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oxidation of pyrite, slowing the production of acid. Second, it will neutralize <br />the acid that is produced. The core samples that exhibited low paste pH's are <br />surrounded by non-acid producing, calcite-bearing rocks. The water that <br />contacts the low-paste pH materials will have first reacted with calcite, and <br />therefore developed a pH-buffer capacity of its own. The groundwater <br />monitoring data indicate that mixed overburden and interburden waters have <br />near neutral pH's. Sample pH's less than 6 are associated only with the lower <br />Dakota coal. Where the coal's permeability is high enough to produce about 5 <br />gpm during sampling, the acid-producing reactions do not appear to be fast <br />enough to maintain a pH of less than 5. Oxidation rates may increase because <br />of the mining process, however, the supply of oxidation is only one of the <br />constraints on the production of acid. Other constraints are influenced by the <br />quantity, of calcite present and the reactivity of the pyrite. The paste-pH test, <br />coriductV.d under oxidizing conditions, indicates that a very small proportion <br />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 <br />at the New Horizon #1 Mine has ever shown an acidic nature. <br />The ability of the calcite in the overburden spoil to neutralize any acid <br />produced is dependent upon a number of factors such as: <br />a) the uniform distribution of calcite in the replaced spoil, b) the higher <br />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 <br />pyrite it the spoil, c) high void channels developing in the spoil at the bottom <br />of the pit which may serve as the primary conduits for flow in the spoil, d) the <br />quantity; of calcite available in the areas needed most, e) other chemistry <br />which may influence the neutralization reactions. <br />For these reasons, there is a possibility that water leaching through the spoil <br />may 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 <br />the Seneca II Coal Mine in northwest Colorado and was the subject of a study <br />by the USGS in 1994. Sampling data gathered through the last 13 years at the <br />New Horizon 1 Mine suggests that some pyrite is oxidizing but is being <br />neutralized, as described below. <br />The analysis of geochemical controls on groundwater quality at the New. <br />Horizon #1 Mine spoil suggests that the water chemistry and concentrations of <br />most elements of concern are controlled by mineral 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 <br />in the iron precipitating as iron oxides. The slightly higher than normal pH of <br />the natural water means that there is sufficient acid-neutralizing ions such as <br />hydroxyl (OH-) or bicarbonate (HC03-) in solution to absorb the acidic <br />hydrogen (H+) ions produced by the pyrite oxidation. Soluble sulfate (S04 -2) <br />30