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0 Spoil Water Chemistry <br />Concerning impacts from the conversion of overburden to spoil, the available data indicate <br />that a small proportion of the overburden may produce acid through the oxidation of pyrite. <br />Based on laboratory tests on overburden cores, calcite is present throughout the overburden. <br />Calcite serves two functions. First, it buffers the pH of the water, which overall tends to slow <br />the oxidation of pyrite, slowing the production of acid. Second, it will neutralize the acid that <br />is produced. The core samples that exhibited low paste pH's are surrounded by non-acid <br />producing, calcite-bearing rocks. The water that contacts the low-paste pH materials will have <br />first reacted with calcite, and therefore developed a pH-buffer capacity of its own. The <br />groundwater monitoring data indicate that mixed overburden and interburden waters have near <br />neutral pH's. Sample pH's less than 6 are associated only with the lower Dakota coal. Where <br />the coal's permeability is high enough to produce about 5 gpm during sampling, the acid- <br />producing reactions do not appear to be fast enough to maintain the pH of the water less than <br />5. Oxidation rates may increase because of the mining process. However, the supply of <br />oxidation is only one of the constraints on the production of acid. Other constraints are <br />imposed by the quantity of calcite present and the reactivity of the pyrite. The paste-pH test, <br />conducted under oxidizing conditions, indicates that a very small proportion of the overburden <br />is likely to produce acid. This overburden is generally located in a thin zone immediately <br />above the coals. No zone near the surface at the New Horizon #2 Mine has ever shown an <br />acidic nature. <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 spoil, <br />b) the higher transmissivity of the spoil to allow irrigation water with higher levels of oxygen <br />to move 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 <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 <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 same <br />strat, suggests that some pyrite is oxidizing but is being neutralized, as described below. <br />The analysis of geochemical controls on groundwater quality at the old New Horizon #1 Mine <br />spoil suggests that the water chemistry and concentrations of most elements of concern are <br />controlled by mineralogic reactions that will resist changes in water chemistry. These <br />geochemical controls also apply to the spoils used as backfill in the Phase 3 areas of New <br />Horizon #2 Mine, due to both mines stripping overburden from the same strata. It appears that <br />SL-12 DRAFT 20AUG10.wPd 40