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Storm water discharges are generated by runoff from land and impervious areas such as paved streets, parking lots, and building rooftops during rainfall and snow events. This runoff often contains pollutants in quantities that could adversely affect water quality.
Most storm water discharges are considered point sources and require coverage by a permit under the National Pollutant Discharge Elimination System (NPDES). The primary method to control storm water discharges is through the use of best management practices.
The NPDES storm water program is organized according to the three types of regulated storm water discharges: industrial, construction, and municipal separate storm sewer systems.
Industrial activities
Activities that take place at industrial facilities, such as material handling and storage, are often exposed to the weather. As runoff from rain or snowmelt comes into contact with these materials, it picks up pollutants and transports them to nearby storm sewer systems, rivers, lakes, or coastal waters.
In order to minimize the impact of storm water discharges from industrial facilities, the NPDES program includes an industrial storm water permitting component. Operators of industrial facilities included in one of the 11 categories of storm water discharges associated with industrial activity that discharge or have the potential to discharge storm water to a municipal separate storm sewer system or directly to waters of the United States require authorization under a NPDES industrial storm water permit.
Construction activities
Storm water runoff from construction activities can have a significant impact on water quality. As storm water flows over a construction site, it picks up pollutants like sediment, debris, and chemicals. Polluted storm water runoff can harm or kill fish and other wildlife. Sedimentation can destroy aquatic habitat and high volumes of runoff can cause stream bank erosion.
The NPDES storm water program requires operators of construction sites one acre or larger (including smaller sites that are part of a larger common plan of development) to obtain authorization to discharge storm water under an NPDES construction storm water permit.
The development and implementation of storm water pollution prevention plans is the focus of NPDES storm water permits for regulated construction activities.
Municipal storm sewer systems
Under the NPDES storm water program, operators of large, medium, and regulated small municipal separate storm sewer systems (MS4s) require authorization to discharge pollutants under an NPDES permit.
Medium and large MS4 operators are required to submit comprehensive permit applications and are issued individual permits. Regulated small MS4 operators have the option of choosing to be covered by an individual permit, a general permit, or a modification of an existing Phase I MS4's individual permit.