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West Elk Mine <br />these naturally occurring rock falls. However, as noted above, the distinction between a natural <br />and mining-induced fall will be difficult to make. <br />Seismic Effects <br />West Elk Mine is located in an area of low seismic risk. The Rulison underground nuclear <br />explosion in 1969, which measured 5.5 on the Richter Scale, generated no known rockfalls or <br />mass movements in or neaz West Elk Mine. Instrumentation in mines in the North Fork Valley <br />indicated little or no subsurface impact from that event, which was the lazgest event within at <br />least the last 100 years in the area. Several mine bumps have been recorded on seismographs <br />with a Richter reading as high as 3.5. Those events, while felt on the surface, have had little <br />impact in underground mines in the area. It can be concluded, therefore, that seismic events <br />would have little or no impact on mining or the environment, and that the mining would not <br />increase the minimal effects of seismicity on the environment. <br />Ground Subsidence <br />Geologic conditions in the azea strongly indicate that no subsidence other than mine subsidence <br />would be expected in the West Elk Mine permit azea. Subsidence is discussed in considerable <br />detail in Section 2.05.6. <br />Other Geoloeic Factors that May Affect Mining Conditions <br />Groundwater <br />Groundwater can be classified in the coal measures as occurring in the active and inactive <br />zones as it relates to the genesis, composition, and storage of the water. Active and inactive <br />zone groundwater systems have been described in the peer reviewed literature by Mayo <br />and others (2004) as follows: "Active zone groundwater flow paths are continuous, <br />responsive to annual recharge and climatic variability and have groundwater residence <br />times "ages" that become progressively older from recharge to discharge area." "Inactive <br />zone groundwater has extremely limited or no communication with annual recharge and <br />has groundwater mean residence times that do not progressively lengthen along the flow <br />path. Groundwater in the inactive zone may be partitioned, occur as discrete bodies, and <br />may occur in hydraulically isolated regions that do not have hydraulic communication with <br />each other." The active water zone includes the near surface waters derived from rain and <br />snowmelt stored in the near surface colluvium and alluvium. Spring and stream flows are <br />variable in correlation to seasonal precipitation variations (A. Mayo, 1998). This water can <br />affect underground mining operations in several ways: <br />1. Surface or near-surface water can percolate into mine workings, roof, or floor, either through <br />weathered and fractured overburden (when the mine workings are very close to the surface or <br />drainages). <br />• <br />1.04-44 Revised November 2004 PR10 <br />