Influence of Forest Floor Covering on Radiocesium Wash-off Associated with Forest Soil Erosion

Abstract

To examine the degree of ¹³⁷Cs wash-off from forest floor, we monitored the amount of particulate and dissolved ¹³⁷Cs in surface runoff water from three m² of experimental plots installed on slopes of 37-39° in four forests with different floor covering in Fukushima Prefecture. Plots were monitored for 145 days from May to October, 2013. A maximum of 1.1% of the ¹³⁷Cs inventory in soil was washed off, with the amounts differing by up to 10-fold between the plots. The largest amount was observed in Japanese cypress forest, followed by deciduous broadleaf forest, Japanese red pine forest and Japanese cedar forest. Most (more than 96%) of the ¹³⁷Cs washed off was associated with particles. The amount of soil loss was relatively large in forests with little understory and/or organic horizon, suggesting that the forest floor covering strongly affected ¹³⁷Cs wash-off. In both suspended solids in surface runoff water and stream water, the activity concentration of ¹³⁷Cs correlated positively with the organic matter amount, suggesting that some of the ¹³⁷Cs in stream came from forest floor and that the organic horizon was an important source of the ¹³⁷Cs. Japanese cypress forest is prone to poor floor covering but covers a small area of Fukushima Prefecture, suggesting that the broadleaf forest floor covering, which is most widely distributed in the area, greatly influences the ¹³⁷Cs wash-off from the forest.