Impact of airborne litterfall on radiocesium redistribution in areas adjacent to forests

Following the 2011 Fukushima Daiichi Nuclear Power Plant accident, radiocesium (¹³⁷Cs) was deposited across forested areas. While internal cycling is well known, lateral transfer via litterfall remains unclear. This study quantified ¹³⁷Cs dispersal from Japanese cedar and deciduous broad-leaved forests using collectors set up to 20 m beyond the forest edge. Over 19 months, litterfall mass and ¹³⁷Cs deposition were measured. Litterfall dispersed several meters beyond forest edges, especially under strong winds, and ¹³⁷Cs deposition decreased exponentially with distance. From 2011 to 2018, estimated cumulative deposition via litterfall reached 15.1 % of the initial fallout at 0.5 m and 0.61 % at 8 m in the cedar forest; in the broad-leaved forest, the values were 2.8 % and 0.31 %, respectively. Litterfall outside of the forest contributes significantly to radiocesium redistribution and should be considered in long-term remediation planning.