2013-11-29_HYDROLOGY - M1977493 - Laserfiche WebLink

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Status Report; Robinson Seep Investigation Climax Molybdenum Company EXECUTIVE SUMMARY This report presents a current status of ongoing water quality monitoring activities at the Climax Mine within the portion of the site located in the Eagle River Watershed, specifically in the vicinity of monitoring well EVMW -1 S. This well is located below Robinson Lake and the Robinson Lake seepage collection system and above Eagle Park Reservoir. Climax Molybdenum Company (Climax) notified the Colorado Division of Reclamation, Mining and Safety (DRMS) and the Eagle River Water & Sanitation District (ERWSD) in early April 2013, that a review of routine monitoring results indicated an apparent increasing trend of certain water quality parameters in monitoring well EVMW -1S. In response, Climax immediately increased the sampling frequency for wells EVMW -1 S, EVMW -1 D, and EVMW -004 to monthly. In addition to the increased frequency of groundwater sampling, Climax began collecting monthly samples from Robinson Lake and the Robinson Lake Seep, and performed additional field investigations to evaluate the spatial extent of impacted groundwater and potential groundwater flow paths between Robinson Lake and the Robinson Lake seepage collection system. A summary of the monthly water quality monitoring results from April through November 2013, and a discussion of the results of the additional field investigation activities are presented in this status report. Increasing trends have only been observed in well EVMW -1 S. In general, increasing trends in constituent concentrations were observed through April 2013 in EVMW -1 S. Concentrations of each of these constituents decreased slightly in May and appeared to have generally stabilized through September. Concentrations of iron and manganese in samples collected from EVMW - 1 S in October and November were higher than those observed over the previous several months, while stable or slightly decreasing trends in concentrations of the other indicator parameters continued. No increases have been observed in the concentrations of indicator parameters cadmium, copper, lead, molybdenum, or zinc over the period of record, and with the exception of molybdenum, concentrations of these constituents are typically not detected above the PQL or MDL. Lower constituent concentrations in monitoring well EVMW -1 S relative to concentrations measured in Robinson Lake and the Robinson Lake Seep indicate the effectiveness of the Robinson Lake seepage collection system in limiting migration of contaminants downgradient of the cut -off wall. The controlled filling of Robinson Lake began in April of 2011 and the increasing trend in concentrations of most constituents in well EVMW -1 S appears to have generally begun during the spring or summer of 2012. Robinson Lake water levels have remained relatively stable since approximately July 2011, but it is likely that the increase in driving head as well as the chemistry of Robinson Lake water (and potentially the quality of the underlying groundwater) contributed to the observed trends in EVMW -1 S. While a short-term increase in constituent concentrations has been observed in shallow groundwater downgradient of the seepage collection system; over the period of record beginning in 1996, concentrations of sulfate, TDS, calcium and magnesium remain lower than previously observed maximum concentrations. Results of the additional field investigation efforts indicate that seepage from Robinson Lake is migrating downgradient. While the central axis of the seepage migration area is directed towards the Robinson Lake seepage collection system, the lateral extent does extend slightly east and west of the cut -off wall. Based on the results of these additional investigations and the observed concentrations and trends in concentrations at monitoring well EVMW -1S, the cut -off Tetra Tech, Inc November 2013