2023-06-05_HYDROLOGY - M1977342

• Permit No: M1977342
• IBM Index Class Name: Hydrology
• Doc Date: 6/5/2023
• Doc Name: Water Monitoring - Groundwater
• From: Clmax Molydenum
• To: DRMS
• Email Name: PSH
• Email Name: JLE
• Media Type: D
• Archive: No

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CMC—Henderson Operations
DRMS 2022 Annual Water Monitoring Report
May 2023
2.2.2 Proposed Point of Compliance Sampling Location: MLGW-15
MLGW-15 is a deeper well nested with MLGW-7 located just downgradient of 1-Dam. No new
trends, deviations, or exceedances were observed in 2022.
Tabular data for MLGW-15 along with applicable NPLs are presented in Table 2. Accompanying
graphs are provided in Trend Evaluations 21-24.
2.2.3 Proposed Point of Compliance Sampling Location: MLGW-17
MLGW-17 is a shallow alluvial well located downgradient of 3-Dam.No new trends, deviations,
or exceedances were observed in 2022.
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 drinking water parameter POC well located in the Aspen Canyon Ranch area.
Beginning in the third trimester of 2019 Triannual sampling, changes to the MLGW-ACR
sampling method likely caused disturbances to the solid-wall well construction indicated by the
increase in iron and sulfate concentrations starting in August 2019 and justified the need for the
well to undergo rehabilitation. Rehabilitation took place in September 2022. The rehabilitation
was performed using standard methods of brush swabbing and chemical treatment. Iron and
manganese concentrations observed from the third Triannual event of 2022 following the
rehabilitation were substantially higher than previous years. It is possible that the abrasiveness of
the swabbing and acid-based chemical treatment exacerbated the existing poor well condition and
the resulting Triannual concentrations were a consequence of the rehabilitation.
Despite the increased iron and manganese concentrations, sulfate remained similar to past
sampling events. This supports the notion that the heightened metal concentrations were a
consequence of the rehabilitation process rather than upstream impacts from the Henderson Mill.
Altogether,this has been an effort to establish an accurate representation of ambient water quality
conditions for water supply downstream of 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. Fluctuations in specific conductivity and sulfate
were observed immediately following the Williams Fork wildfire and likely correlate with land
disturbance from the wildfire activity. WFR-20 will continue to be monitored while the recently
burned area remains susceptible to increased erosion.
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