image: (a) Atomic ratio of 129I/127I in the surface seawater of the NSCS (the white numbers are the station numbers), (b) distribution of salinity (black numbers) and atomic ratio of 129I/127I in the cross-section of the Pearl River estuary area, (c) relationship between the atomic ratio of 129I/127I and salinity in the Pearl River estuary area, (d) relationship between the concentration of 127I and salinity in the Pearl River estuary area.
Credit: ©Science China Press
This study was led by Jinxiao Hou, Dr. Xiaolin Hou, and Dr. Yanyun Wang from the Institute of Earth Environment, Chinese Academy of Sciences. The northern offshore region of the South China Sea hosts one of the highest densities of nuclear power plants along China’s coastline. By systematically collecting seawater samples throughout this area and applying the laboratory’s well-established ultra-trace analytical techniques for ¹²⁹I and ¹²⁷I, the team measured the concentration levels and spatial distributions of both isotopes. By integrating observed ¹²⁹I/¹²⁷I atomic ratios, the transport and dispersion behavior of the anthropogenic radionuclide ¹²⁹I in the northern South China Sea were clarified.
The study demonstrates that riverine input is the dominant source of ¹²⁹I in estuarine and coastal waters, although its influence is limited to within ~100 km of the river mouths and largely restricted to the upper 10 m of the water column. Quantitative source apportionment reveals the following order of contribution to the water-column ¹²⁹I inventory as follows: ocean current input>river input>direct atmospheric fallout. During summer, the prevailing south-westerly monsoon forces the surface ¹²⁹I plume to adopt a fan-shaped pattern over the estuarine shelf, inhibiting southward transport of the Pearl River ¹²⁹I plume and impeding its offshore expansion. Vertically, the buoyancy of the freshwater plume and the presence of upwelling severely limit the downward mixing of ¹²⁹I. In the open sea, surface depletion coupled with conservative behavior at sub-surface depths produces a pronounced subsurface maximum in both ¹²⁹I concentration and the ¹²⁹I/¹²⁷I atomic ratio. These findings provide a scientific basis for forecasting dispersion pathways and impacted areas of accidental radionuclide releases in marginal seas, as well as for assessing environmental radiation security.
See the article:
Hou J, Wang Y, Liu J, Liu Q, Hou X. 2025. Sources, transport, and migration of 129I in the northern South China Sea. Science China Earth Sciences, 68(9): 2913–2923, https://doi.org/10.1007/s11430-025-1639-1
Journal
Science China Earth Sciences