Insights from the salinity origins and interconnections of aquifers in a regional scale sedimentary aquifer system (Adour-Garonne district, SW France): Contributions of δ34S and δ18O from dissolved sulfates and the 87Sr/86Sr ratio. (February 2015)

• Record Type: Journal Article
• Authors: Brenot, Agnès; Négrel, Philippe; Petelet-Giraud, Emmanuelle; Millot, Romain; Malcuit, Eline
• Is Part Of: Applied geochemistry. Volume 53(2015:Feb.)
• Page Start: 27
• Page End: 41
• Publication Date: 2015-02
• Subjects: Environmental geochemistry -- Periodicals; Water chemistry -- Periodicals; Geochemistry -- Social aspects -- Periodicals; Geochemistry -- Periodicals; 551.9
• DOI: 10.1016/j.apgeochem.2014.12.002
• Languages: English
• ISSNs: 0883-2927

Abstract

Highlights: Regional sedimentary aquifer on the Aquitaine Basin (SW France). Dealing with limited number of groundwater wells available. Strong control of evaporite dissolution on groundwater dissolved elements. Guidelines for decision-makers to manage water resources. Abstract: The multi-layered Eocene aquifer is a regional scale sedimentary aquifer system occupying ∼120, 000 km 2 within the Adour-Garonne district (France). Local authorities have recently identified the aquifer as being at risk from extensive irrigation abstractions, threatening the sustainability of this key resource. Because large water abstractions for human activities can significantly influence the natural functioning of such aquifer systems, e.g., with leakage between aquifer layers, which can lead to water quality degradation, the characterization of such large systems constitutes a key point to protect and prevent further deterioration of aquatic ecosystems. This study provides further insight on this large aquifer through a geochemical approach, which addresses the limited number of groundwater wells where sampling is possible. For that purpose, a geochemical analysis combining two isotope systems (δ 34 SSO4, δ 18 OSO4 and 87 Sr/ 86 Sr) has been applied. The Eocene sedimentary aquifer system (detrital to carbonate deposits) is made up of four aquifer layers, Eocene Infra-Molassic sand, Early Eocene, Middle Eocene and Late Eocene, and has a mineralized area north of the Aquitaine Basin, where groundwater shows strong mineralization and anomalous levels of critical substances (SO4, F, etc.), increasing the difficulty of resource exploitation. The extreme heterogeneity of the geochemical composition of the groundwater between the aquifers and within a single aquifer is discussed in terms of the lithological control induced by the lateral variation of facies and interconnections between aquifer layers. Geochemical tools, especially the δ 34 S and δ 18 O from dissolved sulfates and the 87 Sr/ 86 Sr ratio, suggest that evaporite dissolution (both sulfate and halite) is the main process controlling the high salinity levels observed in the groundwater, explaining the spatial variations observed at the aquifer system scale. Isotopic tools also provide new information regarding the interconnections between aquifer layers, supporting the hypothesis that the Eocene aquifer system integrates groundwater from the Oligocene–Miocene aquifer through leakage effects. These new insights will likely help decision-makers adjust their choices when managing quality problems, in particular in the "mineralized area of the Entre-Deux-Mers, " where targeted groundwater wells used for drinking water display anomalous levels of critical substances.