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

When investigating future nitrate (NO₃^-) 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 NO₃^- concentrations. In this study, the different effects on water balance and NO₃^- 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 NO₃^- concentration, the investigation area is divided into 1000 × 1000 m cells. Each cell is assigned a specific NO₃^- input and a NO₃^- degradation capacity. Results show significant variations in NO₃^- 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 NO₃^- 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 NO₃^- input can reduce NO₃^- concentrations, but insufficiently to comply with drinking water guidelines in all regions and aquifers. The model indicates that NO₃^- 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.