2023-01-05_REVISION - C1981019 (13)

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On a smaller than regional scale, some impacts are expected. The sizes of drainage areas for some ephemeral and intermittent creeks within the mined area will be changed, thus altering runoff quantities. Other expected changes include: • Hydraulic transmissivities within backfilled pits will be altered, thus changing the hydrologic balance. • Base flow in creeks may be reduced during mining and for several years after mining. • Peak flows in creeks will be reduced due to pits. • Natural springs might experience increased or decreased flow or be completely eliminated. The Mine's consumption of surface water will possibly affect the agricultural practices on a minor portion of the Good Spring Creek valley floor. However, since Colowyo owns the property to be withdrawn from irrigation and since these lands are not an integral part of any ranching or farming operation, the effect will be minimal. Exhibit 7 of the approved PAP discusses the water rights augmentation plan for the mine. Potential Effects on Surface Water Quality The 1982 Findings Document projected that no significant adverse impacts to surface water quality from mining operations will occur in or adjacent to the permit area. Subsequently, however, the Division has determined that some impacts are anticipated. Spoil spring development and increased salinity due to leaching in the backfilled pit areas is anticipated to be the most significant impact to the hydrologic balance in and adjacent to the mine area. Degraded spoil water will move down gradient through undisturbed strata and emanate as springs on the steep hillsides. This degraded water will mix with flow in receiving waters, including Good Spring Creek. Colowyo has provided a calculation of salinity increases in Good Spring Creek down gradient of the confluence of the West Fork of Good Spring Creek in Section 2.05.6(3)(b)(iii) of Volume 12 of the PAP. Impacts to irrigated areas immediately downstream of monitoring site EFGSC are possible and are discussed in the alluvial valley floor section of this document. In the past, the Division has found that the operations at the Colowyo Mine have been designed to prevent material damage to the hydrologic balance outside the proposed permit area. The Division is in the process of analyzing recent data from water quality sampling of the Yampa River and tributaries potentially impacted by the Colowyo Mine. Three streams could potentially be affected by mining in the Collom area which include Little Collom Gulch, the West Fork of Jubb Creek and Collom Gulch. Evaluations of each stream was made for changes in direct surface runoff, changes in stream base flow amounts, changes in surface water interactions, effects from discharge of water from sediment ponds and effects from discharge of excess dewatering well flows. Potential effects on surface water quality for the Collom area is outlined in Volume 15, Section 2.05.6 in the PAP. Proposed Decision and Findings of Compliance January 5,2023 Permit Renewal No. 8 Page 140