2009-05-19_REVISION - M1977285 (28)

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Hydrogeological Evaluation of Sunday Mines Group Geologic and Hydrologic Conditions 2.4.1.4 Topaz Mine An interpretive hydrogeological cross-section showing the Topaz Mine affected area is included as Figure 2-13. The Topaz Mine affected area overlies the Salt Wash Member of the Morrison Formation. Based on the interpretive hydrogeological framework, this portion of the Salt Wash Member may be saturated, and therefore the Salt Wash aquifer could be present in this area. The Salt Wash Member in this area is truncated to the north by the Brushy Basin Member of the Morrison Formation, which is a confining layer of low permeability shale. There is potential for infiltration from the mine affected area to recharge the Salt Wash aquifer. Flow of this water towards the north into Big Gypsum Valley would be unlikely based on the fault contact juxtaposing the Salt Wash Member against the Brushy Basin Member. However, the potential for westerly flow towards the Dolores River is plausible considering the surficial geology shown previously as Figure 2-1. The Topaz mine decline is also shown on the section. The decline extends toward the south to access mining areas located south of the groundwater divide between Disappointment Valley to the south and Big Gypsum Valley to the north. This orientation is similar that that observed at other Sunday Group Mines where groundwater flow from areas of underground mining is interpreted to be south into Disappointment Valley. . 2.4.2 Potentially Water-Bearing Fracture Zones Potentially water-bearing fracture zones range in size from very small-scale features that primarily influence only the permeability of the rock, to large-scale features that extend for miles and could potentially influence groundwater-flow paths. This section focuses on identifying large scale fracture zones associated with the faults mapped by Cater (1955), because these features have a potential to influence groundwater flow at a larger scale. These faults are shown on the geological map included previously as Figure 2-1. The effect of major fracture zones on groundwater flow is dependent on the rheological characteristics of the displaced rock units. Fault zones within relatively brittle rocks such as massive sandstone are more likely to include zones of closely spaced and interconnected fractures forming a fracture zone. The fracture zones may have a marked influence of hydrogeologic characteristics. Fault zones within shale units such as the bentonite-rich Brushy Basin Member are less likely to influence groundwater, because the shale deforms plastically reducing permeability of the fault zone as compared to the more brittle units (Weir et al. 1983). The interpretive hydrogeologic framework cross-sections presented previously (Figures 2-10 to 2-13) provide information regarding the relative permeability of the fault zones. A hatched pattern identifies portions of faults where two shale units are juxtaposed on either side of the fault. These are areas where the fault is less likely to • have a major effect on groundwater flow. Fault zones that are not hatched identify cm 2-15 T:\84988-Denison Mines\Task Order 3 - OMO Sampling and Analysis Plan\Task 3.14 - Hydrogeology ReporMUnal sunday hydro reporNexITINAL Sunday Hydrogeology Report.doc