2022-02-22_PERMIT FILE - C1980007 (5)

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West Elk Mine 2.04-44 Rev. 11/04- PR10, 04/06- PR10, 09/07- PR12, 10/08- PR14; 01/22- MR459 Seismic Effects West Elk Mine is located in an area of low seismic risk. The Rulison underground nuclear explosion in 1969, which measured 5.5 on the Richter Scale, generated no known rockfalls or mass movements in or near West Elk Mine. Instrumentation in mines in the North Fork Valley indicated little or no subsurface impact from that event, which was the largest event within at least the last 100 years in the area. Several mine bumps have been recorded on seismographs with a Richter reading as high as 3.5. Those events, while felt on the surface, have had little impact in underground mines in the area. It can be concluded, therefore, that seismic events would have little or no impact on mining or the environment, and that the mining would not increase the minimal effects of seismicity on the environment. Ground Subsidence Geologic conditions in the area strongly indicate that no subsidence other than mine subsidence would be expected in the West Elk Mine permit area. Subsidence is discussed in considerable detail in Section 2.05.6. Other Geologic Factors that May Affect Mining Conditions Groundwater Groundwater can be classified in the coal measures as occurring in the active and inactive zones as it relates to the genesis, composition, and storage of the water. Active and inactive zone groundwater systems have been described in the peer reviewed literature by Mayo and others (2004) as follows: “Active zone groundwater flow paths are continuous, responsive to annual recharge and climatic variability and have groundwater residence times “ages” that become progressively older from recharge to discharge area.” “Inactive zone groundwate r has extremely limited or no communication with annual recharge and has groundwater mean residence times that do not progressively lengthen along the flow path. Groundwater in the inactive zone may be partitioned, occur as discrete bodies, and may occur in hydraulically isolated regions that do not have hydraulic communication with each other.” The active water zone includes the near surface waters derived from rain and snowmelt stored in the near surface colluvium and alluvium. Spring and stream flows are variable in correlation to seasonal precipitation variations (A. Mayo, 1998). This water can affect underground mining operations in several ways: 1. Surface or near-surface water can percolate into mine workings, roof, or floor, either through weathered and fractured overburden (when the mine workings are very close to the surface or drainages). 2. In rare cases, water can enter into mine workings through large scale regional fracturing (when deeper mine workings are under fracture-controlled drainages, or the intersections of fracture zones and drainages), or through faults. It cannot be positively determined whether water encountered in the mine might be related to fracture-controlled valleys, unless a strong correlation can be mapped or quantified. Since the F