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acid that is produced should be quickly neutralized. During the mining process, <br />WFC will test the overburden and if acidic layers are encountered, they will be <br />mixed with non - acidic layers to neutralize any acid forming effects. <br />The ability of the calcite in the overburden spoil to neutralize any acid produced <br />is 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 <br />may 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 (CaCO3) 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 />natural water means that there is sufficient acid - neutralizing ions such as <br />hydroxyl (OH -) or bicarbonate (HCO3 -) in solution to absorb the acidic hydrogen <br />(H +) ions produced by the pyrite oxidation. Soluble sulfate (SO4_2) ions are also <br />produced by the pyrite oxidation, and they are quickly taken up by the calcite to <br />produce calcium sulfate or gypsum (CaSO4), which is not very soluble and also <br />precipitates out of solution, especially when the pH of the solution is near <br />normal. The (CO3 -2) anion in the calcite goes in solution to replace the sulfate. <br />Calcium stays as a solid in the new gypsum produced. Overall, the net change to <br />the water quality is not significant as compared to overburden water, but some <br />pyrite has been converted to other solid compounds: gypsum and iron oxides <br />such as limonite. Strong support for this occurring is seen in the water quality <br />comparison of overburden water to spoil water. Well GW -N9 is north of the <br />mined areas of New Horizon 91 and has been unaffected by the mine since the <br />flow gradient is to the southwest. This well is best to use in the comparison. Spoil <br />Spring 1, which developed near the southwestern end of the reclaimed mine best <br />36 <br />