Using isotope data to characterize and date groundwater in the southern sector of the Guaraní Aquifer System

ABSTRACT

The Guaraní Aquifer System (SAG) is the largest transboundary aquifer in Latin America, extending beneath parts of Brazil, Paraguay, Argentina, and Uruguay. This paper presents the results of recent hydrogeological studies in the southern portion of the SAG. Locally, the abundance of surface water bodies precluded the use of conventional hydrological tools to characterize groundwater flows. Geological, hydrochemical and environmental isotope investigations were integrated to postulate a revised hydrogeological conceptual model. The revised geological model has provided a better definition of the geometry of the aquifer units and outlined the relevance of regional faults in controlling flow patterns. The new potentiometric map is consistent with groundwater flow from the SAG outcrops to the centre of the Corrientes Province, where upwards flows were identified. Hydrochemical and isotope data confirmed the widespread occurrence of mixing. Noble gas isotopes dissolved in groundwater (⁴He and ⁸¹Kr/Kr) provided residence times ranging from recent recharge up to 770 ± 130 ka. Groundwater age modelling confirmed the role of the geological structures in controlling groundwater flow. The southern sector of the SAG is a multilayer aquifer system with vertical flows and deep regional discharge near the Esteros del Iberá wetland area and along the Paraná and Uruguay rivers.

KEYWORDS:

• Argentina
• Corrientes and Misiones provinces
• carbon-14
• groundwater age
• Guaraní Aquifer System
• helium-4
• isotope hydrology
• hydrogen-2
• hydrogeochemistry
• krypton-81
• oxygen-18

Acknowledgements

The authors are indebted to Dr Pradeep Aggarwal (formerly at the IAEA) for facilitating the research, to Drs Roberto Kirchheim and Didier Gastmans (Rio Claro University, Brazil) for performing the noble gases sampling, the Ministry of Ecology of Misiones Province, and to the Provincial Directorate of Parks of the Corrientes Province for their strong support with personnel, vehicles, and boats, making most of the fieldwork possible. The authors would like to thank the anonymous reviewers for their useful comments and suggestions.

Disclosure statement

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

Additional information

Funding

The different works were funded by the International Atomic Energy Agency, the Argentina National Agency for Scientific and Technological Promotion (Project PICT-2008-2071), the Spanish Ministry for Economy and Competitivity (Project CGL2009-12910-C03-03, REDESAC), and the Instituto de Hidrología de Llanuras.