Impact of clearcutting on radiocesium export from a Japanese forested catchment following the Fukushima nuclear accident

摘要

Changes in ¹³⁷Cs export over time following clearcutting were investigated in a Japanese forested catchment affected by the Fukushima nuclear accident. A total of 13% of the catchment area was clear-cut 2 years after the accident. Annual suspended solids (SS) export at the catchment outlet increased 1.4 to 2.0 times after clearcutting; however, ¹³⁷Cs export increased slightly (up to 1.1 times), corresponding to 0.21% to 0.30% of the ¹³⁷Cs inventory in the catchment. The smaller change in ¹³⁷Cs export than in SS export was due to a rapid decrease in the activity concentration following clearcutting. This decrease was likely caused by both natural attenuation and SS derived from sources with a low activity concentration in the clear-cut area. Monitoring of the sediment transport from hillslopes in small-scale experimental plots showed that the ¹³⁷Cs yield in the skid trail was 3.6 to 21 times greater than those in clear-cut and unlogged forest floors. This significant ¹³⁷Cs transport was due to greater soil erosion (by up to two orders of magnitude) along the skid trail, despite the lower activity concentration than those in the other plots. This indicates that while skid trails were involved in the rapid decrease of the activity concentration of SS, they were a potential source of the increased export of ¹³⁷Cs and SS. Net ¹³⁷Cs export increased by clearcutting (the export excluding the decrease accompanied by natural attenuation) was estimated to account for only 0.092% of the inventory in the catchment for 2.5 years. These results imply that the impact of clearcutting on ¹³⁷Cs export was temporary in this catchment.