Actinide behavior in a freshwater pond

Long-term investigations of solution chemistry in an alkaline freshwater pond have revealed that actinide oxidation state behavior, particularly that of plutonium, is complex. The Pu(V,VI) fraction was predominant in solution, but it varied over much of the range reported from other natural aquatic environments, in this case, because of intrinsic biological and chemical cycles. A strong positive correlation between plutonium (Pu) - but not uranium (U) - and hydroxyl ion over the observation period, especially when both were known to be in higher oxidation states, was particularly notable. Coupled with other examples of divergent U and Pu behavior, this result supports other evidence that Pu(V), or perhaps a mixture of Pu(V,VI), was the prevalent oxidation state in solution. Observations of trivalent actinide sorption behavior during an algal bloom, coupled with the association with a high-molecular-weight organic fraction in solution, indicated that solution-detritus cycling of organic carbon, in turn, may be the primary mechanism in americium-curium cycling. A significant difference in observed behavior between Am and Cm (25:1 in sediment-water exchange) is primarily attributable to reversible anthropogenic influences. Sorption by sedimentary materials appears to predominate over other factors controlling effective actinide solubility and may explain, at least partially, the absence of an expected strong positive correlation between carbonate and dissolved U. 39 references, 6 figures, 12 tables