Long-Term Transport of Radionuclides in Watersheds: Case Studies from Three Test Beds
Authors
Teresa Baumer1, Fanny Coutelot2, Nimisha Edayilam2, Nancy Merino1, Naomi Wasserman1*, Reid Williams2, Yongqin Jiao1, Daniel I. Kaplan3, Keith Morrison1, Annie B. Kersting1, Carolyn Pearce4, Hilary P. Emerson4, Brian A. Powell2, Mavrik Zavarin1 (zavarin1@llnl.gov)
Institutions
1Lawrence Livermore National Laboratory, Livermore, CA; 2Clemson University, Clemson, SC; 3Savannah River Ecology Laboratory, Aiken, SC; 4Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA
URLs
Abstract
While much progress has been made in the mechanistic understanding of geochemistry of radionuclides, there are still gaps in knowledge regarding their long-term mobility in surface and subsurface environments. The Biogeochemistry at Interfaces has established three test beds at Savannah River Site (SRS), Nevada National Security Site (NNSS), and Hanford Site with different contamination histories, climate, hydrology, and geology to study these processes.
Pond B, a monomictic reservoir, at SRS received cooling water from the R-reactor, which resulted in low-level contamination of anthropogenic radionuclides plutonium (Pu) and caesium-137 (137Cs). Two consecutive years of monitoring demonstrated the occurrence of highly correlated concentration profiles of arsenic, iron, aluminum, plutonium, and dissolved organic matter, all of which increased in concentration by 1 to 2 orders of magnitude within the anaerobic hypolimnion.
Plutonium appears to have become incorporated into the natural iron and carbon cycles with the highest concentrations in water observed at the start of stratification, with the majority released from shallow waters associated with Fe(III)-particulate organic matter.
In contrast to the saturated Pond B system, researchers studied radionuclide transport in an ephemeral wetland at NNSS. Over the last 65 years, the E-Tunnel ponds have continuously collected groundwater discharging from a tunnel where several underground nuclear tests were conducted from the 1950’s to the 1970’s. Variably saturated pond sediment profiles of Pu and 137Cs show that Pu remains immobilized, but 137Cs is largely lost from the pond sediments. Future column experiments will explore the mechanisms behind radionuclide (im)mobilization under wet-dry cycles.
The Z-9 trench vadose zone at Hanford was contaminated with a comparatively larger amount of radionuclides (e.g., ~40 to 150 kg Pu) in an acidic waste containing organic processing solvents. In an effort to better understand Pu migration below the Z-9 trench, researchers undertook a series of bench-scale saturated column experiments using uncontaminated Hanford sediments and Pu in a range of high nitrate, acidic solution compositions with and without TBP in dodecane. These results of this study show that Pu migration is likely driven by weak sorption of aqueous Pu under low pH conditions as well as the formation of Pu-TBP-nitrate complexes in the organic phase at pH < 4. Pu migration in the subsurface will be limited by the natural buffering capacity of the sediments as well as the dispersal of the nitrate plume.