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the pH of the water, which overall tends to slow the oxidation of pyrite, slowing <br />the production of acid. Second, it will neutralize the acid that is produced. The <br />core samples that exhibited low paste pH's are surrounded by non-acid producing, <br />calcite-bearing rocks. The water that contacts the low-paste pH materials will <br />have first reacted with calcite, and therefore developed a pH-buffer capacity of its <br />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 <br />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 <br />to be fast enough to maintain the pH of the water less than 5. Oxidation rates may <br />increase because of the mining process. However, the supply of oxidation is only <br />one of the constraints on the production of acid. Other constraints are imposed by <br />the quantity of calcite present and the reactivity of the pyrite. The paste-pH test, <br />conducted under oxidizing conditions, indicates teat a Yerx gma11'Mrnpo?f ?" ?f <br />the overburden is likely to produce acid. This overburden is generally located in a <br />thin zone immediately above the coals. The acid that is produced should be <br />quickly neutralized. During the mining process, WFC will test the overburden <br />and if acidic layers are encountered, they will be mixed with non-acidic layers to <br />neutralize any acid forming effects. <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, <br />b) the higher transmissivity of the spoil to allow irrigation water with higher <br />levels of oxygen to move quickly through the spoil, resulting in faster <br />breakdown of the pyrite in the spoil, <br />c) high void channels developing in the spoil at the bottom of the pit which may <br />serve as the primary conduit for flow in the spoil, <br />d) the quantity of calcite available in the areas needed most, <br />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 <br />overburden spoil is fully oxidized and removed. This was found to occur at the <br />Seneca II Coal Mine in northwest Colorado and was the subject of a study by the <br />USGS in 1994. Sampling data gathered through the last 13 years at the New <br />Horizon Mine suggests that some pyrite is oxidizing but is being neutralized, as <br />described below. <br />The analysis of geochemical controls on groundwater quality at the New Horizon <br />#1 Mine spoil suggests that the water chemistry and concentrations of most <br />elements of concern are controlled by mineralogic reactions that will resist <br />changes in water chemistry. It appears that any pyrite (FeS2) oxidation gets <br />neutralized by calcite (CaC03) present in the same spoil material. This results in <br />the iron precipitating as iron oxides. The slightly higher than normal pH of the <br />36