Hydrogeochemistry and sustainability of freshwater lenses in the Samborombón Bay wetland, Argentina

Abstract

Freshwater lenses constitute one of the most vulnerable aquifer systems in the world, especially in coastal wetland areas. The objectives of this work are to determine the hydrogeochemical processes that regulate the quality of the freshwater lenses in a sector of the Samborombón Bay wetland, and to assess their sustainability as regards the development of mining activities. A hydrochemical evaluation of groundwater was undertaken on the basis of major ion, trace and environmental isotope data. The deterioration in time of the freshwater lenses in relation to mining was studied on the basis of the analysis of topographic charts, aerial photography and satellite imaging. The results obtained show that the CO_2(g) that dissolves in the rainwater infiltrating and recharging the lenses is converted to HCO₃^-, which dissolves the carbonate facies of the sediment. The exchange of Ca²⁺ for Na⁺, the incongruent dissolution of basic plagioclase and the reprecipitation of carbonate produce a change of the Ca-HCO₃ facies to Na-HCO₃. In depth, the pH increases with the groundwater flow, and the volcanic glassis dissolved, releasing F-and As. Besides, the evapotranspiration processes cause the saline content to increase slightly. As the only sources of drinking water in the region are the freshwater lenses occurring in the shell ridges, mining operations have deteriorated this resource and decreased the freshwater reserves in the lenses. The study undertaken made it possible to develop some preservation, remediation and management guidelines aimed at the sustainability of the water resources in the region.