144
A. Kudo and R.R. Turner
ination and its reclamation. Information on the latter subjects (e.g., Ishikawa and
Ikegaki 1980; Fujiki and Tajima 1992; Hosokawa 1993) has been slow to appear in
English language publications. The purpose of this chapter is to summarize some
important characteristics of the site, the nature and extent of remedial actions
undertaken, and the response of the system to those deliberate actions, and to
natural processes which can achieve similar endpoints. Appropriately, Minamata
Bay was the first mercury-contaminated ecosystem in the world in which a major
restoration has been attempted. Restorations of a similar kind and magnitude are
likely to be contemplated elsewhere and thus the experience at Minamata may
hold valuable lessons.
It was the authors' resolve that no major book on mercury-contaminated sites
should be published without a chapter covering Minamata Bay. As no papers were
presented at the 1996 Hamburg conference which might have served, the authors
elected to prepare this review chapter. Little is presented here which has not
already been published elsewhere in one form or another. The authors were able
to obtain recent data (through 1995) for mercury in fish which allowed extension
of a previously published trend chart. The authors have attempted to assemble in
one document as much information as possible concerning site characterization,
risk assessment, and remediation, and thus provide for easier comparisons with
other mercury-contaminated sites in the world. The authors offer a few opinions
and speculations for which they alone should be held accountable.
1.1
Hydrography of Minamata Bay and Adjacent Yatsushiro Sea
Minamata Bay is a small coastal inlet (about 300 hal of the Yatsushiro Sea (also
called the Shiranui Sea) on the southwestern coast of Kyushu, Japan (Fig. 1). The
only freshwater inflow is a small creek with a flow rate of 0.5 to 1 m 3 /s, most of
which is discharged from a chemical factory. The maximum tidal range is 3.4 m
with an average of 2.23 m, and thus tidal flushing of the bay is considerable. The
bay holds about 30 million m 3 of water and has a theoretical water retention time
of 2.5 days. The relatively steep topography of the lands surrounding the bay is
mirrored in the bay itself which reaches a maximum depth of 30 m and has an
average depth of 10 m. Sediments deposited in the bay have been described as
organic-rich silts and clays. Calculations of natural water velocities in the bay by
Kudo (1992) suggested that bed sediment transport should not be expected but
that suspended sediment transport within and from the bay is possible. However,
Kudo et a1. (1980) pointed out that ship traffic in the bay could resuspend bed
sediments and thus lead to export of some bed sediments to the Yatsushiro Sea.
In addition, sediment within the intertidal zone may be expected to be vulnerable
to res us pension and transport due to wave action and ship wakes.
The Yatsushiro Sea is an inland sea facing the East China Sea with a surface
area of 63 600 ha, an average depth of 20 m, and a maximum depth of 47 m. The
sea holds about 13 km 3 of water, about 10% of which is exchanged daily with the
East China Sea by the tide. Kudo (1992) estimated that surface water velocities
caused by the tide were insufficient to transport bed sediments over most of the
A. Kudo and R.R. Turner
ination and its reclamation. Information on the latter subjects (e.g., Ishikawa and
Ikegaki 1980; Fujiki and Tajima 1992; Hosokawa 1993) has been slow to appear in
English language publications. The purpose of this chapter is to summarize some
important characteristics of the site, the nature and extent of remedial actions
undertaken, and the response of the system to those deliberate actions, and to
natural processes which can achieve similar endpoints. Appropriately, Minamata
Bay was the first mercury-contaminated ecosystem in the world in which a major
restoration has been attempted. Restorations of a similar kind and magnitude are
likely to be contemplated elsewhere and thus the experience at Minamata may
hold valuable lessons.
It was the authors' resolve that no major book on mercury-contaminated sites
should be published without a chapter covering Minamata Bay. As no papers were
presented at the 1996 Hamburg conference which might have served, the authors
elected to prepare this review chapter. Little is presented here which has not
already been published elsewhere in one form or another. The authors were able
to obtain recent data (through 1995) for mercury in fish which allowed extension
of a previously published trend chart. The authors have attempted to assemble in
one document as much information as possible concerning site characterization,
risk assessment, and remediation, and thus provide for easier comparisons with
other mercury-contaminated sites in the world. The authors offer a few opinions
and speculations for which they alone should be held accountable.
1.1
Hydrography of Minamata Bay and Adjacent Yatsushiro Sea
Minamata Bay is a small coastal inlet (about 300 hal of the Yatsushiro Sea (also
called the Shiranui Sea) on the southwestern coast of Kyushu, Japan (Fig. 1). The
only freshwater inflow is a small creek with a flow rate of 0.5 to 1 m 3 /s, most of
which is discharged from a chemical factory. The maximum tidal range is 3.4 m
with an average of 2.23 m, and thus tidal flushing of the bay is considerable. The
bay holds about 30 million m 3 of water and has a theoretical water retention time
of 2.5 days. The relatively steep topography of the lands surrounding the bay is
mirrored in the bay itself which reaches a maximum depth of 30 m and has an
average depth of 10 m. Sediments deposited in the bay have been described as
organic-rich silts and clays. Calculations of natural water velocities in the bay by
Kudo (1992) suggested that bed sediment transport should not be expected but
that suspended sediment transport within and from the bay is possible. However,
Kudo et a1. (1980) pointed out that ship traffic in the bay could resuspend bed
sediments and thus lead to export of some bed sediments to the Yatsushiro Sea.
In addition, sediment within the intertidal zone may be expected to be vulnerable
to res us pension and transport due to wave action and ship wakes.
The Yatsushiro Sea is an inland sea facing the East China Sea with a surface
area of 63 600 ha, an average depth of 20 m, and a maximum depth of 47 m. The
sea holds about 13 km 3 of water, about 10% of which is exchanged daily with the
East China Sea by the tide. Kudo (1992) estimated that surface water velocities
caused by the tide were insufficient to transport bed sediments over most of the