Impact evaluation of typhoons and remediation works on spatiotemporal evolution of air dose rate in two riverside parks in Fukushima, Japan after the Dai-ichi nuclear power plant accident.

This study elucidated the impacts of typhoon events and remediation works on the spatiotemporal evolution of the air dose rate in riverside areas frequented by residents. Spatial distribution of the air dose rate and radiocesium concentration in the sediments were measured in two riverside parks located near each other in Fukushima Prefecture, Japan, for 2015-2020. The air dose rates measured by walk surveys were interpolated using ordinary kriging to generate air dose rate maps, to facilitate a comparison between the results at different points in time during the measurement campaigns. After the typhoons that occurred during 2015-2018, the air dose rate near the riverside in one park decreased, but not in the other, because the erosion and sediment deposition patterns differed between them. This could be due to the presence of a dam upstream, which serves a flood mitigation function. However, the extreme event of typhoon Hagibis in 2019 dropped the air dose rates near the riversides in both parks. In contrast to the typhoon events which affected the riverside areas, remediation works such as decontamination undertaken during 2015-2019 reduced the air dose rates around the garden and lawn areas which are frequently used as recreational sites. Modeling the temporal evolutions in the air dose rates for the entire area of the riverside parks revealed that 35% of the reduction was caused by physical decay of radiocesium on average, 14% by vertical migration of radiocesium in the soil through precipitation, and 51% by the three typhoons and remediation works during 2015-2019. The contribution of 20% from the strongest typhoon Hagibis highlights the fact that floods resulting from large typhoons are effective in causing natural attenuation of air dose rates in riverside parks.