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Overall, there should be no detrimental impacts from the mining operation on groundwater <br />flow and recharge rates. Recharge rates will increase in the spoil and the water should <br />build up in the spoil area as future infiltration occurs. In the pre -mine condition, the <br />overburden aquifers have historically had very low flow rates and also poor quality. These <br />aquifers 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, in the <br />Burro Canyon Formation, are used in surrounding wells and have better water quality and <br />were not affected by the mining and reclamation of this site. Shale layers below the mined <br />coals prevent interaction between the spoil and these aquifers, which get their recharge <br />from long distances away from the mine operation. <br />5.3 Potential impacts of replaced spoil on groundwater quality <br />Since the operation has disturbed the overburden above the Dakota coal seam and has <br />removed the Dakota coal, these are the only two stratigraphic zones that have been <br />affected by this operation. The baseline water quality of the overburden is usually poor <br />with a TDS of over 3000 ppm. The primary potential for impacts to ground water quality <br />occurs from increased water infiltration causing an accelerated oxidation of pyrite in the <br />spoil. Other salts may also dissolve more readily in the more permeable spoil. The minor <br />amounts of sulfuric acid produce can cause lower pH, which then results in higher rates of <br />dissolution of other chemical compounds in the spoil, resulting in higher TDS. Also, the <br />spoil water can infiltrate into the low wall strata of the Dakota Sandstone formation. <br />5.3.1 General Spoil Water Chemistry <br />Concerning impacts from the conversion of overburden to spoil, the available data from the <br />initial mine overburden testing indicate that a small proportion of the overburden may <br />produce acid through the oxidation of pyrite. We also know this can occur because it has <br />occurred in the New Horizon #1 reclaimed area via a very low flow spoil spring with <br />lower pH and high TDS. <br />Based on laboratory tests on overburden cores, calcite is present throughout the <br />overburden. Calcite serves two functions. First, it buffers the pH of the water, which <br />overall tends to slow the oxidation of pyrite, slowing the production of acid. Second, it <br />will neutralize the acid that is produced. The core samples that exhibited low paste pH's <br />are surrounded by non -acid producing, calcite- bearing rocks. The water that contacts the <br />low pH materials will first react with calcite, and therefore developed a pH- buffer capacity <br />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 <br />the lower Dakota coal. Oxidation rates may increase because of the mining process. <br />However, the supply of oxygen 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 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 <br />in a thin zone immediately above the coal. No zone near the surface at the New Horizon <br />#2 Mine has ever shown an acidic nature. <br />SL -14 03DEC12.wpd 57 <br />