2020-06-25_REVISION - C1981019 (6)

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RULE 2 PERMITS 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-D3-07, -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. Other Potential Impacts Flooding and stream flow regime do not appear to have been affected by past mining operations or reclamation, nor are they anticipated to be affected by South Taylor mining. Groundwater availability in the area may potentially be enhanced with the storage of water in the reclaimed pit. Colowyo currently owns all water rights within Taylor Creek and owns over 20% of the appropriated amount (10.83 cfs of the total 51.6 cfs available) of water available in Good Spring Creek. Thus, any potential diminishment of flow will be compensated for by reduced use by Colowyo. There is sufficient capacity for Colowyo to reduce their use of adjudicated water to compensate for potential diminishment of flow in the creek, allowing downstream users full access to their water rights. 2.05.6 (4) Protection of Public Parks and Historic Places No public parks are located within the permit or adjacent areas; therefore no public parks will be affected by the mining operations. Likewise the mining operations will not affect any places listed or eligible for listing in the National Register of Historic Places. 2.05.6 (5-6) Surface Mining near Underground Mining; Subsidence Control No surface mining activities will be conducted within 500 feet of an underground mine. Therefore, there is no subsidence control plan for operations. The Red Wing Mine, a historic underground mine, exists north of the South Taylor pit and is shown on the existing Map 31. South Taylor/Lower Wilson—Rule 2,Page 84 Revision Date: 6/22/20 Revision No.: MR-220