groundwater movement, that some relatively deep boreholes pump groundwater with nitrate concentrations in excess of 20 mgl(-1). Simple numerical modelling suggests that the fraction of modern water pumped is sensitive to the presence of fissures close to the abstraction boreholes and the location of the boreholes relative to superficial deposits. For some scenarios, using realistic superficial deposit geometries and aquifer hydraulic parameters, the proportion of modern water (water that is derived from infiltration that reached the water table since pumping started) could exceed 40% within 15 years of pumping.