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The observed variation in water quality in the backfill wells at Trapper is attributable to differences in the <br />geochemical 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 trends in TDS and sulfate concentrations <br />over time suggesting that the processes of infiltration, carbonate and sulfate mineral dissolution, and <br />pyrite oxidation are resulting in the observed cumulative increases in water quality constituent <br />concentrations but that these processes are moderated by the influence of underburden inflows. The <br />degree and rate of dissolved constituent accumulation exhibited at well GF-11 is higher than at well GD-3 <br />suggesting that infiltration-driven processes are relatively more predominant in this location. At backfill <br />well GF-7, the water quality response has been very similar to that observed in the USGS lysimeter <br />studies suggesting that surface water infiltration processes are not being offset significantly by <br />underburden inflows in this location. In contrast, little to no degradation in water quality is apparent at <br />backfill well GF-5 suggestiing that underburden inflows in this area probably play a much more significant <br />role. <br />The USGS lysimeter studies established that elevated TDS concentrations 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 />isolating them above the water table, diverting surface flows around the backfill locations, and ensuring <br />the materials are covered with a minimum five feet of spoil. The hydraulic properties of the ash materials <br />have been evaluated and indicate that water will preferentially flow around ash and through spoil because <br />of the much higher hydraulic conductivities typical in the spoil. On average, annual infiltration rates at <br />Trapper are quite low (Radian 1981). However, some potential exists for water infiltration to occur directly <br />above ash disposal locations. Any water quality effects resulting from the contact of this water with ash <br />would be minimal and indistinguishable from water quality impacts developing as a result of groundwater <br />contact with spoil. <br />4-238f <br />?Ofjuldi