What Is Stormwater Runoff?

No doubt you’ve seen the brightly colored warnings spray-painted near storm drains. They urge people not to dump anything—household cleaners, paint, etc.—through the steel grates because the water below leads to nearby rivers, streams, lakes and oceans. Those grates, of course, serve as sluices, letting stormwater pass through while keeping out leaves, sticks, and trash.


Unfortunately, harmful pollutants still find their way into stormwater. This is not only a big problem for urban and suburban areas where storm drains are often the main conduits, but in rural areas, where stormwater runoff fouls waterways.


But just what is stormwater runoff? In general, it’s rain and snowmelt that moves over land, streets and other hard surfaces that don’t readily absorb water and it flows into streams, lakes and other bodies of water.

Why is Water Runoff a Problem?

As these waters flow they pick up contaminants, such as pesticides, road salt, petrochemicals and animal waste. In rural areas, they also sweep fertilizers into the water, raising nitrogen and phosphate levels to dangerous levels. Stormwater runoff can also remove sediment from riverbanks, harming underwater grasses and shellfish, and causing flooding.


Water Quality Impact

Stormwater runoff has the potential to impact water quality by allowing the flow of potential pollutants, such as bacteria, pesticides, metals, and more into water sources like streams, rivers, or lakes. The increased pollutants can have a diminishing effect on water quality.



Stormwater runoff contributes to the erosion of riverbanks, lakes and coastlines. When sand, silt and clay are swept from the side of lakes and rivers, they are deposited in the water in the form of sediment. Sediment is made up of loose particles of sand, silt and clay.

Sewage Overflow

In approximately 772 cities across the country, stormwater is collected in the same pipes as domestic sewage and industrial wastewater. It’s not a problem much of the time, as the system transports all of this water to a wastewater treatment plant where it is treated and then discharged. But during heavy storms the wastewater volume can overwhelm the system and cause what are known as combined sewer overflows (CSOs). During CSOs, excess wastewater—containing stormwater and untreated human and industrial waste, toxic materials, and debris—is discharged directly to streams, rivers, or other water bodies.

Localized Flooding

Stormwater runoff, if not properly managed, can flood and damage homes and businesses, disrupt traffic, erode stream channels, and damage or destroy fish and wildlife habitat.


Economic Impact

Stormwater runoff contaminates surface water sources of drinking water. The more contaminated the source water is, the more treatment is required. Treatment, of course, costs money and that money is eventually passed along to consumers. Communities that rely on water resources for recreation and tourism trade can also be hurt economically if deteriorated water quality results in bans on swimming and other water contact activities.


Health Impact

In urban areas, animal waste and fertilizer are the two biggest contributors to excess nutrients in stormwater runoff. Excess nutrients cause algae overgrowths, which may be toxic to humans and pets, as well as reduce water oxygen levels for fish and other aquatic organisms. In addition to animal waste, which is carried to the storm system by stormwater runoff, human waste may enter the stormwater system due to aging and failing infrastructure; backed up sewer lines, known as sanitary sewer overflows (SSOs); and inappropriately maintained septic systems. Human and animal waste contribute bacteria and pathogens to stormwater. In excess, these bacteria and pathogens cause illness and result in closing of swimming areas and impairment of streams that limits other recreational use.
What is Urban Runoff?

Stormwater from city streets and adjacent domestic or commercial properties that carries pollutants into the sewer systems is known as urban runoff. Rapid urbanization, where large swaths of land are paved or hardened to an impervious state, has led to increased volumes of urban runoff across the country and around the world. This means more pollutants are entering our waterways and entering more quickly. According to the U.S. EPA, rapid urbanization and its attendant runoff have degraded water quality and habitat in virtually every urban stream system.


Because of impervious surfaces like pavement and rooftops, a typical city block generates more than five times more runoff than a woodland area of the same size.


The EPA encourages developers and urban planners to use low-impact development measures that conserve natural area, such as riparian, or riverbank, buffers. For existing urban areas, planners should first seek out opportunities to prevent pollutants from entering runoff streams, the EPA suggests.

What Are The Effects of Urbanization and Runoff?

As impervious surfaces cover more land, stormwater has less opportunity to enter the ground and get soaked up by vegetation. Rather, it gets more quickly to waterways carrying with it unnaturally high levels of organic matter that lead to algae blooms and to eutrophication -- the process in which plant life thrives and animals die, due to a lack of oxygen. 

There are other problems, as well. Storm sewer systems concentrate runoff into smooth, straight conduits, which accelerates as it travels underground – so when it eventually exits storm drains into a stream, the excessive volume and power blast out streambanks, damaging streamside vegetation and wiping out aquatic habitat. Also, these increased storm flows often carry higher water temperatures, for example from paved surfaces and rooftops, that are harmful to the health and reproduction of aquatic life. 

The loss of infiltration from urbanization may also cause profound groundwater changes. Although urbanization leads to great increases in flooding during and immediately after wet weather, in many instances it also results in lower stream flows during dry weather. Many native fish and other aquatic life cannot survive when these conditions prevail.

What Are The Differences Between Storm Drains And Wastewater Sanitary Sewers?

A sanitary sewer system channels wastewater (for example, from toilets and washing machines), while a stormwater collection system channels rainwater runoff and snow and ice-melt. In some municipalities, particularly with older systems, stormwater is collected and discharged into the sanitary sewer system. These systems, known as combined sewer systems (CSOs), transport the combined stormwater and wastewater to the sewage treatment plant. However, during heavy rainfall or snowmelt the volume can exceed the system’s capacity and lead to CSOs, in which untreated wastewater is discharged into rivers and streams.

How Can Businesses Reduce Stormwater Runoff Pollution?

There are simple things you can do as a business owner to reduce the pollution entering the storm system. You can:

  • Sweep outdoor areas and throw debris into the trash can, not into a catch basin. Do not use detergents or chemical cleaners to hose down areas and be sure to direct the runoff away from the catch basin
  • Put trash cans and cigarette receptacles in highly trafficked, visible areas including employee break areas
  • Keep the area around dumpsters clean and the lids closed. Place them away from catch basins. Make sure the clean-out plug is properly secured to prevent leaking. Have the containers cleaned on a regular basis and store spill equipment on-site to clean up spills and releases
  • Ensure hazardous materials are disposed of properly
  • Pour wash water into a janitorial sink or a floor drain that is connected to the sanitary sewer system. Never pour wash water onto a parking lot or into catch basins
  • Install and maintain grease traps and never pour cooking oil, grease or fats into sinks, floor drains, solid waste containers or catch basins

How Can Municipalities Reduce Stormwater Runoff?

Of course, the first things municipalities can do to reduce stormwater runoff and attendant pollution is to follow the guidance and regulations set forth in the National Pollutant Discharge Elimination System (NPDES). The NPDES regulates some stormwater discharges from three potential sources: municipal separate storm sewer systems (MS4s), construction activities, and industrial activities. The MS4s—of which there are approximately 7,250 permitted MS4s nationwide, according to the EPA—spell out in detail what municipalities must do to reduce stormwater runoff and minimize pollution. The NPDES MS4 permit requires municipalities to develop and implement a comprehensive Storm Water Management Program (SWMP), including pollution prevention measures, treatment or removal techniques, monitoring (e.g., SUEZ’ Smart Manhole Covers), use of legal authority, and other appropriate measures to control the quality of discharged stormwater. The EPA’s NPDES guidelines can be found here.

MS4s generally have six Minimum Control Measures (MCMs):

  • MCM 1: Public Education and Outreach
  • MCM 2: Public Involvement and Participation
  • MCM 3: Illicit Discharge Detection and Elimination (IDDE) Program
  • MCM 4: Construction Site Stormwater Runoff Control
  • MCM 5: Post-Construction Stormwater Management in New and Redevelopment
  • MCM 6: Municipal Good Housekeeping and Pollution Prevention

Beyond regulation and oversight, municipalities can take proactive steps to reduce stormwater runoff. One of the benefits of these is the use of what is called “green infrastructure.” Section 502 of the Clean Water Act defines green infrastructure as “...the range of measures that use plant or soil systems, permeable pavement or other permeable surfaces or substrates, stormwater harvest and reuse, or landscaping to store, infiltrate, or evapotranspirate stormwater and reduce flows to sewer systems or to surface waters.” (Evapotranspiration is the process by which water is transferred to the atmosphere by evaporation from land and by transpiration from plants.) Green infrastructure reduces and treats stormwater at its source while delivering environmental, social, and economic benefits.


How Can Individuals Reduce Stormwater Runoff?

The U.S. EPA offers these tips for individuals to help them reduce stormwater runoff:

  • Use fertilizers sparingly and sweep up driveways, sidewalks and gutters
  • Never dump anything down storm drains or in streams
  • Vegetate bare spots in your yard
  • Compost your yard waste
  • Use least toxic pesticides, follow labels, and learn how to prevent pest problems
  • Direct downspouts away from paved surfaces; consider a rain garden to capture runoff
  • Take your car to the car wash instead of washing it in the driveway
  • Check your car for leaks and recycle your motor oil
  • Pick up after your pet
  • Have your septic tank pumped and system inspected regularly
Stormwater Runoff Impacts Us All

What Can Recycled Water Be Used For?

Stormwater runoff has an impact on each of us. It affects the food chain. It affects the cost of our drinking water. It affects our health. And it affects our quality of life. Fortunately, we can all play a part in protecting our environment and ourselves from stormwater runoff pollution.

For example, as individuals, we can dispose of trash and harmful liquids properly, and we can plant more trees to soak up storm water. As business owners, we can abide by longstanding guidelines and regulations, and adhere to permitted discharges. We can also employ common sense, knowing that what we pour down storm drains doesn’t magically disappear. For municipal operators, we can adhere to regulations, as well. And we can educate the public through outreach programs about the benefit of stormwater management, including supporting green infrastructure.


Contact a representative to learn more about partnering your business or municipality with SUEZ North America for stormwater management.