Origin of Salinity in a Coastal Karst System: La Clape Massif, SE France

Recent increases in groundwater salinity, exceeding the guidelines for irrigation, prompted this investigation on the origin of mineralisation in the La Clape karst aquifer (SE France). The La Clape massif (Aude) is a typical Jurassic and Cretaceous carbonate aquifer that was deeply karstified during the Messinian crisis. The massif is affected by the vicinity of the Cevennes lithospheric fault, and numerous normal faults related to the Oligocene distension event. Potential sources of salinity are seawater intrusion, upward migration of deep saline waters, as well as dissolution of Triassic evaporites. A contribution from halite can be ruled out on the basis of the measured Br/Cl ratios that are similar to that of seawater and deep saline waters. Sr concentrations suggest mixing with an end member slightly more enriched in Sr than current seawater. Ranging from 0.707852 to 0.708875, 87Sr/86Sr ratios of groundwaters are lower than that of modern Mediterranean seawater (0.709163), and higher than that of the carbonate matrix, ranging from 0.707276 to 0.707477. Those ratios tend towards that of deep saline groundwaters observed at the nearby site of Balaruc. 36Cl/Cl ratios measured on a few samples agree with a mixing hypothesis of superficial karst waters with deep saline water at secular equilibrium with the carbonate matrix. Including dissolution of carbonate and mixing with a saline end member, PHREEQ C modelling confirms that Balaruc-type waters are a likely end member. Calculated parts of mixing range from 0 to 16% of deep saline water. This suggests that seawater intrusion in the karst system is prevented by thick off-shore sedimentary deposits. The deeply faulted and karstified La Clape system is likely to favor the upward movement of deep saline waters. At the regional scale, saline groundwaters of three coastal karst aquifers located along the Gulf of Lion show 87Sr/86Sr ratios tending towards a Sr isotopic ratio of 0.70847. This may suggest a similar origin for the observed salinities in these coastal karst systems, by upward movement of deep saline waters.