2013-11-29_HYDROLOGY - M1977493 - Laserfiche WebLink

Home Browse Search

Status Report, Robinson Seep Investigation Climax Molybdenum Company 4.0 CONCLUSIONS AND RECOMMENDATIONS In summary, with the exception of iron and manganese, the short-term trends of increasing concentrations of several indicator parameters at monitoring well EVMWAS appear to have stabilized at relatively consistent levels, and may even be decreasing. From a long -term perspective, concentrations of primary water quality indicators, including sulfate and TDS at EVMW -1S remain significantly lower than previously observed maximum concentrations at the original monitoring well EVMW. Correlations between Robinson Lake refilling, seepage flow rates, and water chemistry in the lake and seepage collection pond relative to shallow groundwater quality at EVMW -1 S are suggested through the current data evaluation efforts, however additional monitoring may be necessary to confirm these observed trends and correlations. Water quality within the deep bedrock monitoring wells, including EVMW -1 D and EVMW -004, has remained stable. Based on somewhat limited data on the quality of water stored in Robinson Lake, it is likely that re- filling the reservoir following sediment removal and the start-up of operations at the mine may have contributed to a spike in constituent concentrations within the reservoir. This condition coupled with the increasing water level in the lake may have been the primary causes behind the increasing trends observed, over a relatively short period of time, in downgradient groundwater. Recent data from Robinson Lake indicate lower constituent concentrations compared to those observed in April. 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. As discussed in Section 3.3, a likely flow path to the east is considered to be fracture flow through the Minturn Formation. To the west, fractures within the interior of the Elk Mountain Porphyry provide more dispersed flow. Qualitatively it is known that seepage collected through the Robinson Lake Dam toe drain moves relatively quickly to the seepage collection pond through a surface water flow path, and flow through the shallow alluvial aquifer is likely greater than flow through the bedrock system. Based on the results of these additional investigations and the observed concentrations and trends in concentration at monitoring well EVMW -1 S, the cut -off wall and seepage collection system appear to effectively capture surface and shallow, alluvial groundwater flow through the area, however, there is likely some small component of flow through fractured bedrock moving around or under the cut -off wall. Recent stable and decreasing trends for most water quality indicator parameters in the Robinson Lake Seep and EVMW -1 S, and the delineation of the extent of seepage which identified a narrowing and closure of the seepage path north of the cutoff wall, indicate no immediate threat to Eagle Park Reservoir water quality. As a result of the particulars of iron chemistry, 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. For the groundwater conditions identified at the site, iron appears to transfer between these two states. As such, dissolved iron concentrations should not be relied on to evaluate flow pathways and /or source identification. Based on the observed fluctuations of manganese concentrations in deep bedrock wells, a secondary source of manganese, unrelated to seepage from Robinson Lake, may also influence groundwater quality at well EVMW -1 S. The continuation of monthly monitoring of well EVMWAS, as well as monthly sampling of Robinson Lake and Robinson Lake Seep is recommended through at least the end of 2014, as Tetra Tech, Inc. November 2013 40