Stormwater Retention and Urban Runoff Treatment Ponds

What are Stormwater Retention Ponds?

In developed areas where the ground is covered with an impervious surface (i.e., asphalt, concrete, etc.), stormwater retention ponds are designed to catch and hold excess water from large storm events.  Water can be diverted to stormwater retention ponds instead of into the sewer systems.  Water is then released from the pond slowly, instead of the quick surge that would result without water retention.  Artificial wetlands built for stormwater retention can also be used for treatment of urban runoff water.  Beyond the benefits to humans and urban structures, these ponds provide habitat for insects, fish, amphibians, reptiles, birds, and small and medium-sized mammals (both terrestrial and aquatic).  However, stormwater retention ponds may also have negative impacts on native landscapes and habitat.

How are they beneficial?

Stormwater retention or treatment ponds can provide wonderful habitat for many animals and plants, and they also provide recreational opportunities (e.g. the pond at Milliken Park in Scarborough is actually a stormwater retention pond built by the City of Toronto).  Wetlands attract many types of insects, such as butterflies, dragonflies, and damselflies, as well as aquatic water striders, and snails.  These areas also provide wonderful habitat for fish, amphibians, reptiles, mammals, and birds.

Ground cover dominated by asphalt and concrete with minimal natural vegetation causes rainwater to run-off roads and sidewalks into sewers, rivers, and lakes, instead of percolating into the ground.  Stormwater retention ponds can absorb the water, and release it slowly into natural waterways, preventing the drastic increase in water level that can cause (sometimes severe) damage to riverbanks and lakeshores. 

Plants in stormwater retention ponds can filter and treat urban runoff water through filtration.  Heavy metals most frequently found in urban runoff include lead, zinc, cadmium, nickel, and copper, all of which can have adverse affects on humans and other animals.  For example, ingestion of even small amounts of these metals could potentially cause genetic mutations, tumors, and even death.  Water enters the pond or wetland and plants such as Typha latifolia—the common cattail - absorb nutrients and heavy metals or other toxic substances.  Water is then released in better condition than when it entered the wetland.    

In some communities, retention ponds may represent the only wetland available to aquatic species, or they may serve as important linkages between isolated wetlands.

How are they potentially harmful?

If wetlands are not constructed correctly, with sufficient plant diversity and the correct drainage systems, they can be detrimental to ecosystem health.  Certain plant species may form a monoculture, reducing biodiversity and overall fitness of the community to the point that animals in this artificial habitat may be reduced to very few species.  One species may potentially engulf the wetland due to lack of competition or predation from other species.

Plants used to filter heavy metals in stormwater retention ponds or wetlands must be harvested annually to avoid the biomagnification of said heavy metals.  Heavy metals may return to the water column or food web via decomposition or ingestion by another animal.  Put into context, if the sediment contains 1µg of a particular toxin, and a worm buries through that sediment and ingests and stores food, then the worm may contain as much as 10µg of the toxin in its system.  If a fish eats that worm, and stores the energy as fat, then the fish may contain as much as 100µg of the toxin.  By the time the toxin makes its way to the human level, concentrations could be high enough to be hazardous to the person’s health.

Sedimentation may be so strong up stream from a potential stormwater pond or wetland that silt or other thick sediments may build up in these retention ponds and suffocate plants, as well as fish eggs, amphibian eggs, and even fish or amphibian larvae.  The eggs or larvae are essentially coated in fine clay or silt particles, which may prevent sunlight from penetrating and thermoregulating individual eggs.  As well, silt or clay particles may prevent eggs or larvae from taking in oxygen through their egg membranes or gills, respectively. 

Without correct drainage systems or methods, stormwater retention ponds or wetlands may have sudden surges of overflow, defeating their purpose altogether.  They may also dry up between rains, depriving aquatic (or amphibious) species of a suitable habitat.  The proper topography and geology is crucial in any wetland construction, and even more so when the wetland is to be used for retention or treatment purposes.

So what’s the verdict?

Whether we like it or not, stormwater retention ponds or urban runoff treatment ponds make great habitat for plants and animals, and create great aesthetics for humans to enjoy.  Retention ponds can be constructed to both store and filter water and pollutants and to provide wildlife habitat.  This may require a complex of ponds that provide separate functions—water retention in primary filtering ponds that excludes wildlife habitat and secondary ponds downstream that hold water and provide habitat.  If you are interested in constructing a small pond on your property, we would be glad to send you a copy of our book the Urban Outback—Wetlands for Wildlife: A Guide to Wetland Restoration and Frog-Friendly Backyards!  You can obtain a hard-copy of this book by contacting the Adopt-A-Pond Coordinator at 416-392-5999, or at aap@torontozoo.ca.  Also, take a look at Thomas R. Biebighauser’s book A Guide to Creating Vernal Ponds, which can be viewed here: http://www.fs.fed.us/r8/boone/documents/resources/vernal.pdf#search=%22Thomas%20R.%20Biebighauser%20A%20Guide%20to%20Creating%20Vernal%20Ponds%22.  For even more information, visit the Ontario Vernal Pools Association website at: www.ontariovernalpools.org.