Radiocesium Fallout in Surface Soil of Tomakomai Experimental Forest in Hokkaido due to the Fukushima Nuclear Accident

Traces of short- and long-lived fallout 134Cs and 137Cs were found in surface soil (volcanic ash soil) under a cool-temperate deciduous stand at Tomakomai Experimental Forest in Hokkaido, Japan after the Fukushima nuclear accident in March 2011. Most of them were present in the uppermost 5–6 cm of the soil. Mean concentrations of 134Cs and 137Cs were found to be 2.4 (±0.3) and 89 (±2) Bq Kg−1 in May, and 6.9 (±0.4) and 94 (±2) Bq Kg−1 in November 2011, respectively. A small increase in radiocesium concentration may result from biological activity in the uppermost portion of the soil in which fallout nuclides derived from the Fukushima NPP would not have existed in May. They were supposed to be fallen down on the fresh litter layer in the previous year. The results of a sequential extraction experiment with 1 M CH3COONH4 solution showed that desorption of radiocesium from the soil was difficult and not simple ion exchange processes.