When rain falls onto paved urban areas, the water that cannot soak into the ground becomes stormwater runoff. This runoff travels across impervious surfaces, picking up materials before being collected by a complex drainage system. Many people mistakenly believe this water is directed to a treatment plant, cleaned, and then released, similar to household wastewater. However, this is rarely the case; the journey of street runoff is usually a direct path to the environment, carrying everything it collects along the way. Understanding this path reveals why managing this water is a major focus for environmental protection.
From Street Surface to Catch Basin
The initial path of stormwater is largely determined by the design of the street itself. Roads are intentionally built with a slight crown or slope, which directs the water toward the curbs and gutters. These gutters act as channels, using gravity to quickly move the water away from the roadway and into the municipal drainage system. This process is designed to prevent flooding and protect the integrity of the pavement.
The water’s first entry point into the underground network is typically a catch basin, recognized by the grated opening at the curb. This structure functions as an immediate filter for the system. Below the surface grate, the catch basin includes a sump, a chamber designed to allow larger debris and sediment to settle. This prevents items like leaves, trash, and heavy grit from clogging the main underground pipes. The water then flows out through an elevated outlet pipe, continuing its journey into the larger storm sewer network.
Understanding Storm and Sanitary Sewer Systems
The flow of water from the catch basin enters the storm sewer system. Storm sewers are a distinct network of underground pipes and channels engineered solely to transport rainwater and snowmelt. Their primary function is to manage the volume of water to prevent street flooding, and the collected water is typically discharged directly into local waterways without being treated.
In contrast, sanitary sewers collect wastewater from homes and businesses, including water from toilets, sinks, and showers. This wastewater travels to a centralized treatment plant where it undergoes filtration, biological treatment, and disinfection. This separation is crucial for water quality, ensuring that sewage receives necessary treatment before the cleaned water is released.
The main exception is the Combined Sewer System (CSS), found primarily in older urban areas. These historic systems combine both stormwater and sanitary sewage into a single pipe network. During dry weather, the combined flow is directed to a treatment plant, but heavy rainfall can overwhelm the system’s capacity. When this happens, a Combined Sewer Overflow (CSO) occurs, discharging a mixture of untreated sewage and stormwater directly into a nearby water body. This overflow prevents sewage backups into streets and buildings but introduces untreated waste into the environment.
Final Destinations: Discharge and Natural Absorption
Once collected by the storm sewer system, the majority of runoff is directed to the nearest natural water body. This discharge point, or outfall, is often a river, stream, lake, estuary, or the ocean. Although flow and volume are regulated, the water itself is released into the environment essentially untreated, carrying all the pollutants picked up from the street.
A portion of the runoff, however, never enters the pipe system, instead following a path of natural absorption. This occurs where water falls onto pervious surfaces like lawns, tree pits, or shoulders, allowing it to soak into the ground. As the water slowly infiltrates the soil, the earth acts as a natural filter, removing sediments and contaminants and recharging local groundwater sources.
Modern stormwater management increasingly encourages this natural process through the use of green infrastructure. Practices like rain gardens, bioswales, and permeable pavement are designed to capture, filter, and slowly absorb runoff before it can reach a catch basin. These methods mimic natural hydrological cycles, reducing the volume of water entering the sewer system and improving the quality of surface and groundwater.
What Runoff Carries: The Pollution Load
As runoff flows across streets and other impervious surfaces, it picks up various pollutants. The water collects engine oil, grease, and heavy metals, such as copper and zinc, shed from vehicle brake pads and tires. Fine sediment and grit from construction sites and road wear are also mobilized and carried into the drains.
Additional contaminants include pet waste, which introduces harmful bacteria and pathogens into waterways. Nutrient pollution from lawn fertilizers and pesticides is also a significant issue, as it fuels excessive growth of algae once it reaches rivers and lakes. This surge in plant life can lead to an oxygen-depleted environment, harming fish and other aquatic organisms. The direct discharge of this polluted runoff into sensitive aquatic ecosystems highlights the importance of managing street runoff.