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

ABSTRACT

The Tubarão aquifer system constitutes a very complex, multilayered aquifer enclosed in the Paraná 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 São Paulo. An extensive water sampling campaign was carried out followed by hydrochemical and isotopic (δ²H, δ¹⁸O, δ¹³C and ¹⁴C) 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 Tubarão aquifer system.

KEYWORDS:

• Brazil
• carbon-13
• carbon-14
• isotope hydrology‌
• multilayered aquifer
• nitrogen-15
• groundwater residence time
• oxygen-18

Acknowledgments

We would like to thank Dr Gerhard Strauch, Editor-in-Chief of Isotopes in Environmental and Health Studies, and Dr Ricardo Sánchez-Murillo, Associated Editor of this Special Issue, as well as two anonymous reviewers for their valuable comments and suggestion that have improved the final version of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors wish to thank the National Council for Scientific and Technologic Development (CNPq - Brazil) for sponsoring this study via the Universal Project CNPq 473194/2013-9, the São Paulo Research Foundation (FAPESP) for the grant 2018/06666-4, and the International Atomic Energy Agency for approving our participation in the Coordinated Research Project CRP-F33023: ‘Use of Long-lived Radionuclides for Dating Very Old Groundwaters’.