Hydrochemical and isotopic studies in two neighboring river basins in semiarid regions of Minas Gerais

Groundwater is the largest reservoir of fresh water on Earth and the depletion of aquifer systems is a reality, both in semi-arid and humid regions. The hydrology of watersheds is a field of study that is concerned with the movement of waters from the rains and its physical-chemical changes. Groundwater resources in these regions are a sensitive issue, requiring careful management to avoid quality degradation. The present study focuses the groundwater system at the interface of the Pandeiros and Peruaçu river basins in the northwest region of Minas Gerais State. The methods used were the evaluations of the hydrochemistry, as well as isotopic, and physicochemical parameters, allied to geological studies to investigate surface and groundwater, their possible interactions and spatial distribution. The hydrochemical results correlated with the physicochemical measurements show water contrasts in the study area, which is geologically composed from bottom to top by gneisses and granites (Januaria Complex), limestone and siltstones (Bambuí Group), sandstones and eluvial sediments (Urucuia Group), eluvio-coluvial cover and alluvial deposits. It has been observed that waters in the context of sandstones and siliceous sediments are sodium bicarbonate waters, slightly acidic pH and low in TDS, whereas waters in pedological / geologic context of limestones are calcium bicarbonate waters exhibiting basic pH and high TDS. The deuterium (D) and oxygen-18 signatures of the groundwater are compared to the global meteoric water line (GMWL). The average values of the isotopes in the study area are slightly variable, indicating that the isotopic signatures are related to rainfall precipitation. It has been possible to suggest an evaporation line pattern starting from the position of springs, passing though streams and finally ending in the lagoon. The local evaporation line can be represented by the equation δD = 5,8 δ¹⁸O - 2,4. (author)