Small scale temporal distribution of radiocesium in undisturbed coniferous forest soil: Radiocesium depth distribution profiles

The depth distribution of pre-Fukushima and Fukushima-derived 137Cs in undisturbed coniferous forest soil was investigated at four sampling dates from nine months to 18 months after the Fukushima nuclear power plant accident. The migration rate and short-term temporal variability among the sampling profiles were evaluated. Taking the time elapsed since the peak deposition of pre-Fukushima 137Cs and the median depth of the peaks, its downward displacement rates ranged from 0.15 to 0.67 mm yr−1 with a mean of 0.46 ± 0.25 mm yr−1. On the other hand, in each examined profile considerable amount of the Fukushima-derived 137Cs was found in the organic layer (51%-92%). At this moment, the effect of time-distance on the downward distribution of Fukushima-derived 137Cs seems invisible as its large portion is still found in layers where organic matter is maximal. This indicates that organic matter seems the primary and preferential sorbent of radiocesium that could be associated with the physical blockage of the exchanging sites by organic-rich dusts that act as a buffer against downward propagation of radiocesium, implying radiocesium to be remained in the root zone for considerable time period. As a result, this soil section can be a potential source of radiation dose largely due to high radiocesium concentration coupled with its low density. Generally, such kind of information will be useful to establish a dynamic safety-focused decision support system to ease and assist management actions.