Trend and Sources of Dioxin Pollution in Tokyo Bay, Estimated Based on the Statistical Analyses of Congener Specific Data

On the basis of the congener-specific analysis of dioxins in a dated sediment core, surface sediment, and soil samples, the sources and mass balance of dioxins in Tokyo Bay basin, Japan, were estimated. The dioxins in the sediment core showed that their deposition in the bay increased rapidly during the 1960s, peaked in the early 1970s and then decreased gradually. Principal component analysis of the congener-specific data showed that three major sources existed: combustion, pentachlorophenol (PCP), and chloronitrophen (CNP). PCP and CNP are paddy field herbicides used extensively in Japan in the past. The time trends of source contributions were estimated by multiple regression analysis using the source congener profiles. The results revealed that dioxin emission from PCP and CNP herbicides was high in the 1960s and the early 1970s, respectively. The mass balance of dioxins in the basin during the past 45 years showed that only a few percent of total dioxin load from agrochemicals were transported to the bay and accumulated in sediment, indicating that large part of dioxins was still remaining in terrestrial soil and river sediment of the basin and running off gradually.