Release of Mercury at Minamata Japan

One of the earliest and a significant environmental pollution occurred in the 1950s at Minamata Bay in southwestern Kyushu, Japan. In 1956, the first Minamata disease patient was reported initially as suffering from nervous symptoms of an unknown cause in Minamata City, which is located along the Yatsushiro sea coast in Kumamoto Prefecture, Japan. Minamata disease resulted from the release of mercury from an acetaldehyde plant into Minamata Bay. In the plant, acetaldehyde had been synthesized by the hydration of acetylene, and mercury oxide dissolved in sulfuric acid was used as a catalyst.

50 000 _ 45 000 ? 40 000 ■5 35 000 | 30 000 a 25 000 ]| 20 000 | 15000 g 10000 A5 000

Acetaldehyde production Methylmercury emission

Acetaldehyde production Methylmercury emission

50 000 _ 45 000 ? 40 000 ■5 35 000 | 30 000 a 25 000 ]| 20 000 | 15000 g 10000 A5 000

Minamata Discharge Birds

1935 1940 1945 1950 1955 1960 1965 1970 Year

Figure 1 Transition of acetaldehyde production and the release of methylmercury to Minamata Bay from the factory. Data from Nishimura H and Okamoto T (2001) Science of Minamata Disease. Tokyo: Nippon Hyoron-sha (in Japanese).

40 I

1935 1940 1945 1950 1955 1960 1965 1970 Year

Figure 1 Transition of acetaldehyde production and the release of methylmercury to Minamata Bay from the factory. Data from Nishimura H and Okamoto T (2001) Science of Minamata Disease. Tokyo: Nippon Hyoron-sha (in Japanese).

mercury discharges into the bay has led to a reduction in contamination levels in some species.

Once mercury was discharged into the bay it became concentrated to high levels in fish and filter-feeding shellfish by several routes such as bioconcentration and food chain biomagnification. Even before Minamata disease was first recognized by the public in 1956, the local residents had noticed a number of surprising incidents happening in animals and fish. The 'cat dancing disease,' as it was known by the local residents, was particularly shocking. Around 1953, numerous cats, as well as some pigs and dogs, went mad and died. Moreover, fish were found floating on the sea surface and some birds such as the crow and grebe suddenly fell into the sea and drowned.

Specific and qualitative testimonies regarding some of the mercury poisoning observed by local fishermen were summarized in literature. Around 1950, flounder, octopus, and sea bass were floating on the sea surface in the bay, and decrepit fish were caught by hand, indicating severely diminished locomotor activity and escape behavior. Seaweeds began to turn white, and their biomass decreased. Between 1951 and 1952, a wide variety of fish such as sea bream, rockfish, and gilthead began to be found floating on the sea surface; no small prawns were caught, and seaweed production decreased to one-third of previous levels. Furthermore, clam and mate shell decreased in the bay. Between 1953 and 1954, benthic fish and organisms were also found floating on the sea surface and shellfish near the drain of the factory in the bay were killed. Moreover, between 1955 and 1957, the numbers of fish floating on the sea surface in the bay were maximized and began to spread out of the bay. Shellfish inhabiting the bay side of an island completely died.

That a serious disaster occurred in the fish is reflected in the statistics regarding fish catches from Minamata Bay. Fish catches from some fisheries cooperative associations in Minamata are presented in Table 1. As seen in the table, the number of fish catches between 1950 and 1956

drastically decreased. Total fish catches in 1956 had decreased to almost one-fifth of that in 1950-53. From 1950-53 to 1956, the catches of mullet, ribbonfish, octopus, squid, and crab had decreased to almost one-third, whereas those of anchovy, prawn, and oyster had decreased to one-fifth. The number of species living in or near sediment decreased the most, implying that the conditions of sediment in the bay had markedly deteriorated and led to significant ecological damage such as a decrease in biomass. The decrease in catches for migratory species such as anchovy, however, does not necessarily indicate a decrease in population caused by methylmercury. The population size of migratory fish varies according to the year. One reason for the drastic decrease in catches of anchovy, however, could be a reduction in the amount of zooplankton, which resulted from exposure to mercury in the bay.

Ecological damage such as the change in seaweed color and floating fish could be caused not only by methylmer-cury but also, to some extent, by hydrogen sulfide or an anoxic environment resulting from organics discharged from an acetyl cellulose factory located in the same area. An investigation conducted in 1957 showed that the organic matter content in sediment in the Minamata Bay area was very high and correlated well to sulfide. More than 1 mg g _ dry weight of sulfide concentration was recorded in sediment at 1500 m within the drainage of the factory, and 0.2-0.5 mggdry weight was observed at the center ofthe bay in 1957.

Examining the transition ofmercury residues in aquatic organisms from Minamata Bay could provide valuable information for understanding the significance ofthe pollution and its recovery trend. The concentrations of mercury in bivalve, Hormomya mutabilis, between 1959 and 1966 are shown in Figure 2. The highest mercury concentrations were observed in bivalve collected between 1958 and 1959, and the concentrations decreased to approximately 10 ppm after 1961. This decreasing

Table 1 Species-specific fish catches (tons) between 1950-53 and 1956 in Minamata

Avg. 1950-53

1954

1955

1956

Mullet

60

54.4

38

22.1

Anchovy

167.1

101.5

47

26

Konosirus pumctatus

31.7

6.8

6.1

1.2

Ribbonfish

51.9

29.7

24.5

20.2

Muraenesocidae

7.8

6.5

4.7

2.3

Octopus

14.6

9.1

7.6

4.4

Squid

12.3

9.4

5.6

3.9

Prawn

17.7

9.1

5.8

3.5

Crab

5.4

6.4

3.8

2.2

Oyster

10

7.4

5.4

1.6

Sea cucumber

10.3

8.6

6.1

2

Others

70.5

30.4

17.7

6

Total

459.3

279.3

172.3

95.4

Data from Nishimura H and Okamoto T (2001) Science of Minamata Disease. Tokyo: Nippon Hyoron-sha (in Japanese).

Data from Nishimura H and Okamoto T (2001) Science of Minamata Disease. Tokyo: Nippon Hyoron-sha (in Japanese).

120 100 80 60 40 20

1956 1958 1960 1962 1964 1966 1968 Year

Figure 2 The mercury content in bibalve, H. mutabilis, from Minamata Bay. Data from Nishimura H and Okamoto T (2001) Science of Minamata Disease. Tokyo: Nippon Hyoron-sha (in Japanese).

1956 1958 1960 1962 1964 1966 1968 Year

Figure 2 The mercury content in bibalve, H. mutabilis, from Minamata Bay. Data from Nishimura H and Okamoto T (2001) Science of Minamata Disease. Tokyo: Nippon Hyoron-sha (in Japanese).

trend of mercury levels in bivalve suggests that a drastic change in the water quality of the bay had occurred. In fact, the discharge of methylmercury decreased markedly during this period (see Figure 1). The sharp reduction in mercury levels observed in 1959 corresponded to the temporal stop of wastewater release from the plant to the bay between May and July of that year.

An investigation of the levels of localized mercury contamination in the adductor muscle of the mussel Mytilus galloprovincialis collected from four sites around Minamata City from 1993 to 1995 showed that the mercury concentrations were higher (0.026-0.121 ppm, n = 135) at sites near the main fallout of wastewater from the chemical plant in Minamata Bay than at sites 1-5 km from the former sites (0.006-0.028 ppm, n = 52).

The mercury concentrations in tissue of floating fish from the bay caught in 1959 ranged from 1.09 to 52.3 ppm, and the concentrations in the guts were significantly higher than those in muscle. The mercury concentrations of fish caught from the bay were 7-23 ppm in 1961, 0.4-5 ppm in 1963, 0.4-12 ppm in 1965, 0.2-0.4 ppm in 1966, and 0.1-1.3 ppm in 1970. The mercury concentrations in fish have decreased markedly since 1966, and the concentrations of benthic fish are relatively higher than those of surface fish. There are close correlations between the mercury contents of zooplankton and suspended particulate matter, and of sediments and fish muscle, suggesting a pathway from sediment to fish by way of suspended matter and zooplankton. The mercury concentrations declined in fishes inhabiting affected surface waters after industrial discharges of mercury were reduced, although concentrations in fish have remained unacceptably high in some areas. The mercury concentrations in fishes (87 species) from the bay ranged from 0.01 to 1.74 ppm, and 16 species of fish contained greater than 0.4 ppm of mercury in 1989. Monitoring of the mercury concentrations of fish and other marine organisms continues even today in the area around Minamata Bay.

Laboratory studies indicate that fish with residues in muscle of 5-8 ppm in walleyes and 10-20 ppm in salmo-nids are sublethally affected. Symptoms of long-term dietary exposure to methylmercury include incoordination, inability to feed, diminished responsiveness, and starvation. The mercury concentrations of many fish collected in the bay until the early 1960s were above or similar to the levels at which some fish exhibit symptoms of methylmercury intoxication, indicating that fishes in Minamata Bay were poisoned by methylmercury.

The total mercury content in the feathers of 95 stuffed, wild birds collected between 1955 and 1980 from the Minamata Bay area had been measured. The results indicate a strong relationship between the food of birds from Minamata and the mercury content in feathers; the content is highest in fish-eating seabirds and lowest in herbivorous waterfowl. A similar trend has been seen in birds collected from China and Korea, although the concentrations are significantly lower. Relatively high mercury levels were observed in the feather samples from Minamata till the late 1970s, although the discharge of wastewater from the factory stopped in 1968.

In order to deal with the massive mercury pollution in Minamata Bay, several measures have been taken. First of all, regarding the acetaldehyde plant, industrial wastewater containing methylmercury has not been released outside of the plant since 1966 due to the completion of a total circulation system, and the production of acetaldehyde was stopped in 1968.

Even after the release of methylmercury stopped, the bottom sediment in the bay contained considerable levels of mercury. Kumamoto Prefecture removed 1 500 000 m of bottom sediment that contained greater than the regulatory standard (25 ppm of total mercury) by means of dredging and landfill between 1974 and 1990. This project created 58 ha of landfill area at a total cost of approximately 48 billion yen, with the company being responsible for a burden of 30.5 billion yen.

The dredging changed the pattern of mercury movement in the surface sediments of Yatsushiro Sea, and the mercury concentrations in the sediment in the bay were reduced markedly after the dredging. In the postexamination to assess the effects of removal, no more than 20 ppm of mercury was observed from the collected sediment samples in Minamata Bay. One prediction suggests a complete restoration of the surrounding marine environment, Yatsushiro Sea, by the year 2011 if the decrease trend observed between 1985 and 1990 continues.

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