Stormwater Runoff


What is Stormwater Runoff?

Stormwater runoff is the water which does not soak into the soil but rather flows off lawns, streets, paved areas, rooftops, and farm fields during and after a rainstorm. As the water flows across lawns, streets, parking lots, and other surfaces it carries salt, sand, nutrients, pesticides, fertilizers, vegetative debris, oil, grease, litter, and many other pollutants, some possibly toxic, into nearby waterways. Since these pollutants are carried from a wide area by stormwater runoff and cannot be traced to a single pipe, a single point or source, they are called nonpoint source pollutants.

How Does Stormwater Runoff Affect Wisconsin Lakes?

Stormwater runoff affects the water quality of lakes, rivers and streams. The runoff carries sediments, nutrients, and toxins. Each of these nonpoint source pollutants will deteriorate water quality.

Sediment washing into a lake or stream settles out and begins to fill in the basin. Before settling out, sediments will make the water appear cloudy or turbid. High turbidity affects the aesthetics of the body of water as well as the survival of fish and various aquatic plants.

When phosphorus, a nutrient, is delivered to a lake by either urban or rural runoff the growth of algae and aquatic plants in the lake will increase. Algae and aquatic plants are important in providing habitat for fish and wildlife. However, rapid and excessive growth of algae and aquatic plants can deteriorate water quality and can impare recreational enjoyment.

Toxic chemicals such as lead, from gas and auto exhaust, zinc from roof drains and tires, insecticides, herbicides, and other pesticides are carried by stormwater runoff in both urban and rural areas. These materials can affect the "health" of fish and other forms of aquatic life living in lakes and streams.

What Happens to Rainfall Runoff in Developed Areas?

Rainfall runoff is a major problem in many developed areas. This is because a large percentage of land surface in developed areas is covered by buildings and pavement which collect and channel pollutant laden stormwater. Newly developed areas are ususally provided with storm sewers to handle stormwater runoff. Stormwater control ordinances which require that design controls be incorporated into new projects should be adopted by communities.

What are Storm Sewers?

Storm Sewers are pipes laid underground, often below streets, than convey surface water runoff into nearby lakes or streams. Intakes or drains located along curbs and in parking areas collect the runoff water into the pipes for quick transport into the receiving water. A common misperception is that all the water running off streets into a surface collector goes into a sewage treatment plant. It does not. Stormwater usually receives no treatment. Whatever runs off lawns, streets, and parking lots flows directly into lakes and streams, carrying pollutants with it.

What are the Goals of Stormwater Management?

The goals of stormwater management are to:

slow down water flow, lessening soil erosion
encourage runoff's infiltration into the ground, reducing the amount of stormwater that reaches lakes and streams
keep pesticides, oil, and other pollutants off the ground where they can be washed away

How Can You Reduce Stormwater Runoff in Your Community?

The illustration below shows both good and bad stormwater management practices. The good practices are designed to slow up the runoff, encourage water to soak into the ground, and reduce the availabiltiy of polluttant sources. Reducing stormwater runoff will result in cleaner neighborhoods and cleaner lakes.

Other stormwater management practices are non-structural such as:

do not dispose of grass clippings, used motor oil, flushings from radiators, pet wastes, household toxic wastes, etc., by placing them into the gutters or storm sewer inlets
anti-litter ordinances and educational programs
erosion controls included in building codes and subdivision regulations
frequent trash removal and street cleaning
cleaning of catch basins and sewer pipes
controls on herbicide and pesticide usage

We All Pay for Poorly Managed Stormwater

Poorly managed urban stormwater will cost a community both in dollars and environmental damage. Poorly managed stormwater runoff can cause:

flooding of lakes, streams, streets, and homes
erosion of roadbeds, stream banks, and beaches
pollution affecting the quality of lakes, streams, and drinking water.

To reduce costs to a community, stormwater management measures should be included in the design and construction of new developments.

Origins of Nonpoint Source Pollutants

Automotive Traffic
Heavy metals such as lead
Acid-making substances

Construction Activities
Asphalt and paint
Oil and cleaning solvents

Green Space
Farm fields

Airborne Fallout
Smokestack debris
Coal dust
Acid-making substances

Public Refuse
Animal and plant refuse
Street debris

Good Practices

rainwater directed to a grassed area
grassed waterway; allows water to infiltrate
grassed areas

Bad Practices

rainwater directed to a paved area
curb and gutter directed to storm sewer
runoff directed to a lake or stream
parking lot runoff directed to storm sewers


During the last twenty years, urban areas have invested billions of dollars in new wastewater treatment facilities to control water pollution. Despite this effort, many of our local lakes and streams are still plagued with pollution and cannot be used for fishing and swimming. Why? The answer lies in the ways we use our land and in the aftermath of a storm.
When rain falls or snow melts, the runoff washes pollutants off our streets, parking lots, construction sites, industrial storage yards, and lawns. Urban runoff carries a mixture of pollutants from our cars and trucks, outdoor storage piles, muddy construction sites and pesticide spills. Efficient systems of ditches, gutters and storm sewers carry the polluted runoff to nearby lakes and streams, bypassing wastewater treatment systems.

One way of cleaning up polluted urban runoff is to install stormwater treatment facilities. Another less expensive method is to keep pollutants out of runoff.

Keeping It Clean

Keeping pollutants out of stormwater runoff is less expensive than installing stormwater treatment facilities. Here are some ways that you can help prevent stormwater pollution:


Recycle oil
Direct downspouts to lawns
Sweep paved areas to keep waste out of stormwaters
Keep you car tuned, repair leaks
Limit fertilizer and pesticide use, leave grass clippings on lawn
Clean up pet waste
Dispose of toxic wastes properly
Wash your car on your lawn or at a car wash


Enforce construction site erosion control laws
Enact laws requiring stormwater management in new development
Develop and implement a comprehensive stormwater management plan
Sponsor household hazardous waste collections

The potential payoff from better land management practices is high, promising healthier waters, quality water recreation close to home and riverfront development possibilities.

From Streets to Streams

Urban runoff is a relatively recent concern, but it is not an insignificant issue. Although we have less urban area than rural area in Wisconsin, urban areas have more impervious surfaces. That means more water runs off instead of soaking in, and more enters lakes and streams unfiltered by soil or vegetation.

Some of the pollutants found in urban runoff are similar to pollutants found in rural runoff. These are the "conventional" pollutants -- sediment, nutrients, oxygen- demanding materials, and bacteria. Urban areas on a per-acre basis deliver as much or more of these conventional pollutants as rural areas.


Like rural runoff, urban runoff is loaded with sediment. Cities may have less soil erosion than rural areas, but urban areas produce their own distinctive mix of sediment -- flakes of metal from rusting vehicles, particles from vehicle exhaust, bits of tires and brake linings, chunks of pavement, and soot from residential chimneys as well as industrial smokestacks.

The leading sources of sediment in existing urban areas are industrial sites, commercial development, and freeways. But by far the highest loads of sediment come from areas under construction. The Wisconsin Department of Natural Resources (DNR) estimates that an average acre under construction delivers 60,000 pounds (30 tons) of sediment per year to downstream waterways, which is much more than any other land use.

Two factors account for the large amount of sediment coming from construction sites -- high erosion rates and high delivery rates. Construction sites have high erosion rates because they are usually stripped of vegetation and topsoil for a year or more. Typical erosion rates for construction sites are 35 to 45 tons per acre pre year as compared to 1 to 10 tons per acre per year for cropland.

Even more importantly, construction sites have very high delivery rates compared to cropland. During the first phase of construction, the land is graded and ditches or storm sewers are installed to provide good drainage. This also provides an efficient delivery system for pollutants. Typically, 50% to 100% of the soil eroded from a construction site is delivered to a lake or stream, compared to only 3% to 10% of the soil from cropland delivered to lakes or streams.


Runoff from both urban and rural areas is loaded with nutrients such as phosphorus and nitrogen. Phosphorus is the nutrient of greatest concern because it promotes weed and algae growth in lakes and streams. Like sediment, phosphorus concentrations are lower in urban runoff than in rural runoff, but annual phosphorus loads per acre are at least comparable to rural areas.

Because phosphorus compounds attach to soil particles, areas with high sediment loads also produce high phosphorus loads. This means that construction sites are significant sources of phosphorus as well as sediment. Other sources of phosphorus inculde fertilizer spills, leaves and grass left on paved areas, and orthophosphate in vehicle exhaust.

Oxygen Demanding Material

Urban runoff carries organic material such as pet waste, leaves, grass clippings and litter. As these materials decay, they use up oxygen needed by fish and other aquatic life. Shallow, slow-moving waterways are especially vulnerable to fish kills caused by oxygen demand from the organic materials in urban runoff. Indeed, the surge of oxygen demand after a storm dumps organic waste into an urban waterway can totally deplete its oxygen supply. Runoff from older residential areas (with more pavement, more pets, and combined storm and sanitary sewers) carries the highest load of oxygen demanding materials.


The levels of bacteria found in urban runoff almost always exceed public health standards for recreational swimming and wading. Generally, fecal coliform bacteria counts for urban runoff are 20 to 40 times higher than the health standard for swimming. Research shows these high levels of bacteria are typical of runoff from small as well as large cities in Wisconsin. Sources of bacteria in urban runoff include sanitary sewer overflows, pets, and populations of urban wildlife such as pigeons, geese, and deer.

Toxic Pollutants

One of the special challenges of urban watersheds is toxic pollution. Toxic pollutants are substances that may cause death, disease, or birth defects or that may interfere with reproductions, child development or disease resistance. According to DNR studies, the toxic pollutants of most concern in urban runoff are metals, pesticides, polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs).


Metals are the best understood toxic pollutants in urban runoff. They are extensively monitored as part of the National Urban Runoff Program during the early 1980s. Recent data from Wisconsin cities confirms that runoff from small as well as large cities is contaminated with metals such as lead and zinc.

Lead has historically been used as an "indicator" for other toxic pollutants in urban stormwater because it is relatively easy to monitor and its dangers are well documented. Lead is a problem for both humans and aquatic life. Its human health effects include damage to the nervous system and kidneys, high blood pressure and digestive disorders.

Lead can also be toxic to aquatic life. Wisconsin monitoring shows that about 40% of the samples from storm sewer discharges in primarily residential area and 70% of the samples from a commercial area had lead levels high enough to kill aquatic life. Although lead levels still exceed water quality standards, they are much lower today than they were before the shift to unleaded gasoline.

Zinc is another metal in urban runoff which commonly violates water quality standards. While zinc does not create human health problems, it can be toxic to aquatic life. In fact, zinc is even more likely than lead to exceed levels that kill aquatic life.