Surface water bodies are expected to form in several pits at the Getchell Open Pit Mine after mining has ceased due to inflowing surface and ground water. Predicting the long-term geochemical behavior of the pit water is important in assessing potential environmental effects. One of the pits, the Summer Camp Pit, began to develop a pit lake in 1991 when dewatering ceased and the pit was used to store water pumped from underground operations. This provided a field-scale opportunity to identify the controls on lake water chemistry and determine the effects of seasonal mixing events on long-term chemical behavior. During a five-year period (1996)(1997)(1998)(1999)(2000)(2001), a number of physical, chemical and mineralogical characteristics of the lake were monitored with the intent of using this information as a basis for predicting long-term geochemical behavior of future lakes in the other pits. Seasonal and multiyear cycles were identified within the water column. These cycles were influenced by climatic changes and element and sediment loadings of inflow to the lake. Stratification occurred, with the metalimnion or active layer of the lake evolving from a low total dissolved solids (TDS), alkaline water to a high TDS, neutral to mildly acidic water, until turnover occurred due to density variations between the metalimnion and epilimnion, completely mixing the layers. A hypolimnion that formed has the potential to stabilize metals in the basal sediments as sulfide minerals below a chemolimnion in the lake. Longer-term events also appear to involve the hypolimnion.The monitoring program demonstrated the dynamic nature of a pit lake and how the complex limnology can affect seasonal water quality. Such considerations are important in interpreting water quality from pit lakes and in selecting monitoring data to use when constructing mathematical models for predicting changes in water quality.
The Summer Camp Pit (SCP) is situated on the Placer Dome owned Getchell Property in Northern Nevada. The deposit is a typical Carlin type partly oxidized disseminated sulfide-micron gold deposit in strongly deformed and altered Paleozoic metasediments. The pit was mined by a former site owner from March 1990 to December 1991. Although the pit was mostly dry during operations, flows increased as mining deepened the pit, forcing the operators to begin periodic pumping of a small sump in the SW corner of the pit. A small sump developed which due to sulfide oxidation showed low pH and elevated metals and sulfate. To buffer this chemistry the pit was partly filled by water from the underground mining operations approximately 1 mile to the north of the pit and the water level maintained above the oxide-sulfide boundary. The pit lake was monitored at the site for approximately 10 years and during this time the pit was used as part of the site water management strategy. For operational reasons, water was removed from the pit in 2002 and this resulted in exposure of sulfides in the pit wallrock, causing further oxidation and acid generation to occur. The chemistry of contact water created by exposure to these materials exhibited low pH with elevated metals and sulfate concentrations. Getchell Gold Corporation evaluated various options for closure of the pit. Draining and partial backfill provided the most suitable closure alternative for this pit, because it would eliminate the pit lake, reducing the potential for future groundwater impacts and risks to terrestrial and avian wildlife. In order to evaluate the pit backfilling alternative, it was necessary to 1) identify appropriate and available backfill materials, and 2) ensure that those materials do not present an equal or greater risk to groundwater than the existing in-pit materials. The evaluation of potential backfill materials in the vicinity of SCP was based on environmental risk assessment, geochemical testwork and engineering considerations.
At the former Daisy mine gold heap leach operations in Nevada, long term solution draindown is c. 0.25 m 3 /day (0.045 gpm) and hosts arsenic concentrations in the order of 0.27 – 0.61 mg/L. This draindown solution is infiltrated into the ground where it eventually contacts groundwater. To assess the potential for natural attenuation of arsenic in the subsurface and to evaluate the interaction of the draindown solutions with groundwater, a series of predictive geochemical scenarios were undertaken using PHREEQC. These scenarios included varying the draindown rate to assess how this affects groundwater chemistry and varying the proportion of reactive sorbent mineral phases in the sub-surface to mimic the range observed in soil mineralogy. The results of the geochemical predictions demonstrate that arsenic is effectively removed from draindown solutions by adsorption onto soil particles with the net effect that groundwater arsenic concentrations are not increased above baseline levels. The results denote a mass solubility control on solute concentrations during the interaction of heap draindown with groundwater without significant mineral precipitation. Draindown rates measured for the Daisy Heap continue to decline every year and are approaching steady state conditions. Therefore, at the current negligible draindown rates, this geochemical assessment indicates that the interaction of Daisy Heap draindown with groundwater will not alter baseline groundwater chemistry in the vicinity of the operations. Supplementary material: PHREEQC files that support the modelling are available at https://doi.org/10.6084/m9.figshare.c.3587243
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