Abstract
Hot-spring water and mine effluent flow into the Toyohira River, which runs through the center of Sapporo, Japan. In this river, the concentration of arsenic in hot-spring water changes according to the flow regime. This phenomenon must be addressed in water quality management. It was found that arsenic washed out during floods is a non-point-source (diffuse) pollutant. According to the arsenic washed load balance of the river basin as a whole, approximately 20% of the arsenic supplied to the Toyohira River is suspended and sediments in the region downstream of Jozankei. The flow-regime-dependent difference in the washout component is attributed to arsenic accumulation in bottom mud during low-water-level periods. The arsenic concentration in bottom mud is high in the region downstream of Jozankei, a hot-spring resort. Arsenic accumulated in large amounts in the dam is stably supplied to the river. During floods, however, the agitation of bottom mud enhances the washout of suspended arsenic, and the concentration of suspended arsenic washed out increases.
