Forecasting nitrate evolution in an alluvial aquifer under distinct environmental and climate change scenarios (Lower Rhine Embayment, Germany).

When investigating future nitrate (NO) concentrations in groundwater, climate change has a major role as it determines the future water budget and, in turn, the conditions in the aquifer which will finally have a decisive effect on NO concentrations. In this study, the different effects on water balance and NO concentration under three projected climate scenarios - RCP 2.6, RCP 4.5, and RCP 8.5 - are analysed in a water protection area in the Lower Rhine Embayment in Germany. Recharge values were calculated from downscaled precipitation and temperature data for the 21st century in a water budget that considers land use in the evapotranspiration term. Nitrate concentration evolution is estimated using recharge and expected fertilization rates with a lumped-parameter model. In order to be able to map the NO concentration, the investigation area is divided into 1000 × 1000 m cells. Each cell is assigned a specific NO input and a NO degradation capacity. Results show significant variations in NO development projected with the different climate scenarios due to different temperatures and consequently actual ET, and precipitation. Nevertheless, nitrate concentrations clearly increase in all projections. The total NO mass increases most strongly with RCP 8.5 until 2099 (by 89% compared to 2020) and least with RCP 4.5 (by 50%). Further projections show a 20% reduction in agricultural NO input can reduce NO concentrations, but insufficiently to comply with drinking water guidelines in all regions and aquifers. The model indicates that NO input loads should be defined according to future recharge variations governed by climate change. Consequently, a time-varying fertilization rate specific for each region, with their own turnover time and degradation rate, must be estimated to meet pollution environmental goals.