Urban Run-Off as a Source of Contamination in Drinking Water

Not too long ago, urban run-off was considered an insignificant contributor to drinking water contamination. The pollution potential of urban runoff was negligible compared to other sources such as industrial effluent discharge, leachate from dumps and landfills, and other historically significant sources of pollution. Within the last few decades these significant waste discharges have been put under regulatory control and safer disposal practices have been instituted. Many contamination sources are now monitored. With these sources contributing less, urban runoff is now recognized as a significant source of contamination to water. Storm water discharge is now regulated under the Clean Water Act in an attempt to curb the impacts of urban run-off.

Urban run-off consists of water that has drained from man-made non-porous surfaces in densely populated areas. These surfaces consist of roads, freeways, sidewalks, roofed structures, parking lots, airports and industrial sites among others. Any form of precipitation and/or irrigation can 'scour' these surfaces thereby washing away the materials on top of and from which the surfaces are made. Urban terrain is non-porous and does not have the ability to filter or biodegrade contaminants like natural soil does.

Suspended sediment is the primary pollutant in urban runoff which also contains oil, grease, pesticides from turf management, road salts, metals, bacteria and viruses, and toxic chemicals from automobiles among others (1).

Chemicals from urban storm water run-off pose a potential threat to human health and an even greater threat to aquatic organisms, according to a nationwide study (2). In this study seventy-seven of the 127 priority pollutants tested for were found in urban run-off. Lead, selenium, and BHC( a pesticide) were of most concern. But, with dilution, public water treatment, and environmental fate processes at work, the potential for harm from these pollutants can be substantially decreased.

The threat to human health by urban runoff is not only due to materials scoured from surfaces, but also from the infrastructure of the sewer system itself. Storm water systems are often combined with sanitary sewer systems enroute to sewage treatment plants. Excessive storm water can cause this joint system to overflow, resulting in sewage contamination of waterways. Urban runoff is usually collected by storm sewers and discharged directly into waterways so, many sources of discharge go uncontrolled and untreated. Combined systems are cheaper, but the potential to harm health is higher. Some systems have diversions to accommodate heavy flow. According to the EPA approximately 20% of the population is served by combined systems (2). Forty-six percent of the population is served by separate systems.

The Clean Water Act provides control over urban run-off and storm water discharges through the National Pollution Discharge Elimination System Permit Program (NPDES) (3) and management programs developed by the states under the Act. The goal here is to protect public health and aquatic life, and to assure that every facility treats waste water. The types of pollutants regulated are:

1. Conventional pollutants such as those found in household waste; sewage, oil and grease, and detergents.
2. Toxic pollutants particularly harmful to life such as organics (pesticides, solvents, PCBs) and metals (lead, silver, mercury, copper, chromium, and others).
3. Nonconventional pollutants which may require regulation such as nitrogen and phosphorus.

Communities and industry can do a lot to curb the impact urban run-off has on our water resources (1). For example:

New development sites can include pollution prevention strategies in their planning:

Established developments can better manage run-off:

Citizens can play a key role:

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1. Environmental Protection Agency, Non-point source pollution fact sheet. URL: http://www.epa.gov/OWOW/NPS/facts/point7.html

2. Nationwide Urban Runoff Program (NURP) Priority Pollutant Monitoring Project, Summary of Findings. 12 Dec 1983. U.S. Environmental Protection Agency, Washington, DC. Available from National Technical Information Service, Springfield, VA 22161.

3. EPA, Office of Water, Permits. URL: http://www.epa.gov/OWM/wm045000.htm

This page was prepared by T.L. Pedersen , UCD EXTOXNET FAQ Team.
June 1997