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/~ <br />Because of its thickness (90 feet) and low permeability (0.01 ft/yr, 1.0 x 10'8 cm/sec), the <br />Summerville Fottnation is the controlling ltydrogeologic unit beneath Club Mesa. The low <br />permeability of this unit is confirmed by an analysis of past seepage from Tailings Piles 1-2 and 3 <br />and a raffinate spray area on Club Mesa. The tailings disposal system and spray system have <br />existed on Club Mesa since early 1956 and received tailings material until December 1984. Water <br />balance calculations indicate that 250 million gallons of raffinate may have seeped from these <br />tailings ponds and from the spraying system. This seepage migrated downward to the Selt <br />Wash/Summerville contact where the low permeability (0.01 ft/yr) shales of the Summerville <br />Formation stopped the downward migration. Consequently, a saturated zone developed in the Salt <br />Wash directly above the Summerville. This saturated zone, approximately 20 feet (6 meters) in <br />thickness, has not penetrated the Summerville Formation and has not caused mounding of water <br />into the tailings piles. Additionally, the perched zone has not impacted water quality in the Wirigatc <br />Formation as shown by the static concentrations of radionuclides and heavy metals in the <br />Kayenta/W ingate aquifer beneath Club Mesa. No changes in basic ground water chemistry have <br />been observed in the wells completed in the Kayen[a/Wingate aquifer. <br />Geochemical attenuation of potential leachate would occur as liquid is transmitted through the <br />bedrock formations. The Salt Wash Member is about 120 feet thick with sandstones totaling about <br />90 feet in thickness and containing calcium carbonate cement. Geochemically, this calcium <br />carbonate would buffer any solutions exiting the repository. The Salt Wash Member also contains <br />about 30 feet of shale. 'Tests on Salt Wash shales indicate Cation Exchange Capacity (CEC) values <br />ranging from 9.9 meq/100 g to 32 meq/100 g artd a pH of approximately R.4. These tests.indicate <br />that the Salt Wash shales would attenuate contaminants contained in repository seepage. <br />Beneath Club Mesa the direction of groundwater flow is to the northwest with a gradient of 0.01 B. <br />The linear velocity of the groundwater, using a gradient of 0.018, a hydraulic conductivity of 0.12 <br />ft/day (4Z x 10'5 cm/sec) and an estimated effective porosity of 10 percent, is calculated to be about <br />0.022 ft /day (7,7 x 10~ emisec) or about 8 fdyr. Phis calculation indicates that the groundwater <br />flow velocities are low and that it would take more than one thousand years for groundwater <br />beneath the repositories to discharge.into the San Miguel River. <br />5.4 San Mi uel River Valley HvdroEeoloey <br />Groundwater in the San Miguel River Valley occurs in the Kayenta and Wingate Formations. <br />Perched raffinate liquids also occur in the Quaternary deposits in the Club Ranch Ponds area. The <br />groundwater remediation program in the Club Ranch Ponds area has removed substantial quantities <br />of contaminated liquid from the Kayenta/Wingate ayuifer and terrace gravel. Contaminated terrace <br />materials have also been removed from the Club Ranch Ponds. <br />Surface impoundments are located along a terrace north of the San Miguel River. The original <br />surface impoundments were unlined pits that were historically (1960 to 1984) used to manage <br />process water and tailings liquids. T.eakage through the ponds has adversely impacted groundwater <br />in the underlying Kayenta Formation. A groundwater plume of elevated I'DS has been identified <br />and largely delineated (both horizontally and vertically) through. an extensive network of <br />monitoring wells. The ponds were emptied during the late 1980s and the bottom sludges and salCs <br />were removed. <br />Umetco-Uravan DA February 11, 2000 15 <br />6T/ZT 39dd ti'lUf1N dbOD ?J3110D L0ZLb980L6 9Z~TT L00Z/9T/90 <br />