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Hydrogeological Evaluation of Van 4 Mine <br />Geologic and Hydrogeologic Conditions <br />• Monogram Mesa (Cater 1954). At its closest point, the nearest fault is approximately <br />4,000 ft. from the Van 4 Mine affected area. The fault, which intersects the Salt Wash <br />Member, is generally parallel to the ridgeline and located along the slope 500 to 1,000 <br />feet northeast of the topographic divide. There are no intermittent or perennial <br />streams mapped as crossing the fault trace indicating that the only surface water <br />expected to cross the fault would occur as overland flow during occasional rain <br />events or as snowmelt. <br />2.3.3 Direction and Rate of Groundwater Flow <br />Direction of Groundwater Flow <br />On a large scale, groundwater flow in the Van 4 area is towards the Dolores River <br />(Weir et at. 1983). On a smaller scale, topographic variations and structural geology <br />cause local modifications in groundwater flow paths. Groundwater flow in the <br />vicinity of Van 4 Mine is likely towards the southwest following the dip of the <br />hydrogeologic units and the topography of the Bull Canyon area. This interpretation <br />is supported by the presence of Fawn Springs southwest of the Van 4 mine and the <br />intermittent stream within Bull Canyon. Ultimately, this groundwater would be <br />expected to reach the Dolores River. <br />Rate of Groundwater Flow <br />The rate of groundwater flow is discussed below in terms of the average linear <br />groundwater velocity which is a function of the hydraulic gradient, the effective <br />• porosity, and the hydraulic conductivity (Dominico and Schwartz, 1990). Data were <br />presented in Exhibit 2-1 for estimated hydraulic conductivity based on the intrinsic <br />permeability analyses of Jobim (1962). This section presents estimates of the rate of <br />groundwater flow based on the estimates of Jobim (1962), an estimate of hydraulic <br />gradient, and estimated values for the porosity of the Top Rim sandstone. <br />The hydraulic gradient in the area of the Van 4 mine is estimated to be approximately <br />the same as the dip of the hydrogeologic units, because groundwater flow is primarily <br />lateral as a result of the inter-layered aquifers and confining units. This is an <br />approximation, but it is adequate for the purposes of this estimate given the level of <br />detail of available information. Based on a three-point solution in the upper Bull <br />Canyon area using the geologic contacts of Cater (1954), the gradient of the <br />hydrogeologic unit is approximately 0.0125 towards the southwest. <br />Groundwater is present at least seasonally in the general vicinity of the Van 4 Mine <br />area within the Salt Wash Member of the Morrison Formation, as evidenced by the <br />presence of Fawn Springs. A hydraulic conductivity of 1.1 x 104 centimeter per <br />second (cm/sec.) is estimated for this unit based on intrinsic permeability values <br />provided by Jobim (1962). <br />The porosity of the sandstone is unknown; however, Freeze and Cherry (1979) <br />describe an empirical relationship developed by Chilinger (1963), which allows <br />estimation of sandstone porosity based on the textural classification and the intrinsic <br />permeability. Craig et al. (1955) describe the texture of Salt Wash Member sandstone <br />V M?? 2-11 <br />draft final van 4 hydm9sology report.doc