154
A. Kudo and R.R. Turner
an average thickness of approximately 50 cm. Hosokawa (1993) provided an
excellent summary of the details of the dredging operation as well as an outline of
the construction of the revetment cells used to surround the reclamation area.
The Japanese pioneered the use of a sand layer up to 1.5 m thick to stabilize and
compact sludge (Fujino 1977). The technique was used in Minamata Bay to
stabilize the bottom prior to placement of the revetments and to stabilize and
compact the dredge spoil in the reclaimed area (Hosokawa 1993).
4.3
Controlling/Monitoring Effects of Restoration
When the plans to restore Minamata Bay were first presented to the public in the
mid-1970S, there was great fear (and apparently some objection to the project)
that the planned dredging would exacerbate the continuing problem of
unacceptable mercury levels in some biota, result in a reappearance of the
severe poisonings of the past, and cause spreading of the problem into the
adjoining sea. As stated by Hosokawa (1993), "it was essential to get citizen's
support and trust for field works ... We had to use every possible technique in
order to minimize secondary pollution due to the project itself." Ishikawa and
Ikegaki (1980) first detailed some of the ameliorative strategies aimed at
controlling secondary pollution, but noted that the plan was under a provisional
injunction as of October 1979 from some Minamata residents who continued to
doubt its safety.
Some of the main .elements of the ameliorative strategy eventually followed
included: (1) use of boundary nets to exclude nonresident fishes from, and to
retain resident fish within, the bay during the dredging operation, (2)
construction of a temporary cofferdam across the northern opening into the
bay to reduce water circulation in the bay, (3) installation of an underwater sonic
fish-repelling device in front of the only opening in the boundary net which
permitted vessels to enter and exit the bay, (4) use of suction dredges without
cutter heads to minimize resuspension and loss of contaminated sediment at the
working face, (5) active treatment of effluent from the dredge spoil disposal area,
(6) establishment of rigorous performance standards for all operations which
could affect water quality in the bay and beyond. The last element included
continuous monitoring of water turbidity both at the suction head, which was
also fitted with precision depth sounders and video camera, and at the treated
effluent discharge point (Hosokawa 1993). Turbidity had been shown earlier
(Yoshida and Ikegaki 1977) to be a good surrogate for total mercury
concentration and thus continuous turbidity monitoring was a cost-effective
tool to assess, and thus to control, secondary mercury contamination.
An intensive fish-monitoring program was also undertaken in order to track
mercury contents of fish caught both at several locations within the bay and at
key locations outside the boundary nets where fish might be taken for public
consumption (Nakayama et al. 1992). The program included fish (porgy, Sparus
macrocephalux, and croaker, Umbrina) which were cultivated inside the bay and
monitored at lO-day intervals over the duration of the dredging work for total
A. Kudo and R.R. Turner
an average thickness of approximately 50 cm. Hosokawa (1993) provided an
excellent summary of the details of the dredging operation as well as an outline of
the construction of the revetment cells used to surround the reclamation area.
The Japanese pioneered the use of a sand layer up to 1.5 m thick to stabilize and
compact sludge (Fujino 1977). The technique was used in Minamata Bay to
stabilize the bottom prior to placement of the revetments and to stabilize and
compact the dredge spoil in the reclaimed area (Hosokawa 1993).
4.3
Controlling/Monitoring Effects of Restoration
When the plans to restore Minamata Bay were first presented to the public in the
mid-1970S, there was great fear (and apparently some objection to the project)
that the planned dredging would exacerbate the continuing problem of
unacceptable mercury levels in some biota, result in a reappearance of the
severe poisonings of the past, and cause spreading of the problem into the
adjoining sea. As stated by Hosokawa (1993), "it was essential to get citizen's
support and trust for field works ... We had to use every possible technique in
order to minimize secondary pollution due to the project itself." Ishikawa and
Ikegaki (1980) first detailed some of the ameliorative strategies aimed at
controlling secondary pollution, but noted that the plan was under a provisional
injunction as of October 1979 from some Minamata residents who continued to
doubt its safety.
Some of the main .elements of the ameliorative strategy eventually followed
included: (1) use of boundary nets to exclude nonresident fishes from, and to
retain resident fish within, the bay during the dredging operation, (2)
construction of a temporary cofferdam across the northern opening into the
bay to reduce water circulation in the bay, (3) installation of an underwater sonic
fish-repelling device in front of the only opening in the boundary net which
permitted vessels to enter and exit the bay, (4) use of suction dredges without
cutter heads to minimize resuspension and loss of contaminated sediment at the
working face, (5) active treatment of effluent from the dredge spoil disposal area,
(6) establishment of rigorous performance standards for all operations which
could affect water quality in the bay and beyond. The last element included
continuous monitoring of water turbidity both at the suction head, which was
also fitted with precision depth sounders and video camera, and at the treated
effluent discharge point (Hosokawa 1993). Turbidity had been shown earlier
(Yoshida and Ikegaki 1977) to be a good surrogate for total mercury
concentration and thus continuous turbidity monitoring was a cost-effective
tool to assess, and thus to control, secondary mercury contamination.
An intensive fish-monitoring program was also undertaken in order to track
mercury contents of fish caught both at several locations within the bay and at
key locations outside the boundary nets where fish might be taken for public
consumption (Nakayama et al. 1992). The program included fish (porgy, Sparus
macrocephalux, and croaker, Umbrina) which were cultivated inside the bay and
monitored at lO-day intervals over the duration of the dredging work for total