2012-10-01_REVISION - M1977310 (10)

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JD -6 Mine Environmental Protection Plan 34 9.2 Regional Groundwater System 9.2.1 Hydrostratigraphic Units The regional groundwater system in the Paradox Basin consists of an upper Mesozoic sandstone aquifer and a lower Paleozoic carbonate aquifer (Topper, et al., 2003), separated by a thick sequence of Mesozoic - Upper Paleozoic confining beds and salt confining beds (including the Paradox Member of the Hermosa Formation). The hydrostratigraphic units of the Paradox Valley are shown in Table 20. The upper Mesozoic sandstone aquifer has been called the N- aquifer elsewhere on the Colorado Plateau, where the Navajo Sandstone and Wingate Sandstone are the main water bearing units in the N- aquifer over large portions of Utah and Arizona. Although the Navajo Sandstone and Wingate Sandstone, where present, are the main water - bearing units in the Mesozoic sandstone aquifer, these units are missing from portions of the Paradox Valley.In the Paradox Basin, the Mesozoic sandstone aquifer includes interbedded sandstones, siltstones, and shales which are bounded at the top by the Mancos Shale and on the bottom by mudstones, siltstones, and salt beds (Table 20). As reported by Golder (2009a), along the axis of the eastern Paradox Valley: "Although numerous reports [have been] published to address the regional hydrogeology, no published research has focused on the hydrogeology of the eastern Paradox Valley. Many of the regional studies name the Navajo Sandstone, Wingate Sandstone, and the Entrada Sandstone as important bedrock aquifers. However, these formations are either absent or not known to be water - bearing within the project study area, resulting in almost no relevant published information on groundwater ". -- Golder (2009a) The confining beds between the upper Mesozoic aquifer and the lower Paleozoic aquifer include the Dolores Formation (mudstone and fine- grained sandstone which are not water- bearing), Chinle Formation (interbedded shales and siltstones with minor fine- grained sandstone, which may be water- bearing in some places), Moenkopi Formation (mudstone interbedded with minor sandstone, which may yield small quantities of water), and Cutler Formation (fine grained sandstone interbedded with minor conglomerate and mudstone which may yield small quantities of water). The most significant confining beds are the Hermosa Formation, which consists of salt beds (including the Paradox Member). The confining units described in regional studies of the Paradox Basin were defined as aquifers in the hydrogeologic study of the Pinon Ridge site. Golder (2009a) found that groundwater occurs in two units: first at the base of the Chinle Formation at depths of 340 — 400 feet and second at the base of the Moenkopi Formation at depths of approximately 870 feet. For baseline hydrologic characterization in the valley floor, Golder drilled 35 boreholes, of which nine were completed as monitoring wells (MW- series), three were completed as production wells (PW- series), six were completed as observation wells near the production wells, and the remaining holes were groundwater exploratory boreholes (EX- series). The investigation revealed that no groundwater occurred in the alluvium, and "the only known groundwater occurrences within the study area are close to the contact between the Chinle and Moenkopi formations, and close to the contact between the Moenkopi and Hermosa formations" (Golder, 2009a). The Hermosa (salt) formations impeded the downward migration of groundwater flow. The lower Paleozoic carbonate aquifer includes the Leadville Limestone and other porous and permeable limestones and dolomites, which transmits saltwater through fractures. Groundwater in the lower Paleozoic carbonate aquifer is not suitable for human consumption (Topper, et al., 2003). 4148A.120927 Whetstone Associates •