Who's Covered Under the Medium and Large MS4 Stormwater Program?
The definition of medium and large MS4s focuses on the size of the population in the area where the system is located. The population served by combined sewer systems does not figure into this total.
A medium MS4 is a system that is located in an area with a population between 100,000 - 249,999.
A large MS4 is a system that is located in an area with a population of 250,000 or more.
Other MS4s, including those under the 100,000 threshold, are also large and medium MS4s. These systems were designated by NPDES permitting authorities due to interrelationships to other medium and large MS4s.
The Overview of the NPDES Stormwater Program [PDF Format] provides additional information on the definitions of medium and large MS4s.
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