214 Int. J. Water, Vol. 3, No. 3, 2007
Copyright © 2007 Inderscience Enterprises Ltd.
Controlling urban stormwater pollution by
constructed wetlands: a Canadian perspective
A.S. Crowe, Q. Rochfort, K. Exall
and J. Marsalek*
Urban Water Management Section,
National Water Research Institute,
867 Lakeshore Rd, Burlington,
ON L7R 4A6, Canada
Abstract: During the past 20 years, Constructed Stormwater Wetlands
(CSWWs) have attained broad acceptance in Canada as effective measures for
stormwater management. CSWWs are used mainly for improving stormwater
quality by providing sufficient treatment volumes in shallow permanent pools.
This leads to high requirements for land, which is one of the constraints on
CSWWs use. Even though CSWWs perform less effectively in cold
weather, through proper design they can be kept operational through the winter
months. CSWWs attract wildlife, but do not provide high quality habitat.
Consequently, CSWWs and their effects on wildlife need to be monitored and
ecotoxicological risks controlled.
Keywords: Canada; cold climates; constructed stormwater wetlands; CSWW;
ecotoxicity; stormwater management; wildlife.
Reference to this paper should be made as follows: Crowe, A.S., Rochfort, Q.,
Exall, K. and Marsalek, J. (2007) ‘Controlling urban stormwater pollution by
constructed wetlands: a Canadian perspective’, Int. J. Water, Vol. 3, No. 3,
Biographical notes: Allan S. Crowe is a Research Hydrogeologist with the
National Water Research Institute, Environment Canada, in Burlington
(Canada). His research interests focus on the role of groundwater in the
hydrology and geochemistry of coastal environments, including lakes, beaches
and wetlands. Currently, he is assessing the role of groundwater as a
mechanism for E. coli transport and persistence along beaches of the Great
Lakes. He also provides technical assistance to various federal agencies and
committees. He is an Adjunct Professor at the University of Western Ontario
and McMaster University.
Quintin Rochfort has been pursuing research on urban stormwater
best management practices in the Urban Water Management Section,
National Water Research Institute, Environment Canada in Burlington
(Canada) for about ten years. He holds a BSc in Biology and an
MSc in Environmental Engineering from Queen’s University. Constructed
wetlands are his passion – his Masters research addressed the performance of
subsurface-flow constructed wetlands for stormwater treatment and he has
Controlling urban stormwater pollution by constructed wetlands 215
maintained keen interest in constructed wetlands ever since. Currently, he is
working on a study examining aquatic habitat issues in shallow ponds with
large macrophyte presence in the Toronto area.
Kirsten Exall is a Research Scientist at the National Water Research Institute,
Environment Canada in Burlington (Canada). She joined the Institute after
receiving her PhD in Chemistry from Queen’s University in Kingston, Ontario.
Her PhD research dealt with coagulation and flocculation in drinking
water treatment. After joining the Urban Water Management Section at NWRI,
she has been addressing the use of chemical additions in high-rate treatment of
stormwater and combined sewer overflows, drinking water treatment by
membrane processes, quality of urban snowmelt and winter runoff and
wastewater reclamation and reuse.
Jiri Marsalek is Chief of the Urban Water Management Section at the National
Water Research Institute, Environment Canada in Burlington (Canada).
He has studied extensively various urban stormwater management practices
and published research results on stormwater ponds, oil and grit separators,
biofilters and constructed wetlands. His recent contributions include books or
book chapters on stormwater management ponds (IWA), urban water cycle
processes and interactions (UNESCO), integrated urban water management,
transboundary floods, flood risk management, enhancing urban environment
by upgrading and restoration, urban water management and source controls
(all in the NATO Science Series).
Constructed Stormwater Wetlands (CSWWs) are engineered systems designed to mimic
flow control and water quality improvement functions of natural wetlands in urban runoff
control. CSWWs were introduced into the urban drainage practice in the early 1980s and
by the end of the decade, their applicability in urban stormwater management has been
well defined (Livingston, 1989). With respect to flow control, CSWWs function the same
way as shallow stormwater ponds with permanent storage, and often the only differences
between CSWWs and shallow ponds are the smaller depths (in CSWWs the permanent
pool is 0.15–0.3 m) and a greater coverage of surface by aquatic vegetation (in CSWWs
>50%) (OMOE, 2003). In Canadian stormwater management practice, advantages of
these two types of facilities are sometimes combined in hybrid wet pond/constructed
wetland systems, where the deeper pond component functions better during the
winter/spring conditions and the wetland component is better suited for pollutant
removals during the summer months (OMOE, 2003). An example of such a stormwater
management train, comprising a sediment forebay and a riparian wetland upstream
of a stormwater management pond is shown in Figure 1 for two seasons, the early fall
and mid winter. The need to provide a relatively large storage volume in shallow
wetlands results in large land requirements and represents one of the constraints on
CSWWs use. With respect to stormwater quality improvements, CSWWs remove
pollutants first through physical-chemical processes (volatilisation, sedimentation,
adsorption, precipitation and filtration), followed by biological processes (uptake through
plant-soil and plant-water interface, translocation through plant vascular system,
differential pollutant uptake in specific plant parts, non-specific pollutant uptake
216 A.S. Crowe, Q. Rochfort, K. Exall and J. Marsalek
occurring when plants absorb large quantities of nutrients and uptake and immobilisation
by plant litter zones) (Livingston, 1989; Scholz and Lee, 2005).
Figure 1 Stormwater management train comprising a sediment forebay (in the foreground)
and a riparian wetland upstream of a stormwater management pond in two seasons:
early fall and mid winter
Following the CSWWs introduction into the Canadian stormwater management practice,
they have been adopted by many municipalities and provincial authorities as one of the
preferred best management practices frequently applied in new urban developments
(Alberta Environment, 2000; OMOE, 2003; Taylor, 1992). The main purpose of this
paper is to present an overview of the Canadian experience with CSWWs, focusing on
design procedures reflecting the Canadian climate, environmental concerns and practices,
public perceptions, and, selected case studies.
1.1 Constructed wetland design
General design considerations
CSWWs used for stormwater management in Canada are generally designed for
flow control and stormwater treatment, rather than habitat creation (Taylor, 1992).
Thus, constructed wetlands may not appear to the public as attractive as natural wetlands.
This is particularly true for Sub-Surface Flow (SSF) wetlands which use a coarse medium
(gravel, or sand) with high hydraulic conductivity to increase the flow to the root zone of
the plants, where most of the biological treatment occurs. The lack of a wet pool may
keep some wildlife away and such a facility may not be publicly valued as much
as Surface Flow (SF) wetlands or stormwater ponds. A highly-charged clay medium
(often containing iron or aluminium) is commonly used as a substrate in SF wetlands,
because it provides a greater sink for phosphorus removal. However, once the adsorption
sites are occupied, the assimilation capacity is reduced (Lan et al., 1992). Liners of
compacted clay or geotextile membranes can be used to isolate wetlands from the
groundwater. Where the risk of contamination or wetland dessication is low, the wetland
may be designed to drain into underlying soils.
Some form of water level control in CSWWs, by weirs or outlet structures,
is essential to maintain favourable plant growth conditions and a healthy plant
community in the wetland. Plants, such as cattail (Typha latifolia) and common reed
(Phragmites australis), are generally planted as seedlings and allowed to establish
Controlling urban stormwater pollution by constructed wetlands 217
naturally over time (Scholz, 2006). A mixture of plants provides better chance of
establishment, because monocultures may not be well adapted to varying site or water
quality conditions (Taylor, 1992). Many researchers reported that volunteer species
eventually colonise CSWWs (Kadlec and Knight, 1996). Engineered berms and
distribution/collection structures help extend the flow path through the wetland system
and utilise the entire treatment volume, thus increasing contact times and treatment
Various approaches are used in sizing CSWWs. In the Canadian practice, they are
typically designed for specific treatment volumes and large storms are allowed to bypass
such facilities to prevent washout of sediment.
CSWW design for treatment takes advantage of various pollution removal processes,
which will be only briefly summarised here. A number of studies have examined
contaminant removal in CSWWs in Canada (e.g., Farrell and Scheckenberger, 2003;
Fortin et al., 2000; Goulet et al., 2001; Goulet and Pick, 2001; Higgins and Maclean,
2002; SWAMP, 2003) and noted the effects of seasonal temperature and influent quality
variations. Overall removals are further affected by the wetland structure, hydrology,
climate, soils, vegetation and watershed imperviousness (Carleton et al., 2001).
2 Transformations and kinetics in Surface Flow (SF) CSWWs
In SF CSWWs, particulates, along with such adsorbed contaminants as PAHs or
many metals, are generally removed by physical settling and filtration as the water
flows through wetland macrophytes and the litter layer. Alternatively, solids may be
resuspended if the sediment bed is disturbed (Kadlec and Knight, 1996). Pathogens are
removed to some extent through sedimentation or filtration, as well as through chemical
means including oxidation and UV irradiation, or biological mechanisms such as biofilm
adsorption and predation (Werker et al., 2002).
The bulk of the organic carbon load is removed through biomass metabolism and
growth. In aerobic zones, respiration processes dominate, resulting in the production of
CO2. In anaerobic zones, fermentation, methanogenesis and sulphate, iron and nitrate
reduction processes are common, resulting in the production of methane and other low
molecular weight organics, as well as CO2. Trace organics may be biologically degraded
with other carbon species, or they may be removed through volatilisation, photochemical
oxidation, or sorption and sedimentation (Kadlec and Knight, 1996).
Overall, total nitrogen is commonly reduced in CSWWs. Nitrogen transformations
include ammonification (organic nitrogen to ammonia), nitrification (ammonia to
nitrates/nitrites), denitrification (nitrates/nitrites to gaseous nitrogen), nitrogen fixation
(nitrogen gas to ammonia) and nitrogen assimilation (inorganic nitrogen to organic
nitrogen species) (Kadlec and Knight, 1996).
Soluble reactive phosphorus may be taken up by wetland plants or microorganisms,
precipitated by cations present in the water column, or sorbed to soils and
sediments. Solid mineral or organic phosphorus species may either settle with suspended
solids, or become solubilised under reducing conditions. Similarly, phosphorus that
is stored through plant uptake will ultimately be released through decomposition
(Kadlec and Knight, 1996).
Trace metals transported by urban runoff are removed in CSWWs by such
mechanisms as binding to soils, sediments and particulates; precipitation of insoluble
218 A.S. Crowe, Q. Rochfort, K. Exall and J. Marsalek
salts and uptake by plants and bacteria (Kadlec and Knight, 1996). However, CSWW
sediments can act as both a source and a sink for metal species (Kelly-Hooper, 1996).
3 Transformations and kinetics in Sub-Surface Flow (SSF) CSWWs
The substrate bed of SSF CSWWs (with depths <0.6 m) contain both aerobic and
anaerobic zones. Oxygen enters the bed substrate through both direct atmospheric
diffusion and through the plant leaves and roots. As stormwater passes through the bed
substrate, it contacts a mixture of facultative microbes in association with the substrate
and plant roots which act to filter the particulates. The particular media used in CSWWs
may also be chosen for enhanced phosphorus adsorption capacity, such as iron- or
aluminium-rich materials (Kadlec and Knight, 1996).
3.1 Engineering design of CSWWs
Design of CSWWs is guided by design manuals which have been developed by some
Canadian provinces and large municipalities. Perhaps the best known is the Ontario
manual (OMOE, 2003) which specifies the storage volume to be treated, in order to attain
one of the three levels of aquatic habitat protection by Total Suspended Solids (TSS)
• basic, equal to a 60% long-term removal
• normal, equal to a 70% long-term removal
• enhanced, equal to a 80% long-term removal.
The level of protection reflects the applicable status of aquatic habitat to be protected; the
enhanced protection is applied in the case of sensitive habitats; normal protection applies
where conditions for enhanced protection do not exist, and the basic protection applies to
altered water bodies with low potential for rehabilitation. The corresponding water
quality storage requirements were developed from computer simulations of stormwater
management facilities using Southern Ontario climatic data and assuming 24 h drawdown
periods. Examples of treatment volumes are given in Table 1.
Table 1 Water quality storage requirements for constructed stormwater wetlands
Storage volume (mm/ha) for various degrees
of catchment imperviousness
Protection level 35% 55% 70% 85%
Basic (60% long-term TSS removal) 6 6 6 6
Normal (70% long-term TSS removal) 6 7 8 9
Enhanced (80% long-term TSS removal) 8 10.5 12 14
Source: OMOE (2003)
Controlling urban stormwater pollution by constructed wetlands 219
3.2 Design elements
Wetland facilities comprise a number of design elements ensuring proper facility
operation. Basic guidance for designing such elements is offered in Table 2.
Table 2 Design criteria for constructed stormwater wetlands in Ontario, Canada
Design element Design objective Criteria
Sustain wetland operation
(i.e., vegetation and water
5 ha minimum; preferably 10 ha
Treatment volume To protect aquatic habitat 6–14 mm/ha of drained area,
Settling of suspended solids 24 h (12 h if required
by the minimum orifice size)
Forebay Pretreatment (mostly by
Maximum area=20% of the permanent
pool area, minimum depth 1 m,
non-erosive exit velocities
Good flow conditions for
settling, low risk of
Overall ≥ 3 : 1, forebay ≥ 2 : 1
Permanent pool depth Vegetation growth
requirements, quick settling
The average depth should range from
0.15 m to 0.3 m
Active storage depth Providing adequate storage,
≤1.0 m, for storms with return periods
Side slopes Safety 5 : 1 over 3 m above and below
permanent pool; ≤3 : 1 elsewhere
Inlet/outlet Avoid clogging or freezing Minimum diameter = 450 mm, pipe
slope >0.01; submerged inlet obvert
0.15 m below the expected max. ice
depth; outlet orifice size ≥75–100 mm
Maintenance access Access for maintenance Specified by municipality
Buffer Safety Minimum 7.5 m above
maximum water quality/water
quality/erosion/control water level
Source: OMOE (2003)
4 Winter operation of CSWWs
Operation of stormwater management facilities in Canada is adversely affected by
freezing weather and use of road salts during the winter months (Marsalek, 2003).
Such effects include reduced pollutant contaminant removals during winter operation of
CSWWs (e.g., Goulet et al., 2001; Oberts, 1994; SWAMP, 2003) for such reasons
(Oberts, 1994; Werker et al., 2002) as:
220 A.S. Crowe, Q. Rochfort, K. Exall and J. Marsalek
• The thick ice layer that can form and reduce the permanent storage volume of the
wetland. This layer causes the meltwater either to flow above the ice, where it
receives little treatment due to the minimal available settling depth, or to dive below
the ice layer, causing turbulence which scours and resuspends bottom sediments.
Additionally, oxygen transfer between the water and atmosphere is inhibited.
• The reduction in biological activity or die-back of wetland plants at cold
• The alteration of the kinetics of chemical transformations with temperature.
In general, SSF CSWWs are better insulated from cold temperatures than SF CSWWs,
particularly if covered by a layer of plant litter, and are preferred for some applications in
Canada (Werker et al., 2002). The presence of a snow layer covering the surface of a SF
CSWWs can also provide insulation, particularly if water levels drop, creating an
insulating air gap between the snow and water layers (Kadlec and Knight, 1996).
Winter conditions also alter pollutant sources in urban environments, which can
include sand, gravel and road salts that are applied to urban streets, and aircraft de-icing
and anti-icing materials (typically glycols) employed at airports. Road salts enter surface
water, soil and groundwater after snowmelt and can impact water bodies, vegetation and
wildlife. Road salt often contains the anti-caking agent, sodium ferrocyanide (~0.01% by
dry weight), which can transform to toxic free cyanide (HCN) when exposed to light
(Novotny et al., 1999). Particulates added as anti-skid agents (salt : abrasive ratios from
1 : 2 – 1 : 50) add large solids loads to snowmelt and some sands used to control skidding
have been shown to contain relatively high levels of phosphorus and several metals
Elevated concentrations of salts in runoff due to winter road maintenance
activities may impact the efficiency of stormwater management facilities in a number of
ways, including (Amrhein et al., 1992; Marsalek, 2003): changes in water density and
ionic strength, both of which could alter settling and flocculation rates of fine solids;
chemostratification within the wetland; toxicity to biological organisms and, alterations
to binding and transport characteristics of trace contaminants, specifically cation
replacement and leaching of trace metals from soils or sediments affected by salt-laden
runoff. Scholz (2004) found that the presence of high salt concentrations after road
gritting led to a marked reduction in treatment performance for nickel in a SSF wetland
treating concentrated stormwater (gully pot liquor).
A SSF CSWW was constructed for reduction of glycol concentrations in runoff
from aircraft deicing activities at the Edmonton International Airport in Edmonton,
Alberta (Higgins and Maclean, 2002). The wetland was designed without aeration and for
a flow rate of 1300 m3/d. The target for outlet water quality was 25 mg BOD/L and actual
water flow rate through the CSWW was affected by temperature and glycol concentration
(i.e., at lower water temperatures or higher glycol concentrations, flow was reduced).
Other lessons can be gained from Canadian experience with constructed wetlands for
wastewater treatment; many, but not all, transformations exhibit lower reaction rates at
cold temperatures (Werker et al., 2002).
Higgins et al. (2000) described a number of ways in which wastewater constructed
wetlands can be ‘engineered’ to improve contaminant removals under cold operating
temperatures, including the following: aeration of wetland cells to improve ammonia
nitrification rates; use of engineered substrates in SSF CSWWs to improve adsorption,
Controlling urban stormwater pollution by constructed wetlands 221
precipitation or volatilisation of pollutants; addition of energy to compensate for cold
temperatures; chemical addition to promote nitrification, precipitation or sedimentation;
use of alternate vegetation; recycling of effluent or altering feed rates. Pries (1994)
suggested avoiding reduced treatment efficiency in cold weather by storing runoff over
winter months and releasing it into the wetland during the growing season. Finally, some
practical measures ensuring CSWW operation during the winter months were presented
earlier in Table 2 (e.g., protection of inlets/outlets against freezing).
5 Constructed Stormwater Wetlands (CSWW) and wildlife habitat
The combination of aquatic and terrestrial conditions merging at the shore and associated
vegetation in CSWWs present an attractive habitat for birds, mammals, reptiles,
amphibians, fish and invertebrates. However, CSWWs are essentially treatment facilities,
accumulating various pollutants and consequently, the habitat and treatment functions of
CSWWs may be in conflict. Studies on the impact of contaminants on wildlife at
Canadian CSWWs consistently conclude that although a variety of wildlife is found to
inhabit CSWW the diversity of wildlife is much lower than that found in natural
wetlands (Bishop et al., 2000a, 2000b; Free and Mulamoottil, 1983; Kennedy and
Mayer, 2002; Marsalek, 2003; Olding, 2000; Rochfort et al., 2000; van Loon et al., 2000;
Warner and Li, 1998; Wren et al., 1997) and hence CSWWs are not quality wetland
Because of the high inputs of road deicing salt into Canadian CSWWs, the aquatic
plants found in Canadian CSWWs are generally salt-tolerant species, such as Typha
latifolia (common cattail), Lythrum salicaria (purple loosestrife), Phragmites australis
(common reed), Myriophylum spp. (water milfoil), Scirpus validus (bull rush), Carex spp.
(sedge grass), Glyceria spp. (manna grass) (Bishop et al., 2000b; Marsalek, 2003; Mayer
et al., 1999), as well as plant species that are tolerant to other contaminants (Bishop et al.,
2000a, 2000b; Marsalek et al., 1999a). This effect is demonstrated in Figure 2 showing
the number of plant species in constructed wetlands vs. chloride concentrations
(Marsalek, 2003). Olding (2000) examined the different genera of algae (phytoplankton
and periphyton) at CSWWs near Toronto, Ontario and noted that the algae were the
species tolerant to contaminants such as those present in highway runoff or nutrients.
Benthic invertebrate communities, primarily insects (esp. chironomidea, diptera),
annelids (esp. oligochaetea) and mollusks (Hyalella azteca, Caecidotea racovitizai) and
less frequently leeches, water mites and crustaceans, are dominated by contaminant
tolerant species and these have higher population densities than natural wetlands
(Bishop et al., 2000b; Free and Mulamoottil, 1983; Rochfort et al., 2000). The impact
of deicing road salts on vegetation and benthic invertebrates in Canadian CSWWs
increases during the winter and spring when runoff of road salt increase chloride
concentrations in CSWWs (Free and Mulamoottil, 1983; Marsalek, 2003; Marsalek et al.,
1999a; Mayer et al., 1999).
222 A.S. Crowe, Q. Rochfort, K. Exall and J. Marsalek
Figure 2 Number of plant species in constructed wetlands vs. chloride concentration
Source: Marsalek (2003)
CSWWs are stressed aquatic habitats due primarily to contaminants that accumulated in
the CSWWs (Bishop et al., 2000a). If the water and sediment quality change dramatically
and for long periods (i.e., where the wetland is not allowed to recover), species shifts
occur. The CSWW becomes unsuitable habitat for some wildlife because of the
bioaccumulation and biomagnification of contaminants within the food web (Bishop
et al., 2000b; van Loon et al., 2000). Over time the contaminants in the CSWW also may
have adverse effect on local wildlife living in the vicinity of the CSWW (Kennedy and
Mayer, 2002). Berezowsky (1995) and Warner and Li (1998) state that if a properly
functioning wetland is desired, then natural wetlands or restored wetlands should not be
used to treat wastewater because of the detrimental impact from contaminants.
Canadian CSWWs contain many contaminants, and these contaminants often
can be found at levels that are toxic to many species typically associated with natural
wetlands (Bishop et al. 2000b; Dutka et al., 1994a, 1994b; Licsko and Struger, 1996;
Marsalek et al., 1999a, 1999b; Rochfort et al., 2000; Struger et al., 1994; Wren et al.,
1997). A study by Marsalek et al. (1999a) of three CSWWs in Ontario indicated that
CSWWs can exhibit acute toxicity and genotoxicity depending on the source, storm
characteristics, timing of runoff, drainage design and season. Studies have not examined
the source of the toxicity in Canadian CSWW, for example road salt, organics, metals,
pesticides, nutrients, etc. (Marsalek, 2003). Turbidity also creates habitat problems in
CSWW. Turbidity, due to both algae and suspended sediments, leads to a reduction of
sunlight penetrating the water and hence reduces the diversity of submergent vegetation,
wildlife and benthic species (Bishop et al., 2000a; Free and Mulamoottil, 1983; Olding,
2000; Wren et al., 1997).
The lack of wildlife diversity in Canadian CSWW may not be entirely due to
contaminants. Wildlife diversity is also limited by many physical characteristics of
CSWWs in urban areas, such as the size of the CSWWs, percent open water, water level
fluctuations (including periods of no water), local urban noise and human activity, and
ice cover. In general because CSWWs in urban areas are small, they attract fewer wildlife
species (Wren et al., 1997).
Most studies assessing CSWWs and wildlife habitat focus on the detrimental impacts
of CSWW on wildlife; however, wildlife may have a detrimental impact on CSWWs
(Kennedy and Mayer, 2002; Pavo and Duncan, 1996). These studies stated that
CSWW may attract animals that could interfere with the goal of treating stormwater
Controlling urban stormwater pollution by constructed wetlands 223
runoff. A variety of animals that may inhabit CSWW (e.g., beaver, muskrat, otter, carp)
could burrow into the CSWW berms, liners or foundations, block outlets and affect the
CSWW hydraulics, consume the vegetation planted for treatment, introduce other plant
species, and resuspend contaminated sediment. In such cases, portions of the
CSWWs may have to be protected from wildlife by cages. Rochfort et al. (1997) and
Wren et al. (1997) indicate that wildlife habitation of CSWWs may actually increase
bacterial contamination (e.g., E. coli) in CSWWs due to loadings of faecal matter from
wildlife and especially from waterfowl.
Wren et al. (1997) and Bishop et al. (2000b) recommended that if CSWW in Canada
are to be used by wildlife, monitoring of water and sediment must be undertaken to assess
levels of contaminants that could affect wildlife. If contaminants are present in CSWW,
the contaminants must be regularly removed to offer a healthy contaminant-free habitat
for a diverse wildlife community, or wildlife should be discouraged from inhabiting the
CSWW. Wren et al. (1997) proposed a number of protocols that should be put in place
both before the CSWW is constructed and during its operation to protect the health of the
wildlife and the integrity of the wetland as a habitat. The protocols are divided into three
levels of monitoring, of increasing necessity. The first level is the collection of baseline
data to characterise the physical and hydraulic properties of the CSWW and to assess
the basic contaminant information of the CSWW sediment and inflow, to determine if the
CSWW is becoming contaminated. If the CSWW is contaminated, Level I Monitoring
in the various ecosystem compartments is undertaken to assess what vegetation,
wildlife and benthic communities exist in the CSWW and if they may be exposed to
contamination. Level II Monitoring of contaminant exposure and health of populations is
undertaken to provide a detailed assessment that is designed to measure and quantify
health effects in vegetation, wildlife and benthic communities in CSWW due to
In Canada, there are no environmental criteria for levels of contamination in
sediments and water for the protection of aquatic life specifically for CSWWs. However,
there are both federal and provincial aquatic sediment and water quality guidelines for
the protection of aquatic life and these could be applied to Canadian CSWWs to assess
the hazard posed to aquatic wildlife and benthic communities, or used to evaluate
monitoring data (CCME, 2005; OMOE, 1994).
6 Public perception of CSWW in Canada
In the late 1980s, a survey of Canadian government agencies, engineering consulting
firms and nature-based non-governmental organisations with an interest in managing
stormwater runoff, revealed that 52% of respondents did not consider CSWW as a
feasible option for treating stormwater runoff (Carlisle et al., 1991). Specific concerns
included public liability/safety, mosquitoes and the proximity of the wetland and
wildlife to homes. Minor concerns included vandalism, undesirable weeds and algae,
odour and pets chasing birds. Although CSWWs were gaining use in many other
countries, this lack of acceptance in Canada was primarily due to little operational and
educational experience with CSWW in Canada during this time, and that there were local
lakes or streams for the discharge and assimilation urban runoff (Carlisle and
224 A.S. Crowe, Q. Rochfort, K. Exall and J. Marsalek
Now Canadian government agencies, engineering firms and the public view CSWWs
as a viable means of treating urban stormwater runoff, and as an asset to the community.
From a treatment perspective, much of this acceptance has come from considerable
operational experience throughout Canada during the past 20 years. Government agencies
at federal, provincial and municipal levels are promoting CSWW as part of an
environmentally sustainable solution to the treatment of urban stormwater runoff and to
provide wildlife and waterfowl habitat (Environment Canada, 2005; Weatherbe and
Much of the public attitude towards CSWW, has reflected a change in Canadian
society’s attitude towards urban open space from manicured lawns, geometric walkways,
and planned gardens, towards more naturalised areas that consist of naturalised
vegetation and landscaping, wildlife habitat and natural walkways (Amell and Eastlick,
1996). For example, a survey of local residents in Halifax, Nova Scotia, who have small
stormwater runoff wetlands in their communities (Manuel, 2003), revealed that the vast
majority of residents had a positive perception of wetlands in their community.
These residents liked the idea of natural areas and wildlife habitat within their
communities and saw them as a positive asset for their community despite their less than
perfect wetland-habitat conditions (Manuel, 2003). Although many residents felt that the
presence of SW ponds and lakes in their neighbourhood would increase property values,
many residents perceived CSWWs as a source of odour, weeds, etc. (Amell and Eastlick,
1996; Wisner, 1997). To gain public acceptance of CSWWs in neighbourhoods,
municipalities have undertaken landscaping at the CSWW sites to make the sites
more accessible and provide maintenance (Anderson et al., 2002; Wisner, 1997).
Now, CSWW sites in Canada are used by local residents for recreational and natural
activities such as walking, skating, bird watching, feeding birds and nature education
(Hopkinson et al., 1997; Manuel, 2003). But, as shown at a CSWW site in Kitchener
Ontario, the public may be concerned if the wetland becomes too accessible and if trails
are too well maintained (Hopkinson et al., 1997).
There are still public concerns related to CSWWs in Canadian communities.
They may be perceived as visually unappealing, but adding access to the site, such as
trails and some landscaping, will increase public acceptance of CSWWs (Amell and
Eastlick, 1996; Anderson et al., 2002; Wisner, 1997). There is also public concern
with possible odours, weeds, and algae (Manuel, 2003; Pries, 1996; Wisner, 1997)
and the CSWW may become a breeding place for mosquitoes (Anderson et al., 2002;
Manuel, 2003). But if the CSWW is properly engineered to ensure continuous flow
through the CSWW and maintained to prevent blockages, the stagnant water
condition that could lead to these problems would be removed (Pavo and Duncan, 1996;
Pries, 1996). The recent arrival of West Nile virus to Canada and its transmission
to people by mosquitoes (Ford-Jones et al., 2002) has increased public and
municipal concerns about CSWWs becoming breeding areas for mosquitoes. In addition
to taking engineered steps to prevent shallow stagnant water in CSWWs,
mosquito populations can be reduced by introducing mosquito larva-eating fish into
the CSWWs, building nesting sites for mosquito-eating birds (purple martins,
swallows), introducing dragon flies and applying mosquito larvacides (Pavo and Duncan,
1996; Pries, 1996).
Controlling urban stormwater pollution by constructed wetlands 225
7 Constructed Stormwater Wetlands (CSWW): selected Canadian
Canadian examples of the use of constructed wetlands in stormwater management
facilities are plentiful, particularly in Ontario, Alberta and British Columbia. However,
relatively few of these facilities are documented in the literature. Thus, there is a need for
more information on CSWWs, particularly on habitat quality and long-term performance.
Selected examples of Canadian applications of CSWWs, focusing mostly on stormwater
Quebec Ministry of Transport. This site (Serodes et al., 2003) receives runoff from part
of a highway interchange (Autoroute 20 and 30, between Montreal and Quebec City).
It was constructed by modifying existing drainage ditches (adding control structures and
transplanting vegetation), which minimised the cost of installation. It was noted that
the wetlands provided good removal of TSS (effluent concentrations 4–52 mg/L), total
nitrogen (effluent concentrations 0.93–1.6 mg/L), chloride (effluent concentrations
1100–1200 mg/L), conductivity (effluent readings 1900–5400 µS/cm) and Zn (effluent
concentrations 0.013–0.02 mg/L), but Pb (effluent concentrations 0.002–0.05 mg/L) and
Total Phosphorus (TP) (effluent concentrations 0.04–0.11 mg/L) were relatively
unaffected. During the growing season, cells planted with common cattail (Typha spp.)
showed a greater treatment capacity (particularly for metals) than cells planted with
common reed (Phragmites spp.)
TRCA Markham. This Toronto Region Conservation Authority (TRCA) facility
(SWAMP, 2002) is located in a mainly residential area in the Rouge River Watershed in
Markham, Ontario and consists of a sediment forebay, stormwater pond and wetland
system, designed as a treatment train. The facility receives runoff from a 600 ha
residential catchment and can store 151,000 m3 of runoff from a 25 mm rainfall event.
The wetland was planted with common cattail (Typha spp.) and reed canary grass
(Phalaris spp.) and it is used mainly to polish effluent from the stormwater pond.
Analysis of 30 storm events indicates a good facility performance in removing TSS, TP,
Cu, Zn and E. coli (95%, 87%, 85%, 87% and 79% removal, respectively). Higher
concentrations in the effluent in the summer months corresponded to much greater inlet
loadings. Effluent concentrations for winter months were TSS 7.1 mg/L, TP 0.04 mg/L,
Cu 3.7 µg/L, Zn 7.0 µg/L and 10 E. coli/100 mL. In summer effluent concentrations were
TSS 23 mg/L, TP 0.08 mg/L, Cu 8.2 µg/L, Zn 14.1 µg/L and 237 E. coli /100 mL.
Sediment accumulation rates suggested that the facility was slightly oversized so that it
could accommodate any future development upstream, without requiring expansion.
TRCA Aurora. This site (SWAMP, 2003) is located in Aurora, Ontario and receives
runoff from an 82.4 ha drainage basin (30% of which is agricultural land; the rest is
residential). The wetland is 1.2 ha in size and consists of a sediment forebay (40 m3 wet
pool) followed by a number of parallel wetland cells. The wetland did not retain a
permanent wet pool during dry summer months. A greenhouse was installed over 10% of
the wetland (210 m2 in one of the parallel cells) to assess its potential for improving
performance during winter months. Average performance values for the facility
(12 months period, with 29 storm/snowmelt events) were good for TSS, TP, Cu and
E. coli (86%, 58%, 58% and 84% removal respectively), but only marginal for Zn
(17% removal). In summer, effluent concentrations were: TSS 26 mg/L, TP 0.10 mg/L,
226 A.S. Crowe, Q. Rochfort, K. Exall and J. Marsalek
Cu 24 µg/L, Zn 24 µg/L and 475 E. coli/100 mL. Removal for all constituents were
reduced in the winter months to: 46% TSS, 8% TP, 37% Cu, –87% Zn and 42%
E. coli. In winter months, effluent concentrations were: TSS 29 mg/L, TP 0.16 mg/L,
Cu 5.8 µg/L, Zn 64 µg/L and 252 E. coli/100 mL. The wetland that was covered by the
greenhouse showed that although the plant growing period was extended by 16 weeks, no
difference in the pollutant removal performance was noted.
Lincoln Alexander Parkway – Dartnall Road interchange. This facility (Farrell and
Scheckenberger, 2003) is located in Hamilton, Ontario, and forms part of the Red Hill
Creek watershed. Runoff is collected from a 23 ha drainage area and flows through three
wetland cells (1050 m3 permanent wet pool) designed for extended detention. A total of
five years of water quality monitoring were performed (three major storm events were
sampled each year). Performance of the facility was found to be adequate for the
protection of aquatic life downstream, with good removal for TSS, TP, Cu, Zn and E. coli
(64%, 44%, 56%, 40% and 68% removal respectively).
Edmonton International Airport. This facility (Higgins and MacLean, 2002) was
constructed in 2000 near the City of Edmonton, Alberta, to treat winter airport runoff
laden with aircraft deicing fluids (glycol). Discharges of such runoff cause great concern,
because glycol creates a high biochemical oxygen demand and can cause severe
degradation of the receiving waters. Only 10–20% of the deicing fluid can be recovered
on-site, the rest runs off into stormwater management facilities. This situation is
particularly unique to cold climates. Because the site was located at an airport in a cold
climate, a SSF wetland was selected to reduce the use of the wetlands by birds and to
extend the treatment season and efficiency as much as possible. One of the adaptations
required to accommodate the severe cold was to store the runoff over the duration of the
winter and release it for treatment in warmer months. The facility is a 2.7 ha wetland
(6 parallel cells in stage 1 and 6 parallel cells in stage 2) with a gravel substrate
0.7 m deep. Results indicate that the wetland can treat up to 1500 m3/day of glycol
contaminated runoff and still meet the 25 mg/L BOD5 limit.
Stanley Park, Vancouver. This facility (GVRD, 2002) receives runoff from the Stanley
Park causeway, with a traffic volume of 65,000 vehicles per day. It was designed to treat
runoff from the highway as well as provide suitable habitat for birds and other wildlife.
Nature trails and viewing areas have also been incorporated to encourage public access.
The treatment train consists of a sediment forebay, marsh areas, deep pools and wetland
vegetation. A limited survey of water quality performance was conducted and showed
that the facility was able to remove 80% of TSS and 90% of Cu and Zn. Long-term
monitoring is planned for the future.
CSWWs are well established in the Canadian stormwater management practice as one of
the measures facilitating high removals of pollutants from stormwater and providing
aesthetic and recreational amenities. Sufficient technical guidance is available for
designing these facilities for water quality treatment, by providing treatment volumes
between 6 mm/ha and 14 mm/ha in Ontario; in other Canadian climates, different
volumes may be required. Perhaps the greatest disadvantage of CSWWs in stormwater
Controlling urban stormwater pollution by constructed wetlands 227
management is the high land requirement. While it is acknowledged that CSWWs
perform less effectively in cold weather, measures have been developed to keep CSWWs
operational through the winter months, sometimes in hybrid installations with wet ponds.
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concerns have been raised about the quality of wildlife habitat in CSWWs in Canada and
the underlying issues are being addressed. In particular, protocols have been proposed for
monitoring Canadian CSWWs and their impact on wildlife and reducing the risk of
such impacts by enhanced maintenance and control of sources of pollutants, and by
reducing pollutant accumulations in CSWWs. Other perceived problems with CSWWs in
Canada (odour, mosquitoes, weeds, etc.) were caused by lack of experience with
planning, designing and operating these systems and lack of public education about
design objectives of CSWWs and wetland ecology. CSWWs have now gained
widespread public support in Canada and increasingly become part of the urban
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