Wildfires Oppositely Modify Radiocesium Mobility in Forest Litter and Soils: Evidence from Radiologically Contaminated Areas in Fukushima

Wildfires in radiologically contaminated areas have raised concerns about radionuclide redistribution and increased radiation exposure. Most radionuclides, such as radiocesium (¹³⁷Cs), remain in the charred residues and burned soil even after wildfires. This study investigated the impact of wildfires on ¹³⁷Cs speciation in forest litter and soils following the Fukushima nuclear accident. Extraction tests of ¹³⁷Cs from unburned litter and soil artificially combusted at 250, 375, and 500 °C were conducted. ¹³⁷Cs leaching from charred residues and burned soil collected from the field after the wildfires was tested. Sequential extraction was performed on soil obtained after the fire. The results indicated that water-soluble ¹³⁷Cs from burned litter increased with combustion temperature. However, in artificially combusted soil, the release of water-soluble ¹³⁷Cs was significantly reduced. In the field, ¹³⁷Cs leaching from charred residues was lower than that from unburned litter. The fraction of water-soluble ¹³⁷C in burned soil did not differ significantly from that in unburned soil. This suggests that the combustion temperature was relatively low and insufficient to promote significant leaching or soil drying. Furthermore, the combustion of soil organic matter may have altered the exchangeable form of ¹³⁷Cs. Overall, these findings suggest that wildfires enhance ¹³⁷Cs retention in soil, potentially delaying its release.