2022-12-16_PERMIT FILE - M2021067

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Stiles Mining Resource Groundwater Monitoring and Mitigation Plan December 2022 Page 4 one of the monitoring wells per event (seven [7] samples total per event). Additional QA/QC samples(e.g., method blanks, laboratory control samples, matrix spikes, etc.) will be analyzed by the laboratory during batch analyses with results provided in the final laboratory reports. Groundwater samples submitted to the laboratory will be analyzed for thirty (30) applicable WQCC Regulation 41 Interim Narrative Standards (INS) Tables 1 —4 parameters as determined through correspondence with DRMS in the 1 st Quarter of 2022. A complete list of the parameters analyzed is provided in Table 1. The following analytical methods(or comparable methods)will be utilized by the laboratory to report water quality results. ■ Nineteen(19) Metals(Dissolved)by EPA Method 200.8 ■ Two (2)Metals (Dissolved)by EPA Method 200.7 ■ Six(6) Inorganic Anions by EPA Method 300.0+Calculation ■ Chromium by Standard Method(SM) 3500-Cr B +Calculation ■ pH by SM 4500-H-B ■ TDS by SM 2540-C 3.5 SITE-SPECIFIC GROUNDWATER QUALITY BENCHMARKS The objective of the pre-operational groundwater monitoring is to document pre-operational groundwater quality for the applicable WQCC Regulation 41 INS Tables 1 —4 parameters at the Site prior to beginning mining operations. This data will be used to establish the site-specific groundwater quality benchmarks. Per Rule 3.1.7(2)(c)(ii), the lowest WQCC Regulation 41 INS Tables 1 — 4 standard will be utilized as the default numeric protection value (i.e., groundwater quality benchmark), except for parameter(s)where the background groundwater concentration(s) exceed these value(s). For these parameter(s), if applicable, the site-specific groundwater quality benchmark(s) will be the highest concentration reported during the five (5) consecutive quarters of background groundwater sampling events. A list of the applicable default WQCC Regulation 41 INS Tables 1 — 4 standards is presented in Table 1; however, the final list of site-specific groundwater quality benchmarks will be submitted to DRMS in a pre-operational background groundwater quality report once the five (5) quarters of pre-operational groundwater sampling is complete. Once the site-specific groundwater quality benchmarks are established, they will be used for comparison against post-operational groundwater sampling event results to demonstrate protection of existing and reasonably potential future uses of groundwater throughout the operational life of the mine until reclamation has been achieved (i.e., when release of reclamation liability occurs). Any change to parameters on the list (e.g., reduction of analytical parameters) or groundwater quality benchmark values will be submitted to DRMS via Technical Revision for approval prior to implementing.