Source-dependent And Source-independent Controls On Plutonium Oxidation State And Colloid Associations In Groundwater

摘要

Plutonium (Pu) was characterized for its isotopic composition, oxidation states, and association with colloids in groundwater samples near disposal basins in F-Area of the Savannah River Site and compared to similar samples collected six years earlier. Two sources of Pu were identified, the disposal basins, which contained a (240)~Pu/(239)~Pu isotopic signature consistent with weapons grade Pu, and (244)~Cm, a cocontaminant that is a progenitor radionuclide of (240)~Pu. (240)~Pu that originated primarily from (244)~Cm tended to be appreciably more oxidized (Pu(V/VI)L less associated with colloids (~1 kDa - 0.2 μm), and more mobile than (239)~Pu, as suggested by our prior studies at this site. This is not evidence of isotope fractionation but rather "source-dependent" controls on (240)~Pu speciation which are processes that are not at equilibrium, i.e., processes that appear kinetically hindered. There were also "source-independent" controls on (239)~Pu speciation, which are those processes that follow thermodynamic equilibrium with their surroundings. For example, a groundwater pH increase in one well from 4.1 in 1998 to 6.1 in 2004 resulted in an order of magnitude decrease in groundwater (239)~Pu concentrations. Similarly, the fraction of (239)~Pu in the reduced Pu(lll/IV) and colloidal forms increased systematically with decreases in redox condition in 2004 vs 1998. This research demonstrates the importance of source-dependent and source-independent controls on Pu speciation which would impact Pu mobility during changes in hydrological, chemical, or biological conditions on both seasonal and decadal time scales, and over short spatial scales. This implies more dynamic shifts in Pu speciation, colloids association, and transport in groundwater than commonly believed.