2010-10-01_REVISION - C1981008 (11)

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additional infiltration, however, the ditch area is so small (0.87 acres) compared to the area of the spoil (768 acres) and the area of irrigation (422 acres), that the increase in infiltration would be negligible. Therefore, the worst case prediction for flow in the spoil spring at the New Horizon #2 Mine is 952 acre feet per year, while the most realistic flow for the spoil spring is 490 acre feet per year. This is a potential flow of 1.31 cfs (worst case) and 0.68 cfs (realistic case). In both cases, the flow is higher than the previously calculated spoil spring flow from when the New Horizon #2 mine covered a smaller area. That flow was 0.57 cfs. The greater flow at New Horizon #2 mine will lead to a lower impact on area water quality due to dilution. Overall, there should be no detrimental impacts from the mining operation on groundwater flow and recharge rates. It is definite that mound water flow and recharge in the spoil material will be increased, since the overburden aquifers have historically had very low flow rates and also poor quality. These aquifers have not been used for any wells in the surrounding area and cannot be considered a water resource. The ground water aquifers below the Dakota coals, in the Burro Canyon Formation, are used in surrounding wells and have better water quality but they will not be affected by the mining and reclamation of this site. Shale layers below the mined coals prevent interaction between the spoil and these aquifers, which get their recharge from long distances away from the mine operation. 3. Potential impacts of replaced spoil on groundwater quality. Since the operation will disrupt the overburden above the Dakota coals and remove the Dakota coals, these are the only two stratigraphic zones that will be affected by the operation. As described in'the section on overbuden water quality, the pre-mine quality of the overburden water is poor, with TDS generally in the 3000 ppm range and some ions exceeding limits for most water uses. The primary potential for impacts to ground water quality will occur from increased water infiltration causing an accelerated oxidation of pyrite in the spoil. Other salts may also dissolve more readily in the highly permeable spoil. The minor amounts of sulfuric acid produced can cause lower pH, which then results in higher rates of dissolution of other chemical compounds in the spoil, resulting in higher TDS. This water will saturate the spoil at the lowwall and form a spring at the low point. Also, the spoil water can infiltrate into the lowwall strata of the Dakota Sandstone formation. Spoil Water Chemistry Concerning impacts from the conversion of overburden to spoil, 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 present throughout the overburden. Calcite serves two functions. First, it buffers 35