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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 al] 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 ]ower 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 groundwater <br />analyses are from Wells CW-N6; -N13, -N14, -N15; -N16; -N17; -N18, -N19; -N20; -N21; and <br />-N22. Samples from Wells CW-N29, CW-N30, CW-N31, CW-N32, CW-N33 and CW-N34 have not been <br />collected to date. <br />Overburden, Interburden and Underburdert 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 />• d <br />i <br />thi <br />f <br />10 ft i <br />k <br />l <br />th f th <br />it <br />th <br />t <br />t a t <br />o <br />er 30 <br /> gra <br />n <br />ness <br />an <br />e <br />c <br />rom <br />ess n <br />ern par <br />e perm <br />e eas <br />o are <br />v <br />o <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 Ouality 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 U 7.7) occurs sporadically throughout the unit in all core holes except #886E and <br />17-33 Revised 04/11/88 <br />