Long-term variations in the distribution of radioactive Cs in plant, soil, stream bottom sand in a small forest in Fukushima prefecture

Radio-Cs concentrations in fresh leaves/needles, litter, surface soil, and stream sand were continuously investigated in a deciduous broadleaf forest and cedar forest in Namie-town, Fukushima prefecture from June 2012 to June 2016, except for snow-cover periods. The result of a car-borne survey from Fukushima city to Minamitsushima showed that the air dose rate declined faster than the physical attenuation due to decontamination, outside of forests. Radio-Cs concentrations (¹³⁷Cs +¹³⁴Cs) in litter and surface soil in broadleaf forest were constant at 52.0, 102 kBq kg-dry^–1, respectively from 2014. In a cedar forest, however, the radio-Cs concentrations in fresh needles and litter declined from 2012 to 2015, probably because of washing and leaching by throughfall, and radio-Cs was accumulated in surface soil. In broadleaf forest, the buffer depth of radio-Cs in soil (1.26 cm) which indicates the extent of infiltration into deeper layers was greater than in the cedar forest (1.14 cm) in April 2013. However, the buffer depth in the cedar forest overtook that in the broadleaf forest in December, 2015 (1.5 cm in broadleaf forest and 2.6 cm in cedar forest). The radio-Cs values in the stream bottom sand were concentrated in smaller sand (over 2 mm, 3.04; 0.21-2.0 mm, 10.2; under 0.21 mm, 54.5 kBq kg-dry^–1 in downstream near the broadleaf forest and over 2.0 mm, 2.67, 0.21-2.0 mm, 7.95; under 0.21 mm, 41.3 kBq kg-dry^–1 in the upstream area near the cedar forest). It is concerned that a part of them causes the outflow of radio-Cs as suspended sand. The relative radio-Cs concentration ratio between smaller bottom sand and surface soil, which indicates the outflow of radio-Cs from forest via stream declined (2013: 0.54, 2016: 0.29 in downstream and 2013: 1.4, 2016: 0.31 in the upstream region). However, we found that floating male flowers of cedar containing high radio-Cs (23.8 kBq kg-dry^–1) could be another transport media in the spring.