2013-04-10_PERMIT FILE - C1981019 (82)

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RULE 2 PERMITS The Collom Lite pit is likely to be within the watersheds for springs VI, V 10, and V32 (West Fork of Jubb Creek). These springs collectively produce about 0.024 cfs during September, during which monitoring location WFJC was dry, indicating that the spring flow reinfiltrates into the valley fill, or is captured in stock ponds before reaching the monitoring point. The springs contributed about 0.042 cfs in May 2006 to the West Fork of Jubb Creek. Stream flow was measured in May 2005 as 0.10 cfs. No conclusive estimate of the percentage contributed by the springs can be made given the likely difference between hydrologic conditions in those two years. Considering that 0.024 cfs was lost to valley fill or ponds during September, it appears that the contribution of V1, V 10, and V32 to the peak flow at WFJC could be considerably less than the 42 percent implied by a year -to -year comparison. At monitoring point CJC, the May 2005 discharge was 0.15 cfs, implying that the affected springs contribute 28 percent of the flow at that location. Since much of the recharge is from undisturbed areas outside of the Collom Lite pit, the probable reduction is likely to be less than half of this amount and 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 originating from infiltration into the Collom Lite pit would return to normal. Collom Lite Pit Hydrology The Collom Lite pit will have a reclaimed surface area of approximately 880 acres and a pit bottom that inclines predominantly toward the north. The low point in the reclaimed pit topography will be at its intersection with the valley of Little Collom Gulch, at an elevation of approximately 7,275 feet amsl. Resaturation of the reclaimed pit backfill will occur from confined groundwater creating static conditions following dewatering, infiltrating precipitation, seepage of surface water, and groundwater inflow from upgradient areas. The mining activity will not cause any decrease in the hydraulic conductivity or transmissivity of the unmined bedrock units downdip (north) of the pit and the 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. The ground water in the spoils will flow to the north through the pitwall due to the hydraulic gradient in the area. The first component of the pit spoils resaturation will be the reestablishment of the piezometric surface for the confined conditions (as discussed in the WMC study). This elevation in the northern portion of the Collom Lite pit is approximately 7150 feet (amsl). Once the dewatering pumps have ceased pumping, this level should reestablish within a short time period. Some surface water from precipitation will help the establishment of the saturated level. However, this inflow of ground water from the pitwalls and floor will be much quicker than any recharge from any other source. Thus, the ground water from the pitwalls below an elevation of 7150 feet will come into contact with any spoils much faster than any other ground (or surface) water source. This level will be established before mining ceases in Collom, so all possible pit spoil recharge will occur on top of this level. 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 describ�.d below. Collom — Rule 2, Page 136 Revision Date: 9/28/11 Revision No.: PR -03