Trends in mercury concentrations and methylation in Minamata Bay, Japan, between 2014 and 2018.

Methylmercury concentrations in Minamata Bay are high, but the cause is unclear. We conducted a basic study on the behavior of methylmercury in Minamata Bay seawater; the findings suggest that mercury methylation may occur throughout the year in Minamata Bay. Seawater temperature, salinity, and concentrations of dissolved organic carbon were the environmental factors that affected methylation, and the degree of methylation was closely related to bacterial community structure. The concentration of methylmercury in suspended particulate matter was highest 10 m below the surface and decreased with greater depths. We did not observe a correlation between methylmercury concentrations in suspended particulate matter and concentrations of dissolved methylmercury.

The spatial distribution of total mercury in sediments in the Yatsushiro Sea, Japan.

The Yatsushiro Sea in Japan is contaminated with mercury in wastewater discharge from the Chisso Company, which produced acetaldehyde from 1932 onwards. In this study, we investigated the current spatial distribution of total mercury (T-Hg) in sediments, both at the surface and at different depths, using 43 sediment cores and 22 surface sediment samples collected from the Yatsushiro Sea in August 2017. Altogether, 821 surface and core samples were analyzed for their T-Hg concentrations. Most of the mercury that was discharged from Chisso remained either in, or around the exit of, Minamata Bay, but some had been transported from Minamata Bay to the Nagashima Sea. We estimated that almost 51 and 6 tons of mercury had accumulated in the Yatsushiro and the Amakusa Sea areas, respectively. The amount of acetaldehyde produced in Chisso over time was correlated with the T-Hg concentrations in the sediments from the Yatsushiro Sea.

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.

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).

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.

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 Soca/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.