In the Minamata Bay, the remediation project on water and bottom sediment environment was completed in 1990. About 0.8 million m3 of bottom sediment contaminated by mercury was dredged. However, a recent measurement of mercury in the sediments [Tomiyasu et al., 2000] has shown that mercury is spreading around the center and southern parts of the Yatsushiro Sea from the Minamata Bay. Although the concentration of mercury in bottom sediment is lower nowadays (<10ppm), modelling for accurate prediction of mercury dynamics is necessary. In order to predict the mercury dynamics accurately in a coastal area, it is necessary to measure mercury transport in an actual sea.
Thus, the current observation by using ADCP (RD Instruments Workhorse 1, 200k Hz), which was mounted on the side of a fishing boat, was performed on the 2nd of December, 2002 and the 29th of July, 2003 to realize a three-dimensional structure of tidal current in the Minamata Bay. As a result, it was found that a horizontal circulation flow occurred during ebb tide in summer.