Laserfiche WebLink
• 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 TbS for some period of time, until pyrite in the overburden spoil is fully oxidized and removed. <br />This was found to occur at the Seneca II Coal Mine in northwest Colorado and was the subject of a <br />study by the USGS in 1994. Sampling data gathered through the last 13 years at the New Horizon <br />Mine suggests that some pyrite is oxidizing but is being neutralized, as 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 by <br />mineralogic reactions that will resist changes in water chemistry. It appears that any pyrite (FeSz) <br />oxidation gets neutralized by calcite (CaCO3) present in the same spoil material. This results in the iron <br />precipitating as iron oxides. The slightly higher than normal pH of the natural water means that there <br />is sufficient acid-neutralizing ions such as hydroxyl (OH-) or bicarbonate (HCO;) in solution to absorb <br />• the acidic hydrogen (H+) ions produced by the pyrite oxidation. Soluble sulfate (SO, 2) ions are also <br />produced by the pyrite oxidation, and they are quickly taken up by the calcite to produce calcium <br />sulfate or gypsum (CaSO4), which is not very soluble and also precipitates out of solution, especially <br />when the pH of the solution is near normal. The (CO3'2) anion in the calcite goes in solution to replace <br />the sulfate;. Calcium stays as a solid in the new gypsum produced. Overall, the net change to the water <br />quality is not significant as compared to overburden water, but some pyrite has been converted to <br />other solid compounds: gypsum and iron oxides such as limonite. Strong support for this occurring is <br />seen in the water quality comparison of overburden water to spoil water. Well GW-N9 is north of the <br />mined areas 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 the <br />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 be <br />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 solution. <br />This is exactly what is seen between the overburden water and the spoil water for each sampling <br />period except for the August 1998 sample, which must have had outside influence of surface water <br />• (Revised 6/OI) 2.05.6(3) - 25 <br />