Sedimentation Rate and Vertical Distribution of Dioxins in Sediment at Environmental-Quality Monitoring Station No. 35 in Northwest Tokyo Bay

Sediment core samples with a length of about 1 m were collected at station 35, which is located at the intersection of the extended lines of the Arakawa and Tamagawa Rivers in northwest Tokyo Bay. The core samples were cut into pieces of 2.5 cm depth and 137Cs and dioxins were analyzed. There was a clear relationship between the depth and the years of some marked 137Cs fallout events. The sedimentation rate at 0 - 50 cm depths was estimated to be 0.78 cm y-1. On the other hand, the dioxin concentration began to increase in 1946, reached its maximum in 1970, and then gradually decreased. The change in the dioxin concentration was well explained by governmental actions, such as the establishment of environmental laws in the estimated years. The dioxins compositions in the surface sediments of seven stations around the Arakawa and Tamagawa Rivers showed that the sediment at station 35 was derived mainly from the Arakawa River.