Long-Term Decline of 137Cs Apparent Distribution Coefficient in the Soil–Water System as a Result of Decomposition of Glassy Hot Particles in the Fukushima Daiichi NPP Fallout

The apparent distribution coefficient Kd of Fukushima-derived radiocesium is declining with time, which is unusual and unexpected and was never observed for radiocesium in areas contaminated by Chernobyl, Mayak Production Association, or nuclear weapons tests. Normally, Kd slightly increases in the initial period after an accident due to radiocesium fixation by micaceous clay minerals, and thereafter the mean annual Kd value becomes more or less constant. Our hypothesis is that the long-term decline of Kd of Fukushima origin radiocesium in freshwaters can be due to slow decomposition of glassy hot particles (cesium microparticles, CsMPs), resulting in radiocesium leaching. This study was aimed at (i) suggesting a process-level model of 137Cs species transformation in water bodies with allowance for 137Cs leaching from CsMPs, (ii) testing the proposed model using available data on Okuma ponds in close vicinity of the Fukushima Daiichi NPP and Abukuma river, and (iii) estimating the key model parameters such as the equilibrium distribution coefficient Kd∞, initial fraction of CsMPs in the fallout α, and rate constant of radiocesium leaching from CsMPs kl for the water bodies under study. Because 137Cs incorporated in CsMPs is not involved in the exchange with the solution, the solid–liquid equilibrium is attained only for the mobile fraction of 137Cs in the fallout. With time, CsMPs decompose, albeit very slowly, and radiocesium is leached from CsMPs with reestablishment of the quasi-equilibrium. Accordingly, the fraction of dissolved 137Cs increases, and the apparent Kd decreases. Using the observational data on Kd decline and a simplified approach, the rate constant of radiocesium leaching from CsMPs was estimated at 0.02–0.03 year–1 in Okuma ponds and Abukuma river, and it seems to be comparable to the rate constant of the 137Cs radioactive decay. Most likely, the long-term decline of the 137Cs apparent distribution coefficient occurs at locations where the fraction of CsMPs in the fallout exceeds 50%, with their influence being insignificant elsewhere. Presumably, a slower decline of Kd(137Cs) in Abukuma river as compared to Okuma ponds is not associated with slower CsMP decomposition but, most likely, can be attributed to lower CsMP fraction in deposition on the Abukuma River catchment.