Learn what stormwater is, how it differs from wastewater, and the pollutants it carries. This guide covers federal regulations and management practices for controlling runoff.
Stormwater is rain and snowmelt that flows over land or impervious surfaces like paved streets, parking lots, and building rooftops without soaking into the ground. It becomes a pollution problem because it picks up oils, metals, nutrients, and bacteria as it runs over these surfaces and discharges directly into waterways without passing through a treatment plant.
What is stormwater?
Stormwater runoff is generated from rain and snowmelt events that flow over land or impervious surfaces, such as paved streets, parking lots, and building rooftops, and do not soak into the ground. This water is distinct from groundwater, which sits below the surface, and wastewater, which comes from indoor fixtures. The U.S. Environmental Protection Agency defines stormwater specifically as runoff that does not percolate into the soil.
How does stormwater form?
In undeveloped areas, precipitation typically soaks into the ground. When buildings, parking lots, and roads are added to the landscape, the ground cannot absorb the water. Instead, water from rain or snow storms flows over streets, parking lots, and roofs. Urban and suburban areas produce much more stormwater runoff due to the high amount of paved and hard surfaces. This excess water moves toward storm drains or nearby water bodies.
What pollutants does stormwater carry?
Runoff can pick up and deposit harmful pollutants like trash, chemicals, and dirt or sediment into streams, lakes, and groundwater. Construction sites, lawns, improperly stored hazardous wastes, and illegal dumping are all potential sources of stormwater pollutants. Stormwater runoff is often worsened by human activities and can contain nitrogen and phosphorus pollutants from fertilizers, pet waste, and yard waste. Because stormwater flows over hard surfaces directly into a water body or storm drain, there is no opportunity for soil and plants to filter out these pollutants.
| Pollutant Type | Common Sources in Runoff |
|---|---|
| Hydrocarbons and Oils | Vehicle leaks, industrial storage yards, fueling areas |
| Heavy Metals | Brake dust, roofing materials, metal fabrication sites |
| Nutrients | Lawn fertilizers, pet waste, yard waste |
| Bacteria and Pathogens | Pet waste, wildlife, failing septic or sewer connections |
| Sediment and Debris | Construction sites, eroded soil, litter |
How is stormwater different from wastewater?
Unlike sanitary sewers that collect wastewater from homes and businesses and convey it to a wastewater treatment plant, storm sewers are separate pipes that collect storm water runoff from inlets, catch basins, or drains located along street curbs and in parking areas. Most storm water does not go to a wastewater treatment plant for treatment. Water that flows down driveways and streets into a gutter or ditch is not treated and flows directly to a lake, river, or stream. The pollutants in the stormwater are discharged to the receiving water too.
Where does stormwater go after it enters a drain?
Usually, storm water runoff flows overland or is discharged by storm sewers into nearby waterways or treatment devices. Waterways may be stream beds, rivers, or lakes. Treatment devices include rain gardens, swales, infiltration practices, wet ponds, and bioretention systems. Anything that enters a storm sewer system is discharged into the waterbodies used for swimming, fishing, and providing drinking water. In most cases, storm water flows directly to water bodies through sewer systems, contributing a major source of pollution to rivers, lakes, and the ocean.
How does stormwater affect water quality and flooding?
Polluted stormwater runoff can have many adverse effects on plants, fish, animals, and people. Fast-moving stormwater runoff can erode stream banks, damaging aquatic habitat. Increased development across watersheds has made stormwater a significant source of pollution to bays and their rivers. During periods of heavy rainfall or snowmelt, some wastewater systems are designed to occasionally overflow and discharge excess untreated sewage directly to nearby streams, rivers, or other water bodies. These discharges are known as combined sewer overflows and are common in many cities nationwide.
What federal rules regulate stormwater?
The NPDES stormwater program regulates some stormwater discharges from three potential sources: municipal separate storm sewer systems (MS4s), construction activities, and industrial activities. Operators of these sources might be required to obtain an NPDES permit before they can discharge stormwater. This permitting mechanism is designed to prevent stormwater runoff from washing harmful pollutants into local surface waters. Most stormwater discharges are considered point sources and require coverage under an NPDES permit.
What are MS4 and industrial stormwater permits?
MS4s are municipal separate storm sewer systems that collect stormwater from public streets and roads. Industrial facilities under NPDES must also manage their stormwater discharges. In Arizona, for example, operators submit for coverage under general permits for MS4s and construction activities. The operator shall develop a Stormwater Pollution Prevention Plan before submitting the Notice of Intent for permit coverage and prior to conducting any construction activity. These plans must identify the responsible person for on-site implementation and assure compliance with permit terms.
How do you manage and treat stormwater?
The primary method to control stormwater discharges is the use of structural and non-structural best management practices. Green infrastructure works by slowing down the runoff, spreading it out over the land, and slowly soaking it into the ground. Some examples of green infrastructure techniques include rain gardens, pervious pavement, rain barrels, and green roofs. Structural controls like catch basins and detention ponds manage volume and capture solids. For sites requiring advanced treatment, filtration media can be integrated into existing infrastructure to target specific contaminants.
Why do sediment controls miss dissolved pollutants?
Standard catch basins and screens capture particulate matter like sediment and trash, but dissolved metals, hydrocarbons, and bacteria pass through with the water. Sediment control only handles the particulate share of pollutants. Whatever rides on those solids can drop out with them, but dissolved contaminants pass through to the outfall. Treating these requires media or processes that target the dissolved fraction. For example, bacteria removal from stormwater requires treatment matched to specific indicator organisms, as sediment control alone does not stop pathogens from reaching a permitted outfall. Similarly, arsenic removal from stormwater depends on whether the arsenic is dissolved or particulate, because sediment control only handles the particulate share.
If your site discharges to a permitted outfall and you need to address dissolved pollutants, schedule a site assessment to match the right treatment media to your water and infrastructure.