EPA- Stormwater Discharges From MS4′s


Stormwater Discharges From Municipal Separate Storm Sewer Systems (MS4s)


Aerial view of a Municipal Separate Sstorm Sewer System (MS4)

Polluted stormwater runoff is commonly transported through Municipal Separate Storm Sewer Systems (MS4s), from which it is often discharged untreated into local waterbodies. To prevent harmful pollutants from being washed or dumped into an MS4, operators must obtain a NPDES permit and develop a stormwater management program.

  • Phase I, issued in 1990, requires medium and large cities or certain counties with populations of 100,000 or more to obtain NPDES permit coverage for their stormwater discharges.
  • Phase II, issued in 1999, 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.

Generally, Phase I MS4s are covered by individual permits and Phase II MS4s are covered by a general permit. Each regulated MS4 is required to develop and implement a stormwater management program (SWMP) to reduce the contamination of stormwater runoff and prohibit illicit discharges.


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 (including storm drains, pipes, ditches, etc.);
  • Not a combined sewer; and
  • Not part of a Publicly Owned Treatment Works (sewage treatment plant).


EPA’s MS4 Program Highlights

To access the full list of NPDES stormwater information, use the navigation tool box on the right side of this page.

  • Stormwater Phase II Final Rule Fact Sheet Series – Provides an overview of the major elements of the Final Phase II Rule, including small MS4 programs, minimum control measures, and permitting.
  • National Menu of BMPs – Best management practices (BMPs) that can be used to meet the six minimum measures.
  • Urbanized Area Maps – Includes a set of digitized maps for each urbanized area as defined by the 2000 US Census with one map containing an overview of the urbanized area and another detailed map with street level features.
  • MS4 Webcasts – EPA webcasts for local stormwater professionals on the six minimum measures.
  • MS4 Program Evaluation Guidance – Developed to help NPDES authorities evaluate the quality of Phase I and Phase II MS4 programs.
  • Measurable Goals Guidance for Phase II Small MS4s (PDF) (55 pp, 669K) - Designed to help small MS4 operators comply with the measurable goals permitting requirements.
  • MS4 Permit Improvement Guide (PDF) (119 pp, 1.5MB) – The primary purpose of this Guide is to assist permit writers in strengthening municipal separate storm sewer system (MS4) stormwater permits.

Federal Government and Stormwater Fees

  • Federal Government Obligations to Pay Stormwater Fees (PDF) (2 pp, 119K) Exit EPA Site – 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 storm water assessments.
  • Memorandum Clarifying that New Legislation Provides for Stormwater Fees to be Paid from Current Lump-sum Appropriations (PDF) (13 pp, 196K) Exit EPA Site – On March 18, the Departmnet 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.

Fact Sheets on Key Municipal Stormwater Program Issues (Developed by EPA Region 3)

Minimum Control Measures

The MS4 Program contains elements called minimum control measures that when implemented should result in a significant reduction in pollutants discharged into receiving waters. The minimum measures are outlined below:

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