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b) the higher transmissivity of the spoil to allow irrigation water with higher levels of oxygen to move <br />quickly through the spoil, resulting in faster breakdown of the pyrite in the spoil, <br />c) high void channels developing in the spoil at the bottom of the pit which may serve as the primary <br />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 result in a higher <br />level of TDS for some period of time, until pyrite in the overburden spoil is fully oxidized and <br />removed. This was found to occur at the Seneca II Coal Mine in northwest Colorado and was the <br />subject of a study by the USGS in 1994. Sampling data gathered through the last 13 years at the <br />New Horizon Mine suggests that some pyrite is oxidizing but is being neutralized, as described <br />below. <br />The analysis of geochemical controls on groundwater quality at the New Horizon #1 Mine spoil <br />suggests that the water chemistry and concentrations of most elements of concern are controlled <br />by mineralogic reactions that will resist changes in water chemistry. It appears that any pyrite <br />(FeS2) oxidation gets neutralized by calcite (CaCO3) present in the same spoil material. This results <br />in the iron precipitating as iron oxides. The slightly higher than normal pH of the natural water <br />means that there is sufficient acid - neutralizing ions such as hydroxyl (OH -) or bicarbonate (HCO3 ) <br />in solution to absorb the acidic hydrogen (H +) ions produced by the pyrite oxidation. Soluble sulfate <br />(SO4 2) ions are also produced by the pyrite oxidation, and they are quickly taken up by the calcite <br />to produce calcium sulfate or gypsum (CaSO4), which is not very soluble and also precipitates out <br />of solution, especially when the pH of the solution is near normal. The (CO3 -2) anion in the calcite <br />goes in solution to replace the sulfate. Calcium stays as a solid in the new gypsum produced. <br />Overall, the net change to the water quality is not significant as compared to overburden water, but <br />some pyrite has been converted to other solid compounds: gypsum and iron oxides such as <br />limonite. Strong support for this occurring is seen in the water quality comparison of overburden <br />water to spoil water. Well GW -N9 is north of the mined areas of New Horizon #1 and has been <br />unaffected by the mine since the flow gradient is to the southwest. This well is best to use in the <br />comparison. Spoil Spring 1, which developed near the southwestern end of the reclaimed mine best <br />represents the spoil water. Table 2.05.6.(3) -2 shows the chemistry of these waters sampled at the <br />same time. <br />If the pyrite breakdown were occurring without any neutralization, the pH of the spoil water would <br />be lower than overburden water. This has not occurred; however, the replacement of sulfate ion by <br />carbonate ion from the calcite should be seen as an increased carbonate or bicarbonate in <br />Revised September 2014 JR -64) 2.05.6(3) -29 <br />