Chemical and isotopic investigation of rainwater in Southern France (1996–2002): Potential use as input signal for karst functioning investigation

Abstract

Several recharge processes may be defined for karst systems. However, their respective influence is difficult to determine from spring-water chemistry without a good knowledge of the input signal, i.e. rainfall chemistry. Given the little data published concerning rainfall chemistry in the French Mediterranean area, two rainfall gauges networks were installed – one along a transect from the Mediterranean coast to the most elevated part of the Hérault region, which was monitored over two hydrological cycles (1996–1998), and a second on the catchment area of the main karst aquifer of the Gard region (Fontaine de Nîmes karst system), which was monitored over four hydrological cycles (1998–2002). The main objective of this study was to characterize the input signal of Mediterranean karst systems by analysing the chemical and isotopic compositions of rainfall, in particular with reference to spatial, intra-seasonal and inter-annual variations. Rainwater samples were collected and analysed for major ions, trace elements (B, Br and Sr) and stable water isotopes ( δ 18 O, δ 2 H). The major results of this study are the following: the degree of fluctuation in concentrations of major elements and stable water isotopes in the rainwater depend strongly on elevation and distance from sea. Higher rainfall concentrations are systematically associated with areas nearest to the Mediterranean coast. Rainfall of Atlantic and Mediterranean origins may be differentiated – rainfall of Mediterranean origin is characterized by a higher deuterium excess and generally enriched in heavy isotopes. This rainfall is predominant during the autumn.