Environmental Science & Engineering - www.esemag.com - September 2003
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Stormwater management could combat West Nile Virus

By R. Dale Downey, P.Eng., Cumming Cockburn Limited

Can stormwater management ponds be designed to combat the proliferation of the West Nile Virus? The answer is yes, with a little help from Mother Nature. There is a public perception that stormwater management ponds provide abundant habitat for the propagation of mosquitoes. Current research suggests these facilities can and do provide suitable habitat; however, there is little evidence to support them as significant breeding grounds for mosquitoes.

Recent field observations in Canada and the United States conclude that constructed wetlands and stormwater management ponds actually pose a low risk in spreading the West Nile Virus since the mosquito species that are found in wetlands and stormwater management ponds tend not to be the variety that carry the West Nile Virus.

Of the 74 known species of mosquitoes in Canada, the West Nile Virus has been detected in 10 species. Not all species are found in all parts of Canada (57 species are found in Ontario) though field tests suggest that less than 1% of mosquitoes are likely to be infected with the West Nile Virus. Further evidence states that if an infected mosquito bites you, then you have a 0.3% chance of developing mild symptoms. These low percentages may appear insignificant but substantial efforts are presently underway within Canada and the US to manage the spread of the virus to humans and animals.

The focus of this article is to report on the type of measures being implemented by water resource engineers and municipalities in the design, operation and maintenance of stormwater management ponds to help mitigate the spread of the West Nile Virus within our communities.

Current research demonstrates that only female mosquitoes can spread the virus and the most important action we can take to control mosquito populations is to eliminate or treat the highrisk mosquito breeding sites within our urban communities. Birds serve as the host for the virus. The mosquito first acquires the infection by feeding on an infected bird and the virus is then transmitted when the female mosquito bites a person or animal. There is no evidence to support the view that the virus can be spread naturally from one person to another or from birds and mammals to a person.

Which mosquito species actually transmit the West Nile Virus to people and animals? Scientists don’t know for sure; however, many believe that the virus is primarily associated with Culex mosquitoes. They are commonly known as the northern house mosquito, are widespread and have a flight range of less than one kilometre. This means that if you are bitten by one of these mosquitoes, then it likely hatched nearby. Water that is stagnant for more than three to five days, high in organics, within areas of dense vegetation, and free of predators, is a very attractive habitat to the female mosquitoes and well-known as a high risk site, particularly from May to September when the mosquito larvae are hatched. We are all well aware that the adult mosquitoes are most active just after sunset.

There is also uncertainty on how the virus survives our cold winters. Some scientists believe it could be carried inside migrating birds or remain dormant in hibernating mosquitoes.

On May 13, 2003, Ontario Premier Ernie Eves announced new provincial standards for larviciding as part of the government’s $20 Million commitment to combat the West Nile Virus. The Premier’s science–based and multi-tasked action plan was developed from recent experiences of jurisdictions within the United States such as Illinois, New Jersey and New York. The provincial action plan will provide funding to support all costs of purchasing larvicide for use in municipalities in Ontario. In response to this initiative, Ontario municipalities have assessed the risk within their communities and approved by-law amendments and new regulations to eliminate or treat mosquito larvae production areas.

Methoprene is the larvicide of choice in many municipalities in Ontario. It comes in slow-release pellet- like formulations and is administered when the mosquito is in the larvae stage of development. It has been approved by both the provincial and federal governments and is considered safe for humans and the environment.

Stormwater management ponds represent the final stage of a traditional three-stage stormwater management treatment train designed to control and treat the stormwater runoff draining from urban developments. These ponds are an end-of-pipe solution, which means they are located at the end or outlet of the stormwater collection network. The most visible portion of the stormwater collection system are catchbasins, which intercept the water on our roads and deposit it into the underground storm sewer pipes.

The ponds are designed to: capture the excess stormwater to prevent flooding downstream; detain and slow the rate of discharge to help reduce stream channel erosion and habitat degradation; and to capture and hold sediment and other pollutants that are washed from the streets and driveways and into the storm sewers. Recent advances in the design of these facilities have also enhanced their aesthetics and property value within the community.

Most new draft plans of development for residential, commercial and industrial communities in Ontario include a stormwater management pond. Many municipalities have developed guidelines on the proper design, operation and maintenance and monitoring of stormwater ponds; however, many of these guidelines are adopted from the Stormwater Management Planning and Design Manual published by the Ontario Ministry of the Environment and updated in March 2003. Mosquito abatement measures within the stormwater management ponds are not addressed in the current version of the Design Manual.

There are two traditional types of stormwater management ponds: Detention Pond (dry) and Retention Pond (wet). Recent advances in stormwater management have created a combination facility, known as a Hybrid Pond (combined wet and dry plus wetland). A detention (dry) pond is designed to capture stormwater discharged from the storm sewer and release it at a controlled discharge to a designated outlet over a minimum of 24 hours. It is often referred to as an extended detention pond. Most of the time the pond is dry. Its primary function is flood control and erosion control. They were very common in the 1980s; however, many municipalities now discourage their use.

A retention (wet) pond is the same as above except it contains a permanent pool of water and its primary function is water quality treatment, erosion control and flood control. As the name implies the pond always contains a pool of water. This pool assists in the removal of pollutants normally associated with urban stormwater runoff.

A hybrid pond consists of a wet pond element, an extended detention component and a wetland element connected in series. It provides enhanced water quality treatment capabilities. Most municipalities desire this type of pond design and sometimes add accessories such as fountains to enhance its performance and augment the concentration of dissolved oxygen for a healthy fish habitat.

Stormwater management ponds have proven to be a valuable tool to abate some of the water quality pollutants and hydrologic side effects normally associated with the creation of our modern urban communities. A further refinement in the design of these facilities can help to control the spread of the West Nile Virus by eliminating the high-risk mosquito-breeding habitat. The following design features have been or are in the process of being adopted by Cumming Cockburn Limited in the design of stormwater management facilities in Ontario:
Dale Downey is Director, Water Resources, Municipalities and Land Development, for Cumming Cockburn Limited of Richmond Hill, Ontario. Email: ddowney@cclconsultants.com.

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