Stable isotopes (δ2H, δ18O) as tracers of hydrological connectivity in a structurally controlled multilayered aquifer system of the Pampean Plain, Argentina

Abstract

This study combines stable isotope analysis (δ2H, δ18O) with geological and hydrological methods to investigate the hydrological connectivity in the multi-layered aquifer system of the Chucul stream basin in Argentina. The results indicate that the isotopic composition of surface water reflects a range of evaporation levels and surface water-groundwater interactions. This ranges from isolated evaporated lagoons to interconnected lagoons that are sustained by stream flow. The unconfined aquifer demonstrates a clear isotopic zonation, which includes a recharge zone in the upper basin (influenced by isotopically impoverished piedmont waters), an active mixing zone in the mid-basin (mainly influenced by local precipitations), and a regional discharge zone in the lower basin, characterized more depleted waters suggesting that there is upward flow from confined systems with a higher piezometric level and older water. The Confined Aquifer System (CAS) exhibits distinct recharge sources. The western CAS A1/A2 is recharged in the piedmont area, while the deeper eastern systems (CAS C/D) contain older waters, indicating recharge occurred during colder paleoclimatic periods. The study identifies brittle geological structures as the primary control over topography, flow paths, hydraulic relationships, and salinization trends, superimposed by a west-east isotopic precipitation gradient. Additionally, some localized human activities lead to water mixing. These findings provide a conceptual model that is vital for guiding sustainable water resource management and for informing future targeted studies.