Radioactivity Dynamics in forests
（2025）
ＱHow does radiocesium leach from forest leaf litter and humus into water?
ＡA larger amount of radiocesium leaches into water from leaf litter than from humus; the leached fraction from leaf litter is roughly two to three times that from humus. This is considered to occur because dissolved organic matter (DOM) that simultaneously leaches from the leaf litter electrostatically associates with cesium (Cs⁺), thereby facilitating its transfer to the aqueous phase.
Following the accident at the Fukushima Daiichi Nuclear Power Plant, forests that were not decontaminated have retained high concentrations of radiocesium (137Cs), leading to contamination of mushrooms and edible wild plants. On the forest floor lies the “A0 layer,” an accumulation of organic materials composed of freshly fallen leaves (L layer), partially decomposed humus (F layer), and further-decomposed humus (H layer). Transfer of 137Cs in forests is thought to arise when 137Cs that has leached into the aqueous phase from the A0 layer is taken up by organisms. Therefore, to predict the long-term contamination of mushrooms and edible wild plants, it is essential to understand the leaching mechanism from the A0 layer into the aqueous phase. This study investigated how radiocesium leaches into water from these layers.
Figure 1. Overview of sampling and leaching tests.
From the investigation, the amount leached from leaf litter was the greatest—approximately two to three times higher than that from humus. In addition, large amounts of “dissolved organic matter (DOM)” were found to leach into water from leaf litter, and it was shown that this DOM electrostatically associates with cesium and plays a role in transporting it to the aqueous phase.
Figure 2. Leached fractions of radiocesium and alkali metals from leaf litter and humus (a), and pH and dissolved organic carbon (DOC) concentrations in the aqueous phase after the leaching tests (b).
Figure 3. Schematic diagram of the cesium leaching mechanism inferred from the leaching tests.
Through this investigation, it was clarified that the 137Cs leached fraction from leaf litter is particularly high, and that DOM assists this leaching. These findings are indispensable for understanding the transfer of 137Cs into the aqueous phase in forests and for modeling the process by which it migrates from the A0 layer, through the aqueous phase, into biota. Incorporating these insights into prediction models for mushroom and edible wild plant contamination is expected to improve the accuracy of long-term risk assessments.
Related articles
Q Where is radioactive cesium is deposited in forests? How much? How does the amount change with time?
Q Does radioactive cesium move from the ground surface to underground or into groundwater? 【Movement in the water flow in the litter layer】
Q Does radioactive cesium move from the ground surface to underground or into groundwater? 【Movement in underground water】
Q Does radioactive cesium move from the ground surface to underground and into groundwater?【Vertical movement in soil】
References
2) 森林総合研究所(2007): 森林土壌博物館. https://www.ffpri.affrc.go.jp/labs/soiltype/survey/survey_descript.html(2025.11.28アクセス)
3) 寺島元基, 田籠久也, 佐々木祥人(2022): 落葉からの放射性セシウム溶出における腐植化の影響, 日本腐植物質学会第38回講演会要旨集.
4) 寺島元基, 田籠久也, 渡辺勇輔, 佐々木祥人(2023): Chemical equilibrium modelling of radiocesium elution from contaminated litter and its humus into water phase, 33rd Goldschmidt Conference.