Since the passage of the Clean Water Act (CWA) in 1972, the quality of our Nation’s waters has improved dramatically. Despite this progress, however, degraded waterbodies still exist. According to the 2000 National Water Quality Inventory, a biennial summary of State surveys of water quality, approximately 40 percent of surveyed U.S. waterbodies (waters of the United States) are impaired by pollution and did not meet water quality standards. A leading source of this impairment is polluted stormwater runoff from urban and suburban areas.

Concentrated development in urbanized areas substantially increases impervious surfaces, such as city streets, driveways, parking lots, and sidewalks, on which pollutants from concentrated human activities settle and remain until a storm event washes them into nearby storm drains. Common pollutants include pesticides, fertilizers, oils, salt, litter and other debris, and sediment. Another concern is the possible illicit connections of sanitary sewers, which can result in fecal coliform bacteria entering the storm sewer system. Stormwater runoff picks up and transports these and other harmful pollutants then discharges them – untreated – to waterways via storm sewer systems.

In 1987, Congress directed the Environmental Protection Agency (EPA) to develop a regulatory program to address stormwater pollution. In response, EPA issued regulations in 1990 authorizing the creation of a permitting approach under the National Pollutant Discharge Elimination System (NPDES). Specific polluting sources were targeted by the EPA in a two-phase approach (Phase I and Phase II).

In December 2002, the State of Arizona's Department of Environmental Quality (ADEQ) took primacy of the NPDES program in Arizona (except for Indian Tribal lands) under the Arizona Discharge Elimination System (AZPDES) program.


Environmental Protection Agency: Storm Water Phase II Final Rule, an Overview, Fact Sheet Series, EPA 833-F-00-001, January 2000 (revised December 2005).

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Urbanized Areas

The determination of whether a community is regulated under the Phase I or Phase II rule is based on a whether or not the operator of the municipal separate storm sewer system (MS4) serves a large or medium urbanized area or serves a small urbanized area. The Bureau of the Census determines urbanized areas by applying a detailed set of published UA criteria (55 CFR 42592, October 22, 1990) to the latest decennial census data.

The basic unit for delineating an urbanized area's boundary is the census block. Census blocks are based on visible physical boundaries, such as the city block, when possible, or on invisible political boundaries, when not. An urbanized area can comprise places, counties, Federal Indian Reservations, and minor civil divisions (MCDs - towns and townships).

Designations of additional MS4 operators into or within the MS4 program is based on subsequent census years as governed by the Bureau of the Census' definition of an urbanized area in effect for that year and any such boundaries determined as a result of the definition.


Environmental Protection Agency: Storm Water Phase II Final Rule, Urbanized Areas: Definition and Description, Fact Sheet Series EPA 833-F-00-004, January 2000 (revised December 2005).

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Phase I Permits

The Phase I program required operators of "medium" and "large" MS4s, that is, those that generally serve populations of 100,000 or greater, to implement a stormwater management program as a means to control polluted stormwater discharges. The EPA defined a medium or large MS4 to mean all municipal separate storm sewers that are either:

  • Located in an incorporated place with a population of 100,000 or more as determined by the 1990 Decennial Census by the Bureau of the Census (Appendix G of this part);
  • Located in selected counties; or
  • Owned or operated by a municipality other than those described above that the EPA designates as being part of the large or medium municipal separate storm sewer system due to the interrelationship between the discharges of the designated storm sewer and the discharges from municipal separate storm sewers.

In Arizona, that included the Arizona Department of Transportation, City of Glendale, City of Mesa, City of Phoenix, City of Scottsdale, City of Tempe, and Pima County.

EPA Permit Requirements

Requirements for Phase I permittees established by the EPA are detailed in detailed in Title 40, Part 122, Subpart B, Section 122.26 of the Federal Code of Regulations (40 CFR 122.26). This required Phase I permittees to submit a two-part application for coverage through an individual permit under the National Pollutant Discharge Elimination System (NPDES) program.

In general, the requirements that Phase I permittees had to meet were technically challenging and included the following:

  1. Legal Authority – Provide a description of existing legal authority to control discharges to the municipal separate storm sewer system.
  2. Adequate Legal Authority – Demonstrate that the applicant can operate pursuant to legal authority established by statute, ordinance or series of contracts.
  3. Source Identification:
    • Provide a description of the historic use of ordinances, guidance or other controls which limited the discharge of non-storm water discharges to any Publicly Owned Treatment Works serving the same area as the municipal separate storm sewer system.
    • Provide a USGS 7.5 minute topographic map (or equivalent topographic map with a scale between 1:10,000 and 1:24,000 if cost effective) extending one mile beyond the service boundaries of the municipal storm sewer system
    • Provide an inventory, organized by watershed of the name and address, and a description (such as SIC codes) which best reflects the principal products or services provided by each facility which may discharge, to the municipal separate storm sewer, storm water associated with industrial activity
  4. Discharge Characterization:
    • Provide monthly mean rain and snow fall estimates (or summary of weather bureau data) and the monthly average number of storm events.
    • Provide existing quantitative data describing the volume and quality of discharges from the municipal storm sewer, including a description of the outfalls sampled, sampling procedures and analytical methods used.
    • Provide a list of water bodies that receive discharges from the municipal separate storm sewer system, including downstream segments, lakes and estuaries, where pollutants from the system discharges may accumulate and cause water degradation and a brief description of known water quality impacts
    • Provide results of a field screening analysis for illicit connections and illegal dumping for either selected field screening points or major outfalls covered in the permit application.
    • Provide a characterization plan including information and a proposed program to meet the requirements of paragraph (d)(2)(iii)
    • When "quantitative data" for a pollutant are required, the applicant must collect a sample of effluent in accordance with 40 CFR 122.21(g)(7) and analyze it for the pollutant in accordance with analytical methods approved under part 40 CFR 136.
    • Estimate of the annual pollutant load of the cumulative discharges to waters of the United States from all identified municipal outfalls and the event mean concentration of the cumulative discharges to waters of the United States
  5. Management Programs.
    • A description of the existing management programs to control pollutants from the municipal separate storm sewer system.
    • A description of the existing program to identify illicit connections to the municipal storm sewer system.
  6. Fiscal Resources – Provide a description of the financial resources currently available to the municipality to complete part 2 of the permit application
  7. Proposed Management Program – Provide a proposed management program covers the duration of the permit. It shall include a comprehensive planning process which involves public participation and where necessary intergovernmental coordination, to reduce the discharge of pollutants to the maximum extent practicable using management practices, control techniques and system, design and engineering methods, and such other provisions which are appropriate.


Environmental Protection Agency: Storm Water Phase II Final Rule, Small MS4Stormwater Program Overview, Fact Sheet Series EPA 833-F-00-002, January 2000 (revised December 2005).

GPO Access Electronic Code of Federal Regulations (e-CFR) Title 40: Protection of Environment; Part 122.26 – USEPA Administered Permit Programs: The National Pollutant Discharge Elimination System; Subpart B—Permit Application and Special NPDES Program Requirements
Code of Federal Regulations Title 40: Protection of Environment; Part 122.26

ADEQ Permit Requirements

In 2009, the ADEQ began issuing stormwater permits to Phase I MS4 operators. Since these are issued as individual permits, the terms and conditions can vary from one permittee to another. In general, the ADEQ has imposed several more restrictive requirements on Phase I permittees than the federal program had required. Please contact the ADEQ or visit the Members and Sponsors page to contact a Phase I permitted community to determine what permit conditions exist for each individual Phase I permittee.

Contact ADEQ:

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Phase II Permits

The Stormwater Phase II Final Rule was the next step in EPA's effort to preserve, protect, and improve the Nation’s water resources from polluted stormwater runoff. The Phase II program targeted operators of small MS4s and required these operators to implement programs and practices to control polluted stormwater runoff.

EPA Permit Requirements

Operators of small MS4s were identified by the EPA as those located in "urbanized areas" as delineated by the Bureau of the Census that were not already covered by Phase I NPDES stormwater program. Based on the 2000 Census, small MS4 operators were determined to be those serving a residential population of at least 50,000 and an overall population density of at least 1,000 people per square mile.

Operators of regulated small MS4s are required to design their programs to:

  • Reduce the discharge of pollutants to the “maximum extent practicable” (MEP);
  • Protect water quality; and
  • Satisfy the appropriate water quality requirements of the Clean Water Act.

Implementation of the MEP standard required the development and implementation of best management practice (BMPs) and the achievement of measurable goals to satisfy each of six established minimum control measures.

The Phase II Rule required small MS4 operators to develop a stormwater management program to address the following six minimum control measures that, when implemented in concert, are expected to result in significant reductions of pollutants discharged into receiving waterbodies.

  1. Public Education & Outreach
  2. Public Participation & 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

Phase II MS4 operators in Arizona have not been previously permitted under the EPA NPDES program.

ADEQ Permit Requirements

On December 19, 2002, the ADEQ issued its first permit for Phase II regulated operators. Permitting requirements were essentially the same as those defined under the EPA regulations and included:

  • Municipalities that are located wholly or partially in an urbanized area as defined by the 2000 US Census;
  • Municipalities designated by ADEQ including Camp Verde, Cottonwood, Douglas, Fountain Hills, Lake Havasu, Nogales, Sedona, and Sierra Vista; and,
  • Non-traditional Small MS4s such as military bases, universities, hospitals and prisons. This included Arizona State University, Northern Arizona University, University of Arizona, Arizona Department of Corrections, Yuma Marine Corps, Davis Monthan Air Force Base, Arizona State Hospital, Maricopa Community Colleges, Pima Community Colleges, Central Arizona College, Arizona Western College, Yavapai College, and VA Medical Centers in Tucson and Phoenix

Unlike the Phase I permittees, Phase II’s were permitted by the ADEQ under a general permit. According to the ADEQ, general permits are best suited for the regulation of numerous, very similar, and generally smaller facilities and sources that have emissions and discharges that post little environmental risk.

In the 2002 permit, the ADEQ required Phase II permittees to develop a stormwater program that addressed the same six minimum control measures as provided in the federal regulation.


Arizona Department of Environmental Quality: Fact Sheet for the Issuance of the AZPDES Small MS4 General Permit, December 19, 2002.

Arizona Department of Environmental Quality: Permitting: General Permits website.

Arizona Department of Environmental Quality: Permits: Stormwater website; Small Municipal Separate Storm Sewer Systems (Small MS4s).

Environmental Protection Agency: Storm Water Phase II Final Rule, an Overview, Fact Sheet Series, EPA 833-F-00-001, January 2000 (revised December 2005).

Environmental Protection Agency: Storm Water Phase II Final Rule, Urbanized Areas: Definition and Description, Fact Sheet Series EPA 833-F-00-004, January 2000 (revised December 2005).

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General Requirements

In general, Phase I and Phase II MS4s were required to develop and implement an ordinance or other regulatory mechanism to prohibit non-stormwater discharges into the storm sewer system and implement appropriate enforcement procedures and actions. To learn more about local program requirements, please visit the Members and Sponsor page and select the logo of the municipal member of interest.

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Other Links

ADEQ Stormwater Permits

EPA Region 9 Water Program, NPDES Stormwater Permits

International Strormwater BMP Database

Stormwater Managers Resource Center

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To contact a member or report a problem within your community, click on the appropriate link below.