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TECHNICAL REPORTS

Heavy Metals in the Environment

Distribution of Uranium Contamination in Weathered Fractured Saprolite/Shale and Ground Water

^a Environmental Engineering Research Centre, School of Planning, Architecture, and Civil Engineering, Queen's University of Belfast, Belfast BT9 5AG, Northern Ireland, UK
^b Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN, USA 37831-6038
^c Faculty of Earth Systems and Environmental Sciences, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju, 500-757, KR

^* Corresponding author (watsondb{at}ornl.gov)

Received for publication April 8, 2005. The objective of this study was to determine how structure, stratigraphy, and weathering influence fate and transport of contaminants (particularly U) in the ground water and geologic material at the Department of Energy (DOE) Environmental Remediation Sciences Department (ERSD) Field Research Center (FRC). Several cores were collected near four former unlined adjoining waste disposal ponds. The cores were collected, described, analyzed for U, and compared with ground water geochemistry from surrounding multilevel wells. At some locations, acidic U-contaminated ground water was found to preferentially flow in small remnant fractures weathering the surrounding shale (nitric acid extractable U [U_NA] usually < 50 mg kg^–1) into thin (<25 cm) Fe oxide-rich clayey seams that retain U (U_NA 239 to 375 mg kg^–1). However, greatest contaminant transport occurs in a 2 to 3 m thick more permeable stratigraphic transition zone located between two less permeable, and generally less contaminated zones consisting of (i) overlying unconsolidated saprolite (U_NA < 0.01 to 200 mg kg^–1) and (ii) underlying less-weathered bedrock (U_NA generally < 0.01 to 7 mg kg^–1). In this transition zone, acidic (pH < 4) U-enriched ground water (U of 38 mg L^–1) has weathered away calcite veins resulting in greater porosity, higher hydraulic conductivity, and higher U contamination (U_NA 106 to 745 mg kg^–1) of the weathered interbedded shale and sandstone. These characteristics of the transition zone produce an interval with a high flux of contaminants that could be targeted for remediation.

Abbreviations: DO, dissolved oxygen • DOE, Department of Energy • Eh, redox potential • ERSD-FRC, Environmental Remediation Sciences Division-Field Research Center • HIV, hydroxyl-interlayered vermiculite • I, ionic strength • K_d, linear adsorption coefficient • KPA, kinetic phosphorescence analyzer • ORR, Oak Ridge Reservation • RCRA, Resource and Conservation Recovery Act • U_NA, nitric acid extractable uranium • UTEVA, uranium tetravalent actinide • WI, weathering index • XRD, X-ray diffraction