2009-05-19_REVISION - M1981021 (36)

Home Browse Search

Section 7 Groundwater Information likely that groundwater conditions change from unconfined to confined as the depth of the aquifers increase along the northern limb of the Disappointment Valley syncline. The following sections provide additional detail regarding the local groundwater framework in direct vicinity of the various mines within the Sunday Group. The interpreted depth to groundwater is based on observations of groundwater occurring within the Sunday, West Sunday and Topaz mine at a depth of approximately 585 feet (ft), and an assumption that the Dolores River is the local base point for the hydrogeologic system. 7.2.1 Sunday and Carnation Mines An interpretive hydrogeologic framework for the Sunday and Carnation mines is presented in Figure 7-6. This section shows the approximate location of the affected area of the mines, the underground workings, the major hydrogeologic units, major faults and the interpreted potentiometric surface. The affected area of the mines lies above the shale confining layer of the Brushy Basin Member of the Morrison Formation. The potential for significant infiltration through this layer and recharge to groundwater is low in this area. Groundwater is interpreted to be present within the Salt Wash and Navajo aquifers at depth beneath the confining layers of the Brushy Basin Member. Recharge to these aquifers is likely by lateral flow 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. 7.2.2 St. Jude Mine An interpretive section of the St. Jude mine is shown in Figure 7-7. 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 potentiometric surface may have equilibrated over geologic time, it is likely that the 7-4 TA64966-Denison Mines\Task Order 4 - EPP Sunday Mines GroupTINAL EPPTINAL - Environmental Protection Plan Sunday Mines.doc