関東地方整備局管内の河川・湖沼におけるダイオキシン類調査に基づく中川・綾瀬川の結果の考察について

抄録

MLIT has investigated dioxins in first-class rivers from 1999. The nationwide investigation in 2016 showed that dioxin concentrations in ten sites were higher than the monitoring-required concentration, and six sites were from Nakagawa and Ayase River. Therefore, study was conducted to clarify the causes of high concentrations of dioxins in these rivers. As a result, there were strong correlations (r > 0.8) between dioxins detected in the river water and the water quality parameters (SS and VSS), organic contaminants absorbed to suspended matter. The high components of herbicides used in the past was shown from the results of composition analysis of dioxin congeners. Therefore, it is presumed that herbicide residues from rice cultivation in these river basins were agitated when irrigating paddy fields, diffused and suspended in the water, and flowed into the river water through agricultural water channel, causing the high dioxin concentrations.