Characterization and modelling of the nitrate contamination in the chalk aquifer of the Mons Basin (Belgium)

The chalk aquifer of the Mons basin (Belgium) annually produces about 50 Mm³ of drinking water to supply the local demand and export to Brussels city. This 400 km² aquifer, is covered by a mix environment including densely urbanized and agricultural areas. These human activities have induced a severe and long-term nitrate pollution of the groundwater, which varies in time and space, and threatens the sustainability of the water production. Previous work, based on historical data, groundwater sampling, quantification of stable isotope ratios (δ15N, δ18O in NO3 − and δ 11B), groundwater age estimation and statistical analyses demonstrated the combination of various nitrogen sources, including inorganic fertilizer, manure and sewage, with relative intensities, depending on the location in the aquifer. Thanks to this data, and to further understand and manage the pollution, a regional numerical model has been developed. The model is based on the Modflow 6 numerical code, considering flow and solute transport processes, and allowing the implementation of a quadtree grid to represent the complex hydrogeological layering of the aquifer and connections with the river network. All pre- and post-processing is handled by the python3 package Flopy. Nitrogen inputs from diffuse agricultural origin (inorganic fertilizer and manure) are produced by the EPICgrid agro-hydrological model which provides information on the quantity of agricultural nitrate entering the aquifer from 1970 to 2022. Based on the location of the sewage system and nitrogen concentration measured at wastewater treatment plants, contribution to pollution from potential leakage is considered. Scenarios on climate and changes in agricultural practices for the period 2023-2050 are applied to the model. They are used to illustrate the expected trends in nitrate concentrations at several catchment sites.