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River and to just past the town of Nucla to the east. This study area was delineated by a <br /> maximum area of potential influence. <br /> For the Nucla East area, ground water is located within the overburden, interburden or <br /> underburden materials. The overburden includes Unit 1 only, or all material above but not <br /> including the upper Dakota coal seam. The interburden includes Units 2, 3, and 4 which is <br /> all material above the lower Dakota coal seam up to and including the upper Dakota coal <br /> seam. The Dakota coal consists of Unit 5 only, and the underburden, Unit 6, includes only <br /> the material directly below the lower Dakota coal seam. Data used for core analyses are <br /> from core holes #870E, #871E, #872E, #881E, #882E, and #886E. Data used for ground-water <br /> analyses are from Wells GW-N6; -N13, -N14, -N15; -N16; -N17; -N18, -N19; -N20; -N21; and <br /> -N22. Samples from Wells GW-N29, GW-N30, GW-N31 , GW-N32, GW-N33 and GW-N34 have not been <br /> collected to date. <br /> Overburden, Interburden and Underburden Characterization. <br /> Overburden Material . The overburden material consists primarily of thick sequences <br /> of sandstone interbedded with shale, carbonaceous shale, and silt. The sandstone units <br /> grade in thickness from less than 10 ft in the eastern part of the permit area to over 30 <br /> ft in the western part. The shales are most prevalent above the upper Dakota coal and <br /> range in thickness from less than 1 ft to 8 ft (Table 17-6). <br /> Laboratory tests on cores provide information on the chemical properties of the material. <br /> Of prime importance to the water quality are the parameters acid base potential (ABP) and <br /> paste pH. These are discussed in the Controls on Ground Water Quality section. Briefly, <br /> negative values of ABP indicate the potential for the production of acid by oxidation of <br /> pyrite, and the paste pH indicates whether reactive pyrite is present. <br /> The overburden contains low concentrations of naturally-occurring trace elements such as <br /> B, Se, Mo, Fe, Mn, Cu, Pb, Zn, Ni, Cd and Hg. Soil pH analyses show paste pH's range from <br /> 3.7 to 8.9 but average about 7.2. The most acidic pH material ((4.5) occurs only in core <br /> holes #882E and #886E. Generally, the low pH (<5.5) material occurs in the far eastern <br /> part of the site, does not occur in the central or western part, and is usually found in <br /> the shale directly above the upper Dakota coal . In #882E the maximum thickness of the 3.7 <br /> pH zone is about 0.5 ft, and in #886E the 4.3 pH zone is about 4.5 ft thick. The basic pH <br /> material P 7.7) occurs sporadically throughout the unit in all core holes except #886E and <br /> 17-33 Revised 04/11/88 <br />