As in previous diaries it is important to understand that 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).
All of the measurements reported in the study were made at the Low Background Facility at the Lawrence Berkeley National Laboratory in California. Beginning on March 14, 2011 (international pi day for those who are interested in such things http://www.piday.org/) an air sampler was operated at the facility to track the arrival of gamma emitting radionuclides transported in the atmosphere from Fukushima to the San Francisco Bay area. Measurements at LBNL are ongoing but the paper reports measurements made until the end of 2012. Radionuclides in aerosols >0.3 microns were collected on HEPA filters and counted using sensitive gamma radiation spectrometers. Activities of Fukushima radionuclides in air 137-Cs, 131-I, 132-Te tied to the presence of short-lived 134-Cs (t1/2 = 2.06 years) are presented in Bq/m3 in the figure below:
Long term airborne concentrations fission products from March 11, 2011 to the end of 2012 as measured on HEPA filters in Berkeley, CA. Horizontal error bars, when visible, represent filter exposure periods. Missing error bars are smaller than the marker. Plotted uncertainties seen here are statistical only, and a conservative systematic uncertainty of 10% should also be assumed.
Activities peaked shortly after the monitoring program began roughly 14 days after the disaster with peak activities of 131-I of ~15 milliBq/m^3 of air. It is important to note that the filters used to determine 131-I from Fukushima and the Chernobyl incident do not retain gaseous 131-I which can account for 50-70% of the activity. In comparison the activities of the same isotopes arriving to the Bay Area due to Chernobyl fallout after April 1986 are shown plus 103-Ru in the following figure:
Airborne concentrations fission products starting April 26, 1986 following the Chernobyl disaster as measured on a HEPA filter at the LBF in Berkeley, CA. Horizontal error bars, when visible, represent filter exposure periods. Missing error bars are smaller than the visible data marker. Plotted uncertainties seen here are statistical only, and a conservative systematic uncertainty of 10% should also be assumed.
The activities of airborne fission products delivered to San Francisco after April 26, 1986 from Chernobyl are a factor of 10 greater than the same radionuclides transported through the atmosphere from Fukushima in 2011.
The investigators also measured for Fukushima fallout radionuclides in rainwater in Oroville, CA and found peak activities of 131-I of 16 Bq/L on March 24, 2011. Results for rainwater are presented below in units of Bq/L:
Observation of Fukushima fallout in rainwater collected in Oroville, CA over the entire monitoring duration to the end of 2012. 7-Be is shown as a comparison to a naturally produced airborne isotope. Horizontal error bars, when visible, represent filter exposure periods. Missing error bars are smaller than the visible data marker. Plotted uncertainties seen here are statistical only, and a conservative systematic uncertainty of 10% should also be assumed.
90-Sr was not detected (<~9 milliBq/L) in rainwater during the study. This likely reflects both the lower fission yield (5.78%) and volatility of 90-Sr compared to 137-Cs (6.19% yield).
3:51 PM PT: Levels in food, air and water will determine the impacts on ecosystem and human health. Atmospheric transport from Fukushima, at 10-fold less than Chernobyl fallout, should have equal or less impact on the Bay Area than Chernobyl did in 1986. The cumulative effective dose from Fukushima radionuclides in foodstuffs purchased in October 2013 in the Bay Area was near to zero as no short-lived 134-Cs from Fukushima could be detected. Radwatch http://radwatch.berkeley.edu/ is a great site that reports information specific to the Bay Area and have been measuring for Fukushima radionuclides in salmon harvested from Alaska. If you live in the Bay Area you should follow that site.
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