Mercury contamination in the Yatsushiro Sea, south-western Japan: spatial variations of mercury in sediment

Mercury-contaminated effluent was discharged into Minamata Bay from a chemi cal plant over a 20-year period until 1965 (from 1958 to 1959, effluent was discharged into Minamata River), causing Minamata disease. In an effort to characterize the extent of the contamination in the Yatsushiro Sea, the ve rtical and horizontal distributions of mercury in sediment were investigate d. Sediment was sampled at 62 locations in the southern part of the sea fro m 4 to 6 March 1996. In the lower layers of the long cores of sediment, the total amount of mercury was at a relatively uniform low concentration. We interpret these low values to represent the background concentration absent of anthropogenic influence. The background value thus estimated for the Ya tsushiro Sea was 0.059 +/- 0.013 mg kg(-1) (mean +/- S.D., n = 51). The hig hest concentration in each sample ranged from 0.086 to 3.46 mg kg(-1) (mean , 0.57 mg kg(-1)). The higher values were obtained at stations near Minamat a Bay and the Minamata River (the sources of the pollution). Concentrations decreased with distance from the source. An inspection of the vertical pro files of mercury concentration in cores suggested that the deposited mercur y had not been fixed in sediment but had been transported, despite 30 years having past since the last discharge of contaminated effluent. At nine sta tions, extractable inorganic and organic mercury concentrations were determ ined differentially. Inorganic mercury is the predominant species in sedime nt and organic mercury comprising approximately 1% of the total. (C) 2000 E lsevier Science B.V. All rights reserved.