In order to more accurately predict the rates and mechanisms of radionuclide migration from lowlevel waste disposal facilities via groundwater transport, ongoing studies are being conducted at field sites at Chalk River Laboratories to identify and characterize the chemical speciation of mobile, long-lived radionuclides migrating in groundwaters. Large-volume water sampling techniques are being utilized to separate and concentrate radionuclides into particulate, cationic, anionic, and nonionic chemical forms. Most radionuclides are migrating as soluble, anionic species which appear to be predominately organoradionuclide complexes. Laboratory studies utilizing anion exchange chromatography have separated several anionically complexed radionuclides, e.g.,60Co and106Ru, into a number of specific compounds or groups of compounds. Large-volume ultra-filtration experiments have shown that significant fractions of the radionuclides are being transported in these groundwaters in the form of macromolecules having molecular weights ranging from less than 3,000 to 100,000.