2012-09-28_REVISION - M1977307 (23)

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(8) Groundwater Information This section of the permit discusses the potential for environmental exposure of uranium related constituents by the transport of groundwater. To evaluate this potential, the site geology, hydrology, geochemistry of the deposits, and possible groundwater migration pathways are discussed. 8.1 Area Geology The CM -25 mine site (see Figure T1) is located on the northeastern flank of the Paradox Valley anticline. Geologic units in the mine area dip to the northeast at approximately 0.05 (ft /ft) toward the San Miguel River valley. According to Cater et al. (1955), most of the geologic units of interest thin as they approach the flanks of the Paradox Valley anticline. Remnants of the Burro Canyon Formation form highlands located to the southwest of the site. Directly underlying the Burro Canyon Formation is the Brushy Basin Member of the Morrison - the location of the mine portal. This geologic unit is comprised of mostly bentonitic shale but contains some sandstone and conglomeritic lenses (Cater et. al., 1955). This unit's stratigraphy plays an important role in understanding groundwater transport in this region. Bentonite is classified as a montmorillonite clay mineral formed by the weathering of volcanic ash deposited during the Jurassic Period, and characterized by a three layer crystal lattice that exhibits significant swelling on wetting. These types of shales are not only extremely impermeable to water movement but also exhibit high cation exchange capacities that adsorb and prevent the migration of dissolved constituents related to uranium deposits. These same bentonitic shales are found in the Salt Wash Member and restrict the downward movement of dissolved minerals. Table 1: Summary of Selected Properties of Solid Phase Layered Silicates (After Bohn et al., 1979) Component Mineral Type (silicate layers) Cation Exchange Capacity Kaolinite 1:1 1 -10 me /100g Montmorillonite 2:1 80 -120 me /100g The Salt Wash Member of the Morrison Formation is the mineralized zone where uranium and vanadium ores are found. The unit is comprised of interbedded sandstones, shales and mudstones. The sandstone units, particularly the upper sandstone, are mineralized. These sandstones tend to be fine grain and well cemented. Underlying the Morrison Formation is the Summerville Formation that consists of shales and mudstones, and is usually considered an aquitard in the area. The Entrada /Carmel Formations undivided (mapped as the same unit) underlie the Summerville aquitard and provide the first possible aquifers in the area. While the Carmel contains mudstone layers, the Entrada Formations consist dominantly of fine -grain aeolian sandstones. O'Connor Design Group Inc. T - 9