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 Dai-ichi 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 (134Cs), cesium-137 (137Cs) 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 134Cs and 137Cs 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 134Cs and 137Cs 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 134Cs and 137Cs exports from the study watershed were 43 and 72 Bq m−2, respectively. These exports represented approximately 0.07% of the 134Cs and 137Cs 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.