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CLEAR —"— <br />CREEK <br />ASSOCIATES <br />MEMORANDUM <br />TO: Tim Haynes (FMI) <br />FROM: Leah Wolf Martin (AJAX) and Michael Alter (Clear Creek Associates) <br />SUBJECT: Henderson Mill Background Monitor Well Evaluation <br />DATE: December 16, 2015 <br />CC: Miguel Hamarat (FMI) <br />In an April 14, 2015 letter reviewing groundwater Numeric Protection Limits (NPLs) proposed by <br />Climax Molybdenum Company Henderson Operations (Henderson), the Colorado Division of <br />Reclamation Mining and Safety (DRMS) requested review of a possible background monitor well <br />location at the Henderson Mill. DRMS proposed that the review include the need and feasibility <br />of installing and sampling a background monitor well in the Williams Fork alluvium in a location <br />that is upgradient from all possible groundwater seeping from the Tailing Storage Facility (TSF). <br />The DRMS stated purpose of the upgradient monitor well would be to support the analysis of <br />ambient dissolved manganese concentrations and pH measurements at Henderson Mill used to <br />develop NPLs. <br />This memorandum presents relevant technical information regarding the current upgradient <br />location, MLGW-11, and considerations associated with siting an upgradient well in the Williams <br />Fork alluvium. <br />MLGW-11 Background Information <br />Current upgradient monitor well MLGW-11 (see Figure 1) was installed by Henderson and its <br />consultants in November 2007. This upgradient location met Henderson's criteria of being a) <br />within the Henderson Mill permit boundary, b) upgradient of the TSF and other Mill facilities, and <br />c) within the Ute Creek channel. The borehole was drilled to 30 feet below ground surface (ft <br />bgs) and the well screen extends from 10 to 25 ft bgs. As shown on the borehole log in <br />Attachment 1, MLGW-11 was installed in fine to course gravel with some sand, clay and silt, and <br />was characterized as glacial till. Water levels in the well fluctuate between 7 and 10 ft bgs and <br />the groundwater level is approximately 30 ft above East Branch Reservoir. <br />MLGW-11 was originally sampled in December 2007 and May 2008 and these data are <br />summarized in Table 1. The sample results show near neutral pH values of 6.7 and 7.3 standard <br />units (s.u.), sulfate concentrations of less than 50 milligrams per liter (mg/L), and chloride <br />concentrations between 1 mg/L and 2 mg/L. The total dissolved solids (TDS) concentrations <br />were near 300 mg/L and were associated with an alkalinity concentration of approximately 250 <br />mg/L CaCO3. Metals concentrations were generally low or non -detect, except for dissolved iron <br />(0.04 mg/L and 3.36 mg/L) and manganese (1.2 mg/L and 1.7 mg/L). <br />Internal triannual sampling of MLGW-11 began in March 2012 and a summary of these data also <br />is provided in Table 1. Recent water quality data support those collected from this well in 2007 <br />and 2008. Measured pH values in MLGW-11 range between 6.5 and 6.8 s.u. Sulfate and <br />chloride concentrations range from 1.6 mg/L to 2.8 mg/L and <0.14 mg/L to 0.64 mg/L, <br />1 <br />