INTRODUCTION In the terrestrial environment, soils represent the principal repository of the transuranic elements over geologic time. The major factor governing transuranic availability to plants in soils will be the solubility of the transuranics associated with the solid phase, since in order for root uptake to occur a soluble species must exist adjacent to the root membrane for some finite period. The form of this soluble species will have a strong influence on its stability in soil solution and on the rate and extent of uptake, and perhaps, mobility and toxicity in the plant. Furthermore, it is the solubility and form of the element which largely governs mobility in soil. Thus, any assessment of the long-term behavior of the transuranics in the terrestrial environment must be based on determination of the factors influencing solubility and form of soluble species in soil. These factors are illustrated in Fig. 1 and include the concentration and chemical form of the element entering soil, soil properties, aa these influence the elemental distribution between the solid and liquid phase, and soil processes, such as microbial activity, as these influence the kinetics of sorption reactions, transuranic conceptration, and the form of soluble and tnsoluble chemical spectfes. PLANTS 4 The transuranic elements of principal importance in the nuclear fuel cycle (Pu, Am, Cm, Np) may enter the soil through several avenues (Vaughan et al., 1976) including (1) fallout from atmospheric testing, (2) possible airborne particulates passing a filter and liquid effluents, both during reprocessing of spent fuels and fuel fabrication and (3) leaching from waste storage facilities. The major sources of the transuranics may be classified accord- ing to expected initial solubility in soil (Fig. 2). PROPERTIES »> SOIL SOLUTION (SOLUBLE Pu) < SOIL PROCESSES Particulate oxides of the transuranics initially may be expected to be largely insoluble in the soil solution. Ultimately, solubility is expected to be a function of the composition, configuration and equivalent diameter of the particle as well as soil properties and processes. Oxide particles of the highest specific activity and containing the highest concentrations of impurities in the crystal lattice may exhibit greatest solubility. The combination of configuration and equivalent diameter as reflected in surface area exposed to solution will be the other main factor governing oxide solubility. Once solubilized, the transuranic elements will be subject to the chemical reac-— tions governing soluble salts. Hydrolyzable transuranics entering the soil in acid solutions sufficiently concentrated to maintain soluble fons may be expected to be rapidly insolubilized due to hydrolysis on dilution and subsequent precipitation on particle surfaces. These include Pu(EiI, IV, and VI), Am (III, EV}, Cm (111, IV), and Np (111, IV, VI). Conversely, transuranics not subject to marked hydrolysis may be initially more soluble. These,include Pu (¥) and Np (V). Immobilization of these chemical species {Pu0; or NpO2") may occur through cation exchange reactions with particulate surfaces. Complicating this situation, disproportionation and complexation reactions may occur concurrently. Transuranics entering the soil as stable organocomplexes as might occur in the vicinity of a spent fuel separation facility may be initially highly Soluble (Wildung and Garland, 1975). The duration of solubility and mobility in the soil will be a function of the stability of the complex to 128 SOIL 4 SOURCE Fig, 1. the terrestrial Factors influencing transuranic behavior in environment.