2020-06-08_REVISION - C1982057 (31)

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final predicted TDS concentration of 1787 mg/L at WSH7 (SW-S2W-SG7) and 2451 mg/L at WSD5 (SW-S2W-SG5). The projected increase within Sage Creek at WSSF3 (SW-S2W-FG4)was lower at 12% (64 mg/L) and resulted in a predicted concentration of 676 mg/L. The evaluation concluded that the quality of the surface water supplying the AVF's would not be impacted to such an extent that the water uses (irrigation and stock water)would be prohibited or degraded to the point of material damage. This conclusion is supported by the post mine surface water quality at the sample locations within Dry Creek watershed. Hubberson Gulch drains through the 26 acres AVF located approximately a quarter mile southeast of the confluence of Hubberson Gulch and Water Trough Gulch (Section 16 and 21, T5N, R88W). The fields are flood irrigated with Hubberson Gulch water from just upstream of sample point WSH7 (SW-S2W-SG7). The average TDS concentration over the last five years at WSH7 (SW-S2W-SG7)is 1125 mg/L which is approximately 600 mg/L lower than the predicted 1787 mg/L TDS concentration specified in the Probable Hydrologic Consequences analysis. Lower than predicted TDS concentrations were also observed further downstream at WSD5 within Dry Creek where the 1710 mg/L five-year TDS average has been approximately 700 mg/L lower than the predicted 2451 mg/L concentration. These concentrations are significantly lower than the 3567 mg/L pre-mine TDS concentrations measured lower in the watershed at the southwest corner of Section 22, Township 6N, Range 88W. This elevated TDS was attributed to soil salt accumulations from dry land farming in the watershed that are dissolved and transported to the primary drainages. The Sage Creek flood irrigated AVF,located in Section 30, T6N,R87W,receives spoil contributions from both Seneca II-W and Yoast Mine. Although the 970 mg/L five year average TDS concentration measured at nearby stream point WSSF3 (SW-S2W-FG4) exceeds the predicted TDS concentration of 676 mg/L included in the Seneca 11-W permit it is still approximately 1100 mg/L less than the 2118 mg/L post mine concentration predicted in the Yoast Mine PHC(see Tab 17 of Permit C-1994-082). The Yoast Mine PHC concluded that the impact from the increased TDS concentrations in Sage Creek would not have a significant impact in regard to the potential use of the surface water for flood irrigation. Therefore the elevated TDS at point WSSF3 relative to the earlier projected post mine concentrations detailed in the Seneca 11-W PHC do not appear to have impacted the AVE F. Agreement of Observed Hydrologic Impacts with the "Probable Hydrologic Consequences"(PHC)Projected in the Mining Permit The Observed Hydrologic Impacts are generally in good agreement with the Probable Hydrologic Consequences projected in the mine permit. The pre-mine groundwater was marginal to unsuitable for stock and/or irrigation. A single residential well was identified within the mining area and was mitigated per legal agreement. No other residential wells were located within or adjacent to the mine area. Water quality at compliance well DCAL-02 meets all applicable water quality standards except for dissolved iron. As described in Section A above it is likely that the elevated dissolved iron is actually representative of the ambient background concentrations within the Dry Creek Alluvium and is not indicative of offsite impact to the local groundwater system. Compliance bedrock wells were deemed unnecessary due to the limited potential for the mine to negatively impact the quality of bedrock groundwater(see Section A and Technical Revision 63). Therefore, as predicted in the PHC no groundwater users were impacted. Impacts of spoil water to the primary drainages were described in detail in Part E. Spoil water contributions to TDS 21