Understanding the origin of the nitrate contamination in a sandy aquifer (case of the Leefdaal-Puttebos abstraction site)

Groundwater in the pumping wells of the Leefdal-Puttebos abstraction site in Flanders in Belgium are characterized by rising nitrate concentrations that are approaching the drinking water standard of 50 mg/l. In monitoring wells in the immediate vicinity, values of up to 140 mg/l are recorded, suggesting that a further increase cannot be ruled out. These high concentrations and uncertain future trend pose a severe risk to the quality and the security of supply of the extraction. It is therefore important to determine the origin of the nitrates, so that the appropriate measures can be taken to prevent a further increase.
A study, based on 22 groundwater samples and 2 surface water samples, combining classical hydrogeochemical tools, graphical analysis of the results of nitrate and boron isotopes and environmental tracers analyses has been carried out to build a geochemical conceptual model of the studied area, determine the origin of nitrate and the age of groundwater.
The study confirms that a nitrate pollution of the groundwater occurs in the catchment and allows to determine the spatial distribution of the pollution. Based on the isotopes and environmental tracers, it can be concluded that the high nitrate concentrations in the area are caused on one hand by an old pollution due to sewage dating from more than 60 years, and on the other hand to a more recent and spread pollution due to manure spreading on fields. There is no input due to surface water. The pollution due to sewage leaking is expected to be limited at present since the largest part of a new whole network was installed in the past decades, but a decrease in the agricultural use of nitrate in the area is needed in order to stop the nitrate concentrations increase in the groundwater.