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

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Point of Compliance Well Investigation AECOM 7 exhibited a normal, log-normal, or nonparametric distribution. Any non-detect concentrations in the background data were represented for statistical purposes as equal to the detection limit. Calculation of UTLs was limited to constituents with groundwater standards listed in Regulation 41 since these constituents will be the primary driver for determining compliance in future point- of-compliance wells. The site-specific background concentrations estimated using both upgradient and pre-1994 data are summarized in Table 2 alongside the BSGW. Results of the statistical evaluation show that for several constituents, the UTLs calculated are higher than the most stringent groundwater threshold contained in Regulation 41. For example, both the upgradient (0.09 mg/L) and pre- 1994 (0.75 mg/L) background values for manganese are higher than the most stringent manganese groundwater standard (0.05 mg/L). A similar relationship exists between the groundwater standard and at least one background UTL for iron, nitrate as N, combined nitrate plus nitrite as N, selenium, and sulfate. Instances where the background UTL for a constituent are higher than the groundwater standard are common at sites with naturally-occurring, poor quality groundwater. 3.2 Temporary Well Concentration Results The next step after establishing groundwater comparison thresholds was to analyze baseline concentration data from the temporary monitoring wells. The concentration datasets were analyzed by calculating summary statistics for each constituent at each well, including the minimum, maximum, and average concentration values. These statistics are summarized by constituent in Tables 3 through 15. The baseline sampling data were then compared to the groundwater threshold values listed in Table 2. For most constituents, three different comparisons were made: 1. The temporary well concentration data were first compared to the most stringent Regulation No. 41 standard for human health, drinking water, or agricultural uses. 2. The second comparison made was to the background UTLs established using concentration data from the Mine’s upgradient background wells (A6 and A8). 3. The final comparison made was to the background UTLs calculated using pre-1994 baseline data from the downgradient Gossard Well and NGSW. Tables 3 through 15 present results of the comparison for each constituent. The data in the tables are organized from upstream to downstream within each drainage to help illustrate spatial trends across the site. Concentration data for the permanent monitoring wells NGSW, MT-95-02, and Gossard Well are also included in the tables to illustrate how these wells compare to the temporary monitoring well statistics. Average, minimum, and maximum concentrations for the permanent wells were calculated using the approved Mine groundwater monitoring data collected quarterly in accordance with the Mine Permit between January 2015 and March 2020. The comparison to the three groundwater thresholds is represented in the tables as the percentage of concentration values within each well dataset that exceeds the respective comparison standard. For example, the data in Table 7 indicate that 100 percent of manganese samples collected at POC-8 exceeded the most stringent Regulation No. 41 groundwater standard. However, when the manganese concentrations from this well are compared to the upgradient well background value and the pre-1994 downgradient well background value, the percent of manganese concentrations exceeding these thresholds drops to 86 percent and 29 percent, respectively. In this way, the tables are helpful for evaluating whether a future well