Plant and Microbial Processes Influencing Plutonium Mobility through Sediments Under Field Conditions.

Four 11-year field lysimeters were conducted with well characterized forms of Pu(III), Pu(IV) or Pu(VI).  Pu(VI) was the most mobile; Pu(III) and Pu(IV) were appreciably less mobile, with >97% remaining within 5 cm of the source.  XANES and wet chemistry analyses demonstrated that irrespective of whether Pu was placed as Pu(III) or Pu(IV) in the sediment, 11 years later, it was transformed to about 65% Pu(IV) and 35% Pu(III) and >5%Pu(V).  Approximately half of all the Pu moved upward in the lysimeters. A fully-transient flow and reactive transport model with surface-mediated redox reactions showed that this upward migration was primarily attributed to Pu root uptake (and only minimally due to air-content dependent oxidation of Pu, hysteresis, and different root distributions).  Elevated surface sediment Pu concentration were traced to plant pumping up Pu from the subsurface over several years and confirmed with modeling and high sensitivity isotopic TIMS analysis.  However, numerical simulations clearly indicated that there were biogeochemical processes that were still missing from our conceptual model.  Recently we have measured some key microbial processes, plant processes, and Fe(II)-mineralogy processes that influence Pu mobility in the lysimeters and have incorporated them into a numerical tool for hypothesis testing of coupled geochemical processes in complexed systems.  Throughout this study, mathematical models were used to analyze laboratory data. This analytical process can be conceived as a type of computer-aided thinking that when combined with additional experiments can result in new insights. Such an approach has been used to derive future research questions, test hypotheses and to visualize complex, nonlinear, property relationships.