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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,New Horizon will test the overburden and if acidic layers are encountered,they <br /> will 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 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 /> January 2017 (TR-77) 2.05.6(3)-20 <br />