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Hydrogeological Evaluation of Sunday Mines Group <br />Geologic and Hydrologic Conditions <br />Big Gypsum Valley is located within the central portion of the Dolores River Basin, in <br />an area that is among the most and portions of the basin based on analyses presented <br />by Weir et al. (1983). An ephemeral drainage is present within Big Gypsum Valley <br />approximately 1/2 mile north of the Sunday Mines Group. Surface water was not <br />observed in this drainage during site visits in June, August and December, 2008, but <br />the geomorphology of the drainage suggests that ephemeral flows do occur <br />occasionally in response to snow melt or large precipitation events. Intermittent flows <br />may also be present in the lower portion of the drainage. <br />Surface water was not observed during the 2008 site visits, but runoff within the <br />affected areas may occur on an infrequent basis. Runoff occurring within the affected <br />areas of the mines is contained within existing surface water retention basins. These <br />basins are designed to retain 100 percent of runoff generated during a 100 year-24 <br />hour storm event (CDM 2008a; 2008b; 2008c; 2008d). The retained water is generally <br />not discharged from the mine site, because the water evaporates rapidly in the and <br />environment present within Big Gypsum Valley. <br />2.3.3 Groundwater <br />The regional hydrogeology of the Dolores River Basin has been investigated by the US <br />Geological Survey (Weir et al 1983), from which much of this section is derived. Weir <br />et al. (1983) identify two major hydrogeologic units in the basin: an upper unit and a <br />lower unit. The lower unit is hosted by Precambrian and Paleozoic crystalline and <br />metamorphic rocks. This unit is separated from the upper unit by the rock salt of the <br />Paradox Member of the Hermosa Formation. The Paradox Member is essentially <br />impervious to fluid flow. Groundwater within the lower unit is saline, and oil and gas <br />deposits are locally present. The upper unit includes all rock stratigraphically above <br />the Paradox Member, including the clastic sedimentary rocks outcropping in the <br />Sunday Mines Group area. <br />This investigation considers only the upper unit, because it is pertinent to potential <br />groundwater uses in the area and to potential effects of mining on groundwater. The <br />upper unit includes a series of predominantly sandstone aquifers, which are <br />separated by confining units composed dominantly of shale. Weir et. al (1983) <br />identified the following aquifers within the upper unit: <br />¦ Mesozoic sandstone aquifer: <br />¦ Tertiary to Upper Cretaceous aquifer; and <br />¦ Alluvial aquifers. <br />Table 2-5 provides a summary of the hydrogeological characteristics of the aquifers. <br />The Mesozoic sandstone aquifer includes the saturated portions of the Chinle Formation, <br />Wingate Sandstone, the Navajo Sandstone, and the Salt Wash Member of the <br />Morrison Formation. Each of these units is composed dominantly of sandstone. The <br />cm 2-9 <br />T:\64986-Denison Mines\Task Order 3 - DMO Sampling and Analysis Plan\Task 3.14 - Hydrogeology Reporlfinal sunday hydro reporNeATINAL Sunday Hydrogeology Report.doc