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The above statistical computations are not completely definitive. In order for <br />the statistical computations to be strictly valid, the populations sampled must meet <br />several conditions (e.g. normally distributed, independent, etc.). The degree to which <br />these conditions are approached by the populations sampled in this study is not <br />known. Notwithstanding the shortcomings of the calculations, they permit a more <br />rational analysis of the data than would be possible otherwise. <br />Examination of the data in Table 2.5-II indicates no significant degradation <br />in water quality between the two stations with respect To either the common <br />elements or the trace elements listed. As on Trout Creek, the criteria most <br />frequently exceeded are manganese and dissolved iron. <br />2.5.2.3 Aquifers <br />Geologic Setting -The Edna Mine is located in a sequence of deformed <br />sedimentary rocks between the Park Range and the Axial Basin anticline of <br />northwest Colorado. The mine is situated near the southeastern end of the Washakie- <br />Sand Wash structural basin, the axis of which passes near the towns of Yampa and <br />Hayden, Colorado. The mine is located in Upper Cretaceous coal-bearing strata of <br />• the regional Yampa Coal Field and will produce from the Wadge Coal Seam. A <br />generalized stratigraphic section is shown in Exhibit 2.5-14. <br />The formations of particular interest in this study are the Iles and Williams <br />Fork formations. The Iles Formation is a sequence of sandstone, shale and coal beds <br />in the lower half of the Mesaverde Group. The total thickness of the formation is <br />approximately 1500 feet and extends from the base of the Tow Creek Sandstone <br />Member to the top of The Trout Creek Sandstone Member. The Iles Formation is <br />conformable with the Mancos Shale below and the Williams Fork Formation above. <br />The lower two-thirds of The Iles formation is composed of several well-exposed <br />sandstone beds and poorly-exposed shale, siltstone and coal beds. In the Yampa Coal <br />Field the coal beds are referred to as the "lower group" (Bass, et al, 1955). <br />The Tow Creek Sandstone Member at the base is a light brown, fine-grained <br />well-sorted sandstone that is cemented with calcite. This rock unit ranges from 35 <br />to 125 feet Thick and is acliff-forming bed that has been traced through the region. <br />The Double Ledge Sandstone and the Oak Creek Sandstone are two other prominent <br />cliff-forming beds of similar lithology in The lower part of the Iles Formation (Ryer, <br />1977; Kucera, 1962). The Trout Creek Sandstone Member at the top of the formation <br />is named for its exposures near Trout Creek and is persistent through much of <br />Revised 7-81 2.5-43 <br />