Significance of Fukushima-derived radiocaesium flux via river-estuary-ocean system

Highlights

• Distribution and flux of ¹³⁷Cs along river-estuary-ocean system were assessed.

• Geochemical processes of ¹³⁷Cs in estuarine area were confirmed by direct evidence.

• Radiocaesium-rich porewater diffused to estuarine and coastal environments.

• ¹³⁷Cs flux via porewater occupies about 30% of dissolved ¹³⁷Cs flowing into ocean.

Abstract

The environmental dynamics of Fukushima-derived radiocaesium from land to ocean and the impact of its flux on the marine environment are matters of concern because radiocaesium will be continually transported to the open ocean for the next several decades, or possibly more than one hundred years. In order to assess the distribution and flux of radiocaesium in a river-estuary-ocean system, we investigated the activity concentration of radiocaesium in Matsukawa-ura Lagoon, the largest lagoon in Fukushima, where it is very easy to carry out observations with a wide salinity gradient. Activity concentrations of dissolved ¹³⁷Cs are elevated in seawater of low to intermediate salinity. It can thus be inferred that radiocaesium desorbs from suspended particles in an estuarine area. The porewater activity concentration of ¹³⁷Cs in lagoon sediment was about 10 times higher than that in the overlying lagoon water. This direct measurement indicates that a significant amount of radiocaesium in sediment desorbs into porewater. From the results of a mass balance model, dissolved ¹³⁷Cs flux from the lagoon's bottom is 15.3 ± 3.7 times greater than the riverine input, including desorption from particles. In the case of the whole Pacific coast of northeastern Japan (Miyagi, Fukushima, and Ibaraki Prefectures), dissolved ¹³⁷Cs flux into the open ocean, including diffusion of porewater, is estimated to be up to 1.5 times greater than the sum of riverine input and the ongoing release from the Fukushima Dai-ichi Nuclear Power Station's harbor. Consequently, our results suggest that radiocaesium is transported to the open ocean under the control of various processes, not only by desorption from particles but also, for example, by the diffusion of porewater.

Introduction

An urgent public issue demanding greater clarification is the environmental dynamics of anthropogenic radionuclides, including radiocaesium (¹³⁴Cs and ¹³⁷Cs), dispersed throughout the western North Pacific Ocean and extensive land areas following the disaster at the TEPCO Fukushima Dai-ichi Nuclear Power Station (FDNPS) in March 2011. In particular, concerns have been voiced that the radiocaesium deposited on land could be transported through hydrological media into the open ocean (Yoshida and Kanda, 2012). Therefore, it is crucial to investigate the environmental translocation of radiocaesium in hydrological media. For this purpose, many studies have focused on the environmental transfer of radiocaesium from a headwater catchment down to the coastal areas of northeastern Japan (e.g. Ueda et al., 2013; Nagao et al., 2013; Sakaguchi et al., 2015; Eyrolle-Boyer et al., 2016; Naulier et al., 2017; Yamashiki et al., 2014; Kakehi et al., 2016; Kambayashi et al., 2017; Charette et al., 2013). Several previous studies (e.g. Ueda et al., 2013; Nagao et al., 2013; Sakaguchi et al., 2015; Eyrolle-Boyer et al., 2016; Naulier et al., 2017) assumed that suspended particles are the main carrier of Fukushima-derived radiocaesium in rivers. Consequently, the export flux of particulate radiocaesium was also calculated using actual observation results (e.g. Ueda et al., 2013; Yamashiki et al., 2014). However, the riverine input of trace elements is sensitive to the bio-geochemical effect on river-sea systems in estuaries. For example, it has been found that radiocaesium absorbed by suspended particles is deposited in estuarine areas due to flocculating sedimentation (Hirao et al., 2014). Moreover, the increased salinity in estuaries causes desorption of radiocaesium from suspended particles (Sakaguchi et al., 2015; Kakehi et al., 2016). For these reasons, it is necessary to learn more about the dissolved phase of ¹³⁷Cs as well as its particulate phase. However, the literature on the contribution of dissolved ¹³⁷Cs to the export flux of radiocaesium from rivers to the open ocean is limited because it is difficult to carry out extensive observations in estuarine areas. Therefore, more research is needed to understand the translocation mechanisms of radiocaesium in river-estuary-ocean systems and to estimate the export flux of dissolved radiocaesium running to the open ocean.

Riverine input is a major pathway for discharge from land to the ocean. Furthermore, submarine groundwater discharge (SGD) has been recognized as an important source flowing into the ocean (e.g. Zhang and Mandal, 2012; Hatta and Zhang, 2013). As Taniguchi et al. (2002) stated, SGD can be divided into two categories: Submarine fresh groundwater discharge (SFGD) and Recirculated saline groundwater discharge (RSGD). The concentrations of nutrients, trace metals, carbon, and radionuclides such as ²²²Rn and radium isotopes in SGD are often dramatically higher in these discharges than in surface ocean water (e.g. Zhang and Satake, 2003; Rodellas et al., 2014; Kim et al., 2003), and RSGD is greater than SFGD-driven material fluxes (Zhang and Mandal, 2012). Moreover, Charette et al. (2013) raised the possibility that the SGD-derived radiocaesium flux to the ocean increases due to seawater intrusion driven by tidal pumping or seasonal changes in the hydraulic gradient. However, few reports (e.g. Sanial et al., 2017; Otosaka et al., 2020) have been published on subsurface pathways as a source of radiocaesium, partly because it is more difficult to take SGD samples than to take river water samples.

Matsukawa-ura Lagoon is the largest semi-closed shallow lagoon in Fukushima Prefecture, Japan (Fig. 1). It is connected to the Pacific Ocean by a narrow channel, through which inner and outer waters are exchanged. Our previous study revealed the physical and chemical processes in the system of radiocaesium discharge (Kambayashi et al., 2017). However, radiocaesium added by geochemical processes such as desorption from sediment has not been fully examined.

Estuarine areas are bodies of water where fresh water is derived from land drainage seawater. Therefore, radiocaesium deposited on land is transported to coastal zones and the open ocean through estuarine areas connecting land to ocean. In order to gain a better understanding of the transportation of radiocaesium from land to ocean, it is necessary to trace the geochemical process of radiocaesium in an estuarine area as a natural laboratory for obtaining directly observed evidence. The major goals of this study are to explore the distribution of Fukushima-derived radiocaesium in a river-estuary-ocean system and to evaluate the export flux from land to the ocean. To accomplish these goals, we chose to study the brackish lagoon Matsukawa-ura and rivers flowing into the lagoon as a natural laboratory of a river-estuary-ocean system because it is easy to make observations with a wide salinity gradient. Moreover, we determined radiocaesium activity concentration in surface water, overlying water (within 10 cm water depth from the lagoon bottom), and porewater, which we regarded as RSGD based on previous works (e.g. Burnett et al., 2003; Charette and Sholkovitz, 2006).