We must first familiarize ourselves with some of the jargon and units that scientists use when discussing radioactivity. A commonly used unit is the Becquerel (abbreviated as Bq), which represents an amount of radioactive material where one atom decays per second and has units of inverse time (per second). Another unit commonly used is disintegrations per minute (dpm) where the number of atoms undergoing radioactive decay in one minute are counted (so 1 Bq = 60 dpm). When we talk about the radioactivity measured in seawater in seawater the measurements are reported normalized per litre of seawater (Bq/L).

In a previous diary I presented a primer on the major contributors to radionuclide concentrations in average seawater. About 93% of radioactivity in seawater results from the presence of primordial, naturally occurring potassium-40 (K-40) and rubidium-87 (Rb-87). The remaining 7% are radioactive elements deposited to the ocean from past atmospheric nuclear testing. The sum of these activities is about 14 Bq/L on average though there are regional differences that scale with ocean salinity.

Studies of the concentration of radionuclides off the coast of Japan in the aftermath of the disaster have been published since 2011 with some of the most recent work being published or in press in 2013. A study by Povinec and others (2013) measured the concentrations of Cesium-137 (Cs-137), Iodine-129 (I-129) and Tritium (H-3) off the coast of Japan in June 2011. Cs-137 is one of the most important isotopes to monitor for long-term radiological impact because of the large releases involved, relatively long half-life, and its relative high bioavailability. The levels of Cs-137, I-129 and H-3 in seawater following the accident in offshore, surface seawater were 0.002–3.5 Bq/L, 0.00000001–0.0000008 Bq/L and 0.05–0.15 Bq/L, respectively. These represent at maximum an increase above the pre-Fukushima background concentrations of a factor of 1000 (Cs-137), 50 (I-129) and 3 (H-3). A companion study by Casacuberta and others (2013)of the bioaccumulating Strontium-90 (Sr-90) determined concentrations of 0.0008 - 0.085 Bq/L roughly 120 fold greater than Sr-90 background concentrations.

So there is clear enrichment of radionuclide concentrations up to 600 km offshore of the reactors in June 2011 resulting from the release of isotopes to the ocean. If we sum these isotopes together and compare their radioactivity to the natural and fall out background present before the disaster (~14 Bq/L) we see that Fukushima has increased the concentration of radioactivity in the seawater by a maximum of 27%. These concentrations will diminish as the ocean mixes and the isotopes decay.

More recent work that has monitored the concentration of Cs-137 between Japan and Hawaii to track the dispersion of radionuclides from Fukushima was published by Kamenik and others (2013). They found that Cs-137 levels near to Hawaii were similar to what would be expected for pre-Fukushima background concentrations of 0.0017 - 0.0028 Bq/L. Between Japan and Hawaii Cs-137 values were measured that exceeded background Cs-137 by a factor of 2-3 and the southeastern leading edge of the plume traveling westward. These levels are not significant when compared with total radiation levels in north Pacific seawater.

The data we have about concentrations and radioactivity of these isotopes suggest that they do not represent significant exposure risks to human beings through direct contact with seawater or through the consumption of apex predators like Pacific Bluefin Tuna link. Ongoing monitoring of the release and dispersion of radioisotopes, especially those with the potential to biomagnify in marine foodwebs, from Fukushima should remain a high priority for environmental and public health.