This diary is a continuation of a series that aims to communicate what ongoing measurements carried out by the scientific community are telling us about the impact of Fukushima sourced radionuclides on the health of the marine environment and public on the west coast or North America. While a previous post focused on plutonium this diary summarizes what is known about the release of Strontium-90 (90-Sr) a beta-emitting element that is a radiological health concern given its similar chemistry to the nutrient calcium (Ca). Wherever possible I have linked to open-access, peer-reviewed scientific studies. According to measurements of 90-Sr in the air, soil and seawater, releases of 90-Sr were about 30-10,000 fold less than 137-Cs. The release of 90-Sr from Fukushima is similar to the release from the Chernobyl disaster in 1986 and about 600-fold lower than the releases from atmospheric weapons tests that peaked in the mid-1960's.
Those unfamiliar with the units scientists use to discuss radioactivity should consult a previous diary where a brief review is presented.
The total amount of measurements made to determine the beta radiation emitter 90-Sr (half-life 28.9 years) in environmental samples is significantly lower than measurements of the gamma and beta-emitting isotopes 134-Cs (half-life ~2 years) and 137-Cs (~30.2 years) because the analysis of 90-Sr requires significantly more sample processing and handling to separate it from other beta-emitters. What followers here is a brief summary of what measurements indicate with respect to the release of 90-Sr to air, soil and seawater.
90-Sr in the Air
Similar to plutonium isotopes, as described in the modeling study of Shwantes et al. (2012) behind paywall (freely available) 90-Sr is, despite having similar production rates in reactors and a similar half-life, predicted to have lower percentage reactor inventory releases from Fukushima given its lower volatility and mobility compared to 137-Cs.
In the days following the documented, significant releases of radionuclides to the atmosphere, the Fukushima atmospheric plume of contamination was detected over North America by US and Canadian monitoring stations. A study by Smith and others published in 2014 in the Journal of Environmental Protection documented the arrival of the plume and determined that the activities of isotopes of radiological health concern (Iodine-131, 137-Cs, 134-Cs) were present at levels that were 10 times lower than the amounts detected over the San Francisco Bay area in the days following the Chernobyl disaster in 1986. In addition, Smith and colleagues also analyzed rainwater samples collected in central California for the presence of 90-Sr. While 137-Cs was detected in the rainwater at activities of 0.01-0.4 Bq/L with an average of ~0.14 Bq/L no 90-Sr was detected in any sample. The detection limit for 90-Sr was ~0.009 Bq/L so conservatively assuming 90-Sr was present at activities exactly equal to the detection limit the 137-Cs/90-Sr level in rain during the peak of atmospheric fallout from Fukushima was ~11. As we will see below this is likely a minimum ratio and that 90-Sr release was likely much lower than that of 137-Cs.
90-Sr in Soil and Vegetation in Radionuclide Hot Spots in Japan
In a paper published in 2013 Steinhauser and colleagues in the open-access, peer-reviewed journal PLOS One measured the relative activities of 90-Sr and 137-Cs in soil and vegetation samples in areas in Japan known to have received significant atmospheric deposition of Fukushima fallout.
Locations sampled for 90-Sr and Cs isotopes in Japan relative the Fukushima reactors
The activity of 90-Sr was generally 1,000-10,000 times lower than the measured activity of 137-Cs in soil and plants.
Activity concentrations of 90-Sr and 90-Sr + 137-Cs in soil and vegetation samples from radioactive hot spots in Japan.
90-Sr Released to Seawater
Measurements by
Casacuberta and others published in 2013 in the open-access, peer-reviewed journal Biogeosciences documented the peak concentrations of 90-Sr and 137-Cs released to the coastal and open ocean off Japan.
They evaluated the distribution in waters 30-600 km offshore in May-June 2011 after the release had peaked and began to diminish from the reactor sites and found that direct releases of 90-Sr were about 2.6% of 137-Cs releases.
Summary
Given the relatively low volatility and mobility of 90-Sr compared to isotopes like 137-Cs during the disaster at the Fukushima reactors the relative release of 90-Sr to the air, land and seawater were less significant. Overall, the ratio of 137-Cs/90-Sr releases appears to be between about as low as 30 in direct discharge to the ocean where the isotopes were mobilized by waters used to cool the damaged reactors to ~1,000-10,000 in releases to the atmosphere. The total activity of 90-Sr released to the environment from Fukushima is ~1-2 PBq (PetaBq = 10^15 Bq) compared to 600 PBq released during peak atmospheric weapons testing in the 20th century.
Releases of 137-Cs and 90-Sr to the ocean continue at the Fukushima site to this day. Fortunately, ongoing measurements by international investigators indicate that these releases are occurring at rates that are 10,000-100,000 lower than they were in the weeks following the disaster in March-April 2011. Models and measurements indicate that maximum activities of 137-Cs in North Pacific waters near to North America will be between 1 and 30 Bq/m^3 with most measurements suggesting values toward the lower end of the range. According to health professionals these levels, and even the much higher levels in coastal waters off Japan, are unlikely to lead to significant radiological health threats to the public. Given that 90-Sr was released in much lower amounts and does not concentrate in marine biota as significantly as does 137-Cs the radiological health risk posed by this isotope given current conditions in the North Pacific is very low.
Given current conditions at the disaster site it is possible that significant releases of both 90-Sr, 137-Cs and other isotopes could still occur. Releases that approach release rates in Spring 2011 could lead to higher maximum concentrations in the North Pacific that would necessitate a reevaluation of the potential environmental and public health risks. Ongoing monitoring efforts need to improved and expanded to best inform the public in the coming years.