Initial radiocesium deposition on forest ecosystems surrounding the Tokyo metropolitan area due to the Fukushima Daiichi Nuclear Power Plant accident

Abstract

The Fukushima Daiichi Nuclear Power Plant accident in 2011 resulted in releases of enormous amounts of radio­nuclides into the atmosphere. The radionuclides were deposited over a large forested area in the Tohoku and Kanto regions. There were few reports about the initial depositions of radionuclides on forest ecosystems during the main emission period. We investigated the initial radiocesium deposition at various forest sites. The deposition of radiocesium by bulk precipitation during the initial few months (approximately until the end of May just after the accident) ranged from 4.4–42.1 kBq m^-2 while that by throughfall ranged from 2.1–36.6 kBq m^-2. The ratio of radiocesium deposition by throughfall to that by bulk precipitation (D_TF/D_BP) ranged from 0.13–0.66 during the first sampling period (approximately until the end of March just after the accident). In the following sampling periods, the radiocesium input by bulk precipitation decreased rapidly and became undetectable. The D_TF/D_BP ratios in these periods increased and then generally exceeded 1.0, meaning that the forest canopies gradually released the entrapped radiocesium. Atmospheric radio­cesium inputs to forest ecosystems were strongly influenced by the forest canopy interception and temporal retention.