2023-06-05_HYDROLOGY - M1977342 - Laserfiche WebLink

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CMC—Henderson Operations DRMS 2022 Annual Water Monitoring Report May 2023 Henderson has provided updates for the ongoing evaluations and mitigation solutions for No Name Gulch through technical memorandums;most recently,the March 31,2023 "Henderson Mine POC Well MNGW-1 Low pH Status Update—Fourth(4th)Adequacy Review". Tabular data for MNGW-1, along with applicable NPLs, is presented in Table 1. Accompanying graphs are provided in Trend Evaluations 1-4. 2.1.2 Surface Water Sampling Location: BG-20 BG-20 is located upgradient of the Henderson Mine in Butler Gulch and serves as an indicator of background surface water quality.No apparent trends or anomalies were observed in 2022. Tabular data for BG-20 are presented in Table 1 with accompanying Trend Evaluations 5-8. 2.1.3 Surface Water Sampling Location: CC-10 CC-10 is also located upgradient of the Henderson Mine in the West Fork of Clear Creek and serves as another indicator of background surface water quality.No apparent trends or anomalies were observed in 2022. Tabular data for CC-10 are presented in Table 1 with accompanying Trend Evaluations 9-12. 2.1.4 Surface Water Sampling Location: CC-30 CC-30 is located downgradient of the Henderson Mine in the West Fork of Clear Creek and serves as an indicator of surface water quality downstream of Mine development. Manganese and Zinc concentrations recorded during the 5/20/2019 Triannual sampling event were determined to be statistical outliers based on the available data.Henderson performed a statistical analysis to include more recent Triannual data and determined that the 2019 values cannot be considered statistical outliers. Henderson will continue to monitor the manganese and zinc concentrations and perform statistical evaluations as needed. Tabular data for CC-30 are presented in Table 1 with accompanying Trend Evaluations 13-16 2.2 Henderson Mill The Henderson Mill monitoring locations include POC wells MLGW-7, MLGW-ACR, MLGW- 15, and MLGW-17, as well as non-POC surface water locations WFR-20 and WFR-40. Graphical trends are presented as Trend Evaluations 17-40 of this report. 2.2.1 Point of Compliance Sampling Location: MLGW-7 MLGW-7 is a shallow alluvial well nested with MLGW-15 located downgradient of 1-Dam. Henderson continues to observe an overall decreasing trend in dissolved iron, sulfate, and specific conductivity.No other apparent trends, deviations, or exceedances were observed in 2022. Tabular data for MLGW-7 along with applicable NPLs are presented in Table 2. Accompanying graphs are provided in Trend Evaluations 17-20. 5 of 8