Assessment of the influences of groundwater colloids on the migration of technetium-99 at the Paducah Gaseous Diffusion Plant Site in Paducah, Kentucky

摘要

This short report summarizes the influences of groundwater colloids on the migration/transport of (sup 99)Tc at the Paducah Gaseous Diffusion Plant (PGDP) site in Paducah, Kentucky. Limited data suggest that inorganic colloidal materials (e.g., aluminosilicate clay minerals) may not play a significant role in the retention and transport of Tc. Studies by size fractionation reveal that both Tc and natural organic matter (NOM) are largely present in the <3K fraction. The role of NOM on Tc retention and transport is not conclusive on the basis of this study. However, a literature review suggests that Tc is very likely associated with the groundwater organics. The presence of the organic matter could have increased the solubility and cotransport of Tc at the PGDP site. Further studies, applying such techniques as gel chromatography, size exclusion, and spectroscopy, may be useful to determine the association of organic matter with Tc. If Tc is associated with groundwater organics, appropriate protocols for removal of organic matter associated with Tc may be developed. Time and resources were limited so this study is not comprehensive with respect to the role of mobile organic and inorganic colloidal materials on Tc transport in subsurface soils. The redox conditions (DO) of groundwaters reported may not represent the true groundwater conditions, which could have influenced the association and dissociation of Tc with groundwater colloidal materials. Because Tc concentrations in the groundwater (on the order of nCi/L) at the PGDP site is much lower than the solubility of reduced Tc (IV) (on the order of (approximately)10(sup (minus)8) mol/L or parts per billion), regardless of the redox conditions, Tc will stay in solution phase as TC(IV) or Tc(VII). The mechanisms of adsorption/association vs precipitation must be understood under reduced and low Tc conditions so that strategic plans for remediation of Tc contaminated soils and groundwaters can be developed.