Time Dependence of the ¹³⁷Cs Concentration in Particles Discharged from Rice Paddies to Freshwater Bodies after the Fukushima Daiichi NPP Accident

Abstract

The concentration of particulate (137)Cs in paddy fields, which can be a major source of (137)Cs entering the water system, was studied following the Fukushima Daiichi Nuclear Power Plant accident. To parametrize the concentration and to estimate the time dependence, paddy fields covering various levels of (137)Cs deposition were investigated over the period 2011-2013 (n = 121). The particulate (137)Cs concentration (kBq kg-SS(-1)) showed a significant correlation with the initial surface deposition density (kBq m(-2)). This suggests that the entrainment coefficient (m(2) kg-SS(-1)), defined as the ratio between the particulate (137)Cs concentration and the initial surface deposition density, is an important parameter when modeling (137)Cs wash-off from paddy fields. The entrainment coefficient decreased with time following a double exponential function. The decrease rate constant of the entrainment coefficient was clearly higher than that reported for other land uses and for river water. The difference in the decrease rates of the entrainment coefficient suggests that paddy fields play a major role in radiocesium migration through the water system. An understanding of the decrease rate of the entrainment coefficient of paddy fields is therefore crucial to understand the migration of radiocesium in the water system.

Transport and Redistribution of Radiocesium in Fukushima Fallout through Rivers

“…Larger PFU coverage also corresponds to increasing loss of 137 Cs from watersheds during both the first and second periods (Figure C), but the relation is less evident during the second period (Figure G). These data suggest the rapid initial flushing of high concentration 137 Cs from PFU areas as reported in previous studies on the runoff from paddy fields and urban wastewater. , …”

“…The highest 137 Cs activity concentrations occur at the soil surface, and the rapid initial losses of particulate-bound 137 Cs with intense rainfall are concentrated in paddy field and urban areas. In paddy fields, the initial preferential discharge of highly contaminated fine particulate matter is driven by agricultural activity, that is puddling, followed by physical mixing with deeper uncontaminated soil leading to rapid downward migration , and dilution, resulting in the faster rate of decline in riverine particulate 137 Cs activity concentrations . When comparing our results with those for Chernobyl, one of the main differences is the absence of significant ongoing human disturbance in the Chernobyl-affected area after the accident, where forest and abandoned agricultural land are the primary land cover. , …”

“…This analysis suggests that forest areas contributed less to particulate 137 Cs flux and represent more stable contaminant stores . This contrast in radiocesium flux between PFU and forested areas is supported by studies measuring runoff from paddy field, farmland, and small catchments in forested areas. ,,, 137 Cs flux from paddy fields rapidly decreased, while the flux from forested area decreased more slowly.…”

Environmental DNA provides information on sediment sources: A study in catchments affected by Fukushima radioactive fallout

“…However, the identification of the land use contributions was provided with limited resolution so far. Although areas cultivated with rice (Oryza sativa L.) were shown to supply a disproportionate amount of radiocesium contaminated sediment to the region's waterbodies (Yoshimura et al, 2016), uncertainties remain regarding the specific contribution of paddies compared to that of other cultivated fields or that of forested areas to sediment transiting these rivers. Accordingly, the objective of this study is to explore the potential of eDNA as a next generation sediment source fingerprint.…”

Source dynamics of radiocesium-contaminated particulate matter deposited in an agricultural water reservoir after the Fukushima nuclear accident

“…Migration of contaminated particles is accelerated by rice paddy fields that are directly connected to the river network with irrigation systems (Chartin et al, 2013). Accordingly, paddy fields were suggested to be a significant source of particle-bound radiocesium to river systems in the region contaminated by fallout from the FDNPP accident (Lepage et al, 2015;Yoshimura et al, 2016).…”