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-45- Cumulative Hydrologic Impact Assessment - Surface Water (Cont'd) Depletion of Streamflow Natural faults and fractures, as well as those caused by subsidence, could reduce Streamflow by directly diverting surface flow and alluvial ground water into the bedrock ground water system. This effect is undesirable both because of the hydro- logic effects and the mine operation problems it could induce. Experience with underground mines throughout the state, as well as in the North Fork area, indicates that mine inflows are not normally significant when ephemeral streams are undermined. 14ost of the streams to be undermined in the region are ephemeral and would not be expected to be significantly affected by mining. Flow in these ephemeral streams are concentrated in periods of snowmelt and high intensity precipitation events, The stream gradients are steep and their channels contain little alluvium. For all of the above reasons, very little surface water is retained long enough in the stream valleys to infiltrate the ground water systems. Therefore, the amount of water observed reaching the mine workings in insignificant. Perennial streams, however, ma _y experience significant depletions of Flow if mine working are extended adjacent to and/or below the level of the stream valley (e. g. the bedrock aZluvia2 contact). The Lower stream gradient and the thicker alluvial deposits in perennial stream valleys favor flow retention and recharge to the ground water systems. Due to the increased flow retention and ground water recharge in perennial stream valleys, there is a greater potential for stream depletions by mining adjacent to or under these stream valleys. The mine could generate depletions by increasing mine inflows in the area. These mine inflows could be induced either through the mine working encountering existing faults fractures or strata recharged by the stream, or through the mine producing subsidence fractures which extend to the stream valley bottom or to rock strata recharged by the stream. Two mining operations, Pit. Gunnison and Somerset, will undermine perennial drainages and one mining operation, Blue Ribbon will be mining adjacent to a perennial drainge tributary to the North Fork of the Gunnison. Both the Blue Ribbon and the Somerset mines have surface facilities and under- ground mine workings located in the Hubbard Creek Drainage. The combined mining operations of the Blue Ribbon and Sorr~rset mines may impact the quality of surface water flowing in Xubbard Creek and in the irrigation ditches supplied by Hubbard Creek) the Deer Trail Ditch and the hlajnik and Klein Ditch. The quantity of flow may depleted throuyh the dewatering of faults, fractures and rock strata which are in direct or indirect communication with the alluvium or stream bottom of Xubbard Creek. The Somerset Mine has recently mined beneath Xubbard Creek. Inspection by Ch1LRD hydrologists of the underground workings showed this portion of the mine to be making significant amounts of water. At the present time it is uncertain whether or not surface floors are being depleted. However, since there is a greater concentration of inflows and increased discharges associated with the inflow in the Somerset mine workings below Xubbard Creek, there is strong evidence that the U.S. Steel Somerset Mine may already be depleting about 31 .I gpm (44,784 gpd) of the flow in Hubbard Creek. (See Exhibit E8-3361 "Water Sources in the Somerset Mine" within the Somerset Mine Permit Application). The Blue Ribbon mine workings have not yet impacted the quantity of water in Xubbard Creek. However, as mining with- in the Blue Ribbon Mine proyresses down dip and towards Hubbard Creek, fractures and low displacement faults, which are in communication with the Hubbard Creek alluvium and streafnbed, could be intersected and thus could cause some depletion of Streamflow,