Shigeki Masunaga

Institute of Environmental Science and Technology, Yokohama National University
(79-7, Tokiwadai, Hodogaya-ku, Yokohama-city, Kanagawa 240-8501 Japan)

Abstract

Based on the detailed dioxin (PCDD/DF) analyses of dated sediment cores collected from Lake Shinji and Tokyo Bay, the past and present states of dioxin pollution in the Japanese watershed were described. The dioxin concentration and profile recorded in the sediment cores showed that the pollution originating from pentachlorophenol (PCP) and chloronitrofen (CNP) herbicides was severe in the 1960s and 1970s. From the decrease of dioxin deposition in the lake after intensive herbicide use, the dioxins accumulated in the soil of the basin were estimated to have decreased by 0.9-1.4% per year or to have a half-life of 50-77 years. This indicated that dioxin run-off from agricultural fields would continue for a long time in the basin. The contributions of PCP, CNP and incineration to the TEQ concentration in the present surface sediment of the lake were estimated to be about 60, 10 and 30%, respectively. These results indicated that the present government policy on the reduction of dioxin emissions from combustion sources would do little for the immediate remediation of dioxin pollution in the Japanese water environment.

Key words: dioxin, herbicide, sediment core, principal component analysis, source