(PU)-P-239,240 transport into the Arctic Ocean from underwater nuclear tests in Chernaya Bay, Novaya Zemlya

Radionuclide measurements have been conducted on sediment, seawater and bio ta samples collected in Chernaya Bay, on the southern coast of Novaya Zemly a, the site of two underwater nuclear tests conducted in the 1950s. Pu-239, Pu-240 levels in sediments from the central region of Chernaya Bay exceed c oncentrations of 15,000 Bq/kg, and are among the highest ever reported for the marine environment. It is estimated that approximately 11 TBq of Pu-239 ,Pu-240 from the tests has been retained in the sediments of Chernaya Bay. Plutonium from Chernaya Bay is distinguished by Pu-240/Pu-239 atom ratios o f 0.03 that are much lower than ratios of 0.18 typical of global fallout. H igh levels of Cs-137 (Bq/kg) and Co-60 (Bq/kg) were also measured in surfac e sediments in the central regions of Chernaya Bay near the presumed epicen tre of the explosions. Applications of a biodiffusion model to excess Pb-21 0 Sediment depth profiles indicate that the distribution of Pu-239,Pu-240 i s governed mainly by sediment mixing in this low sedimentation rate (< 0.1 cm/yr) regime and, as a result, most of the Pu-239,Pu-240 has been retained in the upper 20 cm of the sediment column. Elevated levels of Pu-239,Pu-24 0 measured in Macoma (104 Bq/kg), Fucus (15 Bq/kg) and polychaete (1292 Bq/ kg) from Chernaya Bay, indicate that Pu-239,Pu-240 levels in the benthos ar e comparatively high and that significant uptake has occurred in the food c hain. Although levels of Pu-239,Pu-240 in bottom water from Chernaya Bay ar e high (4.2 Bq/m(3)), restricted exchange over the fjord sill limits the pr esent rates of Pu-239,Pu-240 transport from contaminated sites in Chernaya Bay into the eastern Barents Sea. However, low Pu-240/Pu-239 atom ratios me asured in sediment cores collected throughout the eastern Barents Sea indic ate that significant offshore transport of plutonium from Chernaya Bay has occurred in the past, probably at the time of the original nuclear tests. T he large difference in end member Pu-240/Pu-239 atom ratios for Chernaya Ba y fallout (0.03) and atmospheric fallout (0.18) has been exploited to estim ate that 2 TBq of Pu-239,Pu-240 in Barents Sea sediments was originally der ived from Chernaya Bay. Further, a plume of low Pu-240/Pu-239 ratio plutoni um, distributed in a northwestward direction, is evident in sediments along the southern coastline of Novaya Zemlya, indicating that an additional qua ntity of Chernaya Bay plutonium may have been transported into the Arctic O cean. (C) 2000 Elsevier Science Ltd. All rights reserved.