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CMC—Henderson Operations <br /> DRMS 2021 Annual Water Monitoring Report <br /> May 2022 <br /> Introduction <br /> The GWMP establishes a plan for groundwater monitoring at the Henderson Mine and Mill for the <br /> protection of groundwater quality pursuant to Rule 3.1.7(5) of the Mineral Rules and Regulations <br /> of the Colorado Mined Land Reclamation Board for Hard Rock, Metal, and Designated Mining <br /> Operations and the Colorado Water Quality Control Commission (WQCC) standards. Henderson <br /> has prepared this report in accordance with the requirements of Section 7.1 of the GWMP to <br /> summarize results of 2021 water monitoring activities. <br /> 1.0 Discussion of Annual Water Monitoring Data <br /> This section provides a summary of the annual water monitoring data collected in 2021 in <br /> accordance with the GWMP for each permit-identified POC well and non-POC long-term surface <br /> water monitoring location at the Henderson Mine and Mill. Monitoring is conducted three times <br /> per year as stipulated in the GWMP as follows: <br /> • During the April through May spring run-off period; <br /> • During the summer months of July and August; and <br /> • During the September through December low-flow period. <br /> To provide a better data set for trending purposes, the 2021 water quality data has been appended <br /> to the previous four years of data. This data can be seen in trend evaluation graphs as well as in <br /> the data tables. Note that for trending the analytical data, results reported below the laboratory <br /> detection limit are shown as a value of zero.All monitoring locations are depicted in Figures 1 and <br /> 2. <br /> Outliers are identified, as needed, using either the Dixon's or the Rosner's statistical method <br /> depending on the available number of data points. Although data from prior reporting years is <br /> presented herein for trending and discussion purposes, outlier and NPL assessments/discussions <br /> are limited to current reporting year data. <br /> 2.1 Henderson Mine <br /> Henderson Mine monitoring locations include POC well MNGW-1 and surface water locations <br /> BG-20, CC-10 and CC-30. Graphical trends for MNGW-1, BG-20, CC-10 and CC-30 are <br /> presented as Trend Evaluations 1-16 of this report. <br /> 2.1.1 Point of Compliance Sampling Location: MNGW-1 <br /> MNGW-1 is a shallow alluvial well located downgradient of the Henderson Mine operations. <br /> Values of pH below the established NPLs were seen during monitoring events in 2021: 6.3 <br /> standard units (s.u.) on 6/2/21, 6.2 s.u. on 8/13/21, 6.3 s.u. on 11/30/21, and 5.6 s.u. on 12/21/21. <br /> In accordance with the GWMP, Henderson has provided notification to the DRMS upon <br /> reoccurring pH exceedances for POC wells, when necessary. <br /> No new trends or deviations relative to historical data were observed in 2021.Henderson requested <br /> reverting the monitoring schedule back to a Triannual basis as stated in the GWMP. Due to an <br /> increase in pH exceedances relative to historical data, DRMS requested that Henderson continue <br /> monitoring field parameters at MNGW-1 on a monthly basis while Henderson continues to <br /> investigate the relationship between the low pH at No Name Gulch and the groundwater chemistry <br /> at MNGW-1. <br /> 4of8 <br />