Relationship between particle size and radiocesium in fluvial suspended sediment related to the Fukushima Daiichi Nuclear Power Plant accident

Mineral composition characteristics of radiocesium sorbed and transported sediments within the Tomioka river basin in Fukushima Prefecture

The deposited radiocesium in the Fukushima river basin is transported in the river systems by soil particles and redistributed in the downstream areas. Although predicting the behaviors of minerals that adsorb radiocesium and of radiocesium dissolved in river water within the river systems is essential, the dominant mineral species that adsorb radiocesium have not yet been comprehensively identified. We identify herein such mineral species by investigating the ¹³⁷Cs distribution and the mineral species in each size fraction that are found in the bedload sediments from an upstream reservoir to an estuary within the Tomioka river basin located east of Fukushima Prefecture in Japan. In the fine sand sediment, which is the dominant fraction in terms of the ¹³⁷Cs quantity in the river bedload, the ¹³⁷Cs concentrations of the felsic and mafic minerals are comparable to that of micas. The mafic minerals contain 62% of the ¹³⁷Cs in the fine sand fraction in the upstream area, while the felsic minerals contain the highest quantities of ¹³⁷Cs in the downstream area. These results suggest that the quantification of the mineral species and the ¹³⁷Cs concentration of each size fraction are critically important in predicting the behaviors of the minerals and radiocesium within the Fukushima river basin in the future.

Modeling watershed-scale 137Cs transport in a forested catchment affected by the Fukushima Dai-ichi Nuclear Power Plant accident

The Fukushima nuclear accident in 2011 resulted in ¹³⁷Cs contamination of large areas in northeast Japan. A watershed-scale ¹³⁷Cs transport model was developed and applied to a forested catchment in Fukushima area. This model considers ¹³⁷Cs wash-off from vegetation, movement through soils, and transport of dissolved and particulate ¹³⁷Cs adsorbed to clay, silt and sand. Comparisons between measurements and simulations demonstrated that the model well reproduced ¹³⁷Cs concentrations in the stream fed from the catchment. Simulations estimated that 0.57 TBq of ¹³⁷Cs was exported from the catchment between June, 2011 and December, 2014. Transport largely occurred with eroded sediment particles at a ratio of 17:70:13 of clay, silt, and sand. The overall ¹³⁷Cs reduction ratio by rainfall-runoff wash-off was about 1.6%. Appreciable ¹³⁷Cs remained in the catchment at the end of 2014. The largest rate of ¹³⁷Cs reduction by wash-off was simulated to occur in subwatersheds of the upper catchment. However, despite relatively low initial deposition, middle portions of the watershed exported proportionately more ¹³⁷Cs by rainfall-runoff processes. Simulations indicated that much of the transported ¹³⁷Cs originates from erosion over hillsides and river banks. These results suggested that areas where ¹³⁷Cs accumulates with redeposited sediments can be targeted for decontamination and also provided insight into ¹³⁷Cs transport at the watershed scale to assess risk management and decontamination planning efforts.

Linking sedimentary total organic carbon to 210Pbex chronology from Changshou Lake in the Three Gorges Reservoir Region, China

The influences of total organic carbon (TOC) and total nitrogen (TN) on Lead-210 (²¹⁰Pb) dating have recently been of increasing concern in lacustrine research. Sediment core from Changshou Lake in the Longxi catchment was investigated for influence of TOC on ²¹⁰Pb dating. Lead-210 excess (²¹⁰Pb_ex), Cesium-137 (¹³⁷Cs) activities, TOC, TN, and particle size were measured. We proposed a dating index based on ¹³⁷Cs chronology and particle size distribution of the lake sediment profile and rainfall erosivities calculated from Longxi catchment metrological records. Increasing trends in TOC and TN were specifically caused by commercial cage fish farming after 1989. The statistically significant correlation between ²¹⁰Pb_ex activity, TOC (0.61, p = 0.04) and TN (0.51, p = 0.04), respectively explained post-1989 ²¹⁰Pb scavenging. The ²¹⁰Pb_ex activity was closely related with coupled peaks of TOC and TN from mass depth 5-10 g cm^-2. Higher TOC/TN ratio (8.33) indicated submerged macrophytes and native aquatic algal growth as main source of carbon from enhanced primary productivity because of massive fertilizer use and coherent climate warming. The study supported key hypothesis on vital role of fertilizer usage and algal derived TOC in controlling sedimentary ²¹⁰Pb_ex activity at Changshou Lake sediment. ¹³⁷Cs profile and erosive events as time markers provided reliable and consistent sedimentation rate of (1.6 cm y^-1). ²¹⁰Pb_ex activity decayed exponentially after peak at mass depth 5.68 g cm^-2. Therefore, violation of ²¹⁰Pb dating primary assumptions made it inappropriate for sediment dating at Changshou Lake. TOC content must be considered while using ²¹⁰Pb as dating tool for lake sediment profiles.

Temporal changes in dissolved 137Cs concentrations in groundwater and stream water in Fukushima after the Fukushima Dai-ichi Nuclear Power Plant accident

The concentration of dissolved ¹³⁷Cs in groundwater and stream water in the headwater catchments in Yamakiya district, located ∼35 km north west of Fukushima Dai-ichi Nuclear Power Plant (FDNPP), was monitored from June 2011 to July 2013, after the earthquake and tsunami disaster. Groundwater and stream water were sampled at intervals of approximately 2 months at each site. Intensive sampling was also conducted during rainstorm events. Compared with previous data from the Chernobyl NPP accident, the concentration of dissolved ¹³⁷Cs in stream water was low. In the Iboishi-yama catchment, a trend was observed for the concentration of dissolved ¹³⁷Cs in stream water to decline, which could be divided into two phases by October 2011 (a fast flush of activity as a result of rapid washoff and a slow decline as a result of soil fixation and redistribution processes). The highest ¹³⁷Cs concentration recorded at Iboishi-yama was 1.2 Bq/L on August 6, 2011, which then declined to 0.021-0.049 Bq/L during 2013 (in stream water under normal water-flow conditions). During the rainfall events, the concentration of dissolved ¹³⁷Cs in stream water increased temporarily. The concentration of dissolved ¹³⁷Cs in groundwater at a depth of 30 m at Iboishi-yama displayed a decreasing trend from 2011 to 2013, with a range from 0.039 Bq/L to 0.0025 Bq/L. The effective half-lives of stream water in the initial fast flush and secondary phases were 0.10-0.21 and 0.69-1.5 y, respectively in the three catchments. The effective half-life of groundwater was 0.46-0.58 y at Koutaishi-yama and 0.50-3.3 y at Iboishi-yama. The trend for the concentration of dissolved ¹³⁷Cs to decline in groundwater and stream water was similar throughout 2012-2013, and the concentrations recorded in deeper groundwater were closer to those in stream water. The declining trend of dissolved ¹³⁷Cs concentrations in stream water was similar to that of the loss of canopy ¹³⁷Cs by throughfall, as shown in other reports of forest sites in the Yamakiya district.

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

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in radiocesium fallout contaminating coastal catchments of the Fukushima Prefecture. As the decontamination effort progresses, the potential downstream migration of radiocesium contaminated particulate matter from forests, which cover over 65% of the most contaminated region, requires investigation. Carbon and nitrogen elemental concentrations and stable isotope ratios are thus used to model the relative contributions of forest, cultivated and subsoil sources to deposited particulate matter in three contaminated coastal catchments. Samples were taken from the main identified sources: cultivated (n = 28), forest (n = 46), and subsoils (n = 25). Deposited particulate matter (n = 82) was sampled during four fieldwork campaigns from November 2012 to November 2014. A distribution modelling approach quantified relative source contributions with multiple combinations of element parameters (carbon only, nitrogen only, and four parameters) for two particle size fractions (<63 μm and <2 mm). Although there was significant particle size enrichment for the particulate matter parameters, these differences only resulted in a 6% (SD 3%) mean difference in relative source contributions. Further, the three different modelling approaches only resulted in a 4% (SD 3%) difference between relative source contributions. For each particulate matter sample, six models (i.e. <63 μm and <2 mm from the three modelling approaches) were used to incorporate a broader definition of potential uncertainty into model results. Forest sources were modelled to contribute 17% (SD 10%) of particulate matter indicating they present a long term potential source of radiocesium contaminated material in fallout impacted catchments. Subsoils contributed 45% (SD 26%) of particulate matter and cultivated sources contributed 38% (SD 19%). The reservoir of radiocesium in forested landscapes in the Fukushima region represents a potential long-term source of particulate contaminated matter that will require diligent management for the foreseeable future.

Temporal changes of radiocesium in irrigated paddy fields and its accumulation in rice plants in Fukushima

About half of the total paddy field area, which is the dominant agricultural land in Fukushima Prefecture, was contaminated by radiocesium released by the Fukushima Daiichi Nuclear Power Plant accident. In this study, we investigated the temporal changes of radiocesium in soil, irrigation water, and rice plant in two adjacent rice paddies, with and without surface-soil-removal, in Fukushima Prefecture for over three years (2012-2014) after the nuclear accident. Our results showed that radiocesium migrated into 24-28 cm soil layers and that the activity concentration of radiocesium in paddy soils showed a significant reduction in 2014. The newly added radiocesium to paddies through irrigation water contributed only a maximum value of 0.15% and 0.75% of the total amount present in control and decontaminated paddies, respectively, throughout the study period. The radiocesium activity concentration in suspended sediment in irrigation water exponentially decreased, and the effective half-lives (Teff) for (137)Cs and (134)Cs were 1.3 and 0.9 years, respectively. Additionally, the average suspended sediment concentration in irrigation water increased between 2012 and 2014, suggesting that enhanced soil erosion had occurred in the surrounding environment. Radiocesium accumulation in rice plant also decreased with time in both paddies. However, the concentration ratio of radiocesium for rice plant in the decontaminated paddy increased compared with control paddy, despite approximately 96% of fallout radiocesium removed in paddy soil. Further analysis is required to clarify the reasons of high concentration ratio of radiocesium for rice plant in the decontaminated paddy.

Behaviour of radiocaesium in coastal rivers of the Fukushima Prefecture (Japan) during conditions of low flow and low turbidity--Insight on the possible role of small particles and detrital organic compounds

To investigate riverine transfers from contaminated soils of the Fukushima Prefecture in Japan to the marine environment, suspended sediments, filtered water, sediments and detrital organic macro debris deposited onto river beds were collected in November 2013 within small coastal rivers during conditions of low flow rates and low turbidity. River waters were directly filtered on the field and high efficiency well-type Ge detectors were used to analyse radiocaesium concentrations in very small quantities of suspended particles and filtered water (a few mg to a few g). For such base-flow conditions, our results show that the watersheds studied present similar hydro-sedimentary behaviours at their outlets and that the exports of dissolved and particulate radiocaesium are comparable. Moreover, the contribution of these rivers to the instantaneous export of radiocaesium to the ocean is similar to that of the Abukuma River. Our preliminary results indicate that, in the estuaries, radiocaesium concentrations in suspended sediments would be reduced by more than 80%, while radiocaesium concentration in filtered waters would be maintained. Significant correlations between radiocaesium concentrations and radiocaesium inventories in the soils of the catchments indicate that there was at that time little intra and inter-watershed variability in the transfer processes of radiocaesium from lands to rivers at this regional scale. The apparent liquid-solid partition coefficient (KD) values acquired for the lowest loads/finest particles complement the values acquired by using sediment traps and highlight the strong capacity of the smallest particles to transfer radiocaesium. Finally, but not least, our observations suggest that there could be a significant transfer of highly contaminated detrital biomass from forest litter to the downstream rivers in a rather conservative way.

Cumulative history recorded in the depth distribution of radiocesium in sediments deposited on a sandbar

We collected sediments deposited on a sandbar from the surface to 20 cm in depth in the Abukuma River to clarify the history of radiocesium derived from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. We analyzed the (137)Cs concentration in the sediments from size-fractioned samples as well as bulk samples. The depth distribution of (137)Cs showed the highest concentration in the deepest sediment layer (18-20 cm) studied, which indicates that sediments with a lower (137)Cs concentration were transported and deposited on sediments having a higher (137)Cs concentration. At the same time, the depth distribution suggests a decrease in radioactivity in provenance areas of the sediments. Analysis of the size-fractioned sediments indicated that the three sediment layers at 4-6 cm, 16-18 cm and 18-20 cm intervals had similar size distribution of (137)Cs and grain size composition although the concentration levels of (137)Cs were different according to their bulk concentrations. The size distribution of (137)Cs also supported the possibility that the decrease in (137)Cs concentration in bulk sediments above 18 cm is due to a decrease in the level of radioactivity in the catchment area. A comparison of the size distribution of (137)Cs between the sediment layers above and below 18 cm suggested that the (137)Cs concentration in the transported fine sediment particles decreased more with time than the (137)Cs concentration in the coarse particles, reflecting the selective transport of the finer particles. The results of this study demonstrated that sediment layers deposited on a sandbar retained the cumulative history of the fluvial transport of radiocesium after the FDNPP accident.

Size-dependent distribution of radiocesium in riverbed sediments and its relevance to the migration of radiocesium in river systems after the Fukushima Daiichi Nuclear Power Plant accident

We investigated the particle size distribution of radiocesium in riverbed sediments after the Fukushima Daiichi Nuclear Power Plant accident. Riverbed sediments were collected in the Abukuma River system in Fukushima and Miyagi Prefectures. The collected sediments were separated into 11 fractions, ranging from granular size (>2000 μm) to clay size (<2 μm) fractions. Cesium-137 concentrations were higher in the smaller particle size fractions, possibly reflecting specific surface areas and the mineralogy, in particular the clay mineral content. A gap in (137)Cs concentration was observed between the silt size and sand size fractions of riverbed sediments at downstream sites, whereas riverbed sediments at an upstream site did not show such a concentration gap. It is likely that selective transport of small particles in suspended state from upstream areas resulted in an accumulation of radiocesium in downstream areas.