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Threats to Sources of Drinking Water and Aquatic Ecosystem Health in Canada

11. Urban Runoff

Flooding and erosion impacts of urban runoff can be lessened by effective stormwater management.J. Marsalek,1 M. Diamond,2 S. Kok3 and W.E. Watt4

1Environment Canada, National Water Research Institute, Burlington, ON
2University of Toronto, Department of Geography, Toronto, ON
3Environment Canada, Great Lakes Sustainability Fund, Burlington, ON
4Queen's University, Department of Civil Engineering, Kingston, ON


Current Status

Rainfall and snowmelt in urban areas are converted into urban runoff, which is transported by sewers, drainage channels and streams, and ultimately discharged to receiving waters as urban stormwater in areas serviced by storm sewers or as combined sewer overflows (CSOs) in older areas with combined sewers. During transport, the runoff quality is degraded by various pollutants, materials and thermal energy from the urban environment. CSOs represent a mixture of stormwater, raw municipal sewage and scoured sewer sludge, and their composition is comparable to that of untreated sewage. The pollution of both stormwater and CSOs greatly varies during and between rainfall events, ranging from severe (usually during early phases of runoff, also referred to as the first flush) to low, towards the end of runoff events (Marsalek et al. 1993).

Urban runoff has been recognized as a significant environmental detriment during the past 30 years. Both stormwater and CSO discharges cause physical, chemical, biological and combined effects on receiving waters, either of acute or cumulative nature (Harremoes 1988), and seriously impair beneficial water uses in many locations (House et al. 1993).

In new urban developments, some mitigation of flooding and erosion impacts has been achieved by stormwater management practiced during the past 25 years, and some improvements in water quality have been achieved by stormwater quality enhancement practices over the past decade. However, the long-term performance of stormwater management facilities is uncertain. In older areas, requiring retrofit, hardly any progress has been made. Most Canadian municipalities with combined sewers pursue CSO abatement programs, but progress is relatively slow because of high costs (Chambers et al. 1997).

Almost 80% of Canadians live in urban areas (about 25 million in 2001; Statistics Canada 2000). The process of urbanization and associated activities increase runoff flows and degrade runoff quality. In terms of discharge volume and solids load, urban runoff significantly exceeds those associated with municipal wastewater (Chambers et al. 1997). Just in the Canadian Great Lakes region, urban runoff discharges annually in the order of 105 tonnes of suspended solids, 104 tonnes of chloride, 103 tonnes of oil and grease, and 102 to 103 tonnes of trace metals (Marsalek and Schroeter 1989). Concerns about CSO pollution are similar, with most significant pollutants being pathogens (typically assessed by indicator organisms), solids, oxygen-demanding substances, nutrients and chemicals from small industrial and commercial sources.

The evidence of serious impacts of these discharges was found in one half of the Areas of Concern in the Canadian Great Lakes region, in which stormwater and CSO discharges caused medium-high pollution problems impeding the delisting of these areas (Weatherbe and Sherbin 1994). Elsewhere in Canada, the cities with similar pollution problems include Vancouver, Edmonton, Winnipeg, Windsor, Hamilton, Toronto, Ottawa, Montreal, Quebec City and Halifax. Some examples of urban runoff impacts on water quality, aquatic ecosystems, and human health have been reported in the literature and are listed below.

Impacts on water quality are exerted by combinations of physical, chemical and microbiological factors (Chambers et al. 1997; House et al. 1993):

  • Physical factors include flow (the effects of which are flooding, erosion, habitat washout), sediment (causing habitat destruction, interference with water quality processes, impacts on aquatic life, transport of contaminants), thermal energy (causing thermal pollution, loss of cold water fisheries) and densimetric stratification (causing the impairment of mixing).
  • Chemical factors include biodegradable organics in CSOs (contributing to dissolved oxygen depletion), and nutrients (contributing to eutrophication), trace metals, chloride, POPs, pesticides and hydrocarbons, often occurring in complex chemical mixtures in stormwater and CSOs (contributing to acute and chronic toxicity, and genotoxicity).
  • Microbiological factors include bacteria and viruses of fecal origin in stormwater and CSOs (causing beach closures and contamination of shellfish).

The factors listed above can adversely affect the aquatic ecosystem by alterations of chemical dynamics, energy dynamics, food web (trophic dynamics), dispersal and migration of species, disturbance of ecosystem development, loss of critical species, reduced biodiversity, and reduced genetic diversity (Lijklema et al. 1993).

Impacts on human health can be attributed to: (a) contamination of drinking water sources, e.g., by trace substances (Makepeace et al. 1995), (b) contamination of fish and shellfish by pathogens and trace metals in municipal effluents (Chambers et al. 1997), (c) contamination of recreational waters by fecal pollution due to discharges of CSOs and stormwater during wet weather (Health and Welfare Canada 1992), and (d) provision of breeding grounds for disease vectors (e.g., West Nile virus and encephalitis).

It should be recognized that urban runoff quality and its effects are linked to other water quality issues, as shown in Fig. 1. With respect to pollution sources, urban runoff conveys some municipal sewage (in the case of CSOs) and some pollution from industrial sources (accidental spills, illicit discharges, grey waters). Treatment of stormwater and CSOs produces sediment and sludge, which are often disposed of at landfill sites. Urban runoff conveys POPs, pesticides, EDS, pathogens and microorganisms from various urban sources. Stormwater and CSO discharges may contribute to eutrophication and possibly acidification of receiving waters. Finally, urban runoff is impacted on by climate change, with respect to runoff quantity and its distribution in time and space, runoff quality, and operation of runoff control and treatment facilities.

Figure 1 Linkages between urban runoof and other water quality issues

Fig. 1. Linkages between urban runoff and other water quality issues (dark and grey arrows represent primary and secondary linkages, respectively).

Some appreciation of the relative significance of pollutant and/or associated pollution processes can be obtained from U.S. EPA assessment of water quality conditions in the U.S.A. (US EPA 2000). In impaired rivers and streams, the leading pollutants/processes were ranked as follows: 1. siltation (leading in 13.2% of the 850,000 river miles assessed), 2. pathogens (bacteria) (12.3%), 3. nutrients (10.0%), 4. oxygen-depleting substances (8.1%), 5. metals (7.2%), pesticides (5.3%), 6. habitat alterations (5.2%), and 7. thermal modifications (5.2%). With minor exceptions, all these pollutants/processes are associated with urban runoff. The ranking for impaired lakes was as follows: 1. nutrients (leading in 19.9% of the 17.4 million assessed lake acres), 2. metals (12.1%), 3. silt (6.7%), 4. oxygen-depleting substances (6.3%), 5. suspended solids (4.6%), 6. noxious aquatic plants (3.8%), and 7. excess algal growth (3.6%).


Water and sediment quality, habitat and other uses of urban receiving waters are expected to degrade due to cumulative effects of both controlled and uncontrolled stormwater and CSO discharges from both existing urban areas and future developments.

A long-term demographic trend in urban population, due to overall population increase, and migration from rural to urban areas, will increase demand for water services, including drinking water supply, drainage infrastructure, wastewater management, and protection of receiving waters. Population projections indicate a 5-million increase in the Canadian population in the next 15 years, with most of this increase (80-90%) occurring in urban areas (Statistics Canada 2000). Meeting these demands will become even more challenging, because of increased per capita resource consumption and emissions (e.g., car emissions, heating and air conditioning, personal care products, household and garden chemicals) leading to higher pollution loads and more constituents in stormwater and CSOs. The relative significance of these two diffuse sources is increasing with improved control of point source pollution.

The long-term trend in under-funding of renewal and replacement of drainage infrastructures is expected to continue for at least two decades and will continue to contribute to degradation of receiving waters.

Emerging Issues

It is now recognized that current practices of urban development are not environmentally sustainable with respect to receiving water quality and ecosystem integrity, when assessed on watershed and long-term bases (ASCE 1998; Rijsberman and van de Ven 1999).

The current investment in water planning, management and infrastructure is inadequate to meet the demands of increased population, increased per capita emissions, increased expectations on urban water uses, and increased needs for rehabilitation/ replacement of existing infrastructures. An example of inadequate funding can be documented by the aging infrastructure and insufficient maintenance (maintenance programs and their funding are not well established, and there is limited public accountability for privately owned drainage structures and facilities). Poorly maintained drainage systems represent environmental liabilities.

The lack of research is an emerging issue, because of disparity in the severity of problems and challenges posed by new issues, and the current levels of investment in urban water research.

Examples of these upcoming challenges include:

  • Climate change; projected changes in precipitation and temperature over the life of some drainage structures may lead to their overloading and poor performance, increased erosion and transport of sediment and landscape contaminants by runoff (resulting in adverse impacts on receiving waters), siltation in receiving waters, disturbance of constructed ecosystems (ponds and wetlands), and hydraulic conveyance problems in coastal areas due to rising lake and sea levels.
  • Delayed recognition of impacts (e.g., gradual accumulation of secondary and cumulative impacts in urban developments with and without stormwater control).
  • New chemicals, e.g., environmental estrogen, pharmaceuticals, personal care products, and other substances often found in complex chemical mixtures in surface waters and municipal effluents.
  • Spread of infectious diseases by vectors inhabiting urban wetlands and impoundments.
  • Potential GMO use in urban landscaping and associated effects on the environment.

Impacts on urban waters will also occur due to additional activities outside of urban boundaries. Adding to these stresses are upstream and downstream development and water uses (e.g., boating).

Knowledge and Program Needs

The knowledge requirements span the breadth of fields of science, social science, engineering, planning and management.

The first requirement is a better understanding of urban runoff and associated processes in terms of:

  • Sources, source inventories, pathways and fate of contaminants as well as microbial pollutants in the urban environment.
  • Regional diversity in processes due to climate, surficial geology, urban development practices, etc.
  • Effectiveness of control measures in protection of receiving waters and both aquatic and terrestrial ecosystems, and human health, by policies and source controls, site best management practices (BMPs), community BMPs, and watershed-level measures. This knowledge should contribute to the eventual substitution of effluent criteria with ecological risk assessment of receiving waters.
  • Increased vulnerability of ecosystems by secondary effects of stormwater management measures (risk of contamination of groundwater, heating of ponds and wetlands, release of contaminants from sediments, and risk to aquatic life and wildlife through uptake of contaminants and exposure to pathogens).
  • Cumulative and combined effects of urban effluents on receiving waters and their ecosystems, with respect to intermittent exposures to varying concentrations.

To acquire this understanding as well as to establish time trends, the following data/monitoring needs are identified:

  • Chemical and microbiological descriptors of stormwater and CSOs (deficiencies in the available data--limited data on such constituents as pesticides and nutrients; inorganic and limited trace organic [POPs] data are 20 to 25 years old, since then analytical capabilities have improved and sources of these chemicals have changed; insufficient geographic coverage; and, minimal data are available on CSOs).
  • Status/performance of the existing drainage systems (extending from catchment headwaters to receiving waters).
  • Cumulative long-term impacts of urbanization (e.g., geomorphologic changes and habitat degradation).
  • New chemicals and/or chemicals newly identified under CEPA (EDS in CSOs and runoff, including natural and synthetic hormones and certain industrial chemicals that have been identified in sewage effluents and are capable of estrogenic effects; pharmaceuticals and personal care products; road salts; and, used crankcase oil in runoff).

The second requirement is research on the integrated management of urban water, including better management of runoff and CSOs, in support of total urban water cycle management (Lawrence et al. 1999). Products of this research would include:

  • Sustainability criteria for urban water management.
  • Advancement of pollution source controls in urban areas, including research on roles of public education, awareness and participation in source control programs.
  • Protocols for protection of urban drinking water supplies from all hazards including urban runoff.
  • New approaches to urban development based on a scientific understanding of processes and providing maximum protection of the environment.
  • Adaptive approaches to urban water management, in light of climate change and other uncertainties.

The third requirement is research in support of infrastructure renewal, which would include:

  • Development of national standards for environmentally and economically efficient design and operation of urban drainage systems (considering structure life expectancies and discount rates).
  • Assessment of alternative modes of infrastructure ownership and operation (ownership and asset management, provision of services by the public or private sector, establishment of drainage utilities/agencies, and user service fees).

To deal effectively with the existing and emerging issues in management of urban waters, the Federal Government should provide leadership by initiating and directing research on integrated water quality management. This would entail establishing strategic alliances of all levels of government, academia, the public, NGOs, and the private sector. The proposed research would be based on broad applications of information technology and would provide tools to assist municipalities in implementing sustainable practices in water management. Towards this end, the following two recommendations are made:

  • Develop a knowledge base and tools on urban water processes, data and monitoring, integrated urban water management and planning infrastructure renewal. The progress under this initiative would be accelerated by developing a policy on integrated urban water management, considering the whole watershed and integrating the needs of the urban population and the protection of urban ecosystems.
  • The initiative targeting urban areas should be part of a broader initiative on integrated water quality management, encompassing principal sources, both point (industrial and municipal, landfills) and non-point (agricultural and urban), and principal receiving water stressors (including algal toxins, EDSs, trace metals, nutrients, pathogens and microbial pollutants, pesticides, POPs, GMOs, eutrophication, acidification, and those associated with climate change).


  • American Society of Civil Engineers (ASCE), Water Resources Planning and Management Division and UNESCO International Hydrological Programme IV Project M-4.3 Task Committee on Sustainability Criteria. 1998. Sustainability criteria for water resource systems. ASCE, Reston, Virginia, U.S.A.
  • Chambers, P.A., M. Allard, S.L. Walker, J. Marsalek, J. Lawrence, M. Servos, J. Busnarda, K.S. Munger, K. Adare, C. Jefferson, R.A. Kent and M.P. Wong. 1997. Impacts of municipal wastewater effluents on Canadian waters: a review. Water Qual. Res. J. Canada 32: 659-713.
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  • Health and Welfare Canada. 1992. Guidelines for Canadian recreational water quality. Health and Welfare Canada, Ottawa, ON, ISBN: 0-660-14239-2.
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