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The ability of the calcite in the overburden spoil to neutralize any acid produced is <br />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 levels of <br />oxygen to move quickly through the spoil, resulting in faster breakdown of the pyrite in <br />the spoil, <br />c) high void channels developing in the spoil at the bottom of the pit which may serve as <br />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 result in <br />a higher level of TDS for some period of time, until pyrite in the overburden spoil is fully <br />oxidized and removed. This was found to occur at the Seneca II Coal Mine in northwest <br />Colorado and was the subject of a study by the USGS in 1994. Sampling data gathered <br />through the last 13 years at the New Horizon #1 Mine, north of New Horizon #2 in the <br />same strata, 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 old New Horizon #1 <br />Mine spoil suggests that the water chemistry and concentrations of most elements of <br />concern are controlled by mineralogic reactions that will resist changes in water chemistry. <br />These geochemical controls also apply to the spoils used as backfill in the Phase III areas <br />of New Horizon #2 Mine, due to both mines stripping overburden from the same strata. It <br />appears that any pyrite (FeS oxidation produces sulfuric acid and ferrous sulfate, which <br />are both soluble. The pH is lowered but the waters get neutralized by calcite (CaCO if <br />present, in the same spoil material. This results in the iron precipitating as iron oxides. The <br />slightly higher than normal pH of the natural water means that there is sufficient acid - <br />neutralizing ions such as hydroxyl (OH -) or bicarbonate (HCO - ) in solution to absorb the <br />acidic hydrogen (H +) ions produced by the pyrite oxidation. Soluble sulfate (SO -2 ) ions are <br />also produced by the pyrite oxidation. Some sulfate ion reacts with dissolved calcium to <br />produce calcium sulfate or gypsum (CaSO which can precipitate out of solution, but <br />water analyses from the site shows that both of these ions can stay in solution to a large <br />extent. Overall, the net change to the water quality is not significant as compared to <br />overburden water, but some pyrite has been converted to other solid compounds: gypsum <br />and iron oxides such as limonite. <br />5.3.2 SL -14 Bond Release Area Groundwater Quality Pre -Mine to Post -Mine Comparison <br />In order to determine what impact mining and reclamation activities have on the area in <br />this bond release application, a comparison between undisturbed ground water and <br />disturbed ground water must be made. Undisturbed ground water is water that has passed <br />only through in -situ overburden on the New Horizon Mine. Data for this water can either <br />SL -14 03DEC12.wpd 58 <br />