Distribution of 239,240Pu and 238Pu concentrations in sediments from the Ob and Yenisey Rivers and the Kara Sea

Abstract

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 ( 239,240Pu, 238Pu/ 240Pu and 241Pu/ 239,240Pu) have yielded information not only on the time horizons for sedimentary deposits but also on the sources of Pu. We have measured 238Pu, 239,240Pu and 137Cs concentrations in the surficial sediments of the Ob and Yenisey River (Russia) and the Kara Sea. The downcore variations of 238Pu and 239,240Pu concentrations have also been measured in some sediment cores. A comparison of the sediment core inventories of 239,240Pu along with the 238Pu/ 239,240Pu activity ratios with those expected from global fallout at the study sites allows us to estimate the relative amounts of reactor-derived 238Pu and 239,240Pu from the dumped reactor sites in the study area. In surficial sediment samples, the 239,240Pu concentrations vary between 9.4 and 627 mBq kg −1, with a mean of 250 mBq Kg −1. The 238Pu/ 239,240Pu 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 238Pu/ 239,240Pu 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 238Pu to the sediments in the study area. In sediment cores, the maximum depths at which 239,240Pu 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).