Modelling the effect of forest cover in mitigating nitrate contamination of groundwater: A case study of the Sherwood Sandstone aquifer in the East Midlands, UK

As the second largest aquifer in the United Kingdom, the Sherwood Sandstone is threatened by high nitrate concentrations and water shortages. Land-use change from arable agriculture to woodland could help mitigate agricultural diffuse pollution and this study provides an evaluation of the effects of woodland cover on groundwater quality. Six land-use change scenarios focusing on two public-supply borehole capture zones on the unconfined Sherwood Sandstone aquifer in Nottinghamshire were examined in predictive simulations of nitrate concentrations until 2025. Groundwater flow modelling (MODFLOW) and mass transport modelling (MT3DMS), incorporating outputs from a groundwater recharge model and an export coefficient model enabled a quantitative comparison between different scenarios on groundwater quality. Results revealed that the greatest decrease by 2025 in nitrate concentration (35%) was associated with the entire target zone covered with forest, whereas a decrease dependent on the application of best agricultural practice achieved a 20% reduction. Conversion of 80 ha and 240 ha of agricultural land to woodland in two borehole capture zones (with areas of 210 ha and 484 ha, respectively) could potentially restrict nitrogen losses such that the EU groundwater nitrate limit (50 mg L−1) could be achieved in abstracted groundwater. It is concluded that the combination of forest creation and application of best agricultural practice in a borehole capture zone is a recommended approach to control nitrate pollution in the long term and provides scientific support for pollution control strategies that rely on land-use change to woodland to achieve water quality objectives.