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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 concentrations 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 % 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% 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 in discrete 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 <br />spoil body and reduce the volume of spoil material available for oxidation and leaching processes. This <br />physical setting might reduce the duration of time elevated water quality constituent levels are expressed <br />by effectively reducing the mass of sulfate available to be leached from the spoil. Reduction estimates of <br />10% to 25% are reasonable (equating to elevated TDS duration estimates of 540 and 450 years <br />respectively). <br />Three of the four backfill wells installed to date at Trapper (GD-3, GF-5, and GF-11) exhibit better water <br />quality characteristics than would be expected if accumulated backfill waters were derived primarily from <br />infiltration. USGS lysimeter studies (Water Resources Investigations Reports 86-4350 and 92-4187) <br />documented typical TDS concentrations in the range of 3500 to 4500 mg/I with sulfate concentrations <br />averaging aroung 2500 mg/I. These concentrations were developed over short periods of time (2 to 3 <br />years). Backfill wells associated with the lysimeters in Report 924187 were shown to exhibit relatively <br />better water quality with TDS concentrations around 25% lower than lysimeter levels and sulfate <br />concentrations around 20% lower. The relatively better water quality in the backfill wells was attributed to <br />the diluting effects of higher quality underburden inflows. <br />4-238e <br />f Of <br />01 12A,