Recharge processes in karstic systems investigated through the correlation of chemical and isotopic composition of rain and spring-waters

Four karstic springs in southern France, along with rainwater in the same area, were monitored during two hydrological cycles. Stable isotopic ratios (δ18O and δD) allow the contribution of the rain waters from the previous periods (discretised as winter or summer period) to the spring water to be estimated. These computations indicate that heavy rainfall events during the autumn season contribute for 56 ± 7% and 68 ± 9% of the spring water discharge during the following winter and summer seasons, respectively. During the low flow period, residence time does not exceed 1 hydrological cycle, for a large part of the water. Stable isotopic ratios (δ18O and δD) combined with Cl concentration allow the evapotranspiration coefficients to be estimated although the recharge surface is not known and hydrological balance can not be computed. Except for one spring, the evapotranspiration coefficients range from 0.54 to 0.38 (46–62% volume reduction), even during the high flow period. The short residence times, and the strong evapotranspiration coefficients whatever the period (winter or summer) are interpreted as the result of the major role of the epikarst reservoir in the karst recharge functioning over direct or diffuse infiltration.