Green islands for blue water

In collaboration with Jon MacMull, Supervisor, Marketing & Communications, Credit Valley Conservation

Stormwater management ponds have long been the infrastructure of choice for managing runoff from roads, parking lots and other impermeable surfaces. While stormwater ponds do a good job of slowing the surge of water during a rain storm and collecting sediment, they often do little to treat pollution. Further, standing water often experiences an increase in temperature. Warm water has lower levels of dissolved oxygen and can have a detrimental impact on aquatic species living in streams receiving water from stormwater ponds.

In order to mitigate these negative impacts, the Credit Valley Conservation (CVC) is monitoring the benefits of floating islands in stormwater management ponds. One pond in particular (which drains into Fletcher’s Creek in Brampton) has been the focus of an extensive monitoring program over the past two years.

Floating islands are engineered vegetated rafts made of natural or inert materials. The roots dangle into the water, and absorb nitrates, phosphorus and ammonia. Floating islands have also been found to be effective at removing total suspended solids and dissolved organic matter.

The islands also shade the water as the vegetation grows. This can lower water temperatures, which is especially important in the centre of larger ponds where shoreline vegetation does not provide shade.

Results from the Fletcher’s Creek stormwater management pond have demonstrated a variety of benefits. There is less layering of warmer and cooler water during dry periods and storms. CVC saw lower water temperatures in the top layers of the pond from the shading effect of the islands. Better mixing of water and shading enhances the quality of water leaving the pond and helps to protect aquatic life downstream. Additionally, well mixed ponds are known to limit the re-suspension of pollutants, reducing the level of pollution entering Fletcher’s Creek.

By themselves, floating Islands aren’t the solution to stormwater issues but they are proving to be another tool in working towards better management of water in our cities.

For more information, view the entire Fletcher’s Creek monitoring report.

This article was published in The Umbrella Stormwater Bulletin Issue 45.

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