Temporal trends of ¹³⁷Cs activity concentration in pond waters in the vicinity of Fukushima Dai-ichi nuclear power plant

Closed and semi-closed water bodies, such as lakes and ponds, are important water resources in Fukushima area and they are the most sensitive environments to radioactive contamination after the Fukushima Dai-ichi nuclear power plant accident. Wakiyama et al. (2017) investigated ¹³⁷Cs activity in water and bottom sediment in four ponds; Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ), and Kashiramori (KM), within 10 km zone from the FDNPP during 2015–2016. This study follows up their observation to address longer time trends of ¹³⁷Cs activity concentration in pond waters and to show speciation of ¹³⁷Cs in soil and bottom sediment. Mean total ¹³⁷Cs activity concentration in water ranged from 2.5 to 29 Bq L⁻¹. There was not found steady trend in the activity concentration of total and particulate ¹³⁷Cs for four ponds. The concentration of dissolved ¹³⁷Cs was usually low in winter during the entire observation period in all four ponds. A tendency to a decrease in the ¹³⁷Cs activity concentration in suspended sediments was found for four ponds and the decreasing rate constants, including radiological decay, on SU, FS, IZ and KM were 0.33, 0.53, 0.29 and 0.25 yr⁻¹, respectively. The results of sequential extractions of soil and bottom sediment samples showed higher proportion of bioavailable ¹³⁷Cs, i.e., exchangeable and organic bound ¹³⁷Cs, in bottom sediment than in the soil.

1 Introduction

Closed and semi-closed water bodies, such as lakes and ponds, are the most sensitive environments to radionuclide contamination. More than 3700 individual irrigation ponds of different sizes are located in Fukushima Prefecture, which are radiologically contaminated after the Fukushima Dai-ichi nuclear power plant (FDNPP) accident. The Tohoku Regional Agricultural Administration Office of the Ministry of Agriculture, Forestry and Fishery (MAFF) and the Fukushima Prefectural government (MAFF and Fukushima prefecture, 2015) investigated 2679 ponds in Fukushima Prefecture and reported their water and sediment contamination levels.

The long-term behaviour of radiocesium in a closed and semi-enclosed water system affected by the Chernobyl accident can serve as a basis for long-term prediction of changes in environmental radioactive contamination in Fukushima. For example, Putyrskaya et al. (2009) presented the results of more than ten years of observation on the behaviour of ¹³⁷Cs in several lakes affected by the Chernobyl accident and elaborated a model calculation of temporal variations in the concentration of ¹³⁷Cs activity concentration of the lake water. However, the meteorological and geographical conditions in these areas are different from those in the Fukushima area, and the applicability of these results requires verification (Konoplev et al., 2016).

Wakiyama et al. (2017) demonstrated radiological contaminations in four ponds in the vicinity of FDNPP based on one-year observation. Although they suggested seasonal variation of dissolved ¹³⁷Cs activity concentration of water, long-term trends remained unclear due to short period of their observations. This study shows longer temporal trends of ¹³⁷Cs activity concentration in pond waters, as a continuation of observations undertaken by Wakiyama et al. (2017). In addition, we evaluated contamination of bottom sediment and its speciation.

2 Materials and method

2.1 Study site

Selected irrigation ponds of Suzuuchi (SU), Funasawa (FS), Inkyozaka (IZ) and Kashiramori (KM) are located in the city of Okuma within a radius of 10 km from FDNPP (Fig. 1). The surface area of these ponds of SU, FS, IZ and KM was 4100, 10 700, 6500 and 8100 m², respectively, and ambient dose equivalent rates measured on March 2014 around SU, FS, IZ and KM were 24, 6.0, 11 and 3.3 µSv h⁻¹, respectively (Wakiyama et al., 2017). The average annual precipitation and the average annual air temperature in this region were 1493 mm and 12.3 ^∘C, respectively, according to Namie weather station data from 1976 to 2015.

Figure 1 Location of study sites in Fukushima Prefecture and in Okuma town.

2.2 Sampling and analyses of pond water

We had collected 2 L of water samples from the four ponds approximately once a month since August 2015 to August 2017. Then, water samples were filtered using 0.45 µm-pore cellulose acetate membrane filter units in the laboratory. Suspended sediment (SS) concentrations in water (SSC, kg L⁻¹) were quantified by dividing weight of SS by volume of filtered water. Suspended sediments and filtrates were measured with Ge detectors to quantify the ¹³⁷Cs activity concentration in SS (C_Cs-SS, Bq kg⁻¹) and dissolved ¹³⁷Cs activity concentration (C_Cs-dis, Bq L⁻¹), respectively. Measurements of ¹³⁷Cs activity were done with standard electrode coaxial Ge detectors (CANBERRA GC4020, Canberra, USA) with relative efficiency of 42.6 % at the Institute of Environmental Radioactivity, Fukushima University. Measurements were taken for 90 000 s for ¹³⁷Cs activity concentrations in SSs and 240 000 s for dissolved ¹³⁷Cs activity concentrations to ensure minimal statistical errors, which were <5 % and <10 %, respectively.

Table 1 Summary of ¹³⁷Cs activity concentration in pond waters.

^* SD means standard deviation (1σ).

Particulate ¹³⁷Cs activity concentration in water (C_Cs-par, Bq L⁻¹), total ¹³⁷Cs activity concentration in water (C_Cs-tot, Bq L⁻¹), and the apparent distribution coefficient (K_d, L kg⁻¹), were calculated by following equations.

$$\begin{array}{}\text{(1)}& {\displaystyle }& {\displaystyle }{C}_{\mathrm{Cs}\text{-}\mathrm{par}}=C_{\mathrm{Cs}\text{-}\mathrm{SS}}\times \mathrm{SSC}\text{(2)}& {\displaystyle }& {\displaystyle }{C}_{\mathrm{Cs}\text{-}\mathrm{tot}}=C_{\mathrm{Cs}\text{-}\mathrm{par}}+C_{\mathrm{Cs}\text{-}\mathrm{dis}}\text{(3)}& {\displaystyle }& {\displaystyle }{K}_{\mathrm{d}}=C_{\mathrm{Cs}\text{-}\mathrm{SS}}÷C_{\mathrm{Cs}\text{-}\mathrm{dis}}\end{array}$$

Figure 2 Temporal variations in total, particulate, dissolved ¹³⁷Cs activity concentration in the different ponds.

2.3 Sampling and analyses of soil and bottom sediment

Soil and bottom sediment samples were collected in June 2016. Soils at a depth of 30 cm using a liner core sampler DIK-110C (DAIKI, Japan) with a plastic cylinder insert 5 cm in diameter. In laboratory, surface soils, to the depths of 3–5 cm, were cut for every 1–2 cm, and then remained subsurface soils were cut by 3–5 cm. Each soil sample was dried at 50 ^∘C for at least three days, then ground and homogenized in a mortar. Three cores of bottom sediment were obtained using 4 cm diameter polyvinyl core tubing in each pond. The bottom sediment cores were taken from a boat and the sampling depth ranged from 33 to 56 cm. Bottom sediments to the depth of 10 cm were cut for interval of 2.5–5 cm, and subsurface bottom sediments were cut by 5–10 cm.

Each bottom sediment sample was dried at 50 ^∘C for at least three days, then ground and homogenized in a mortar. The homogenised samples of both soil and bottom sediments were packed into 100 mL containers and measured to determine its ¹³⁷Cs activity concentration (Bq kg⁻¹). The measurements were done with standard electrode coaxial Ge detectors (CANBERRA GC4020, Canberra, USA) with relative efficiency of 42.6 % at the Institute of Environmental Radioactivity, Fukushima University. Measurements were taken up to 85 600 s to ensure a minimal statistical error of <5 % for almost samples. Eight samples taken from the deep portion of the bottom sediment had low ¹³⁷Cs activity concentrations, at a minimum of 1.83 Bq kg⁻¹, and high relative error, at 12.8 % maximum.

The proportion of ¹³⁷Cs in the exchangeable fraction, the organic fraction and the fraction strongly bound to the particles in the soil and bottom sediment were evaluated by a sequential extraction. Soil samples for quantifying the proportion were prepared by mixing up 0–1 and 1–2 cm of soil samples which are described in Sects. 2–3. The soil samples were passed through 2 mm-mesh sieves. Bottom sediment for quantifying the proportion were additionally taken by an Ekman-Berge sampler on June 2016, simultaneous to bottom the core sampling. The bottom samples were dried at 50 ^∘C.

The exchangeable fraction of the soil and bottom sediment were extracted with 1 M CH₃COONH₄ and the extract was measured for ¹³⁷Cs activity. Subsequently, organic matter was decomposed with mixture of 5 % of H₂O₂ solution and nitric acid. The extract was measured for ¹³⁷Cs activity. The measurements were done with same Ge detectors for soil and bottom sediment samples. Measurements were taken up to 53 000 s to ensure a minimal statistical error of <5 %. Only for the organic-bound fraction of ¹³⁷Cs in bottom sediment from IZ, relative error was 14.6 % because of low ¹³⁷Cs activity.

3 Results and discussion

3.1 ¹³⁷Cs activity concentrations in pond water

The observation results are presented in Table 1. Particulate ¹³⁷Cs accounted for 73 %–88 % of total ¹³⁷Cs, on average. The mean apparent K_d value ranged from 1.4×10⁵ to 6.0×10⁵ L kg⁻¹. These values were higher than those observed in the Chernobyl area by at least one order of magnitude, as previous authors suggested (e.g., Tsukada and Ohse, 2016). These results show a stronger affinity of ¹³⁷Cs for sediment particles in Fukushima.

Figure 2 shows temporal variations in total, particulate and dissolved ¹³⁷Cs activity concentration in pond water. Both total and particulate ¹³⁷Cs activity concentrations varied significantly throughout observation, and the temporal variations were similar. These similar variations were attributed to high proportions of particulate ¹³⁷Cs activity concentration, as shown in Table 1. In contrast, the concentration of dissolved ¹³⁷Cs was usually low in winter during the entire observation period in all ponds. These results confirm that the seasonal cycle exists on dissolved ¹³⁷Cs in ponds, as it was previously found in the Chernobyl-affected area (e.g., Putyrskaya et al., 2009).

Decreasing trends of ¹³⁷Cs activity concentration in SSs were found for all ponds (Fig. 3). The temporal trend of ¹³⁷Cs activity concentration in SS was expressed by the following equation,

$$\begin{array}{}\text{(4)}& C_{\mathrm{Cs}\text{-}\mathrm{SS}}\left(t\right)=C_{\mathrm{Cs}\text{-}\mathrm{SS},\mathrm{0}}\exp (-\mathit{\lambda }t)\end{array}$$

where t is the elapsed time since the accident (year), C_Cs-SS(t) is ¹³⁷Cs activity concentration in SSs at time t (Bq kg⁻¹), $C_{Cs-SS,\mathrm{0}}$ is initial ¹³⁷Cs activity concentration in SSs at t=0 (Bq kg⁻¹) and λ is rate constant including radiological decay (yr⁻¹). The rate constant on SU, FS, IZ and KM were 0.33, 0.52, 0.29 and 0.25 yr⁻¹, respectively. The corresponding periods of half-reduction for SU, FS, IZ and KM were 2.1, 1.3, 2.4 and 2.8 years, respectively. Since ¹³⁷Cs activity concentration in SS was more variable than that dissolved ¹³⁷Cs activity concentration, the temporal variation in apparent K_d seemed to coincide with changes in the ¹³⁷Cs activity concentration in SS.

Figure 3 Temporal variations in ¹³⁷Cs activity concentration of suspended sediment (Cs-SS) and apparent distribution coefficient K_d in the different ponds.

3.2 ¹³⁷Cs in soil and bottom sediment

The vertical distributions of ¹³⁷Cs activity concentration in soil and bottom sediment are relatively similar for the all study sites (Fig. 4). As indicated by Wakiyama et al. (2017), the mean ¹³⁷Cs inventory in the soil around SU, FS, IZ and KM were 6.8, 4.6, 2.9 and 0.42 MBq m⁻², respectively. Similarly, mean ¹³⁷Cs inventory in bottom sediment in SU, FS, IZ and KM were 13, 8.9, 1.6 and 1.1 MBq m⁻², respectively. The ¹³⁷Cs activity concentration both in the soils and bottom sediments decreased with depth.

Figure 4 Vertical distribution of ¹³⁷Cs activity concentration in soil (a) and bottom sediment (b).

The depth at which the soil contains 90 % of the inventory of ¹³⁷Cs in the soil around the SU, FS, IZ and KM were 5, 7.6, 9.7 and 6.7 cm, respectively. The depth at which the soil contains 90 % of the inventory of ¹³⁷Cs in bottom sediment on SU, FS, IZ and KM were 30, 17, 8.3 and 12.5 cm, respectively. Deeper penetration of ¹³⁷Cs into bottom sediments than in the soil were found with the exception of IZ.

At least 95 % of ¹³⁷Cs in soil was strongly bound to sediment particle in soil, while the ¹³⁷Cs content in bottom sediment ranged from 42 % to 72 % (Fig. 5). The low proportions of ¹³⁷Cs strongly bound with particle in bottom sediment, suggest that the bioavailability of ¹³⁷Cs was increased in bottom sediments.

Figure 5 Proportion of exchangeable, organic and particle bound fractions of ¹³⁷Cs in soil of 0–2 cm depth (a) and bottom sediment by an Ekman–Berge sampler (b).

Putyrskaya et al. (2009) demonstrated that percentage of exchangeable ¹³⁷Cs was 5.7 % in average in 2006, based on sequential extraction of bottom sediment taken in lakes affected during the Chernobyl accident. Although the depth and water flow regime of ponds, located near Chernobyl, differed from ours, a high proportion of ¹³⁷Cs in the exchangeable and organic fractions in Fukushima study sites suggested that the bioavailability of ¹³⁷Cs in Okuma town ponds is still high.

These mobilities may have led to the vertical migration of ¹³⁷Cs and controlled the fluctuation in dissolved ¹³⁷Cs activity concentration. The effect of biological activities on ¹³⁷Cs behaviour should be high in the spotlight.

4 Summary

We investigated the behaviour of ¹³⁷Cs in four small ponds around Fukushima Dai-ichi nuclear power plant as a following-up of observation by Wakiyama et al. (2017). Continuous monitoring of ¹³⁷Cs activity concentration in water confirmed the presence of a seasonal cycle of dissolved ¹³⁷Cs activity concentration and newly revealed tendencies to a decrease trends in ¹³⁷Cs activity concentration in suspended sediment for all ponds.

In addition, the sequential extractions of sediment samples showed a higher proportion of ¹³⁷Cs in the exchangeable and organic fractions in bottom sediments than in soils and this indicates an increase in the bioavailability of ¹³⁷Cs in bottom sediment. The effect of biological activity should be tested to better understand ¹³⁷Cs activity in ponds.