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Unsaturated Transport Processes in Undisturbed Heterogeneous Porous Media: II. Co-Contaminants

Environmental Sciences Division, Oak Ridge National Lab., P.O. Box 2008, Oak Ridge, TN 37831-6038

J. D. O'Dell

Dep. of Plant and Soil Science, Univ. of Tennessee, Knoxville, TN 37901-1071

*Corresponding author.

ABSTRACT

Department of Energy facilities involved in defense-related activities have generated huge quantities of low-level radioactive mixed waste during the past several decades. The waste is composed of organically complexed contaminants, also known as co-contaminants, which are typically disposed in shallow land burial sites. The objective of this study was to provide an improved understanding of the geochemical processes controlling co-contaminant transport in heterogeneous, unsaturated subsurface media. Large undisturbed columns were isolated from a proposed waste site consisting of fractured saprolitic shale, and the steady-state unsaturated transport of Co(II)EDTA^2–, Co(II)EDTA^–, and SrEDTA^2– was investigated at –10 cm pressure head. Subsurface Fe and Al sources effectively dissociated the SrEDTA^2– co-contaminant and Sr was transported as a reactive, uncomplexed species. The EDTA readily complexed with Fe and Al, resulting in significant solid-phase modification of the porous media via chelate-enhanced dissolution and redox alterations. Displacement of Co(II)EDTA^2– through the subsurface media was characterized by a MnO₂-mediated oxidation of the co-contaminant with subsequent formation of Co(III)EDTA^–. The latter co-contaminant was an extremely stable complex that was transported through the subsurface as a single, reactive entity and exhibited an overall retardation that was similar to the uncomplexed contaminant Co²⁺. Modeling results using equilibrium and nonequilibrium formulations of the convective-dispersive equation suggested that a large portion of the transported Co(III)EDTA^– was controlled by time-dependent sorption reactions with the solid phase. Although the solid-phase retention of Co(III)EDTA^– and Co²⁺ were similar, the sorption kinetics of the former were more sluggish relative to Co²⁺ and contaminant transport was accelerated in the presence of EDTA.

Joint contribution from Oak Ridge National Lab. and the Univ. of Tennessee. This research was funded by the Subsurface Science Program of the Ecological Research Division, Office of Health and Environmental Research, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems. Publication no. 4068.

Received for publication July 8, 1992.

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