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 alluvia l water is situated ina fore land area. The alluvial aquifer is rechar ged by seepage from the intake area of the river Var and by subsurface infiltration from its fore land area. Chemical and isotopic tracing o f both sources is well contrasted, in accordance with the very differe nt geology and relief in the two places. O-18, silica, and sulphate co ntents were chosen to calculate percentages of recharge from the Var r iver and the embanking Pliocene pudding stones. Differences among rech arge estimates, calculated from the three tracers are low, about 10%. Ground-water areas where component contributions to recharge are maxim um (over 70% for Var inflow and over 40% for pudding stone inflow) are considered feeding areas. Discharge of leakage from pudding stones wa s 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 t otal water budget of the alluvial aquifer. Chemical surveying of alluv ial water, during a water year, displays the constancy of ground-water inflows over time, ranging from those when inflow surface water is no nexistent in the river Var during low-water periods to those when it i s massive in spate. Thus, during periods without influence from the ri ver Var, the inflows from the embanking formation sustain discharge of alluvial water and influence its chemical makeup. Taking these result s into account, the protection area of the lower river Var water resou rce must be enlarged to include a part of the intake area of the adjac ent reservoirs.