Dynamics of dissolved radiocaesium in forest catchments contaminated by the accident of Fukushima Dai-ichi Nuclear Power Station

A huge amount of radionuclides such as radiocaesium were dispersed and deposited on the territorial area of Fukushima Prefecture, Japan, after the accident at the TEPCO Fukushima Daiichi Nuclear Power Station (FDNPS) on March 2011. Since 70% of the area of Fukushima Prefecture is covered by forest, it is important to clear the distribution, behavior and runoff process of radiocaesium in this area. In this study, we have monitored activity concentrations of dissolved radiocaesium in stream water collected from forest catchments in order to elucidate the behavior of dissolved radiocaesium in this area. There is a very strong correlation between annual mean activity concentration of dissolved ¹³⁷Cs and mean air does rate in catchment area. These results indicate that the activity concentration of dissolved radiocaesium is depended on the radiocaesium inventory around catchment area in the river. Also, dissolved ¹³⁷Cs had seasonal variation and peaked in summer season. The activity concentrations of dissolved ¹³⁷Cs collected in upstream and downstream related to water temperature. This result suggests that one of the sources of dissolved radiocaesium in stream water and river water are desorption from litter layer. However, activity concentrations of dissolved radiocaesium in midstream of Saruta River unrelated to water temperature, suggesting that some part of radiocaesium is likely bonded to an exchangeable fraction of particles, which is easy to leach into the water caused by ion exchange. We conclude that the bio-geochemical processes such as desorption from litter and particles is important processes of radiocasium dynamics in forest catchment. (author)