Pu, Cs-137 and excess Pb-210 in Russian arctic sediments
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abstract
The activity ratios of Pu and radiocesium isotopes have been used to delineate the major sources (such as global and close-in (debris) fallout, nuclear fuel reprocessing and fabrication plant effluents) in the environment. We have measured 238Pu, 239,240Pu, 137Cs, and excess 210Pb concentrations in 107 surficial sediments as well as in 5 sediment cores collected in the summer months of 1993 and 1994 from the Ob and Yenisey Rivers (Russia) and the Kara sea. A comparison of the sediment core inventories of 239.240pu and 137Cs, along with the 238Pu/239,240 Pu activity ratios, with those expected from global fallout allows us to estimate the relative amounts, if any, of reactor-derived 238Pu and 239,240Pu from the dumped reactor sites in the study area. In surficial sediment samples collected in 1993 and 1994, the 239,240Pu concentrations varied between 4.2 and 856 mBq kg-1, with a mean of 239 mBq kg-1. In samples with a measurable 238Pu, the 238Pu/ 239,240 Pu activity ratios varied between 0.010 and 0.069, with an average value of 0.035 0.014. 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 ( 5%) additional sources of 238Pu to the sediments in the study area. The inventories of Pu in the 5 sediment cores from the study area varied between 2.67 0.67 and 24.5 2.2 Bq m-2, with a mean value of 8.83 Bq m-2. The 137Cs concentrations in the upper 3 cm of the sediments varied between below detection limit to 71.4 Bq kg-1, with a mean of 14.9 Bq kg-1. The 137Cs inventories in the 5 sediment cores varied between 156.7 28.3 and 1600 153.3 Bq m-2, with a mean value of 583.3 Bq m-2. The mean ratio of inventories of Pu to that of 137Cs, 0.015, is comparable to the values in other places in the Arctic region. There is a significant correlation between total organic carbon and concentrations of 137Cs, 239,240Pu and 210Pb, suggesting that organic matter-enriched fine particles are a likely carrier phase for these nuclides. The ratio of 137Cs/ 239,240Pu also suggests that most of these nuclides are derived from global fallout.
