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•~ Site-SOecif is Hvdrogeolocv Introduction. Bedrock geologic units monitored at the Yoast Mine during the baseline period include in descending order, the Lennox overburden, Lennox coal, Wedge overburden, Wadge coal, Wa dge underburden, Wolf Creek coal, and Wolf Creek underburden. Saturated quaternary alluvium along Sage Creek, Grassy Creek, and annand Draw Was also monitored during the baseline period. The bedrock units, commonly referred to as the middle coat group, comprise the basal portion of the Williams Fork Formation of the upper Cretaceous Mesaverde Group (see Figure 2, Tab b, Geology). This assemblage is bounded below by the Trout Creek sandstone (uppermost member of the Iles Formation), and above by the middle shale member of the Williams Fork Formation, which is assumed to Lie within the Lennox overburden. The Twenty-Mile Sandstone lies stratigraphically above the middle coal group, and outcrops as prominent sandstone cliffs north of the Yoe st permit boundary. The middle coal group outcrops in the southern and southeast portion of the Yoast permit boundary, forming a '~V" shaped pattern along the axis of the Fish Creek anticline (see Exhibit 6-1, Teb b, • Geology). Towards the north, the 4illiams Fork formation (including the middle coal group) dips extensively. The following sections describe the local hydrogeologic characteristics of the bedrock units (middle coal member-Williams Fork Formation) and alluvium monitored at or proximate to the Yoast mining area during the baseline period, although portions of each unit monitored are either partially or completely saturated with ground eater, the low hydraulic properties of the units monitored locally, in combination with marginal to poor eater quality, preclude classification of these geologic units as aquifers. Both locally and regionally, the only significant geologic units that may be classified as aquifers are the lower Trout Creek sandstone and the Twenty mile sandstone. Well yields and water quality in these units are of sufficient quantities for using ground water for beneficial use; however, neither unit will be directly disturbed by mining. The Trout Creek sandstone exists 300 io 400 feet below the Wadge coal seam and will be used for a water supply for the Yoast Mine. The Tuent y-mile sandstone exists 600 feet above the Wadge seam, and therefore does not exist in any Wadge coal mining areas. The outcrops of both units are shown on Figure 1 of Tab 7. Robson and Stewart, 1990 gives a detailed . description of both units, including po[entiometric contours for the Twent y-mile sandstone. 5 Revised 06/19/95