2009-10-30_REVISION - C1982056 - Laserfiche WebLink

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The major portion of the Fish Creek Alluvial Valley Floor is not flood irrigated but is assumed to be subirrigated based on vegetation types, alluvial water depths and late growing season color infrared photography. Material damage to subirrigated vegetation would not occur because the majority of discharge to the aquifer occurs during spring and early summer when mine discharge is diluted by high runoff volumes. Predicted alluvial water salinity on Fish Creek is approximately 1 mmhos/cm. Well 008-AU-3 on Fish Creek corresponds more closely with surface water quality and is thought to be a more representative site than 006-AW-2. Based on 008-AU-3, a measurable increase in salinity of alluvial aquifers is projected, but the predicted conductivity would be well below the 3 mmhos/cm material damage suspect level for sub-irrigation. Water quality measured in well 006-AW-2 exceeds the suspect level in both the existing and predicted case, but mine discharge water would actually dilute the salinity of 006-AW-2 water, resulting in an improvement in water quality due to mining. Following mining and reclamation, the mine workings would fill with ground water, and water passing through the mine workings would again discharge to the stream/alluvial aquifer system. This water would be expected to have a degraded water quality (3200 mg/l TDS at the source), but concentrations would decrease as the affected water flows to the discharge area. The ground water discharge would be further diluted by flow in the stream/alluvial aquifer system. Because of the low flows involved (0.02 cfs), water quality degradation in the stream/alluvial aquifer system would not be sufficient to cause material damage to the waters which supply the Fish Creek alluvial valley floor. The proposed operation would preserve, throughout the mining operation, the essential hydrologic functions of the alluvial valley floor. The essential hydrologic functions of the Fish Creek alluvial valley floor have been identified as having the capacity to support subirrigated and flood irrigated agricultural activities. Mining beneath the Fish Creek AVF has been limited to panels 5, 6 and 7 of the Southwest Mining District and did not result in dewatering of the stream/alluvial system. Also, as discussed previously, no material damage would result from changes in the quality of surface and ground water that supply the Fish Creek alluvial valley floor. The essential hydrologic functions of the Fish Creek alluvial valley floor would not be affected by the proposed operation. Undermining of a small area of Fish Creek (section 25, T5N, R87W) has been completed and no identifiable impacts were observed in the data. Monitoring of Fish Creek above and below this area continues as per the approved plan. Undermining of Fish Creek is proposed in this Permit Revision in Longwall panels 13L, 14L, and 15L in Section 19, Township 5 North, Range 86 West, and Sections 24 and 35, Township 5 North, Range 87 West. Similar response to undermining of Fish Creek is expected as observed when other portions of Fish Creek were undermined previously. Fish Creek will be monitored upstream and downstream from the under-mined area, as required in the approved plan. Trout Creek The boundaries of the Trout Creek alluvial valley floor which would potentially be affected by the proposed operation and to which these findings apply include irrigated pasture 8S, which encompasses the Middle Creek/Trout Creek Confluence, and all alluvial deposits downstream on Trout Creek to its confluence with the Yampa River, including irrigated pastures 7, 8N, 9, 10, 11, 12E, 12W, 14, 15S, and 15N, as depicted on Map No. 1 of Exhibit 42 of the permit application. Alluvial deposits on Trout Creek meet alluvial valley floor criteria based on extensive acreage of flood irrigated hayland, potential for flood irrigation and a presumed capability to support subirrigated vegetation. 31