Ocean dynamic processes causing spatially heterogeneous distribution of sedimentary caesium-137 massively released from the Fukushima Daiichi Nuclear Power Plant

Abstract

Abstract. Massive amounts of anthropogenic radiocaesium 137Cs that were released into the environment by the Fukushima Daiichi Nuclear Power Plant accident in March 2011 are widely known to have extensively migrated to Pacific Ocean sediment off of eastern Japan. Several recent reports have stated that the sedimentary 137Cs is now stable with a remarkably heterogeneous distribution. The present study elucidates ocean dynamic processes causing this heterogeneous sedimentary 137Cs distribution in and around the shelf off Fukushima and adjacent prefectures. We performed a numerical simulation of oceanic 137Cs behaviour for about 10 months after the accident, using a comprehensive dynamic model involving advection–diffusion transport in seawater, adsorption and desorption to and from particulate matter, sedimentation and suspension on and from the bottom, and vertical diffusion transport in the sediment. A notable simulated result was that the sedimentary 137Cs significantly accumulated in a swath just offshore of the shelf break (along the 50–100 m isobath) as in recent observations, although the seabed in the entire simulation domain was assumed to have ideal properties such as identical bulk density, uniform porosity, and aggregation of particles with a single grain diameter. This result indicated that the heterogeneous sedimentary 137Cs distribution was not necessarily a result of the spatial distribution of 137Cs sediment adsorptivity. The present simulation suggests that the shape of the swath is mainly associated with spatiotemporal variation between bottom shear stress in the shallow shelf (< 50 m depths) and that offshore of the shelf break. In a large part of the shallow shelf, the simulation indicated that strong bottom friction suspending particulate matter from the seabed frequently occurred via a periodic spring tide about every 2 weeks and via occasional strong wind. The sedimentary 137Cs thereby could hardly stay on the surface of the seabed with the result that the simulated sediment-surface 137Cs activity tended to decrease steadily for a long term after the initial 137Cs migration. By contrast, in the offshore region, neither the spring tide nor the strong wind caused bottom disturbance. Hence, the particulate matter incorporated with 137Cs, which was horizontally transported from the adjacent shallow shelf, readily settled and remained on the surface of the sediment just offshore of the shelf break.

Migration of radioactivity in multi-fraction sediments

“…It can be the result of both local heterogeneities of sediment features [11] and the bottom topography [39]. To separate the topography effect from the effects of transport, erosion and deposition of contaminated sediments on the exchange of activity between the seafloor and water, consider the idealized case of a straight channel with a bottom depression (Fig.…”

“…Unlike other radioactivity transport models [2,11], the exchange rate W pw ð0;1Þ (mass transfer coefficient [7]) is estimated by boundary layer theory [34] and corrected for surface roughness [10] as…”

“…Table 1 shows the important features of a number of models of radioactivity transport in the marine environment. Diffusion in pore water and bioturbation of the sediments are considered in only a few models [2,11] of the migration of sedimentary radioactivity. The continuous profile of multi-fraction sediments with inhomogeneous porosity in most of models was substituted by an averaged distribution of a single fraction of sediments.…”

Decline in radiocesium in seafloor sediments off Fukushima and nearby prefectures

“…Several studies have investigated the 137 Cs budget in the sediment off Fukushima (Ambe et al 2014;Black and Buesseler 2014;Otosaka and Kato 2014;Higashi et al 2015), but because the sampling area and timing differed among these studies, it is difficult to compare the data to clarify the fate of FDNPP-derived 137 Cs in the sediment. In the Irish Sea, MacKenzie et al (1994) reported that sediment contaminated by radionuclides from the Sellafield nuclear fuel reprocessing plant is redistributed mainly with silt-sized sediment particles, and dissolution of 137 Cs from the sediment may also play an important role in post-depositional migration of 137 Cs (e.g., Cook et al 1997;MacKenzie et al 1998).…”

“…Long-term monitoring of bottom waters just above the seafloor and field experiments carried out off the Fukushima coast revealed that the combination of wave and current action caused by meteorological disturbance was a key process in the transport of suspended particulate material (Yagi et al 2015;Buesseler et al 2015). On the basis of a numerical simulation, Higashi et al (2015) suggested that the shape of the high-concentration band in the southern area (the "hotspot swath") mainly reflected spatiotemporal variation in bottom shear stress between the shallow shelf (<50 m water depth) and the area offshore of the shelf break. These proposals are all based on data obtained during a relatively restricted time span after the accident (~1 year), so longterm trends of 137 Cs deposition and transport in the area are not yet known.…”