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Flood - Environment - Saguenay : An exceptional fjord!

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Flood pictureThe Saguenay Flood of July 1996 was of great interest for our oceanographers. Such a natural event occurs rarely and it revealed certain interesting elements of the Fjord. After a few hours of very intensive rain in middle of the vacation period, lakes swelled, rivers overflowed and dams and bridges were carried away. About 10 million tons of mud, gravel and various debris were washed away into Baie des Ha ! Ha ! and the north arm of the Fjord. The sand and gravel settleflood pictured quickly at the mouth of the rivers but the fine mud covered the polluted sediments over a vast area. A layer of sediments was formed by clay originating from wooded and agricultural areas where pollution was very low. These sediments were therefore free of mercury or industrial PAHs. In Baie des Ha ! Ha !, between 5 and 75 centimetres of new sediments settled over a few days. It is a rare historic event! For the benthic fauna living on the bottom and buried under several centimetres of these sediments, the torrent was a real disaster. Certain animals that could easily move, such as fish, escaped by fleeing the devastated area, while the other species that were unable to adapt or to move sufficiently rapidly, were buried.

sediments sampledCores of the sediments sampled in Baie des Ha ! Ha ! clearly show the boundary between the former surface layer and the new and sometimes thick layer, depending on where the samples were taken. The most important question is the following: Will the pollutants remain buried under the recent sediments or will they surface in the near future? This is an important subject for study that involves a vast environmental scope. Initially, the stability of the new layer must be determined.

The Baie des Ha ! Ha ! bottom is sloped a few degrees and this could cause underwater landslides. Our geologists are trying to determine if the sediment layer is sliding or if it is consolidating gradually. Data are still insufficient to allow drawing a conclusion.

The other crucial question is the following one: Will the uncontaminated sediments dilute the former contaminated sediments? This may occur through remobilisation where certain metals may migrate to the uncontaminated sediments by a series of chemical reactions. The behaviour of mercury in such circumstances was examined. The most recent data indicate that the mercury is stable and that it remains well buried at the bottom.

Flood pictureWhat about the colonisation process in the devastated zones? Organisms were destroyed but many species gradually recolonised the zones. The benthos biologists continually monitored these species and succeeded in understanding this true recolonisation of an entirely new zone. The process is rapid and soon the fauna will have claimed all its rights on the marine capital of the Saguenay Region.

The most important question remaining is the following one: Will the pollutant burying be permanent? Presently, the answer is affirmative: recolonisation is diversified and faster than foreseen. After four years of observation, it has been established that the mercury and the PAHs have not yet migrated to the new sediment layer. However, we must be cautious because the chemical and geochemical processes are very slow. The sediment layer gets more compact and the species organize and structure themselves. Normally, the new sediment layer should be able to support full growth of the benthic ecosystem over the next few years.

Source: Émilien Pelletier, professor for the Canada Marine Ecotoxicology Research chair, ISMER.

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