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<br />percent OW-N20 water (pH 4.68) and 20 percent GW-N21 (pH 7.17) water would have a pH of <br />1 5.85: a minor maaynt of di lotion increases the pH by moro than one unit. khan the low-pH <br />water i5 allowed to equilibrate with calcite (which is vary comson in the overburden), the <br />pH can exceed 7.D. <br />Svm,ary. The available data indicate that a small proportion of the overburden may <br />produce acid through the oxidation of pyrite, Based on laboratory tests on overburden <br />cores, tales to Ss almost ubiquitous, Calcite nerves two functions. F{rst, it buffers the <br />pH of the water, wht ch overall tends to al ow the oxidation of pyrite, slowing the <br />production of acid. Second, it M1] neutralize the acid Chat is produced. Tha core <br />sampl as thst exhibited tow paste pH's (using df sEilled water) are surrounded by non-acid <br />producing, ealcite-beari ng rocks. The weier that contacts the low-pasta pH maters ale will <br />have fi rat reacted with talcs te, antl therefore tlavel oped a pH-buffer capacity oP its own: <br />the paste-pH teat may overastima to the potanif al for acid production because of the use of <br />d1sC111 ad water in the test, <br />The ground water moot tort ng data indicate ghat mixed overburden and interhurden waters <br />. have near neutral pH's. Sample pH's leas then 6 are associated only M th the loner Dakota <br />• coal. khere the coal's permeability is high enough to protluee abouC 5 gpm during <br />sampling, the acid-product ng reacti one do not appear to be fasi enought to nai ntain the pH <br />of the water less than 5. <br />Oxidation rates may increase because of Shs mi ning process. Ho+rever, the supply of <br />oxidation fs only one of the constraints an the production of acid. Other constraints are <br />imposed by the quantt ty of calcite present, and the reactivity of the pyrite. The <br />paste-pH test, conducted ynder oxidizing Bondi !tons, Sndi Cates' that a very small <br />proportion of the overburden fa likely to produce acid. The acid that fs produced will be <br />quickly neutralf zed. <br />Geochemlcal lbdeling. <br />Introduction. The geochemS cal apectatian code pHREEOE was used to investigate <br />nhethar preeipitatlon and/or diasoldti on of minerals influences nester chemistry. Malysea <br />01 aanplea iron a variety of envi roro~ents in the Nuela and Nuela East areas were chosen <br />for study. Results are prosanted /n Table 17-10. The water analyses are gS Ven in the <br />17-44 Revised 04/17/88 <br /> <br />REVISED MARCH 2006 Attachment 2.05.6(3)-2-52 <br />