VERTICAL DISTRIBUTION OF MERCURY IN BOTTOM SEDIMENTS AND PARTICLE SIZE EFFECT ON MIGRATION OF MERCURY-CONTAINING SEDIMENTS IN THE YATSUSHIRO SEA

In 1977, the Minamata Bay Pollution Prevention Project was initiated to dispose of sedimentary sludge containing over 25 ppm of total mercury (T-Hg) after the outbreak of Minamata Disease. The sediments containing the high concentration Hg were dredged, but nevertheless, the residual Hg in Minamata Bay has attracted much attention. Moreover, some studies have even indicated that the Hg remaining in the sediments near the bay has migrated to the Yatsushiro Sea. In this study, the vertical distribution of mercury concentration across sediment layers at different sites was investigated. Developed numerical modeling was used to evaluate the impact of sediment particle size on the migration of Hg-containing sediments from Minamata Bay to the Yatsushiro Sea as well. According to the results, mercury migrated from Minamata Bay to the Yatsushiro Sea, where the mercury migrating to the southwest of the Yatsushiro Sea is more concentrated. Additionally, there were significant differences in mercury concentrations at various depths and locations. The total mercury content varied with the particle size. The migration of mercury-containing sediments with larger particle size was slower and the migration range was limited.