Emmanuelle Petelet-Giraud

Drinking water quality in agricultural rural areas remains locally a challenge even all the effort made by local authorities to restore the groundwater resources quality, especially regarding nitrates. In Plourhan, a ~2000 inhabitants, about 10 km from the sea, NW France, the drinking water is pumped in a natural spring emerging from the Brioverian basement. The nitrate concentrations exceed the 50 mg/L standard for drinking water supply, and thus needs to be diluted to be delivered to the population. Over the last 15 years, a large programme of measures was undertaken in order to reduce the NO3 concentration, including the purchase of agricultural parcels around the spring, moving progressively from mixed farming and livestock to fallows and meadows, and thus drastically change the local land use. Despite all these efforts, nitrate concentrations only decrease very slowly and remain above the 50 mg/L standard.In this context, the objective of this study is to better understand the transfer of nitrates at the basin scale, by studying flow paths, geochemical reactions, transit times that are key parameters to estimate the vulnerability and the recovery-time of the critical zone. In that way, a geochemical and isotopic approach is applied at the basin scale. Major elements analysis of the groundwater reflect the drained contrasted lithologies as metasediments (pelites & sandstones) and amphibolite, with a large spatial heterogeneity of the NO3 concentrations, ranging from a few mg/L to more than 50 mg/L. Nitrogen and oxygen isotopes of nitrates (δ15N-NO3 and δ18O-NO3) suggest that denitrification can occur locally in some wells presenting low or intermediate  NO3 contents, whereas other wells present high or low NO3 concentrations without any evidence of denitrification processes. The mean residence time of groundwater is assessed through CFCs and SF6 dissolved gas measurements. Some wells preferentially in amphibolite, present water with low recharge temperature (around 6°C while the mean recharge temperature in Britany is 11-12°C) correlated with low CFCs/SF6 values indicating that some very old groundwater (last glaciation :  -19/17 k yrs) exists in the reservoir. Other ones in metasediments have modern water or a mixing between an old and a present day recharge. These results, together with structural and lithological detailed geological field mapping, help to draw up the conceptual model of the aquifer functioning regarding nitrates transfer in the critical zone.  This work is part of the POLDIFF study that benefits from the funding of BRGM and the French Loire-Bretagne water Agency.

Surface water and groundwater contamination by a chemical element or molecule may have several origins. For instance, AMPA can originate from agricultural practices as a degradation product of glyphosate, but also from wastewater effluents as a degradation product of phosphonates. The discrimination of the origins and vectors of contamination in the environment is both an environmental and a societal issue, and is required to define integrated water resources management at the catchment or water body scale by implementing appropriate measures to effectively struggle against pollution. The objective of this study is to define a methodology to identify a "domestic wastewater" component within surface waters and groundwater. As there is no ideal tracer, i.e. conservative, persistent in water compartments, present in quantity above the detection limit and originating from a single type of pollution source, we propose a multi-tracer approach (chemical and isotopic) to identify and validate the relevance of foreseen tracers, in order to overcome this limitation. Among the relevant tracers of wastewater, some may be used for their intrinsic or combined discriminant power: i.e. Tracer of detergents: boron concentration and isotopes; Pharmaceuticals tracers: e.g. carbamazepine, ibuprofen, gadolinium anomaly; Lifestyle tracers: e.g. caffeine, sweeteners. The studied catchment is impacted both by diffuse agricultural pollution and punctual wastewater inputs from small capacities wastewater treatment plants without tertiary treatment process, representative of rural areas. Potential combination of suitable tracers is discussed combining various micropollutants and isotopic signatures.

En raison de la tendance observée de la baisse des précipitations, la RMR et, dans une plus large mesure, le nord-est du Brésil, sont identifiés comme un « point chaud » représentatif des enjeux globaux liés aux changements climatiques et à leurs effets sur les processus hydrologiques (précipitation, évaporation, ruissellement et infiltration dans les sols vers les couches géologiques dites aquifères) (Bates et al., 2008). Ville côtière située à 8° sous l’équateur, Recife est également la capitale de l’État le plus sec du Brésil, le Pernambuco. Recife est ainsi une ville doublement vulnérable quant à la disponibilité d’une eau de qualité car elle est confrontée à la fois au risque de remontée du niveau marin induisant une salinisation des eaux souterraines et à la diminution chronique des précipitations. Cette diminution affecte la recharge des lacs de barrage, dont Recife dépend à plus de 85 % pour l’approvisionnement en eau, ainsi que la recharge naturelle des aquifères par infilt...