Abstract
The authors analyzed the water quality of five rivers in the Kaname River basin and correlated it with the sewerage system and land use. The development of a sewerage system and reduction in agricultural land were found to affect water quality. The water quality of all river basins improved with time, and the improvement was seen as a gradual reduction in the pollution loads from the unsewered population and agricultural land. Temporal variations in the water quality of each river could be used to characterize them in terms of the expansion of the sewerage system and decrease in agricultural land use. Multiple regression analysis revealed that, in most cases, the unsewered population had a greater impact on water quality; nevertheless, the change in agricultural land use also had some influence. Unlike total nitrogen (TN), chemical oxygen demand and total phosphorus exhibited good correlations with the pollution loads from the unsewered population and agricultural land. Atmospheric nitrogen (N) was presumed to impact the TN concentration in rivers. To maintain the water quality in the Kaname River basin, the development of a sewerage system could be a crucial factor; however, the contamination due to atmospheric N also needs to be considered.
