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RIPDES Stormwater Municipal Separate Storm Sewer Systems (MS4s)

In December 2003, the Rhode Island Pollutant Discharge Elimination System (RIPDES) Program issued the first General Permit for Stormwater Discharge from Small Municipal Separate Storm Sewer Systems (MS4s) and from Industrial Activity at Eligible Facilities Operated by Regulated Small MS4s. Under the General Permit MS4s are required to submit an Annual Report documenting progress made towards achieving the requirements of the program, as well as reporting on ongoing maintenance of their stormwater system. MS4s are required to make their Annual Report available to the public, and many choose to post it on their website. The permit expired in 2008, and has since been administratively continued until a new permit is issued. For updates on the status of the MS4 General Permit reissuance, go to the For Permittees/MS4 Operators Page.

Who is regulated under Phase II?

Under the Phase II Stormwater Rules adopted in 2003, 33 municipalities were regulated based on urbanized or densely populated area, 1 municipality was regulated due to the issuance of a TMDL and 7 non-municipal/non-traditional entities were regulated based on the size of the population served. For more detailed information, see the "Who is Regulated under Phase II?" Page.

The Phase II Rule automatically covers all small MS4s located in "urbanized areas" (UAs) as defined by the Bureau of the Census. As the U.S. Census data changes the applicability of the program also changes. The RIPDES Program is required to reevaluate who is regulated following each census and designate any additional MS4s as the urbanized areas (UAs) and densely populated areas (DPAs) change. Click here to view the most recent map of stormwater regulated areas in RI based on census data. For more detail click here to view EPA's Urban Areas Map

The 2010 census places a portion of the Town of Hopkinton in a category that will require coverage under the MS4 program. This designation becomes official when the RIPDES Regulations are amended to include the 2010 U.S. Census data and the MS4 permit is re-issued.

Note that there are other ways that the stormwater regulated area can be expanded, including Special Resources Protection Waters (SRPWs) and impaired waterbodies with approved TMDLs. These areas are not indicated on the census-based map above.

Each municipality/MS4 has designated one person to be their Stormwater Coordinator. For a list of contacts, please visit MS4 Stormwater Coordinators List

Historical Overview of Stormwater Phase II

The National Pollutant Discharge Elimination System (NPDES) Phase I stormwater program, initiated in 1990, required operators of "medium" or "large" Municipal Separate Storm Sewer Systems (MS4s) (MS4s that serve a population of 100,000 or greater), to obtain permits and implement a stormwater management program as a means to control polluted discharges from these activities. On December 8, 1999, the NPDES Stormwater Program was expanded (Phase II) to require operators of small MS4s (MS4s serving populations of less than 100,000, located within Urbanized Areas,) to obtain permits and establish a stormwater management program that is intended to improve waterbodies by reducing the quantity of pollutants that can enter storm sewer systems during storm events

The Phase II rule described six minimum control measures, which regulated small MS4s needed to implement. These minimum control measures were typically implemented by applying one or more Best Management Practices (BMPs) appropriate to the source, and/or necessary to address problems specific to the MS4. To assist MS4s with the development of their programs, EPA developed a National Menu of Stormwater Best Management Practices. The menu was intended to provide guidance to regulated small MS4s as to the types of practices they could use to develop and implement their stormwater management programs. The menu was intended as guidance only. More information about Phase II requirements can be found on the Requirements: Six Minimum Control Measures page.

Why Manage Municipal Stormwater Systems?

In Rhode Island there is a strong correlation between the amount of impervious area in a watershed and the waterbody within that watershed being listed as impaired by the RIDEM.

Increased volume, temperature, and rate of runoff from impervious surfaces, and the concentration of pollutants in the runoff can cause changes in hydrology and impact water quality.

At greater than 10% impervious area, we begin to see:

  • Water quality issues
  • Impacts to biological communities
  • Increased flooding
  • Stream erosion
  • Loss of recreational uses
  • Shellfish bed closures
  • Beach closures
  • Reduced baseflow and recharge


Effective management of stormwater runoff offers a multitude of benefits, including protection of wetlands and aquatic ecosystems, improved quality of receiving waterbodies, conservation of water resources, protection of public health, and flood control.

For more information, e-mail:
Margarita Chatterton
Jennifer Stout