An investigation of infiltration processes in a Mediterranean agricultural soilwas carried out using environmental and artificial tracing. The objective was to assessthe impact of the different vertical flow processes on the recharge of the underlyingunconfined aquifer. A long term isotopic survey of rainwater, soil water and groundwaterenabled us to estimate the seasonal influence of irrigation on the groundwater recharge.According to the rainfall time series over the previous months, the monthly proportionsof irrigation ranged from 60% to 98%. These results point out the sensitivity of suchshallow groundwater systems in the Mediterranean region, submitted to climatevariability and depending on artificial recharge. At plot scale, preferential flow processeswere investigated from a simulated rainfall using bromide as tracer (110 mm over2h 52 min). The impact of this irrigation on the saturated zone was detected about 2 hafter the start of the experiment. The hydrological contribution of this local infiltrationto the groundwater was low owing to the high transmissivity of the saturated zone andthe regional extent of the aquifer. But according to the concentration observed in soilwater, solutes fluxes may be significant and local, short-term overtaking of allowablestandards for water quality may occur. A rational management of shallow groundwaterresources in agricultural regions should consider more explicitly these preferential flowprocesses in soils.
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