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March 7998 Slump Investigation and Mitigation Report ~ Page S 3.0 SUMMARY OF HYDROGEOLOGY The mine permit area generally lies within the Hayden Gulch watershed, which is struc[urally between the Sand Wash Basin and the White River Uplih (Grose, 1972). Geologic strata in this region dip predominantly to the northeast from 4 to 8 degrees. The geologic forma ions consist of several sedimentary units including the Lewis Fork Formation overlying the Williams Fork formation, which in turn overlies the Iles Formation. The mined coal units and the former highwall were within the Williams Fork formation. The Williams Fork formation includes a 600-foot upper unit and a 1,000 foot lower unit. The upper unit consists of the five mined coal units interbedded with siltstones, mudstones, and sandstones. The lower unit consists of marine shale, sandstone, siltstone, mudstone, shale and coal seams. At the base of the upper unit, approximately 200 to 300 feet below the mined zone, is the massive Twentymile sandstone. Underlying the lower unit is the massive 100-foot thick Trout Creek sandstone, which is part of the Iles Formation. The principal water-bearing zones in the vicinity of the mine are recent alluvial deposits, coal beds, and the massive Twentymile and Trout Creek sandstone units. The Twentymile and Trout Creek sandstone units constitute significant regional aquifers and are present below the coal seams mined at the Hayden Gulch Coal Mine. In the permi[ area, groundwater flows to the northeast (Morrison-Knudsen, 1980). Recharge primarily comes from rainwater saturating the units where they outcrop just [o the south of the permit area. A regional seep and spring survey performed in 1980 (Morrison-Knudsen, 1980), revealed the presence of several seeps or springs adjacent to the valley floors in the vicinity of the Hayden Gulch Coal Mine; none were observed in the vicinity of the slump. However, in [he 1991 Annual Reclamation Report, there is reference to a spring monitoring point RSP-5 chat was apparently located approximately 200 feet downhill of the slump (HGCC, 1992). In 1991 and subsequent years, the spring was reported as dry. The area around [he spring was apparently regraded which may have blocked off or buried the source of the spring. This indicated that water may have discharged higher up on the hillsides prior to mine development and reclamation. Mozztgo>nery Watson Americas * PO. Boz 774018 "Steamboat Springs, Colorado "80477 (970J 879-6260 R`dA~9.mp Nmt•nan~L.mp Repon 1/}1/9H dv