Akihide Tada

Mercury speciation driven by seasonal changes in a contaminated estuarine environment

In this study, seasonal changes of mercury (Hg) species in the highly variable estuary of Soča/Isonzo River (northern Adriatic Sea) were investigated. Samplings were performed on a seasonal basis (September 2009, May, August and October 2010) and Hg species (total Hg, methylmercury (MeHg), dissolved gaseous Hg (DGM)) in waters, sediments and pore waters were determined. In addition, a range of ancillary parameters were measured (salinity, nutrients, organic carbon (OC), nitrogen species). Hg values were interpreted using these parameters and hydrological conditions (river flow, wave height) around the time of sampling. There were no significant changes in Hg load from river to the gulf, compared to previous studies. The load was temporarily higher in May 2010 due to higher river flow. Wave height, through changing hydrostatic pressure, was most likely to cause resuspension of already deposited Hg from the bottom (August 2010). The estuary is a net source of DGM to the atmosphere as suggested by DGM profiles, with salinity, redox potential and organic matter as the most probable controls over its production. MeHg is produced in situ in sediment or in water column, rather than transported by river, as indicated by its correlation with OC of the marine origin. Calculated fluxes for THg and MeHg showed sediment as a source for both the water column. In pore waters, OC in part affects partitioning of both THg and MeHg; however other factors (e.g. sulphide and/or oxyhydroxides precipitation and dissolution) are also probably important.

Tracking the fate of mercury in the fish and bottom sediments of Minamata Bay, Japan, using stable mercury isotopes.

Between 1932 and 1968, industrial wastewater containing methylmercury (MeHg) and other mercury (Hg) compounds was discharged directly into Minamata Bay, Japan, seriously contaminating the fishery. Thousands of people who consumed tainted fish and shellfish developed a neurological disorder now known as Minamata disease. Concentrations of total mercury (THg) in recent fish and sediment samples from Minamata Bay remain higher than those in other Japanese coastal waters, and elevated concentrations of THg in sediments in the greater Yatsushiro Sea suggest that Hg has moved beyond the bay. We measured stable Hg isotope ratios in sediment cores from Minamata Bay and the southern Yatsushiro Sea and in archived fish from Minamata Bay dating from 1978 to 2013. Values of δ(202)Hg and Δ(199)Hg in Yatsushiro Sea surface sediments were indistinguishable from those in highly contaminated Minamata Bay sediments but distinct from and nonoverlapping with values in background (noncontaminated) sediments. We conclude that stable Hg isotope data can be used to track Minamata Bay Hg as it moves into the greater Yatsushiro Sea. In addition, our data suggest that MeHg is produced in bottom sediments and enters the food web without substantial prior photodegradation, possibly in sediment porewaters or near the sediment-water interface.

Distribution and characteristics of methylmercury in surface sediment in Minamata Bay.

This study was carried out to evaluate the present-day chemical properties of methylmercury in surface sediment in Minamata Bay where a dredging project was completed 28years ago. Present-day sediment from Minamata Bay consists of sandy silt, and the average loss-on-ignition in surface sediment was 7.0±2.3%. The average methylmercury concentrations in the upper sediment layers were significantly higher than those in the lower sediment layers. Currently, the concentrations in sediments in Minamata Bay do not exceed the Japanese regulatory standard value for mercury. The average concentration of methylmercury in Minamata Bay surface sediment was 1.74±1.0ng/g on a dry weight basis (n=107). The methylmercury concentration in Minamata Bay surface sediment was almost 16 times higher than that in surface sediment from Isahaya Bay surface sediment, which was 0.11±0.045ng/g on a dry weight basis (n=5).

A NEW TYPE BREAKER FORMING A GIANT JET AND ITS DECAYING PROPERTIES

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

Coastal Current Observation in the Area of Abrupt Topographic Change with DBF Ocean Radar

A DBF ocean radar can detect the surface current pattern every 15 minutes. The tidal current and the residual current were investigated through the continuous observation of the DBF (digital beam forming) radar in the middle-west area of Ariake Bay in autumn 2005. For the validation of the surface current data by the DBF ocean radar the ship-board ADCP measurements were also conducted at spring tide. As the result of the comparison of both surface current data, the correlation coefficient and the standard error of the current speed are 0.89 and 7.23cm/s, and those of the current direction are 0.92 and 35.89 degree. The harmonic analysis of the long-term data by the DBF ocean radar clarifies the spatial pattern of the principal tides and the residual currents. The residual currents in 15days flow south to southeastward caused mainly by the tidal residual currents due to the topographic asymmetry.

Chemical characteristics of dissolved mercury in the pore water of Minamata Bay sediments

Methylmercury concentrations in fish from the historically polluted Minamata Bay remain higher than in fish from other coastal seas around Japan. To obtain a better understanding of this phenomenon, the chemical characteristics of pore water from Minamata Bay sediments were investigated. Samples were taken from two stations over a 1-year period. Total average values in the pore water at the two stations for dissolved total mercury and methylmercury concentrations were 6.64±4.93 and 2.69±2.07ng/l, respectively. The pore water was centrifuged at 1000rpm to 3000rpm. The highest ratio of dissolved methylmercury to dissolved total mercury exceeded 60% for pore water centrifuged at 3000rpm. Furthermore, because total average values of Log Kd of total mercury and methylmercury in sediment (St1 and St2) were 5.42 and 2.32 (Lkg-1), methylmercury in Minamata Bay sediment is more eluted than other mercury species.

Development of technologies to preserve the water environment and support its sustainable use in Asia

Through this project, successful applicants from Korea and China will be accepted into the Graduate School of Engineering Masters Course, that was established in April 2011, at Nagasaki University. They will be trained to become highly skilled professionals with practical knowledge who can contribute to solving water environment conservation issues, including sustainable water use, in East Asia.

The mechanism of bottom water DO variation in summer at the northern mouth of Isahaya Bay, Japan

Recently, bivalves have been massively killed by anoxia or hypoxia in summer at the northern part of Isahaya Bay, Japan, which constituted a major problem for fisheries. However, the mechanism behind the occurrence of hypoxic water masses is unclear. It is known that the bottom water dissolved oxygen (DO) in this area is affected by the inflow of seawater into the northern mouth of Isahaya Bay. To understand the mechanism of hypoxia, it is necessary to determine the physical processes that cause changes in the bottom DO concentrations in this area. This study shows that there is a neap-spring tidal variation in bottom DO due to a change in vertical tidal mixing, and it also suggests that the decrease in bottom DO was generated by a baroclinic flow, which is due to the internal tide, and a shear flow, which is induced by the external tide in the bottom boundary layer. In addition, our study suggests that the source of cold and hypoxic water that appears in the bottom layer at low tide is the inner area of the Ariake Sea.

Consideration of impact factors on data acquisition for ocean radar with VHF band

An ocean radar with a VHF band is a useful tool for surveying coastal currents in a wide area. We have conducted a field observation of currents in Ariake Bay, the large inner sea located in the western part of Japan. Although the number of valid data acquired (hereafter, data acquisition) varies temporally and spatially, the mechanism of such variation is not yet well understood. Our aim in this study is to investigate the impact factor affecting data acquisition through field observations of current, salinity and wind in Ariake Bay. As a result, the data acquisition and the maximum measurement distance of the radar decreased owing to the decreases in salinity and wind speed. After receiving much rainfall, the data acquisition decreased rapidly in accordance with the decrease in surface salinity influenced by run-off from adjacent rivers and a flood control dike. Also, when wind speed became less than 2 m/s, the intensity of the signals received was not sufficient for the detection of current velocity. In conclusion, salinity and wind speed are important factors for steady-current observation using ocean radars.

Characteristics of Surface Currents around the Mouth of Isahaya Bay using DBF Ocean Radar

This study deals with the field observations on coastal surface currents by means of both Digital Beam Forming marine radar (DBF ocean radar) and Acoustic Doppler Current Profilers (ADCP) in order to realize characteristics of flow around the mouth of Isahaya Bay. In particular, the former has started since 2005. On the other hand, the latter has carried out seven times since 2001. As a result, it is found out that the feature of the M2 consituent flow detected by the DBF ocean radar was widely coincided well with other ADCP field observation data. Not only the river runoff in the head of Ariake Sound but also a flush flow from the sea-dike gate contribute to the surface currents around the mouth of Isahaya Bay significantly.