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The ABP's range from -25 in #886E (Unit 4) to +20 in #881E (Unit 3). The more acidic <br />• material is associated with high pyrite concentrations which are highest in #886E (Unit 4, <br />1.5 percent), #882E (Unit 4, 1.38 percent), and in #881E (Unit 2, 3.8 percent). The zones <br />of highest pyrite content occur almost exclusively directly above either the upper or <br />lower Dakota coal seams. <br />Underburden Material. The underburden (Unit 6) consists of the sandstone and <br />associated shale directly beneath the lower Dakota coal seam (Table 17-8). All core holes <br />stop in various parts of the sandstone so underburden thicknesses cannot be accurately <br />defined or computed. Trace elements found in low concentrations and considered to be <br />naturally occurring are B, Fe, Mn, Ca, Pb, Zn, Ni, Ca and Hg. Analyses of soil pH show a <br />range from 7.3 to 8.0 and ABP calculations range from -12.4 in #881E to +67,8 in #870E. <br />The pyrite content ranges from .03 to .80 percent in #881E. <br />Controls on Cround Water Quality. <br /> Introduction. The chemistry and quality of the ground water is determined by <br /> chemical reactions (such as dissolution of pyrite, calcite and other minerals) and by <br />• dilution with irrigation water. In the Nucla East area, the water is marginal to poor <br /> quality, and is not suitable for drinking water. The total dissolved soiids content and <br /> sulfate concentrations are high, and drinking water standards for nitrate, cadmium and <br /> selenium are exceeded in some areas. Secondary drinking water standards for iron, sulfate <br /> and manganese are exceeded throughout the area. <br />This section will address the controls on water chemistry by several approaches. The <br />geochemical speciation code PHREEOE (Parkhurst, et al ., 1980) will be used to analyze <br />mineral solubility controls. Some of the coals and interburdens contain high <br />concentrations of pyrite, and low paste pH's have been measured. The impact of acid <br />production and evidence of its impacts will be addressed. The effects of dilution by <br />irrigation will be investigated using data from GW-N9. Because of the potential for <br />mobilizing trace metals with the production of acid through oxidation of pyrite, the <br />potential for production of acid will be addressed first. <br />Mechanisms for Production of Acid. The iron disulfide minerals pyrite and marcasite <br />• are thermodynamically unstable under oxidizing environments. In coal-forming <br />environments, iron sulfide (mackinawite) and disulfides (pyrite and marcasite) are formed <br />17-37 Revised 04/11/88 <br />