Are Urban Effluents Responsible for Metal Contamination of the St. Lawrence River?

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Are Urban Effluents Responsible for Metal Contamination of the St. Lawrence River?

With the exception of silver, metals do not originate from municipal effluents.

A study was conducted in 2000 and 2001 by Environment Canada, in collaboration with Fisheries and Oceans Canada, to assess the contribution of urban effluents to the contamination of the St. Lawrence River by more than twenty different metals.

The table below presents the estimated inputs of metals in urban effluents. Although based on the analytical results of effluent samples from the Montreal wastewater treatment plant, these estimates represent the contribution of all urban effluents discharged into the river upstream of Quebec City.

Metal	Quantity in urban effluents (%)
Silver (Ag)	60
Bismuth (Bi) Cadmium (Cd) Copper (Cu) Molybdenum (Mo) Zinc (Zn)	8 to 13
Aluminum (Al) Arsenic (As) Barium (Ba) Cesium (Cs) Chromium (Cr) Cobalt (Co) Iron (Fe) Lead (Pb) Manganese (Mn) Nickel (Ni) Rhenium (Re) Rubidium (Rb) Strontium (Sr) Thorium (Th) Tungsten (W) Uranium (U) Vanadium (V) Zirconium (Zr)	< 3

Samples of Montreal urban effluent and river water at Lévis were drawn between July 2000 and July 2001. Sediments were also sampled from the shores of Île au Boeuf, downstream of Montreal.

Are Metals Toxic to Aquatic Biota?

The contribution of effluents to the metal contamination of the St. Lawrence River is relatively low, quantitatively speaking. But the toxicity to aquatic organisms of the metals released in various chemical forms to receiving waters is not well understood.

However, a team of research scientists at Environment Canada has found that the physical and chemical characteristics of receiving waters influence the bioavailability of metals near the effluent outfall, as well as their distribution within the dispersion plume. This could have a major impact on the exposure pathways of various contaminants to aquatic organisms.

The effluent of the City of Montreal undergoes primary treatment, which consists of screening out the larger solids and abrasive materials like sand and other heavy particles using grit chambers. Since 1988, a physical/chemical treatment has also been applied, which consists of adding a coagulant, such as ferric chloride or alum, to destabilize the fine particles. An anionic polymer is injected at the outlet of the grit chamber to agglomerate the destabilized particles into flocs, which settle rapidly in sedimentation basins.

Though this physical/chemical treatment reduces metal quantities in SPM, their sources must still be determined precisely to guide mitigation measures and to improve the quality of the aquatic environment.

Generally speaking, our results show that the contribution of effluents to the metal contamination of the St. Lawrence River is low, except in the case of silver, for which 60% of the load is associated with effluents.

Urban effluent discharges to the St. Lawrence River

Metals are transported in the dissolved form and in the particulate form associated with suspended matter in the water. Particulate metal concentrations are generally lower in urban effluents than in the St. Lawrence River. This was true for the majority of metals analysed in the particulate phase, except for copper, zinc, molybdenum, cadmium, bismuth and silver. We estimate that silver concentrations in suspended particulate matter (SPM) in Montreal effluent are 50 times higher than in SPM in the St. Lawrence River.

Concentrations of dissolved metals, however, are generally higher: chromium, copper, manganese, cobalt, nickel, zinc, molybdenum, silver, cadmium, lead and bismuth are all five to ten times higher in Montreal effluents than in the water of the St. Lawrence.

Metal sources are sometimes difficult to determine, given that metals are naturally present in all bodies of water. Because measured concentrations of dissolved metals in the St. Lawrence River do not exceed the quality criteria for the protection of aquatic life, they are thought to be sourced in tributary rivers and the eroding banks and bed of the river. Only the concentrations of certain metals in the particulate phase can be traced to anthropogenic inputs when compared to natural levels in the earth’s crust.

Indeed, Environment Canada scientists have found that the concentration of lead (Pb) in the St. Lawrence River is three times higher than in uncontaminated environments. They estimate that roughly 90 tonnes of the Pb transported annually in SPM in the St. Lawrence is of human origin. However, urban effluents contribute only 8 to 13% of the Pb detected in the river and are therefore not a significant source of this contaminant.

Present-day concentrations of chromium, nickel, copper, zinc and cadmium in SPM are also higher in the St. Lawrence than in preindustrial sediment. In other words, industrial wastewater discharges and other anthropogenic sources of metals add to the contamination of the St. Lawrence River.

These results contribute to the St. Lawrence water quality monitoring project, which works, among other things, to assess the environmental impacts of urban effluent discharges to the St. Lawrence River.

Literature

Cossa, D., T.-T. Pham, B. Rondeau, S. Proulx, C. Surette, and B. Quémerais. 1998. Tracking Contaminants in the St. Lawrence River: Summary of the Mass Balance Study of Contaminants in the St. Lawrence River. Environment Canada –Quebec Region, Environmental Conservation, St. Lawrence Centre.

Gagnon, C. and I. Saulnier. 2003. Distribution and fate of metals in the dispersion plume of a major municipal effluent. Environmental Pollution 124(1): 47–55.

Gobeil, C., B. Rondeau, and L. Beaudin. 2005. Contribution of municipal effluents to metal fluxes in the St. Lawrence River. Environmental Science & Technology 39(2): 456–464.

Rondeau, B. 2002. Water Quality in the Fluvial Section: Contamination by Toxic Substances. Fact sheet in the “Monitoring the State of the St. Lawrence” series. Environment Canada –Quebec Region, Environmental Conservation, St. Lawrence Centre.

Rondeau, B., D. Cossa, P. Gagnon, T.-T. Pham, and C. Surette. 2005. Hydrological and biogeochemical dynamics of the minor and trace elements in the St. Lawrence River. Applied Geochemistry 20(7): 1391–1408.