IAEA advises that Japanese authorities today reported data on radiation samples collected 30 kilometres off shore of the Fukushima Daiichi nuclear power plant on 24 March, and the levels of iodine-131 and cesium-137 showed slight variations from data collected at the same locations on 23 March (see previous update.

A vessel from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) collected water samples at eight points 30 kilometres from the coastline and found measurable concentrations of iodine-131 and cesium-137. The iodine concentrations measured were about at Japanese regulatory limits, and the cesium levels were well below those limits.

The IAEA’s Marine Environmental Laboratory in Monaco has received the data and offered this preliminary analysis:

Dilution, both into deeper layers and by dispersion along the prevailing ocean currents will lead to a rapid decrease of the initial surface contamination. For the short term, iodine-131 will be the relevant radionuclide as far as doses are concerned, but for the long term, cesium-137 will be the more important radionuclide in the marine environment. It will be possible to follow this nuclide over long distances for several years.

It can be expected that radionuclides will take months or years to reach other shores of the Pacific. The main transport of contamination takes place by atmospheric transport over long distances.