A MULTIDISCIPLINARY APPROACH TO MODELING OF OPEN-PIT LAKES AND THEIR ENVIRONMENTAL IMPACTS

When surface mines close, pit lakes are often allowed to form in the open-pit that remains. One of the biggest concerns with filling a pit with water is the development of water/rock interactions that can potentially produce acid rock drainage (ARD). Based on studies of forming and existing pit lakes, the water quality of these lakes tends to be poor and can adversely affect the surrounding environment. For this reason, an accurate understanding and prediction of the geochemical system is necessary to prevent and mitigate potential environmental impacts. To fully model this type of a system, multiple modeling platforms and methods must be utilized. This project applied five separate models to fully characterize the pit lake system and was used to predict the pit’s lake chemistry over time and to determine if there would be any long-term environmental impacts caused by the formation of a lake. This study was performed as part of the planning activities to help operate the mine and implement engineering controls to achieve clean-closure at the end of the mine life.

Initially, hydrologic modeling was performed to establish the water balance of the system. Next a groundwater model was constructed to simulate the dewatering system of the pit during the mine’s operation. This provided the starting point for the filling of the pit lake which was simulated as an extension of the dewatering model. This model was further used to examine the interactions of the lake with the surrounding hydrologic system. A limnological model was applied to simulate the temperature profile of the lake and to determine the potential mixing behavior that may exist once the pit is filled with water. Finally, a geochemical model was used to determine the overall water chemistry of the lake and the level of treatment that will be required to meet effluent standards. This multidisciplinary approach allowed for the full characterization of all physical and chemical processes in the pit lake system that could not be acheived with just geochemical modeling. Limiting a pit lake study and predictive model to just the geochemical aspects will likely result in overlooking significant inputs to the system and possibly an underestimation of the water quality impacts.