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What Is Stormwater?

Stormwater refers to a heavy quantity of water, such as rain or snow, that falls to the surface of the Earth. When it reaches the surface, stormwater becomes polluted as it picks up, carries, and transports various pollutants (oil, grease, chemicals, sediment, nutrients, pathogens) along streets, drains, open channels, and storm sewer systems. Most of the untreated runoff eventually is discharged into nearby waterbodies. However, in combined sewer systems, stormwater flows with wastewater and is treated at a water resource recovery facility.

In urban areas, stormwater management is especially important because of decreases in natural land cover and the expansion of impervious surfaces, such as rooftops, sidewalks and roadways. These surfaces exacerbate runoff because they change the permeability of the landscape — preventing rainwater from soaking in or infiltrating the soil. Some of the impacts of stormwater include flooding, sewer overflows, and nonpoint source pollution.

Stormwater is managed using storm sewers as well as stormwater controls, which include green infrastructure and low-impact development.

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