About Our Stormwater Program
Municipal Separate Storm Sewer System
In the Valley, most municipalities operate two separate drain systems-a sanitary sewer, which carries wastewater to a water treatment plant, and a Municipal Separate Storm Sewer System (MS4). The MS4 conveys stormwater runoff through drains, streets and open channels, directly discharging the water into retention basins, washes, rivers, or lakes. This water does not go to a water treatment plant.
Did you know that stormwater runoff is one of the leading causes of water pollution in the United States?
The U.S. Environmental Protection Agency has estimated that over 30% of pollutants in our nation's waters are the result of stormwater runoff.
In an effort to address this problem, revisions were made to the Clean Water Act (CWA), creating the National Pollutant Discharge Elimination System (NPDES). In Arizona, the program is administered by the Arizona Department of Environmental Quality (ADEQ) and is called the Arizona Pollutant Discharge Elimination System (AZPDES).
Construction sites that meet the criteria are required to obtain permit coverage under the Arizona Construction General Permit (PDF) . Municipalities operating separate storm sewer systems within the local urbanized area (PDF) which have been designated by the EPA or the Director of ADEQ must get permit coverage under Arizona's AZPDES authority.
Phase I & Phase II Permits
There are two types of municipal permits: Phase I and Phase II. Phase I permits are for large and medium-sized MS4s; Phase II permits are for small MS4s. Maricopa County is designated as a small MS4 and must obtain stormwater discharge permit coverage under Phase II Small MS4 General Permit AZG2016-002 (PDF).
Minimum Control Measures
Elements of the program include a Stormwater Management Plan (SWMP) that incorporates Best Management Practices (BMPs) associated with six Minimum Control Measures as outlined by the federal rule:
1. Public Education and Outreach
2. Public Participation and Involvement
3. Illicit Discharge Detection and Elimination
4. Construction Site Runoff Control
5. Post-Construction Runoff Control
6. Pollution Prevention/Good Housekeeping for Municipal Operations
To meet the minimum requirements of the Phase II permit, the Maricopa County Stormwater Quality Management and Discharge Control Regulation (PDF) was enacted. This regulation prohibits non-stormwater discharges and illicit connections to the County's MS4. Runoff from construction sites, as well as long-term, permanent runoff control is also regulated by the County.