Spatial variability and the fate of cesium in coastal sediments near Fukushima, Japan

Quantifying the amount of cesium incorporated into marine sediments as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident has proven challenging due to the limited multi-core sampling from within the 30 km zone around the facility; the inherent spatial hetero-geneities in ocean sediments; and the potential for inventory fluctuations due to physical, biological, and chemical processes. Using Pb-210, Th-234, Cs-137, and Cs-134 profiles from 20 sediment cores, coastal sediment inventories were reevaluated. A Cs-137 sediment inventory of 100 +/- 50 TBq was found for an area of 55 000 km(2) using cores from this study and a total of 130 +/- 60 TBq using an additional 181 samples. These inventories represent less than 1% of the estimated 15-30 PBq of cesium released during the FDNPP disaster. The time needed for surface sediment activities (0 to 3 cm) at the 20 locations to be reduced by 50% via sediment mixing was estimated to range from 0.4 to 26 yr. Due to the observed variability in mixing rates, grain size, and inventories, additional cores are needed to improve these estimates and capture the full extent of cesium penetration into the shallow coastal sediments, which was deeper than 14 cm for all cores retrieved from water depths less than 150 m.