The Source of Refractory Organic Matters in Lake Biwa: Estimation by Biodegradation Assay in Generation Sources and Box Model

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 fl ow using the box model, the generation or infl ow 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.