2013-11-29_HYDROLOGY - M1977493 - Laserfiche WebLink

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Status Report Robinson Seep Investigation Climax Molybdenum Company 2.3 Long Term Trends A series of graphs presenting long term constituent trends in groundwater and seepage water quality are presented as Figure 2.7. Similar to the short term analysis, concentrations of sulfate, TDS, and dissolved manganese, iron, calcium, magnesium, and molybdenum are presented in individual graphs. Because zinc concentrations in EVMW-1S are typically non - detect this data has not been presented in this section. As referenced previously, monitoring well EVMW was decommissioned and replaced by EVMW-1S (although EVMWA S was completed approximately 10 feet deeper into the underlying, weathered bedrock); together these wells provide a record back to 1996. Water quality data over the entire period of record for well EVMW show low to non - detectable concentrations of primary constituents, with the exception of manganese, iron, molybdenum, TDS and sulfate. Overall these five constituents have downward trends at EVMW over this period ending in November 2011. Concentrations of dissolved iron over this period fluctuated widely over relatively short time intervals which differs from the observed trends of other constituents. Relatively small shifts in pH and redox potential (Eh) can cause significant changes in iron speciation between the insoluble ferric (Fe 3+) and the soluble ferrous (Fe 2+) forms of iron. These fluctuations of dissolved iron in EVMW are likely the results in minor shifts in pH and /or Eh, suggesting that dissolved iron may not be a good indicator of overall water quality due to fluctuating dissolution /precipitation reactions. Analytical results from bedrock monitoring well EVMW-1D and the Robinson Lake Seep are also presented on the series of graphs in Figure 2.7. In addition, Robinson Lake Water Levels are presented to show the relationship of the timing of filling Robinson Lake to the changes in downgradient groundwater quality at EVMW -1 S. As stated in the preceding section, the controlled filling of Robinson Lake began in April of 2011 and the increasing trend in concentrations of the above cited constituents (with the exception of dissolved iron and perhaps to some degree dissolved manganese) 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 -1S. While a short-term increase in constituent concentrations at EVMW -1S has been observed; over the period of record presented in Figure 2.7 (beginning in 1996), concentrations of sulfate, TDS, manganese, calcium and magnesium remain lower than previously observed maximum concentrations. Molybdenum concentrations in shallow groundwater below the Robinson Lake seepage collection system continue to follow a decreasing trend over the entire period of record, irrespective of the potential influences of Robinson Lake. These general observations related to long -term trends do not hold when evaluating the measured concentrations of dissolved iron and manganese at well EVMW -1S. Dissolved iron concentrations appear to have been increasing steadily over a longer time frame than most other constituents and surpassed previously observed concentrations in June 2012. Dissolved manganese concentrations have been steadily approaching historic maximum levels, and while they appeared to have been stabilizing through September, the sharp increase in October and November are inconsistent with the changes in other indicator parameters. It appears that there are other conditions or factors potentially unrelated to upgradient chemistry and hydrologic conditions that may be influencing iron and manganese at EVMW-1S. It is currently unclear Tetra Tech, Inc November 2013 21