This study aims to clarify the mass balance of pollutants during both dry periods and storm events and to discuss the effects of some strategies such as pollutant removal, land use planning and new drainage systems by simulation. Three subjects are discussed in this paper. First, the amount of pollutants entering Lake Biwa from an urban area have been roughly estimated by using data collected by the local government. Second, many additional samples were collected from road surfaces, house roofs and parking lots to consider the role of land use in pollutant runoff. Third, some ongoing BMP projects in an urban area are introduced. As a result, some ideas on how to solve the problem of diffuse pollution in urban areas have been obtained.