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have not been used for any wells in the surrounding area and cannot be <br />considered a water resource. The ground water aquifers below the Dakota coals, <br />in the Burro Canyon Formation, are used in surrounding wells and have better <br />water quality but they will not be affected by the mining and reclamation of this <br />site. Shale layers below the mined coals prevent interaction between the spoil and <br />these aquifers, which get their recharge from long distances away from the mine <br />operation. <br />3. Potential impacts of replaced spoil on groundwater quality. <br />Since the operation will disrupt the overburden above the Dakota coals and <br />remove the Dakota coals, these are the only two stratigraphic zones that will be <br />affected by the operation. As described in the section on overburden water <br />quality, the pre -mine quality of the overburden water is poor, with TDS generally <br />in the 3000 ppm range and some ions exceeding limits for most water uses. The <br />primary potential for impacts to ground water quality will occur from increased <br />water infiltration causing an accelerated oxidation of pyrite in the spoil. Other <br />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 <br />higher rates of dissolution of other chemical compounds in the spoil, resulting in <br />higher TDS. This water will saturate the spoil at the lowwall and form a spring <br />at the low point. Also, the spoil water can infiltrate into the lowwall strata of the <br />Dakota Sandstone formation. <br />Spoil Water Chemistry <br />Concerning impacts from the conversion of overburden to spoil, the available <br />data indicate that a small proportion of the overburden may produce acid through <br />the oxidation of pyrite. Based on laboratory tests on overburden cores, calcite is <br />present throughout the overburden. Calcite serves two functions. First, it buffers <br />the pH of the water, which overall tends to slow the oxidation of pyrite, slowing <br />the production of acid. Second, it will neutralize the acid that is produced. The <br />core samples that exhibited low paste pH's are surrounded by non -acid <br />producing, calcite- bearing rocks. The water that contacts the low -paste pH <br />materials will have first reacted with calcite, and therefore developed a pH- buffer <br />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 <br />6 are associated only with the lower Dakota coal. Where the coal's permeability <br />is high enough to produce about 5 gpm during sampling, the acid - producing <br />reactions do not appear to be fast enough to maintain the pH of the water less <br />than 5. Oxidation rates may increase because of the mining process. However, <br />the supply of oxidation is only one of the constraints on the production of acid. <br />Other constraints are imposed by the quantity of calcite present and the reactivity <br />of the pyrite. The paste -pH test, conducted under oxidizing conditions, indicates <br />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 <br />35 <br />