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Dissolved radiocesium concentrations in river water in a high‐dose‐rate forest watershed in Fukushima Prefecture were investigated under base flow and storm flow conditions. Under base flow conditions, dissolved 137Cs concentrations in water (Bq/L) were relatively high in summer, and these levels were higher than particulate 137Cs concentrations (Bq/L). Under storm flow, particulate 137Cs concentration became dominant as the suspended solid concentration increased. Throughout the monitoring period, dissolved 137Cs concentrations in water (Bq/L) were higher under storm flow than base flow conditions and were positively correlated with runoff intensity. Factors influencing changes in dissolved 137Cs concentrations were investigated by measuring the 137Cs concentration of suspended solid (Bq/kg) and dissolved 137Cs of unsaturated soil water, throughfall, and rainfall, together with other main solute concentrations. The 137Cs concentration per unit weight of suspended solids in river water was not strongly correlated with runoff intensity. Additionally, dissolved 137Cs concentrations of soil water, groundwater, and rainfall were not detected, while higher dissolved 137Cs concentrations were detected in throughfall than river water. K+ concentrations were higher under storm flow than base flow, and dissolved organic carbon increased toward the peak flow rate. These findings suggested that one main factor influencing generation of dissolved 137Cs in the river water was leaching from organic material in flooded areas. However, further investigation is needed to clarify the dominant source of dissolved 137Cs in river water.

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Floor Covering on Radiocesium Wash-off Associated with Forest Soil Erosion. J Jpn For Soc 97: 63 69 To examine the degree of Cs wash-off from forest floor, we monitored the amount of particulate and dissolved Cs in surface runoff water from three m of experimental plots installed on slopes of in four forests with different floor covering in Fukushima Prefecture. Plots were monitored for days from May to October,. A maximum of of the Cs inventory in soil was washed off, with the amounts differing by up tofold between the plots. The largest amount was observed in Japanese cypress forest, followed by deciduous broadleaf forest, Japanese red pine forest and Japanese cedar forest. Most more than of the Cs washed off was associated with particles. The amount of soil loss was relatively large in forests with little understory and or organic horizon, suggesting that the forest floor covering strongly affected Cs wash-off. In both suspended solids in surface runoff water and stream water, the activity concentration of Cs correlated positively with the organic matter amount, suggesting that some of the Cs in stream came from forest floor and that the organic horizon was an important source of the Cs. Japanese cypress forest is prone to poor floor covering but covers a small area of Fukushima Prefecture, suggesting that the broadleaf forest floor covering, which is most widely distributed in the area, greatly influences the Cs wash-off from the forest.