2021-06-02_HYDROLOGY - M1977342 (2)

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CMC—Henderson Operations DRMS 2020 Annual Water Monitoring Report May 2021 2.2.3 Proposed Point of Compliance Sampling Location: MLGW-17 MLGW-17 is a deep alluvial well located downgradient of 3-Dam.Although a trend of increasing dissolved iron and zinc concentrations was noted previously, most of the 2019 and all of the 2020 results indicate an overall decrease in concentrations. Tabular data for MLGW-17 along with applicable NPLs are presented in Table 2. Accompanying graphs are provided in Trend Evaluations 25-28. 2.2.4 Point of Compliance Sampling Location: MLGW-ACR MLGW-ACR is a domestic water supply POC well located in the Aspen Canyon Ranch area. Starting in the third trimester of 2019,changes to the MLGW-ACR sampling method likely caused disturbances to the solid-wall well construction indicated by the increase in iron concentrations and established the need for the well to undergo rehabilitation. The current results presented are not believed to be representative of background water quality. Well rehabilitation will be conducted. Altogether, this has been an effort to establish an accurate representation of the ambient groundwater conditions far downstream from Henderson operations. Henderson will continue to evaluate the data collected at MLGW-ACR following future sampling events to better understand these data. Tabular data for MLGW-ACR along with applicable NPLs are presented in Table 2. Accompanying graphs are provided in Trend Evaluations 29-32. 2.2.5 Surface Water Sampling Location: WFR-20 WFR-20 is located upgradient of the Henderson Mill in the Williams Fork River and serves as an indicator of background surface water quality.A small increase in specific conductivity and sulfate during the second and third trimesters, respectively, during the summer and low-flow months might correlate with influence from Williams Fork wildfire activity. Firefighting efforts and rainfall can mobilize suspended and dissolved material into surface water. Although an increase in pH and metal concentrations was not observed, WFR-20 will continue to be monitored while the newly burned area remains susceptible to increased erosion. Tabular data for WFR-20 are presented in Table 2 with accompanying Trend Evaluations 33-36. 2.2.6 Surface Water Sampling Location: WFR-40 WFR-40 is located downgradient of the Henderson Mill in the Williams Fork River.Although still within historical levels,pH and sulfate concentrations measured in 2020 increased relative to 2019 which may relate to the Williams Fork fire activity. Increased pH and major ion loading into adjacent surface water are common responses from runoff interacting with a recently burned area. WFR-40 will continue to be monitored to determine potential influences from the recent fire activity. Tabular data for WFR-40 are presented in Table 2 with accompanying Trend Evaluations 37-40. 4of5