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

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 MnO 2 ‐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 Co 2+ . 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 Co 2+ were similar, the sorption kinetics of the former were more sluggish relative to Co 2+ and contaminant transport was accelerated in the presence of EDTA.