Influence of non-equilibrium sorption on the vertical migration of 137Cs in forest mineral soils of Fukushima prefecture

Vertical migration of radiocesium is a key issue in soils impacted by Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Among radioactive substances deposited on terrestrial ecosystems, ¹³⁴Cs (with half-life 2.07 years) and ¹³⁷Cs (with half-life 30.2 years) were dominant and have by far the most radiological significance.

This work investigates the importance of non-equilibrium sorption on the vertical migration of ¹³⁷Cs in field conditions. The equilibrium-kinetic (EK) sorption model was selected as a non-equilibrium parameterization embedding the K_d approach. It supposes the existence of two types of sorption sites. The first sites are at equilibrium with solution; whereas for the second sites, kinetics of the sorption and desorption are taken into consideration.

We focused our study on four ¹³⁷Cs soil contamination plots measured in a young cedar stand situated around 35 km northwest of the FDNPP. Profiles were sampled at four different dates (2013, 2014, 2016, and 2018) by measuring ¹³⁷Cs activity in both organic (humus + litter layer) and mineral soil layers reaching a maximum depth of 20cm.

To successfully simulate the ¹³⁷Cs transfer throughout these soil profiles, the input flux at the top of the mineral soil surface was reconstructed from global monitoring data from the forest stand and a first-order compartment model for the organic layer.

Our results showed that the inclusion of non-equilibrium sorption slightly improves the realism of simulated ¹³⁷Cs profiles compared to the equilibrium hypothesis. While both models were able to reproduce the overall vertical distribution throughout the profiles, the persistent contamination at the surface was closer to the measured value with the EK approach. As a consequence, the K_d model overestimated the contamination into deeper layers and therefore overestimated the migration velocity of ¹³⁷Cs. Fitted sorption parameters suggested a fast sorption kinetic (1 - 7 hours) and a pseudo-irreversible desorption rate (3.2 - 3.4 x 10⁶ years), whereas equilibrium sorption (4.0 x 10^-3 L kg^-1 on average) only affected a negligible portion of ¹³⁷Cs inventory.

To further distinguish the models behaviors, short and long term simulations were conducted. By June 2011, EK parameters fitted on our plots realistically reproduced different profiles measured in the same forest study site. Predictive modeling of ¹³⁷Cs profiles in soil suggested a strong persistence of the surface ¹³⁷Cs contamination by 2030, with exponential profiles consistent with those reported after the Chernobyl accident.

These results prove that the choice of the sorption model is critical in post-accidental situations. An equilibrium approach can result in an underestimation of ¹³⁷Cs residence time in the surface. Whereas a kinetic approach, by distinguishing different sorption and desorption rates, is able to reproduce the slow evolution of ¹³⁷Cs soil profiles with time that is already observed in the case of Chernobyl contaminated areas 30 years after the accident. Non equilibrium sorption parameters 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.