Stormwater is a ubiquitous part of our environment, often flowing unnoticed across our landscapes. It represents a significant element of the natural water cycle, yet its journey through developed areas transforms its character and impact. Understanding what stormwater is and where it travels helps illuminate its significance, particularly as human development alters natural processes.
What Stormwater Is
Stormwater is defined as any precipitation, including rain, snowmelt, or hail, that flows over land or impervious surfaces rather than soaking into the ground. In natural environments like forests, much of this precipitation is absorbed by the soil, replenishing groundwater. However, in urban and suburban settings, surfaces like rooftops, roads, parking lots, and sidewalks prevent water from infiltrating the soil, causing it to run off. Stormwater is different from wastewater, which is water used in homes and businesses that carries sewage and requires treatment before being released back into the environment. Stormwater, in contrast, is not treated before its discharge.
Where Stormwater Goes
After falling on impervious surfaces, stormwater begins its journey as runoff, flowing across rooftops, streets, and parking lots. It then collects in gutters and flows into storm drains. These storm drains are part of an extensive underground pipe network known as the storm sewer system. Unlike sanitary sewers, which transport wastewater to treatment plants, storm sewers are separate systems that carry stormwater directly to local waterways without any treatment.
This untreated water is then discharged into natural bodies of water such as streams, rivers, lakes, and eventually oceans. In older urban areas, some communities may have combined sewer systems where both stormwater and wastewater flow through the same pipes. During heavy rainfall, these combined systems can be overwhelmed, leading to combined sewer overflows (CSOs) where a mix of untreated wastewater and stormwater is discharged directly into waterways to prevent flooding.
The Environmental Impact of Stormwater
Stormwater runoff has significant environmental consequences because it picks up various pollutants from surfaces as it flows. These pollutants include chemicals like pesticides and fertilizers from lawns, oil and grease from vehicles, trash, sediment, and bacteria from pet waste or failing septic systems. Excess nutrients from fertilizers can lead to algal blooms, which deplete oxygen levels in the water and create “dead zones” that suffocate marine life. Sediment can cloud water, reducing sunlight for aquatic plants and smothering habitats, while heavy metals and other toxic chemicals can poison aquatic organisms and accumulate in the food chain.
Beyond water quality, large volumes of untreated stormwater can cause increased erosion of stream banks, damaging aquatic habitats and altering natural stream flows. It can also contribute to localized flooding in urban and suburban areas, posing risks to property and infrastructure. Polluted stormwater can present health risks to humans through contaminated drinking water sources, or through exposure to bacteria and viruses when engaging in recreational activities in affected waters.