Propagation of agricultural and sewage nitrate in a chalk aquifer through regional numerical modeling of groundwater flow and solute transport in the Mons Basin, Belgium

This study tackles the nitrate pollution of groundwater in a chalk aquifer (Mons Basin, Belgium) using a numerical modeling approach based on MODFLOW 6 and Flopy. Previous research focused on hydrogeochemical surveys and isotopic measurements in this area has revealed the implication of diverse anthropogenic sources (mineral fertilizer, manure, and sewage) explaining nitrate contamination. Using numerical modeling, this research studies and demonstrates how the pollutant migrates through the saturated aquifer. A 3D multi-layer steady-state model is developed to simulate the regional mean groundwater flow in the aquifer. A transient solute transport component is coupled to the steady-state flow model. The MODFLOW 6 model inputs included simulated results from the EPIC GRID hydrological model, which calculates the amount of nitrogen of agricultural origin reaching the saturated groundwater, and hypothetical nitrate leakages from the sewage network. The model was calibrated to historical piezometric levels and nitrate concentrations, and used for predictive purposes between 2023 and 2050, using six different scenarios of diffuse agricultural inputs. The scenarios included three climate scenarios coupled with two agricultural practice scenarios. The model was also used to simulate the potential impact of addressing the contamination by sewage. Although the number of scenarios remains limited, results generally showed a stronger influence of the climate scenarios considered compared to the tested changes in agricultural practices. A more exhaustive range of scenarios remains to be tested, but the developed numerical model remains efficient and flexible to address the questions related to regional-scale nitrate contamination.