How isotopic hydrogeochemical tools can help policy makers to target priority area for drinking water preservation?
Résumé
The main objective of the Water Framework Directive (2000/60/EC) is to prevent further deterioration, protect and enhance the status of aquatic ecosystems in Europe. In details, WFD enforces member states to identify the hydrosystems that should be protected for present or future use as drinking water. In this context, water policy makers of the Rhône-Méditerranée-Corse district (south-eastern part of France, covering 1/5 of the French territory) plan to list and delimit all the areas related to groundwater that present outstanding interests regarding quality and quantity for drinking use. Discussion of Results Our study is focused on two groundwater bodies located on the Rhône-Méditerranée-Corse district that have been identified as being of primary importance and/or at risk by the policy makers. These groundwater bodies (Isere alluvial aquifer and molasse aquifer) are used for drinking water supply for the large cities of Lyon, Grenoble and Albertville, to a lesser extent. Objectives were to characterize and better understand the origin of water and dissolved elements of these aquifers in order to identify the areas of major interest and protect them in priority. For that purpose, combined geochemical analysis of major and trace elements, and isotopes (δ18O and δ2H of water, 3H, δ34SSO4 and δ18OSO4 of sulfates, 87Sr/86Sr) have been successfully applied. This approach, in addition to geological and hydrogeological information, allow to identify groundwater units in each aquifer that i) present an uniqueness functioning and recharge and ii) permit low cost production for drinking purpose (water treatments reduced). For instance, the Isere alluvial aquifer presents high concentrations up to 3µg/L in As and Sb due to weathering of local rocks in the Alpes mountains. Hydrogeochemical knowledge, and especially isotopic tools, allow to target the groundwater units whose recharge area are less controlled by high As and Sb contents.