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waters have near neutral pH. Sample pH's less than 6 are associated only with the lower Dakota <br />coal. Where the coal's permeability is high enough to produce about 5 gpm during sampling, the <br />acid - producing reactions do not appear to be fast enough to maintain the pH of the water less <br />than 5. Oxidation rates may increase because of the mining process. However, the rate of <br />oxidation is only one of the constraints on the production of acid. Other constraints are imposed <br />by the quantity of calcite present and the reactivity of the pyrite. The paste -pH test, conducted <br />under oxidizing conditions, indicates that a very small proportion of the overburden is likely to <br />produce acid. This overburden is generally located in a thin zone immediately above the coals. <br />The acid that is produced should be quickly neutralized. <br />The ability of the calcite in the overburden spoil to neutralize any acid produced is dependent <br />upon a number of factors such as: <br />a) The uniform distribution of calcite in the replaced overburden, <br />b) The higher transmissivity of the backfill allows irrigation water with higher levels of <br />oxygen to move quickly through the spoil, resulting in faster breakdown of the pyrite in the <br />backfill, <br />c) High void channels developing in the backfill at the bottom of the pit which may serve as the <br />primary conduit for flow in the backfill, <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 backfill 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 <br />backfill suggests that the water chemistry and concentrations of most elements of concern are <br />controlled by mineralogical reactions that will resist changes in water chemistry. It appears that <br />any pyrite (FeS2) oxidation gets neutralized by calcite (CaCO3) present in the same spoil <br />material. This results in the iron precipitating as iron oxides. The slightly higher than normal pH <br />of the natural water means that there is sufficient acid - neutralizing ions such as hydroxyl (OH -) <br />or bicarbonate (HCO3_) in solution to absorb the acidic hydrogen (H +) ions produced by the <br />pyrite oxidation. Soluble sulfate (SO4_2 ) ions are also produced by the pyrite oxidation, and they <br />are quickly taken up by the calcite to produce calcium sulfate or gypsum (CaSO4), which is not <br />Section 2.05.6(3) Page 20 November 2011 <br />