Upward movement of plutonium to surface sediments during an 11-year field study.

An 11-year lysimeter study was established to monitor the movement of Pu through vadose zone sediments. Sediment Pu concentrations as a function of depth indicated that some Pu moved upward from the buried source material. Subsequent numerical modeling suggested that the upward movement was largely the result of invading grasses taking up the Pu and translocating it upward. The objective of this study was to determine if the Pu of surface sediments originated from atmosphere fallout or from the buried lysimeter source material (weapons-grade Pu), providing additional evidence that plants were involved in the upward migration of Pu. The (240)Pu/(239)Pu and (242)Pu/(239)Pu atomic fraction ratios of the lysimeter surface sediments, as determined by Thermal Ionization Mass Spectroscopy (TIMS), were 0.063 and 0.00045, respectively; consistent with the signatures of the weapons-grade Pu. Our numerical simulations indicate that because plants create a large water flux, small concentrations over multiple years may result in a measurable accumulation of Pu on the ground surface. These results may have implications on the conceptual model for calculating risk associated with long-term stewardship and monitored natural attenuation management of Pu contaminated subsurface and surface sediments.

Tritium and plutonium in waters from the Bering and Chukchi seas.

During the summer of 1993, seawater in the Bering and Chukchi Seas was sampled on a joint Russian-American cruise [BERPAC] of the RV Okean to determine concentrations of tritium, 239Pu and 240Pu. Concentrations of tritium were determined by electrolytic enrichment and liquid scintillation counting. Tritium levels ranged up to 420 mBq L(-1) and showed no evidence of inputs other than those attributed to atmospheric nuclear weapons testing. Plutonium was recovered from water samples by ferric hydroxide precipitation, and concentrations were determined by thermal ionization mass spectrometry. 239+240Pu concentrations ranged from < 1 to 5.5 microBq L(-1). These concentrations are lower than those measured in water samples from other parts of the ocean during the mid-1960's to the late 1980's. The 240Pu:239Pu ratios, although associated with large uncertainties, suggest that most of the plutonium is derived from world-wide fallout. As points of comparison, the highest concentrations of tritium and plutonium observed here were about five orders of magnitude lower than the maximum permissible concentrations allowed in water released to the off-site environs from licensed nuclear facilities in the United States. This study and others sponsored by the International Atomic Energy Agency and the Office of Naval Research's Arctic Nuclear Waste Assessment Program are providing data for the assessment of potential radiological impacts in the Arctic regions associated with nuclear waste disposal by the former Soviet Union.

Hanford-derived plutonium in columbia river sediments.

Mass spectrometry data on plutonium isolated from Columbia River sediments exhibit mean ratios of plutonium-240 to plutonium-242 consistent with those observed for integrated global fallout. Ratios of plutonium-240 to plutonium-239 show marked deviations from accepted fallout values, suggesting a second source of plutonium-239. This additional plutonium-239 arises from the decay of neptunium-239 produced in reactor effluent water from the old plutonium production reactors located on the Hanford Reservation. An estimated 20 to 25 percent of the total plutonium inventory in sediments behind McNary Reservoir, the first downriver site of fine sediment accumulation below the Hanford Reservation, is ascribed to reactor operations.