Identification of the Distribution and Sources of Pu in the Northern South China Sea: Influences of Provenance and Scavenging

To investigate the distribution and source of plutonium in the northern South China Sea, surface sediments, including samples from the shore, shelf, slope, and Pearl River estuary, were collected to analyze plutonium (Pu) isotopes with sector field inductively coupled plasma mass spectrometry (SF-ICP-MS). In this study, we demonstrate that the distributions of 239+240 Pu activities and 240 Pu/ 239 Pu atom ratios both showed an increasing trend from the shore to the inner shelf and a decreasing trend from the inner shelf toward the outer shelf and slope. This variation trend was different from those observed in previous studies in the same study area, and the obvious dividing line was the vicinity of the 50 m isobath. All of the 240 Pu/ 239 Pu atom ratios in the studied surface sediments were higher than the global fallout value (0.178 ± 0.019), which might indicate that the Pu in the northern South China Sea (NSCS) was a mixture of global fallout Pu and Pacific Proving Ground (PPG) Pu. Using a simple two end-member mixing model, the contribution of Pu from the PPG was approximately 60–71% in the NSCS and approximately 49–69% in the Pearl River estuary. We also study the different characteristics of the Pu scavenging process in different areas, which, in combination with the terrain, sediment composition affected the distribution of 239+240 Pu activities and 240 Pu/ 239 Pu atom ratios in the NSCS. In fact, according to the low accumulation rate of sediments in the shore area near western Guangdong Province (WGC), the Pu in surface sediments best retained information on Pu scavenging in the 1950s. Finally, due to particular interactions, the vicinity of the 50 m isobath is considered the priority for future environmental risk assessment.