2020-03-24_PERMIT FILE - C1981019A (2)

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RULE 2 PERMITS To project the potential impact to Good Spring Creek, a weighted TDS loading between the historic low flow at NUGSC (0.85 cfs and 1,050 mg/L TDS) (Table 2.04.7-34) and the projected spoils (0.06 cfs and 3,400 mg/L (worst case) and 1,796 mg/L (likely case) TDS; Table 2.04.7-31) was performed. Worst case (pit spoil aquifer TDS concentrations): ((0.85 cfs x 1050 mg/L) + (0.06 cfs x 3400 mg/L))/0.92 cfs = 1192 mg/L Reasonable case (groundwater immediately downgradient from pit spoil): ((0.85 cfs x 1050 mg/L) + (0.06 cfs x 1796 mg/L))/0.92 cfs = 1087 mg/L Thus, the base flow of Upper Good Spring Creek is calculated to have between 37 and 142 mg/L increase in total dissolved solids, or an increase of between 3.5% and 13.5% caused by the projected contribution from the pit springs. The increase in TDS in the base flow at Lower Good Spring Creek (with the base flow of 1.8 cfs and TDS of 1187 mg/1 placed into the above calculations) would be between 20 mg/L and 71 mg/L, or between 1.6% and 6% of TDS increase. Peak flow TDS increases would be less than these values. Based upon analyses performed by Williams and Clark (1994) at the Seneca II Mine, the dominant anion would most likely be sulfate and that the oxidation of the pyrite would be the main source of TDS in the spoil pit water. Oxidation of minor pyrite in the spoil could produce soluble sulfate at the South Taylor pit, which will be the dominant ion causing the increased TDS. The duration of the elevated TDS can be predicted based upon the oxidation of pyrite in the reclaimed spoils pit aquifer. Saturation indices (SI) were calculated for the average constituent concentrations in well 84 -0 -OB (Williams Fork Formation well) and the Streeter Well. The SI is used to determine if a mineral will dissolve into or precipitate from solution. A negative SI indicates that the water is undersaturated with respect to the mineral and, if present, the mineral should dissolve. If the SI is positive, the water is supersaturated with respect to the mineral, and the mineral should precipitate from solution. An SI near zero indicates a condition near equilibrium. Table 2.05.6-3 presents the SI for the wells at Colowyo Mine. The SIs presented in this table are very similar to those determined by Williams and Clark (1994). Calcite and dolomite have positive saturations indices in the sampled wells; therefore, the water is saturated with respect to these minerals and it is not anticipated that an increase in TDS would occur. Sulfate minerals (gypsum and epsomite) have negative SIs; therefore, the water is not saturated with respect to these minerals and increases in TDS would occur if sulfate minerals were present in the spoil. This is consistent with the increase in sulfate in the Streeter Well (1,960 mg/L) as compared to Good Spring Creek (average of 600 mg/L). The average pyritic sulfur concentration in the spoils is 0.09 percent in borehole 97-15, the only borehole in South Taylor with every interval analyzed for pyritic sulfur. The pyritic sulfur concentrations in boreholes 83-133-07, -10, -12, and -14 were measured at only selected intervals biased towards high pyrite; the arithmetic mean of these samples is 0.45% pyritic sulfur. Based upon the exhaustion time for 0.20 percent pyrite of 300 years (Williams and Clark 1994), the time of the elevated TDS discharge would be between 150 and 600 years. The actual duration would be reduced in direct proportion to the amount of "piping" that occurs as a result of channel formation within the spoils. This type of flow is documented at other mines, and has reduced the amount of pyrite oxidized in the spoil. Prediction of the amount of piping that will occur is not possible, but assuming that 25 percent of the spoil pile would be bypassed by piping, then the duration of elevated TDS concentrations would be reduced by 25 percent to 110 to 450 years. South Taylor/Lower Wilson — Rule 2, Page 85 Revision Date: 4/7/17 Revision No.: RN -07