2009-06-08_REVISION - M1997032 (25)

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Hydrogeological Evaluation of Van 4 Mine Geologic and Hydrogeologlc Conditions generally towards the southwestJ. An ephemeral drainage is present west of the mine, which flows generally towards tL south into Bull Canyon. Surface water was not observed during the August 2008 site visit to the mine, but runoff within the affected areas i within the affected areas of the r detention basins. These basins ai during a 100 year-24 hour storm not discharged from the mine sii environment present in the area 2.2.3 Groundwater The regional hydrogeology of tfi Geological Survey (Weir et al. 1!: derived. Weir et al. (1983) identi. upper unit and a lower unit. The crystalline and metamorphic roc rock salt of the Paradox Membei impervious to fluid flow. Groun deposits are locally present. The pper unit includes all rock units stratigraphically above the Paradox Member, tding the clastic sedimentary rocks outcropping in ay occur on an infrequent basis. Runoff occurring nes is contained within existing surface water designed to retain 100 percent of runoff generated vent (CDM 2008). The retained water is generally because the water evaporates rapidly in the and the mine. Dolores River Basin has been investigated by the US 33), from which much of the following discussion is y two major hydrogeologic units in the basin: an lower unit is hosted by Precambrian and Paleozoic :s. This unit is separated from the upper unit by the of the Hermosa Formation, which is essentially lwater within the lower unit is saline, and oil and gas the Van 4 Mine area. This report focuses on the upper uses in the area and to potential includes a series of predominan confining units composed domi following aquifers within the ul iit, because it is pertinent to potential groundwater ects of mining on groundwater. The upper unit sandstone aquifers, which are separated by fitly of shale. Weir et al. (1983) identified the r unit: • ¦ Mesozoic sandstone aquifer: ¦ Tertiary to Upper Cretaceous ¦ Alluvial aquifers. er; and Table 2-5 provides a summary of the hydrogeological characteristics of the rock units comprising the Mesozoic and Tertiary to Upper Cretaceous aquifers. The Mesozoic sandstone aquifer in 'ludes the saturated portions of the Chinle Formation, the Wingate Sandstone, the Navajo Sandstone, and the Salt Wash Member of the Morrison Formation as well as other less permeable units. The Chinle Formation, the Wingate Sandstone, the Navajo Sandstone, and the Salt Wash Member of the Morrison Formation are composed dominantly of sandstone. The Mesozoic Sandstone Aquifer is underlain by rock salt of the Paradox Member and is overlain by bentonitic shale of the Morrison Formation, which are both confining layers. ?0 2-6 • dreft final van 4 hyd.9-1.9y r port:jo.