2017-03-23_PERMIT FILE - C1981019A

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RULE 2 PERMITS In Collom Creek, the springs potentially affected by mining operations (V27, V28, SPRC-02 and SPRC- 04) produced a combined average flow of about 0.057 cfs, a maximum flow of about 0.13 cfs and a minimum flow of about 0.002 cfs during the baseline monitoring period. It is not expected that new springs will develop in Collom Creek during the post mining period. Potential Impacts to Streams The three streams potentially affected by mining include Little Collom Gulch, the West Fork of Jubb Creek and Collom Creek. An evaluation of each stream was made for the following potential impacts from mining operations: • Changes in direct surface runoff to streams from storm flow and snowmelt • Changes in stream base flow amounts • Changes in surface water and groundwater interactions • Effects from discharge of water from settling ponds • Effects from discharge of excess dewatering well flows Little Collom Gulch Little Collom Gulch is an ephemeral stream throughout its entire length, has a drainage area of about 2.9 square miles (WMC, 2005) and flows south to north through the center of the mine footprint. The area of Little Collom Gulch within the pit footprint is about 0.74 square miles and the area within the spoil pile footprint is about 0.59 square miles for a total area of about 1.33 square miles. Thus, the disturbance is about 46% of the total watershed area. Several clean water diversion structures are planned in Little Collom Gulch upstream of the pit to intercept and safely reroute storm flows around the mine area. The water collected in these structures will come from undisturbed areas. As described in Section 2.04.7 (2) no flow was observed in Little Collom Gulch during any of the sampling events. The direct surface runoff from 25% of the drainage area of Little Collom Gulch will be intercepted by the pit and will be either lost to evaporation or be utilized for dust control within the pit. The surface runoff from 20% of the drainage area of Little Collom Gulch will be incident upon the out of pit spoil pile. This runoff will be captured by one of the five sediment ponds (See Map 4113) and will either be lost to surficial evaporation or be discharged according to CPDES requirements to Little Collom Gulch or Collom Gulch. Another 8% (0.24 square miles) of the watershed will be disturbed by the facilities area and report ultimately to the Section 25 pond and will be lost either to evaporation or discharged to Little Collom Gulch. The runoff intercepted by the Little Collom Gulch clean water diversion structures upstream of the mine pit (0.78 square miles of drainage) will be redirected to either Collom Gulch or the West Fork of Jubb Creek and not be impacted by mining activities. Surface water flows in Little Collom Gulch have not been observed so impacts to direct runoff in Little Collom Gulch are expected to be minimal. Since Little Collom Gulch does not normally contribute to the direct surface water runoff in Collom Creek, the overall effects on the streamflow in Collom Creek are expected to be insignificant. During the post -mining period, the Little Collom Gulch surface drainage pattern will be re-established to pre -mine density. There is currently a small amount of recharge to the shallow valley fill groundwater that occurs from precipitation and surface runoff in Little Collom Gulch. This source of recharge will be eliminated during Collom —Rule 2, Page 113 Revision Date: 12/30/16 Revision No.: TR -108