Hidrogeologia dos aqüíferos cársticos cobertos de Lagoa Santa, MG

• Ano de defesa: 2005
• Autor(a) principal: Paulo Fernando Pereira Pessoa
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; UFMG
• Palavras-chave em Português: Aquíferos cársticos; Hidrogeologia; Monitoramento hidrológico; Meio ambiente; Hidrogeologia Aspectos ambientais; Aquiferos; Engenharia sanitária
• Link de acesso: http://hdl.handle.net/1843/ENGD-6LBJ3W

Hydrogeological studies have been conducted at a specific portion of the classical karstic area of Lagoa Santa, in the state of Minas Gerais, Brasil. Part of the studied area is comprised within a significant strip of the county with the same name, and coincides with the limestoneexploration area of the Lapa Vermelha mine. Resources implemented in the site and in the whole of its surrounding area were employed to gather the basic information necessary for the development of the research.The scenery of the study area differs from the traditional regional karst. It is characterized by a karstic aquifer system, covered by a thick soil mantle, and is situated on a representative strip on the edge of the depositional carbonate platform of the Bambuí sedimentary basin, atthe SE limit of the São Francisco craton. The system is made of high concentration limestone, interstratified among layers of impurecarbonates of the Bambuí Group in a piling system. It has a smooth bedding dip ranging from 5 to 10 degrees and is intersected by foliations and joints of the low-angle push systems dating back to the action of sin-to-post-depositional Brasiliana tectonics. The local aquifer system has a high concentration of calcite, which is interbeded by impurelimestone layers in the likes of an interestratal karst system. Carbonate dips low to the east and is transected by foliations and joints created by the overthrusting events in the Brasilian Cycle from the east as a part of the whole Folded overthrusting belt that comes from Espinhaço Ridge. These deformation processes was developed in a thin-skinned decollement faulting carbonate strata resting on crystalline rocks of an entirely different structural character. These kinds of events have been associated the relief with these structural controls, which ones have controlled karst processes to subsurface, creating disperse and localized expressions of dissolution perceived on the geographic scenerySpecific analytic tools for understanding the hydrogeochemical and hydrodynamical control mechanisms of these karstic aquifer systems were employed. About 57 probe holes, drilled up to the base level, helped characterize the topographic formation of the crystalline floor. Itsdisposition indicated the presence of front and side ramps which molded the tectonic style reflected in this domain.Field-based hydrogeologic, hydrochemic and hydrodinamic data were collected in the LagoaSanta mantled karst aquifer, and indicates that most of groundwater flows are dependent of the epicarstic zone. By using quantitative dye tracing techniques, it was possible to recognize flux routes betweenswallow holes and springs, as well as analyses of the hydraulic behavior of the karstic system. Estimated underground flow speeds of up to 80 m/h in turbulent flows conditions inside conduit systems were recognized. Seasonally, the exploration operation reports at the Lapa Vermelha mine have helped observe, via the open pit outlet system, an extensive network of deep fissures and conduits confirmed by the hydraulic connections detected by the measuring devices installed in the monitoring wells. The directions of the underground flow routes, according to the interpolation of the water levels distributed in 30 monitoring wells, have been identified by the execution of potentiometric maps. For the studied area the groundwater divides of the karstic aquifers were surveyed at different observation periods, which helped define the collection area of the Tadinho spring. Along eighteen months of measurements, twenty double-chamber wells installed at different alignments in the study area allowed for the presence of a developed epikarstic zone by observing the water level variation both in the porous environment and in karstic aquifers. This zone specially controls the whole management of the meteoric recharges, thus dictating the expansion rules of the dissolution processes, laterally, under the thick soil covers, and conditioning, along time, the distribution of recharges and the flow regime for the springs.A model of the functioning of the karstic aquifer system was proposed, departing from the coupling between the carbonate rock packet and the structuring of the crystalline basal complex. One can notice the presence of an underlying and extremely active karstic aquifer under the thick weathering mantle. Its control is mostly dictated by the distribution conditions of the pure calcitic limestone occurrence strips, whose evolution in the epikarstic zones ismore noticeable. Structurally, the conditioning of the karstification processes are linked to the presence, mostly, of the intersection planes between the interstratal contacts, generally N-Sdirection, with the planes of the joints and tectonic fabric etched on the carbonates in a parallel way. By using automatic sensors, the hydrodynamical conditions of the karstic aquifers were characterized from the records of water level variations in the monitoring wells. The behaviors of underground flows in conduits and fissures were differentiated. It was also possible to verify an intense interconnection network of flows in the karstic system. Theserecords were compared with pluviometric events and it was possible to establish a correlation between points with the same hydraulic characteristics of the karstic aquifers. In this sense, by using the results of such measuring devices, it was possible to observe, during dry periods, that the aquifer system restricts its flow domain in the form of underground hydric circulation corridors aligned with the interstratal tectonic ccontact strips, preferentially. The collection areas of the Tadinho and Cafundó surgencias were calculated based on the hydric balance, and are around 5,6 km2 and 0,95 km2, respectively. A plausible contourcondition for its limits was sketched with the aid of the potentiometric map. The recession coefficients calculated according to the Worthington norms (1991) allowed to classify the surgencia system of Tadinho under the base flow pattern, underflow type, and the Cafundó and Carrapato as overflow surgencias situated at the beginning of the main discharge domain. The hydrochemical studies carried out in fourteen sampling posts, distributed along the Tadinho aquifer basin, allowed for the identification of an integrated aquifer system, hydraulically, which chemically evolves between its concentrations in alkalinity ofbicarbonates and calcium, and diminishing its corrosion potential, finally culminating in a chemical balance before the discharge zones at the Tadinho spring. The calculation of the mean karstic denudation rates measured at the Tadinho spring indicated an erosive dynamics for the same aquifer system of around 22,5 ± 4 mm/ka.