Plutonium, 137Cs and U in some pond and lake sediments from areas surrounding the Semipalatinsk Nuclear Test Site: With emphasis on anomalously high U accumulation

Sediment core samples were collected up to a depth of 25–40 cm from three ponds (P. Korosteli, P. Mihairov and P. Alkat) and two lakes (L. Kanoneruka and L. Semanaika) located in widely separated regions outside the former Semipalatinsk Nuclear Test Site (SNTS) in Kazakhstan. The ¹³⁷Cs, Pu and U concentrations with depth were determined in samples divided at 1 cm intervals from the top of each core. These sediment cores were dated by the excess ²¹⁰Pb method. The reservoirs with low sedimentation rates of 0.038–0.41 g·cm^–2·y^–1 permitted, but not in detail, the estimation of the depositional history of close-in fallout of ¹³⁷Cs and Pu within these regions. The sediments accumulating an anomalously high ²³⁸U concentration of 250–400 Bq/kg were also found for two of the five reservoirs, in which their ²³⁴U/²³⁸U and ²³⁵U/²³⁸U activity ratios were 1.3–2.0 and around 0.047 (nearly the same as that of natural U), respectively. Such U enrichment at the subsurface with a thin layer of lower-U sediment at the surface is mainly due to infiltration of lake water containing U from the ground water flowing into the lake or pond, followed by reduction of U(VI) to U(IV) at the redox boundary.The authors would like to express their gratitude to research staff of the Kazakh Scientific Research Institute for Radiation Medicine and Ecology for sediment and soil sampling. This work was supported by a Grant in Aid for Scientific Research from the Ministry of Education and Culture of Japan, Monbusuo International Scientific Research Program during the period of 1995–2003, represented by Profs. M. Hoshi and M. Yamamoto.