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Contamination - Environment - Saguenay : An exceptional fjord!

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The case of mercury | Aromatic hydrocarbons | Organochlorinated compounds | Beluga contamination

The case of mercury

Mercury is a typical pollutant in the Saguenay River and was the first problem reported. The first scientific reports between 1970 and 1975 were effectively very alarming. The famous Saguenay shrimp was contaminated 10 to 20 times higher than the Canadian limit. From then on, this shrimp was used to monitor the pollution level of the Fjord water. The mercury level found was 12 ppm (parts per million), however nobody reported this to the public. No other analyses were made on shellfish or fish.

Saguenay shrimp

When significant levels of mercury were found in the shrimp, scientists immediately analysed the sediments. Mercury was suspected because the Scandinavian shrimp (Pandalus borealis) is a species that feeds mainly on the seabed. This fact allows to conclude that the bioaccumulated mercury originated from the seabed. The scientists rapidly found high levels of mercury in the sediments of the Fjord’s deeper areas.

If mercury is introduced through effluents in an aqueous environment, it may be adsorbed on metal oxides, clays and other mineral particles and then slowly settle on the bottom. Two phenomena may then occur; remobilisation or burial. Remobilisation is a technical term in geochemistry that means that a pollutant, in this case mercury, may reappear and recirculate. If mercury is not adsorbed on oxide particles or clay, it can adhere to organic matter. These particles consist of living or dead bacteria, plankton and various debris from decomposing plants and organisms. When mercury is dispersed via organic products, a very particular biochemical phenomenon may occur. Bacteria can convert inorganic mercury into methylmercury. Although inorganic mercury has relatively low toxicity on most aquatic organisms and human beings, methylmercury however is extremely toxic. It can be bioaccumulated by the plankton at the beginning of the aquatic food chain. Methylmercury eventually settles to the bottom and is buried but can be released and return in the water. The mechanisms of reintroducing mercury in the inorganic and methylated states will continue although the pollution source no longer exists. Even though waste effluents have ceased, the mercury will recycle itself and natural elimination will be completed only decades later. The notion of elimination time linked with other metals and pollutants in the Fjord waters will be discussed later on.

Industrialization and urbanization are the main cause of the release of numerous other pollutants such as lead, zinc, copper and cadmium. These toxic metals are generally associated with industrial activity. Whether via heavy and light industry or motor vehicles, one inevitably finds all these metals in marine environments. They are also present in the Fjord but at a lower level than mercury. During the 70's, these metals were found in the sediments, but here again, no analyses were carried out on the organisms living in the Fjord.

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