Nitrate contamination in the chalk aquifer of the Mons Basin (Belgium), characterization by hydrochemical and isotopic analyses

The problem of nitrate contamination of groundwater is twofold. First, this pollutant degrades the quality of the water needed for human consumption and second, in excessive quantities, it disturbs the balance of ecosystems. While discrimination of the origin of this pollutant is a fundamental step in mitigation strategies, the multitude and mixing of nitrate sources generally makes this process difficult.

The Mesozoic chalk aquifer of the Mons basin (Belgium) covers an area of over 400 km². From a hydrogeological perspective, this aquifer is largely exploited for public water production (50 million m³/year) to answer the local demand but also with significant volume transfers to Brussels city and other regions. Nevertheless, year after year, an increase in nitrate concentration has been observed in several water catchments and is increasingly threatening the sustainability of some production sites. The land-uses in the area are various including fields, pastures, urban areas and industrial sites. Therefore, this diversity creates difficulties in identifying the origin of nitrate and mitigate the pollution. Finally, historical measurements of nitrate concentration in groundwater suggest the presence of denitrification processes along specific interfaces such as confined – unconfined limits.

The characterisation of the pollution and associated nitrate sources was carried out through multiple sampling campaigns covering the different land use zones and confined/unconfined areas. Classical hydrochemical analyses were performed to define the extent of the nitrate pollution, to locate potential denitrification zones and to highlight correlations with other major ions. In parallel, analyses of the stable isotopes of nitrate (δ¹⁵N and δ ¹⁸O) and boron (δ ¹¹B)were carried out. These isotopic ratios differ according to the chemical processes in which they were involved and allow 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.

The results of the sampling campaigns support some preliminary hypotheses while raising new questions. First, regarding the geographical distribution of nitrate, the agricultural areas in the south and west are the most affected. However, the highest local concentration peaks are generally found in urban areas, in urban zones of near former industrial sites. Furthermore, as expected, nitrate is generally absent from groundwater in confined areas of the aquifer. This observation is reinforced by higher iron concentrations and a lower redox potential in these zones. Regarding nitrate sources, isotope analyses reflect the influence of inorganic fertilisers in the most agricultural areas. At the same time, contamination due to sewage leakage seems to be significant in some specific large areas, also suggesting possible actions to mitigate the pollution.