Global fallout Pu isotopes, 137Cs and 237Np records in the sediments of Lake Xingkai and their response to environmental changes in the catchment

Abstract

Plutonium (Pu) isotopes, neptunium-237 (237Np) and cesium-137 (137Cs) in the environment have emerged as important tools for promoting geochemical studies. To elucidate their source terms, deposition process, and the geochemical behavior, as well as their response to catchment environmental changes in lacustrine environment, 239, 240Pu, 137Cs and 237Np were determined for the first time in a sediment core collected from the Lake Xingkai. The 240Pu/239Pu atom ratios (0.163–0.178) in all sediment samples corresponded well with the typical global fallout value of ∼0.18, indicating that there were no other significant radioactive source inputs. The activity concentrations of 239+240Pu, 137Cs, and 237Np were 0.010–1.030 Bq kg−1, 6.4–30.1 Bq kg−1, and (0.020–2.195) × 10−3 Bq kg−1, respectively. They presented similar vertical distribution patterns, with a typical global fallout peak (15–16 cm) corresponding to the year of 1964. On the other hand, the 237Np/239Pu atom ratios exhibited a wide range (0.207–1.306), and showed a trend of increasing values with depth, implying different geochemical behaviors of these two elements. Using the advection-dispersion equation model (ADE), the obtained apparent dispersion coefficient of 237Np (0.58 ± 0.09 cm2 yr−1) was higher than that of 239+240Pu (0.38 ± 0.07 cm2 yr−1), indicating that Np has greater mobility than Pu. The total global fallout inventories of 239+240Pu, 137Cs, and 237Np were 145, 4592, and 0.291 Bq m−2, respectively. Approximately 46% of the total global fallout was derived from riverine input, indicating that severe soil erosion occurred in the catchment of Lake Xingkai in the past decades.