Abstract
In Lake Biwa, the gap between biochemical oxygen demand (BOD) and chemical oxygen demand (COD) is widening, so the increase in the amount of refractory organic matter has been of concern. The objective of this study is to estimate the source and the cause of the increase in the amount of refractory organic matter. In order to investigate the material flow using the box model, the generation (or inflow) load in the watershed and internal production are calculated by monitoring COD and total organic carbon (TOC) by biodegradation assay in drainage from each pollution source and lake water. As a result, it is revealed that although the generation load of organic matter has been reduced by domestic and industrial measures, the main cause is the reduction of labile organic matter. Model results indicate that autochthonous organic matter is the dominant source of refractory organic matter in the lake but is at the same level as the allochthonous organic matter in the case of the dissolved fraction. It is most likely that the change in the amount of the autochthonous organic matter caused the increase in the amount of refractory dissolved organic matter.
