DISTRIBUTION OF PU-239,PU-240 AND PU-238 CONCENTRATIONS IN SEDIMENTS FROM THE OB AND YENISEY RIVERS AND THE KARA SEA

The major sources of plutonium isotopes in the environment are from nuclear weapons testing via global and close-in (debris) fallout, nuclear fuel reprocessing and Fabrication plant effluents. Measurements of differences in the Pu isotopic ratios ((PU)-P-239/(PU)-P-240, (PU)-P-238.240/(PU)-P-239.240) and (PU)-P-241/(PU)-P-239.240) have yielded information not only on the time horizons for sedimentary deposits but also on the sources of Pu. We have measured (PU)-P-238, Pu-239,Pu-240 and Cs-137 concentrations in the surficial sediments of the Ob and Yenisey Rivers (Russia) and the Kara Sea. The downcore variations of Pu-238 and Pu-239.240 concentrations have also been measured in some sediment cores. A comparison of the sediment core inventories of Pu-239.240 along with the Pu-238/Pu-239.240 activity ratios with those expected from global fallout at the study sites allows us to estimate the relative amounts of reactor-derived Pu-238 and Pu-239.240 from the dumped reactor sites in the study area. In surficial sediment samples, the Pu-239.240 concentrations vary between 9.4 and 627 mBq kg(-1), with a mean of 250 mBq kg(-1). The Pu-238/Pu-239.240 activity ratios vary between 0.009 and 0.065 with an average value of 0.034 +/- 0.003. This range can be compared to the average Pu-238/Pu-239.240 activity ratio of 0.030 for the year 1993 from nuclear weapons testing and SNAP fallout obtained from soil studies, indicating very little additional sources of Pu-238 to the sediments in the study area. In sediment cores, the maximum depths at which Pu-239.240 was detected varied between 6 and 12 cm. These depths yield average apparent sedimentation rates in the coastal Kara Sea between 1.5 and 3.0 mm yr(-1) (assuming Pu input since 1952, and no sediment mixing).