Reactive Plutonium (Pu) Transport in the Vaodse Zone: Interactions of Experiments, Soil Physics and Computer-Aided Thinking.

Making classical predictions, i.e. unambiguous mathematical projections based on independent measurements, is extremely difficult in natural systems.  This is particularly true for the vadose zone with its heterogeneity, nonlinear property relationships, complex boundary conditions and active biochemical processes.  Using soil physics to analyze experiments, however, is not equivalent to classical predictions.  The analysis process can be conceived as a type of computer-aided thinking that when combined with additional experiments can result in new insights.  This process, that involved the use of the van Genuchten functions, is illustrated by the analysis of 11-year Pu lysimeter experiments at the Savannah River Site which showed anomalous distributions below the source with migration above the source.  The initial conceptual model of the transport process was based on steady-state and then fully transient soil water movement coupled to REDOX reactions between reduced and oxidized Pu species.  Simulations from this model yielded reasonable below-source transport, but little above-source transport, with surprisingly little difference between steady-state and transient formulations.  The conceptual model was then modified to include Pu absorption by plant roots and upward movement in the transpiration stream. Resulting simulations were consistent with Pu activity concentrations in the top 20 cm of soil and suggested Pu transport into the shoots of the annual grasses and an accumulation on the soil surface.  Such a residue has now been verified by isotope ratio analysis, and experiments on corn have verified rapid Pu transport in the transpiration stream.  However, a remaining failure of the conceptual model is a continuing inability to explain Pu migration in detail within a few cm of the source.  This motivated a 2^nd conceptual model modification wherein spreading in the immediate source vicinity is due to microbe-mediated transport that mimics diffusion but resists soil water advection.  Related experimental studies are ongoing.