Compartimentação e modelagem hidrogeológica do sistema aquífero cárstico-fissural da região cárstica de Lagoa Santa

Abstract

Hydrogeological conceptual and numeric models are the best way to represent the knowledge of groundwater regime and, specially in karstic systems, that constitute a very complex environment this kind of modelling is a challenge. In this context, this work was developed in the most studied karst in Brazil, in the metropolitan region of Belo Horizonte, Minas Gerais, including the Lagoa Santa Karst Environmental Protection Area. In an area of 600 Km², a refinement of the conceptual hydrogeological model (CHM), in vertical and horizontal compartments, in the fissured-karstic aquifer system was proposed and, in a second moment, a numerical hydrogeological model (NHM) was developed using the software Feflow. The CHM worked as a basis for the NHM, that had as an objective the simulation of flow routes obtained in tracer studies of previous studies and the underground flow behavior focusing on the karstic aquifer. In the methodology applied the horizontal compartmentalization was based on geoprocessing data that ratify the existence of karstification processes (caves, high flowing springs, tracer routes, etc.), being delimited three subcompartments (Escrivânia-Gordura, Palmeiras-Jaguara and Palmeiras-Jaguara-Samambaia) within the karstic hydrogeological basins in the study area. The vertical compartmentalization was based on water inlets described in wells in the area, specific capacity data and cover thickness evaluation. It was proposed that the first 30 m depth in carbonates correspond to the most karstified zone that composes the karstic aquifer itself, overlapped to the fissured-karstic aquifer. In the crystalline basement it was defined that the fissure aquifer would be productive until the depth of 60m below the fresh basement, where most water intakes are located. With increase depth, the basement become an aquifuge that acts as an impermeable base of the system. Six hydrogeologic units were proposed: granular aquifer (covers), aquiclude (pelites of Serra de Santa Helena Formation), karstic and fissured-karstic aquifer (carbonates of Sete Lagoas Formation) and fissure aquifer and aquifuge (Belo Horizonte migmatitic-granite gneiss complex). On the other hand, the hydrogeological modeling was done in steady state after the boundary conditions definition and the 3D geological modelling in the software Leapfrog. The numerical hydrogeological model achieved satisfactory parameters, being able to represent the study area hydraulic heads and the discharges monitored in springs. As results, possible flow routes to springs that have unknown flow components were suggested and the calculated water exchanges in the karstic aquifer were evaluated. The main work conclusions showed that the hydrogeological behavior it is not bonded to the lithostratigraphic units, but it can vary within a same unit or has characteristics that reflect the combination of two different units. Moreover, the karstic conduits representation using the discrete features tool allowed the representation of the local groundwater flow, mainly related to the importance of external water entrances to explain the high flow rates observed in karstic springs.