抄録
Hot-spring water and mine effluent flow into the Toyohira River, which runs through the center of Sapporo, Japan. In the river, the concentrations of components from hot-spring water and mine effluent change according to the flow regime. This phenomenon must be addressed in water quality management. We found that when the water level is low, the arsenic originating from hot-spring water is washed out as a point-source (dilution) pollutant or together with polluted water with a constant arsenic concentration. In the region downstream of the dam, the arsenic washed out during flood is a non-point-source (diffuse) pollutant. The flow-regime-dependent difference in the washout component is attributed to arsenic accumulation in the bottom mud during low water levels. The arsenic concentration in the bottom mud is high in the region downstream of Jozankei, a hot-spring resort. The arsenic accumulated in large amounts in the dam is stably supplied to the river. During flood, however, the agitation of the bottom mud enhances the washout of suspended arsenic, and the concentration of suspended arsenic washed out is high.
