Three Gorges Probe

Chapter 5 – Potential Methyl Mercury Contamination in the Three Gorges Reservoir

by Alan Penn, M.Sc.

Background on Methyl Mercury Contamination

The CYJV study indicates that inorganic mercury is present in the sediment, soil, and vegetation that would be subject to flooding by the Three Gorges Project. Canadian experience with the impoundment of rivers to create reservoirs for hydroelectric dams demonstrates that methyl mercury, a central nervous system toxin, is formed through bacterial synthesis in flooded soils and vegetation. The methyl mercury so produced is accumulated by fish, and is thus a potential health hazard for consumers of fish.

The process of methyl mercury production occurs naturally in lakes and rivers but certain new reservoirs in northern Manitoba and northwestern Quebec in Canada have resulted in a four to six-fold increase in methyl mercury concentrations in fish. In particular, methyl mercury contamination has become a significant issue at the La Grande hydroelectric complex (Phase I of the James Bay Project) which was built by Hydro-Québec in northwestern Quebec between 1970 and 1984, and will continue to play a role in the Quebec government’s plans to proceed with additional hydroelectric development in the same region.

Prior to hydroelectric development in these regions of Canada, methyl mercury contamination was a concern because of regionally elevated levels of mercury found in fish. In the 1970s, methyl mercury concentrations as high as 100 milligrams per kilogram (mg/kg) were found in the hair of some Cree Indian fishermen. The World Health Organization sets the tolerance limit for human exposure to methyl mercury at 6 mg/kg; therefore, the high levels found in the Crees have prompted questions about long-term toxicity and the possible effects of fetal exposure.

Since the construction of Phase I of the James Bay Project, limits have been placed on fish consumption in order to control exposure to methyl mercury. Recent surveys of adult Crees over 40 years old reveal that approximately 5 percent of this adult population have in excess of 25 mg/kg in their hair.

In the case of the Three Gorges Project, CYJV has identified methyl mercury contamination as a potentially major water quality impact and proposes a monitoring program for soil, vegetation, and fish – including more scientific studies, post-impoundment monitoring of fish, and restricting consumption should mercury levels in fish become a health risk.

CYJV suggests several conditions which would act to reduce the extent of methyl mercury contamination: high annual flows through the reservoir; low organic matter in soils and absence of peat in the reservoir region; removal of vegetation before flooding; and the water’s resistance to acidification. Factors which would act to increase the likelihood of contamination are cited as dissolved oxygen depletion in the reservoir, mercury present in soils and vegetation, and the transport of mercury with suspended sediment.

Key Criticisms of CYJV’s Discussion of Methyl Mercury

  • In the past two decades, methyl mercury contamination associated with North American water resource development projects has stimulated research on the factors which influence the rate of methyl mercury production and breakdown over flooded terrain, and its subsequent accumulation by fish. CYJV does little to assess the extent to which Canadian or United States research on methyl mercury in the aquatic environment could be used in the evaluation of the Three Gorges Project. Without further documentation on the nature of available research, the CYJV text on this subject must be considered inadequate, and potentially misleading.

  • In any event, applying experience from water resource development in Manitoba and Quebec to the very different geological setting of the Three Gorges Project has its own limitations. Even in the absence of flooding to create a reservoir, current understanding of geochemical factors which influence the bioavailability of inorganic mercury and production of methyl mercury is quite limited. These technical limitations which make impact assessment in this area rather uncertain, were not brought out by CYJV in its study.

  • The rate of accumulation of methyl mercury by fish ultimately depends on the feeding ecology (i.e., feeding patterns and habitat) of the fish species present and the nature of the food chain. There is little in the CYJV study to indicate how these biological factors would influence the nature of anticipated methyl mercury contamination in the case of the Three Gorges Project.

  • CYJV did not conduct a risk assessment for plausible scenarios of methyl mercury contamination in consumers of fish from the Three Gorges reservoir. The current health status of the population and the role of fish in the diet (actual or potential) are not addressed. Also, the role of fish as a subsistence food within the local economy is unclear. In these circumstances, it is especially difficult to assess the extent to which methyl mercury contamination should be considered a significant public health issue. There appears to be no fundamental reason that such a risk assessment could not have been undertaken.

  • CYJV’s treatment of methyl mercury as an environmental issue in the feasibility study, not unexpectedly, reflects Hydro-Québec’s approach to, and experience with, Phase I of the James Bay Project. Hydro-Québec’s guiding principle* seems to be that ecological impacts, by their very nature, are difficult to predict. Therefore, the argument runs, rather than rely on prior assessments, ecological and social consequences should be monitored as they unfold and responded to by the proponents, where necessary, with appropriate remedial measures. This leaves a great deal of discretion and control in the hands of proponents and places local people, who are directly affected by ecological impacts, in a disadvantaged position.

  • In this setting, this also means that it is difficult to deal directly with ecological concerns, such as methyl mercury contamination, in the initial project design. A more fundamental concern is that the overall approach to hydroelectric development reflects an underlying lack of conviction about the relevance of initial environmental impact assessments and about the value of acquiring practical experience through those assessments before decisions are made.



Sources and Further Commentary


*Hydro Quebec’s approach is set out in Section 8 of the James Bay and Northern Quebec Agreement, the aboriginal land claims settlement associated with the James Bay Project.

Categories: Three Gorges Probe

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