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respectively. Dissolved iron concentration range from 0.124 mg/L to 1.06 mg/L and dissolved <br />manganese concentrations range from 0.662 mg/L to 3.7 mg/L. <br />Summary and Initial Recommendations <br />Based on its location and boring log, MLGW-11 appears to be installed in the Quaternary <br />sediments aquifer consisting of Ute Creek alluvium and glacial drift/till in the vicinity of the TSF. <br />This hydrostratigraphic unit has been the focus of previous and current hydrogeologic <br />investigations downgradient of the TSF and is monitored by downgradient point of compliance <br />(POC) wells MLGW-7, MLGW-15, and MLGW-17. <br />Previous pH measurements from MLGW-11 in 2007 and 2008, as well as more recent <br />measurements collected during triannual monitoring since 2012 indicate naturally low pH values <br />in the Quaternary sediments aquifer. As shown on Figure 2, the pH values measured in this well <br />are within the range of pH values measured in other Site wells including POC wells (Climax <br />Molybdenum Company, 2014). <br />In addition, the MLGW-11 data indicate naturally elevated concentrations of dissolved <br />manganese within the Quaternary sediments aquifer. As shown on Figure 3, dissolved <br />manganese concentrations in MLGW-11 have exceeded concentrations measured in the POC <br />monitor wells. It appears based on our review, that the dissolved manganese NPL of 0.42 mg/L, <br />as documented in the Groundwater Management Plan TR -16 (Climax Molybdenum Company, <br />2012), did not consider the results from background monitor well MLGW-11 and was instead <br />based on statistical analysis of data collected from one well (MLGW-7 data collected between <br />1993 and 2011). <br />Based on our review, MLGW-11 appears to be an adequate upgradient location to monitor <br />ambient groundwater conditions within the Quaternary sediments aquifer. In addition, aerial <br />photo and Site reconnaissance has not identified a more suitable upgradient location with the <br />Quaternary sediments aquifer. <br />Based on this evaluation, the following recommendations are made for future monitoring of <br />upgradient groundwater conditions at the Henderson Mill. <br />• Continue to include MLGW-11 in the internal triannual monitoring program along with <br />downgradient POC wells. <br />• Consider re-evaluation of ambient concentrations for dissolved manganese and <br />potentially update the NPL based on inclusion of data from MLGW-11 and POCs wells <br />MLGW-15 and MLGW-17, in addition to MLGW-7. <br />Note that DRMS specified that an upgradient monitor well location be considered in the Williams <br />Fork alluvium. The Williams Fork alluvium may be a different hydrostratigraphic unit than the <br />Quaternary sediments aquifer in the vicinity of the TSF. A background well location in the <br />alluvium along the Williams Fork River also could be influenced by surface water. It is unclear <br />how representative an alluvial well near the river would be of ambient groundwater conditions in <br />the Quaternary sediments aquifer. <br />References <br />Climax Molybdenum Company, Henderson Operations, 2012. Technical Revision (TR -16) <br />to Permit M-1977-342 Groundwater Management Plan. April 2012. <br />Climax Molybdenum Company, Henderson Operations, 2014. 5 -Quarter Water Quality Date and <br />Baseline Parameters Report. May 2014. <br />2 <br />