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Surficial geology in the area consists largely of consolidated sedimentary rock of the upper <br />Cretaceous Mesaverde Group. A veneer of colluvial/alluvial material is present in drainages, <br />and becomes more significant along the major streams in the area. <br />The Seneca II Mine extracted coal from the Middle Coal Group of the Williams Fork <br />Formation, a member of the Mesaverde Group. The Williams Fork Formation, which <br />conformably overlies the 1,550-foot thick Iles Formation, is a 1,100 to 2,000-foot-thick <br />sequence of interbedded shales, sandstones, sandy shales and coal. <br />The lower-most economically recoverable coal member of the Middle Coal Group is the Wolf <br />Creek Coal, which averages 15 feet thick at the Seneca II Mine site. The underlying Trout <br />Creek Sandstone, considered to be a regional aquifer, is separated from the Wolf Creek Coal <br />by about 70 feet of carbonaceous shale, sandstone, siltstone, and clean shale. <br />Overlying the Wolf Creek Coal is about 150 feet (stratigraphically) of lenticular sandstone, <br />siltstone, shale and thin coal seams. This interburden is overlain by the Wadge Coal, a nine <br />to ten foot thick coal seam. The Wadge overburden is a moderately thick sequence of <br />sandstone, siltstone and shale. About 50 feet above the Wadge Coal is the 2.5 to 5.5 foot <br />thick Lennox Coal. This poor quality coal was recovered in the southern portion of the permit <br />area. Overlying the Lennox Coal is a thin sandstone cap which is overlain by a thick shale <br />unit. <br />Three coal seams in the Williams Fork Formation have been mined at the site. In ascending <br />order, these seams are the Wolf Creek, Wadge and Lennox. The Lennox Coal is <br />discontinuous and was recovered where economically feasible. <br />Ground Water <br />Aquifers identified within and adjacent to the permit area include the Trout Creek Sandstone, <br />Wolf Creek Coal, Wadge Coal and overburden, resaturated spoils and the alluvial bodies <br />associated with local drainages. A discussion of groundwater is provided in the PHC <br />discussion of this document. <br />Surface Water <br />The original Seneca II Mine permit area is located within two adjacent drainage basins: <br />Grassy Creek and Fish Creek. Both of these are sub-basins of the upper Yampa River basin <br />in northwestern Colorado. A discussion of surface water conditions at the Seneca II Mine is <br />provided in the Probable Hydrologic Consequences discussion of this document. The former <br />Wadge mining area lies in the Fish Creek drainage, while the Wolf Creek mining area lies <br />within the Grassy Creek drainage. Both of these drainages are tributary to the upper Yampa <br />River. <br />Tributaries to Fish Creek have historically exhibited ephemeral characteristics, but <br />mining-related groundwater discharges may increase the flow in these tributary streams. This <br />4