Mercury is found everywhere in the environment–in air, water, soil, plants, animals and fish. There are natural (e.g. volcanoes) and human (e.g. burning fossil fuels) sources of inorganic mercury contributing to the global mercury cycle.
Methylmercury, an organic form of mercury, is created when microorganisms that live in aquatic environments react with the inorganic mercury that exists naturally in soil and plants. Once formed, methylmercury enters the food chain.
The process of converting inorganic mercury to methylmercury temporarily speeds up when a new reservoir is created due to the rapid decomposition of soil and vegetation previously on dry land.
Methylmercury levels rise up the food chain
When we begin filling the Site C reservoir in 2023, there will be temporary changes in fish methylmercury levels. We predict fish methylmercury levels to increase by an average of three to four times the baseline level in the newly created reservoir, and slowly return to a new baseline after approximately 20 to 30 years.
Based on Health Canada's guidelines and BC Hydro's predictions of fish methylmercury levels, people who eat two or fewer servings of fish a month, on average, from the Site C reservoir do not need to be concerned about the amount of methylmercury in the fish. However, people who eat more than the recommended number of servings of fish from the Site C reservoir, or eat more than one kind of fish, may need to limit their consumption of large predatory fish, such as bull trout and lake trout, to stay within Health Canada's safe level of methylmercury.
At their highest, levels of methylmercury in fish from Site C are expected to be similar to levels in fish found in many lakes and rivers elsewhere in Canada and lower than some types of fish sold in stores and restaurants. In general, predatory fish (fish that eat smaller fish) are more likely to have higher levels of methylmercury due to accumulation through the food chain.
The relatively low methylmercury levels expected in Site C reservoir fish are due to the low pre-existing levels of mercury and to the project's design. Because Site C will rely on the existing Williston Reservoir for most of its water storage, the extent of flooded land – which drives methylmercury production – at Site C is small compared to many reservoirs.
BC Hydro is working with Indigenous groups, communities and health authorities to develop a methylmercury monitoring plan for Site C to verify predicted levels of mercury in fish. The plan will include regularly measuring methylmercury levels in local fish and collecting information on how much fish people are eating. This information will be communicated, in partnership with health authorities, to Indigenous groups and the general public.