2010-12-17_REVISION - C1981019 (108)

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RULE 2 PERMITS capacity of the bedrock units to transmit groundwater will not diminish. Consequently, the recharge and upgradient inflow entering the pit area will re -enter the bedrock units on the downdip side of the pit and flow in directions similar to the pre- mining flow configuration. It is very unlikely that groundwater resaturating the reclaimed pit will accumulate to an elevation high enough to cause it to discharge to the valley fill or to the surface in Little Collom Gulch. That could occur only if the capacity of the bedrock units downdip of the reclaimed pit to transmit groundwater flow were to be diminished compared to the pre- mining condition. A range of time necessary to resaturate the reclaimed pit backfill to that point can be calculated from the pit topography, the volume of materials to be resaturated, and the rate of recharge to the reclaimed pit backfill, as described below. The volume of materials that must be resaturated to bring the water level in the pit up to 7,290 feet amsl is calculated to be about 5.2 x 10 cubic feet (ft). Assuming 20 percent effective porosity, 1.05 x 10 ft of water (24,000 acre -feet) must infiltrate from the surface and from the Williams Fork Formation to fill the pit to this level. WMC (2006) estimated groundwater recharge from precipitation in the Collom Lite pit area to be 1.1 inches per year. For the Collom Lite pit area of 880 acres, that translates to 9,627 ft /day. WMC (2006) also estimated the horizontal groundwater flow rate through the Collom Gulch drainage area to be about 19,000 cubic feet per day. If all of that is assumed to enter the upgradient side of the reclaimed Collom Lite pit, and infiltration of seepage from surface water is assumed to be about 0.016 cfs, then approximately 30,000 ft /day of recharge would contribute to resaturation of the pit backfill. Given these numbers, it would take approximately 96 years to resaturate the pit backfill to an elevation of 7,290 feet amsl. If the water - bearing units updip of the pit area are considered to have been completely dewatered and therefore not be a source of groundwater inflow to the backfilled pit after mining, the time to resaturate the pit backfill from precipitation recharge and surface water seepage alone would be about 300 years. These calculations do not account for any outflow of groundwater from the backfilled pit into the bedrock units on the downdip side of the pit. Such outflow would occur, however, and would either prevent the pit backfill from becoming resaturated up to the 7,290 feet amsl low point (the most probable scenario) or further extend the time necessary to resaturate the backfill up to the 7,290 feet elevation. Groundwater accumulating in the pit backfill and flowing into the deeper bedrock units on the downdip side of the reclaimed pit would likely have the same characteristics as the water in the Streeter Fill well or the Streeter pond or in similar spoil springs (Williams and Clark, 1994). Analytical data for these sampling points are summarized on Table 2.04.7 -49. The above information suggests that it is possible that a reclaimed pit aquifer (if one develops) will flow entirely into the undisturbed strata and that there will be no or limited discharge into the surficial valley fill from the reclaimed pit. Whether groundwater in the reclaimed pit backfill discharges into the bedrock of the Williams Fork Formation or into valley fill, it will eventually contribute to the valley fill aquifer and springs tributary to Collom Gulch south of the outcrop of the Trout Creek Sandstone near the northern side of the permit revision area. Potential Surface Water Quantity Impacts As described above, Little Collom Gulch is ephemeral, and showed no evidence of surface flow during 18 months of baseline monitoring. Elimination of springs within Little Collom Gulch will therefore have no measurable effect on surface water quantity in Little Collom Gulch. There may be an effect on peak flows in the West Fork of Jubb Creek, due to the elimination of recharge to three springs adjacent to the Collom Lite pit, but this effect is not expected to be measurable or statistically significant. Once the mining has been completed and the pit has been saturated, the contributions to surface water from springs Collom — Rule 2, Page 129 Revision Date: 1/23/09 Revision No.: PR -03