Ecosystem Monitoring of Radiocesium Redistribution Dynamics in a Forested Catchment in Fukushima After the Nuclear Power Plant Accident in March 2011

Abstract

The accident at the Fukushima Daiichi Nuclear Power Plant in March 2011 emitted 1.2 × 1016 Bq of cesium-137 (137Cs) into the surrounding environment. Radioactive substances, including 137Cs, were deposited onto forested areas in the northeastern region of Japan. 137Cs is easily adsorbed onto clay minerals in the soil; thus, a major portion of 137Cs can be transported as eroding soil and particulate organic matter in water discharge. Dissolved 137Cs can be taken up by microbes, algae, and plants in soil and aquatic systems. Eventually, 137Cs is introduced into insects, worms, fishes, and birds through the food web. To clarify the mechanisms of dispersion and export of 137Cs, within and from a forested ecosystem, we conducted intensive monitoring on the 137Cs movement and storage in a forested headwater catchment in an area approximately 50 km from the Nuclear Power Plant. Two major pathways of 137Cs transport are as follows: (1) by moving water via dissolved and particulate or colloidal forms and (2) by dispersion through the food web in the forest-stream ecological continuum. The 137Cs concentrations of stream waters were monitored. Various aquatic and terrestrial organisms were periodically sampled to measure their 137Cs concentrations. The results indicate that the major form of exported 137Cs is via suspended matter. Particulate organic matter may be the most important carrier of 137Cs. High water flows generated by a storm event accelerated the transportation of 137Cs from forested catchments. Estimation of 137Cs export from the forested catchments requires precise evaluation of the high water flow during storm events. The results also suggested that because the biggest pool of 137Cs in the forested ecosystem is the accumulated litter and detritus, 137Cs dispersion is quicker through the detritus food chain than through the grazing food chain.