A chemical and isotopic approach to investigate groundwater dynamics in a coastal aquifer

•Groundwater is Ca/Na-HCO3 in the sand-dune barrier and NaCl in the continental plain.•Isotopic content variability and unsaturated zone thickness are related.•Evaporation processes prior to infiltration were detected.•The mixing processes detected are natural and anthropogenic.•Saltwater intrusion of marine origin was confirmed by the isotopic method. The hydrogeological processes in two different zones of a sandy coastal aquifer of the province of Buenos Aires, Argentina, have been assessed on the basis of hydrochemical tools and environmental isotopes (2H and 18O). In general, groundwater in the sand dune barrier is either a Ca-HCO3 or a Na-HCO3 type, whereas in the coastal plain it is a NaCl type. In general terms, the isotopic composition of groundwater matches the average isotopic composition of the rainfall, which would indicate that there is no significant fractionation during the recharge process. However, the interpretation of data allowed the detection of different processes— rapid infiltration, selective infiltration, evaporation, mineral dissolution and mixing—in certain locations. The last process mentioned includes both natural lateral and vertical interface mixing, as well as the one generated by anthropogenic action (saline intrusion). There is a relationship between the variability of the isotopic contents and the thickness of the unsaturated zone. In the northern zone, its lesser thickness causes greater variability in isotopic contents, whereas in the southern zone a greater thickness is suggested by more homogeneous, clustered values. In both zones, selective infiltration was observed; they were represented by both highly depleted and highly enriched isotopic contents, as well as evaporation processes occurring prior to infiltration. Besides, in both zones it was possible to recognise the effects of the displacement of the freshwater–saltwater interface towards the continent as a consequence of intensive groundwater exploitation. The mixing percentage of salt water is about 51% for both conservative elements oxygen-18 and chloride. The importance of the association of hydrochemical and isotopic methodologies stands out as a tool in the sustainable management of groundwater resources in coastal sand-dune regions.