by Jay T. Cullen

This post is part of an ongoing series that endeavors to report measurements of Fukushima derived radionuclides in the environment to help determine the likely impact on ecosystem and public health in western North America. One of the goals of the InFORM project is to provide quality measurements of Fukushima derived radionuclides in the North Pacific to help verify model predictions of ecosystem and public health impacts of the disaster. The purpose of this post is to summarize results of a recent peer reviewed study by Kaeriyama and colleagues published in Environmental Science & Technology who measured radioactive isotopes of cesium (137-Cs half life ~30 yr and 134-Cs half life ~ 2 yr) in the western North Pacific Ocean to help track the location and movement of the Fukushima contaminated seawater plume.

They measured the depth distribution of 134-Cs and 137-Cs from August 2011 until March 2013. Measurements indicate Fukushima isotopes had spread as far to the south as 18°N along 135°E longitude at 300 meters depth by September 2012. They estimate that 9.0% of the Cs from the Fukushima disaster is being transported to the south into the subtropical western Pacific Ocean. This result supports and is consistent with a previous study which suggested significant amounts of Fukushima derived radionuclides are being transported south towards the tropics at depths centered around 300 meters. Measurements are thus indicating that previous models have likely overestimated the eastward transport of Fukushima radioactive elements and thus the maximum activity concentrations that will impact the west coast of North America and highlight the utility of trace concentrations of Cs as a tool to build a better understanding of ocean circulation.

A massive release of radionuclides to the North Pacific Ocean from the Fukushima nuclear power plant triple reactor meltdowns occurred in March and April 2011 through direct discharge of contaminated water 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. Because, 134-Cs has a relatively short half-life (~2 years) it serves as an unequivocal fingerprint of a Fukushima source. It and 137-Cs (half-life = ~30 years) were released in a 1:1 ratio in large quantities and therefore pose a potential radiological threat to organisms.

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).

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. Kaeriyama and colleagues occupied a series of stations in the Pacific to the south and west of the Fukushima power plant between August 2011 and March 2013 and measured the activity concentration of Cs isotopes in the upper 1000 meters of the ocean:

The authors found that winter cooling of surface water in 2011 caused sinking of contaminated water and southwestern transport of the Fukushima plume below the Kuroshio centered on a depth of 300 meters. At stations W09 (north of Kuroshio Extension, KE), W11 (KE) and W13 (south of KE) (see Figure panels k and l above) vertical profiles of 137-Cs were measured.

Higher activities of 137-Cs up to about 40 Bq/m^3 are found in surface waters north of the Kuroshio and cooling, sinking and southward transport of this water is evident given the subsurface peaks in activity at 150 and 300 m underneath and to the south of W09. Oceanographers call this water, that cools seasonally and sinks away from the surface in this area, North Pacific Subtropical Mode Water (NPSTMW). Significant Fukushima derived Cs is present in the NPSTMW. What you can also see is that by November 2012 (open circles) the heart of the contaminated plume carrying the bulk of radionuclides that were released in March-April 2011 has moved to the south of stations W09, W11 and W13 with much lower activity water (detection limit to 5 Bq/m^3) behind it now found at the stations.

Farther to the south and west the plume was detected as far south as 18°N along 135°E longitude at 300 meters depth by September 2012. This represents the southwest transport of Fukushima radionuclides associated with NPSTMW in the area (see stations A00-A10 in panel j of the station map above).

Taken together the measurements show that in Fall 2011 cooling and sinking led to the formation of NPSTMW that carried Fukushima derived Cs to the south and west of the Kuroshio centered on a depth of 300 m that had traveled as far south as 18°N north latitude by September 2012. Using all of their measurements Kaeriyama and colleagues estimate that about 2 PBq of radiocesium from Fukushima or about 9% of the total release was moving south toward the subtropics rather than east in the North Pacific towards North America. The following figure (Figure S9) is taken from the supplemental material for the paper and shows the intrusion of 134-Cs with mode water to the south of Japan:

These results are consistent with the results of Kumamoto and colleagues which also found as much as 10-60% (uncertainty related to total amount of release) of Fukushima Cs in mode water being transported to the south.

Measurements in the north western Pacific, therefore, suggest that previous model based studies may have overestimated the transport of the Fukushima plume to the east towards North America and that maximum activities in the plume that will reach North America in the coming 2 years will be lower than previously thought. Continued measurements of Fukushima derived radionuclides in seawater through the InFORM project will be critical towards understanding the potential for negative ecosystem and public health impacts on the west coast and promise to improve our understanding of Pacific Ocean circulation.