2018-11-19_PERMIT FILE - C2010089A (6)

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through the oxidation of pyrite. Based on laboratory tests on overburden cores, calcite is present throughout the overburden. Calcite serves two functions. First, it buffers the pH of the water, which overall tends to slow the oxidation of pyrite, slowing the production of acid. Second, it will neutralize the acid that is produced. The core samples that exhibited low paste pH are surrounded by non-acid producing, calcite -bearing rocks. The water that contacts the low -paste pH materials will have first reacted with calcite, and therefore developed a pH -buffer capacity of its own. The groundwater monitoring data indicate that mixed overburden and interburden waters have near neutral pH. Sample pH's less than 6 are associated only with the lower Dakota coal. Where the coal's permeability is high enough to produce about 5 gpm during sampling, the acid -producing reactions do not appear to be fast enough to maintain the pH of the water less than 5. Oxidation rates may increase because of the mining process. However, the rate of oxidation is only one of the constraints on the production of acid. Other constraints are imposed by the quantity of calcite present and the reactivity of the pyrite. The paste -pH test, conducted under oxidizing conditions, indicates that a very small proportion of the overburden is likely to produce acid. This overburden is generally located in a thin zone immediately above the coals. The acid that is produced should be quickly neutralized. The ability of the calcite in the overburden spoil to neutralize any acid produced is dependent upon a number of factors such as: a) The uniform distribution of calcite in the replaced overburden, b) The higher transmissivity of the backfill allows irrigation water with higher levels of oxygen to move quickly through the spoil, resulting in faster breakdown of the pyrite in the backfill, c) High void channels developing in the backfill at the bottom of the pit which may serve as the primary conduit for flow in the backfill, d) The quantity of calcite available in the areas needed most, e) Other chemistry which may influence the neutralization reactions. For these reasons, there is a possibility that water leaching through backfill may result in a higher level of TDS for some period of time, until pyrite in the overburden spoil is fully oxidized and removed. This was found to occur at the Seneca II Coal Mine in northwest Colorado and was the subject of a study by the USGS in 1994. Sampling data gathered through the last 13 years at the New Horizon Mine suggests that some pyrite is oxidizing but is being neutralized, as described below. The analysis of geochemical controls on groundwater quality at the New Horizon #1 Mine backfill suggests that the water chemistry and concentrations of most elements of concern are controlled by mineralogical reactions that will resist changes in water chemistry. It appears that any pyrite (FeS2) oxidation gets neutralized by calcite (CaCO3) present in the same spoil material. This results in the iron precipitating as iron oxides. The slightly higher than normal pH Section 2.05.6(3) Page 20 March 2018 (TR -16)