2009-05-19_REVISION - M1977285 (28)

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Hydrogeological Evaluation of Sunday Mines Group Geologic and Hydrologic Conditions through the aquifer from adjacent areas where the sandstones are exposed at the surface. An area of submerged mine workings is shown on the south end of the section, south of the interpreted groundwater divide between Disappointment Valley to the South and Big Gypsum Valley to the north. This portion of the mine was excavated during a prior phase of mine development, and these workings are currently inaccessible. Groundwater in this area likely flows towards the south into Disappointment Valley. Communication of groundwater between the Salt Wash aquifer and the underlying Navajo aquifer is unlikely, because of the presence of the confining layer formed by the Summerville Formation shale. 2.4.1.2 St. Jude Mine An interpretive section of the St. Jude mine is shown in Figure 2-11. The affected area of the St. Jude mine is also located on shale of the Brushy Basin Member of the Morrison Formation, which would restrict potential recharge to groundwater in this area. Along the trace of the cross-section, significant direct recharge to groundwater is unlikely in the central and southern portions of the section; however recharge may occur to the Dakota Aquifer in the Big Gypsum Valley area north of the mine affected area. • Extensive faulting is present in the St. Jude mine area, which may lead to local compartmentalization of groundwater. The interpretive potentiometric surface in this area assumes that the water table has equilibrated over geologic time. Although the potentiomentric surface may have equilibrated over geologic time, it is likely that the confining units restrict groundwater flow between aquifers, unless the aquifers are directly adjacent as a result of fault displacements. 2.4.1.3 West Sunday Mine An interpretive hydrogeologic cross-section showing the West Sunday mine is included as Figure 2-12. The West Sunday Mine affected area is located east of the series of faults on the periphery of the Gypsum Valley anticline, and the development rock storage area overlies the Burro Canyon Formation. Where saturated, the Burro Canyon Formation is part of the Dakota aquifer, and there is potential for groundwater recharge to the Dakota aquifer from the mine affected area. The Salt Wash and Navajo aquifers are overlain by the Brushy Basin Member shale, and therefore significant recharge from the mine affected area to those aquifers is unlikely. The cross-section shows the West Sunday mine decline extending to the south towards Disappointment Valley. Groundwater is present in the base of the West Sunday mine, which is south of the groundwater divide between Disappointment Valley to the South and Big Gypsum Valley to the north. Therefore, groundwater in the underground mining area would be expected to flow southward into 0 Disappointment Valley. cm 2-14 TA64986-Denison Mines\Task Order 3 - DMO Sampling and Analysis Plan\Task 3.14 - Hydrogeology ReporlUinal sunday hydro reporNeATINAL Sunday Hydrogeology Repon.doc