2021-08-17_REVISION - M1977342 (4)

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TR-34 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 TSF 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 in the vicinity of the Henderson Mine surface facilities occurs within colluvial deposits near and adjacent to the West Fork of Clear Creek. Groundwater flow is generally from west to east from the upper end of the drainage to the lower end. Upgradient of the confluence with Woods Creek,the colluvium pinches out and groundwater is forced to surface into the West Fork of Clear Creek(Climax Molybdenum Company, 2012).The Precambrian Silver Plume Granite is generally impermeable and non-water bearing (Ajax/Clear Creek Associates, 2013). 8.3.2. Mill Three primary hydrostratigraphic units have been 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 TSF, 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. The primary groundwater flow path is generally from southwest and towards the Williams Fork River to the northeast. Glacial and Alluvial Materials Field data from test pits and borings advanced prior to and after tailing 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 clays, and silts underlain by semi-consolidated siltstones, sandstones, conglomerates and claystones. Data from test pits and borings within the Troublesome indicate that the presence of groundwater within this unit is highly variable. A site study conducted by Woodward-Clyde (1983) concluded that this formation is not considered to be a continuous aquifer because of the limited extent of the sand layers in the formation which would preclude significant groundwater flow. Idaho Springs Formation and Silver Plume Granite Climax Molybdenum Company August,2021 Henderson Operations Page 31 of 45