Which MS4s are regulated by the NPDES Stormwater Program?
For regulatory purposes, EPA’s NPDES Stormwater Program regulates “medium,” “large,” and “regulated small MS4s.” A medium MS4 is a system that is located in an incorporated place or county with a population between 100,000 – 249,999. A large MS4 is a system that is located in an incorporated place or county with a population of 250,000 or more. In addition, some MS4s that serve a population below 100,000 have been brought into the Phase I program by an NPDES permitting authority and are treated as medium or large MS4s, independent of the size of the population served. A regulated small MS4 is any small MS4 located in an “urbanized area” (UA), as defined by the Bureau of the Census, or located outside of a UA and brought into the program by the NPDES permitting authority.
