2024-11-04_PERMIT FILE - M2024056 (6)

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Evans Mining Resource Groundwater Monitoring Plan October 2024 Page 3 of groundwater throughout the operational life of the mine until reclamation has been achieved (i.e., when release of reclamation liability occurs). 2.3.2 Baseline Groundwater Quality Sampling Results Results of the nine (9) quarters of baseline groundwater sampling events conducted to-date along with a comparison against the lowest WQCC Regulation 41 INS Table 1 — 4 standard for each analyte is presented in Table 3. As shown, results for seven (7) analytes during the baseline monitoring period to-date have exceeded their lowest WQCC Regulation 41 INS Tables 1 — 4 standards (Aluminum, Iron, Manganese, Nitrate [NO3], Total Nitrate-Nitrite [NO2+NO3], Sulfate, and Uranium). Therefore,the highest concentration reported for these analytes during the baseline groundwater monitoring period will be used as the site-specific groundwater quality benchmark. Results for all other analytes are below their respective WQCC Regulation 41 INS Tables 1 — 4 standards. The final site-specific groundwater quality benchmark values will be submitted in the Baseline Data Summary Report to DRMS. 3.0 PREDICTED IMPACTS TO HYDROLOGIC BALANCE To predict the extent of impacts to the hydrologic balance from the two (2) slurry walls, groundwater modeling was conducted(see Appendix G-3 of the DRMS Permit Application). The predicted impacts to the hydrologic balance were localized areas of groundwater mounding(+1.03 feet to the west and+0.46 feet to the south)and groundwater shadowing(-0.07 feet to the east and -0.36 feet to the north) after the initial slurry wall. The predicted impacts after installation of the second slurry wall indicate groundwater mounding impacts of+1.16 feet to the west and +1.20 feet to the south and groundwater shadowing impacts of-0.29 feet to the east and-0.91 feet to the north. Though the extent of groundwater mounding and shadowing is minimal, the adjacent property to the west of the mine is being utilized by the City of Aurora as part of a water augmentation project in which they are injecting water into the subsurface. To mitigate groundwater mounding to the west,a French drain will be installed along the western boundary of the Site along with an infiltration gallery to facilitate groundwater flow north of the Site. The predicted impacts after installation of the French drain and infiltration gallery indicate groundwater elevation will decrease-1.83 feet to the west after the first slurry wall is installed and groundwater elevation will decrease -0.80 feet after the second slurry wall is installed. Therefore, the French drain and infiltration gallery will not negatively impact the City of Aurora water augmentation project. The predicted impacts as a result of the French drain and infiltration gallery in the other directions remain minimal with localized groundwater mounding of+0.17 feet to the east and groundwater shadowing of-1.08 feet to the south and -0.24 feet to the north after installation of the first slurry wall.The predicted impacts after the second slurry wall is installed indicate minimal amounts of groundwater mounding(+0.01 feet to the east, +0.10 feet to the south, and +0.08 feet to the north). These predicted impacts to the hydrologic balance are within the general range of seasonal fluctuation at the Site and are not anticipated to have an adverse effect to the surrounding area. Since no other slurry walls are near the Site,these slurry walls will not prohibit groundwater from reaching the South Platte River. As the slurry walls will hydraulically separate the mining operation from the surrounding alluvial aquifer,no impacts to groundwater quality are anticipated.