2013-11-29_HYDROLOGY - M1977493 - Laserfiche WebLink

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Status Report Robinson Seep Investigation Climax Molybdenum Company In general, concentrations of sulfate, TDS, manganese, zinc, molybdenum and calcium are significantly lower in EVMW -1 S than observed in Robinson Lake Seep and Robinson Lake. These data indicate the overall effectiveness of the Robinson Lake seepage collection system in limiting migration of contaminants downgradient of the cut -off wall. Magnesium concentrations at the Robinson Lake Seep are higher than in Robinson Lake, but are generally similar to concentrations observed at EVMW -1 S. Over the relatively short time period represented by these data, the trends in concentrations of sulfate and TDS in monitoring well EVMW -1 S appear to generally correlate with similar trends of these constituents in the Robinson Lake Seep. While a slight increasing trend is observed in the dissolved manganese concentration at EVMW -1S, the magnitude of concentrations observed in Robinson Lake and the Robinson Lake Seep are significantly greater. No increasing trends in the concentrations of zinc and molybdenum have been observed in monitoring well EVMW -1S, and in fact molybdenum concentrations appear to be continuing to decrease. Decreases in constituent concentrations in the Robinson Lake Seep are observed in the data from late spring sampling events in both 2012 and 2013. This temporary improvement in water quality is likely the result of the influence of spring snowmelt captured within the seepage collection system. Although only a limited data set from Robinson Lake is available, a significant improvement in water quality within the reservoir has been observed over the period April through July 2013. Most of the constituent concentrations of concern have increased from July to November, but remain lower than those levels observed in April. Short term trends in dissolved iron indicate an overall steady increase of concentrations in well EVMW -1 S since October 2011, with a marked increase in this trend since October 2013. This increase has occurred independent of other constituent trends within this and other wells, as well as the apparent absence of dissolved iron in Robinson Lake and the Robinson Lake Seep. Based on initial interpretation of available data, this increase was attributed to potential microbial activity in saturated sediments immediately down gradient of the cut -off wall. Increasing sulfate, alkalinity, and TDS concentrations at EVMW -1S, in conjunction with the increasing dissolved iron concentrations may suggest some form of microbial reduction in this aquifer. To further investigate the iron detections, initial monitoring enhancements included the collection of Oxygen- Reduction Potential (ORP) and Dissolved Oxygen (DO). Field parameters indicate that pH increases slightly from Robinson Lake to Robinson Seep. pH values in EVMW - 1 S are similar to Robinson Seep while the deeper bedrock wells are slightly higher. While pH readings are similar, both Oxygen- Reduction Potential (ORP) and Dissolved Oxygen (DO) show distinct differences between Robinson Lake and Seep, and EVMW -1 S, with the latter indicating that a reducing environment exists. Additionally, alkalinity values are also higher in EVMW -1 S compared with samples upstream of the cutoff wall. Figure 2.3 is an iron stability diagram; utilizing data from September 2013, illustrating that Robinson Lake, Robinson Lake Seep, EVMW -1 D and EVMW -004 are within the stability field for iron hydroxide precipitation, while EVMW -1 S is in the ferrous iron field typical of dissolved iron. This presence of a lower redox value indicates a different chemical condition in EVMW -1 S compared with the other samples. This may be due to the proximity of saturated sediments immediately around the well, resulting in reducing conditions and thus allowing for the formation of dissolved iron. Tetra Tech, Inc. November 2013 12