National Pollutant Discharge Elimination System (NPDES)

Stormwater Discharges from Municipal Sources


In addition to the MS4s information below, see other stormwater municipal discharge material:

Polluted stormwater runoff is commonly transported through municipal separate storm sewer systems (MS4s), and then often discharged, untreated, into local water bodies.

An MS4 is a conveyance or system of conveyances that is:

  • owned by a state, city, town, village, or other public entity that discharges to waters of the U.S.,
  • designed or used to collect or convey stormwater (e.g., storm drains, pipes, ditches),
  • not a combined sewer, and
  • not part of a sewage treatment plant, or publicly owned treatment works (POTW).

To prevent harmful pollutants from being washed or dumped into specific types of MS4s, operators might be required to obtain NPDES permits and develop stormwater management programs (SWMPs).

Phase I MS4s

Phase II MS4s

Issued in 1990, the Phase I regulation requires medium and large cities or certain counties with populations of 100,000 or more to obtain NPDES permit coverage for their stormwater discharges.

  • There are approximately 750 Phase I MS4s
  • Generally, Phase I MS4s are covered by individual permits

Issued in 1999, the Phase II regulation requires regulated small MS4s in urbanized areas, as well as small MS4s outside the urbanized areas that are designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges.

  • There are approximately 6,700 Phase II MS4s
  • Generally, Phase II MS4s are covered by general permits.

NPDES permits for regulated MS4s require permittees to develop a SWMP, which describes the stormwater control practices that will be implemented consistent with permit requirements to minimize the discharge of pollutants from the sewer system.

Urbanized Area Maps for NPDES MS4 Phase II Stormwater Permits

Developing an MS4 Program

Owners/operators of regulated municipal separate storm sewer systems (MS4s) are required to develop, implement, and enforce a stormwater management program (SWMP). The focus of the SWMP is to describe how the MS4 will reduce the discharge of pollutants from its sewer system and addresses these program areas:

Use the guidance and references on this page to help you develop your SWMP.


Long-Term Stormwater Planning

  • Community Solutions for Stormwater Management: A Guide for Voluntary Long-Term Planning - Guide to assist states and local governments in developing and implementing effective long-term stormwater plans. This document describes how to develop a comprehensive long-term community stormwater plan that integrates stormwater management with communities' broader plans for economic development, infrastructure investment and environmental compliance. 

Permit Resources

Fees and Funding

Federal Government and Stormwater Fees

  • Federal Government Obligations to Pay Stormwater Fees — On January 4, 2011, President Obama signed into law "An Act to Amend the Federal Water Pollution Control Act to clarify Federal responsibility for stormwater pollution," Pub. L. No. 111-378, 124 Stat 4128 (2011) to clarify that reasonable service charges payable by federal agencies, as described in section 313(a), include certain stormwater assessments.
  • Memorandum Clarifying that New Legislation Provides for Stormwater Fees to be Paid from Current Lump-sum Appropriations — On March 18, 2011, the Department of Justice/Office of Legal Counsel released a memorandum to clarify that language in section 313(c)(2)(B) of the Clean Water Act contained in new legislation obligating federal agencies to pay stormwater managements fees does not impose a specific appropriation requirement. Stormwater assessments are payable from annual—including current—lump-sum appropriations.

Funding Sources and Resources

Fact Sheets and Guidance

  • Funding Stormwater Programs — This fact sheet for municipalities provides information on alternatives for funding their stormwater programs.
  • Guidance for Municipal Stormwater Funding — This document was developed in January 2006 by the National Association of Flood and Stormwater Management Agencies (NAFSMA) under a grant provided by EPA to provide funding guidance for stormwater utilities.

Training and Webcasts

EPA provides webcasts for stormwater professionals on the minimum control measures as well as joint EPA-Federal Highway Administration (FHWA) webcasts on transportation stormwater management.

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