Sediment quality in the fluvial section of the St. Lawrence, 2019
Status: Overall, sediment quality in the upstream portion of the fluvial section between 2004 and 2014 was good, but in the Contrecœur area it was considered poor (figure 1).
Temporal changes: The concentrations measured for the major substances associated with 20th-century industrial activity remained high in 2014 and exceeded the concentration thresholds, primarily in the Contrecœur area.
Figure 1. Sampling locations and sediment quality index in the sectors being monitored, 2004–2014
Figure 1 shows all the locations where samples were collected during this project, and the level of contamination at each of them. The samples were taken along the shores of the islands and the St. Lawrence. Locations included points north of the îles de Boucherville, south of île Sainte-Thérèse, and north of île Bouchard. Four samples were taken along the north shore of the St. Lawrence. About 20 were taken in the îles de Contrecœur, along the south shore of the St. Lawrence and in the channels between the islands.
The samples with the highest levels of contamination (shown in red) were those taken just downstream from the City of Montreal effluent outfall, along the south shore of île Sainte-Thérèse (2 samples), north of île Bouchard (1 sample), offshore from Lavaltrie (1 sample), and in the channels between the îles de Contrecœur (about 10 samples), particularly in the channel between île au Dragon and the south shore of the St. Lawrence.
The samples with lower levels of contamination (a total of 19, shown in yellow) were scattered throughout the section; they did not occur close together except to the south of Île Sainte-Thérèse and in the Contrecœur area.
The other samples collected (shown in green) were not contaminated. Most of them were located upstream from the City of Montreal effluent outfall, upstream from the îles de Contrecœur, and north of île Bouchard.
The two pie charts summarize the contamination in the fluvial section. The first chart presents the results for the fluvial section minus the Contrecœur area. It shows that about two thirds of the samples were not contaminated, one quarter were contaminated, and about 10% were very contaminated. The second pie chart shows the results for the Contrecœur area. About 30% of the samples were not contaminated, about 20% were contaminated, and 50% were very contaminated.
The fluvial section of the St. Lawrence is located between the cities of Montreal and Sorel and is dotted with numerous islands. The upstream portion of the fluvial section includes the confluence with the waters of the Ottawa River, which flow into the St. Lawrence via rivière des Milles Îles and rivière des Prairies.
The environmental issue of sediment quality is associated with discharges of wastewater from the City of Montreal and the municipalities in its northern and southern suburbs. In addition, the industrialized areas in the east end of Montreal and the Port of Montreal, just upstream, are sources of contamination from metals, polychlorinated biphenyls (PCBs), hydrocarbons and butyltins.
Îles de Contrecœur National Wildlife Area, located across from the municipality of Contrecœur, is made up of 22 islands surrounded by wetlands. This protected area is home to many species of birds, reptiles and mammals. However, it is vulnerable to contamination from industrial activities being carried out along the shores in this part of the St. Lawrence.
Only the portion between Montreal and Contrecœur was covered in this study; samples could not be taken between Contrecœur and Sorel because the currents were too strong.
Sediment quality criteria and indices
The sediment quality criteria used in this document (TEL: threshold effect level) are taken from EC and MDDEP (2007). They are defined on the basis of observed biological effects on benthic and pelagic organisms and contaminant concentrations measured in sediment.
The quality indices are calculated by dividing the measured concentration of each substance (mercury, metals, PCBs and PAHs) in each sample by the TEL quality criterion for the substance in question. An index higher than 1 indicates that the concentration exceeds the criterion and biological effects may be observed, whereas an index lower than 1 indicates that sediment quality is good. Mean quality indices were calculated for metals (except mercury) and PAHs.
The overall indicator status is based on the proportion of uncontaminated sites and contaminated sites relative to the total number of sites characterized. This proportion defines an overall status that ranges from good to poor.
Hydrology and sedimentology
The velocity of currents is relatively high, exceeding 1 m/sec in some places. The water in the St. Lawrence flows directly over postglacial clay deposits left by the Champlain Sea more than 8,000 years ago. However, fine sediments representative of the particles circulating in those waters have been deposited along the north and south shores of the St. Lawrence and the shores of various islands. Sediments are deposited only in certain places: small bays, areas covered with aquatic plants, or areas within navigation infrastructure that are sheltered from currents (figure 2).
Figure 2. Sediment substrate in the fluvial section
Figure 2 is a map of the sediment substrate in the fluvial section between Montreal and Contrecœur. The part of the riverbed shown in yellow represents postglacial clay, sometimes covered with sand. The pale orange along the north and south shores of the St. Lawrence and the shores of various islands indicates shoreline sediment covered with submerged vegetation. The green areas in the channels between the îles de Boucherville, the îles de Contrecœur and the other islands in the fluvial section indicate permanent sediment covered in submerged or emergent vegetation. The red areas at the eastern tip of each island or group of islands, and at the mouths of the rivière des Mille Îles and rivière des Prairies, indicate fluvial sediments deposited on sand spits. Lastly, the pink area just north of the îles de Contrecœur indicates shoreline sediments covered with emergent vegetation.
Sediment quality status
Sediments in the fluvial section are composed mostly of sand, with a bit of silt and clay. That composition, which is low in fine particulates, does not tend to accumulate contaminants. For that reason, sediment quality in the fluvial section was considered good when the samples from the Contrecœur area were excluded from the calculations (table 1).
Concentrations of metals, mercury and PAHs were below the TEL quality criterion. However, concentrations of chrome, copper and zinc exceeded the TEL by more than 30%, reaching maximum values of 165 µg/g, 118 µg/g and 913 µg/g, respectively. The highest concentrations of metals and mercury were found along the northwest shores of île Bayol, île Chalut and île Bouchard and in the channels of the îles de Contrecœur (figures 3 and 4).
Among the 13 PAHs measured in this study, between 20% and 40% of the concentrations exceeded the TEL. Anthracene, benz(a)anthracene, benzo(a)pyrene and phenanthrene showed the highest concentrations. PAHs were found downstream from the City of Montreal effluent outfall on the south shore of île Sainte-Thérèse, and around the îles de Boucherville and the îles de Contrecœur (figure 5).
A little over half of the samples (53%) exceeded the TEL for PCBs. The highest concentrations were distributed throughout the fluvial section (figure 6).
Lastly, PBDEs were mainly located downstream from the City of Montreal effluent outfall (figure 7) – a predictable result, since they are present in municipal wastewater.
Table 1. Median and maximum concentrations of substances analyzed in sediments for Lake Saint-Louis, the fluvial section, Lake Saint-Pierre and the Contrecœur area
TEL quality criterion
Fluvial sector, excluding Contrecoeur
* The analysis results from Lake Saint-Louis and Lake Saint-Pierre will be published at a later date.
Spatial analysis of contamination
Freshets in the spring and high water levels in the fall cause sediment resuspension, consequently, it is not possible to draw a reliable picture of the evolution of the contamination because there is too little permanent accumulation. However, it is possible to compare the median and maximum concentrations in the fluvial section with those of sediment samples taken upstream in the northern part of Lake Saint-Louis and downstream in Lake Saint-Pierre. Those comparisons can be used to assess the impact of the sources of contamination in the fluvial section.
Figure 3. Mercury concentrations in sediments in the fluvial section of the St. Lawrence, 2004–2014
Figure 3 shows the mercury concentrations found in this part of the fluvial section of the St. Lawrence. The most contaminated samples (0.49 microgram per gram or higher) were taken from the channel between île au Dragon and the south shore of the St. Lawrence in the Contrecœur area (3 samples) and off the south shore of île Sainte-Thérèse, downstream from the Montreal effluent outfall (one sample). The rest of the samples showed little or no contamination, except for two on the north side of île Bouchard and one on the south side of the îles de Boucherville which exceeded the TEL of 0.17 microgram per gram of dry sediment.
Figure 4. Metals index for sediments in the fluvial section of the St. Lawrence, 2004–2014
Figure 4 shows the contamination index for metals, calculated for the fluvial section of the St. Lawrence. The samples with the highest indices (more than 3.2 times the TEL criterion), were all taken in the Contrecœur area between île au Dragon and the south shore of the St. Lawrence, except for one taken upstream of those islands. The other samples with a high contamination index (i.e., greater than 1) were mainly from locations in the channels of the îles de Contrecœur. There were also four points north of île Bouchard and two upstream from the City of Montreal effluent outfall. The other samples had index values of less than 1, indicating little or no contamination.
Sediments in the fluvial section, other than those in the Contrecœur area, are depleted of organic matter and fine particulates. That limits adsorption of contaminants by particles, despite significant inputs from upstream sources such as the City of Montreal effluent outfall and the Port of Montreal.
Contamination from metals, which is primarily associated with mineral matter, tended to decrease from upstream (Lake Saint-Louis) to downstream (Lake Saint-Pierre). However, it was not possible to identify any such trend for contaminants associated with organic matter, such as mercury, PCBs, PAHs and PBDEs (table 1).
The maximum concentrations were higher in the fluvial section than in the lakes for copper, lead, zinc, PCBs, total PBDEs and the majority of PAHs, including benzo(a)pyrene. Those concentrations, which were sometimes extreme, are evidence of the high inputs of contaminants (table 1). The higher concentrations of chrome may be due to the fact that the postglacial clay covering the riverbed in the fluvial section is enriched in this metal.
Figure 5. PAH index for sediments in the fluvial section of the St. Lawrence, 2004–2014
Figure 5 shows the PAH contamination index. None of the samples had an index greater than 15.27 times the TEL criterion. However, in all the samples from île au Dragon and the south shore of the St. Lawrence in the Contrecœur area, the index was greater than 1. There were also values greater than 1 for two samples taken upstream from the îles de Contrecœur, three taken downstream from the City of Montreal effluent outfall and the south side of Île Sainte-Thérèse, and two from the îles de Boucherville. All the other samples had indices of less than 1, indicating little or no PAH contamination.
The contamination in the Contrecœur area was located in a channel between the south shore and the îles de Contrecœur. The sediment there was contaminated by mercury and heavy metals such as chrome, lead and zinc. Mercury levels were 3 to 4 times higher than the threshold concentration that has effects on aquatic organisms (TEL criterion). The highest concentrations of chrome and lead were 6 times higher than their TEL, while concentrations of zinc were as high as 2,180 µg/g, more than 18 times higher than its criterion (table 1).
In addition, very high concentrations of butyltins exceeding 2,000 ng Sn/g were found at the outlet of the île au Dragon channel. As yet, no source has been identified for these contaminants, which have historically been used as biocides in paint for boat and ship hulls (Pelletier et al., 2014). Further studies have been conducted and will be published at a later date.
Figure 6. PCB concentrations in sediments in the fluvial section of the St. Lawrence, 2004–2014
Figure 6 shows the PCB concentrations measured in fluvial section sediments. Only one sample had a concentration greater than 280 nanograms per gram; it was taken north of île Chalut, upstream from île Bouchard. The other high concentrations (above the TEL criterion for PCBs) were distributed throughout the entire study area. The only locations where the concentrations did not exceed the TEL were the upstream portion of île Sainte-Thérèse (upstream from the City of Montreal effluent outfall) and the upstream portion of the îles de Contrecœur. This map shows that the majority (53%) of the samples were contaminated at levels that exceeded the TEL of 34 nanograms per gram for PCBs.
Figure 7. BDE 99 concentrations in sediments in the fluvial section of the St. Lawrence, 2004–2014
Figure 7 shows concentrations of BDE 99, which, of all the PBDEs, is considered the most harmful to aquatic wildlife. Among the 29 samples analyzed, 11 exceeded the TEL criterion of 0.4 nanogram per gram. The highest concentrations were found in the Contrecœur area, between île au Dragon and the south shore of the St. Lawrence. The other samples with high concentrations came from downstream of the City of Montreal effluent outfall: along île Sainte-Thérèse and along île Bouchard, across from Lavaltrie.
Currently, sediment quality is monitored by sampling surface sediments and using the samples to characterize annual inputs of contaminant-bearing particles. The results of this monitoring are used to produce an up-to-date picture of contaminant concentrations in the aquatic environment that could affect the habitats of many benthic organisms. However, there are not many locations in the fluvial section where sediments are deposited over time. For the past few years, particles have been captured by sediment traps placed on the riverbed, and this method has proven effective for studying emerging substances of interest such as PBDEs, siloxanes and a number of others directly associated with municipal wastewater (Armellin et al., 2018; Isabel et al., in writing). Therefore, it is important to continue monitoring and surveillance of these substances using appropriate techniques to measure their impacts on the aquatic environment.
EC and MDDEP, 2007. Criteria for the Assessment of Sediment Quality in Quebec and Application Frameworks: Prevention, Dredging and Remediation. Environment Canada and Ministère du Développement durable, de l’Environnement et des Parcs du Québec, 39 pages.
Pelletier M., M. Desrosiers, S. Lepage and Y. de Lafontaine, 2014. Butyltins in Sediments of the St. Lawrence River. State of the St. Lawrence fact sheet, Environment Canada. ISBN: 978-0- 660-21501-3, 7 pages.
Armellin A., M. Pelletier, L. Champoux, M. Desrosiers et Y. Paradis, 2018. PBDE, contaminants of emerging concern accumulating in St. Lawrence food web. ISBN: En14-322/2018F, 10 p.
Isabel L., M. Pelletier, A. Armellin, D. McGoldrick, P. Martin, S. Moore, and S. Backus. Influence of waste water effluents on the bioaccumulation and biomagnification of methyl-siloxanes in the St. Lawrence River. (Scientific article, in writing).
Fresh Water Quality Monitoring and Surveillance
Environment and Climate Change Canada
My thanks to Michel Arseneau, Germain Brault, Claude Lessard and Simon Blais of Environment and Climate Change Canada for their contributions to the fieldwork, and to Mélanie Desrosiers of the Centre d’expertise en analyse environnementale of the Ministère de l’Environnement et de la lutte contre les changements climatiques du Québec. Thanks also to Simon Blais for the financial contribution from the Environmental Protection Operations Directorate under the St. Lawrence Action Plan. Lastly, special thanks to Nathalie Gratton of Environment and Climate Change Canada for her contribution to producing this fact sheet.