Radiocesium concentrations in epigeic earthworms at various distances from the Fukushima Nuclear Power Plant 6 months after the 2011 accident

We investigated the concentrations of radiocesium in epigeic earthworms, litter, and soil samples collected from forests in Fukushima Prefecture 6 months after the Fukushima Dai-ichi Nuclear Power Plant accident in 2011. Radiocesium concentrations in litter accumulated on the forest floor were higher than those in the soil (0-5 cm depth). The highest average 134+137Cs concentrations in earthworms (approximately 19 Bq g−1 of wet weight with gut contents and 108 Bq g−1 of dry weight without gut contents) were recorded from a plot that experienced an air dose rate of 3.1 μSv h−1, and earthworm concentrations were found to increase with litter and/or soil concentrations. Average 134Cs and 137Cs concentrations (with or without gut contents) were intermediate between accumulated litter and soil. Different species in the same ecological groups on the same plots had similar concentrations because of their use of the same habitats or their similar physiological characteristics. The contribution of global fallout 137Cs to earthworms with gut contents was calculated to be very low, and most 137Cs in earthworms was derived from the Fukushima accident. Transfer factors from accumulated litter to earthworms, based on their dry weights, ranged from 0.21 to 0.35, in agreement with previous field studies.