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5) Potential impacts of replaced spoil on Groundwater quality. 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 of the overburden water is poor, with TDS generally in the 3000 <br />ppm range and some ions exceeding limits for most water uses. The primary potential for impacts <br />to ground water quality will occur from increased water infiltration causing an accelerated oxidation <br />of pyrite in the spoil. Other salts may also dissolve more readily in the highly permeable spoil. The <br />minor amounts of sulfuric acid produced can cause lower pH, which then results in higher rates of <br />dissolution of other chemical compounds in the spoil, resulting in higher TDS. This water will <br />saturate the spoil at the lowwall and form a spring at the low point. Also, the spoil water can <br />infiltrate into the lowwall strata of the Dakota Sandstone formation. <br />Spoil Water Chemistry <br />Concerning impacts from the conversion of overburden to spoil, the available data indicate that a <br />small proportion of the overburden may produce acid through the oxidation of pyrite. Based on <br />laboratory tests on overburden cores, calcite is present throughout the overburden. Calcite serves <br />two functions. First, it buffers the pH of the water, which overall tends to slow the oxidation of <br />pyrite, slowing the production of acid. Second, it will neutralize the acid that is produced. The core <br />samples that exhibited low paste pH's are surrounded by non-acid producing, calcite -bearing rocks. <br />The water that contacts the low -paste pH materials will have first reacted with calcite, and therefore <br />developed a pH -buffer capacity of its own. The groundwater monitoring data indicate that mixed <br />overburden and interburden waters have near neutral pH's. Sample pH's less than 6 are <br />associated only with the lower Dakota coal. Where the coal's permeability is high enough to <br />produce about 5 gpm during sampling, the acid -producing reactions do not appear to be fast <br />enough to maintain the pH of the water less than 5. Oxidation rates may increase because of the <br />mining process. However, the supply of oxidation is only one of the constraints on the production <br />of acid. Other constraints are imposed by the quantity of calcite present and the reactivity of the <br />pyrite. The paste -pH test, conducted under oxidizing conditions, indicates that a very small <br />proportion of the overburden is likely to produce acid. This overburden is generally located in a thin <br />zone immediately above the coals. The acid that is produced should be quickly neutralized. During <br />the mining process, WFC will test the overburden and if acidic layers are encountered, they will be <br />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 a <br />number of factors such as: <br />a) the uniform distribution of calcite in the replaced spoil, <br />b) the higher transmissivity of the spoil to allow irrigation water with higher levels of <br />oxygen to move quickly through the spoil, resulting in faster breakdown of the pyrite in <br />the spoil, <br />July 2016 (TR -74) 2.05.6(3)-20 <br />