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Stable (127I) and radioactive (129I) iodine concentrations in surface sediments of brackish lake near nuclear facility.

Y. Satoh,S. Ueda

Published 2025 in Science of the Total Environment

ABSTRACT

Marine sediment is the largest iodine reservoir in the Earth's crust, acting as environmental pool of anthropogenically produced radionuclides. The iodine concentrations in the sediments, river-derived particles, settling particles, plankton, and water were monitored in a brackish lake adjacent to the Pacific Ocean to determine factors regulating the concentrations of stable (127I) and radioactive (129I) iodine in marine coastal surface sediments. Geochemical parameters such as the 127I to C (I/C) ratio, C to N (C/N) ratio, and stable isotopes of carbon (δ13C) were measured. The sediment was sampled at two locations: the river mouth (west site) and the lake center (center site). 127I concentrations in the solid samples were approximately ranked as settling particles ≒ plankton > sediments ≒ river-derived particles. 129I concentrations ranked as river-derived particles > sediments ≒ settling particles ≒ plankton. The 127I and 129I concentration differences were caused by the anthropogenic 129I input via atmospheric deposition during the temporary operation of a nearby spent nuclear fuel reprocessing plant. The 127I and 129I sediment concentrations were estimated from their concentrations and mixing ratios to determine whether they were input by river-derived and settling particles. The mixing ratios were estimated using C/N ratio and δ13C, as organic matter is the dominant iodine carrier in marine sediments. The results of the estimation using the mixing ratio and iodine concentrations in river-derived and settling particles showed that the actual measured values of 127I and 129I concentrations in sediments at the center site ranged in the estimated values, whereas those at the west site did not. This finding indicated that an iodine source was not considered at the western site, which was likely fresh terrestrial plant materials. Additionally, plant materials minimally contributed to the sediment iodine at the center site, where the dominant source was settling particles. The sedimentary iodine concentrations in coastal areas were mainly regulated by terrestrial (river-derived particles and fresh plant materials), marine (planktonic particles) inputs, and topographic factors.

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