CHARACTERIZATION OF RIVER WATER POLLUTION AND ASSESSMENT OF WATER QUALITY IMPROVEMENT BY SEWERAGE EXTENSION USING RIVER BASIN CATEGORIZATION BASED ON LAND USE INDICATORS

This study aimed to assess the relationship between land use indicators and the levels of water pollution in Japanese rives and water quality improvement by extension of sewerage in river basins. A total of 101 river basins were categorized into four groups based on their three types of land uses, namely forest, farmland and built environments. Nationwide, the largest number of river basins were categorized into Group 3, which is characterized by high forest and farmland coverages. This group of rivers have not experienced serious pollution, while water pollution had been serious in the river basins that belong to high area coverages by built environments. Nonetheless, there were differences in the levels of water pollution even among the river basins belonging to the same groups; namely, the river basins having high population densities and high basin areas per base flows were more prone to water pollution than others. As opposed to the river basins having high development areas, which is the sum of the built environments and farmlands, the river basins having high forest areas were more found to be easier and faster in extending sewerage coverage and thus more effective in improving water quality in rivers by extension of sewerage systems.