Mercury Contamination of Minamata Bay
149
tions reported for other mercury-contaminated marine systems (see review in
Kudo et al. 1980) tend to be somewhat higher than those reported for Minamata
Bay, suggesting possible low recovery of methylmercury from Minamata Bay
sediments during analysis, rather than an anomalous content of this form of
mercury. Even today, the extraction and analysis of this form of mercury from
sediments is not straightforward (Bloom et al. 1996; Liang et al. 1996).
Sakamoto et al. (1995) reported results of mercury speciation analyses, based
on selective sequential chemical extraction, applied to Minamata Bay sediments.
The forms reported included "organic"mercury, mercury(II)oxide, mercury(II)
sulfide, and residual mercury. In this case, "organic" mercury represented
sediment mercury extractable with chloroform and analyzed as total mercury,
and thus does not necessarily represent alkylated forms. In a similar vein it is
prudent to consider all the forms reported from such sequential extraction
techniques as "operationally defined". The samples (3 sites, 26.7 to 35.6 mg/kg
total Hg) analyzed by Sakamoto et al. were collected from Minamata Bay in
September 1983 and from Yatsushiro Sea (15 sites, 0.43 to 5-49 mg/kg total Hg) in
January 1987. The predominant form of mercury in all samples was HgS (42-93%
of total Hg). Organic Hg in Minamata Bay samples ranged from 0.35 to 0.85 mgt
kg, while those from the Yatsushiro Sea ranged from 0.4 to 0.12 mg/kg. Organic
Hg generally accounted for more of the total mercury in Yatsushiro Sea
sediments (2.6 to 12.8%) than in Minamata Bay sediments (1.1 to 2.4%).
2.2
Redistribution of Sediment Mercury Beyond Minamata Bay
An annual monitoring effort to track possible redistribution of mercury in
sediments beyond Minamata Bay into the Yatsushiro Sea began in 1975 (Kudo
et al. 1980) and has been continued (Kudo and Miyahara 1988, 1992; Miyahara
et a!. 1988; Kudo 1992). The most recent sampling campaign (23rd in the series)
took place in June 1996. Twenty-four locations extending to more than 50 km
from Minamata Bay were established in 1975 and then resampled using identical
methods and analytical techniques. Analysis of the results of this monitoring by
the involved investigators has shown substantial movement of mercury out of
Minamata Bay and redeposition in the Yatsushiro Sea. Kudo (1992) estimated
that the average annual movement of mercury out of the bay was 3.7 tons, and
thus over several decades since the bay was contaminated a significant fraction
(perhaps 10 to 30%) of the 150 to 225 tons of mercury in bay sediments was
dispersed and "diluted" in the adjoining sea. Following the initial surveys in 1975
the level of mercury in sediment from the Yatsushiro Sea closest to Minamata
Bay increased dramatically. At the station located 8 km from the Hiyakken-mon
(where bay and creek meet) sediment mercury levels increased in the sequence,
0.61, 3.5, and 10 mg/kg, in 1975, 1978, and 1984, respectively (Kudo 1992). More
recent data (unpub!.) collected annually since 1990 have shown a gradually
decreasing trend at all stations.
Kumagai and Nishimura (1978) also estimated transport of mercury-contaminated sediment out of Minamata Bay. They used data for depth profiles of
149
tions reported for other mercury-contaminated marine systems (see review in
Kudo et al. 1980) tend to be somewhat higher than those reported for Minamata
Bay, suggesting possible low recovery of methylmercury from Minamata Bay
sediments during analysis, rather than an anomalous content of this form of
mercury. Even today, the extraction and analysis of this form of mercury from
sediments is not straightforward (Bloom et al. 1996; Liang et al. 1996).
Sakamoto et al. (1995) reported results of mercury speciation analyses, based
on selective sequential chemical extraction, applied to Minamata Bay sediments.
The forms reported included "organic"mercury, mercury(II)oxide, mercury(II)
sulfide, and residual mercury. In this case, "organic" mercury represented
sediment mercury extractable with chloroform and analyzed as total mercury,
and thus does not necessarily represent alkylated forms. In a similar vein it is
prudent to consider all the forms reported from such sequential extraction
techniques as "operationally defined". The samples (3 sites, 26.7 to 35.6 mg/kg
total Hg) analyzed by Sakamoto et al. were collected from Minamata Bay in
September 1983 and from Yatsushiro Sea (15 sites, 0.43 to 5-49 mg/kg total Hg) in
January 1987. The predominant form of mercury in all samples was HgS (42-93%
of total Hg). Organic Hg in Minamata Bay samples ranged from 0.35 to 0.85 mgt
kg, while those from the Yatsushiro Sea ranged from 0.4 to 0.12 mg/kg. Organic
Hg generally accounted for more of the total mercury in Yatsushiro Sea
sediments (2.6 to 12.8%) than in Minamata Bay sediments (1.1 to 2.4%).
2.2
Redistribution of Sediment Mercury Beyond Minamata Bay
An annual monitoring effort to track possible redistribution of mercury in
sediments beyond Minamata Bay into the Yatsushiro Sea began in 1975 (Kudo
et al. 1980) and has been continued (Kudo and Miyahara 1988, 1992; Miyahara
et a!. 1988; Kudo 1992). The most recent sampling campaign (23rd in the series)
took place in June 1996. Twenty-four locations extending to more than 50 km
from Minamata Bay were established in 1975 and then resampled using identical
methods and analytical techniques. Analysis of the results of this monitoring by
the involved investigators has shown substantial movement of mercury out of
Minamata Bay and redeposition in the Yatsushiro Sea. Kudo (1992) estimated
that the average annual movement of mercury out of the bay was 3.7 tons, and
thus over several decades since the bay was contaminated a significant fraction
(perhaps 10 to 30%) of the 150 to 225 tons of mercury in bay sediments was
dispersed and "diluted" in the adjoining sea. Following the initial surveys in 1975
the level of mercury in sediment from the Yatsushiro Sea closest to Minamata
Bay increased dramatically. At the station located 8 km from the Hiyakken-mon
(where bay and creek meet) sediment mercury levels increased in the sequence,
0.61, 3.5, and 10 mg/kg, in 1975, 1978, and 1984, respectively (Kudo 1992). More
recent data (unpub!.) collected annually since 1990 have shown a gradually
decreasing trend at all stations.
Kumagai and Nishimura (1978) also estimated transport of mercury-contaminated sediment out of Minamata Bay. They used data for depth profiles of