148
A. Kudo and R.R. Turner
map (see Fig. 1 in Yoshida and Ikegaki 1977) of the areal distribution of mercury
in the bay and the adjacent Yatsushiro Sea (see Fig. 1 in Nishimura and Kumagai
1983). In 1973, the mercury level in sediments exceeded 15 mg/kg everywhere in
the bay, and a level as high as 600 mg/kg was found at the innermost part of the
bay (Nishimura and Kumagai 1983). Later (1983 and 1987), analyses of surface
sediments from Minamata Bay (3 locations) and the Yatsushiro Sea (15 locations)
were reported by Sakamoto et al. (1995). Values for the samples from the bay
were 27 to 30 mg/kg, while those from the sea were 0.41 to 5.5 mg/kg. Hosokawa
(1993) provided a detailed map (see his Fig. 9) of the areal distribution of
mercury in bay sediments after completion of all dredging in 1988. The
maximum value was 8.75 mg/kg but most values were less than 5.0 mg/kg.
Published information on the distribution of mercury with depth in Minamata
Bay sediments is sparse. Fujiki and Tajima (1992) present some data (see their
Table V) which indicate that the thickness of contamination in the 1960s was in
excess of 3 m in some locations. Similarly, Ishikawa and Ikegaki (1980) mention
that contaminated deposits in the innermost portion of the bay were 4 m thick.
This large thickness implies a very rapid rate of bulk sedimentation in some
portions of the bay, or perhaps unfortunate selection of coring sites over
underwater dredge spoil. In any case, other authors (e.g., Kudo et al. 1980) give
the thickness of contamination as "over 50 cm". Unfortunately, the data
presented by Fujiki and Tajima (1992) are insufficient (depth intervals are too
large) to detect any chronological pattern in mercury deposition in the bay. Nor
have any other published data been found in the English language literature
which would permit a temporal reconstruction of mercury deposition in the bay.
Such a sediment "record" was probably present in undisturbed bay sediments,
and would have been valuable to retrieve, but has now been almost certainly
obliterated by the bay-wide dredging which began in 1980.
2.1
Forms of Mercury in Sediments
Some of the earliest measurements of the forms of mercury in Minamata Bay
sediments appear to have been made in the early 1960S. Fujiki and Tajima (1992)
cite references as early as 1964 as sources of information to the effect that
"mercury compounds in the muds were inorganic, such as mercury sulfide and
oxide, and the highest concentration of methyl mercury in the mud was 0.015
ppm as mercury (wet weight),'. Once methylmercury was identified as the main
contaminant of concern in fish and shellfish, it became the focus of further
measurements in sediment but little was ever found (Fujiki et al. 1977; Yoshida
and Ikegaki 1977; Nishimura and Kumagai 1983; Fujiki and Tajima 1992) in bay
sediments using the methods available through the mid-1970S. For example,
Nishimura and Kumagai (1983) note that alkylmercury was not detected
(detection limit 0.001 mg/kg) in any of 263 sediment samples from Minamata
Bay and Yatsushiro Sea. In minor contrast to this report, Hosokawa (1993) states
that, while most sediments contained less than 0.005 mg/kg methylmercury, the
maximum concentration was 0.03 mg/kg. Sediment methylmercury concentra-
A. Kudo and R.R. Turner
map (see Fig. 1 in Yoshida and Ikegaki 1977) of the areal distribution of mercury
in the bay and the adjacent Yatsushiro Sea (see Fig. 1 in Nishimura and Kumagai
1983). In 1973, the mercury level in sediments exceeded 15 mg/kg everywhere in
the bay, and a level as high as 600 mg/kg was found at the innermost part of the
bay (Nishimura and Kumagai 1983). Later (1983 and 1987), analyses of surface
sediments from Minamata Bay (3 locations) and the Yatsushiro Sea (15 locations)
were reported by Sakamoto et al. (1995). Values for the samples from the bay
were 27 to 30 mg/kg, while those from the sea were 0.41 to 5.5 mg/kg. Hosokawa
(1993) provided a detailed map (see his Fig. 9) of the areal distribution of
mercury in bay sediments after completion of all dredging in 1988. The
maximum value was 8.75 mg/kg but most values were less than 5.0 mg/kg.
Published information on the distribution of mercury with depth in Minamata
Bay sediments is sparse. Fujiki and Tajima (1992) present some data (see their
Table V) which indicate that the thickness of contamination in the 1960s was in
excess of 3 m in some locations. Similarly, Ishikawa and Ikegaki (1980) mention
that contaminated deposits in the innermost portion of the bay were 4 m thick.
This large thickness implies a very rapid rate of bulk sedimentation in some
portions of the bay, or perhaps unfortunate selection of coring sites over
underwater dredge spoil. In any case, other authors (e.g., Kudo et al. 1980) give
the thickness of contamination as "over 50 cm". Unfortunately, the data
presented by Fujiki and Tajima (1992) are insufficient (depth intervals are too
large) to detect any chronological pattern in mercury deposition in the bay. Nor
have any other published data been found in the English language literature
which would permit a temporal reconstruction of mercury deposition in the bay.
Such a sediment "record" was probably present in undisturbed bay sediments,
and would have been valuable to retrieve, but has now been almost certainly
obliterated by the bay-wide dredging which began in 1980.
2.1
Forms of Mercury in Sediments
Some of the earliest measurements of the forms of mercury in Minamata Bay
sediments appear to have been made in the early 1960S. Fujiki and Tajima (1992)
cite references as early as 1964 as sources of information to the effect that
"mercury compounds in the muds were inorganic, such as mercury sulfide and
oxide, and the highest concentration of methyl mercury in the mud was 0.015
ppm as mercury (wet weight),'. Once methylmercury was identified as the main
contaminant of concern in fish and shellfish, it became the focus of further
measurements in sediment but little was ever found (Fujiki et al. 1977; Yoshida
and Ikegaki 1977; Nishimura and Kumagai 1983; Fujiki and Tajima 1992) in bay
sediments using the methods available through the mid-1970S. For example,
Nishimura and Kumagai (1983) note that alkylmercury was not detected
(detection limit 0.001 mg/kg) in any of 263 sediment samples from Minamata
Bay and Yatsushiro Sea. In minor contrast to this report, Hosokawa (1993) states
that, while most sediments contained less than 0.005 mg/kg methylmercury, the
maximum concentration was 0.03 mg/kg. Sediment methylmercury concentra-