UNSATURATED TRANSPORT PROCESSES IN UNDISTURBED HETEROGENEOUS POROUS-MEDIA .2. COCONTAMINANTS

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)EDTA2-, Co(III)EDTA-, and SrEDTA2- was investigated at -10 cm pressure head. Subsurface Fe and Al sources effectively dissociated the Sr-EDTA2- 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)EDTA2- 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.