Stormwater is rain and snowmelt that flows over land surfaces instead of soaking into the ground. In natural landscapes with soil and vegetation, most precipitation absorbs into the earth. But on paved streets, parking lots, rooftops, and other hard surfaces, water has nowhere to go but across the surface, picking up pollutants along the way and eventually draining into rivers, lakes, and coastal waters. That runoff, and the system of pipes and channels that carries it, is what cities and environmental agencies mean when they talk about stormwater.
Why Hard Surfaces Change Everything
The core problem with stormwater is impervious surfaces. Concrete, asphalt, and roofing don’t absorb water the way soil and root systems do. In a natural watershed, most rainfall infiltrates the ground, recharging groundwater and feeding streams slowly over time. As a landscape becomes more developed, that balance shifts dramatically. More water runs off the surface, less soaks in, and streams receive larger, faster surges during storms.
This isn’t a minor shift. Along just the western coast of the United States, researchers estimate that hundreds of billions of kilograms of suspended solids wash off land surfaces into the Northern California Current system each year. Those solids include silt, decaying organic matter, vehicle exhaust particles, pavement wear, oils, building materials, and paint residue. Every new parking lot or subdivision that replaces open land accelerates this pattern.
What Stormwater Carries With It
Stormwater looks like ordinary rainwater running down a street, but by the time it reaches a storm drain it contains a surprisingly complex mix of contaminants. The most commonly reported pollutants are nutrients (especially phosphorus), heavy metals, bacteria, and compounds from burned fuel and vehicle exhaust. Metals like lead, copper, zinc, mercury, cadmium, and arsenic show up regularly in urban runoff samples. Phosphorus alone has been measured at concentrations up to nearly 800 micrograms per liter, high enough to trigger algae blooms in receiving waters.
Beyond those well-known pollutants, stormwater also carries pesticides from lawns and landscaping, including common herbicides, fungicides, and insecticides like neonicotinoids. Researchers have detected pharmaceuticals such as the diabetes drug metformin, acetaminophen, and antibiotics. Caffeine and nicotine appear frequently. Industrial chemicals show up too: corrosion inhibitors, plastic-related compounds like bisphenol A (detected in 90% of samples in one study), and wood preservatives. The sheer variety of contaminants makes stormwater one of the most chemically diverse pollution sources flowing into American waterways.
Storm Sewers vs. Sanitary Sewers
One of the most important things to understand about stormwater is that it typically travels through a completely separate pipe system from your household wastewater. Most cities maintain two distinct underground networks. The sanitary sewer collects water from your bathroom, kitchen, and laundry, then routes it to a treatment plant where it’s cleaned before being discharged. The storm sewer collects rain and snowmelt from streets, rooftops, and parking lots through those metal grates you see along curbs.
Here’s the critical difference: water in the storm sewer system generally receives little or no treatment before it empties directly into local rivers, streams, and coastal waters. When someone dumps motor oil in a street gutter or over-applies fertilizer on a lawn, those pollutants flow through storm drains and straight into natural waterways. The roughly 7,250 permitted municipal storm sewer systems across the country are required under federal law to implement pollution prevention measures, but the infrastructure itself is designed to move water quickly, not to clean it.
Health Risks of Contact With Stormwater
Swimming, wading, or fishing in waters heavily affected by stormwater runoff carries real health risks. The primary concern is gastrointestinal illness caused by pathogens. Fecal bacteria from pet waste, wildlife, and leaking sanitary sewers mix into runoff and end up in recreational waterways. Epidemiological studies have consistently found a relationship between stormwater exposure and waterborne illness, particularly at beaches and swimming areas near storm drain outfalls.
Specific pathogens of concern include Campylobacter (a common cause of food poisoning), Giardia (which causes severe diarrhea), and various viruses that persist in water longer than bacteria do. One study found infection risk as high as 100% for a population regularly ingesting Giardia-contaminated stormwater. Eating fish from stormwater-contaminated waterways also poses risks from accumulated heavy metals. These aren’t hypothetical dangers. They’re documented in studies across the U.S., Australia, and the Netherlands.
Environmental Damage to Waterways
Beyond human health, stormwater degrades the ecosystems it flows into. Large volumes of fast-moving runoff erode stream banks and scour streambeds, destroying habitat for fish and aquatic insects. Suspended sediment smothers the rocky bottoms where many species lay eggs. Nutrient loads from fertilizers feed explosive algae growth, which depletes oxygen in the water and can kill fish.
Temperature is another factor. Water that flows across sun-baked pavement in summer arrives at streams significantly warmer than natural groundwater would, stressing cold-water species like trout and salmon. Heavy metals and organic chemicals accumulate in sediment over time, creating long-term contamination that persists even if new pollution is reduced.
How Cities Manage Stormwater
Federal regulations under the Clean Water Act require cities and construction sites to obtain permits for stormwater discharge. These permits mandate comprehensive management programs that include pollution prevention, monitoring, and treatment or removal techniques. But traditional stormwater management, which relies on pipes and concrete channels to move water away as fast as possible, is increasingly being supplemented by a different approach called green infrastructure.
Green infrastructure works by mimicking natural processes. Rather than rushing water into pipes, these systems slow it down, filter it, and let it soak into the ground. Common examples include:
- Rain gardens: Shallow, planted depressions that collect runoff from rooftops and sidewalks, then filter it through layers of soil, sand, and gravel.
- Bioswales: Vegetated channels along roads that slow water with plants and mulch, filtering pollutants as runoff flows through. They’re especially well suited for highways and residential streets.
- Permeable pavement: Concrete, asphalt, or interlocking pavers designed to let water pass through the surface and into gravel layers below. One project in Sultan, Washington eliminated over $260,000 in construction costs by using pervious concrete instead of traditional pavement with separate drainage.
- Green roofs: Rooftop vegetation that absorbs and stores rainfall, then releases it slowly through evaporation, reducing the volume entering storm drains.
What You Can Do at Home
Individual properties contribute to stormwater runoff, and small changes add up across a neighborhood. Rain barrels or cisterns attached to your downspouts capture roof runoff and store it for later use watering your garden or lawn. This keeps water out of the storm system during heavy rains when it matters most.
Replacing sections of your driveway or walkway with permeable pavers lets water infiltrate instead of running off. Even simpler, directing downspouts onto lawn areas rather than driveways gives water a chance to soak in naturally. Planting a small rain garden in a low spot on your property creates a natural filter for runoff. Reducing fertilizer and pesticide use on your lawn directly cuts the chemical load washing into storm drains. And keeping leaves, grass clippings, and pet waste out of the street prevents them from becoming part of the next storm’s pollution load.