Geochemical evolution, residence times and recharge conditions of the multilayered Tubarao aquifer system (State of Sao Paulo - Brazil) as indicated by hydrochemical, stable isotope and 14C data

The Tubarao aquifer system constitutes a very complex, multilayered aquifer enclosed in the Parana basin (central-southern part of Brazil). Despite the relatively low productivity of wells, groundwater represents an important source of water for the very populated and industrialized zones of the State of Sao Paulo. An extensive water sampling campaign was carried out followed by hydrochemical and isotopic (delta H-2, delta O-18, delta C-13 and C-14) studies, aiming at a better understanding of the aquifer's geochemical evolution, recharge processes, and its groundwater residence times. Two main hydrochemical facies were recognized and divide the aquifer in two portions. The shallow portion - the active hydrological zone of the aquifer - is characterized by the Ca-HCO3 water type, evolving as a system open to atmospheric CO2. Mean residence times are typically lower than 5000 years. The lower portion is mostly characterized by the stagnant, Na-HCO3 water type, evolving under closed system conditions. Residence times average up to 15,000 years, but can reach 44,000 years, which indicates the exploitation of (possibly non-renewable) fossil waters. This study contributes to the establishment of proper policies regarding the sustainable groundwater exploitation of the Tubarao aquifer system.