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percent CW-N20 water (pH 4,68) and 20 percent CW-N 21 (pH 7.17) water would have a pH of <br />• 5.85; a minor amount of dilution increases the pH by more than one unit. When the low-pH <br />water is allowed to equilibrate with calcite (which is very common in the overburden), the <br />pH can exceed 7.0. <br />S unwary. 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, calcite is almost ubiquitous. Calcite serves two functions. First, it buffers the <br />pH of the water, which overall tends to slow the oxidation of pyrite, slowing the <br />production of acid. Second, it will neutralize the acid that is produced. The core <br />samples that exhibited low paste pH's (using distilled water) are surrounded by non-acid <br />producing, calcite-bearing rocks. The water that contacts the low-paste pH materials will <br />have first reacted with calcite, and therefore developed a pH-buffer capacity of its own; <br />the paste-pH test may overestimate the potential for acid production because of the use of <br />distilled water in the test. <br />The ground water monitoring data indicate that mixed overburden and interburden waters <br />have near-neutral pH's. Sample pH's less than 6 are associated only with the lower Dakota <br />• coal. Where the coal's permeability is high enough to produce about 5 gpm during <br />sampling, the acid-producing reactions do not appear to be fast enought to maintain the pH <br />of the water less than 5. <br />Oxidation rates may increase because of the mining process. Honever, the supply of <br />oxidation is only one of the constraints on the production of acid. Other constraints are <br />imposed by the quantity of calcite present, and the reactivity of the pyrite. The <br />paste-pH test, conducted under oxidizing conditions, indicates that a very small <br />proportion of the overburden is likely to produce acid. The acid that is produced will be <br />quickly neutralized. <br />Geochemical Modeling. <br />Introduction. The geochemical speciation code PHREEpE was used to investigate <br />whether precipitation and/or dissolution of minerals influences water chemistry. Analyses <br />of samples from a variety of environments in the Nucla and Nucla East. areas were chosen <br />• for study. Results are presented in Table 17-10. The water analyses are given in the <br />17-44 Revised 04/11/88 <br />