2020-06-25_REVISION - C1981019 (6)

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RULE 2 PERMITS 1. All pit groundwater will have chemistry similar to Streeter Pond, Streeter Well, or published pit spoil geochemistry 2. All pit groundwater will eventually enter the Good Spring Creek surface water regime 3. The quantity of water entering Good Spring Creek would match assumptions in the Potenital Surface Water Quantity Impacts section. The South Taylor Pit will likely have geochemical characteristics similar to the water quality in the Streeter Well, the Streeter Pond, and other spoil pit aquifers (Williams and Clark, 1994), since the lithology is relatively homogenous across the area. The TDS in the Streeter Well is 3,750 milligrams per liter (mg/L), and TDS in pit spoil wells nearby average 3,400 mg/L(Williams and Clark, 1994). TDS concentrations in the Streeter Pond averaged 1,786 mg/L in 2005 and TDS concentrations in aquifers immediately downgradient from nearby pit spoils averaged 1,796 mg/L (Table 2.04.7-31). Wells located a half mile downgradient from pit spoil averaged 900 mg/L (Williams and Clark, 1994). An estimate of TDS loading from backfilled spoils discharge into Good Spring Creek was developed based on a simple mass balancing based on the projected increased TDS of the water contributing to Good Spring Creek. Calculated impacts of this groundwater into the alluvial and surface water flow regime at Good Spring Creek are shown here. A calculated spoil pit maximum discharge estimate of 0.06 cfs enters Good Spring Creek during base flow, and 0.6 cfs enters during peak flow. Therefore, a maximum of 7% of the base flow and 5% of the peak flow to Good Spring Creek at the NUGSC sampling point would be contributed from the pit outflow at steady state. (These percentages are approximately twice what the springs above NUGSC actually contribute to the creek flows.) 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/l 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. South Taylor/Lower Wilson—Rule 2,Page 83 Revision Date: 6/22/20 Revision No.: MR-220