Isotopic and chemical investigations of a small river to constrain groundwater recharge quantity and quality
Karine Lefebvre1,2, Florent Barbecot
2, Elisabeth Gibert-Brunet
1, Marie Larocque
2, Marina Gillon
3 and François Hardy
4, (1)GEOPS-UMR 8148, CNRS - Université Paris Sud, orsay, France, (2)GEOTOP-UQAM, Departement of Earth and atmospheric sciences, Montréal, Canada, (3)1114 UAPV-INRA EMMAH, University of Avignon, Avignon, France, (4)Syndicat Mixte du Parc Naturel Régional de la Haute Vallée de Chevreuse, Chevreuse, France
Contact First Author: Karine Lefebvre; karine.lefebvre@u-psud.fr
English Abstract:
Groundwater recharge is largely known to be spatially heterogeneous especially on watersheds impacted by human activities (drained surfaces, urban areas, release of effluents of waste water treatment plant, etc.). Depending on land uses occurring in recharge areas, the quality of the groundwater can undergo a marked deterioration, inducing the degradation of related aquatic ecosystems in groundwater discharge areas. Both the location of recharge areas and the quantification of the recharge rates are then crucial for the management of watersheds and the preservation of the ecosystems. The objective of this work was to use isotopic and geochemical tools to locate recharge of the Fontainebleau Sands aquifer in the small watershed of the Rhodon stream (26 km²) southwest of the Paris Basin (France). In this region, groundwater recharge is often impacted by agricultural practices with high nitrates concentrations.
A water and mass balance model was developed to quantify groundwater inflows and mass fluxes to the river, based on the spatial evolution of the river chemistry (222Rn, δ13CTDIC, δ18Owater, δ2Hwater and major ions). Along the course of the stream (10.5 km), two important groundwater contribution areas were identified between 2 and 5.5 km and after 8 km from the spring. In sub-watersheds corresponding to drained surfaces discharging in the stream at each sampling point of the stream, the mean quality of the regional groundwater and the calculated quality of groundwater inflow to the river with mass balance approach have been compared. Despite mass fluxes strongly balanced for conservative tracers (Cl-, K+, Mg2+), there are some significant differences for the non-conversative elements (NO3-, SO42-). These differences were compared to land uses to constrain the most important areas of groundwater recharge. This approach allowed the estimation of recharge rates and quality for each area to determine the quality of the groundwater discharge.
