Water

Mercury

Mercury is a toxic element that enters the environment from natural processes (such as volcanic eruptions, vapor from oceans and the weathering of soils and rocks) and industrial activities (such as metal smelting, waste incineration, coal-fueled power plants).

Higher than normal levels of mercury can be found in fish inhabiting newly created hydro reservoirs. This mercury has a dual origin: part of it is released when soils and rocks, which naturally contain mercury, are flooded; the other part originates from the flooding of topsoil where mercury has accumulated from atmospheric emissions from industrial activities.

The creation of reservoirs frees the mercury trapped in soils and rocks. Bacteria then transform this relatively inert toxic mercury into a biologically more active form (methyl mercury), which penetrates the food chain and especially accumulates in the fatty tissue of predatory fish (such as northern pike, walleye, lake trout).

Well-established monitoring and mitigation procedures are in place to avoid health risks from mercury accumulation. Utilities and governments have carried out extensive research on mercury for many years. Recent studies have included monitoring of mercury content in fish in reservoirs, the impact of river flows on fish mercury levels and research on mitigation measures to reduce the mercury content of fish in new reservoirs. Industry and government studies suggest that the effect of flooding vegetation on mercury levels gradually decreases over time, with mercury levels returning to values similar to those measured in fish in surrounding natural lakes. This occurs after 10 to 30 years, depending on the fish species.

Source of image: Manitoba Hydro

Manitoba Hydro designed the Limestone generating station to limit flooding to three square kilometres, lowering habitat changes and mercury contamination.