Depositional fluxes and residence time of atmospheric radioiodine (131I) from the Fukushima accident

Activities of radioiodine (131I) along with 210Pb and 210Po in time series precipitation samples were measured to determine the depositional fluxes of 131I in the Southern United States and its removal rate and residence time in the atmosphere during the Fukushima nuclear accident. Radioiodine released from the Fukushima accident reached the Southern United States within 11 days, giving rise to a concurrent 131I peak and anomalous 210Po/210Pb ratios in the precipitation samples. The cumulative 131I depositional flux was 4.6 ± 0.2 Bq m−2 during the maximum fallout. The removal rate of 131I out of the atmosphere, derived from a definite 131I integral model, ranged from 0.03 to 0.14 d−1 with an average of 0.08 ± 0.02 d−1, which corresponds to a residence time of 131I in the atmosphere of 12 ± 3 days, consistent with the resident timescale constrained by the 210Po/210Pb disequilibrium technique. These results support our hypothesis that radioiodine was removed from the atmosphere by precipitation within two weeks. It seemed that regions reachable by 131I transport within two weeks from Fukushima Japan would receive much more fallout, whereas places outside that distance would be relatively less polluted with radionuclides. - Highlights: ► Concurrent 131I peak and anomalous 210Po/210Pb ratios were observed in the Southern United States after the Fukushima nuclear accident. ► Cumulative 131I depositional flux was 4.6 ± 0.2 Bq m−2 during the fallout. ► Wet precipitation was the main removal pathway, resulting in detectable 131I activities in surface waters. ► The residence time of atmospheric 131I was estimated to be 12 ± 3 days.