239,240Pu transport into the Arctic Ocean from underwater nuclear tests in Chernaya Bay, Novaya Zemlya

Radionuclide measurements have been conducted on sediment, seawater and biota samples collected in Chernaya Bay, on the southern coast of Novaya Zemlya, the site of two underwater nuclear tests conducted in the 1950s. 239,240Pu levels in sediments from the central region of Chernaya Bay exceed concentrations of 15,000 Bq/kg, and are among the highest ever reported for the marine environment. It is estimated that approximately 11 TBq of 239,240Pu from the tests has been retained in the sediments of Chernaya Bay. Plutonium from Chernaya Bay is distinguished by 240Pu/239Pu atom ratios of 0.03 that are much lower than ratios of 0.18 typical of global fallout. High levels of 137Cs (Bq/kg) and 60Co (Bq/kg) were also measured in surface sediments in the central regions of Chernaya Bay near the presumed epicentre of the explosions. Applications of a biodiffusion model to excess 210Pb sediment depth profiles indicate that the distribution of 239,240Pu is governed mainly by sediment mixing in this low sedimentation rate (<0.1 cm/yr) regime and, as a result, most of the 239,240Pu has been retained in the upper 20 cm of the sediment column. Elevated levels of 239,240Pu measured in Macoma (104 Bq/kg), Fucus (15 Bq/kg) and polychaete (1292 Bq/kg) from Chernaya Bay, indicate that 239,240Pu levels in the benthos are comparatively high and that significant uptake has occurred in the food chain. Although levels of 239,240Pu in bottom water from Chernaya Bay are high (4.2 Bq/m3), restricted exchange over the fjord sill limits the present rates of 239,240Pu transport from contaminated sites in Chernaya Bay into the eastern Barents Sea. However, low 240Pu/239Pu atom ratios measured in sediment cores collected throughout the eastern Barents Sea indicate that significant offshore transport of plutonium from Chernaya Bay has occurred in the past, probably at the time of the original nuclear tests. The large difference in end member 240Pu/239Pu atom ratios for Chernaya Bay fallout (0.03) and atmospheric fallout (0.18) has been exploited to estimate that 2 TBq of 239,240Pu in Barents Sea sediments was originally derived from Chernaya Bay. Further, a plume of low 240Pu/239Pu ratio plutonium, distributed in a northwestward direction, is evident in sediments along the southern coastline of Novaya Zemlya, indicating that an additional quantity of Chernaya Bay plutonium may have been transported into the Arctic Ocean.