Unraveling the water exchange dynamics of the Guarani aquifer system with underlying and overlying aquifers in Uruguay

The Guarani Aquifer System (GAS) ranks among the world's largest aquifer systems, stimulating extensive research into its stratigraphy and hydrodynamic properties improving understanding into groundwater flow dynamics. Previous geochemical studies have concentrated on the hydrochemical and isotopic characteristics of the GAS, highlighting the significant role of progressive rock-water interactions in shaping its hydrochemical evolution. This study builds on previous works by incorporating hydrogeochemical and isotopic data from both the overlying Serra Geral Aquifer System (SGAS) and the underlying Paleozoic aquifers (Pre-GAS), coupled with geochemical simulations, to provide a more holistic understanding of GAS dynamics in Uruguay. Our findings reveal that the GAS is intricately interconnected with both the SGAS and Pre-GAS, forming a complex, multilayered aquifer system characterized by significant groundwater exchange among these units. Contrary to expectations, the GAS outcrop zone functions primarily as a discharge area due to its topographical position and proximity to the Tacuarembó River, with recharge occurring mainly through leakage from the SGAS. These results challenge the prevailing groundwater circulation models focused solely on the GAS and carry important implications for groundwater management, numerical flow modeling, and the estimation of groundwater residence times.