A catchment-scale approach to understand the nitrate legacy issues in Permo–Triassic sandstones in the Eden Valley, UK

摘要

Nitrate water pollution, which is mainly caused by agricultural activities, remains an international problem. It has been identified as a major threat to the implementation of the EU Water Framework Directive. It could take decades for nitrate leached from the soil to discharge into freshwaters due to nitrate long time-lag in the groundwater system. Nevertheless, this nitrate legacy issue has generally been ignored in the environmental water management. Efforts have been made to tackle the agricultural diffuse groundwater pollution in the Eden Valley, UK. However, it is necessary to investigate the impacts of historical nitrate loading from agricultural land on the trend in nitrate concentrations in the Permo–Triassic sandstones, which form the major aquifers in the study area. For this purpose, this paper presents an approach to representing nitrate transport in the groundwater system at the catchment scale but in simplified ways. This method considers the spatio-temporal nitrate loading from agricultural land, the impact of low-permeability superficial deposits on nitrate movement, the nitrate transport via both the intergranular matrix and fractures in the unsaturated zones, and nitrate transport and dilution in the saturated zones. It requires modelled recharge values, together with published aquifer properties and local geological information. Monte Carlo simulations were undertaken to analyse the sensitivity of parameters, and to calibrate the model using nitrate monitoring data. Time series of annual nitrate concentrations from 1925 to 2150 were generated for four aquifer zones in the Eden Valley, i.e. ‘St Bees Sandstones’, ‘Silicified Penrith Sandstones’, ‘Non-silicified Penrith Sandstones’ and ‘interbedded Brockram Penrith Sandstones’. The results show that the nitrate concentrations in the first three aquifer zones keep rising until their peak values are reached at their turning-point years, while the last aquifer zone has a declining trend in nitrate concentration. These results can help policy makers understand how the historical nitrate loading from agricultural land affect the evolution of the quality of groundwater and groundwater-dependent surface waters. The model developed in this study requires relatively modest parameterisation but provides a framework for informing the long-term impact and timescale of different scenarios introduced to deliver water-quality compliance.