The purpose of this diary is to report results from a recently published, peer reviewed study (behind paywall) examining the degree of Fukushima contamination in fungi and soil of western North America. The diary is the most recent contribution to an ongoing series which aims to provide evidence from scientific studies assessing the impact of the Fukushima Daiichi disaster on the environment and the health of residents of North America. Trappe and colleagues measured the activity of cesium isotopes (134-Cs half life ~ 2 years; 137-Cs half life ~30 years) in wild mushrooms, soil and leaf litter of the west coast from California up to Vancouver Island. The conclusions of the study were as follows:
- No activity measurements exceeded levels thought to impact human health
- 137-Cs activity increased in fungi and soil towards the north
- 134-Cs increased to the south in leaf litter
- Chanterelles did not significantly bioconcentrate Cs isotopes
- 137-Cs and 134-Cs activities were highly variable from sample to sample
- 137-Cs levels largely reflected non-Fukushima sources from either atmospheric weapons tests in the last century or the Chernobyl disaster in 1986
The study by Trappe et al. (2014) was published in the Canadian Journal of Forest Research in August 2014 and can be found here. The authors began with the following hypotheses which motivated the research:
- activity levels of Cs isotopes in wild edible mushrooms were below the U.S. Food and Drug Administration derived intervention limits of 1200 becquerels per kilogram (Bq kg–1)
- Cs activity would be higher in samples farther north due to jet stream influenced precipitation patterns as most deposition of radioisotopes is associated with rain that predominant jet stream patterns bring to the Pacific northwest rather than to California
- Chanterelles would bioaccumulate Cs isotopes at levels above those of their substrates (transfer factor > 1.0)
To test these hypotheses they collected samples of fungi, soil and leaf litter from a variety of sites up and down the west coast of North American approximately 6–10 months after the Fukushima accident. These samples were subsequently analyzed for Cs isotopes at the Oregon State University Radiation Center using high purity germanium detectors that quantify gamma radiation spectra. All activities were decay corrected to April 1, 2011 and reported as Bq kg-1.
Results were as follows.
Hypothesis #1: activity levels of Cs isotopes in wild edible mushrooms were below the U.S. Food and Drug Administration derived intervention limits of 1200 becquerels per kilogram (Bq kg–1)
The following figure summarizes measurements made in Chanterelle mushrooms, soil and leaf litter.
In all cases the summed activities of 134-Cs (which because of its short half life is a fingerprint for Fukushima fallout) and 137-Cs were well below the US Food and Drug Administration intervention level of 1200 Bq kg-1. One Chanterelle sample collected in Bamfield, British Columbia was a significant outlier at 157 Bq kg-1. Including this outlier the average 137-Cs activity for Chanterelles was 12.3 Bq kg-1 with mean activities higher in the Pacific north west (15.1 Bq kg-1) compared to California (0.4 Bq kg-1). The authors hypothesis holds in this case.
Hypothesis #2: Cs activity would be higher in samples farther north due to jet stream influenced precipitation patterns as most deposition of radioisotopes is associated with rain that predominant jet stream patterns bring to the Pacific northwest rather than to California
The authors found that 137-Cs activity tended to increase towards the north as the following figures showing the 137-Cs content of soil and Chanterelles with latitude demonstrate.
Unlike 137-Cs, 134-Cs activities were comparably lower than 137-Cs and were highest in the south and decreased towards the north or showed little trend with latitude. The following figure demonstrates this trend in leaf litter samples.
These trends and relative activities give us important information. Firstly, given that the cesium isotopes were released to the atmosphere in roughly a 1:1 ratio (134-Cs/137-Cs = 1) in March and April 2011 the low decay corrected activity ratio of 134-Cs/137-Cs in mushrooms, soil and leaf litter in the study suggest that significant amounts of 137-Cs did not come from Fukushima. This 137-Cs is likely present from atmospheric weapons testing in the 20th century or the Chernobyl disaster in 1986 which both released much larger amounts of 13-Cs to the environment.
The trend towards higher activities of 134-Cs in the south refutes the authors hypothesis that Fukushima fallout would be higher in the north given average precipitation patterns. However, at the time the main Fukushima plume arrived at the North American west coast March 17-20, 2011, the jet stream was positioned much farther to the south than average and brought 4.4 cm of rain to San Francisco (maps can be accessed here). During that same period Corvallis, Oregon, reported only 1.9 cm and here where I live in Victoria, British Columbia only 1.2 cm of rain fell. The anomalous path of the jet stream during the plumes arrival is thus consistent with the observations that 134-Cs levels and Fukushima fallout were slightly higher in California than in the Pacific north west in 2011.
Little evidence was found that Chanterelle mushroom species were bioaccumulating either 134-Cs or 137-Cs higher than levels in the soil in which they grew. The average concentration factor (mushroom activity/soil activity) for 137-Cs was 1.0 while the median was 0.4. For 134-Cs the concentration factor was on average 1.4 with a median of 0.7. Some samples had high concentration factors for both isotopes (6.0-6.5) however the majority of samples were below 1.0. This data suggests that Hypothesis #3 does not hold.
The activities of Cs isotopes in mushrooms, soil and litter from before and after the Fukushima disaster began are compared in the following table.
The authors conclude from this comparison that the much of the activity they detected in their study predated the Fukushima disaster. This is in keeping with measurements which suggest that the levels of fallout from the Fukushima disaster in western North America was about a factor of 10 lower than from the Chernobyl disaster and many orders of magnitude less significant than weapons testing fallout in the 20th century.