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• ~ Potential impacts of replaced spoil on groundwater auality. Since the operation will disrupt <br />the overburden above the Dakota coals and remove the Dakota coals, these are the only two <br />stratigraphic zones that will be affected by the operation. As described in the section on overbuden <br />water quality, the pre-mine quality ofthe overburden water is poor, with TDS generally in the 3000 ppm <br />range and some ions exceeding limits for most water uses. The primary potential for impacts to <br />ground water quality will occur from increased water infiltration causing an accelerated oxidation of <br />pyrite in the spoil. Other salts may also dissolve more readily in the highly permeable spoil. The minor <br />amounts of sulfuric acid produced can cause lower pH, which then results in higher rates of dissolution <br />of other chemical compounds in the spoil, resulting in higher TDS. This water will saturate the spoil <br />at the lowwall and form a spring at the low point. Also, the spoil water can infiltrate into the lowwall <br />strata of the Dakota Sandstone formation. <br />S,ooil Water Chemistry <br />Concerning impacts from the conversion of overburden to spoil, the available data indicate that a small <br />proportion of the overburden may produce acid through the oxidation of pyrite. Based on laboratory <br />tests on overburden cores, calcite is present throughoutthe overburden. Calcite serves two functions. <br />First, it buffers the 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 samples that <br />• exhibited low paste pH's are surrounded bynon-acid producing, calcite-bearing rocks. The waterthat <br />contacts the low-paste pH materials will have first reacted with calcite, and therefore developed a pH- <br />buffer capacity of its own. The groundwater monitoring data indicate that mixed overburden and <br />interburden waters have near neutral pH's. Sample pH's less than 6 are associated only with the lower <br />Dakota coal. Where the coal's permeability is high enough to produce about 5 gpm during sampling, <br />the acid-producing reactions do not appear to be fast enough to maintain the pH of the water less than <br />5. Oxidation rates may increase because of the mining process. However, the supply of oxidation is <br />only one of the constraints on the production of acid. Other constraints are imposed by the quantity <br />of calcite present and the reactivity of the pyrite. The paste-pH test, conducted under oxidizing <br />conditions, indicates that a very small proportion of the overburden is likely to produce acid. This <br />overburden is generally located in a thin zone immediately above the coals. The acid that is produced <br />should be quickly neutralized. During the mining process, WFC will test the overburden and if acidic <br />layers are encountered, theywill be mixed with non-acidic layers to neutralize any acid forming effects. <br />The ability of the calcite in the overburden spoil to neutralize any acid produced is dependent upon <br />a number of factors such as: <br />a) the uniform distribution of calcite in the replaced spoil, <br />. (Revised 6/01) 2.05.6 (3) - 29 <br />