Behavior of Radiocesium in Sediments in Fukushima Coastal Waters: Verification of Desorption Potential through Pore Water

Concentrations of 137Cs in seawater, seabed sedi-ment, and pore water collected from the area around Fukushimawere investigated from 2015 to 2018, and the potential of coastalsediments to supply radiocesium to the bottom environment wasevaluated. The 137Cs concentration in the pore water ranged from33 to 1934 mBq L−1 and was 10−40 times higher than that in theoverlying water (seawater overlying within 30 cm on the seabed).At most stations, the 137Cs concentrations in the overlying waterand the pore water were approximately proportional to those in thesediment. The conditional partition coefficient between pore waterand sediment was [0.9−14] × 102 L kg−1, independent of the yearof sampling. These results indicated that an equilibrium of 137Cs between pore water and sediment has been established in a relatively short period, and 137Cs in the pore water is gradually exported to seawater near the seabed. A simple box model estimation based on these results showed that 137Cs in the sediment decreased by about 6% per year by desorption/diffusion of 137Cs from the seabed