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Sediment Quality of the St. Lawrence River
According to studies by Environment Canada, surface sediments in the St. Lawrence River are generally less contaminated than they were 20 years ago as a result of effluent treatment measures. Yet, concentrations of some substances have remained unchanged or are even increasing.
With urbanization, the opening of the St. Lawrence Seaway in the late 1950s and the construction of hydroelectric dams, the St. Lawrence River has undergone significant industrial development. Industrial and municipal effluents discharged to the river have contaminated the sediments of this aquatic ecosystem with toxic substances such as metals and polychlorinated biphenyls (PCBs).
Sediment quality is assessed using criteria established for the protection of benthic organisms and the management of contaminated sediment (EC and MDDEP, 2007). The criteria define two levels of contamination: a threshold effects level (TEL) and a probable effects level (PEL).
- Concentration < TEL = effects seldom observed
- TEL < Concentration > PEL = effects occasionally observed
- Concentration > PEL = effects often observed
Mercury, which is used in many 20th-century industrial processes--is thought to be the dominant inorganic contaminant in sediment in the St. Lawrence River. Yet concentrations of this toxic substance have been on the decline for some 20 years, since the implementation of pollution-reduction plans for effluent discharges.
Industrial activities in the Cornwall-Massena region in the last century were a significant source of sediment contamination in Lake Saint-François. However, since 1989, mercury levels have fallen by 65% and 56% on the north and south shores, respectively. Although mercury contamination seems to be gradually declining, high concentrations (> PEL) are still present in the north, particularly around Hamilton Island.
Sediment Mercury Concentrations in Lake Saint-François
in 1989 and 2008
Concentrations of metals such as copper, zinc and lead have decreased by up to 50% since 1989, and PCB concentrations have decreased by approximately 95% since 1979. In 2008, the sediments of Lake Saint-François were not highly contaminated by metals, except in the northern upstream part of the lake, where mercury and zinc remain. With respect to PCBs, the southern part of the lake remains contaminated.
PCBs, Dioxins and Zinc in Surface Sediments
in Lake Saint-François in 2008
Other newly detected substances
Polybrominated diphenyl ethers, or PBDEs, are used as flame retardants in household products. Researchers have detected PBDEs in St. Lawrence sediments over the past few decades, and their concentrations are on the rise. Municipal effluents and atmospheric inputs are the main sources. The highest concentrations are found primarily in the upstream part of the lake and are believed to originate in the Great Lakes. Although their use in manufacturing is now prohibited by regulations, PBDEs are still present in many products that were manufactured before the regulations came into effect.
Dioxins (TCDDs) and furans (TCDFs) are organic chemicals produced by the heating of organic compounds, such as PCBs. The main sources are the incineration of industrial waste and pulp and paper manufacturing. The presence of dioxins and furans in the sediments of Lake Saint-François appears to be widespread, and scientists have detected these substances in all locations where analyses were conducted. The results show relatively high concentrations in the southern part of the lake and downstream from Cornwall.
Tributyltin (TBT) is used mainly as an antifouling agent in boat paints. Although this product has been regulated in Canada since 1989, it is still widely used in other countries. Harbour areas and marinas are generally contaminated with TBT. In Lake Saint-François, the highest concentrations are in the sediments of a marina located on the north shore at Bainsville, Ontario.
It is impossible to determine the impact of TBT and PBDEs on benthic organisms because, to date, no quality criteria have been officially adopted in Canada for these substances.
Located downstream from Lake Saint-François, Lake Saint-Louis has also seen a decline in mercury levels in surface sediments. Environment Canada scientists observed an average decline of 70% in mercury concentrations between 1985 and 2003. Levels are now below the most stringent quality criteria, except in one area in the south part of the lake.
Sediment Mercury Concentrations in Lake Saint-Louis
in 1985 and 2003
In 2003, in addition to mercury, sediments in Lake Saint-Louis contained polycyclic aromatic hydrocarbons (PAHs) and PCBs at concentrations exceeding the probable effects level (PEL). The south sector, i.e., the mouth of the Saint-Louis River, is the most heavily contaminated sector of the lake. The source of the contamination is likely a tributary, the Saint-Louis River, as well as industrial effluent discharged to the lake.
Concentrations of PCB, PAH and arsenic
in surface sediment in Lake Saint-Louis in 2003
Arsenic concentrations rising in the north
While most chemical contaminants appear to be declining over time, there are some that appear to be rising. They include arsenic, which has been detected in sediments in the north part of the lake at twice the concentration present in 1985. On the basis of their analyses, Environment Canada scientists suggest that the increase is associated with a natural, rather than anthropogenic, source. The presence of arsenic may be due to the geology of the area.
Chemicals to monitor
As in Lake Saint-François, PBDEs and TBT were first detected in Lake Saint-Louis sediments in 2003. On average, PBDE and TBT concentrations are lower than those measured in the Great Lakes and Lake Saint-Pierre, with the highest concentrations being observed in the north part of the lake and in the stretch of the Ottawa River west of Île Perrot. The highest TBT concentrations are in sediments in the Dorval marina and near the navigation channel, south of Île Perrot.
The chemical quality of surface sediments in Lake Saint-Pierre has been improving since 1976. Environment Canada scientists have observed a decline of over 90% in organic contaminant levels (PCBs) and over 50% in inorganic contaminant levels (Cu, Hg, Cd and Cr) in the north part of the lake. Mercury concentrations are now below the TEL for benthic organisms, except in Chenal aux Castors north of Dupas Island, where they are above the TEL. The highest chromium, copper and nickel concentrations are in the south part of the lake.
Mercury Concentrations in Surface Sediment Samples Collected Between 1976 and 2004 in Lake Saint-Pierre
© Michel Arseneau, Environment Canada
Average PCB concentrations had fallen by 90% in 2004, but remained high in the channels of the Sorel Delta and in the area north of the navigation channel, without exceeding the PEL. Heavy metal concentrations were close to natural levels, except for chromium and mercury, which exceeded the PEL at some locations.
Chromium, PBDE and Mercury Concentrations in Surface Sediments
in Lake Saint-Pierre in 2003
* The range of concentrations of PBDEs is not associated with levels of toxicity to benthic organisms. No quality criteria for PBDEs is in effect today in Canada.
What is Environment Canada doing?
Monitoring sediment quality in the St. Lawrence supports Environment Canada's mandate to protect and conserve aquatic ecosystems. Findings can be used to evaluate changes that have occurred in recent decades in the concentrations and spatial distribution of chemical contaminants in St. Lawrence surface sediments.
In the context of environmental action, the Îles de la Paix area is currently undergoing additional monitoring work to better identify the most contaminated sites and to find other potentially problematic contaminants. Analyzing emerging substances is a new challenge, but one that is important for the monitoring of sediment quality.
EC and MDDEP – Environment Canada and the Ministère du Développement Durable, de l’Environnement et des Parcs. 2007. Criteria for the Assessment of Sediment Quality in Quebec and Application Frameworks: Prevention, Dredging and Remediation. 41 pp.
Pelletier, M. 2010. Suivi de la qualité des sédiments au lac Saint-François. Science and Technical Report. Environment Canada, Science and Technology Branch, Water Quality Monitoring in Quebec.
Pelletier, M. 2009. Suivi de la qualité des sédiments au lac Saint-Louis. Science and Technical Report. Environment Canada, Science and Technology Branch, Water Quality Monitoring in Quebec.
Pelletier, M. 2008. Toxic Contamination in Sediments – Lake Saint-Louis: Where TwoRivers Meet. PDF fact sheet in the series "Monitoring the State of the St. Lawrence River" on the St. Lawrence Plan web site. Environment Canada, Science and Technology Branch, Water Quality Monitoring in Quebec and Ministère du Développement Durable, de l’Environnement et des Parcs du Québec.
Pelletier, M. 2005. Toxic Contamination in Sediments – Lake Saint-Pierre: Last Stop before the Estuary. Fact sheet in the series "Monitoring the State of the St. Lawrence River." Environment Canada, Quebec Region, and the Ministère du Développement Durable, de l’Environnement et des Parcs.
Pelletier, M. 2002. Toxic Contamination in Sediments – Lake Saint-François: A Century-Old Story. Fact sheet in the series "Monitoring the State of the St. Lawrence River." Environment Canada, Quebec Region, Environmental Conservation, St. Lawrence Centre.
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