1. IN SITU MEASUREMENT ON COASTAL CURRENT CHARACTERISTIC AND WATER QUALITY DYNAMICS IN MINAMATA BAY Izaya Tateyama*, A Tada, S Yano, A Matsuyama, Y Ohbuchi * Graduate Student, Graduate school of Science and Technology

2. Introduction Small amount of mercury remains in bottom mud of Minamata Bay. Recent measurement result (Tomiyasu et al., 2000) has shown that mercury is spreading around the center and southern parts of Yatsushiro Sea from Minamata Bay. In order to elucidate the mercury dynamics accurately in a coastal area, it is necessary to measure mercury transport in an actual sea.

3. Outline of field observation Current observation using ADCP In order to reveal a three-dimensional structure of tidal currents during spring tide in Minamata Bay, the current observation using ADCP was performed. Current measurements were collected 4 times during one tidal cycle. the 2nd of December in 2002 the 29th of July in 2003 Minamata Bay M-1 ， M-9 and M-10 M-1-M-10 and F-1

4. Long-term simultaneous measurement on tidal currents and turbidity At the time of establishment At the time of removal Winter8cm7cm Summer1cm0cm the 11th of December in 2004 to the 30th of January in 2005 the 17th of August to the 17th of September in 2005 Outline of field observation

5. Observation on various water quality indexes Measurement on the vertical profiles of SS and Total-Hg in SS by water sampling These observations were carried out at three Stations (No.3, No.6 and No.11) once every four weeks Outline of field observation

6. inflow from Fukuroura and the northern mouth into Minamata Bay. horizontal distributions of velocity vectors outflow into Yatsushiro Sea occurred in the western side of Minamata Bay a topographic eddy was generated the maximum current velocity of ebb tide (a) the 2nd of December in 2002(b) the 29th of July in 2003

7. along M-1 along M-4 Vertical structures of the maximum current velocity the outflow into Yatsushiro Sea occurred in the western side of Minamata Bay was extended over a depth of 8 m. It is realized that the eddy becomes weak in the direction of sea bottom to a depth of 7 m at least. the maximum current velocity of ebb tide in the 29th of July in 2003 Distance(m) Depth(m) Distance(m) Depth(m)

8. spring tide in winter spring tide in summer an eastern current was dominant to a tide level of 10 m above the sea bottom. (inflow from Yatsushiro Bay) western current near the sea bottom was predominant. (outflow from Minamata Bay ) a western current near the sea bottom was predominant. (outflow from Minamata Bay ) Temporal variations of vertical profile on east-west component Water level(m) m/s

9. N S W E N S W E N S W E N S W E 1.8m above sea bottom 5.8m above sea bottom 9.8m above sea bottom 13.8m above sea bottom The major axis of M 2 excels in ENE - WSW Tidal current ellipses(winter season)

10. N S W E m/s N S W E N S W E N S W E 1.8m above sea bottom 5.8m above sea bottom 9.8m above sea bottom 13.8m above sea bottom The major axis of M 2 excels in ENE – WSW at1.8m.5.8m and 9.8m above sea bottom The major axis at a depth of 13.8 m turns clockwise Tidal current ellipses(summer season)

11. -3 -2 0 1 2 3 2004/12/11 14:00 2004/12/12 14:00 2004/12/13 14:00 2004/12/14 14:00 潮位 (m) 0 5 10 15 20 25 30 濁度 (mg/l) 潮位 濁度 spring tide in winter spring tide in summer It is found out that the turbidity measured in summer is higher than the one in winter. The turbidity is prone to rise from the maximum current velocity of ebb tide to low tide. Turbidity water level turbidity water level turbidity Tidal level(m) Turbidity(mg/l)

12. the existence of the strong thermal stratification at a water depth of 5 m. influence of the inflow of fresh water from Minamata River. Vertical distribution of water temperature and salinity The 18th of August in 2005 The 17th of September in 2005

13. Influence by rainon the day before the observation day. On the vertical profiles of SS in water body and Total-Hg in SS components The measured SS in water body gradually falls and converges on the constant value, about 5 mg/l. Total-Hg in SS(ng/l)

14. Conclusion A topographic eddy was generated in the northern part of Minamata Bay at the maximum current velocity of ebb tide in summer because the northern mouth of Minamata Bay is very narrow. The most dominant tidal constituent in Minamata Bay is M 2 tide. The major axes of M 2 tidal ellipses in winter correspond to the direction of ENE- WSW at Station No.6. It is found out that the turbidity measured in summer is higher than the one in winter.