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• Draw, respectively. These increases in TDS will be additional magnesium/sodium-sulfate type water <br />which may result in a slight increase in salinity. These projected changes in TDS levels will in nd way <br />affect the present and potential uses of the surface water and are so small that they may not be <br />measurable. <br />Imaact of spoil water quality on the ground and surface water quality The available data <br />indicate that a small proportion of the overburden may produce acid through the oxidation of pyrite. <br />Based on laboratory tests on overburden cores, calcite is almost ubiquitous. Calcite serves two <br />functions. First, it buffers the pH of the water, which overall tends to slow the oxidation of pyrite, <br />slowing the production of acid. Second, it will neutralize the acid that is produced. The core samples <br />that exhibited low paste pH's are surrounded by non-acid producing, calcite-bearing rocks. The water <br />that contacts the low-paste pH materials will have first reacted with calcite, and therefore developed <br />a pH-buffer capacity 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 the <br />lower Dakota coal. Where the coal's permeability 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 df 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 likely <br />to produce acid. The acid that is produced will be quickly neutralized. During the mining process, <br />WFC will test the overburden and if acidic layers are encountered, they will be mixed with non-acidic <br />layers to neutralize any acid forming effects. <br />Potential imaacts of replaced soil on groundwater duality. The analysis of geochemical <br />controls on groundwater quality suggests that the water chemistry and concentrations of most <br />elements of concern are controlled by mineralogic reactions that will resist changes in water <br />chemistry. Production of acid may occur in very local settings and is probably most prevalent in the <br />coal which will be mined. Calculations indicate that neutralization of the acid will occur rapidly with <br />mixing of water, or with movement of acidic water into calcite-bearing rocks. Also, the analysis <br />conducted indicates that chemical changes are not likely to occur. <br />(REVISED 9/99) 2.04.7-48 <br />