2009-04-20_GENERAL DOCUMENTS - M2009076 (6)

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The Front Range was arched during the Paleozoic, as indicated by the sedimentary record on its flanks (Lovering and Goddard, 1950, p. 29, 30) but this arching did not leave a recognizable record in the Idaho Springs district. During the Laramide orogeny in Late Cretaceous and early Tertiary time, the Front Range highland was arched and uplifted (Lovering and Goddard, 1950, p. 57-60). In the Idaho Springs district this arching was accompanied by the formation of the regional joint system and was accompanied or followed by the successive emplacement of nine varieties of porphyritic igneous rock - fast, primarily in irregular plutons, thick dikes, and lenticular masses, and later, primarily in long narrow dikes. After most of the igneous activity but before the emplacement of the latest variety, a network of faults formed that trends east, east-northeast, and northeast. The north-west-trending faults were then reactivated. Openings along the faults were filled with ore and gangue minerals, and, as indicated by the brecciation of minerals in many veins, fault movements continued during mineralization. This arching, jointing, and faulting evidently took place under compressive stresses that were oriented east-northeast. Late in the stage of mineralization, a few north-trending faults formed, and renewed movements occurred on some northwest-trending faults. The movement patterns indicate that this late faulting occurred under tensional stress oriented east-northeast. Since the Laramide disturbance the region has undergone deep erosion. The steep valley walls, narrow canyons, and fast-moving streams are indications that erosion continues today. The coarse alluvium in Clear Creek Canyon may, in part, be outwash from late Pleistocene valley glaciers that did not extend into the district. The continuous downhill movement of loose material is locally indicated by the colluviul creep debris sheets, though these, in part, may have formed in late Pleistocene time (Harrison and Wells, 1959, p. 26). (USGS Bulletin 1208, Economic Geology of the Idaho Springs District - Clear Creek and Gilpin Counties, Colorado, Moench, Robert H. and Drake, Avery Ala, Jr., 1966). D. There are no holocene faults or other seismic events in this area that pose any geological hazards or slope instability. Precipitation As reported by NOAA's Denver/Boulder Weather Service Office, the average annual precipitation for the Georgetown monitoring station in 2006 was 20.37 inches. Additionally, the pan evaporation rate for the area (as reported by the Western Regional Climate Center) ranges between 35.5 - 40.5 inches annually. 3.1.7 Floodplain According to the most current FEMA Flood Plain Map (reference FEMA Map #0801900093D effective March 19, 2007 in Appendix H) the proposed disposal site is in Zone X. Zone X is defined as follows: Areas of 0.2% of annual chance flood; areas of 1.0% annual chance flood with average depths of less than 1 foot or with drainage areas of less than 1 square mile; and areas protected by levees from 1% annual chance flood. 2.1.5 Aquifer Recharge Area This area is not a significant aquifer recharge area. 3.2.5 (11) Groundwater Travel Calculation The rocks located at the proposed site themselves have essentially no porosity or permeability. However, fractures due to jointing and faulting provide the only groundwater transmissivity and relative porosity. These are directly hydraulically connected to the drainage system of the Big Five Tunnel located in the vicinity underneath the proposed disposal site. The Big Five Tunnel drainage is monitored, collected, and pumped to the Argo Tunnel Treatment Facility located in Idaho Springs. 3.3.1 FAA Notification There are no airports within 5 miles of the proposed disposal site. • 18