Présentations Colloques

Oral Presentation
Session 6.03: Innovative tools to improve understanding of groundwater recharge processes
Sebastien Santoni
Stable isotope composition of rainwater as a proxy to delineate recharge processes to Mediterranean coastal aquifers
Understanding groundwater behaviour means considering the rainwater role in the aquifer recharge processes. Mediterranean coastal areas often display intense touristic, industrial and agricultural activities and groundwater resources are usually over exploited due to low seasonal recharge conditions. This problematic is particularly strategic for southern Corsica (France) where the only available, but not sufficient, water resource is the Bonifacio aquifer. Insofar as no seawater intrusion is yet detected in this aquifer, the isotope composition of groundwater is expected to reflect the signal of rainwater. The Bonifacio aquifer corresponds thus to an interesting study site to observe the recharge processes of Mediterranean coastal aquifers. It is a calcareous plateau filling a fractured granitic depression. Precipitation have been monthly sampled for 18O and D from January 2013 to December 2014. Groundwater have been sampled during the same period and with the same frequency. The survey allows a local meteoric water line to be identified for Bonifacio, characterized as follows D = 7.39 (A0.19) 18O + 6.03 (A0.53) considering deuterium excesses higher than 3 ‰ and rainfall amount higher than 10 mm. This equation is consistent with that of precipitation found in Sicily by Liotta et al. (2013). The rainwater 18O and D values range respectively from -8.31‰ and -53.9‰ during winter to -3.69‰ and -21.6‰ during summer. Same seasonal variations with lower intensities are observed in groundwater. Indeed, groundwater 18O and D means are about respectively -5.97‰ and -35.8‰ but samples from the surrounding granites and from the plateau recharge area displays temporal variability up to A1.27‰ for 18O and A8.6‰ for D. Nevertheless, the presence of important mixing with older groundwater implies less variability for the deepest aquifer. Stable isotope composition of rainwater is thus a relevant proxy to delineate recharge processes to the aquifer but a good definition of the flow conditions is necessary to interpret the signals.