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• 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 />The water quality constituent concentrations from the natural springs West Buzzard, Flume, Coyote, <br />Wapiti and Cottonwood are representative of the values anticipated during low flow conditions when all of <br />the surface water is derived from groundwater discharge unaffected by mining operations. The previously <br />documented concentrations from the Colt Seep and the current concentrations encountered at the <br />Johnson Gulch and No Name springs, derived from the backfill aquifers in these areas, are higher than <br />the typical natural low flow concentrations. The groundwater being discharged to the surface from spoil <br />springs is expected to contain higher TDS concentrations for a period potentially extending several <br />hundred years after mining under the worst~ase scenario as previously described. <br />• These groundwater discharges are expected to increase the surface runoff water quality constituent <br />concentrations to levels wmparable to those found in the backfill during periods when other sources of <br />surface runoff are not contributing significant quantities of water to the. Historic TDS and sulfate <br />concentrations from spoil springs within the project area have ranged from 1100 to 4100 mg/I and 400 to <br />2300 mg/I, respectively. Some spoil springs will exhibit elevated water quality constituent concentrations <br />for an extended period as pyrite oxidation continues to occur over the next several hundred years. The <br />relative impact to discharged surface water quality from spoil aquifers is significant during low fbw <br />conditions. Typically, low flow conditions prevail for most of the year with seasonal higher flows mostly <br />associated with snowmelt runoff in April, May and June. Although higher TDS concentrations are evident <br />during the low flow periods and are important with respect to the localized flows in Johnson Gulch, overall <br />the flow magnitudes during these periods are small and of little significance to the Yampa River. The <br />Johnson Gulch base flow water quality has been affected by the backfill aquifer but these higher <br />concentrations have not affected the use of this water. <br />The water quality in the Upper Williams Fork aquifers is generally better as compared to the shalk„v <br />alluvium downgradient of the mine. The conductivity in the Pyeatt shallow alluvial system is approximately <br />twice the baseline value of the Third White Sandstone in the Pyeatt drainage. The spoil aquifer will mainly <br />be connected to the Third White Sandstone in this area. The initial spoil water quality will likely be similar <br />. to the Pyeatt shallow alluvial system water. Trapper will continue to evaluate spoil water quality through a <br />spoil spring survey program. See Permit Section 4.8.7 for the program description. <br />4-238e ^~+ti:.io;+: T.t'~y <br />Af;C.i Ci ~ ly~L~~= <br />