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

Tatsuhiro Nishikiori,Mirai Watanabe,Tetsuo Yasutaka,Seiji Hayashi,Tim A Mousseau

doi:10.1371/journal.pone.0212348

Abstract

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.

Highlights

The Great East Japan Earthquake and the following tsunami damaged the Fukushima Daiichi Nuclear Power Plant (FDNPP) on March 11, 2011 and resulted in the emission of a large amount of 137Cs (8.8–50 PBq) to the atmosphere [1], thereby contaminating a wide area of eastern Japan
At 2 years after the accident, more than 75% of the deposited 137Cs accumulated in the forest floor [12]. 137Cs in the forest floor has been retained in the top soil [13,14,15,16] due to strong adsorption/fixation by clay minerals [3] and assimilation by fungal mycelium [17,18]
The inventories and activity concentrations in soil did not differ significantly between the plots based on the Steel-Dwass test (Table 2), the skid trail had an order of magnitude lower values than those of the other plots

Summary

Introduction

The Great East Japan Earthquake and the following tsunami damaged the Fukushima Daiichi Nuclear Power Plant (FDNPP) on March 11, 2011 and resulted in the emission of a large amount of 137Cs (8.8–50 PBq) to the atmosphere [1], thereby contaminating a wide area (mainly forest) of eastern Japan. These results suggest that 137Cs removal from forest ecosystems is limited in Japan

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Conclusion