Characterization of the nitrate contamination in the chalk aquifer of the Mons Basin (Belgium) using hydrochemical and isotopic analyses

Whether it concerns the management of drinking water reserves or the preservation of natural ecosystems, a good understanding of the nitrate groundwater contamination is crucial. In particular, identifying its origin is often difficult, due to the variety and combinations of nitrate sources. This task is nevertheless decisive for mitigating the pollution and predicting its spatial and time evolution. The chalk aquifer of the Mons sedimentary Basin, which is part of the South Belgium strategic groundwater reserves, is affected by nitrate contamination. Composed of Mesozoic chalk, it covers an area of more than 400 km², characterized by mixed land use activities including fields, pastures, urban areas and industrial sites. The aquifer is intensively exploited for water public distribution (50 million m³/year), to answer the local demand but also with significant volume transfers to Brussels city and other regions. Some parts of the aquifers are highly contaminated by nitrate, with suspicion of denitrification processes along specific interfaces such as confined – unconfined limits. To characterize the pollution and related nitrate sources, sampling campaigns have been performed throughout the aquifer, covering the different land use areas and the confined/unconfined compartments. Conventional Classical hydrochemical analyses were performed to define quantify the extent of the nitrate pollution, to locate potential denitrification zones and to highlight correlations with other major ions. In parallel, a analyses quantification of the 15N and 18O isotopes in nitrate was were conductedcarried out. The These abundance of these isotopesisotopic ratios differs according to the chemical processes in which they were involved and allow to differentiate. The objective is to differentiate different sources of nitrate, including mineral or organic fertilisers, household waste degradation in landfills and possible leakage from sewer systems in urban areas. In addition, the amount of 11B in each sample is measured as it allows to better distinguish sewage, fertilizers or manure e correlated with the possible contamination by wastewater. First results show significant variations in nitrate concentration at the scale of the aquifer, mainly explained by landuse activities and the presence of confining units above the chalk aquifer. The spatial distribution of the nitrate concentration and differences between nearby sampling points suggests the presence of spontaneous denitrification zones. Correlations between nitrate, iron and organic carbon concentrations are emerging and are interpreted to characterise denitrification processes. Planned numerical modelling, based on those field results, intend supporting the management of groundwater resources in this type of aquifer