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• 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°h 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 92187) <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). BaGfill wells associated with the lysimeters in Report 92-4187 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 />The observed variation in water quality in the backfill wells at Trapper is attributable to differences in the <br />geochemicel and physical processes dominating the groundwater systems at each site. Sites GD-3 and <br />GF-11 are similar in that they both show moderate increasing Vends in TDS and sulfate concentrations ' <br />• overtime suggesting that the processes of infiltration, carbonate and sulfate mineral dissolution, and pyrite <br />oxidation are resulting in the observed cumulative increases in water quality constituent concentrations but <br />that these processes are moderated by the influence of underburden inflows. The degree and rate of <br />dissolved constituent accumulation exhibited at well GF-11 is higher than at well GD-3 suggesting that <br />infiltration-driven processes are relatively more predominant in this location. At backfill well GF-7, the <br />water quality response has been very similar to that observed in the USGS lysimeter studies suggesting <br />that surface water infilVation processes are not being offset significantly by underburden inflows in this <br />location. In contrast, little to no degradation in water quality is apparent at backfill well GF-5 suggestiing <br />that underburden inflows in this area probably play a much more significant role. <br />The USGS lysimeter studies established that elevated TDS concenVations can develop as spoil aquifers <br />are resaturated following mining. Sulfate comprises a large percentage of the dissolved solids load that <br />develops. Sources of sulfate in backfill aquifers include the dissolution of gypsum and the oxidation of <br />pyrite. The naturally occurring concentrations of these minerals in overburden are more than sufficient to <br />account for the documented sulfate levels contributing to elevated TDS concentrations. Gypsum is <br />common in the area and only small amounts can account for the sulfate levels found in leachates. Pyrite <br />concentrations encountered in the area are also sufficient to account for all of the sulfate measured in <br />. spoil waters. <br />Combustion byproducts utilized as backfill also contain potential sources of sulfate. However, the <br />management practices employed are designed to minimize the hydrologic exposure of these materials by <br />hc!'iSiOi,: r~''~~ <br />4-238d Apprcv~CT 2 S 2004 <br />