Tracing groundwater dynamics using a multi-isotope (δ18O, δ2H, 14C, 3H, and 4He-81Kr) approach in the complex groundwater system of the Baixo Pardo Grande Basin

The combined use of multiple isotopes is a powerful tool for the assessment of complex groundwater systems (CGS), because the differentiated information carried by the isotopes helps to track the adaptations promoted by the interconnection between the reservoirs. In this study, we used the hydrogeological context of the Baixo Pardo Grande Basin as a CGS. The basin is located in the northeastern portion of the São Paulo State and inserted within the geological Paraná Basin framework. The main hydrogeological units are: Guarani, Serra Geral and Bauru aquifer systems (thereafter GAS, SGAS and BAS). The GAS is completely confined by the SGAS, which is a semi-confined aquifer overlain by the unconfined BAS in the western portion of the basin. The database consists of one hundred groundwater samples, from eight different studies, that present stable isotope (δ¹⁸O and δ²H) and twenty of them also present estimated residence time calculated from radiocarbon (¹⁴C), tritium (³H), and noble gases (⁴He-⁸¹Kr chronometer). The GAS has an isotopic fingerprint of δ¹⁸O = -8.60 ± 0.78 ‰ and δ²H = -57.74 ± 4.44 ‰ (28 samples), while SGAS has δ¹⁸O = -7,26 ± 0.78 ‰ and δ²H = -47.18 ± 5.07 ‰ (36 samples) and BAS has δ¹⁸O = -7.21 ± 0.51 ‰ and δ²H = -47.10 ± 2.65 ‰ (36 samples). Groundwater isotopic composition were compared to the Local Meteoric Water Line (LMWL) from Rio Claro GNIP station (δ²H = 8.05 ± 0.1 δ¹⁸O + 13.08 ± 0.5 ‰), the closest to the study area, and groundwater δ¹⁸O isoscapes were constructed for each aquifer using the kriging spatial interpolation method. The residence time was estimated to be over 22,600 years up to 127,000 years for the GAS (7 samples), modern up to 2,500 years for the SASG (7 samples), and over 30 years up to 60 years for the BAS (6 samples). In general, the younger groundwater, recharged after 1986, has a more enriched isotopic fingerprint from shallower wells (depths less than 170 m). The more recent groundwater is from BAS and SGAS, recharged after 1990, and located near the outcrop limits of the aquifer units. BAS and SGAS exhibit similar isotopic fingerprints, located aligned to the LMWL of Rio Claro or below it. This suggests direct recharge from rainfall and the occurrence of evaporation process during recharge. Meanwhile, the older groundwater has a more depleted isotopic composition, mostly from SGAS and GAS, and deeper wells (ranging from 170 m up to 1,200 m). The depleted isotopic signatures from BAS and SGAS can be spatially related to the more depleted isotopic signature from GAS. In the case of BAS, it may be related to past subsidence events in the southwestern part of the basin near the Turvo River, which may allow the mixing with deeper groundwater. For SGAS, the depleted signatures are clustered in the eastern part of the basin where the SGAS is unconfined and the GAS is shallower. This supports the hypothesis of groundwater mix between SGAS and GAS due to similar water heads.  Therefore, the multi-isotope approach highlights important aspects of the CGS in the basin. These insights can promote a more sustainable groundwater resource management, particularly for the GAS, a non-renewable reservoir connected to unconfined aquifers. It is important to note that these findings may also be supported by other types of data, such as potentiometric maps, hydrochemical and hydrodynamic descriptions.