Estimation of Diffuse Pollutant Loads in Fine Weather using GIS and Monthly Water Quality Data of Rivers in Lake Biwa Basin

Abstract

In this study, we propose a methodology to determine quantitatively diffuse pollution loads by using GIS (Geographical Information System) and monthly water-quality data. This methodology was applied to 26 river basins, which are contained in the Lake Biwa basin. Firstly, we collected information concerning point source pollutant loads in this basin in detail and entered them into GIS. For example, concerning industrial effluents, we collected the daily average effluent volume from 1, 657 factories, and the quality of about 99% of the effluents was recorded. Moreover, based on their sewer-treatment type, 1, 789 villages were divided into 5 categories. With these detailed data, we could estimate the daily average amount of pollutant loads from the point sources in each sub-basin using the information collected in GIS. Secondly, we estimated non-point pollutant loads in each river basin from these calculated results and measured pollutant loads. The quality of the observed water is considerably affected by various complex processes such as rain, irrigation, and snow melting. Not taking these influences into account, the quality of the observed river water shows the essential pollution loads generated by human activities. We defined this pollution load as ‘own basic pollutant load’, including pollutants from non-point sources, such as forests. Then, we tried to compare these pollutants with loads based on the geographical information of each basin including land use, geographical features, and vegetation. In this way, the characteristics of non-point pollutant loads could be evaluated, and potential pollution loads were estimated.