Migration path and isotope tracing of 137Cs and 239 + 240Pu in estuary sediments: a case study of Liao River estuary in China

Abstract

Currently, the distribution characteristics and transport processes of the radionuclides 137Cs and 239 + 240Pu in the sediments of estuaries are still a controversial issue. Thus, in the present study, we investigated the distribution characteristics, sources and migration path of 137Cs and 239 + 240Pu in sediment cores of the Liao River estuary (LRE), China, and evaluated the relative contributions of the Pacific Proving Grounds (PPG) and riverine sources of Pu for the sediments in this area. We used 137Cs and Pu isotopes for dating the sediments and estimated the sedimentation rates by the two radionuclides in the LRE. Seven sediment samples were collected using a box corer from the Liao River estuarine wetland and tidal flat in the LRE in October 2012 and April 2015, respectively. The activities of 137Cs in the various samples were determined by γ spectrometry using HP-Ge detectors with 60% relative counting efficiency. Approximately 2–5 g of sediment were spiked with 242Pu (ca. 1 mBq) as a chemical yield tracer for Pu isotopic analysis at the School of Radiation Medicine and Protection, Soochow University. The measurement of Pu isotopes (239Pu, 240Pu, 242Pu) was performed by a sector field ICP-MS. In order to quantitatively differentiate the relative proportions of global fallout and PPG, we employed a two-end member mixing model to estimate their contributions. The average values of 137Cs, 239 + 240Pu activity concentrations, and 240Pu/239Pu atom ratios in the surface sediment samples of the LRE were 6.727 ± 0.251 mBq g−1, 0.294 ± 0.024 mBq g−1, and 0.188 ± 0.049 (1σ), respectively. The average 240Pu/239Pu atom ratios ranged from 0.180 ± 0.034 to 0.199 ± 0.021 in sediment cores from the east and west sides of the LRE. For core LT-2 values for 137Cs and 239 + 240Pu concentrations were from below the detection limit: 3.380 ± 0.414 and 0.036 ± 0.007 to 0.105 ± 0.007 mBq g−1. The mean 240Pu/239Pu atom ratio 0.217 ± 0.050 (1σ) in sediment core LT-2 lies between the global fallout and PPG close-in fallout. We found that atmospheric fallout is the main source of Pu in sediment cores from the east and west sides of the LRE. For core LT-2, atmospheric fallout and riverine input (~ 73.4%) are the major sources of Pu with the source of the rest of Pu (~ 26.6%) attributed to the PPG via the Tsushima Warm Current and the coastal water of the East China Sea. The sedimentation rates (means 0.62–0.8 cm year−1) estimated by the two radionuclides were in good agreement and without any statistically significant difference.