Municipal Separate Storm Sewer System (MS4) Discharges: Am I Regulated?

Explains which MS4s are required to obtain permit coverage for their stormwater discharges.

Municipal Separate Storm Sewer System (MS4) permit requirements were implemented in two phases: Phase I for large and medium MS4s and Phase II for small MS4s.

• Click here for information from EPA about stormwater discharges from municipal sources

Phase I MS4 Requirements
Phase II MS4 Requirements

Phase I MS4 Requirements

If you operate a medium or large MS4 which serves or is located in an incorporated place or county with a population of 100,000 or greater (based on the 1990 U.S. Decennial Census), then you operate a regulated Phase I MS4 and you must apply for an individual permit*.

*Please note that all Phase I MS4s are already permitted and there are no new Phase I designations.

Phase II MS4 Requirements

If you operate a small MS4:

located in an urbanized area (UA), as defined by the 2000 or 2010 U.S. Decennial Censuses,
located in an “urban area with a population of at least 50,000 people”, as defined by the 2020 U.S. Decennial Census, or

located outside of a UA or “urban area with a population of at least 50,000 people” and are brought into the program on a case-by-case basis (i.e., “formally designated”) by the TCEQ,

then you operate a regulated small MS4 and must obtain authorization or apply for a waiver, as applicable.

To determine if you are located in a UA or “urban area with a population of at least 50,000 people”, you can visit the US Census Bureau’s TIGERweb webpage and find an interactive map of these areas in a city near you.