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Stormwater means stormwater runoff, snow melt runoff, surface runoff and drainage. Stormwater runoff may pick up and transport sediment, oil, and other pollutants. If not managed properly these pollutants can affect the quality of surface waters making them unsafe for drinking, fishing, swimming, or other activities.


Clean Water Act (CWA)

In 1972, Congress passed the Federal Water Pollution Control Act Amendments "to restore and maintain the chemical, physical, and biological integrity" of the Nation's waters. The Clean Water Act (CWA) is a 1977 amendment to the Federal Water Pollution Control Act of 1972. The CWA set the basic structure for regulating discharges of pollutants to waters of the U.S.

National Pollution Discharge Elimination System (NPDES)

The NPDES permit program was introduced in 1972 and regulates discharges to "waters of the U.S." Any discharge from a "point source" into a "waters of the U.S." must have an NPDES permit. Stormwater discharges are regulated by NPDES permits. NPDES permits contains limits on allowed discharges, monitoring and reporting requirements, and other provisions to ensure that discharges do not harm water quality or public health. See EPA website for additional information

Idaho Transportation Division of Highways (ITD) has coverage under three different types of NPDES permits, including:

  • Construction General Permit (CGP) - Construction activities that disturb greater than one acre of land and have the potential to discharge stormwater to a waters of the U.S.
  • Municipal Separate Storm Sewer Systems (MS4)- Stormwater drainage from roads or property that drain into stormwater conveyance systems owned by the state, a city, a town, a village, a public university, a public hospital, a military base, a correctional facility or other public entity that discharges to waters of the U.S.
  • Multi-Sector General Permit (MSGP)- Industrial activities that have the potential to discharge stormwater to a waters of the U.S., including sand and gravel mining. All EPA covered industrial activities are listed in Appendix D of the MSGP.

Two Phases of NPDES Program

Phase I:
Developed in 1990 by the EPA, this phase addresses sources of stormwater runoff that have the greatest potential to negatively impact water quality. The EPA requires NPDES permit coverage for stormwater discharges from:
Phase II:
Published in December 1999, this phase revised the "No Exposure Exclusion" for certain industrial facilities under Phase I of the NPDES Stormwater Program and established potential waivers for small construction activities. Phase II also requires NPDES permit coverage for stormwater discharges from:
  • Regulated Small MS4s which are any MS4s not covered by Phase I and located in "urbanized areas" of a certain population
  • Construction activity disturbing one to five acres of land with the potential to discharge to waters of the U.S.

CGP page MSGP page MS4 page NPDES Link

Page Last Modified: 11/13/2012 10:34:16 AM

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Idaho Transportation Department
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