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Geology and Topography The Seneca II Mine is located on the Tow Creek anticline, a local structure within the regional Sand Wash swctural basin. The local stratigraphic sequence, in ascending order, is the Mancos Shale; the Mesaverde group, which may be subdivided into the lies and Williams Fork Formations; the Lewis Shale; and the Lance Formation, all of which aze upper Cretaceous in age. Overlying the Lance Formation are the Tertiary Fort Union and Wasatch Formations. Quaternary alluvial deposits, generally derived from the Mesaverde Group, are locally present in stream valleys. Recent colluvial slides and slumps may also be present in canyons and gulches throughout the area. Surficial geology in the azea consists lazgely of consolidated sedimentary rock of the upper Cretaceous Mesaverde Group. A veneer of colluvial/alluvial material is present in drainages, and becomes more significant along the major streams in the area. The Seneca II Mine extracts coal from the Middle Coal Group of the Williams Fork Formation, a member of the Mesaverde Group. The Williams Fork Formation, which conformably overlies the 1,550-foot thick Des Formation, is a 1,100 to 2,000-foot-thick sequence of interbedded shales, sandstones, sandy shales and coal. The lower-most economically recoverable coal member of the Middle Coal Group is the Wolf Creek Coal, which averages 15 fee[ thick at the Seneca II Mine site. The underlying Trout Creek Sandstone, considered to be a regional aquifer, is separated from the Wolf Creek Coal by about 70 feet of carbonaceous shale, sandstone, siltstone, and clean shale. Overlying the Wolf Creek Coal is about 150 feet (stratigraphically) of lenticular sandstone, siltstone, shale and thin coal seams. This interburden is overlain by the Wadge Coal, anine- to ten-foot thick coal seam. The Wadge overburden is a moderately thick sequence of sandstone, siltstone and shale. About 50 feet above the Wadge Coal is the 2.5- to 5.5-foot thick Lennox Coal. This poor quality coal was recovered in the southern portion of the permit area. Overlying the Lennox Coal is a thin sandstone cap which is overlain by a thick shale unit. Three coal seams in the Williams Fork Formation have been mined at the site. In ascending order, these seams are the Wolf Creek, Wadge and Lennox. The Lennox Coal is discontinuous and was recovered where economically feasible. Ground Water Aquifers identified within and adjacent to the permit area include the Trout Creek Sandstone, Wolf Creek Coal, Wadge Coal and overburden, resaturated spoils and the alluvial bodies associated with local drainages. A discussion of groundwater is provided in the PHC discussion of this document.