Uranium and plutonium isotopes and their environmental implications in surface sediments from the Yangtze River catchment and estuary

Abstract

The U-series and Pu isotope signatures in the environment have emerged as important tools for promoting geochemical studies. To enhance the understanding of U and Pu sources and behaviours in river systems, this study examined 234U, 235U, 236U, 238U and 239Pu, 240Pu isotopes in surface sediments collected from the Yangtze River catchment and estuary in China, for the first time. The 234U, 235U, 238U and 236U concentrations were measured in the range of 8.29–20.6 Bq/kg, 0.352–0.789 Bq/kg, 7.39–16.4 Bq/kg and (3.05–21.1) × 107 atoms/g. Additionally, the 236U/238U, 234U/238U and 235U/238U atom ratios were in the range of (1.43–6.85) × 10−8, (6.08–7.01) × 10−5 and (7.49–8.08) × 10−3 respectively. For all samples, the 239+240Pu activity concentrations ranged from 0.01 to 3.40 Bq/kg. In the Yangtze River catchment, 240Pu/239Pu atom ratios presented typical global fallout values (0.168–0.201); while in the Yangtze River estuary, higher values of 0.216–0.264 located between global fallout and PPG (Pacific Proving Grounds) close-in fallout values. From west to east, the 234U, 235U, 236U and 238U activities generally exhibited decreasing trend from the upstream parts of the river to the estuary. The values of 236U concentrations and 236U/238U, 240Pu/239Pu, 236U/239Pu atom ratios located in the range of global fallout level in environmental samples from other regions, therefore, it may be concluded that 236U in surface sediments of the Yangtze River catchment and estuary are mainly derived from global fallout.