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Section 8 <br />Groundwater Quality <br />including arsenic, molybdenum and selenium. The presence of these elements in <br />groundwater in close proximity to the underground workings is thought to be related <br />to naturally occurring enrichments of metals, and development of equilibrium <br />conditions between the ore minerals and the local groundwater. As discussed <br />previously, underground mine development activities such as tunneling, excavating <br />rock, and ventilating the underground workings may also affect the concentration of <br />the naturally enriched elements in groundwater. The extent of effects to groundwater <br />quality attributable to naturally occurring geochemical conditions versus <br />anthropogenic effects related to underground mine development is unknown. <br />The location and orientation of the monitoring wells were initially designed to also <br />evaluate the persistence of any mining-related affects to groundwater quality in the <br />down-gradient direction. This was based on an interpretation that the hydraulic <br />gradient in the area was coincident with topography (i.e. a generally southward <br />gradient). However, a mine dewatering pump is located approximately 200 feet west <br />of the well nest within the lower portion of the West Sunday mine workings. <br />Therefore, it is unlikely that any of the wells are measuring direct effects of the <br />underground workings; because a local cone of depression is likely present with an <br />inward gradient surrounding the dewatered mine workings. Therefore the direction <br />of groundwater flow in this area is into the underground workings on both the north <br />side where monitoring well WS-4 is located and the south side of the workings where <br />monitoring wells WS-1, WS-2, and WS-3 are located. <br />• The groundwater quality data show a marked difference in samples from monitoring <br />well WS-4 as compared to WS-1, WS-2 and WS-3. The locations of the underground <br />monitoring wells are shown in Figure 8-1. The collars of the wells are located very <br />close to the base of the Top Rim sandstone, and the rock units dip gently towards the <br />south. The collar location and the downward angle of drill hole WS-4 causes the <br />screened interval to be located in an intermediate siltstone and shale of the Salt Wash <br />member rather than the Top Rim Sandstone. The screened interval of monitoring <br />wells located on the south side of the underground mine workings are located in the <br />Top Rim Sandstone, which hosts the uranium-vanadium mineralization as well as the <br />other associated elemental enrichments. <br />Groundwater quality data shown in Table 8-2 display a marked difference between <br />wells screened in the Top Rim Sandstone (WS-1, WS-2 and WS-3) and the well <br />screened in the underlying intermediate siltstones and shales (WS-4). Water quality <br />from WS-4 is very saline with TDS of 6,990 mg/L and contains high concentrations of <br />chloride, sodium and sulfate. This water also contains relatively lower concentrations <br />of the ore metals uranium and vanadium and associated elements such as arsenic, <br />molybdenum and selenium. The wells screened in the Top Rim sandstone show <br />relatively higher concentrations of the ore metals and associated elements. This water <br />is also relatively saline with TDS ranging from 1,300 to 1,710 mg/L, but the water <br />contains much lower concentrations of sodium, chloride and sulfate. <br />. The groundwater data collected to date supports the hydrogeological framework for <br />the area developed by Weir et al. (1983), which describes groundwater flow moving <br />8-4 <br />T:164986-Denison MinesUask Order 4 - EPP Sunday Mines Group1FINAL EPP%FINAL - Environmental Protection Plan Sunday Mines.doc