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DEPARTMENT OF ENVIRONMENTAL PROTECTION
Bureau of Point and Non-Point Source Management
Designation Criteria for Regulated Small Municipal Separate Storm Sewer
Upon publication of notice as final in the
Pa. Title 25, Section 92.a.32(c), Pennsylvania Clean Streams Law, and
Federal Clean Water Act, 40 CFR § 122.26(b)(16), 40 CFR § 123.34 and
40 CFR § 122.32.
The action undertaken by this guidance will be to identify designation
criteria for regulated small MS4s.
This Guidance serves to set forth the criteria that the Department of
Environmental Protection (DEP) uses to designate a regulated small MS4.
By providing a guidance document, DEP can work in cooperation with the
potential MS4 communities and “non-traditional” MS4s which may be
introduced into the EPA Phase II Stormwater Program. The guidance will
also serve as a public education tool in describing the process of how
MS4s will be designated.
The guidance is applicable to potential MS4 municipalities in Urban Areas
(UAs), municipalities outside UAs which are designated pursuant to DEP
criteria, municipalities which are physically interconnected to other MS4s,
and entities which may be designated following third party petition. The
guidance is also applicable to “non-traditional” MS4s such as large
airports, public institutions, educational facilities, and highways and
thoroughfares in UAs as determined by DEP.
The policies and procedures outlined in this guidance document are
intended to supplement existing requirements. Nothing in the policies or
procedures shall affect regulatory requirements.
The policies and procedures herein are not an adjudication or a regulation.
There is no intent on the part of DEP to give these rules that weight or
deference. This document establishes the framework within which DEP
will exercise its administrative discretion in the future. DEP reserves the
discretion to deviate from this policy statement if circumstances warrant.
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303(d) Listed water
– Section 303(d) of the Clean Water Act requires states to list all impaired
waters not supporting uses even after appropriate and required water pollution control
technologies have been applied. (Source: DEP website:
– Pennsylvania Department of Environmental Protection is the permitting authority
authorized to implement the federal NPDES program in Pennsylvania. (Source: General)
– United States Environmental Protection Agency. (Source: General)
– High Quality (HQ) surface waters have quality which exceeds levels necessary
to support propagation of fish, shellfish, and wildlife and recreation in and on the water by
satisfying certain regulatory criteria and Exceptional Value (EV) waters are high quality waters
which satisfy antidegradation requirements in the regulations. (Source: Pa. Title 25, Chapter 93,
Section 93.1 Definitions)
– Phase I MS4 located in an incorporated place or county with a population of
250,000 or greater. (Source: EPA Fact Sheet 2.1, January 2000, revised June 2012)
– Phase I MS4 located in an incorporated place or county with a population
between 100,000–249,999. (Source: EPA Fact Sheet 2.1, January 2000, revised June 2012)
– “Municipal separate storm sewer system” which is a conveyance or system of
conveyances (including roads with drainage systems, municipal streets, catch basins, curbs,
gutters, ditches, man-made channels, or storm drains).... Refer to full definition at 40 CFR
– An authorization, license or equivalent control document issued by the
Administrator or DEP to implement the requirements of 40 CFR Parts 122-124 (relating to EPA
administered permit programs: the National Pollutant Discharge Elimination System, state
program requirements, and procedures for decision making) and the Federal Act. (Source: Pa.
Title 25, Chapter 92, Section 92a.2 Definitions)
Phase II Final Rule
– Requires that operators of Phase II-designated small MS4s and small
construction activity apply for NPDES permit coverage, most likely under a general rather than
individual permit, and to implement stormwater discharge management controls (known as “best
management practices” (BMPs)). (Source: EPA Fact Sheet 1.0, January 2000, revised
– Any MS4 not already covered by the Phase I stormwater program. Small MS4s
include federally-owned systems, such as military bases and MS4s regulated as discussed in the
technical guidance document. (Source: EPA Fact Sheet 2.1, January 2000, revised June 2012)
Total Maximum Daily Load (TMDL)
– The sum of individual waste load allocations for point
sources, load allocations for nonpoint sources and natural quality and a margin of safety
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expressed in terms of mass per time, toxicity or other appropriate measures. (Source: Pa.
Title 25, Chapter 96, Section 96.1 Definitions)
Urbanized Area (UA)
– A densely settled core of census tracts and/or census blocks that have
populations of at least 50,000, along with adjacent territory containing non-residential urban land
uses as well as territory with low population density included to link outlying densely settled
territory with the densely settled core. It is a calculation used by the Bureau of the Census to
determine the geographic boundaries of the most heavily developed and dense urban areas.
(Source: EPA Fact Sheet 2.1, January 2000, Revised June 2012)
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Designation Criteria for Regulated Small MS4s
Pennsylvania Department of Environmental Protection
EPA’s National Pollutant Discharge Elimination System (NPDES) stormwater permitting program
labels municipal separate storm sewer systems (MS4s) as small, medium or large for the purpose of
regulation. Phase II of EPA’s stormwater program automatically designates small MS4s with any
portion of their jurisdiction located within an Urbanized Area (UA) and requires them to obtain an
NPDES permit to discharge to surface waters. A UA is determined by the latest decennial census and
defined by the Census Bureau as an area with a population greater than 50,000 and a population density
of 1,000 people per square mile. However, it is possible for jurisdictions outside of a UA to become
regulated under the NPDES MS4 program as well.
This document describes the conditions under which an entity may be regulated as a small MS4.
Designation of MS4s:
Small MS4s located within a UA are categorized as “regulated.” The Permitting Authority for
Pennsylvania’s NPDES permitting program, including the MS4 permitting program, is the Pennsylvania
Department of Environmental Protection (DEP). An MS4 may be designated by DEP as regulated as
Automatic Designation (based on UA as described in the introduction) – Once a small MS4 is
designated into the program based on the UA boundaries, it cannot be removed from the program
on the basis that subsequent decennial UA calculation shows that the MS4 is no longer within the
UA boundaries. However, designated small MS4s may be eligible for waivers from permitting,
Designation by Criteria – The Phase II Final Rule required NPDES Permitting Authorities (i.e.,
DEP) to develop a set of criteria and apply them at a minimum to all small MS4s located outside
of a UA serving a jurisdiction with a population of at least 10,000 and a population density of at
least 1,000 people/square mile.
Designation by Interconnection – DEP may designate any small MS4 located outside of a UA
that contributes substantially to the pollutant loadings of a physically interconnected MS4
already regulated by the stormwater program.
Designation Following Petition – At any time a petition may be filed by a person or entity
requesting that an MS4 be designated as regulated. When a petition is filed, DEP must make a
final determination on whether or not to designate the MS4 as regulated within 180 days after its
Designation by DEP as required by EPA’s regulations at 40 CFR § 122.26(b)(16)(iii) to systems
within urban areas such as military bases, large hospital or prison complexes, large airports, and
highways and thoroughfares (non-traditional). Permits are issued to such systems when the
regional office Clean Water Management Program staff determines pursuant to the additional
designation criteria and Bureau of Point and Non-Point Source Management staff concurs it
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would not be reasonable to expect the affected municipality to accept MS4 responsibility for the
stormwater discharges from non-traditional sources.
Additional Designation Criteria:
DEP will use census data and environmental data to identify MS4s that have the potential to be
regulated based on the designation options listed above. Additionally, once an MS4 is identified as
meeting one of the above criteria, DEP will analyze the potential for stormwater discharges from the
MS4 to exceed water quality standards based on the guidance provided by the federal regulations
(40 CFR 123.35(b)(1)(ii)).
Discharge to sensitive waters
303(d) Listed waters impaired, or likely to be impaired, by stormwater
Waters with an established TMDL
HQ or EV waters
Waters used for drinking water or recreation
Critical habitat for plant or animal species
Population density or growth potential
Contiguity to an urbanized area
Will directly contribute to a regulated MS4
Significant contributor of pollutants to waters of the United States/Commonwealth of
DEP will determine if the discharge from the MS4: will exceed water quality standards, impairs
designated uses, or has significant biological and ecological impacts.
As a result of the designation criteria explained above, small MS4s designated as regulated are
required to apply for coverage under an NPDES MS4 stormwater permit within 180 days after
receiving formal notification from DEP.
DEP has published a Storm Water Best Management Practices (BMP) Manual to provide
guidance for the design and implementation of stormwater management programs.
For More Information:
A reference map identifying the Census 2010 urbanized areas within your jurisdiction can be found on
. Regulated MS4s located wholly or
partially within the automatically designated UA or those MS4s additionally designated by DEP will be
required to obtain NPDES permit coverage and develop a Stormwater Management Program for that
portion of the MS4 located within the designated area.
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