Spatial variation in low-level 134Cs in the coastal sediments off central Honshu in the Sea of Japan: implications for delivery, migration, and redistribution patterns

In 2014 and 2015, we examined the spatial distribution of cesium-134 (half-life: 2.06 years) from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in marine sediments within coastal-basin areas (water depths of 40-520 m) off central Honshu Island (the main island of Japan) in the Sea of Japan. The Cs-134 concentrations in both the surface sediment (0-1 cm depth) and whole-core inventory exhibited wide variations, and were highest at the site closest to the Agano River Estuary area (6.7 Bq/kg-dry and 886 Bq/m(2), respectively). This indicates that Cs-134 in coastal areas was delivered by riverine suspended solids (SS). Given the spatial variation in Cs-134 concentrations, we believe that Cs-134 partially migrated northeastward within -50 km along Honshu Island (at water depths shallower than similar to 140 m), and southwestward, including the Sado Basin area. This is predominantly attributable to the transport of SS by bottom currents and unsteady downward delivery onto the steep slopes of the basin. The total amount of Cs-134 in the study area in 2014 was estimated at approximately 0.6 TBq (decay-corrected to March 11, 2011, date of FDNPP accident).