Filipino scientists working with Japanese experts have detected elevated levels of the radioactive isotope iodine-129 in seawater from the West Philippine Sea, with concentrations 1.5 to 1.7 times higher than in other parts of the country.
The findings come from a study published in Marine Pollution Bulletin . Researchers analysed 119 surface seawater samples collected across Philippine waters, including the West Philippine Sea, the Philippine Rise, the Sulu Sea and other parts of the archipelago.
Samples were gathered during multiple research expeditions aboard the RV Panata of the University of the Philippines Marine Science Institute in May 2017, June 2019, September 2019, and April to May 2021.
The research team included scientists from the Department of Science and Technology–Philippine Nuclear Research Institute (DOST-PNRI), the University of the Philippines Marine Science Institute’s Geological Oceanography Laboratory, and the Micro Analysis Laboratory, Tandem Accelerator (MALT) at the University of Tokyo.
Iodine-129 is a long-lived radionuclide with a half-life of 15.7 million years. It is produced mainly through human nuclear activities such as nuclear weapons testing, nuclear fuel reprocessing and nuclear accidents. Because it remains stable in seawater and spreads widely through ocean circulation, it is widely used as an environmental tracer.
Although the Philippines has no active nuclear power plants and no nuclear weapons programme, the West Philippine Sea showed consistently higher iodine-129 levels than other Philippine sampling sites. The researchers concluded that no domestic nuclear activities are responsible for the elevated concentrations.
Instead, the isotope is believed to have originated in northeastern China and been transported southward by major ocean currents, particularly the Yellow Sea Coastal Current and the Chinese Coastal Current.
Likely sources include historical nuclear weapons tests and nuclear fuel reprocessing facilities in China, some of which are linked to releases from Europe. Contributions from major nuclear incidents, including the Chernobyl disaster, may also be involved.
According to the study, iodine-129 likely accumulated in soils in northern China, was washed into rivers, entered coastal seas such as the Bohai and Yellow Seas, and was later carried south by ocean circulation into the West Philippine Sea and the wider South China Sea.
The researchers noted that further oceanographic modelling is needed to fully confirm these transport pathways. But the results are consistent with recent Chinese studies that traced iodine-129 in the Yellow Sea to the same historical sources.
Importantly, the team stressed that the detected levels of iodine-129 pose no risk to human health or marine ecosystems. The isotope’s behaviour in seawater makes it valuable for studying large-scale ocean circulation and water mass mixing in the South China Sea.
The findings also demonstrate the transboundary nature of radioactive materials in marine environments, showing how contaminants released in one region can travel vast distances through interconnected ocean systems.
Funding for the study was provided by the DOST National Research Council of the Philippines (NRCP) and the DOST Philippine Council for Agriculture and Resources Research and Development (PCAARRD).
The research builds on earlier Philippine work using iodine-129 in coral cores to reconstruct historical nuclear influences, reinforcing its dual role as an indicator of human nuclear activity and a tool for understanding long-term environmental change.
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