Three-dimensional finite difference numerical model of the Getchell Mine and pit lakes, Humboldt County, Nevada: Comparison of modeled versus historical rates of pit filling and prediction of future pit filling rates

Several gold mines in Nevada have large open pit mining operations located below the water table which will partially or totally till with water once dewatering ceases. Operational and regulatory decisions are being based on results of numerical groundwater flow models used to predict future pit lake filling rates. The predictive ability of these models has been called into question because no studies have yet been completed that demonstrate that modeled pit filling rates reasonably reflect actual rates. Although the Getchell Main and North pits are currently pumped in order to dewater the underground mining operation, the Getchell Mine was inactive and the pits filled with water between 1968 and 1982 as the groundwater table recovered from previous pumping of mine dewatering wells. For this investigation, the construction of conceptual and numerical groundwater flow models were made to predict the rate of pit filling following 1968 and into 1982 at which time the South pit was overflowing with water. The filling rates of the pits from 1968 to 1982 have been modeled with MODFLOW, a three dimensional finite difference groundwater flow model. Existing hydrologic data have been incorporated into the model to simulate the filling of the historic North, Center and South Getchell pits. With the filling rates of these three pit lakes being adequately reproduced with the numerical model, greater confidence can now be placed on the results of forward modeling that predict the filling of the two resultant larger pit lakes.