2021-07-21_PERMIT FILE - C1981019 (5)

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Point of Compliance Well Investigation AECOM 8 sample from an individual location would achieve compliance with the various comparison thresholds that could apply to the monitoring program. AECOM also created comparison figures (Figures 2 through 12) illustrating which temporary well locations meet the BSGW. Figures were prepared for the constituents that had at least one sample result from a temporary well that exceeded the most stringent Regulation 41 criteria. This list includes arsenic, pH, iron, manganese, nitrite as N, selenium, sulfate, and TDS. On each figure, the well symbols are shown in green if the average concentration value for the well is below the standard, and are coded in red if the average concentration exceeds the standard. The label for each well location also lists the average and maximum concentrations reported in the monitoring dataset, and the percentage of values that exceed the standards. For any figure that had a “red-coded” well indicating a consistent exceedance of the groundwater standard, AECOM also created a second figure comparing the average concentration values to the background UTL for that constituent. A brief summary of each figure is provided below. • Figure 2 – Dissolved Arsenic: The wells on this figure are consistently coded green because the average arsenic concentration at each well meets the Regulation 41 arsenic standard. Arsenic will therefore have minimal influence on the siting of future compliance wells. • Figure 3 – Field pH: Like arsenic, the wells on this figure are coded green because the average pH value at each well meets the Regulation 41 standard. Therefore, pH will have minimal influence on the siting of future compliance wells. • Figures 4 and 5 – Dissolved Iron: The average dissolved iron concentration at two wells in the Good Spring Creek drainage (POC-8 and POC-9) exceeds the 0.3 mg/L Regulation 41 iron standard (Figure 4). When the upgradient background UTL is used for comparison, only the result from POC-9 exceeds the background value (Figure 5). No comparison could be made to pre-1994 baseline concentrations because there were no pre-1994 iron data available from the Gossard Well or NGSW to calculate background UTLs (Table 2). • Figures 6, 7a, and 7b – Dissolved Manganese: Average dissolved manganese concentrations in the temporary monitoring wells were consistently above the most stringent Regulation No. 41 criteria. As a result, all of the temporary well symbols on Figure 6 are coded red. When the upgradient background UTL is used for comparison (Figure 7a), compliance with the threshold value is only achieved at one temporary well location: POC-3 in the Taylor Creek Drainage. However, further analysis shows that at ten of the temporary wells, average manganese concentrations are below the background value calculated using pre-1994 baseline data from Gossard Well and NGSW (Figure 7b). The only wells that do not meet this higher threshold include POC-5 and POC-6 in the Wilson Creek drainage, and POC-9 and POC-12 in the Good Spring Creek drainage. • Figure 8 – Nitrite: The wells on this figure are coded green because the average nitrite concentration at each well meets the Regulation 41 nitrite standard. Nitrite therefore will not influence the siting of future compliance wells. • Figure 9 – Dissolved Selenium: Average selenium concentrations generally meet the most stringent Regulation 41 criteria except at POC-16 in the Wilson Creek drainage. However, POC-16 is located over a mile downgradient of the Mine Permit boundary, and is unlikely to be selected as a permanent point-of-compliance well location. AECOM did not prepare a UTL comparison figure for selenium because the selenium background values shown in Table 2 are the same or lower than the groundwater standard. • Figures 10, 11a, and 11b – Sulfate: Average sulfate concentrations exceed the Regulation 41 standard in both the temporary and permanent wells (Figure 10). However, as shown