Transport and Redistribution of Radiocesium in Fukushima Fallout through Rivers

Abstract

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released the most significant quantity of radiocesium into the environment since Chernobyl, and detailed measurements over the initial 5 years provide new insights into fluvial redistribution of radiocesium. We found that the high initial activity concentration of 137Cs-bearing suspended sediment in rivers was followed by a steep exponential decline (λ1) which extended to approximately 1 year after the accident, while the rate of initial decline in radiocesium activity concentration in water was an order of magnitude higher than rates measured after Chernobyl. Fluvial transport of 137Cs to the ocean from the Abukuma river totaled 12 TBq between June 2011 and August 2015 and almost all this radiocesium (96.5%) was transported in the particulate form. The primary sources of 137Cs were paddy fields, farmland, and urban areas [plaque-forming unit (PFU)], discharging 85% of the exported 137Cs from 38% of the watershed area. After 1 year, activity concentrations were lower and exhibited a more gradual secondary decline (λ2) which was associated with reduced radiocesium losses from PFU areas, while forest areas continue to represent more stable contaminant stores.

Temporal Change in Radiological Environments on Land after the Fukushima Daiichi Nuclear Power Plant Accident

“…In fields covered with sufficient vegetation, the discharge rate of radiocesium is low, whereas that in the fields with small plants is relatively high. In some test fields, it was observed that radiocesium discharge rate was drastically pro-moted by weeding, and cultivation tends to accelerate radiocesium migration; in particular, soil puddling before rice planting has played an important role in discharging radioceisum deposited in paddy fields [50]. Therefore, the discharge of radiocesium from farmland contributes significantly to migration of radiocesium at a large scale as discussed next.…”

“…The decreasing tendency of radiocesium concentration in rivers was faster than that observed in the Chernobyl accident, and this is considered to have been due to the difference in environmental conditions [50]. Areas surrounding Chernobyl are dominated by fields and forests, whereas those surrounding the Fukushima NPP include urban and farmland (with paddies).…”

“…Long-distance transport of radiocesium is mostly governed by migration in rivers. Even though the discharge rate of radiocesium from individual land areas is small (generally < 1% of deposited radiocesium per year) [43,50,51], where a dominant river gathers water from a wide drainage basin, the amount of accumulated radiocesium becomes large. For example, the total discharged 137 Cs in the Abukuma River from 2011 to 2015 was estimated to be 12 TBq [43].…”

“…For example, the total discharged 137 Cs in the Abukuma River from 2011 to 2015 was estimated to be 12 TBq [43]. Radiocesium is transported in both dissolved and particulate form in water; it was found that particulates contributed the most to radiocesium migration by river water [43,49,50]. The radiocesium concentrations both dissolved in water and in the suspended sediments have been decreased drastically in the years since the accident; the normalized radiocesium concentration of suspended sediments declined by one to two orders of magnitude from 2011 to 2016 [50,51,52].…”

“…Statistical analysis has implied that the decreasing tendency of radiocesium is related to the proportion of urban and farmland areas in the basin. In the Abukuma River drainage basin, 85% of the discharged radiocesium originated from the 38% of the region that is composed of farmland and urban areas, and the faster decline in radiocesium concentration in Fukushima is considered to have been due to the fact that radiocesium concentration in sediments discharged from urban and farmland area decreased rapidly [50].…”