Revised On: Sept. 18, 2023 -  6:20 p.m.

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 

Permit requirements established in 1990 by the Phase I Rule for the National Pollutant Discharge Elimination System (NPDES) Stormwater Program changed in 1999. Since this change, MS4s required to apply for new AZPDES permit coverage are categorized as Phase II MS4s. Phase I MS4s established in 1990 maintain the original classification.

Once stormwater flows reach an MS4 conveyance, pollutants and solids transported from impermeable surfaces (e.g., streets and parking lots) or resulting from erosion do not receive physical, biological, or chemical treatment in a wastewater treatment plant prior to discharge to a protected surface water. In the absence of treatment, Phase I MS4s are required by their AZPDES permits to perform routine monitoring of stormwater for pollutants of concern. The permit also requires the Phase I MS4 to decrease the discharge of pollutants, to the maximum extent practicable, that cause or contribute to an exceedance of an applicable surface water quality standard. Actions taken to decrease the discharge of pollutants can be found in the Stormwater Management Program (SWMP) document prepared by each Phase I MS4. The SWMP is posted on the city, county, or agency website for each Phase I MS4.

Arizona has eight Phase I MS4s regulated under the AZPDES program:

• Arizona Department of Transportation
• City of Glendale
• City of Mesa
• City of Phoenix
• City of Scottsdale
• City of Tempe
• City of Tucson
• Pima County

2011 – 2020 Phase I MS4 Report 

Municipalities and other organizations regulated under Phase I MS4 AZPDES permits collect and discharge stormwater runoff that occur inside their defined service area. 

In 2023, ADEQ analyzed the last 10 years' worth of data provided by the eight Phase I MS4s. This was done to better understand the extent to which pollutants of concern are entering Arizona’s surface waters through stormwater. The findings revealed that on average, Phase I MS4s release more pollutants compared to individually permitted publicly owned treatment plants (POTWs) and industrial wastewater plants. For example,  E. coli, copper, lead and zinc are discharged into Arizona’s surface waters at concentrations three times or more than those discharged from POTWs and industrial wastewater plants.  

2011 – 2020 Phase I MS4 Report | View/Download > 

There are multiple ways to reduce the amount of harmful substances in stormwater. One approach is green stormwater infrastructure (GSI), which involves using natural systems to capture, clean, and let stormwater slowly seep into the ground. Another method is to update existing municipal stormwater systems to capture and store more stormwater in order to fix existing maintenance issues and solve flooding problems | Learn More About GSI (EPA) > | Learn More About GSI in Arizona >