Frequently Asked Questions (FAQs)
Methylmercury and Hydropower
What is mercury?
Mercury is a naturally occurring chemical element that belongs to the category of heavy metals. It is the only metallic element that is a liquid at standard pressure and temperature. The most visible and common form is the heavy, odourless, silvery liquid found in thermometers and called elemental mercury. Once known as quicksilver, mercury (Hg) exists in various forms, organic and inorganic.
Under certain conditions, in the presence of microorganisms, mercury present in the environment can be transformed from an inorganic form to an organic form and vice versa. One of the organic forms, called methylmercury (MeHg), is easily absorbed by living organisms such as fish.
Where does mercury come from?
Elemental mercury occurs naturally in the crust of the earth. It is present throughout the environment.
How is mercury released into the environment?
Mercury can be distributed into the environment through natural processes or human activity. For example, volcanoes, forest fires and the natural weathering and deterioration of rock can release mercury. Mining, the burning of coal, and waste incineration are some of the man-made sources. Mercury released by natural or man-made sources naturally finds its way into vegetation, animals and humans.
What is methylmercury?
Methylmercury (MeHg) is an organic form of mercury. It is a compound made of mercury (Hg) carbon (C) and hydrogen (H).
How does methylmercury get into the food chain?
Methylmercury present in lakes and rivers is taken up by plankton and insects living on the bottom. These are then eaten by fish. Smaller fish are eaten by larger ones, which are eaten by predators (birds or mammals). Humans in turn eat the large fish and the animals that feed on fish. The mercury concentration increases along the food chain, from plankton to humans; the scientific term for this process is “bioaccumulation.” Consequently, non-predatory fish feeding on plankton and insects will have lower levels of methylmercury than predators that feed on other fish. The older and larger fish will have higher mercury levels than younger and smaller ones.
All species of fish from natural lakes or rivers contain some methylmercury even in the absence of human activity; the concentration of methyl mercury depends on where the fish live, their size and age.
In rivers, lakes and reservoirs, the concentration of mercury in free water is extremely low and exposure of local populations through the consumption of drinking water is not a concern: it is the consumption of certain species of fish and fish-eating animals that may need to be managed.
What happens in reservoirs?
In reservoirs, terrestrial matter is flooded. The mercury naturally present in the soil and vegetation becomes available to be transformed into methylmercury by the action of certain types of naturally occurring bacteria.
The flooding of forests and other vegetation results in a rapid decomposition of organic matter during the years immediately following the impoundment, boosting the processes that transform mercury into methylmercury and increasing bioaccumulation.
After reservoir impoundment, methylmercury levels increase in fish in the reservoir and also downstream of the dam. Mercury levels in fish reach levels that are generally between 2 and 8 times the levels found in fish in nearby, unaffected waterbodies. The increase is temporary. The methylmercury levels in fish typically peak 4 to 14 years after flooding and then diminish. They generally return to baseline levels after approximately 10 to 35 years. These results are based on measurements made in hydropower reservoirs over time around the world.
We are thus not faced with a new source of mercury, but rather with a redistribution to the aquatic environment of mercury that was already present on land.
Is methylmercury in hydro reservoirs a new issue?
No. The issue of methylmercury and hydroelectric development has been extensively studied for decades and is now well understood.
Is methylmercury a risk to human health for those living near hydroelectric reservoirs?
Proximity to a reservoir – on its own – does not raise a health concern. However, to avoid potential health concerns, exposure to methylmercury through diet should not be allowed to exceed certain levels. Government health agencies around the world establish target (safe) levels of methylmercury in food. They also determine maximum levels that are considered safe in human blood and hair, to identify individuals that may be at increased risk of experiencing mercury-related effects. In Canada, these target levels are established by Health Canada. These health agencies consider that when methylmercury levels are below these target levels, health effects are extremely unlikely and that benefits of eating locally-caught fish outweigh the extremely small risks that exposure to such levels may cause.
Reservoirs are treated on a case-by-case basis and consumption advisories are issued only if and when there are concerns related to methylmercury concentrations in fish flesh.
What is a ‘consumption advisory’?
A consumption advisory that is issued for a specific food item like fish means that the amount eaten and the consumption frequency should be reduced to avoid potential health risks. The advisory generally does not mean that the food item should not be eaten at all. Rather officials apply the precautionary principle and specify the amounts and frequencies that should not be exceeded. Consumption advisories may differentiate between segments of the population and apply only to some segments, for example, youth and woman of childbearing age.
The same document is also available here.
Can methylmercury bioaccumulation be avoided
All known measures that could potentially reduce methylmercury concentrations in reservoirs have been assessed by the hydropower industry. For example: intensive fishing, increasing the pH or stripping the soil. To date there is no realistic solution, for various reasons including potential harmful side effects and economic or technical impracticability for large-scale application.
More specifically, the effectiveness and acceptability of soil removal as a solution to avoid the release of methylmercury in the environment has been evaluated. Soil removal for a hydroelectric development is unprecedented and the benefits and potential negative impacts of completely clearing the reservoir of all trees, vegetation and organic soils and managing the excavated soil are not known and would likely cause other negative impacts to the surrounding environment and fish habitat.
It is not surprising that the bioaccumulation of mercury cannot be eliminated as it is a natural phenomenon that takes place in natural water bodies and is amplified whenever there is flooding for example where there are beaver dams.
What mitigation measures have hydropower projects developers put in place to manage the risks related to methylmercury?
To date, the only acceptable mitigation for the additional exposure to methyl mercury in humans due to temporary increases in methylmercury levels in fish living in reservoirs and fish-eating animals is the use of consumption advisories developed in collaboration with the public health agencies and used in combination with communication and information programs. These advisories are site specific. They are often combined with studies that measure concentration increases through time in order to assess the time and level of the concentration peak and, later, to monitor the decrease. This allows the monitoring and updating of consumption advisories.
The same document is also available here.