Radiocesium dynamics in the aquatic ecosystem of Lake Onuma on Mt. Akagi following the Fukushima Dai-ichi Nuclear Power Plant accident

Highlights

• Radiocesium dynamics of ecosystem in Lake Onuma after FDNPP accident was assessed.

• ¹³⁷Cs decay was estimated using samples collected from 2011 to 2016.

• ¹³⁷Cs levels in wakasagi almost reached a state of dynamic equilibrium.

• Parts of aquatic ecosystems exhibited different decay processes with wakasagi.

• ¹³⁷Cs contamination in lake water affected aquatic organisms.

Abstract

Understanding ecosystem dynamics of radionuclides is necessary to ensure effective management for food safety. The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on March 11, 2011 released large amounts of radiocesium (¹³⁴Cs and ¹³⁷Cs) and contaminated the environment across eastern Japan. In this study, we aimed to elucidate the temporal dynamics of ¹³⁷Cs in the aquatic ecosystem of Lake Onuma on Mt. Akagi. The effective ecological half-life (T_eff) of ¹³⁷Cs in fishes, western waterweed (Elodea nuttallii), seston (phytoplankton and zooplankton), and lake water was estimated using survey data of ¹³⁷Cs concentration collected from 2011 to 2016, and single- and two-component decay function models (SDM and TDM, respectively). The decay processes of ¹³⁷Cs concentrations in wakasagi (Hypomesus nipponensis), pale chub (Zacco platypus), phytoplankton, and total ¹³⁷Cs concentrations of the water column (WC) in the lake were well suited by the TDMs. The T_eff in the fast component of the TDMs in these samples ranged from 0.49 to 0.74 years. The T_eff in the slow component of the TDMs could converge towards the physical half-life of ¹³⁷Cs. Nearly five and a half years after the FDNPP accident, we concluded that ¹³⁷Cs concentrations approached a state of dynamic equilibrium between some aquatic organisms (wakasagi, pale chub, and phytoplankton) and the environment (lake water). However, the decay processes of ¹³⁷Cs concentrations in Japanese dace (Tribolodon hakonensis), western waterweed, zooplankton, and particulate- and dissolved-forms in the WC were better predicted for the SDM. The total ¹³⁷Cs concentrations in inflowing river and spring waters were one to two orders of magnitude lower than lake water under normal flow conditions. However, particulate ¹³⁷Cs contamination level in the river water was high after heavy rains. Overall, ¹³⁷Cs contamination levels have significantly decreased in Lake Onuma, but monitoring surveys should be continued for further understanding of the reduction processes.

Introduction

The accident at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) of the Tokyo Electric Power Company that occurred following the Great East Japan Earthquake on March 11, 2011 released large amounts of radioactive materials including radiocesium (¹³⁴Cs and ¹³⁷Cs) into the environment (Hirose, 2016, Steinhauser et al., 2014). In particular, the FDNPP accident-derived radiocesium contaminated most parts of eastern Japan through atmospheric transport, as confirmed by simulation (Morino et al., 2011, Morino et al., 2013) and airborne monitoring surveys (NRA, 2017). Radiocesium has long physical half-life (T_phy) (T_phy of ¹³⁴Cs = 2.06 years, T_phy of ¹³⁷Cs = 30.2 years) and accumulates in the muscle of vertebrates including fishes (Fukuda et al., 2013, Malek et al., 2004, McCreedy et al., 1997, Yamamoto et al., 2014). FDNPP accident-derived radiocesium deposited on inland waters has accumulated to high concentrations in freshwater fishes through bioconcentration in the ecosystem (Arai, 2014a, Arai, 2014b, Matsuda et al., 2015, Mizuno and Kubo, 2013, Tsuboi et al., 2015, Wada et al., 2016, Yoshimura and Yokoduka, 2014). A strict standard for allowable ¹³⁴Cs plus ¹³⁷Cs level of 100 Bq kg^− 1 wet weight in general food was enforced from April 1, 2012 under the Food Sanitation Law in Japan, although a provisional standard on ¹³⁴Cs plus ¹³⁷Cs contamination level of 500 Bq kg^− 1 wet weight had been applied following the FDNPP accident (Gilmour et al., 2016). Hence, it is important to understand the distribution and dynamics of radiocesium contamination across regions to prevent internal and external exposure of humans to radiocesium.

Following the Chernobyl Nuclear Power Plant (CNPP) accident in 1986, the biological impact and dynamics of ¹³⁷Cs in aquatic ecosystems were extensively investigated and discussed (IAEA, 2006). Numerous studies on ¹³⁷Cs contamination in freshwater fishes have been carried out in European countries (Elliott et al., 1993, Håkanson et al., 1989, Jonsson et al., 1999, Saxén et al., 2010, Smith et al., 2000, Ugedal et al., 1995). Despite low ¹³⁷Cs concentration in water, freshwater fishes exhibit high ¹³⁷Cs concentrations due to bioconcentration. In particular, ¹³⁷Cs concentrations in freshwater fishes inhabiting closed lakes have declined at slower rates in comparison to fishes in rivers or open lakes (Bulgakov et al., 2002, Rask et al., 2012, Sarkka et al., 1995, Saxén et al., 2010, Saxén and Ilus, 2008).

In Gunma Prefecture, relatively high concentrations of ¹³⁴Cs plus ¹³⁷Cs (30–100 kBq m^− 3) was deposited after the FDNPP accident (Hirose, 2016, NRA (Japan Nuclear Regulation Authority), 2017). The Gunma prefectural government initiated measurement of radiocesium concentrations in agricultural, forestry, livestock, and fishery products for food safety control immediately after the accident. In August 2011, 640 Bq kg^− 1 wet weight of ¹³⁴Cs plus ¹³⁷Cs was detected in wakasagi (Hypomesus nipponensis) from Lake Onuma on Mt. Akagi in the Gunma Prefecture (Suzuki et al., 2016, Suzuki and Tsunoda, 2013, Mori et al., 2017). This was higher than levels in wakasagi from other lakes in Gunma, Tochigi, or Fukushima Prefectures where similar radiocesium contamination levels were confirmed by airborne monitoring surveys (MAFF (Japan Ministry of Agriculture, Forestry and Fisheries), 2017, Mori et al., 2017, Suzuki and Tsunoda, 2013, Wada et al., 2016). Mt. Akagi area and Lake Onuma are popular tourist destinations, and wakasagi fishing is an important tourist attraction from autumn to winter. The radiocesium contamination of wakasagi has significantly damaged tourism in the area. Therefore, it is particularly important for local residents around Lake Onuma to elucidate the mechanism and forecast the future of radiocesium contamination in the area. Furthermore, Mt. Akagi area has relatively heavy rainfall from June to September because the climate of Japan is largely under the influence of the East Asian monsoon (Kondo and Hamada, 2011, Kono, 1993). Thus, the measurement of radiocesium in this area under the East Asian monsoon climate may be useful in understanding the dynamics and contamination of radioactive materials in the environment.

When ¹³⁷Cs contamination occurs in natural ecosystems, radiological risks can be evaluated from the duration of ¹³⁷Cs persistence in populations of certain species in the biota. For such cases, the effective ecological half-life (T_eff) or ecological half-life (T_eco) is used to assess ¹³⁷Cs dynamics in the environment (Iwata et al., 2013, Jonsson et al., 1999, Pröhl et al., 2006, Smith and Beresford, 2005, Smith et al., 2000). In addition, the concentration ratio (CR) value, which is the ratio of the ¹³⁷Cs concentration in aquatic organisms to that in the lake water, is a useful environmental parameter (IAEA (International Atomic Energy Agency), 2010, IAEA (International Atomic Energy Agency), 2004, Kaeriyama et al., 2015).

In the previous report of our group (Mori et al., 2017), ¹³⁷Cs was measured for soil and lake sediment on Mt. Akagi surrounding Lake Onuma by sequential extraction, and abundance ratios of soluble and insoluble species were estimated by determining the radiocesium concentrations of each sample. In this study, we measured radiocesium levels in aquatic organisms (fishes and aquatic plant) in Lake Onuma since August 2011 to evaluate ¹³⁷Cs contamination levels and temporal changes in the aquatic resources. In addition to aquatic species, radiocesium concentrations were determined in the environment (lake water) and potential food resources (seston), all of which can affect the radiocesium concentration of aquatic organisms. Though many studies on the dynamics of radiocesium concentrations in fishes have been reported (e.g. Iwata et al., 2013, Ugedal et al., 1995, Wada et al., 2016), studies on the dynamics in lake water and seston have been limited. To predict the future of ¹³⁷Cs contamination levels in aquatic organisms and lake water, we derived the T_eff for ¹³⁷Cs by constructing decay function models and the CR value of ¹³⁷Cs based on samplings performed from 2011 to 2016 in the lake. Additionally, this paper reports our monitoring results of radiocesium concentrations in inflowing spring and river waters, including impact of the heavy rainfall in September 2013.