INTERIM GUIDANCE ON CENSUS ELIMINATION OF "URBANIZED

AREA" DEFINITION

The Phase II National Pollutant Discharge Elimination System (NPDES) regulations automatically cover,
on a nationwide basis, any MS4 "located in an urbanized area as determined by the latest Decennial
Census by the Bureau of the Census" (unless the MS4 qualifies for a waiver) 40 CFR § 122.32(a)(1).
"Urbanized areas/' as defined by the Census Bureau when the Phase II regulations were issued,
comprise areas "that together have a minimum population of 50,000 people." 64 Fed. Reg. 68722,
68751 (December 8. 1999).

On March 24, 2022, the Census Bureau finalized revisions to its criteria for defining urban areas based
on the results of the 2020 Decennial Census. As part of that action, the Census Bureau ceased
distinguishing between different types of urban areas, which include "urbanized areas." This means that
the Decennial Census, starting in 2020 and into the future, will not identify "urbanized areas." Because
the Phase II regulations are written to cover MS4s located in "urbanized area[s] as determined by the
latest Decennial Census," questions have arisen about what effects the Census Bureaus' new change has
on which systems are considered regulated small MS4s moving forward.

EPA is currently evaluating next steps to provide clarity on this issue, including whether revisions to the
Phase II stormwater regulations may be appropriate. In the meantime, EPA provides the following as
interim guidance:

Will currently regulated small MS4s brought into the program during a previous decennial census
continue to be regulated in light of the change in Census Bureau criteria?

Yes. MS4s that have been regulated continue to be regulated. EPA continues to interpret its regulations
to include small MS4s that are already regulated as a result of a previous census. As stated in the
preamble to the Phase II stormwater regulations: "Additional designations based on subsequent census
years will be governed by the Bureau of the Census' definition of an urbanized area in effect for that
year. Based on historical trends, EPA expects that any area determined by the Bureau of the Census to
be included within an urbanized area as of the 1990 Census will not later be excluded from the
urbanized area as of the 2000 Census. However, it is important to note that even if this situation were to
occur, for example, due to a possible change in the Bureau of the Census' urbanized area definition, a
small MS4 that is automatically designated into the NPDES program for storm water under an urbanized
area calculation for any given Census year will remain regulated regardless of the results of subsequent
urbanized area calculations." 64 Fed. Reg. 68722, 68751 (Dec. 8. 1999).

What actions should permitting authorities take with respect to new or expanded small MS4s based on
the 2020 Census?

The Census Bureau has not yet published mapping data based on the 2020 Census.1 Prior to the
publication of these data, EPA does not expect permitting authorities to make formal designations of
new or expanded small MS4s based on the 2020 Census. Additionally, even after publication of the 2020

1 As of the release of this guidance, the Census Bureau expects to publish its new urban area data in December
2022. For information on the release of this data, including any changes to the expected publication schedule, see
https://www.census.gov/programs-survevs/geographv/guidance/geo-areas/urban-rural.html.

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Census data, permitting authorities may delay decisions regarding the designation of new or expanded
small MS4s based on the 2020 Census until EPA has provided direction on this issue. A permitting
authority is not, however, precluded from using its designation authority2 to regulate additional or
expanded small MS4s to protect water quality.

What actions should permitting authorities take with respect to currently regulated small MS4s?

Where a current small MS4 permit is set to expire, or where a small MS4 permit has already expired
without a replacement permit, to reduce permit backlog EPA recommends that the permitting authority
move forward with reissuing the permit based on the currently designated small MS4s (based on the
2010 Census), along with any additional MS4s that the permitting authority designates based on its
belief that they should be permitted to protect water quality under 40 CFR § 123.35(b). EPA
acknowledges that this may mean issuing a permit without the inclusion of new or expanded small MS4s
that could be regulated in the future if EPA subsequently changes the regulated MS4 definition.

DISCLAIMER: This interim guidance does not impose any new legally binding requirements on EPA, States, Tribes,
territories, or the regulated community. It does not confer legal rights or impose legal obligations upon any
member of the public. In the event of a conflict between this guidance and any statute, regulation, or permit, this
document would not be controlling. Interested parties are free to raise questions and objections about this
guidance's substance, and about its applicability to a particular situation. EPA retains the discretion to adopt
approaches on a case-by-case basis that differ from those described in this guidance, where appropriate.

2 Under the Section 402(p)(2)(E) of the Clean Water Act, permitting authorities may regulate additional stormwater
discharges "... for which the Administrator or the State, as the case may be, determines that the stormwater
discharge contributes to a violation of a water quality standard or is a significant contributor of pollutants to
waters of the United States." The NPDES regulations further explain that additional stormwater discharges may be
designated for regulation where the permitting authority determines either "that storm water controls are needed
for the discharge based on wasteload allocations that are part of "total maximum daily loads" (TMDLs) that
address the pollutant(s) of concern" or "that the discharge, or category of discharges within a geographic area,
contributes to a violation of a water quality standard or is a significant contributor of pollutants to waters of the
United States." 40 CFR § 122.26(a)(9)(i)(C), (D). The regulations also require State permitting authorities to develop
a process and criteria to designate additional small MS4s, other than the Census-designated MS4s, as appropriate
to protect water quality. 40 CFR § 123.35(b).

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