2011-08-09_REVISION - C1981019 (10)

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• RULE 2 PERMITS 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 approxiamtely 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 described below. The volume of materials that must be resaturated to bring the water level in the pit up to 7,275 feet amsl is calculated to be about 5.2 x 10 cubic feet (ft). Since recharge will occur rapidly up a estimated elevation of 7150 feet, the pit spoil aquifer will develop on top of this level in the northern portion of the Collom Lite pit. The volume of materials below this level is calculated to be 1.0 x 10 ft This decreases the volume of materials to be saturated to 4.2 x 10 ft Assuming 20 percent effective porosity, 8.4 x 10 ft of water (19,284 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. Other studies show a potential infitration rate of up to 18 percent of annual precipitation. This would translate to an estimated annual recharge rate of up to 3.24 inches per year. Using the highest potential infiltration rate for the Collom Lite pit area of 880 acres, that translates to approximately 28,255 ft /day (0.65 acre - feet/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 (0.68 acre - feet/day) of recharge would contribute to resaturation of the pit backfill. This equals a total recharge of approximately 1.33 acre - feet/day recharge after mining is complete. Given these numbers, it would take approximately 39 years to resaturate the pit backfill to an elevation of 7,275 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 Collom — Rule 2, Page 136 Revision Date: 8/5/11 Revision No.: PR -03