The role of organic complexants and microparticulates in the facilitated transport of radionuclides

Abstract

This progress report describes the results of ongoing radiological and geochemical investigations of the mechanisms of radionuclide transport in groundwater at two low-level waste (LLW) disposal sites within the waste management area of the Chalk River Laboratories (CRL), Ontario, Canada These sites, the Chemical Pit liquid disposal facility and the Waste Management Area C solid LLW disposal site, have provided valuable 30-to 40-year-old field locations for characterizing the migration of radionuclides and evaluating a number of recent site performance objectives for LLW disposal facilities. Thii information will aid the NRC and other federal, state, and local regulators, as well as LLW disposal site developers and waste generators, in maximizing the effectiveness of existing or projected LLW disposal facilities for isolating radionuclides fiom the general public and thereby improving the health and safety aspects of LLW disposal.These studies have focused on identifying the physicochemical species of mobile radionuclides in groundwater at these field locations and characterizing their behavior in the sub-surface environment. Field and laboratory studies have shown that the mobile radionuclide species, including 55Fe, @Coy '06Ru, *=Sb, and 23g3%'u, are generally anionic in nature and being sequestered by naturally occurring and/or man-made complexing materials, such as fulvic and humic substances. Ion chromatographic separations of contaminated groundwater have identified a number of individual mobile chemical species of @Co and '06Ru, indicating the presence of a complex mixture of these sequestered radionuclides being transported in the groundwater. Further physico-chemical characterization of these mobile radionuclides using large-volume hollow-fiber ultrafiltration techniques have confirmed that a significant fiaction of the "soluble" radionuclides are present as intermediate molecular weight compounds corresponding to size ranges typical of fulvic and humic organo-metallic macromolecules. Optical spectroscopic techniques for characterizing the organo-metallic radionuclide species have also provided confirmatory evidence of complexation of radionuclides by humic materials.These studies have shown, through the use of multiple state-of-the-art characterization methodologies, the consistent relationship between migrating radionuclides and naturally occurring humic and fulvic materials in CRL groundwater. Their ubiquitous presence in sampled waters, substantial capacity for the complexation of cationic radionuclides under the existing geochemical conditions, and general lack of attraction for most mineral surfaces in the aquifer medium suggest that these species are primarily responsible for the facilitated transport of radionuclides in this region. As the CRL site serves as a viable model for LLW disposal sites in humid environments, the results of this investigation have farreaching implications in the fields of facility design, maintenance, and remediation.On-going field and laboratory studies are con...