Impact of infiltrating irrigation and surface water on a Mediterranean alluvial aquifer in France using stable isotopes and hydrochemistry, in the context of urbanization and climate change

The alluvial aquifer in the lower Durance valley, southern France, constitutes the main source of drinking water for the city of Avignon. This aquifer is amply recharged by both the Durance River water and irrigation return flows. In the context of diminishing area for cultivation and of climate change, preserving the water resources in this region is essential. Effective management requires adequate knowledge of the recharge sources and groundwater flow. For this purpose, a field study was conducted during the period 2010–2011. Samples of groundwater from the shallow aquifer, rainfall and surface water were analyzed for chemical and stable isotope composition—oxygen (δ¹⁸O) and hydrogen (δ²H)—to characterize the groundwater flow and major recharge sources. The results of the groundwater hydrochemical investigation indicate that the predominant geochemical processes taking place along the main groundwater flow path are highly affected by land cover and human activities. Spatial variation in the isotopic signature and total dissolved solids of the groundwater highlights different flow patterns and identifies the different recharge zones. Using the contrast in isotopic mixing proportions between irrigation water and regional precipitation, the relative contribution and spatiotemporal distribution of the different sources of recharge can be determined. By synthetizing all available data, a conceptual model is proposed, providing a basis for integrated modelling of the hydrosystem according to likely future scenarios of land-use and/or climate change.