2012-10-05_REVISION - M1977342 (12)

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TR -18 to Permit M- 1977 -342 Environmental Protection Plan The Henderson Mill and adjacent facilities are constructed on the Idaho Springs Formation and Silver Plume Granite. The tailings storage area is located on the western slope of the Williams Fork River Valley and is constructed primarily on the Troublesome Formation although some areas overlay glacial and alluvial deposits. 8.3. Site Hydrogeology 8.3.1. Mine Groundwater occurrence at the Henderson Mine is primarily limited to a thin, well- defined lens of alluvium which is bounded on all sides by the Precambrian Silver Plume Granite Formation. Groundwater occurrence within the Precambrian Silver Plume Granite is limited. The low permeability of the granite is evident in the Mine workings where groundwater inflow has remained unchanged in the 36 year life of the Henderson operation. Additionally, because groundwater is pumped from the Mine workings to the surface for treatment, increased exposure of sulfides to oxidation through the underground mining activities does not impact groundwater quality near the underground workings. Groundwater flow direction within the alluvium generally flows from the upper end of the drainage to the lower end. Upgradient of the confluence with Woods Creek, the alluvium pinches out and groundwater is forced to surface into the West Fork of Clear Creek. See Table 12 for current Mine monitoring locations. 8.3.2. Mill Hydrogeologic conditions at the Henderson Mill were investigated by advancing seven borings into the alluvium and weathered bedrock in the fall of 1993. Of the seven borings, six borings were completed as monitoring wells (designated as GW -2 through GW -7). Based on the site geology, boring logs and observation of groundwater levels, three primary hydrostratigraphic units can be identified at the Henderson Mill site: 1) unconsolidated glacial and alluvial deposits, 2) the Troublesome Formation, and 3) the Idaho Springs Formation and Silver Plume Granite. The following sections summarize the hydraulic characteristics of each hydrostratigraphic unit. Within and downgradient of the tailings storage facility, groundwater primarily occurs within the glacial and alluvial deposits, while little groundwater flow is present in the Troublesome Formation, Idaho Springs Formation and Silver Plume Granite. Glacial and Alluvial Materials Field data from test pits and borings advanced prior to and after tailings deposition (Woodward - Clyde, 1983, Hydrokenetics, 1993) show that the groundwater levels within the glacial and alluvial materials are hydraulically connected. Since both the glacial and alluvial materials consist of gravels, sands and clay deposits, and are hydraulically connected, these materials are considered a single hydrostratigraphic unit. The groundwater levels measured within the glacial and alluvial materials vary considerably across the site. When correlated to geologic data, it is evident that the variability of the groundwater levels can be attributed to multiple perched water zones present within pervious layers which overlay impervious layers. Therefore, the groundwater levels and hydraulic properties of this hydrostratigraphic unit are expected to be highly variable. Troublesome Formation The Troublesome Formation has been documented to contain discontinuous sands, gravels, lensed Climax Molybdenum Company Henderson Operations October, 2012 Page 26 of 40