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A <br /> Project Descriptions AdrianBrown <br /> p <br /> EVALUATION OF MINE PIT REFILLINQ NEW MEXICO <br /> AdrianBrown was retained to perform the groundwater and geochemical evaluation of the post-mining <br /> conditions at an open-pit lake as a result of a citizen-led Clean Water Act suit. The mine pit was <br /> dewatered during operation and,on cessation of mining,began to refill. Because the local rock <br /> contained sulfide minerals such as pyrite, concern was raised regarding the ultimate quality of pit lake <br /> water and the potential for adverse impacts on local water resources. <br /> AdrianBrown evaluated the extensive pre-existing information on the project and developed a <br /> conceptual model of groundwater and surface water flow. This model was refined and calibrated to be <br /> able to account for observed rates of filling. A geochemical component was added to this model to <br /> account for the introduction of chemical mass into the lake along with water to ultimately produce a <br /> model of the changes in water quality in the existing pit. The coupled hydrologic and geochemical <br /> model was subsequently used to model the future conditions for surface and groundwater quality in the <br /> vicinity of the mine. <br /> Our approached utilized a mass-balance algorithm for water inflows and outflows,which also allowed <br /> an accounting of chemical mass reporting to the lake and the effects of evaporative concentration. <br /> Geochemical equilibrium speciation modeling was used for the historical pit lake during early refilling to <br /> produce empirical relationships for the solubility of principal phases controlling the concentrations of <br /> sulfate(gypsum)and calcium/alkalinity(calcite). <br /> The results produced by the model were presented to the citizen intervenor group,and to an independent <br /> technical panel selected by the mining interest and the citizen group in a mediation forum. The <br /> evaluation and model of the refilling pit lake was accepted as appropriate and established as a <br /> management tool whereby future chemical evolution of the pit lake could be evaluated. As a <br /> management tool,the model will allow identification of significant departures from expected behavior, <br /> triggering action that will ensure environmental protection of groundwater and surface-water resources. <br />