This diary is part of an ongoing series that aims to report measurements of Fukushima derived radionuclides in the North Pacific Ocean to help determine the likely impact on ecosystem and public health in western North America. The purpose of this diary is to report the results of a recently published study by Kumamoto and colleagues in the open-access journal Scientific Reports. The study measured the activity of Fukushima derived cesium (Cs), a tracer for other radionuclides, in the upper 1000 meters of the western Pacific Ocean along the 149 degree E meridian in winter 2012. These measurements indicate that 10-60% of the total Fukushima derived 134-Cs in the North Pacific has been transported to the south at a depth of ~300 m below the surface. This result is surprising as most models suggest that transport would be primarily to the east toward North America. The study demonstrates that the amount of Fukushima derived radionuclides being transported to the east towards North America is lower than predicted by previous models and provides important information on the circulation of the ocean.
The disaster at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), precipitated by the earthquake and tsunami on March 11, 2011, resulted in a massive release of radionuclides to the North Pacific Ocean by direct discharges from the plant and by deposition of radionuclides released to the atmosphere. While a suite of radionuclides were released, 134-Cs is a useful tracer of Fukushima impact. 134-Cs has a relatively short half-life (~2 years) that unequivocally fingerprints a Fukushima source. It was also released in large quantities and therefore poses a potential radiological threat to organisms. 134-Cs was released along with 137-Cs (half-life = ~30 years) in a 1:1 ratio from Fukushima.
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 14,000 Bq/cubic meter (m^3) 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 13,000 Bq/m^3 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 22,000 Bq/m^3 (Persian Gulf) and 15,000 Bq/m^3 (eastern Mediterranean). All other amounts of radioactivity resulting from human activities and disasters should always be discussed by recognizing these background values.
Estimates of direct release to the ocean were on the order of 11-15 PBq (10^15 Bq) while the deposition of Cs to the surface of the ocean were about 5.8-30 PBq. In 2012 the authors of the study occupied a series of stations along 149degree E as shown in the figure below:
Location of stations sampled for 134-Cs by Kumamoto et al. (2014)
In addition the surface plume of radionuclides that has been modeled
in surface currents heading to the east toward North America depth distributions of 134-Cs in the western Pacific show that a concentrated plume of Fukushima derived radionuclides has been transported to the south at a depth of 300 meters:
Cross sectional views of 134Cs activity (Bq/m^3) with depth and latitude along 149 degree East
Based on the integration of the activity of Cs over the depth the authors estimate that about 6 Pbq are present in the subsurface feature being transported to the south. This represents on the order of 10-60% of the total radiocesium that was introduced to the Pacific by the disaster. This helps to explain the lower activities being measured in the eastern Pacific compared to what models predict and suggests that maximum activities on the west coast of North America will likely fall toward the lower end of model predictions that were in the range of 2-30 Bq/m^3
. Simply stated more of the radioactive elements released from Fukushima to the Pacific Ocean are headed south rather than east to North America in the plume than previously thought. The impact of radioactive elements from Fukushima on the west coast of North America will be less significant than models have predicted.
More direct measurements of radioactive elements in the North Pacific Ocean will help to determine what activities are likely on the west coast of North American as the plume arrives from 2013 onward. The measurements or radionuclides in seawater, combined with measurements of radioactive elements in marine organisms, will help to assess the risk of exposure of west coast residents to radionuclides from Fukushima.