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October 2015 MLGW-ACR GW Quality Memo♦1 <br /> 1.0 INTRODUCTION <br /> This Memorandum has been prepared for Climax Molybdenum Company's Henderson Mill <br /> Operation (Henderson) to summarize the continued assessment of water quality in point of <br /> compliance (POC) groundwater monitor well, .MLGW-ACR, located downgradient of <br /> Henderson Mill's Tailing Storage Facility (TSF). Henderson's facilities,specifically their TSF, <br /> are located within the Ute Creek Basin of the Williams Fork River Valley,south of Parshall,in <br /> Grand County, Colorado. Figure 1 presents a regional map showing the location and <br /> geographic setting of the Henderson Mill, TSF area, and location of POC wells including <br /> MLGW-ACR. Monitoring and reporting requirements are pursuant to the Division of <br /> Reclamation, Mining, and Safety (DRMS)-approved Henderson Groundwater Management <br /> Plan (GWMP), formally submitted as Technical Revision 16 (TR-16) to the Henderson Mine <br /> and Mill Reclamation Permit No. M-1977-342 (Climax Molybdenum Company,2012). <br /> 1.1 BACKGROUND AND OBJECTIVES <br /> MLGW-ACR was established in the GWMP as the POC for water supply related parameters. <br /> This well was designated as the POC well as a result of rulemaking hearings by the Water <br /> Quality Control Commission (E 5 CCR 1002-33). As shown on Figure 1, MLGW-ACR is <br /> located approximately 3 1/2 miles north (downgradient) of the TSF. When the GWMP was <br /> approved, there were insufficient data to establish numeric protection limits (NPLs) and the <br /> GWMP specified that additional baseline monitoring be performed prior to development of <br /> NPLs. Five(5) calendar quarters of baseline data were collected between fourth quarter 2012 <br /> and fourth quarter 2013. These baseline data were reported in the 5-Quarter Water Quality <br /> Data and Baseline Parameters Report(Baseline Parameters Report;Henderson and Aquionix, <br /> 2014). As noted in this report, MLGW-ACR results were compared against the domestic <br /> water supply standards specified in Table 1 and Table 2 of the Colorado Water Quality Control <br /> Commission Basic Standards for Groundwater (CBSG). <br /> All of the results were below the standards with the exception of dissolved iron and <br /> manganese,both secondary aesthetic standards. Both constituents exceeded the CBSG Table <br /> 2 standards in two of the five baseline monitoring events. Iron exceeded the standard of 300 <br /> micrograms per liter (µg/L) in the fourth quarter 2012 and first quarter 2013 with <br />