2017-03-23_PERMIT FILE - C1981019A

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RULE 2 PERMITS There are times of year, mainly during the winter and spring seasons, when there may be an excess of water produced from the mine dewatering system. The excess water will be discharged to Collom Gulch and the West Fork Jubb Creek with an approve CDPHE permit. It is unknown but may at times equal 300 gpm, divided between both drainages. If each drainage receives 150 gpm at times, then the increase flow to the drainages will be a maximum of 0.3 cfs (effects to surface water on Collom Gulch and West Fork Jubb Creek discussed previously in this section). WMC (2006) developed a regional hydrogeological model of the Collom area. This model was calibrated to existing conditions and the transient response observed during a 30 day pumping test in the bedrock units that will be mined. Based on the modeling and the results of the WMC (2005) study the groundwater level drawdown from mine dewatering potentially will extend some distance to the south of the mine but should not extend further than the drainage divide in the southern portion of the Collom area. Drawdown effects may extend laterally to Collom Creek on the west side of the mine and the West Fork of Jubb Creek on the east side of the mine. There is potential for drawdown in groundwater levels in bedrock units that extend beyond these two stream valleys but the drawdown is anticipated to be less than 50 ft and may be lower if there is vertical hydraulic communication between the valley fill aquifers and the underlying bedrock units. Groundwater levels in the bedrock will recover after mining but may vary from the pre -mining conditions. Potential changes in the final groundwater levels are discussed below in the section on pit backfill re -saturation. Potential interactions with snrinas and seers The drawdown in groundwater levels caused by the mine dewatering activities may affect springs and seeps that are fed by groundwater discharge. The area where springs and seeps are affected will probably be bounded by the southern extent of the pit footprint to the south, the West Fork of Jubb Creek to the east, Collom Creek to the west, and one to two miles north of the pit highwall. The springs that are potentially affected by the dewatering operations are discussed in the section on springs and seeps above. Potential interactions with valley fill and streams There is some groundwater flow in the valley fill deposits associated with Little Collom Gulch that eventually enters Collom Creek at the confluence between Little Collom Gulch and Collom Gulch. Recharge to shallow groundwater in the Little Collom Gulch valley fill will be reduced because the recharge area for valley fill groundwater south of the pit highwall will be eliminated during mining. This may result in an approximate 50% reduction in shallow groundwater flow in the Little Collom Gulch during mining. The amount of groundwater flow in the Little Collom Gulch valley fill is estimated to be about 2,060 ft3/d (17 ac-ft/yr). This estimate is based on a hydraulic conductivity of the valley fill that averages 33 ft/d (WMC, 2005), a gradient of 0.025 ft/ft, a saturated thickness of valley fill of 25 ft (based on the measurements in valley fill monitoring well MLC -04-1 located near the mouth of Little Collom Gulch), and an estimated lateral extent of the saturated valley fill of 100 ft. WMC (2006) estimates that the total valley fill groundwater flow is about 18,850 ft3/d to the north in Collom Gulch below the confluence with Little Collom Gulch. Of this amount, about 12,000 ft3/d is flowing in valley fill aquifer and an additional 6,900 ft3/d is groundwater flow that discharges to the stream as base flow. Thus, if the valley fill groundwater inflow from Little Collom Gulch is reduced by 50% from 2,060 to 1,030 ft3/d, this would only reduce the total groundwater flow out of Collom Gulch by about 5%. Collom —Rule 2, Page 117 Revision Date: 1/6/17 Revision No.: TR -109