2012-10-01_REVISION - M1977311 (12)

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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 SR -13A mine site is located on the southwestern limb of the Disappointment Valley syncline near the axis of the syncline (see Figure TI). Geologic units in the mine area steeply dip to the northeast at approximately 0.15 (ft/ft) toward the axis of the syncline. Remnants of the Burro Canyon Formation form highlands to the east and directly adjacent to the site. Directly underlying the Burro Canyon Formation is the Brushy Basin Member of the Morrison Formation. This geologic unit is comprised of mostly bentonitic shale but contains some sandstone and conglomeritic lenses (Cater, 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 CEC 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. The upper sandstone unit of the Salt Wash is where the mine portal is located. 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 eolian sandstones. Underlying the Entrada/Carmel Formation is the Navajo Sandstone, another eolian fine grain clean quartz sandstone. The siliceous cemented Kayenta Formation, commonly a resistant bench - forming unit, consists of sandstones, mudstones and conglomerates that O'Connor Design Group Inc. T - 9