What gets into our lake?
In this special report, John Hicks, a landscape architect and vice-chair of the York Environmental Stewardship Council, talks about the need for effective management of stormwater ponds for the health of Lake Simcoe and local wildlife
Stormwater ponds function as temporary storage or detention ponds. They are primarily designed and constructed “to reduce downstream flooding and erosion by controlling the peak flow, frequency of flow, and the velocity of stormwater.”
They are also designed for the most part to trap and settle much of the solid material carried by the stormwater sediment, which improves water quality and helps reduce contaminant loads into rivers and lakes. If they are designed properly, aquatic vegetation — both emergent and submergent species — is installed and it serves as a living biological filter, retaining fine sediment and absorbing the contaminants bound to the sediment.
Unfortunately, the ponds may also contain many toxic contaminants. Although some of these contaminants biodegrade within the pond, others are persistent and accumulate in the muddy sediment at the pond bottom. In spite of the fact that the ponds are designed to protect downstream areas by containing toxic materials, the accumulation of these contaminants within the pond offers a threat to local wildlife using the ponds as habitat.
This can be the case unless the ponds are properly managed.
Trace metals and organic compounds bind with solids that settle to the bottom of the pond as sediment. As this muck accumulates, the concentration of metal and organic contaminants can exceed levels that have toxic effects on the organisms that feed in the sediment. They can accumulate in the tissues of animals that live in the water or in the preditors that consume the animals. In order to prevent this from occurring, it is necessary to clean out the ponds periodically and dispose of the sediment. Without this control, the contaminants flush out into streams and lakes with every rainstorm.
Urban storm runoff also brings natural and human sources of phosphorus into the lake. Stormwater ponds intercepting this flow are small relative to the overall Lake Simcoe watershed, and the rapid growth of phosphorus loadings from urban stormwater runoff make this source of phosphorus one of the most important to control. There is also much research to be done on the use of aquatic plants in the pond to reduce the levels of contamination.
Some measures can be taken to improve the functioning of stormwater ponds such as adding "constructed wetlands," which are built to improve downstream water quality.
Combining ponds with constructed wetlands generally provides increased water storage time, allowing lighter particles such as clays to settle out of the stormwater. The aquatic vegetation designed into the wetland further improves water quality through the uptake of phosphorus, nitrogen and various toxic compounds.
In a study conducted in 1997 in the greater Toronto area, water samples were collected from 15 stormwater ponds and analyzed for metals, chlorides and nitrogen. Most notably, levels of chlorides (from road salt) increased during the winter, while concentrations of phosphorus and nitrates increased in the spring and autumn, due to fertilizer use in residential areas. More disturbing, the Canadian Water Quality Guidelines for the protection of wildlife were exceeded for copper, lead, zinc and chromium at several sites.
The findings showed that stormwater ponds do not provide good quality habitat for fish and wildlife due to the potential for contamination.
The overall recommendation was for proper stormwater pond management, including water and sediment quality monitoring. We have much work to do in understanding how these ponds function, and the interrelationships between plant, animal and the products that enter them. With the possibility of flushing stored contaminants into Lake Simcoe with every severe storm, the ponds require constant monitoring so that the levels of toxins never exceed the guidelines.
This would be a giant step toward good stewardship of our lake which becomes the ultimate repository.