The absence of any detectable 134-Cs (an unambiguous fingerprint isotope of Fukushima contamination) in the seawater samples indicates that as of November 2014 these locations covering the length of the British Columbia coast have not be affected by ocean currents carrying Fukushima contamination.
The detection limit for 134-Cs averages ~0.2 Bq m-3
Newly added results come from seawater samples collected in collaboration with citizen scientists at the following locations in British Columbia, Canada during November 2014.
Samples were processed and the amount of gamma emitting isotopes determined using a high purity germanium detector. We look primarily for radioisotopes of cesium (134-Cs half life ~2 years and 137-Cs half life ~ 30 years) for the following reasons:
134-Cs has a half life that is short enough that all other human sources to the environment have decayed away making it an ideal tracer for Fukushima contamination
next to the short lived Iodine-131 (half life ~ 8 days), Cs isotopes were released in greatest activity to the environment from Fukushima and would be most likely to represent a radiological health risk given their chemistry and propensity to be taken up by the biota
other isotopes were released in much lower amounts from Fukushima relative to Cs (see other posts here and search for plutonium and strontium for example) and would therefore be much more difficult to detect
because they are gamma emitters (unlike Pu isotopes and 90-Sr which emit alpha and beta radiation respectively) they are relatively easy and resource efficient to detect
The absence of detectable 134-Cs indicates that waters near these locations spanning the length of British Columbia have not been contaminated with Fukushima radioactive elements transported across the Pacific by prevailing currents as of Nov 2014. The presence of 137-Cs is due to historical sources of this human made isotope owing to atmospheric nuclear weapons tests in the 20th century and contamination from the Chernobyl disaster in 1986. You can read about the levels of 137-Cs in the North Pacific pre-Fukushima here.
More results will be published as they become available.
This short blog summarizes an open access paper published today reporting results from a Canadian monitoring program tasked with documenting the arrival of ocean borne Fukushima contamination along the North American Pacific coast. This diary is part of an ongoing effort to communicate the best science available on the impacts of the Fukushima Dai-ichi meltdowns on the environment. High quality measurements to look for Fukushima derived radiocesium were made in seawater in the North Pacific and Arctic Oceans from 2011 to early 2014. The authors concluded that:
Fukushima derived radiocesium was first detected 1500 km west of British Columbia Canada in June 2012
Contamination was detected on the continental shelf (near coastal waters) in June 2013
By February 2014 Fukushima radiocesium was present at levels similar to preexisting weapons testing derived 137-Cs
These same models predict that total radiocesium levels from weapons testing fallout and Fukushima will likely reach maximum values of ~3-5 Becquerel per cubic meter (Bq m-3 of seawater in 2015-2016 and then decline to fallout background level of ~1 Bq m-3 by 2021
Fukushima will increase northeastern Pacific water to levels last seen in the 1980’s but does not represent a threat to environmental or human health
The Integrated Fukushima Ocean Radionuclide Monitoring (InFORM) project is a network involving academic, governmental, and non-governmental organizations, as well as citizen scientists. InFORM is acquiring data to support a thorough radiological impact assessment for Canada’s west coast stemming from the Fukushima Dai-ichi nuclear power plant (FD-NPP) accident, and to effectively communicate these results to the public. The first measurements undertaken as part of the InFORM project to look for Fukushima derived radionculides were made on 19 fish in collaboration with the Nisga’a First Nation. Sockeye salmon (Oncorhynchus nerka) and Steelhead Trout (Oncorhynchus mykiss) were caught off the west coast of Canada in Summer 2014 as they were returning to the Nass River in northern BC.
Measurements of radioactive elements in these fish are reported in the data table below and are available for download through Health Canada:
Samples Combined (average)
Numbers with “<” indicate that levels were below the detection limit of the analysis and numbers inside parenthesis “( )” report the uncertainty associated with the measurement.
What are we measuring and why?
The triple meltdowns at the Fukushima Dai-ichi nuclear power plant (NPP) released many different radioisotopes to the environment, however only a very few of these are both measurable and unique to Fukushima. A reliable finger-print radioisotope for Fukushima is Cesium-134 (134-Cs half life ~ 2 years). This is because 134-Cs is only produced in nuclear reactors and it has a relatively short half-life, so that 134-Cs from other human sources, like the Chernobyl NPP disaster in 1986, are no longer present in the environment. Other isotopes such as Cesium-137 (137-Cs half life ~30 years) are not positive indicators of Fukushima since they were also a products of atmospheric testing in the 20th century and Chernobyl and are still present in the environment from these legacy sources.
How scientists talk about radioactivity in the environment
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). The measurements above represent that numbers of Bq detected in a kilogram of fish flesh.
Measurements of Sockeye Salmon and SteelheadTrout
We measured the activities of cesium radioisotopes 134-Cs and 137-Cs that were released in large quantities from the Fukushima Dai-ichi Nuclear Power Plant disaster in 2011. We also measured naturally occurring radioisotopes Potassium-40 (40-K) and Polonium-210 (210-Po) that always contribute doses of radiation to human consumers of marine fish. Samples of sockeye salmon and steelhead returning to the Nass River in northern BC, obtained from the Nisga’a First Nation, were analyzed and none were found to contain detectable levels of Fukushima derived radionuclides. By adding together the signals obtained for all of the samples we calculate an average activity concentration of 0.27 Bq/kg for 137-Cs. This represents residual 137-Cs in the North Pacific largely from atmospheric nuclear weapons testing in the last century. At present, Fukushima derived radionuclides cannot be detected. Nuclear weapons testing fallout (137-Cs) can be detected in BC fish at levels that represent a fraction of the radiation exposure owing to naturally occurring radionuclides neither of which represent a dangerous health risk to consumers in Canada. Results for a further ~80 fish collected this summer and fall from various returning runs up and down the BC coast will be reported as samples are analyzed.
19 fish samples, flesh only (9 Sockeye Salmon and 10 Steelhead Trout) were obtained from the Nisga’a First Nation
A sub-sample of ~125g of wet mass (average wet mass (measured) = 126.67 g) was taken and placed in a sample counting container
Gamma-radiation emitting isotopes were measured by gamma spectroscopy using a high purity germanium detector and each sample being counted for 6 hours
Planar BE5030 high purity germanium (HPGe) detector with relative counting efficiency of 46%
counting geometry = Parkway Jar (Polyethylene, active volume = 129 mL) placed in a polyacrylate sample holder (2.95 mm thickness)
counting efficiency was determined by applying an empirical efficiency curve, determined from a multi-nuclide (12) standard (Eckert and Ziegler Analytics, SRS: 79535-411) of similar density (1.15 g/cm^3, Parkway Jar format) spanning energies of 46.5 – 1836.1 KeV
true coincidence summing (where applicable) is accomplished by extracting detailed decay scheme data from the UniSampo-Shaman nuclide library
Spectral summation was done by adding all the spectrum into a single one and then reanalysed using UniSampo-Shaman gamma spectral analysis software from Baryon Oy, Ltd., Finland, with decay correction to the catch date of August 28, 2014