Estimation of Spatial and Temporal Variability of Recharge Fluxes to an Alluvial Aquifer in a Fore Land Area by Water Chemistry and Isotopes

The Var Valley (Southern France) is a typical setting in which alluvial water is situated in a fore land area. The alluvial aquifer is recharged by seepage from the intake area of the river Var and by subsurface infiltration from its fore land area. Chemical and isotopic tracing of both sources is well contrasted, in accordance with the very different geology and relief in the two places. 18 O, silica, and sulphate contents were chosen to calculate percentages of recharge from the Var river and the embanking Pliocene pudding stones. Differences among recharge estimates, calculated from the three tracers are low, about 10%. Ground‐water areas where component contributions to recharge are maximum (over 70% for Var inflow and over 40% for pudding stone inflow) are considered feeding areas. Discharge of leakage from pudding stones was calculated by the use of the Hantush formula on the whole surface of these areas: 0.53 ± 0.2 m 3 /sec, which corresponds to 1/5 of the total water budget of the alluvial aquifer. Chemical surveying of alluvial water, during a water year, displays the constancy of ground‐water inflows over time, ranging from those when inflow surface water is nonexistent in the river Var during low‐water periods to those when it is massive in spate. Thus, during periods without influence from the river Var, the inflows from the embanking formation sustain discharge of alluvial water and influence its chemical makeup. Taking these results into account, the protection area of the lower river Var water resource must be enlarged to include a part of the intake area of the adjacent reservoirs.