A scanning electron microscope image of mineral colloids containing plutonium filtered from groundwater. Mineral colloids consist of clay and zeolites, common secondary minerals found in rocks at the Nevada Test Site. (LLNL)

Five years from now, scientists will be able to better determine how, when, and why plutonium moves in soil and groundwater. The way to predict how plutonium is transported in groundwater away from a site is by looking at the dominant geochemical processes that control plutonium's (Pu) behavior in the subsurface at environmental levels. But that isn't always so easy.

A $6 million five-year proposal funded by the Department of Energy's Office of Science, Biological and Environmental Research (BER), will allow about a dozen Lawrence Livermore National Laboratory (LLNL) scientists to study Pu transport at concentration levels at the picomolar to attomolar scale. Plutonium can move on small particulates, called colloids, which are often found in groundwater, but the conditions that control whether Pu migrates or remains immobile are not well understood.

That's where LLNL's team comes in. Annie Kersting, director of the Lab's Glenn T. Seaborg Institute and manager of the study, said previous experiments of plutonium movement in the subsurface were performed at Pu concentrations orders of magnitude higher than those observed in the field. Now they will do experiments at much lower, environmental conditions.

Ultimately, the research will help in the development of conceptual models for Pu transport across the Department of Energy (DOE) complex. The model will provide DOE with the scientific basis to support decisions for the remediation and long-term stewardship of legacy sites.

“We can provide DOE an understanding of when and how Pu is transported in groundwater,” Kersting said. “What process did it go through to get there, could it affect people downstream of DOE sites, and how can we predict and monitor Pu contamination?”

Taking advantage of the facilities housed at Livermore's institutes, the team will be able to measure minute amounts of Pu, similar to the concentrations found in soil and groundwater, with the Lab's secondary ion mass spectrometer (NanoSIMS), the Transmission Electron Microscope (Titan SuperSTEM), and CAMS (an accelerator mass spectrometer). “With these tools, we'll be able to identify where the Pu is sited on a colloid, study the behavior of Pu at extremely low concentrations and identify the geochemical processes controlling its mobility,” Kersting said.

Mavrik Zavarin, the lead scientist, said it's likely the team will discover that Pu behavior is very much dependent on concentration. “At very low concentrations, Pu will interact with groundwater, minerals and microbes in ways we could not have predicted based on past experiments,” he said. “Our team at LLNL has a unique opportunity to study Pu chemistry under environmentally relevant conditions.”

Laboratory results will be compared to field samples taken from three DOE sites: the Nevada Test Site, Rocky Flats, and the Hanford Site. Pu-containing colloids will be collected and characterized in terms of their inorganic surfaces, associated organic compounds/coatings, and possible microbial associations.

(Lawrence Livermore National Laboratory)