Overview

Stormwater runoff from an urbanized area is commonly transported through Municipal Separate Storm Sewer Systems (MS4s). Urbanized areas are associated with different activities that contribute pollutants to stormwater runoff. Additionally, urbanized areas are characterized by large amounts of impervious surfaces such as streets, sidewalks, driveways, parking lots, and roof tops. Stormwater runoff from impervious surfaces travels faster which results in damage to the waterbodies and causes flood­ing and hydrologic changes to the receiving waters. Elevated pollutant levels can impair waterbodies, degrade biological habitats, and pol­lute drinking water sources.

Discharges from MS4s are regulated under the Clean Water Act and OPDES Act as point source discharges. MS4 regulations were developed and implemented in two phases. Implementation of MS4 Phase I began in the early 1990s and required medium and large cities or certain counties with populations of 100,000 or more to obtain NPDES permit coverage for their stormwater discharges. The MS4 Phase II regulations became effective on March 23, 2003 and required regulated small MS4s in urbanized areas, as well as small MS4s outside the urbanized areas that are designated by DEQ, to obtain permit coverage for their stormwater discharges.

Phase I MS4s are covered by individual permits and Phase II MS4s are covered by the general permit OKR04. Each regulated MS4 is required to develop and implement a stormwater management program (SWMP) to reduce the contamination of stormwater runoff and prohibit illicit discharges to the MS4.

MS4 Phase I Program

Currently, there are two cities, City of Oklahoma City and City of Tulsa, which are regulated under the Phase I program.

MS4 Phase II Program

There are 51 small cities/counties which are regulated under the MS4 Phase II Program. A Regulated Phase II MS4 is an MS4 that is located wholly or partially within an Urbanized Area (UA) boundary as defined by the latest decennial U.S. Census and not a large or medium municipality. Regulated Phase II MS4s include systems that are operated by traditional communities such as municipalities, (such as counties, cities or towns) as wel as non-traditional communities such as military installations, universities and community development districts. Phase II MS4s located outside of UAs may also become regulated by designation by DEQ according to designation criteria set forth in the rules.An urbanized area (UA) is a central place (or places) and the adjacent densely surrounding territory that together has a minimum residential population of 10,000 and a minimum average density of 1,000 people/square mile. All UA calculations and mapping are done by the U.S. Bureau of the Census. The determination of which MS4s are automatically designated as regulated Phase II MS4s is based solely on the boundaries of U.S. Bureau of the Census urbanized areas –not political boundaries. You may view a list of regulated MS4 Phase II cities and counties which have been determined based on 2010 Census by U.S. Bureau of the Census.

For a full list of regulated MS4s within Oklahoma, please view: List of Regulated MS4 Operators

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