Radioactive cesium discharge in stream water from a small watershed in forested headwaters during a typhoon flood event

Abstract

We investigated radioactive cesium (Cs) export in stream water during a typhoon flood event (No. 1204, Guchol) in a small forested headwater catchment (1.2 ha), which was contaminated by the Fukushima Daiichi Nuclear Power Plant (FDNPP) disaster. The experimental watershed is covered with hardwood and conifer trees. Hourly stream water samples were collected by an automatic water sampler, and turbidity was measured every 10 min. Downstream transport of radioactive Cs was recorded through the typhoon, during which the total rainfall was 168 mm. Total cesium-134 ( 134 Cs), cesium-137 ( 137 Cs) and suspended solid (SS) concentrations and turbidity were elevated on the rising limb of the hydrograph. When maximum rainfall intensity, maximum runoff peak were observed, and maximum radioactive Cs and SS concentrations occurred. Total 134 Cs and 137 Cs concentrations were 1.5 and 2.4 Bq L −1 , respectively, at peak flow. However, concentrations of radioactive Cs, SS and turbidity dropped sharply on the receding limb. After filtration (0.7 μm), the concentrations of 134 Cs and 137 Cs were either not detectable (ND) or below 0.09 Bq L −1 . Concentrations of dissolved radioactive Cs in stream water were very low, and radioactive Cs was mainly discharged with suspended materials. Total 134 Cs and 137 Cs exports from the study watershed were 43 and 72 Bq m −2 , respectively. These exports represented approximately 0.07% of the 134 Cs and 137 Cs fallout around the study site (180 kBq m −2 ). We therefore concluded that discharge of radioactive Cs from forested headwaters was negligible even though a typhoon flood event.

Radiocesium transfer from hillslopes to the Pacific Ocean after the Fukushima Nuclear Power Plant accident: A review

“…In this catchment, the particulate fraction only accounted for 40% of the radiocesium migration in base flow conditions. In addition, Shinomiya et al (2014) found that 92e97% of radiocesium is transferred with the particulate fraction during significant rainfall events (Typhoon Guchol), compared to between 12 and 96% in base flow. From modelling and monitoring research, Mori et al (2014), determined that the particulate fraction transported more than 95% of the 137 Cs, slightly above the range calculated by Yamashiki et al (2014) (82e93%).…”

“…At the plot or hillslope scale, Yoshimura et al (2015a) determined that between 0.07 and 3.2% of the catchment 137 Cs inventory was exported with wash-off, with a mean annual export of 0.7% for their 7 USLE plots. For a forested experimental catchment, Shinomiya et al (2014) reported that only 0.07% of the initial radiocesium inventory was exported during a typhoon event. In an outline of Fukushima related research projects, indicated that less than 1% of the radiocesium inventory is transferred from hillslopes and discharged from catchments.…”

“…For example, Shinomiya et al (2014) monitored radiocesium discharge from a small forested catchment (0.012 km 2 ) in Abukuma River basin. The authors found that maximum rainfall intensity and runoff coincided with the maximum radiocesium (3.9 Bq L À1 ) and suspended solid export.…”

“…Although 80% of the fallout from this atmospheric release was over the ocean, 20% of fallout was over terrestrial Japan (Gro€ ell et al, 2014;Kawamura et al, 2011;Kobayashi et al, 2013). The terrestrial fallout from the FDNPP accident occurred in a different environmental context than previous major accidents, occurring predominantly in a coastal mountainous region subject to frequent typhoons (Hilton et al, 2008;Shinomiya et al, 2014).…”

Do forests represent a long-term source of contaminated particulate matter in the Fukushima Prefecture?

“…Conversely, soil erosion and radiocesium export from forest environments is thought to be limited (Shinomiya et al, 2014;Yoshimura et al, 2015) as a thick litter layer of organic matter covers the soil surface, minimizing rainfall driven soil erosion. A key question for ongoing radiocesium management is whether contaminated forest landscapes represent a potential source of radiocesium to downstream and downslope environments.…”