GENERAL52957 - Laserfiche WebLink

Home Browse Search

Twentymile Coal Company owns a significant portion of [he lands in and adjacent to [Ite Fish Creek AVF. Twentymile Coal Company operates a cattle operation on these lands and manages the land. through i[s ranch manager, Trout Creek Ranch, as undeveloped rangeland. TCC does not utilize the Fish Creek AVF as cropland. 7CC projects that impacts to the Fish Creek AVF will not si_nificantly impact the cattle operation. TR-14 approved the undermining and subsidence of a small portion of Fish Creek due to longwall extraction in panels 5, 6 and 7 in the Southwest Mining District. No farming activities are currently undertaken along Fish Creek in this area. Subsidence would not preclude the farming of this area. The proposed mining operations would not cause material damage to the quantity and quality of surface and ground water that supply the alluvial valley floor. The potential for subsidence cracking at the surface and subsequent interception and reduction of flow in Fish Creek in the WMD is minimal. Depths of subsidence cracks produced by tensile stress are generally less than 50 feet. Due to the presence of thick, fine-grained sediments in this area, significant surface cracking is not anticipated. Any cracks which may develop in the stream channel will rapidly heal as alluvial materials collapse in the voids and stresses are relieved due to adjacent subsidence movements. Mining has been completed in section 25, TSN, R87W. Hydrologic monitoring data and analysis for this area are included in Exhibit 7j of the permit application. An analysis of the data collected is also included. No changes in Fish Creek flow volumes in lower Fish Creek were observed. Surface water quality should not be impacted by subsidence. A slight increase in sediment load may occur as the stream channel gradient is slightly modified due to localized channel slope changes. However, this increase will be localized and is within normal sediment load increases due to spring runoff and normal flow volume increases along Fish Creek. No change due to potential degradation of surface water via ground water quality changes is projected since only a very small portion of the streamflow comes from adjacent bedrock groundwater discharge. Surface water quality data collected along Fish Creek, and presented by the applicant in Exhibit 7j, reveal no significant changes in water quality as a result of undermining and subsidence. Some increases in permeability of the aquifer units above the subsided area was expected up to 600 feet above the Foidel Creek workings. However, after completion of mining, the permeability of the disturbed strata should return to close to pre-mining levels. Review of the data and analysis of Exhibit 7j reveals that shallow bedrock wells completed below the alluvium did show dewatering and casing damage due to subsidence. However, the dewatering was possibly attributed to the increased permeability created by tensile stresses within 50 feet of the surface. Water levels showed rapid decline followed by slow recovery and have now recovered to nearly pre-mining levels. The Fish Creek alluvial aquifer should not be affected due to its fine-grained nature and the newly 1000 feet of separation from the mine workings. Alluvial wells showed slight impacts (see GWAP-7) but it is not clear whether these small water level changes are due to water loss to the bedrock below or to re- consolidation of the alluvial body. Regardless, no changes in Fish Creek alluvial water levels in lower Fish Creek have occurred. No impact to groundwater quality is predicted. Subsidence could cause inter-communication of aquifers. The two aquifers of sirtnificance above the workings are the Fish Creek alluvial aquifer and the Twentymile Sandstone aquifer. No interaction between these aquifers was predicted due to their physical separation. C: W H B\C82056\RN03\RN03FI N D. doc 30