Radiocesium leaching from litter during rainstorms in the Fukushima broadleaf forest

Forests are important sources of dissolved radiocesium (¹³⁷Cs) discharge downstream. To improve understanding of dissolved ¹³⁷Cs discharge processes during rainstorms, we investigated the relationship between rainfall-runoff hydrological processes and the discharge of ¹³⁷Cs leached from litter. Leaching tests were conducted with broadleaf litter collected in the area where saturated overland flow was generated during rainstorms in a broadleaf-tree-dominated forest. According to the leaching test results, the ¹³⁷Cs leaching rate was higher in the early stage of the test and decreased afterward. There was no significant difference in the overall results between the agitation and non-agitation cases. The ¹³⁷Cs leaching rate from litter after the 24-h test was up to 33.7%. A large proportion of the original ¹³⁷Cs activity was present even after the tests, as leaching from litter during rainstorms in the headwater area could be an additional source of dissolved ¹³⁷Cs in the stream water. If mixing of ¹³⁷Cs originating from groundwater, soil water, and rainfall with the hydrological processes is assumed, differences between the observed and estimated ¹³⁷Cs in the surface runoff water became larger under high flow conditions. This analysis indicates additional ¹³⁷Cs loading on surface runoff water during rainstorms, where saturated surface area can expand as the surface runoff rate increases. Contact area between surface runoff and litter accumulated on the forest floor should increase and accelerate ¹³⁷Cs leaching from the litter. Therefore, ¹³⁷Cs leaching in the saturated surface area that is temporarily formed during rainstorms can play a principal role in dissolved ¹³⁷Cs discharge during rainfall-runoff events. Contaminated litter in the temporally saturated region of forested headwaters is an important factor contributing to elevated levels of dissolved ¹³⁷Cs during rainstorms in the Fukushima area.