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Frequently Asked Questions


    • This is the federal stormwater program. In 1972, the NPDES program was essentially the regulatory mechanism of the Clean Water Act (CWA) which the Environmental Protection Agency (EPA) used to focus on industrial process wastewater and municipal sewage discharges. These sources were identified as being responsible for severely degraded water quality. In 1987, in an effort to mitigate diffuse sources of pollution conveyed in stormwater runoff, Congress enacted the Water Quality Act, an amendment to the CWA a comprehensive program to regulate stormwater.

      EPA promulgated the Phase I stormwater rule on November 16, 1990. This rule targeted stormwater discharges from communities with a population of at least 100,000 (termed by EPA as medium and large municipal separate storm sewer systems or MS4s), as well as stormwater discharges associated with certain industrial activities and construction sites impacting five acres or more of land. In Arizona, there were 8 entities targeted by the Phase I stormwater program. They included, Phoenix, Mesa, Tempe, Glendale, Scottsdale, Tucson, Pima County and the Arizona Department of Transportation (ADOT).

      The Phase II stormwater rule was promulgated on December 8, 1999 and focused on small MS4s. Small MS4s were communities serving a population of less than 100,000 or communities greater than 100,000 (based on the 2000 census) not captured by the Phase I program that are located in an “urbanized” area or designated by the permitting authority. In addition to the MS4s, Phase II also focused on federal and state facilities including prisons, hospitals and universities, municipally owned/operated industrial facilities and lastly construction sites greater or equal to one acre and less than five acres in size.

      It is the unincorporated areas of Maricopa County within the urbanized area that are the areas regulated by the Phase II permitting program.

    • The AZPDES program is the Arizona version of the federal NPDES program. The Arizona Department of Environmental Quality (ADEQ) is the authority responsible for regulating the AZPDES program in Arizona.

    • Stormwater runoff is the water that runs off surfaces, such as rooftops, paved streets, highways, parking lots, and the desert itself. It can also come from hard grassy surfaces, such as golf courses, lawns, parks, and play fields.

    • A municipal separate storm sewer system means a conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains): owned or operated by a” …”County”…”or other public body, including special districts such as a flood control district”…”designed or used for collecting or conveying storm water and which is not a combined sewer and which is not part of a Publicly Owned Treatment Works (POTW).

      The County’s MS4 system consists mainly of flood control conveyances, drainage ditches, catch basins, storm drains, County owned streets, curbs and gutters.

    • Stormwater runoff from lands modified by human activities can harm surface water and, in turn, cause or contribute to an exceedance of water quality standards by changing natural hydrologic patterns, accelerating natural stream flows, destroying aquatic habitat, and elevating pollutant concentrations and loadings. Such runoff may contain high levels of contaminants, such as sediment, suspended solids, nutrients (phosphorus and nitrogen), heavy metals, pathogens, toxins, oxygen-demanding substances (organic material), and floatables.

      Nationwide, polluted stormwater runoff has been identified as a leading cause of problems in nearly 40 percent of surveyed U.S. waters, which currently do not meet water quality standards. Over land or via storm sewer systems, polluted runoff is discharged, often untreated, directly in local waters. When left uncontrolled, this water pollution can lead to the destruction of fish, wildlife, and aquatic life habitats; a loss in aesthetic value; and threats to public health.

    • If your construction site is located in the Urbanized Area of Unincorporated Maricopa County and will disturb one or more acres or is less than one acre but is part of a larger plan of development, Stormwater Plan approval by Maricopa County is required prior to the start of land disturbance.

    • A permit is required for land disturbance equal to or greater than one acre. Land disturbance of less than one acre constituting a part of a larger plan of development are also regulated. Activities for which a permit is required include land development and redevelopment to include clearing or grubbing, leveling, construction of new or additional impervious or semi-pervious surfaces such as driveways, roadways, parking lots, recreation features, construction of new buildings or additions to existing buildings and installation of stormwater management facilities or appurtenances thereto.