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Sediments
Mercury Concentrations in Sediments in the Great Lakes and the St. Lawrence River
Background
The Great Lakes–St. Lawrence Basin covers a total surface area of 1 235 000 km2 and can be divided into three sections: the Great Lakes, the St. Lawrence River, and the marine area, which includes the Estuary and the Gulf of St. Lawrence. This system is one of the main arteries into the North American heartland, supplying a population of several tens of millions of people with goods and services and providing industry with access to the sea.
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| Magella Pelletier, St. Lawrence Centre Lake Ontario, near the Niagara River |
Geomorphology of lakes in the Great Lakes–St.Lawrence system |
Lake |
Length (km) |
Width (km) |
Area (km2) |
Average depth (m) |
Volume (km3) |
| Superior | 563 |
257 |
144 691 |
147 |
12 100 |
| Michigan | 494 |
190 |
93 860 |
85 |
4 920 |
| Huron | 332 |
245 |
81 340 |
59 |
3 540 |
| St. Clair | 42 |
39 |
1 600 |
3 |
4.2 |
| Erie | 388 |
92 |
35 696 |
19 |
484 |
| Ontario | 311 |
85 |
26 435 |
86 |
1 640 |
| Saint-François | 50 |
4.7 |
235 |
6 |
2.8 |
| Saint-Louis | 23 |
6.5 |
148 |
3 |
0.5 |
| Saint-Pierre | 35 |
15 |
500 |
3 |
1.5 |
Over the past century, discharges of liquid and solid waste from industrial, agricultural and domestic sources have introduced a multitude of toxic substances into the waters of the Great Lakes and the St. Lawrence. Many of these contaminants bonded to the particles suspended in water, only to be deposited on the water bottom based on their size and the speed of the current. The accumulation of these particles in successive layers constitutes a veritable historical record of the natural and anthropogenic events that have shaped the environment in this watershed. It is becoming vital that we understand and are able to assess the extent of the contamination in these sediments — all the more so since the diets of many organisms depend on the energy they contain.
Mercury (Hg) is recognized as one of the most widespread toxic contaminants of industrial source and is found in high concentrations in the environment. It is still a threat to the health of aquatic organisms today (see sidebar). This is the reason that mercury, by means of sediment quality assessment criteria, serves as an indicator of the quality of the environment (see boxed text). This fact sheet offers a synthesis of data acquired between 1968 and 2004 on the contamination of sediment by mercury and presents an updated snapshot of sediment quality over the past few decades.
Sediment Quality Assessment Criteria |
The chemical quality of sediment is generally assessed by means of criteria relative to the degree of toxicity to benthic organisms. Two levels are generally employed: a base concentration that produces an effect in organisms (the Threshold Effect Level, TEL), and a concentration that produces a probable effect in organisms (the Probable Effect Level, PEL). These concentrations were established by the Canadian Council of Ministers of the Environment (CCME) and are routinely used by the different stakeholders involved in sediment management activities. |
Mercury |
Mercury is a chemical element that is present in some minerals in the earth’s crust. The natural quantities of mercury released to the environment are fairly small, however, and do not constitute a genuine threat to benthic organisms or humans. Unfortunately, the industrial activities of the past century have contaminated the environment — our air, water, soil and sediment. Mercury is derived mainly from coal combustion, paint manufacturing, the chlor-alkali industry and pesticide use in agriculture. It has also figured in the manufacture of pulp and paper, thermometers, electrical equipment, dental amalgams and some pharmaceutical products. Despite the tremendous advances that have been made in reducing emissions of mercury, it continues to escape from coal-fired power plants, incinerators, cement plants and steel mills. When present in large concentrations in the environment, mercury is particularly harmful to aquatic organisms and human beings. It is especially damaging in the form of methylmercury, which attacks the nervous system. Mercury is persistent, breaking down very slowly in the environment. Metallic mercury can be transformed by the bacteria present at the bottom of lakes and rivers and in the sludges generated by wastewater treatment plants into a more toxic form such as methylmercury (CH3Hg+). This substance can accumulate in the tissues of organisms and cause irreparable damage. Because of its high degree of toxicity and the deleterious effects on biota, mercury has been classified as a Priority Substance by Environment Canada. |
Date modified: 2008/05/01 – Important Notices
