Mobility of Radionuclides in the Smear Zone.

NRC staff is reviewing remediation approaches for radionuclide contaminants in the subsurface, and their performance over time may need to extend to 1000 years. An area of uncertainty is the “smear zone,” the zone of local water-table fluctuation which includes the capillary fringe. This uncertainty is a reflection of the variability of hydraulic, chemical, thermal and microbial processes and conditions which affect the mobility of radionuclides. This review focuses on sequestration of radionuclides using in situ bioremediation where the smear zone may constitute a long-term intermittent source of contaminants to the saturated zone. Radionuclides retained in the smear zone can be subject to remobilization by recharge. This remobilization is episodic and may be caused by nutrients, organic carbon and dissolved minerals and gases carried by recharge. These chemical and microbial processes are the result of rapidly altered oxygen content in both liquid and vapor phases, or of strong biological gradients across the smear zone. Our focus is on radionuclides subject to redox changes and complexation. The foremost uncertainty involves uranium which has very low solubility as U(IV) but is soluble as U(VI). Other radionuclides of concern are technetium-99 and iodine-129, long-lived fission products and redox sensitive, with oxidized forms being readily transported in groundwater. Sampling of solid-phase minerals, as well as monitoring of aqueous and vapor phases, are needed to understand and model smear zone processes to confirm long-term performance. The principal initiating event that may influence oxidation and remobilization is surface flooding. These episodic events may fully saturate the smear zone and expose sequestered radionuclides to oxygenated water, particularly those adsorbed within small pores. Flooding will change the local hydraulic and chemical gradients affecting mobility. The practical concern is how to monitor prior to and following a flooding event, and to model repeated events to assure long-term sequestration.