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Cleaning up the river

The Little Saint-Pierre River, an ecosystem in a deplorable condition.

Following a biological analysis of the site, we learned that the flat surface and meager vegetation of the wet areas do not hold run-off water efficiently. Run-off from the site and the adjacent residential neighborhoods spills into the ditches that cross the site and then into the Saint-Pierre collector – a storm sewer which empties into the Lachine Canal. The ecosystem also suffers badly from the poor quality of the water and the absence of habitat in the river bed and vegetation on the banks. The Saint-Pierre River’s substratum is essentially mud, and its bed is extremely unstable. The extreme variations in its flow and levels are not winning natural conditions for this ecosystem. In addition, samples taken from water table near the Little Saint-Pierre River have heavy concentrations of fecal coliforms, zinc, phosphorous and copper. This indicates that there are major sources of pollution upstream. The Réseau de suivi du milieu aquatique (RSMA) (Water Monitoring Network), confirms that the “ most unfavourable conditions” are to be found in the golf course ditch, which it describes as an unacceptable open sewer.

Re-establishing the Petit Saint-Pierre River’s flood plain might be the principal step to be taken to stimulate the biodiversity of the site.

 
The Little Saint-Pierre River today
 

It is important to add that the results of the analyses of the Little Saint-Pierre River’s water quality raise concerns about the capacity of this watercourse to sustain life in its current state as an open almost-sewer. The plan envisions bringing the Little Saint-Pierre River back to life by growing the river’s natural habitat, increasing the amount of shade, improving water quality and controlling the water flow and flood level. The restoration projects will be multi-stage and will begin with cleaning up the river to the point where it can be home to wildlife and welcome recreational activities. Following that, we would use sumps and diversions to create artificial wetlands to filter the polluted water. These wetlands could become the main factor in stimulating biodiversity regeneration in the eastern sector of the site.

  Wetlands restored to health: habitat and natural filters
 

  Photo : © Assassi | Courtesy of BNIM
 

The complementary efficiency of a constructed wetland

With a view to serving both regenerating wetlands and residential areas, the site will have constructed wetlands to purify water in a targeted fashion within a full water retention and treatment system. Surface permeability will be maximized to reduce the run-off created by storms and constructed wetlands will absorb the excess water.

Constructed wetlands are shallow ponds with impermeable bottoms and are covered with vegetation. This vegetation cleans the water by a natural process that combines sedimentation, aeration, predation and assimilation by plants. Filtering marshes are among the most efficient ways in existence for treating water and their performance even exceeds the criteria governing municipal treatment systems.

The collected rainwater will be treated in several stages and reused to meet needs (toilets and irrigation) and to keep the water circulating in the same pools to maintain a consistent level during dry spells or to reduce icing-over in winter. In the final development stage, the water will be sent to a “snow-melt” area consisting of wetlands set up to treat the water from the spring run-off. The snow-melt will pass through sedimentation trenches before spreading through the marsh with its halophytes - plants that thrive in saline soils. The improvement of the quality of the snow-melt before it goes into the city’s sewers will reduce the quantity of pollutants that flood into the city’s water treatment system in the spring.

  Constructed wetlands: efficient water treatment and quality natural habitat
 

 


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