Urban Area Maps for Phase II (Small) Municipal Separate Storm Sewer Systems (MS4s)

Small MS4 Urbanized Area Clarification Rule

The designation criteria for small MS4s has changed following the U.S Census Bureau’s recent urban area mapping revisions. In 2022 the Census Bureau discontinued the use of the term and its practice of publishing the location of “urbanized areas” with the 2020 Census and future censuses.

The Census Bureau’s longstanding definition of urbanized areas has now been clarified by a new EPA rule—finalized June 7, 2023— National Pollutant Discharge Elimination System (NPDES) Small MS4 Urbanized Area Clarification . The rule replaced the term “urbanized area” in Phase II MS4 regulations with “urban areas with a population of at least 50,000,” This change allows TCEQ and other permitting authorities to use 2020 Census and future census data consistent with existing longstanding regulatory practice.

Urban Area vs. Urbanized Area

Before the 2020 Census, the Census Bureau identified urban areas across the U.S. and sub-classified each as one of the following:

• “urbanized area”
  • Population of 50,000 people or more within the urban area boundary
• “urban cluster”
  • Population less than 50,000 people within the urban area boundary. In the 2020 decennial and future censuses they no longer sub-classify an urban area into either an “urbanized area” or “urban cluster”. Instead, the census refers to all qualified areas as “urban areas” regardless of population size.

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2020 Urban Areas in Texas

Any operator of a small MS4 within or partially within an urban area with a population of at least 50,000 will be regulated under the Texas Pollutant Discharge Elimination System (TPDES) 2024 Phase II (Small) MS4 General Permit TXR040000. Phase I MS4s and Texas Department of Transportation areas already regulated by a TPDES individual permit are excluded from this general permit. See the table below for these 2020 Urban Areas in Texas—effectively “urbanized areas” as they were previously called.

You can also use TIGERweb —a Census Bureau interactive map—to see the locations of the 2020 urban areas in Texas with a population of 50,000 or more to determine where the Phase II (Small) MS4 regulations apply.

You may also find more details about the impact to newly regulated and existing MS4 operators below.

Table 1: 2020 Urban Areas in Texas with a Population of 50,000 or More People

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2024 TPDES Phase II (Small) MS4 General Permit and the 2020 Decennial Census

TCEQ published public notice of the proposed 2024 TPDES Phase II (Small) MS4 General Permit on August 18, 2023, and it is tentatively scheduled to be renewed on January 24, 2024. The general permit is expanded to apply to MS4 operators in urban areas with a population of 50,000 or more people based on the 2020 Decennial Census. This change in applicability criteria will result in including newly regulated MS4 operators and impact some existing MS4 operators.

Newly Regulated MS4 Operators

Any operator of a previously unregulated small MS4 within or partially within a 2020 urban area with a population of 50,000 or more people will be regulated under the proposed 2024 TXR040000 General Permit. Operators will have 180 days from the date the general permit renewal is issued to prepare and apply for coverage or a waiver, as applicable.

If you believe your MS4 is within a 2020 urban area with a population of 50,000 or more and will be newly regulated, please contact the TCEQ Stormwater team at 512-239-4671 for more information.

Existing Regulated MS4 Operators

MS4 operators currently regulated by the TXR040000 General Permit will continue to be regulated and must apply for coverage or a waiver, as applicable, within 180 days of the general permit renewal.

In some cases, urban areas with a population of 50,000 or more people may have grown or decreased in size from the 2010 to the 2020 decennial censuses. Where the areas have grown, MS4 operators may now have expanded regulated areas to address in their Stormwater Management Program and permit authorization. Where the areas have decreased in size, however, the MS4 operator’s regulated areas will not change.

Once a portion of the MS4 is regulated, it will always remain regulated regardless of changes in future decennial censuses.

Although the regulated area may not decrease in size, the population the MS4 operator serves may have decreased in the 2020 Decennial Census. MS4 operators should include the population based on that census in the application they submit. This will determine the applicable MS4 level under the general permit. MS4 operators may be newly eligible for a waiver or may seek coverage under a different level based on the new population established by the 2020 Decennial Census.

Contact us if you have any questions about the Phase II (Small) MS4 General Permit or the 2020 Decennial Census changes.

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