Using δ15N and δ18O values to identify sources of nitrate in karstic springs in the Paris basin (France)

In many parts of the world, groundwater quality deterioration is occurring often due to changes in land use along with overexploitation of some aquifers. Some of the drinking water distributed in Paris (France) is collected from a karstic Cretaceous chalk aquifer at two groups of springs located 80 km south of Paris, in the Lunain catchment. Although geographically close to each other, these springs (Sp1 and Sp2) have quite different hydrogeochemical characteristics. In Sp1, an increase in the NO3- concentrations of 18 mg L−1 (from 16 mg L−1 in 1965 to 34 mg L−1 in 2009) has been observed and in Sp2, NO3- concentrations have increased by 26 mg L−1 (from 20 mg L−1 in 1965 to 46 mg L−1 in 2009). The origin of the NO3- was, however, unclear. The stable isotopic composition of NO3- was used to identify the sources causing the increase of NO3- concentrations and to assess seasonal variations. The isotopic composition of two potential sources including agricultural effluents and wastewater was determined. Likewise, two additional NO3- sources, the chalk groundwater and the Lunain River water, were considered, following the hydrogeological context of the Lunain catchment. Determination the concentrations and the δ15N and δ18O values of spring water NO3- and the use of a Bayesian isotopic mixing model SIAR (Stable Isotope Analyses in R) allowed determination of the proportional contribution of respective NO3- sources in the springs. In spring Sp1, all the potential NO3- sources (4) contributed almost equally during the low flow period, but higher NO3- contributions from the agricultural source and the chalk groundwater were observed during the high flow period. For the Sp2 group of springs, elevated contributions of NO3- from the agricultural source and from the chalk aquifer were observed during the high flow period as well as during the low flow period. Knowledge of NO3- sources contributing to these springs is important for the implementation of remediation measures to protect the water resource and avoid further water quality deterioration.