Research papersPartitioning groundwater recharge between rainfall infiltration and irrigation return flow using stable isotopes: The Crau aquifer

•Stable isotopes of water implemented in a mixing model yield recharge proportions.•Geostatistics are used to produce an equivalent homogeneous reservoir.•Updated piezometric and aquifer geometry maps are produced.•The global groundwater budget is thus inferred.

This study reports an assessment of the water budget of the Crau aquifer (Southern France), which is poorly referenced in the literature. Anthropogenically controlled by a traditional irrigation practice, this alluvial type aquifer requires a robust quantification of the groundwater mass balance in order to establish sustainable water management in the region. In view of the high isotopic contrast between exogenous irrigation waters and local precipitations, stable isotopes of water can be used as conservative tracers to deduce their contributions to the surface recharge. Extensive groundwater sampling was performed to obtain δ18O and δ2H over the whole aquifer. Based on a new piezometric contour map, combined with an updated aquifer geometry, the isotopic data were implemented in a geostatistical approach to produce a conceptual equivalent homogeneous reservoir. This makes it possible to implement a parsimonious water and isotope mass-balance mixing model. The isotopic compositions of the two end-members were assessed, and the quantification of groundwater flows was then used to calculate the two recharge fluxes (natural and irrigation). Nearly at steady-state, the set of isotopic data treated by geostatistics gave a recharge by irrigation of 4.92 ± 0.89 m3 s−1, i.e. 1109 ± 202 mm yr−1, and a natural recharge of 2.19 ± 0.85 m3 s−1, i.e. 128 ± 50 mm yr−1. Thus, 69 ± 9% of the surface recharge is caused by irrigation return flow. This study constitutes a straightforward and independent approach to assess groundwater surface recharges including uncertainties and will help to constrain future transient groundwater models of the Crau aquifer.