Groundwater flow and nitrate migration in a Dutch-Belgian chalk catchment; observed and future concentrations

Abstract

In the catchment of the Noor brook, by the Dutch/Belgian border,  excess precipitation with high nitrate concentrations (80–120 mg·l–1) recharges the Cretaceous multi-aquifer system under the plateau and foothill (agricultural area). The nitrates are transported through the aquifer (median  > 50 mg·l–1 under the plateau) towards springs and wetlands of the Noorbeemden nature reserve. The major spring has a  concentration of 70–80 mg·l–1. The concentration in the Noor brook is somewhat lower (median 46 mg·l–1) because of denitrification in the wetlands. The groundwater flow and transport model FLONET/TRANS was used to simulate the development of the nitrate distribution in the last 50 years. The general  distribution in the northern part of the catchment could be reasonably well predicted, whereas the concentrations in the southern part were underestimated by about 20 mg·l–1. The simulated future trend in the northern part of the catchment in the next 100 years shows that a reduction of nitrate leaching by 50% is required to level off the currently still increasing concentrations. A dramatic decrease of leaching by 75% is needed to achieve pre-1950’s concentrations. This implies that the management plan of the Noorbeemden Nature Reserve, which is complied by the Nature Conservation Organisation, should not aim at botanical restoration in the first decades, even if severe restrictions in nitrogen application are imposed on the farmers now.