Sedimentation areas along the Fukushima coast for Cs-bearing microparticles from the Lagrangian particle tracking

13 years after the Fukushima nuclear accident, the coastal area in front of the Fukushima Dai-ichi Nuclear Power Plant remains contaminated. First, it relates to the bottom sediments, which were contaminated with Cs-137 during the accidental and post-accidental releases. Additional contamination of bottom sediments could be caused by river runoff of Cs-bearing microparticles, which were formed in the initial phase of the accident, dispersed in the atmosphere, and fell into the watershed of nearby rivers. Cesium may be preserved in such particles for a long time due to its insoluble characteristics. River runoff carries suspended particles along with Cs-bearing microparticles to the coastal areas of the ocean, especially during heavy rains.

In the study, we applied the Lagrangian particle tracking model Parcels to identify places of potential deposition of such Cs-bearing microparticles in the Fukushima coastal area. As input data we used

• 3D circulation data from the detailed ROMS-based ocean circulation model customized to the Fukushima coast for the period 2013-2016;
• Estimates of released Cs-bearing microparticles from rivers during heavy rains in selected period;
• Estimates of microparticles’ sizes define the vertical velocity of their falling in the water column.

Lagrangian particles simulated the transport of Cs-bearing microparticles, which were released from the Abakuma River mouth, by the coastal currents taking into account Stokes settling velocity. Maximums in the distribution of such particles on the bottom identified areas with possible accumulation of Cs-bearing microparticles in the coastal area after heavy rains. Distributions for different particles’ sizes obtained in simulations were analyzed.