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Stormwater Pollution: What You Need to Know

Don't let the bad stuff down your drains.

Editor’s note: Welcome to our series on Stormwater Management! We hope you find these PIG-exclusive articles to be helpful in explaining the basics of stormwater regulations and what you can do to help protect your storm drains.

Understanding stormwater pollution can help you manage your stormwater plan and effectively monitor what’s going down your storm drains. Use the links below to jump to different sections of this article that apply to you and your facility:

Table of Contents:

What is Stormwater Pollution?

Why is Stormwater Pollution a Problem?

What Causes Stormwater Pollution?

What Governs Stormwater Pollutant Discharges?

Types of Stormwater Pollutants

What is Stormwater Pollution?

Stormwater pollution is anything that is picked up and carried by stormwater runoff that can be considered harmful to rivers, lakes, streams and other natural water resources. Sometimes the pollutants are visible and obvious, like trash and debris, but other forms of pollution can’t be seen at all, such as oils, chemicals and dissolved heavy metals.

Why is Stormwater Pollution a Problem?

Stormwater pollution is a problem because without safeguards in place, any pollutants that enter a storm drain can directly harm the environment. This can result in violations of the EPA’s Clean Water Act and other stormwater regulations. On their own, rainwater and snowmelt usually don’t cause many problems except in areas prone to flooding. But unlike wastewater that is treated before being discharged, stormwater is not treated before it enters streams, rivers, lakes or coastal waters.

What Causes Stormwater Pollution?

Stormwater pollution can sometimes be tracked to industrial and municipal sources, which the Environmental Protection Agency (EPA) collectively calls “point sources.” Some examples of contaminants that can cause stormwater pollution are dredged soil, solid waste, incinerator residue, sewage, garbage, sewage sludge, munitions, chemical wastes, biological materials, radioactive materials, heat, wrecked or discarded equipment, rock, sand, cellar dirt and industrial, municipal and agricultural waste. (By law, a pollutant is not sewage from vessels or discharges incidental to the normal operation of an Armed Forces vessel, or certain materials injected into an oil and gas production well.)

Pollution from residential and other sources is called “non-point source pollution” and includes pollutants generated from activities such as improperly disposed residential wastes, non-commercial vehicle washing, lawn fertilizing and gardening.

The EPA does not have jurisdiction over homeowners or residential properties, so pollution caused by non-point sources often goes unchecked and continues to contribute to water impairment. The EPA does govern stormwater pollutant discharges from industrial and municipal point sources, significantly limiting the pollutant loads from these sources.

What Governs Stormwater Pollutant Discharges?

The Clean Water Act is the body of regulation that governs pollutant discharges into the nation’s waters. Pollutants impair water quality by making the water unfit for its intended purposes. These purposes vary, but they can include drinking water, process water for industrial use, swimming and recreation uses and suitable habitat for aquatic life.

Each state determines the uses for each of their waterbodies, as well as the effluent limits they will enforce to prevent the waters from becoming more polluted and those that will improve each impaired waterbody.

Types of Stormwater Pollutants

The EPA’s definition of stormwater pollutants is very encompassing, but pollutants can widely be grouped into three categories: conventional, toxic and non-conventional.

Conventional Pollutants

Conventional pollutants are a group of water pollutants that municipal sewage treatment facilities are able to treat and remove. This helps to prevent them from causing human health problems in drinking water. However, because stormwater is not treated in sewage treatment plants, conventional pollutants can cause a variety of water quality issues in receiving waters.


Toxic and Priority Pollutants

While publicly owned treatment works (POTWs) and sewage treatment plants are able to remove conventional pollutants from water, most are not able to remove toxic or priority pollutants. When this is the case, they pass through water treatment systems and enter larger waterways where they accumulate in the sediment at the bottom of the waterbody or attach to solids that are suspended in the water.

Although some toxic and priority pollutants can originate from non-commercial sources, most are exclusive to industrial processes, such as paint and dye manufacturing, mining, smelting and finishing and plating of metals.

The EPA has developed a list of 65 toxic pollutants [40 CFR 401.15] and 129 priority pollutants [40 CFR 423, Appendix A] that accumulate in water and are detrimental to both human and aquatic health. Below are some examples of these pollutants:


Non-Conventional Pollutants

Other water pollutants that aren’t conventional, toxic or priority are grouped as non-conventional pollutants. Like conventional pollutants, they can stem from industrial, municipal and residential sources.


Whether they rob the water of oxygen, bio-accumulate in aquatic animals, cause algae blooms or otherwise compromise water quality, pollutants that enter stormwater jeopardize the nation’s water resources. The tight controls that are placed upon industrial and municipal point sources through National Point Source Discharge Elimination System (NPDES) Permits are one of the methods used to control and prevent further damage to our nation’s waters.

Some Best Management Practices to help meet these goals include drain covers to temporarily seal off a drain; storm drain filters and filtration “socks” for passive protection that still allows clean water to flow; and absorbent spill kits for fast response to emergency spills.

Keep reading our Stormwater Management Series for more expert advice and storm drain solutions from New Pig.

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