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of pyrite in the spoil. Other salts may also dissolve more readily in the highly <br />permeable spoil. The minor amounts of sulfuric acid produced can cause lower pH, <br />which then results in higher rates of dissolution of other chemical compounds in the <br />spoil, resulting in higher TDS. This water will saturate the spoil at the low-wall and <br />form a spring at the low point. Also, the spoil water can infiltrate into the low-wall <br />strata of the Dakota Sandstone formation. <br />Spoil Water Chemistry <br />Concerning impacts from the conversion of overburden to spoil, the available data <br />indicate that a small proportion of the overburden may produce acid through the <br />oxidation of pyrite. Based on laboratory tests on overburden cores, calcite is present <br />throughout the overburden. Calcite serves two functions. First, it buffers the pH of the <br />water, which overall tends to slow the oxidation of pyrite, slowing the production of <br />acid. Second, it will neutralize the acid that is produced. The core samples that <br />exhibited low paste pH's aze surrounded bynon-acid producing, calcite-bearing rocks. <br />The water that contacts the low-paste pH materials will have first reacted with calcite, <br />and therefore developed a pH-buffer capacity of its own. The groundwater monitoring <br />data indicate that mixed overburden and interburden waters have near neutral pH's. <br />Sample pH's less than 6 aze associated only with the lower Dakota coal. Where the <br />coal's permeability is high enough to produce about 5 gpm during sampling, the <br />acid-producing reactions do not appeaz to be fast enough to maintain a pH of less than <br />5. Oxidation rates may increase because of the mining process, however, the supply <br />of oxidation is only one of the constraints on the production of acid. Other constraints <br />are influenced by the quantity of calcite present and the reactivity of the pyrite. The <br />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 <br />located in a thin zone immediately above the coals. No zone near the surface at the <br />New Horizon #1 Mine has ever shown an acidic nature. <br />The ability of the calcite in the overburden spoil to neutralize any acid produced is <br />dependent upon a number of factors such as: <br />a) the uniform distribution of calcite in the replaced spoil, b) the higher transmissiviry <br />of the spoil to allow irrigation water with higher levels of oxygen to move quickly <br />through the spoil, resulting in faster breakdown of the pyrite in the spoil, c) high void <br />channels developing in the spoil at the bottom of the pit which may serve as the <br />primary conduits for flow in the spoil, d) the quantity of calcite available in the areas <br />needed most, e) other chemistry which may influence the neutralization reactions. <br />For these reasons, there is a possibility that water leaching through the spoil may <br />result in a higher level of TDS for some period of time, until pyrite in the overburden <br />spoil is fully oxidized and removed. This was found to occur at the Seneca II Coal <br />Mine in northwest Colorado and was the subject of a study by the USGS in 1994. <br />Sampling data gathered through the last 13 yeazs at the New Horizon 1 Mine suggests <br />2a <br />