Skip to main content
U.S. flag

An official website of the United States government

Here’s how you know

Dot gov

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

HTTPS

Secure .gov websites use HTTPS
A lock (LockA locked padlock) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

    • Environmental Topics
    • Air
    • Bed Bugs
    • Chemicals and Toxics
    • Climate Change
    • Emergency Response
    • Environmental Information by Location
    • Environmental Justice
    • Greener Living
    • Health
    • Land, Waste, and Cleanup
    • Lead
    • Mold
    • Pesticides
    • Radon
    • Science Topics
    • Water Topics
    • A-Z Topic Index
    • Laws & Regulations
    • By Business Sector
    • By Topic
    • Compliance
    • Enforcement
    • Guidance
    • Laws and Executive Orders
    • Regulations
    • Report a Violation
    • Environmental Violations
    • Fraud, Waste or Abuse
    • About EPA
    • EPA Administrator
    • Organization Chart
    • Staff Directory
    • Planning, Budget, and Results
    • Jobs and Internships
    • Headquarters Offices
    • Regional Offices
    • Lab and Research Centers
National Pollutant Discharge Elimination System (NPDES)
Contact Us

Stormwater Discharges from Municipal Sources


  • OVERVIEW
  • Developing an MS4 Program
  • Resources

Recent Developments
  •    Green Infrastructure Permit Compendium (pdf)
  • Recorded Webinar: Clean Watersheds Needs Survey Stormwater Training for State Stormwater and Green Infrastructure Staff

Quick links to relevant information on stormwater discharges from municipal sources: 

  • Community Solutions for Stormwater Management: A Guide for Voluntary Long-Term Planning 
  • Stormwater Discharges from Transportation Sources
  • Proposed National Rulemaking to Strengthen the Stormwater Program
  • National Menu of Best Management Practices (BMPs) for Stormwater
  • Stormwater Maintenance

National map of regulated MS4s
View a larger version of this map

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 MS4s, certain operators are required to obtain NPDES permits and develop stormwater management programs (SWMPs). The SWMP describes the stormwater control practices that will be implemented consistent with permit requirements to minimize the discharge of pollutants from the sewer system.

Phase I MS4s
Phase II MS4s

The 1990 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 855 Phase I MS4s covered by 250 Individual Permits

The 1999 Phase II regulation requires small MS4s in U.S. Census Bureau defined urbanized areas, as well as MS4s designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges. Phase II also includes non-traditional MS4s such as public universities, departments of transportation, hospitals and prisons.

Most of the 6,695 Phase II MS4s are covered by statewide General Permits, however some states use individual permits. There are three Watershed Permits that cover 3 Phase I and 40 Phase II MS4s.

Urbanized Area Maps for NPDES MS4 Phase II Stormwater Permits
 

  • NPDES Home
  • About NPDES
  • All NPDES Program Areas
  • Animal Feeding Operations
  • Aquaculture
  • Forest Roads
  • Industrial Wastewater
  • Municipal Wastewater
  • National Pretreatment Program
  • Pesticide Permitting
  • Stormwater
    • Construction Activities
    • Industrial Activities
    • Municipal Sources
    • Transportation Sources
    • Oil and Gas Permitting
    • EPA's Residual Designation Authority
    • Rules and Notices
  • Water Quality Trading
  • NPDES Technical Resources
Contact Us to ask a question, provide feedback, or report a problem.
Last updated on June 28, 2022
United States Environmental Protection Agency

Discover.

  • Accessibility
  • Budget & Performance
  • Contracting
  • EPA www Web Snapshot
  • Grants
  • No FEAR Act Data
  • Plain Writing
  • Privacy
  • Privacy and Security Notice

Connect.

  • Data.gov
  • Inspector General
  • Jobs
  • Newsroom
  • Open Government
  • Regulations.gov
  • Subscribe
  • USA.gov
  • White House

Ask.

  • Contact EPA
  • EPA Disclaimers
  • Hotlines
  • FOIA Requests
  • Frequent Questions

Follow.