Mechanisms of dissolved-form 137Cs runoff from forest source watersheds

The concentration of dissolved-form ¹³⁷Cs in forested rivers is known to increase during rainstorms, and direct leaching from litter and soil water is considered to be a factor. There have also been many studies showing that competing ions such as K⁺ and NH₄⁺ promote the elution of ¹³⁷Cs. However, there are no examples of detailed measurements of ¹³⁷Cs concentrations and water quality characteristics of stream water and water passing through litter during actual rainstorms. In this study, stream water, throughfall,  water passing through litter, and groundwater were sampled in a small watershed in Fukushima Prefecture, Japan, which was affected by the Fukushima Daiichi Nuclear Power Plant, to measure dissolved ¹³⁷Cs and dissolved organic carbon (DOC), K⁺and NH₄⁺. NH₄⁺ was not detected in stream water. The average concentrations of dissolved¹³⁷Cs, DOC and K⁺ were 6.36 (mBq/L), 0.51 (mg/L) and 0.14 (mg/L), respectively, while the concentrations of ¹³⁷Cs and DOC doubled to 13.38 (mBq/L), 1.13 (mg/L) during rainfall event and the K⁺concentrations remained unchanged (0.15mg/L). The concentrations of ¹³⁷Cs and DOC  in the water passing through the litter were 50 and 30 times higher than in the stream water, respectively, suggesting that the high concentrations of dissolved ¹³⁷Cs at the time of runoff were formed by leaching from the litter rather than by the presence of competing ions. The amount of ¹³⁷Cs and K⁺eluted from the litter increased in the order of near-channel, saturated zone at run-off and slope, while the amount of DOC eluted from the litter was lower near the channel. These results suggest that ¹³⁷Cs, K⁺and DOC release from near-channel litter is lower than that from litter on the slope because of the progress of leaching due to the occurrence of saturated surface flow.