Compartimentação dos fluxos do sistema hidrogeológico cárstico do Grupo Bambuí a partir dos isótopos ²H, ¹⁸O e ³H na região de Lagoa Santa, Minas Gerais

Abstract

The study area (505 km²) includes the Lagoa Santa Karst Environmental Protection Area (APA Carste de Lagoa Santa), worldwide known for the unique karst environment. In the São Francisco Sedimentary Basin context, its embasement and the Bambuí Group outcrop, the latter represented by the Sete Lagoas Formation metalimestone (Pedro Leopoldo and Lagoa Santa members) and Serra de Santa Helena Formation metapelites. As this region has been experiencing progressive anthropic pressure, the karst-fractured aquifers, naturally vulnerable, are objects of research in order to support a groundwater monitoring network. Thus, the purpose of this study was to develop a hydrogeological, isotopic, and hydrochemical compartmentation model for the hydrogeological karst system of Lagoa Santa region. For this objective, sampling was carried out for isotopic analysis of δ²H, δ¹⁸O and tritium concentration from five types of water point: meteoric, lagoon, stream, spring and well waters. Physical-chemical, hydrochemical, geological, hydrogeological and hydraulic parameters were also used. The isotopic and hydrochemical signatures of each type of point were investigated according to their respective focus. Meteoric waters were used to delineate the Local Meteorological Water Line (LMWL), which obtained a high precision; the lagoons showed strong evaporation; and the streams had signatures similar to those of the springs. Focusing on groundwater, spring and well water data were also subjected to spatial and multivariate statistical analyses. Springs were separated into two groups according to their position in relation to the LMWL: the first one with a major contribution of direct precipitation and the second with a significant contribution of waters that have undergone evaporation. Karst springs from the same hydrogeological basin showed high similarity to each other, revealing that, in addition to topographic elevation and discharge rate, both tritium concentration and electrical conductivity differentiate springs from distinct basins. In relation to well water, it was observed that where the Pedro Leopoldo Superior Member and the Serra de Santa Helena Formation outcrop are areas with relatively lower concentrations of tritium. This suggests a shorter residence time caused by the lower permeability of these units, affecting the time of their recharge and water conduction. The deep aquifer recharge form, direct and rapid through massifs, was proven by tritium and stable isotopes, contrasting with the older waters of the springs. The physical-hydrogeological compartments were delimited according to drainage basins, dye tracer tests and potentiometry. The isotopic-hydrochemical compartments were defined considering tritium concentration and scores resulting from the first principal component, generating two main compartments: one of faster and more qualitative vulnerable water, and another of slightly slower water, but with greater quantitative vulnerability. Isotopic tracers proved to be a powerful technique for characterizing aquifer recharge origins and processes and also for flow stratification. In addition to confirming the hydrogeological divisions of the study area, the two isotopic-hydrochemical compartments, influenced by rock composition, karstification development, and stratigraphic and structural configurations, are valuable indicators of the different contexts of water circulation. The proposed compartmentalization will be fundamental to guide a more efficient groundwater resources management.