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<br /> <br />• <br />Irnaad of spoil water quality on the ground and surface water quality, The available data indicate <br />that a small proportion of the overburden may produce acid through the oxidation of pyrite. Based <br />on laboratory tests on overburden cores, calcite is almost ubiquitous. Calcite serves iwo functions. <br />First, it buffers the pH of the water, which overall tends to slow the oxidation of pyrite, slovring the <br />production of acid. Second, it will neutralize the acid that is produced. The core samples that <br />exhibited low paste pH's are surrounded by non-acid producing, calcite-bearing rocks. The water <br />that contacts the low-paste pH materials v~ill have first reacted v~ith calcite, and therefore developed <br />a pl-buffer capacity of its own. The groundwater monitoring data indicate that mixed overburden <br />and interburden waters have near neutral pH's. Sample pH's less than 6 are associated only v~ith <br />the lower Dakota coal. Where the coal's penneability is high enough to produce about 5 gpm during <br />sampling, the acid-producing reactions do not appear to be fast enough to maintain the pH of the <br />water less than 5. Oxidation rates may increase because of the mining process. However, the <br />supply of oxidation is only one of the constraints on the production of acid. Other constraints are <br />imposed by the quantity of calcite present, and the reactivity of the pyrite. The paste-pH test, <br />conducted under oxidizing conditions, indicates that a very small proportion of the overburden is <br />likely to produce acid. The acid that is produced will be quickly neutralized. During the mining <br />process, WFC v~ill test the overburden and if acidic layers are encountered, they v~ill be mixed with <br />non-acidic layers to neutralize any acid forming effects. <br />Potential impacts of replaced spoil on groundwater qualilX: The analysis of geochemical controls <br />on groundwater quality suggests that the water chemistry and concentrations of most elements of <br />concern are controlled by mineralogic reactions that v~ill resist changes in water chemistry. <br />Production of acid may occur in very local settings and is probably most prevalent in the coal which <br />will be mined. Calculations indicate that neutralization of the acid v~ill occur rapidly with mixing of <br />water, or v~ith movement of acidic water into calcite-bearing rocks. Also, the analysis conducted <br />indicates that chemical changes are not likely to occur. <br />Effects of mining on the local georrbrphology. Impacts from mining on the local geomorphology v/ill <br />be long term, but appear to be of minimal significance. The reestablished reach of the drainage <br />which will be mined will result in a shorter, slightly steeper stream channel. The potential for <br />increased sediment loads in the drainage (once pond 007 is removed) should be offset by the stable <br />banksides and the relatively small change in overall gradient. The increased runoff and consequent <br />erosion potential on disturbed basins in the mining area due to the temporary loss of topsoil stricture <br />(Revised ¢S96) 2.04.7-45 <br />