Speciation of mercury in water at the bottom of Minamata Bay, Japan

Total mercury (T-Hg) and methylmercury (MeHg) concentrations were determined in water at the bottom, in the suspended solid, and in the surface sediment of Minamata Bay to assess the remobilization of mercury from the sediment into the water column. The water and sediment samples were taken from the bottom of the bay using a gravity core sampler at nine locations in October 2002 and six locations in April 2005. The average concentration of T-Hg and the proportion of MeHg in the sediment were 3.71 ± 1.90 mg/kg and 0.27 ± 0.28%, respectively. The water contained 1.80 ± 1.00 ng L− 1 of T-Hg, which was higher than the value reported for the upper-middle depth of Minamata Bay. The results suggest that the sediment is an important source of mercury in the water of Minamata Bay. The percentage of MeHg in the water at the bottom was 50.7 ± 24.6%, also considerably higher than in the upper-middle layer of the water column, suggesting that MeHg may be the predominant form of mercury released from the sediment into the water. The percentage of MeHg was lower in the resuspended sediment than in the surface sediment. The amount of mercury eluted from the sediment into the water was estimated at 0.46 kg and 0.11 kg per year for T-Hg and MeHg, respectively.