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• Investigations Report 92-4187) include the introduction of waters containing dissolved COZ and Oz gases <br />into the system (rainfall and snowmelt), the dissolution and precipitation of carbonate and sulfate minerals, <br />the oxidation of pyrite, and cation exchage reactions. These chemical reactions are influenced by the <br />prevailing pH. At Trapper, It is typical in spoil aquifers derived from sedimentary rock dominated <br />groundwater systems for carbonate dissolution to effectively buffer the production of acids and thereby <br />maintain near neutral pH levels. The groundwater developing in these aquifers is effectively buffered and <br />therefore the potential for trace metals to be mobilized is low. The subsequent dilution of spoil-influenced <br />groundwater by unoxygenated groundwater, or the influence of microbial activity can also affect <br />equilibrium water quality. <br />The TDS and sulfate concenfrations in Johnson Gulch backfill well GF-7 are noticeably higher than the <br />TDS and sufate levels in backfill well GF5 in the No Name drainage. The water quality characteristics of <br />backfill wells GD-3 and GF-11 fall between these two exfremes. Pyrite oxidation has been determined to <br />bean important contributing source of increased TDS and sulfate levels in spoil water. Variation in the <br />percent of pyrite contained in the spoil is expected to result in variation in the TDS and sulfate <br />concentrations observed in the backfill aquifers, at least initially. This type of variation was noted in the <br />USGS study at Seneca and may contribute to the variation observed to date at Trapper. Over time, <br />groundwaters can be anticipated to accumulate sulfate and other dissolved constituents to the point of <br />• saturation given the prevailing geochemicel conditions. The maximum TDS concentration estimated by <br />the USGS at Seneca based on undiluted lysimeter samples was approximately 5000 mg/I. This level <br />represents a reasonable estimate of the peak TDS value to be expected at Trapper and is consistent with <br />the maximum levels collected on-site to date. <br />Using the same spoil unit volume and sulfate concentration removal rate as Williams and Clark (USGS <br />1994), the approximate estimates of time for higher TDS and sulfate concentrations to persist range from <br />300 to 900 years for the 0.1 to 0.3 percent average pyritic sulfur levels respectively. These estimates <br />indicate that higher sulfate and TDS concenVations from some of the backfill aquifers at Trapper Mine <br />may, in the absence of any ameliorating factors as discussed below, last for several hundred years under <br />a worst-case scenario in which the entire mass of pyrite available in the spoil body is oxidized. The <br />average overall value for percent pyritic sulfur derived from core hole analyses at Trapper is 0.21 % by <br />weight. However, pyrite is only 53°~ pyritic sulfur by weight; the remaining 47°~ consisting of iron. <br />Therefore, a pyritic sulfur concentration of 0.21 °k by weight equates to a total pyrite concentration of <br />approximately 0.4°~ by weight. According to Williams and Clark (Table 18), a spoil unit with a pyrite <br />content of 0.4°k can be expected to generate high TDS concentrations for approximately 600 years. <br />• Backfilled spoils are commonly graded to a degree with coarser rock fragments and boulders tending to <br />accumulate indiscrete zones as a result of the mine backfill process. Channelization or piping of <br />groundwater flow through these zones in the backfill materials could inhibit complete saturation of the spoil <br />body and reduce the volume of spoil material available for oxidation and leaching processes. This <br />Ravision: T ~a <br />4-238c Approved: <br />