Identifying the significant factors that influence temporal and spatial trends in nitrate concentrations in the Dorset and Hampshire Basin Chalk aquifer of Southern England

Abstract

Nitrate data collected by the Environment Agency over a 30 year period from groundwater monitoring points in the Dorset and Hampshire Chalk aquifer are used to investigate the temporal and spatial evolution of nitrate concentration in the saturated zone. Time series of nitrate data are compared with effective precipitation and groundwater level data aggregated over groundwater bodies to identify long-term trends. Seasonal variability in nitrate concentrations is more pronounced in the east of the aquifer, with winter peaks taking the nitrate concentration over the 11.3 mg N l −1 Drinking Water Standard. Phase lags between effective precipitation, groundwater level and nitrate peaks of 2–3 months suggest that piston flow is the main transport mechanism for nitrate to the saturated zone. Multiple linear regression of the groundwater body data indicates that seasonal cycles are superimposed on a longer-term trend of rising nitrate concentrations. The relationship of groundwater level with nitrate concentrations is more direct, and has less of a time lag than that with effective precipitation. Groundwater level is therefore a better predictor of seasonal changes in nitrate concentrations in the model than effective precipitation. Contours of equal nitrate concentration produced for the aquifer using 5 year means between 1976 and 2006 show that nitrate concentrations have increased by a mean value of 30% and up to 115% at one site, over the 30 year record. Statistical modelling of over 8000 nitrate measurements indicates that nitrate varies significantly with time, borehole depth and groundwater level, and between Chalk formation, land-use and groundwater body. Arable and Urban land-uses are significantly more likely to be associated with higher groundwater nitrate concentrations than Managed Grassland.