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Peabody Sage Creek Mine (C-2009-087) <br />MT- 2 <br />and rangeland. These low hills extend to higher elevation ridges and steep slopes in the eastern and <br />southwestern parts of the permit area. The perennially flowing Grassy Creek crosses the permit area <br />from south to north, and is joined by its two ephemeral tributaries, Little Grassy Creek and <br />Scotchman's Gulch, near the northern boundary of the permit area (Little Grassy Creek has perennial <br />flow in its lower part as a result of coal spoil spring inflow from Pond 002 on lands transferred from <br />the Seneca II Mine to the PSCM.) Grassy Creek flows into the Yampa River two miles north of the <br />permit area. The Yampa River is one of the largest tributaries of the upper Colorado River system. <br />Geology <br />Section 2.04.5 of the PSCM permit application describes the geology and hydrology in the permit and <br />surrounding areas, which includes information from the following two U.S. Geological Survey Water - <br />Resources Investigations Reports (WRIRs): Hydrology and Geochemistry of a Surface coal Mine in <br />northwestern Colorado, WRIR 92-4187, by R.S. Williams, Jr. and G.M. Clark and Geohydrologic <br />Evaluation of the upper part of the Mesaverde Group, Northwestern Colorado, WRIR 90-420, by S.G. <br />Robson and Michael Stewart. <br />The PSCM permit area extends from the crest of the Tow Creek anticline westward to the axial area <br />of the Hayden syncline. This area is on the southern edge of the regional Sand Wash structural basin. <br />The local stratigraphic sequence that is exposed at the surface is of Cretaceous age. The sequence is, <br />in ascending stratigraphic order, the Iles Formation, Williams Fork Formation, and Lewis Shale. The <br />Iles and Williams Fork Formations are part of the regionally extensive Mesaverde Group. Quaternary <br />alluvial deposits, generally derived from the Mesaverde Group, are locally present in stream valleys. <br />Recent colluvial slides and slumps may also be present in canyons and gulches throughout the area. <br />The Seneca II Mine extracted coal on approximately 1,800 acres of land that was transfered to the <br />PSCM. Coal seams were mined from the lower member of the Williams Fork Formation. The lower <br />member of the Williams Fork Formation is approximately 300 feet thick in the permit area. <br />Approximate thicknesses of the coal seams, in ascending stratigraphic order are: the Wolf Creek <br />seam, 15 ft; the Wadge seam, 10 ft; and the Lennox seam, 4 ft. Shale, siltstone, and sandstone <br />separate the coal seams. The Wadge seam is approximately 150 above the Wolf Creek seam and <br />approximately 40 feet below the Lennox seam. The PSCM will mine coal from the Wadge seam in the <br />Hayden syncline. <br />Groundwater <br />Groundwater hydrology information can be found in section 2.04.5 of the permit application. A <br />"hydrostatigraphic unit" is a group of stata which has hydraulic continuity. Hydrostatigraphic units in <br />and adjacent to the permit area that may serve as aquifers are (in ascending stratigraphic order): the <br />Trout Creek Sandstone member of the Iles Formation, coal and sandstone beds in the lower member <br />of the William Fork Formation, the Twentymile Sandstone member of the Williams Fork Formation, <br />Page 4 of 12 <br />