Infiltration processes and impact on shallow groundwater in agricultural dry land areas

An investigation of infiltration processes in a Mediterranean agricultural soil was carried out using environmental and artificial tracing. The objective was to assess the impact of the different vertical flow processes on the recharge of the underlying unconfined aquifer. A long term isotopic survey of rainwater, soil water and groundwater enabled us to estimate the seasonal influence of irrigation on the groundwater recharge. According to the rainfall time series over the previous months, the monthly proportions of irrigation ranged from 60% to 98%. These results point out the sensitivity of such shallow groundwater systems in the Mediterranean region, submitted to climate variability and depending on artificial recharge. At plot scale, preferential flow processes were investigated from a simulated rainfall using bromide as tracer (110 mm over 2h 52 min). The impact of this irrigation on the saturated zone was detected about 2 h after the start of the experiment. The hydrological contribution of this local infiltration to the groundwater was low owing to the high transmissity of the saturated zone and the regional extent of the aquifer. But according to the concentration observed in soil water, solutes fluxes may be significant and local, short-term overtaking of allowable standards for water quality may occur. A rational management of shallow groundwater resources in agricultural regions should consider more explicitly these preferential flow processes in soils. (author)