An MS4 is a structural system owned by a public entity with the purpose of collecting and conveying stormwater that discharges to waters of the United States (WOTUS). Stormwater conveyed by an MS4 never connects to a sewer or passes through a wastewater treatment plant prior to discharge to surface waters.

There are two categories of MS4s that are required to maintain Arizona Pollutant Discharge Elimination System (AZPDES) permit coverage for discharge to protected surface waters: 

• Phase I MS4s, also referred to as Large or Medium MS4s, and
• Phase II MS4s, also referred to as Small MS4s 
  

In 1999, the EPA created the Phase II Rule, which expanded the National Pollution Discharge Elimination System (NPDES) Stormwater Program to include stormwater discharges from regulated MS4s not covered by the Phase I MS4 program. 

MS4s are automatically designated and required to apply for the Phase II general permit if they are located in an urbanized area with a population of at least 50,000 as defined by the U.S. Census Bureau. Additionally, MS4s with a minimum population density of 10,000 people per square mile, certain non-traditional MS4s (e.g., hospitals, public universities, departments of transportation, and prisons), MS4s that contribute substantial pollutant loads to impaired or protected surface waters, and those MS4s designated by ADEQ are required to apply for the general permit. 

The Phase II MS4 general permit requires the MS4 to develop and fully implement a stormwater management program (SWMP) that contains appropriate management practices in each of the six minimum control measure categories:

• Public Education and Outreach
• Public Involvement and Participation
• Illicit Discharge Detection and Elimination (IDDE)
• Construction Site Stormwater Runoff Control
• Post-Construction Site Stormwater Runoff Control
• Pollution Prevention for Municipal Employees
  

Actions taken to decrease the discharge of pollutants can be found in the Stormwater Management Program (SWMP) document prepared by each Phase II MS4. The SWMP is posted on the city, county, or agency website for each Phase II MS4.