Mercury Contamination of Minamata Bay
147
controversy continues even today (Nakanishi 1992) as to what fraction of the
mercury in biota in the bay originated from direct discharges of methylmercury
from the process and what fraction was produced in the bay via biomethylation.
The latter mechanism was unknown and unsuspected prior to 1968 (Jensen and
Jernolov 1969). Unquestionably, methylmercury was shown (see review by Fijiki
and Tajima 1973) to be an unintentional byproduct of acetaldehyde production,
but analytical chemical technology available in the late 1950S and early 1960s
precluded identification and quantitation of methylmercury in the wastewater
effluent. Irukayama (1966) was the first to positively identify methylmercury in
sludge samples from the Chis so plant. In addition, Fijiki and Tajima (1992) note
that wastewater from the vinyl chloride production process in May 1968
contained 40 ~lg/l total mercury and 0.3 ~lg/l methylmercury (0.75% methyl).
Research conducted by these same authors in the 1960s and early 1970S did not
support the hypothesis that inorganic mercury was methylated in bay sediments.
Instead, their results pointed to direct discharges as the origin of methylmercury
in fish and shellfish.
As is so well known today, exceedingly small quantities of methylmercury can
contaminate biota to levels unsafe for consumption by humans and wildlife. If
one assumes that only 0.1% of the total mercury released from the chisso plant
was methylmercury, this quantity represents an enormous mass of methylmercury with the potential to contaminate easily all the fish and shellfish in the bay to
levels far exceeding those observed during the height of the epidemic. As
discussed subsequently, the rapid rate at which the mercury content of fish and
shellfish in the bay decreased after action was taken to halt further direct
discharges suggests that direct discharges of methylmercury may indeed have
been the most important aspect of the episode. Biomethylation of inorganic
mercury in the bay, while unimportant during the height of the epidemic, most
likely increased in importance after direct discharges of mercury were halted.
2
Distribution of Mercury in Sediments
The distribution of mercury in bay sediments began to be studied in detail in
1963 (see Tables IV and V in Fujiki and Tajima 1992), with studies continuing
well into the early 1970S. Information summarized from other sources by Kudo
(1992) indicates that the first measurements of mercury in sediment took place in
1960. One of these early samples was from a creek near the Chis so plant and
contained over 2010 mg/kg total mercury on a "wet weight" basis. Other samples
collected at the same time indicated decreasing concentrations with distance
from the mouth of the creek out into the bay. For example, total mercury (wet
basis) decreased in the sequence 133, 59, 40, 19, and 12 mg/kg at 0.4, I, 2·5, 3,
3.5 km, respectively, from the mouth of the creek (see Fig. 14.10 in Takizawa
1979). Extensive measurements (342 locations) of mercury in bay sediments were
undertaken in 1973 by the Kumamoto Prefecture and resulted in the first detailed
147
controversy continues even today (Nakanishi 1992) as to what fraction of the
mercury in biota in the bay originated from direct discharges of methylmercury
from the process and what fraction was produced in the bay via biomethylation.
The latter mechanism was unknown and unsuspected prior to 1968 (Jensen and
Jernolov 1969). Unquestionably, methylmercury was shown (see review by Fijiki
and Tajima 1973) to be an unintentional byproduct of acetaldehyde production,
but analytical chemical technology available in the late 1950S and early 1960s
precluded identification and quantitation of methylmercury in the wastewater
effluent. Irukayama (1966) was the first to positively identify methylmercury in
sludge samples from the Chis so plant. In addition, Fijiki and Tajima (1992) note
that wastewater from the vinyl chloride production process in May 1968
contained 40 ~lg/l total mercury and 0.3 ~lg/l methylmercury (0.75% methyl).
Research conducted by these same authors in the 1960s and early 1970S did not
support the hypothesis that inorganic mercury was methylated in bay sediments.
Instead, their results pointed to direct discharges as the origin of methylmercury
in fish and shellfish.
As is so well known today, exceedingly small quantities of methylmercury can
contaminate biota to levels unsafe for consumption by humans and wildlife. If
one assumes that only 0.1% of the total mercury released from the chisso plant
was methylmercury, this quantity represents an enormous mass of methylmercury with the potential to contaminate easily all the fish and shellfish in the bay to
levels far exceeding those observed during the height of the epidemic. As
discussed subsequently, the rapid rate at which the mercury content of fish and
shellfish in the bay decreased after action was taken to halt further direct
discharges suggests that direct discharges of methylmercury may indeed have
been the most important aspect of the episode. Biomethylation of inorganic
mercury in the bay, while unimportant during the height of the epidemic, most
likely increased in importance after direct discharges of mercury were halted.
2
Distribution of Mercury in Sediments
The distribution of mercury in bay sediments began to be studied in detail in
1963 (see Tables IV and V in Fujiki and Tajima 1992), with studies continuing
well into the early 1970S. Information summarized from other sources by Kudo
(1992) indicates that the first measurements of mercury in sediment took place in
1960. One of these early samples was from a creek near the Chis so plant and
contained over 2010 mg/kg total mercury on a "wet weight" basis. Other samples
collected at the same time indicated decreasing concentrations with distance
from the mouth of the creek out into the bay. For example, total mercury (wet
basis) decreased in the sequence 133, 59, 40, 19, and 12 mg/kg at 0.4, I, 2·5, 3,
3.5 km, respectively, from the mouth of the creek (see Fig. 14.10 in Takizawa
1979). Extensive measurements (342 locations) of mercury in bay sediments were
undertaken in 1973 by the Kumamoto Prefecture and resulted in the first detailed