Avaliação do balanço hídrico e modelagem numérica hidrogeológica da formação de lago em cava : o caso da mina de Águas Claras

Abstract

Mining activities, especially open-pit mining, significantly alter the natural composition of the landscape, resulting in various consequences such as the formation of pits that can reach the groundwater level. To enable continued mining, techniques for lowering the groundwater level are employed, mainly through the construction and pumping of borewells, impacting both groundwater flows and related surface water bodies. The decommissioning of these mining activities proposes different alternatives for the post-operational use of the pits, such as the formation of lakes. Specifically, the Mina de Águas Claras (MAC) serves as a valuable case study for this analysis, having operated for 29 years and subsequently transitioning to the formation of a lake in the pit over the past 24 years following the cessation of pumping activities. The study of MAC, which includes the analysis of extensive hydrometeorological monitoring data and the development of a numerical hydrogeological model, aims to understand the limitations and potentials of computational models for predicting lake formation in mining pits, addressing the contribution of different variables of the hydrological cycle and the sensitivity of applied hydrodynamic parameters. This analysis underwent a discussion on the characterization of the area, defining the main hydrogeological units and groundwater flow, as well as estimating the percentages of the hydrogeological cycle components contributing to lake filling, which indicated the predominance of groundwater flow responsible for 43% of the accumulated lake volume by the year 2023. Finally, a numerical hydrogeological model was created in the Feflow program, to analyze not only the calibration of the numerical model from the pre-lowering period to the current situation of lake filling, but also an assessment of the model's sensitivity to appropriate calibration of parameters governing local groundwater flow and the use of average rainfall rates for system recharge and investigation of final lake filling. The model indicated a high sensitivity of pit filling to the storage employed for pit voids, as well as to rainfall rates directly contributing to lake filling. Furthermore, the simulation of final lake filling indicated a final filling period between the years 2050 and 2052, when the lake reaches the spill point at Córrego Águas Claras, at elevation 1098 m. This prediction, however, is highly sensitive to the rainfall contribution rate employed, with a percentage of 40% of the average rainfall used in the 100-year simulation applied for this result.