Modeling dynamics of 137Cs in forest surface environments: Application to a contaminated forest site near Fukushima and assessment of potential impacts of soil organic matter interactions

Abstract

A process-based model for 137Cs transfer in forest surface environments was developed to assess the dynamic behavior of Fukushima-derived 137Cs in a Japanese forest. The model simulation successfully reproduced the observed data from 3year migration of 137Cs in the organic and mineral soil layers at a contaminated forest near Fukushima. The migration of 137Cs from the organic layer to the mineral soil was explained by the direct deposition pattern on the forest floor and the turnover of litter materials in the organic layer under certain ecological conditions. Long-term predictions indicated that more than 90% of the deposited 137Cs would remain within the top 5cm of the soil for up to 30years after the accident, suggesting that the forest acts as an effective long-term reservoir of 137Cs with limited transfer via the groundwater pathway. The model was also used to explore the potential impacts of soil organic matter (SOM) interactions on the mobility and bioavailability of 137Cs in the soil–plant system. The simulation results for hypothetical organic soils with modified parameters of 137Cs turnover revealed that the SOM-induced reduction of 137Cs adsorption elevates the fraction of dissolved 137Cs in the soil solution, thereby increasing the soil-to-plant transfer of 137Cs without substantially altering the fractional distribution of 137Cs in the soil. Slower fixation of 137Cs on the flayed edge site of clay minerals and enhanced mobilization of the clay-fixed 137Cs in organic-rich soils also appeared to elevate the soil-to-plant transfer of 137Cs by increasing the fraction of the soil-adsorbed (exchangeable) 137Cs. A substantial proportion (approximate 30%–60%) of 137Cs in these organic-rich soils was transferred to layers deeper than 5cm decades later. These results suggested that SOM influences the behavior of 137Cs in forests over a prolonged period through alterations of adsorption and fixation in the soil.