Deciphering surface water-groundwater connectivity using chemical and stable isotope tracers (H and O) in the headwaters of São Francisco River, Brazil

Marcela Aragão de Carvalho Ramos; Carolina Stager Quaggio; Vinícius Dos Santos; Vladimir Eliodoro Costa; Camila de Vasconcelos Muller; Gustavo Barbosa Athayde; Roberto Kirchheim; Didier Gastmans

doi:10.1016/j.scitotenv.2024.177971

Deciphering surface water-groundwater connectivity using chemical and stable isotope tracers (H and O) in the headwaters of São Francisco River, Brazil

Sci Total Environ. 2024 Dec 11:958:177971. doi: 10.1016/j.scitotenv.2024.177971. Online ahead of print.

Authors

Marcela Aragão de Carvalho Ramos¹, Carolina Stager Quaggio², Vinícius Dos Santos³, Vladimir Eliodoro Costa⁴, Camila de Vasconcelos Muller⁵, Gustavo Barbosa Athayde⁶, Roberto Kirchheim⁷, Didier Gastmans⁸

Affiliations

¹ Laboratory of Water Resources and Environmental Isotopes, Center of Environmental Studies, São Paulo State University (UNESP), Rio Claro, SP, Brazil. Electronic address: marcela.aragao@unesp.br.
² Laboratory of Water Resources and Environmental Isotopes, Center of Environmental Studies, São Paulo State University (UNESP), Rio Claro, SP, Brazil. Electronic address: stager.quaggio@unesp.br.
³ Laboratory of Water Resources and Environmental Isotopes, Center of Environmental Studies, São Paulo State University (UNESP), Rio Claro, SP, Brazil. Electronic address: vinicius.santos16@unesp.br.
⁴ Center of Stable Isotopes, Institute of Biosciences (UNESP), Botucatu, SP, Brazil. Electronic address: vladimir.costa@unesp.br.
⁵ Laboratory of Hydrogeologic Research, Federal University of Paraná (UFPR), Curitiba, PR, Brazil. Electronic address: camilavmuller@ufpr.br.
⁶ Laboratory of Hydrogeologic Research, Federal University of Paraná (UFPR), Curitiba, PR, Brazil. Electronic address: gustavo.athayde@ufpr.br.
⁷ Geological Survey of Brazil (SGB - CPRM), São Paulo, SP, Brazil. Electronic address: roberto.kirchheim@sgb.gov.br.
⁸ Laboratory of Water Resources and Environmental Isotopes, Center of Environmental Studies, São Paulo State University (UNESP), Rio Claro, SP, Brazil. Electronic address: didier.gastmans@unesp.br.

PMID: 39667165
DOI: 10.1016/j.scitotenv.2024.177971

Abstract

This study assessed the chemical composition of major elements and stable isotopes (H and O) in precipitation, groundwater, and surface water in São Francisco 1 (SF1) sub-basin (approximately 14,000 km²), located at the São Francisco River headwater region, fractured aquifer system region with a complex geologic framework. Both groundwater and surface water exhibited low mineral content, with average electrical conductivity of 147.2 ± 99.4 μS.cm^-1 and 65.7 ± 78.7 μS.cm^-1, respectively. The ionic abundance (mEq.L^-1) followed Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ for cations and HCO₃^- > SO₄^2- > Cl^- > NO₃^- > F^- > PO₄^3- for anions, with most samples being calcic and/or magnesian bicarbonates. Chemical composition was primarily influenced by rock-water interactions and wet-season precipitation in surface water. The isotopic composition of precipitation showed a seasonal pattern, with lower values in the wet season (-63.87 ‰ for δ²H and - 9.86 ‰ for δ¹⁸O) and higher values in the dry season (-5.12 ‰ for δ²H and - 2.12 ‰ for δ¹⁸O). Groundwater remained constant (-43.71 ‰ for δ²H and - 6.70 ‰ for δ¹⁸O), while surface water varied seasonally (-44.95 ‰ for δ²H and - 7.07 ‰ for δ¹⁸O in the wet season, and - 38.12 ‰ for δ²H and - 6.14 ‰ for δ¹⁸O in the dry season), reflecting wet-season precipitation inputs. Consistent groundwater Evaporation Line (EL) slopes across seasons suggest that the recharge was gradual and slow. The Mann-Whitney test for chemical and isotopes tracers pointed out that surface water-groundwater connectivity was present in both seasons, although enhanced in the wet season. In Lower SF1, identical distributions (p = 1) of NO₃^-, δ²H and δ¹⁸O underscore the stronger connectivity and mixing within this compartment. Groundwater was the primary source of streamflow, contributing from 60 % to 85 %, and wet-season precipitation was another source of surface water.

Keywords: Hydrochemistry; Isotope hydrology; Surface water–groundwater interaction; São Francisco River basin; Water stable isotopes.