Behavior of dissolved radiocesium in river water in a forested watershed in Fukushima Prefecture

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 Cs-137 concentrations in water (Bq/L) were relatively high in summer, and these levels were higher than particulate Cs-137 concentrations (Bq/L). Under storm flow, particulate Cs-137 concentration became dominant as the suspended solid concentration increased. Throughout the monitoring period, dissolved Cs-137 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 Cs-137 concentrations were investigated by measuring the Cs-137 concentration of suspended solid (Bq/kg) and dissolved Cs-137 of unsaturated soil water, throughfall, and rainfall, together with other main solute concentrations. The Cs-137 concentration per unit weight of suspended solids in river water was not strongly correlated with runoff intensity. Additionally, dissolved Cs-137 concentrations of soil water, groundwater, and rainfall were not detected, while higher dissolved Cs-137 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 Cs-137 in the river water was leaching from organic material in flooded areas. However, further investigation is needed to clarify the dominant source of dissolved Cs-137 in river water.