2020-06-04_PERMIT FILE - C1981008A (2)

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Containment of Pit Inflow and Impacts on Water Ouality All runoff and pit pumpage from disturbed areas will be routed through sedimentation pond 007. The pond is designed and constructed to impound runoff and pit pumpage from areas disturbed by mining and provide sufficient residence time to insure that the pond discharge water chemistry meets the effluent requirements specified in the NPDES Permit. A review of the chemical and flow data indicates that the potential for any discharge from Pond 007 to exceed receiving stream or federal standards is minimal. Past history of mine operations at the Nucla Mine indicate very few exceedances of the standards over the years of operations. As previously discussed, highest pit inflow is predicted to occur in year 5 at approximately 5,600 cubic feet per day. To assess the impact of this inflow on Calamity Draw and the San Miguel water quality, it was assumed that the entire flow was discharged and that the TDS level for the overburden aquifer was representative of the chemical load. The duration of the water quality impacts is relatively short term (5 years) and the significance of the impact is negligible as TDS increases in the range of 0.08 to 1.5 percent are projected on the San Miguel River and Calamity Draw, respectively. These increases in TDS will be additional magnesium/sodium-sulfate type water which may result in a slight increase in salinity. These projected changes in TDS levels will in no way affect the present and potential uses of the surface water and are so small that they may not be measurable. Impact of Spoil Water Quality on the Ground and Surface Water Quality The available data indicate that a small proportion of the overburden may produce acid through the oxidation of pyrite. Based on laboratory tests on overburden cores, calcite is almost ubiquitous. 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's 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's. 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 supply 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. The acid that is produced will be quickly neutralized. During the mining process, New Horizon will test the overburden and if acidic layers are encountered, they will be mixed with non -acidic layers to neutralize any acid forming effects. Potential Impacts of Replaced Spoil on Groundwater Quality The analysis of geochemical controls on groundwater quality suggests that the water chemistry and concentrations of most elements of concern are controlled by mineralogic reactions that will resist changes in water chemistry. Production of acid may occur in very local settings and is probably most prevalent in the coal, which will be mined. Calculations indicate that neutralization of the acid will occur rapidly with mixing of water, or with movement of acidic water into calcite -bearing rocks. Also, the analysis conducted indicates that chemical changes are not likely to occur. Page 2.04.7-32 March 2017 (TR -77)