Since the Fukushima Dai-ichi disaster on March 11, 2011 and with more frequency recently, there are many reports in the media of the potential impact of radioactivity from Japan causing environmental harm to sea life and people on the west coast of North America. While this is not a comprehensive treatment of the issue my intent here it to briefly summarize what the scientific community knows about radioactive elements and radioactivity in the ocean and in so doing put the risk associated with the Fukushima disaster here in North America in perspective.
Scientists use a variety of units to measure radioactivity. A commonly used unit is the Becquerel (Bq for short) 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).
Almost all the radioactivity in seawater is the result of primordial, naturally occurring radionuclides that have been transported or deposited in the oceans by natural processes like the erosion of the continental crust. There is spatial variability in the amount of radioactivity in the ocean that mostly relates to differences in salinity where the dilution of seawater with freshwater reduces the overall activity of the radioactive elements. The average radioactivity of seawater is about 14 Bq/L of which 88% is from naturally occurring potassium-40 (K-40). About 7% is from anthropogenic fallout from atmospheric nuclear weapons testing and nuclear accidents like Chernobyl (1986) and Fukushima Daiichi (2011). So there is about 13 Bq/L of natural radioactivity on average is the oceans. In high salinity areas (where conservative elements that scale with salinity like K and U have the highest concentration) activity can be as high as 22 Bq/L (Persian Gulf) and 15 Bq/L (eastern Mediterranean). All other amounts of radioactivity resulting from human activities and disasters should always be discussed by recognizing these background values.
For the purpose of putting the radioactivity released to the ocean by the Fukushima disaster in perspective lets consider isotopes of the alkali metal Cesium. Cesium behaves like a salt in seawater and is very similar to potassium (K+). The radioactive element Cesium 137 (Cs-137) was released in large quantities from Fukushima into the Pacific link. Pre-Fukushima activities of Cs-137 were approximately 1 mBq/L in the NW Pacific and about 0.9 mBq/L in the NE Pacific off the coast of British Columbia where I live. That represents on the order of 0.007% of the radioactivity in a litre of seawater. Measurements of Cs-137 were made after the disaster at 50 stations 40-600km from the coast of Japan. At 40 km from the reactor site Cs-137 was elevated to up to 3500 times the background level or ~3.5 Bq/L (to 25% if the total naturally occurring background) and at 600 km Cs137 activity was 0.3 Bq/L (2% of natural radioactivity). Research scientists did not have to take any precautions while handling seawater, sediment and biological samples collected during the study because the radioactivity was so low. Release estimates of other potentially harmful isotopes like Strontium 90 are much lower (a factor of 10 or more less) than Cs but must be monitored if the situation changes at the disaster site.
Talk of plumes of radioactivity being broadcast across the Pacific must take into account that the background radioactivity of seawater is about 14 Bq/L. It is important that although one can detect isotopes from the reactor in the environment the absolute levels are very low and will be lower as the ocean mixes, and the isotope decays.
We must recognize Fukushima Daiichi for what it is, a disaster resulting from the application of nuclear technology. The impact is immense at the site and consequences for the terrestrial environment are dire. There is impact in the ocean as well. For example, bottom dwelling fish near the reactors are so contaminated that they can't be sold or consumed and the local effects near and in the reactors are acute and terrible. But when I read that the west coast of north America is now dying because of radionuclides leaked from Fukushima I have a responsibility to communicate to the public that this is not so.
Radioactivity that we are exposed to here every day, by being on or in the water or consuming seafood is the same as if the terrible events at Fukushima never took place.