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The Seneca II Mine extracted coal from the Middle Coal Group ofthe Williams Fork Formation, a member of the Mesaverde Group. The Williams Fork Formation, which conformably overlies the 1,550-foot thick Iles 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 ofthe Middle Coal Group is the Wolf Creek Coal, which averages 15 feet 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 lenticulazsandstone, 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. Grouud 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. Surface Water The Seneca II Mine is located within two adjacent drainage basins: Grassy Creek and Fish Creek. Both of these aze sub-basins of the upper Yampa River basin in northwestern Colorado. A discussion of surface water conditions at the Seneca II Mine is provided in the Probable Hydrologic Consequences discussion of this document. The former Wadge mining azea lies in the Fish Creek drainage, while the Wolf Creek mining azea lies within the Grassy Creek drainage. Both of these drainages are tributary to the upper Yampa River. Tributaries to Fish Creek have historically exhibited ephemeral chazacteristics, but mining-related groundwater discharges may increase the flow in these tributary streams. This is discussed in the Probable Hydrologic Consequences (PHC) section ofthe permit application package (Volume 4, Tab 7). The increase in flow is due to the changing of the ground water system from a structurally controlled artesian aquifer to a topographically controlled unconfined system. This change increases the amount of groundwater available for dischazge into the surface water system. At the Seneca II Mine this may increase the flow of Grassy Creek, Little Grassy Creek, and the unnamed tributaries of Fish Creek. 4