Mercury
Your Health and the Environment

Mercury in the Environment

Mercury and its compounds can be toxic to living organisms at very low concentrations in aquatic and terrestrial ecosystems. Since mercury is an element, it cannot be created or destroyed and it persists in the environment. In addition, mercury can bioaccumulate in organisms. This ongoing accumulation in the body tissues of various species leads to biomagnification in the predator species as they consume organisms lower down in the food chain.

This section covers topics such as how mercury affects fish and wildlife and the concentrations of mercury present in various components of the environment such as air and freshwater.

Q47. How much does Canada contribute to worldwide mercury releases?
Total global mercury emissions from human sources are currently thought to be approximately 2400 tons per year. In 2000, the Canadian contribution to global releases was approximately 8 tons.

It is assumed that a large portion of the mercury present in the atmosphere today is a result of many years of releases stemming from anthropogenic activities. The natural component of the total atmospheric burden is difficult to estimate, but is probably in the order of 25% to 50%. Anthropogenic activities have thus increased the levels of mercury in background air by roughly a factor of 3. Current estimates indicate that total global mercury releases to the atmosphere are in the area of 5000 metric tons peryear, of which roughly 50% are of anthropogenic origin.

Q48. How are anthropogenic (human) mercury emissions transported through the atmosphere?

• Because mercury evaporates relatively easily, mercury emissions can be transported on wind currents, either as a vapour or bound to particles.
• Elemental or metallic mercury vapour can circulate in the atmosphere for a year or more and can travel long distances. In contrast, oxidized or reactive mercury has an atmospheric residence time of less than two weeks due to its solubility in water and its reactivity.
• As a result, mercury emissions from industrial point sources might remain localized in the environment, or might be transported regionally and even globally. In addition, mercury is thought to participate in a global distillation phenomenon that transfers mercury and other chemical emissions from equatorial, subtropical and temperate regions to the polar regions via the "grasshopper effect". When this phenomenon takes place, the emitted substance reenters the atmosphere by volatilizing after initial deposition and continues over time to "hop" through the environment until there is insufficient solar energy to revolatilize the substance. This favours contaminant accumulation in the colder polar regions.

Mercury from the atmosphere reaches land and surface waters either through wet deposition in precipitation or through dry deposition bound to particles, and can also enter water bodies as runoff from soil or through groundwater. In surface waters and sediments, biological processes can transform mercury into methyl mercury - a highly toxic form that can accumulate in living organisms and biomagnify up the food chain.

Q49. How is mercury transformed in the environment?

• Natural transformations and environmental pathways of mercury are very complex and are greatly affected by local conditions.
• There are two main types of chemical reactions in the mercury cycle that convert mercury through its various forms: oxidation-reduction and methylation-demethylation.
• In oxidation-reduction reactions, mercury is either oxidized to a higher valence state through the loss of electrons (e.g. from elemental mercury as Hg⁰ to the more reactive form as Hg²⁺) or mercury may be reduced to a lower valence state by gaining electrons.
• Mercury is transformed into methyl mercury when the oxidized, or mercuric species (Hg²⁺), gains a methyl group (CH₃). The methylation of Hg²⁺ is primarily a natural, biological process resulting in the production of highly toxic and bioaccumulative methyl mercury compounds that build up in living tissue and increase in concentration up the food chain.

Q50. What is polar sunrise depletion?
A Canadian atmospheric researcher, Dr. Bill Schroeder, has shown in recent studies the rapid oxidation of Hg⁰ vapour to Hg²⁺ in Arctic surface air during and after the polar sunrise at Alert, Nunavut Territory, and this has more recently been observed in Pt. Barrow, Alaska. This reaction is thought to occur photochemically (in the presence of sunlight) and in the presence of reactive chemicals released from sea salt (for example, bromine and chlorine ions). As a result, a pulse of reactive mercury enters the Arctic environment when the short-lived growing season is beginning. It remains a research question to determine what fraction of this reactive mercury is converted to toxic methyl mercury and taken up by animals and plants.

It is suggested that significant atmospheric mercury is being deposited to the snow pack during the period following polar sunrise. It has been found that, in the springtime, the accumulation rate of mercury in the snow is enhanced by a factor of four. Elevated concentrations of mercury have been found in snow melt-water collected near Arctic communities. Concentrations of mercury ranged from 2.1 nanograms per litre at Baker Lake to 237 nanograms per litre at Cambridge Bay. A nanogram is one-billionth (10^-9) of a gram. Future research will help to assess the environmental fate and impacts of this significant influx of mercury into the Arctic ecosystem.

Q51. How does mercury affect fish and wildlife?
Mercury is toxic, persistent, and can build up or bioaccumulate in living organisms, inflicting increasing levels of risk on higher order species (see Q.12). Although the long-term effects of mercury on whole ecosystems are unclear, the survival of affected populations and overall biodiversity could be at risk.

Fish:
Methyl mercury is bound tightly to fish protein when absorbed through the gills or when contaminated food sources are eaten. In some cases, methyl mercury levels in higher trophic level (food chain) fish species such as freshwater bass, walleye and pike, and marine shark and swordfish, can be up to a million times greater than in the surrounding water. In general, levels of mercury increase with fish size and age. Levels also vary by species and location. Bioaccumulation in fish is influenced by the amount of methyl mercury present, which is in turn affected by local biogeochemical processes.

Birds:
Piscivorous (fish eating) predators such as loons, merganser ducks, osprey, eagles, herons and kingfishers generally have high concentrations of mercury in their systems. Mercury has been detected in common loons from Alaska to Atlantic Canada, and blood concentrations have been correlated with levels in prey fish species. Research indicates that loon blood mercury concentrations increase from west to east across Canada and the United States, with the highest levels in Southeast Canada. High levels of mercury are associated with impaired loon reproductive success as well as with growth related problems. These problems can lead to an increased death rate and a decreased birth rate, resulting in a reduction in the abundance of natural populations.

Mammals:
Mercury has been found in predatory mammals such as otters from south central Ontario. It is thought that elevated mercury levels in otters may cause early mortality due to toxicity and behavioural changes. While the reproduction and behaviour of bird species is generally affected by exposure to methyl mercury, mammals most often suffer neurological effects. The severity of the toxic effects will depend on the degree of exposure and can range from a slight impairment to reproductive failure or death.

Q52. Is mercury present in the air and at what concentration?
In 1996 Environment Canada initiated the Canadian Atmospheric Mercury Measurement Network (CAMNet). Currently, there are stations in rurally representative locations across Canada measuring gaseous elemental mercury on a continuous basis. Typical ambient air concentrations at these sites range from 1.5 to 1.7 nanograms per cubic metre. The concentrations show a seasonal variation and, when air from industrialized regions arrives at a site, are correlated with other pollutants. Studies are also being undertaken in urban areas to further investigate the behaviour of mercury.

Q53. Is mercury present in dust & soil and at what concentration?
Concentrations of mercury in Canadian soils are in the range of 0.01 to 0.4 milligram per kilogram of dry weight, though higher levels tend to be reported in areas of ore deposits, spills, landfills and metal processing plants.

Q54. Is mercury present in precipitation and at what concentration?
Results at seven Canadian Atmospheric Mercury Measurement Network sites in 2001 showed an average concentration of 6.8 nanograms of mercury per litre of precipitation, with higher values recorded in the summer. Concentrations and deposition were highest in summer and lowest in winter. Deposition is a function of the concentration of mercury in precipitation and the amount of precipitation. The average annual deposition rate in 2001 for the seven sites noted above was 5.5 micrograms of mercury per square metre per year.

Q55. Is mercury present in freshwater and at what concentration?
Based on information largely from Ontario and Quebec, levels for methyl mercury are usually less than 1 nanogram per litre in natural surface waters, although concentrations up to 4.1 nanograms per litre have been reported. Methyl mercury concentrations are higher (0.6 nanogram per litre) in water draining from wetland areas relative to concentrations in water from other areas (0.03 nanogram per litre). Generally, methyl mercury accounts for less than 10 percent of total mercury found in surface water (see Q. 7).

Q56. Is mercury present in sediment and at what concentration?
Concentrations of mercury in freshwater and marine sediments vary considerably.

Background levels in lakes and rivers average 0.07 milligram per kilogram of dry weight. Mercury levels in contaminated lake sediments are as high as 15 milligrams per kilogram of dry weight, and as high as 25 milligrams per kilogram of dry weight for contaminated river sediments

In marine environments (coastal and estuary), background concentrations range from 0.010 to 0.521 milligram per kilogram of dry weight, while concentrations up to 23 milligrams per kilogram of dry weight have been measured in contaminated marine sediments.