2013-11-29_HYDROLOGY - M1977493 - Laserfiche WebLink

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Status Report, Robinson Seep Investigation Climax Molybdenum Company piezometers is similar to that observed in Robinson Lake samples, with the exception of the disparity in dissolved iron concentrations. A review of the stability diagram for iron (Figure 2.3) indicates that a decrease in Eh would result in the dissolution of iron minerals. Although accumulated sediment was removed from Robinson Lake to increase capacity, iron sediments may still be present that can provide a source of dissolved iron. Seepage from Robinson Lake appears to be entering both the unconsolidated sediments and bedrock before migrating towards the Robinson Lake seepage collection system. Once dissolved iron - bearing water surfaces and enters the Robinson seepage collection pond, it likely precipitates as ferrihydrite, based on the stability diagram (Figure 2.3). At sampling point EV- 21, located immediately below the cut -off wall, a water sample was collected from near surface, alluvial groundwater at a depth of 9 feet below ground surface and dissolved iron was not detected. However, dissolved iron is present at higher concentrations in monitoring well EVMWA S, which has a screened interval that extends into weathered bedrock seven feet below the bottom of the cut -off wall. These observations may suggest that iron is remaining in dissolved form in the deeper portions of the shallow alluvial /fractured bedrock hydrostratigraphic unit. Further support for the presence of dissolved iron through the fractured bedrock is provided by the presence of dissolved iron in the seep at EVS -006, a point of discharge for groundwater. In addition, iron bacteria, identified in other locations within the Climax property, may reduce ferric iron, which is insoluble, to ferrous iron, which is soluble. This mechanism could account for variations of dissolved iron in the groundwater. Tetra Tech, Inc November 2013 35