Assessment of cation exchange as conditioning processes of water chemistry in freshwater lenses

Freshwater lenses are groundwater sources of limited dimensions that can be usually found in a variety of climates worldwide. These aquifers' quality is important for socioeconomic development, being cation exchange one of the most important geochemical processes that can change the water geochemistry. This study aims to assess the cation exchange processes that determine the chemistry of freshwater lenses in a multilayer aquifer type, considering the center-east of the Pampean Region (Argentina) as a case study. Water samples were taken from the freshwater lenses at different depths to analyze major ions in the laboratory. In addition, geological profiles were made along with the extraction of sediment samples for X-ray diffractometry (XRD) and laboratory tests to analyze the cation exchange capacity. The results show that water stored in the lenses has a vertical facies variation from Ca-HCO₃ to Na-HCO₃ . According to the laboratory results, the change of water facies mainly occurs in the clayey sediments that divide the carbonate bioclastic material above and the loessic sediment below, being cation exchange the most important process. PRACTITIONER POINTS: Cation exchange is the main geochemical process regulating groundwater chemistry. Hydrochemical changes determine the quality of freshwater lenses. Na/Ca exchange is mainly regulated by the groundwater flow into the bioturbated clay. Batch exchange tests were also carried out to quantify the Na/Ca exchange processes.