Long-term assessment of airborne radiocesium after the Fukushima nuclear accident: re-suspension from bare soil and forest ecosystems

Abstract

Abstract. The long-term effect of 137 Cs re-suspension from contaminated soil and forests due to the Fukushima nuclear accident has been quantitatively assessed by numerical simulation, a field experiment on dust emission flux in a contaminated area (town of Namie, Fukushima prefecture), and air concentration measurements inside (Namie) and outside (city of Tsukuba, Ibaraki prefecture) the contaminated area. In order to assess the long-term effect, the full year of 2013 was selected to study just after the start of the field experiments. The 137 Cs concentrations at Namie and Tsukuba were approximately 10 −1 -1 and 10 −2 -10 −1 mBq m −3 , respectively. The observed monthly median concentration at Namie was 1 to 2 orders of magnitude larger than that at Tsukuba. This observed difference between the two sites was consistent with the simulated difference, indicating successful modeling of 137 Cs re-suspension and atmospheric transport. The estimated re-suspension rate was approximately 10 −6 day −1 , which was significantly lower than the decreasing rate of the ambient gamma dose rate in Fukushima prefecture (10 −4 -10 −3 day −1 ) as a result of radioactive decay, migration in the soil and biota, and decontamination. Consequently, re-suspension contributed negligibly in reducing ground radioactivity. The dust emission model could reproduce the air concentration of 137 Cs in winter, whereas the summer air concentration was underestimated by 1 to 2 orders of magnitude. Re-suspension from forests at a constant rate of 10 −7 h −1 , multiplied by the green area fraction, could explain the air concentration of 137 Cs at Namie and its seasonal variation. The simulated contribution of dust resuspension to the air concentration was 0.7-0.9 in the cold season and 0.2-0.4 in the warm season at both sites; the remainder of the contribution was re-suspension from forest. The re-suspension mechanisms, especially through the forest ecosystems, remain unknown. This is the first study that provides a crude estimation of the long-term assessment of radiocesium re-suspension. Additional research activities should investigate the processes/mechanisms governing the re-suspension over the long term. This could be achieved through conducting additional field experiments and numerical simulations.

The seasonal variations of atmospheric 134,137Cs activity and possible host particles for their resuspension in the contaminated areas of Tsushima and Yamakiya, Fukushima, Japan

“…These concentrations are at least two orders of magnitude higher than those observed in East Japan before the FDNPP accident (~1 μBq/m 3 ; Igarashi et al 2015). Several studies showed that this atmospheric radiocesium could not be attributed to the direct emissions/leakage from the FDNPP site (e.g., Igarashi et al 2015;Kajino et al 2016) but could be supplied by secondary emissions of the deposited radiocesium, which is called resuspension to the atmosphere (e.g., Igarashi 2009;Igarashi et al 2015;Kajino et al 2016). Igarashi et al (2015) concluded that the delayed primary emissions from the FDNPP could not be a major source of the current (till March 2015) radiocesium in the atmosphere based on the atmospheric observations in Tsukuba and the emission inventory data from Tokyo Electric Power Co., Inc. (TEPCO) (2012).…”

“…In this study, the sampling sites are located in low mountainous areas and are surrounded by forests, and thus, it is natural that the increases of bioaerosols significantly contribute to resuspension, unlike in urban areas. Kajino et al (2016) simulated resuspension from the soil (Ishizuka et al 2017) and that from vegetation (details were showed in the "Introduction" section), succeeding in reconstructing the 137 Cs activity concentration levels that we observed. In their results, most of atmospheric 137 Cs in summer/autumn was supplied by biogenic emissions, which is consistent with our results.…”

“…Igarashi et al (2015) concluded that the delayed primary emissions from the FDNPP could not be a major source of the current (till March 2015) radiocesium in the atmosphere based on the atmospheric observations in Tsukuba and the emission inventory data from Tokyo Electric Power Co., Inc. (TEPCO) (2012). Kajino et al (2016) also showed that resuspension could predominantly contribute to the atmospheric radiocesium concentrations in the year 2013 using a 3D aerosol model simulation and the emission data from TEPCO.…”

“…However, their study only focused on the resuspension from winter to spring; the seasonal variations and other processes of resuspension were not discussed. A recent work by Kajino et al (2016) has revealed that the forest ecosystem could be another source of radiocesium during the warm seasons, in addition to that of dust suspension. These authors applied a novel scheme for radiocesium resuspension from the forest (as the green fraction) that operates according to forest activity as measured by the normalized difference vegetation index (NDVI) satellite data (Gutman and Ignatov 1998).…”

“…These authors applied a novel scheme for radiocesium resuspension from the forest (as the green fraction) that operates according to forest activity as measured by the normalized difference vegetation index (NDVI) satellite data (Gutman and Ignatov 1998). However, Kajino et al (2016) showed a model simulation application and did not reveal the actual host material of radiocesium. Recently, Ochiai et al (2016) added new observational results of the monthly radiocesium concentrations (September 2012 to December 2014) at a contaminated area in western Tsushima that is near our present observation site.…”