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

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− > SO₄2− > Cl− > NO₃− > F− > PO₄3− 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. © 2024 Elsevier B.V.