A storm drain, often visible as a grated inlet on the side of a street, is designed to efficiently channel rainwater and surface runoff away from roads and developed areas. These systems are primarily engineered to prevent flooding by collecting water that cannot be absorbed by pavement and other impervious surfaces. The water that flows into these drains typically bypasses any treatment facilities, moving directly into the natural environment.
The Primary Destination: Untreated Waterways
The network of pipes beneath the streets funnels the collected water to a discharge point known as an outfall. This outfall is the final exit where the storm drain system releases its contents directly into the nearest natural water body. The receiving waters can be a local river, stream, lake, estuary, or even the ocean, depending on the municipality’s geography.
The design prioritizes the rapid removal of large volumes of water to mitigate flooding risks. This means the water does not go through the filtering, chemical disinfection, or biological processing standard for household wastewater.
Storm Drains Versus Sanitary Sewers
The public often confuses storm drains with sanitary sewers, mistakenly believing that all water entering a drain is treated. Sanitary sewers are a completely separate system of underground pipes designed to carry wastewater exclusively from sinks, toilets, showers, and industrial sources.
Storm drains are part of a separate storm sewer system, or MS4 (Municipal Separate Storm Sewer System), intended only for rainwater runoff. This dual system prevents treatment plants from becoming overwhelmed by the sheer volume of water during a heavy rainstorm.
An exception exists in older urban areas where combined sewer systems (CSOs) are still in use, integrating both storm and sanitary flows into a single pipe. During normal, dry weather, all flow in a CSO system goes to the treatment plant. However, during intense storms, the volume of water can exceed the system’s capacity, causing the mixture of untreated sewage and stormwater to overflow directly into local waterways.
The Environmental Impact of Runoff
Because the water entering storm drains is not treated, any pollutant picked up from streets and yards is carried directly into the receiving water body. This polluted runoff is considered a leading source of water quality impairment across the country. The runoff collects fine sediment, which clouds the water, making it difficult for aquatic plants to grow and destroying habitats by smothering organisms on the bottom.
One environmental concern is the presence of nutrients, primarily nitrogen and phosphorus, contributed by lawn fertilizers and pet waste. These excess nutrients accelerate the growth of algae, leading to dense blooms. When these large algal populations die and decompose, they consume dissolved oxygen in the water, creating “dead zones” that cannot support fish and other aquatic life.
Stormwater carries hydrocarbons (oil and antifreeze) from leaking vehicles, and heavy metals (copper and zinc). A single quart of motor oil, for example, has the potential to contaminate millions of gallons of water. These substances are toxic to aquatic plants and animals and can accumulate in the food chain.
Bacteria and other pathogens from pet and wildlife waste are washed into the drains, creating a public health hazard that can force the closure of swimming areas and beaches after rainfall events. Litter, including plastics and cigarette butts, also enters the system, posing a direct threat to wildlife through entanglement or ingestion.