2022-02-09_PERMIT FILE - M2021046 (7)

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A stormwater detention pond is proposed to control stormwater drainage at the property. Based on the Mile High Flood District's Urban Storm Drainage Criteria Manual Volume 2,the stormwater detention pond should be sized to temporarily store a minimum of 10%of the 100- year flood flow to achieve a reduction of 10%of the 100-year flood flow through the structure. The drainage above the property that will flow through the affected area is estimated to be approximately 330 acres in size. The 100-year precipitation event for the property area is estimated to be 4.75 inches over a 24-hour period. Accordingly,the 100-year flood event could introduce approximately 130 acre-feet to the drainage, but a significant portion of this precipitation will infiltrate the subsurface as opposed to flow in the drainages as live flow. Based on a conservative estimation that 50%of the precipitation of to 100-year flood event infiltrates and the remaining 50% results in live stream flow,the stormwater detention pond should be sized for the storage of up to approximately 6.5 acre-feet or 2.1 million gallons should be adequate to control the potential stormwater runoff. There are no stream gages on Windy Gulch or Dawson Gulch to confirm the flow assumptions presented above. 2.7.5.2 Dewatering Tanks Two 5000 gallon tanks are proposed to receive dewatering water not utilized in the ore processing. Excess water from the tanks, not used in the mill process will be discharged in the natural drainage. As mentioned above and based on the monitoring well data, saturated fractures exist in the subsurface at the property. As the mine is constructed,the mine workings will intercept and drain those saturated fractures which are hydraulically connected to the mine workings. Water that flows into the mine will be evacuated using a permanent pumping station that will be constructed at the bottom of the mine. Mine inflow water will be connected at the bottom of the mine in a system of overflow pools with connecting drain holes to allow for initial settling of the inflow water with the cleanest water from the last overflow pool being pumped to the surface for clarification and reuse. Water that is not reused will be discharge to a settling pond system at the surface that will further manage sediment from the dewatering. Dewatering of the mine may initially require dewatering rates as high as 80 gpm. On average, dewatering rates will be approximately 55 gpm based on the modelling. These estimates are conservatively high in that they assumed the immediate dewatering of the mine form the bottom of the mine and assumes that the Precambrian material responds to pumping as a porous media and not a fractured rock aquifer. If the fractures in the Precambrian material are not connected, dewatering rates will be much lower once the fractures drain. For the purpose of sedimentation pond sizing,the pond should be designed around a discharge rate of 55 gpm. Based on the Mile High Flood District's Urban Storm Drainage Criteria Manual Volume 3,the sedimentation pond should include a minimum storage volume of approximately 1.8 acre-feet or 600,000 gallons to allow for the sedimentation of the discharge water. This volume represents the water quality capture volume for the property location and should be sufficient to manage the dewatering water and limited ore process discharge if it occurs. The proposed 100-foot by 150-foot sedimentation pond should be adequate for the detainment of discharges from mine dewatering and the ore process. 44