Relationship between Outflow of Radiocesium and Forest Floor Conditions at a Decontaminated Forest in Fukushima

Splashed particles due to raindrop erosion are observed at a radioactively contaminated deciduous broadleaf forest in the Abukuma Mountains, located approximately 35 km northwest of the Fukushima Daiichi Nuclear Power Plant. Morgan Splash Cups, which allow splashed particles to be measured, were installed at three experimental plots with different forest floor cover conditions for one month in October 2016, and from late September to the end of October 2017. Forest floor cover conditions are assessed, and collected data are analyzed to examine correlations among rainfall characteristics, forest floor cover rate, grain size parameters (sorting and skewness), splashed particle weight, and 137Cs transport. The results reveal no significant correlations between rainfall intensity or gross precipitation and grain size parameters. Similarly, no significant correlation is found between forest floor cover and grain size parameters. However, when comparing grain size parameters of plots with forest floor cover rates below 60% and those above 90%, it is observed that a plot with a cover rate exceeding 90% exhibited a larger standard deviation in grain size parameters. When normalizing splashed particle weight by gross precipitation, the experimental plot with a forest floor cover rate exceeding 90% showed a tendency for variability to converge, compared to plots with a cover rate below 60%. This indicates that the variation in 137Cs transport is smaller at the plot with a cover rate exceeding 90%. These findings suggest that the effects of forest floor cover on raindrop erosion are not a simple correlation. Instead, the protective effects of the forest floor become significantly higher within a specific cover range (60-90%), leading to a shift in the rainsplash behavior of forest floor soil.