Annual and weekly cycles of radioactivity concentration observed in Fukushima City.

Weekly cycles in the radioactivity concentration of radiocesium measured in Fukushima City, Japan, are found embedded in an annual cycle of the same. The radioactivity concentration and fallout of radiocesium measured in Fukushima City during the period from June 2011 until July 2013 are analyzed to evaluate mainly their time dependences. Although on average the two quantities decrease at paces much faster than those of local dose rates, both of them show annual cycles, peaking during winter and spring and bottoming otherwise. These annual cycles, which are somewhat out of phase compared to those in Europe, are formed due to weathering. However, the local radioactivity concentration shows weekly cycles also, increasing on weekdays and decreasing at weekends. The causes of these weekly cycles are presumably vehicular traffic and precipitation. Therefore, both human and natural activities are playing important roles at different timescales on atmospheric migration of radiocesium deposited in Fukushima City.