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

Concentrations of Cs-137 in seawater, seabed sediment, and pore water collected from the area around Fukushima were investigated from 2015 to 2018, and the potential of coastal sediments to supply radiocesium to the bottom environment was evaluated. The Cs-137 concentration in the pore water ranged from 33 to 1934 mBq L-1 and was 10-40 times higher than that in the overlying water (seawater overlying within 30 cm on the seabed). At most stations, the Cs-137 concentrations in the overlying water and the pore water were approximately proportional to those in the sediment. The conditional partition coefficient between pore water and sediment was [0.9-14] x 10(2) L kg(-1), independent of the year of sampling. These results indicated that an equilibrium of Cs-137 between pore water and sediment has been established in a relatively short period, and Cs-137 in the pore water is gradually exported to seawater near the seabed. A simple box model estimation based on these results showed that Cs-137 in the sediment decreased by about 6% per year by desorption/diffusion of Cs-137 from the seabed.