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Sorption hypotheses and models are required for the prediction of 137 Cs migration in soils contaminated after nuclear reactor accidents and nuclear weapons tests. In assessment models, the K d (distribution coefficient) hypothesis for sorption, which assumes that sorption is instantaneous, linear and reversible, has often been coupled with the convection-diffusion equation (CDE) to model 137 Cs migration. However, it fails to describe 137 Cs migration velocities which often decrease with time. Alternative equilibrium-kinetic (EK) hypotheses of 137 Cs sorption/desorption have been suggested by laboratory experiments but have not been fully validated in field conditions. This work addressed the influence and magnitude of non-equilibrium 137 Cs sorption in field conditions by reinterpreting, with an inverse approach, series of 137 Cs profiles measured in mineral soils of forest plots located in Fukushima Prefecture (2013-2018). Our results show that the inclusion of non-equilibrium sorption significantly improves, compared to the equilibrium hypothesis, the realism of simulated 137 Cs profiles. Fitted sorption parameters suggest a fast sorption kinetic (half-time of 1-7 h) and a pseudo-irreversible desorption rate (half-time of 3.2 × 10 0 -3.4 × 10 6 years), whereas equilibrium sorption (4.0 × 10 -3  L kg -1 on average) only affects a negligible portion of 137 Cs inventory. By June 2011, such EK parameters fitted on our plots realistically reproduced profiles measured in the same forest study site (Takahashi et al., 2015). Predictive modeling of 137 Cs profiles in soil suggests a strong persistence of the surface 137 Cs contamination by 2030, with exponential profiles consistent with those reported after the Chernobyl accident. This study demonstrates that hypotheses and parameters of 137 Cs sorption can be partially inferred from in situ measurements. However, further experiments in controlled conditions are required to better estimate the sorption parameters and to identify the processes behind non-equilibrium sorption., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident led to the contamination by radiocesium ( 137 Cs) of large drained areas. Cesium-137 concentrations in rivers result from complex transfer processes, depending on multiple forcings. Better knowledge of the factors controlling these concentrations is therefore a prerequisite to improve predictions of 137 Cs transfers within river catchments. This study aimed at analyzing the spatial and temporal variability of 137 Cs concentrations in rivers and identifying the key factors controlling their variability. Published values of 137 Cs concentrations in rivers in the north of FDNPP were collected, characterizing 122 sampling sites from May 2011 to October 2014. It resulted in three datasets: dissolved concentrations C W (Bq/L), concentrations in suspended sediment C SS (Bq/kg) and total concentrations C T (Bq/L). The resulting database reflected a large variety of catchments and hydrological conditions. Observed 137 Cs concentrations varied by 2-4 orders of magnitude and were poorly explained (R 2  = 0.13-0.38) by the average contamination density. Indices summarizing the complex spatial and temporal properties of the catchments were proposed as candidate explanatory variables of concentrations in rivers. They were selected by stepwise regression for each dataset (C W , C SS , C T ). For the three datasets, the selection and combination of 5-10 indices significantly better explained this variability (R 2  = 0.69-0.83). Deposit indices were identified as first drivers of concentrations in rivers. A deposit index was selected for each dataset, indicating no effect of the contamination distribution for C W , whereas C T and C SS required considering the distribution of contamination and connectivity, as well as the presence of dams for C SS . The others selected variables significantly contributed to explain the concentration variability. This meta-analysis emphasizes the importance of structural (e.g. slope, land-cover) and functional (e.g. delay, season, rainfall) properties in the dissimilarities of catchments responses, stressing that assessments could be improved by including more these properties in models., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

To investigate riverine transfers from contaminated soils of the Fukushima Prefecture in Japan to the marine environment, suspended sediments, filtered water, sediments and detrital organic macro debris deposited onto river beds were collected in November 2013 within small coastal rivers during conditions of low flow rates and low turbidity. River waters were directly filtered on the field and high efficiency well-type Ge detectors were used to analyse radiocaesium concentrations in very small quantities of suspended particles and filtered water (a few mg to a few g). For such base-flow conditions, our results show that the watersheds studied present similar hydro-sedimentary behaviours at their outlets and that the exports of dissolved and particulate radiocaesium are comparable. Moreover, the contribution of these rivers to the instantaneous export of radiocaesium to the ocean is similar to that of the Abukuma River. Our preliminary results indicate that, in the estuaries, radiocaesium concentrations in suspended sediments would be reduced by more than 80%, while radiocaesium concentration in filtered waters would be maintained. Significant correlations between radiocaesium concentrations and radiocaesium inventories in the soils of the catchments indicate that there was at that time little intra and inter-watershed variability in the transfer processes of radiocaesium from lands to rivers at this regional scale. The apparent liquid-solid partition coefficient (KD) values acquired for the lowest loads/finest particles complement the values acquired by using sediment traps and highlight the strong capacity of the smallest particles to transfer radiocaesium. Finally, but not least, our observations suggest that there could be a significant transfer of highly contaminated detrital biomass from forest litter to the downstream rivers in a rather conservative way., (Copyright © 2015 Elsevier Ltd. All rights reserved.)