Investigating nitrate transport in a thick sandstone unsaturated zone based on integrated modelling – the Eden Valley, UK

Pollution of water by nitrate is not only an environmental issue but also an economic one and a threat to human health. In the Eden Valley (EV), groundwater nitrate concentrations are very variable distributed and have a rising trend, with some groundwater exceeding the limit of 50mg NO3 l-1 demanded by the EU Nitrate and Water Framework Directive. This study was undertaken to investigate the lag time between nitrate loading and entering groundwater, and explore the linkage between the nitrate in groundwater and its historic surface application. This involved the application of integrated nitrate transport modelling in the unsaturated zone (USZ) of Permo-Triassic sandstones (PT-SS). Three process-based models were integrated in the study. A map of the USZ thickness of the PT-SS was derived from a groundwater flow model using GIS-Groundwater – a GIS groundwater flow model; the historic and future arrival time of nitrate at the water table were modelled using a GIS nitrate transport model in the USZ; and a GIS model for nitrate dilution in the saturated zone was developed to validate the results. The modelled thickness of the PT-SS USZ in the Eden Valley varies from 0 – 183m and hence this imposes a large range of nitrate transport time (lag time) in the USZ (up to c. 60 years). The variable USZ thickness is one of the major reasons for the observed variable distribution of nitrate groundwater concentrations in the EV. The peak nitrate loading at around 1983 has affected most of the study area, and will arrive at the water table in the areas with thicker USZ within the next 35 years. The results of this study, especially the lag time between nitrate surface loading and entering the groundwater, are significant in supporting decision making. The integrated model developed in this study is readily-transferable to other regions or countries for any diffuse conservative pollutant, and can be used for studying the impact of climate and landuse changes on groundwater nitrate pollution