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A variety of analyses were conducted on selected sediment samples collected from two wells (299 W15-46 and 299-W15-48) drilled near the 216-Z-9 Trench to elucidate the form and potential for Pu and Am to be mobilized under present conditions and those that could be expected in future remediation scenarios. Analyses included moisture content, determination of the less than sand size fraction (silt plus clay), carbon analysis, SEM/EDS analysis, microwave-assisted acid digestions for total element analysis, and extraction tests using Hanford groundwater as the leachate. Results of the extraction tests were used as input to conduct equilibrium geochemical modeling of the solutions with Geochemist’s Workbench®. Geochemical modeling results for Pu were evaluated in terms of recent conclusions regarding the solubility and redox reactions of Pu by Neck et al. (2007a, 2007b). It was found that the highest concentrations of Pu and Am were associated with sediments of low silt/clay content and occur above silt/clay rich layers within the sediment profile. It was also found that the Pu and Am were relatively enriched in the silt/clay portion of these samples. The fact that the highest concentrations of Pu and Am occurred in sediments with low silt/clay contents suggests that waste solutions had perched on top of the low permeability silt/clay rich layers and interactions with the high silt/clay layers was minimal. SEM/EDS analysis indicated that the Pu and Am in these sediments does not occur as discrete micron size particles, and therefore must occur as mononuclear or polynuclear/ nanoclusters size particles adsorbed throughout the sediment samples. Leaching of these samples with Hanford groundwater indicates that release of Pu and Am from the sediments is correlated most significantly with the acidity of the water and not the initial concentrations of Pu and Am in the sediments. Only extracts that were acidic after contact with the sediments (pH 4.3 to 5.4) contained detectable concentrations of extractable Pu and Am. Water extracts from samples containing high concentrations of TBP suggest that if the TBP degradation products DBP and MBP are available in these sediments, they do not significantly increase the extractability of Pu or Am. Geochemical modeling results suggest that the concentrations of Am in water in contact with these sediments is not controlled by the solubility of Am(OH)3(c), but rather by desorption of Am that has been previously adsorbed to the sediments during the period of active wastewater disposal. Sediment extracts that had measureable concentrations of Am only occurred in samples that were fairly acidic (pH 4.3 to 4.6), indicating that Am will remain effectively sequestered to sediments when pH conditions approach those of normal Hanford groundwater (mildly alkaline, ~ pH 8). The geochemical modeling results indicate that Pu in acidic extracts is significantly undersaturated with respect to PuO2(am). However, recent reviews of Pu solubility and redox reactions suggest that the data used for these calculations is incomplete (Neck et al. 2007a, 2007b). The results of Neck et al. (2007a, 2007b) suggest that Pu concentrations in solutions in contact with the 216-Z-9 Trench sediment samples might be controlled by a mixed valent solid phase [(PuV)2x(PuIV)1-2xO2+x(am)] with various dissolved Pu(V) complexes and Pu(IV)O2(am) colloids or nanoclusters being the dominant species in solution for typical Hanford groundwater conditions. Adsorption is likely to have a major impact on the mobility of these species (Neck et al. 2007a, 2007b; Clark et al. 2006; Kaplan et al. 2006; Powell et al. 2005). Further research is planned to verify these hypotheses

Plutonium Mobility Studies: 216-Z-9 Trench Sample Analysis Results