2014-11-18_HYDROLOGY - M1977493 - Laserfiche WebLink

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Status Report; Robinson Seep Investigation Climax Molybdenum Company 5.0 SHORT AND LONG TERM TRENDS A series of graphs presenting short term (between January 2011 and September 2014) and long term (between January 1996 and September 2014) constituent trends (iron, manganese, sulfate, and TDS) in groundwater and seepage water quality are presented as Figure 5 and Figure 6. All wells were sampled quarterly through March 2013 and, beginning in April 2013, the sampling frequency of monitoring wells EVMW-1S, EVMW-1D and EVMW -004 increased to monthly. Additionally, monthly sampling of Robinson Lake and Robinson Lake Seep has been conducted since April 2013. Also, monthly monitoring (as weather and access conditions allowed) of EVS- 006 (surface location near Robinson Lake Seep) and the toe drain piezometers, RL -17 and PZ- 04 started in August 2014. Monitoring well EVMW was replaced by monitoring well EVMW -1 S (constructed approximately 10 feet deeper). Below is a brief discussion related to the relevant constituents of interest based on these historical trends. 5.1 Sulfate and Total Dissolved Solids Sulfate concentrations mimic TDS concentrations at all sampling locations. Despite the increase in sulfate concentrations observed in monitoring well EVMW-1S in March 2013 and April 2013 (Figure 5), a decreasing trend has been observed since and sulfate concentrations have remained lower than maximum observed concentrations in historic monitoring well EVMW. Sulfate concentrations in Robinson Lake and Robinson Lake Seep (Figure 6) remain above that of monitoring well EVMW -1 S. The decrease in sulfate and TDS from May to July 2013 and again in June through August 2014 in Robinson Lake is likely seasonal fluctuation due to dilution following snow melt. Sulfate and TDS concentrations observed in the two piezometers (RL -17 and PZ -04) located at the toe of Robinson Lake Dam show consistently similar values to Robinson Lake suggesting infiltration and seepage through and /or below the dam (Figure 5). Sulfate and TDS concentrations in the two piezometers have decreased since June 2013. 5.2 Iron Observed dissolved iron concentrations do not follow the concentration trends of other constituents. Dissolved iron concentrations appear to steadily increase in monitoring well EVMW - 1S since October 2011, recording a maximum value in November 2013 (4.71 mg /L). Concentrations have since decreased and, as of September 2014, are at 1.77 mg /L. Based on long -term observations, concentrations of iron (observed in the long term graphs; Figure 5) have fluctuated significantly, possibly due to small shifts in pH and Eh (redox potential) that can lead to changes in iron speciation between insoluble ferric and soluble ferrous species. The phase - stability diagram (Figure 7) shows that groundwater conditions lay near this phase shift boundary between these species and thus susceptible to minor changes in Eh and /or pH. With the exception of EVMW -1 S, EVMW -004, and RL -17, all sampling sites lay well within the stability field for insoluble iron hydroxide, while PZ -04 lies very near this transitional boundary. The lower redox condition indicates differences in the chemical reducing environment in EVMW- 1S and the two piezometers. The distinct shift in EVMW -004 into the soluble ferrous field between August and September 2014 sampling is unclear, and may represent an anomaly in the September 2014 data. Total iron concentrations in EVMW-1S (total iron concentration is 2.00 mg /L) show that the majority of the iron occurs as the soluble ferrous species. Tetra Tech November 2014