Abstract
Radioactive cesium contamination caused by Fukushima nuclear disaster was investigated in suburban forest patches in Kashiwa city, Chiba prefecture, central Japan. Air dose rate at 1 m aboveground in summer to winter of 2011 was 0.3-0.4 μSv/h either inside or outside the forest patches. Radiocesium concentration of twigs and old leaves of evergreen broad-leaves and conifers was 1.2-8.8 kBq/kg, while that of current leaves of evergreen broadleaved and deciduous trees was 0-2.8 kBq/kg. Most ground beetles collected in summer to autumn of 2011 showed lower concentration than 5 kBq/kg, while mushrooms showed higher concentration with the maxium value of 61 kBq/kg. From measurements of tree branches and wood discs collected from thinned trees, total deposition of radiocesium in the forest patches was estimated. The radiocesium concentration was highest in outer bark of both conifers and deciduous broadleaves and in old leaves of conifers. The contamination level was so high that firewood and compost made in the stand could not be used. Deposition of radiocesium on aboveground parts of forests was estimated as 5.7 kBq/m^2 in a mixed forest of Hinoki and deciduous broadleaves and 3.7 kBq/m^2 in a deciduous oak forest, respectively. Belowground deposition was 85 kBq/m^2 in the deciduous stand. The total amount of radiocesium above- and below-ground was estimated to be 90 kBq/m^2. The soil measurement made in January of 2013 in Konara patches showed that most of radiocesium moved to A layer of the soil, therefore decontamination by litter clearing seemed difficult in these forests.
