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c) high void channels developing in the spoil at the bottom of the pit which may serve as the <br />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 result in a <br />higher level of TDS for some period of time, until pyrite in the overburden spoil is fully oxidized <br />and removed. This was found to occur at the Seneca II Coal Mine in northwest Colorado and was <br />the subject of a study by the USGS in 1994. Sampling data gathered through the last 13 years at <br />the New 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 #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 <br />results in the iron precipitating as iron oxides. The slightly higher than normal pH of the natural <br />water means that there is sufficient acid -neutralizing ions such as hydroxyl (OH-) or bicarbonate <br />(HCO) in solution to absorb the acidic hydrogen (H+) ions produced by the pyrite oxidation. <br />Soluble sulfate (SO4-2) ions are also produced by the pyrite oxidation, and they are quickly <br />takenup by the calcite to produce calcium sulfate or gypsum (CaSO4)' which is not very soluble <br />and also precipitates out of solution, especially when the pH of the solution is near normal. The <br />(CO3-2) anion in the calcite goes in solution to replace the sulfate. Calcium stays as a solid in the <br />new gypsum produced. Overall, the net change to the water quality is not significant as <br />compared to overburden water, but some pyrite has been converted to other solid compounds: <br />gypsum and iron oxides such as limonite. Strong support for this occurring is seen in the water <br />quality comparison of overburden water to spoil water. Well GW -N9 is north of the mined areas <br />of New Horizon #1 and has been unaffected by the mine since the flow gradient is to the <br />southwest. This well is best to use in the comparison. Spoil Spring 1, which developed near the <br />southwestern end of the reclaimed mine best represents the spoil water. Table 2.05.6.(3)-2 shows <br />the chemistry of these waters sampled at the 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 <br />by carbonate ion from the calcite should be seen as an increased carbonate or bicarbonate in <br />solution. This is exactly what is seen between the overburden water and the spoil water for each <br />sampling period except for the August 1998 sample, which must have had outside influence of <br />surface water flows since its total dissolved solids is much less than any other sample. All other <br />ions in solution are more or less proportional to the level of TDS in the sample. Bicarbonate, on the <br />other hand, has consistently increased by approximately 33%. <br />July 2016 (TR -74) 2.05.6(3)-21 <br />