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carbonate and sulfate minerals, and hence are controlled by the solubilities of calcite <br />• and gypsum, Equilibrium with calcite and gypsum is well-demonstrated. <br />Neutralization Calculations. Acidic environments occur in the loner Dakota coal, and <br />may exist in the interburden between the upper and lower coals, On the other hand, <br />calcite is quite common in surrounding materials. PHREEOE was used to investigate the <br />neutralization of acidic waters, and the mixing of acidic and neutral waters. <br />The sample with the lowest pH (CW-N20, July 12, 1986) was chosen to represent the acidic <br />water. The results of the geochemical modeling of saturation indices is presented in <br />Table 17-10. A further step in the analysis consisted of equilibrating this water with <br />calcite. The calculations indicated that 2,6x10-3 moles (0,26 gms) of calcite per liter <br />of water would be in the equilibration process, and the pH would increase from 4.68 to <br />7,15. <br />The modeling of the mixing process was performed in three steps, and the resulting pH's <br />are given in Table 17-11. In the analysis, water from CW-N 20 (pH 4,68) was mixed with <br />water from CW-N21 (pH 7.17), a monitoring well in the overburden. First, no mineral <br />• precipitation or dissolution was modeled. Next,~because the CW-N20 sample appears to be <br />at equilibrium with gibbsite, and the precipitation of gibbsite is a source of acidity, <br />its precipitation was modeled. Third, when the water moves into surrounding rocks, <br />calcite should dissolve, gibbsite and gypsum should precipitate. The third analysis <br />allowed these reactions to occur to equilibrium. Mixing proportions were varied in steps <br />of 20 percent. <br />If no mineral reactions occur, mixing of 20 percent GW-N21 water with 80 percent GW-N20 <br />water increases the pH by over one pH unit and neutralization occurs rapidly. If gibbsite <br />precipitates, the pH increase is not as great at these proportions. But, if calcite <br />dissolves, and gypsum and gibbsite precipitate, the pH increases to above 7. The <br />neutralization is due to calcite dissolution rather than the mixing process. The analysis <br />indicates that at greater proportions of CW-N21 water, the pH is actually lower in the <br />third case but is caused by the GW-N21 water not being completely at equilibrium with <br />calcite and gypsum. <br />• These calculations indicate that neutralization of acid should occur very readily in the <br />geochemical setting found at Nucla East, and that acidic waters should not typically <br />result from mining. <br />17-50 Revised 04/11/88 <br />