According to 40 CFR 122.26(b)(8), “municipal separate storm sewer means a conveyance or

system of conveyances (including roads with drainage systems, municipal streets, catch basins,

curbs, gutters, ditches, man-made channels, or storm drains):

(i) Owned or operated by a State, city, town, borough, county, parish, district,

association, or other public body (created to or pursuant to State law)...including

special districts under State law such as a sewer district, flood control district or

drainage district, or similar entity, or an Indian tribe or an authorized Indian tribal

organization, or a designated and approved management agency under section 208 of

the Clean Water Act that discharges into waters of the United States.

(ii) Designed or used for collecting or conveying storm water;

(iii) Which is not a combined sewer; and

(iv) Which is not part of a Publicly Owned Treatment Works (POTW) as defined at 40

CFR 122.2.”

United States Office of Water EPA 833-F-99-003

Environmental Protection (4203) April 1999 Agency Fact Sheet 2.1

Storm Water Phase II

Proposed Rule

Who’s Covered? Designation and

Waivers of Regulated Small MS4s

This fact sheet is based on the Storm Water Phase II Proposed Rule. Therefore, the information provided herein is subject to

change upon publication of the final Phase II rule in November 1999. A revised series of fact sheets will be provided at that

time. A comprehensive list of the current fact sheets is in the text box at left.

Who Would Be Affected by the Proposed Phase II Small MS4 Program?

The Storm Water Phase II Proposed Rule would apply to public owners or operators of regulated

small municipal separate storm sewer systems (MS4s), which are determined by specific criteria

as discussed in this fact sheet. To aid in determining whether an MS4 would meet these criteria, this

fact sheet reviews the definition of an MS4 and distinguishes between small, medium, and large

MS4s. Conditions under which a small MS4 may be designated as a regulated small MS4, as well as

the conditions for a waiver from the Phase II program requirements, are outlined. This fact sheet also

attempts to clarify possible implementation issues related to determining one’s status as an owner or

operator of a regulated Phase II small MS4.

What Is a Municipal Separate Storm Sewer System (MS4)?

What constitutes an MS4 is often misinterpreted and misunderstood. An MS4 is not meant to

refer to municipally-owned storm sewer systems only, but rather it is a term of art with a much

broader application. Owners or operators of MS4s, in addition to local jurisdictions, could be State

and Federal departments of transportation, universities, local sewer districts, hospitals, military bases,

and prisons. Also, an MS4 is not necessarily merely a system of underground pipes. It also can

include roads with drainage systems, gutters, and ditches. The existing regulatory definition of an

MS4 is provided below.

The Phase II Proposed Rule would add Federal systems, such as military bases and prisons, to the

existing regulatory definition of an MS4 by changing paragraph 122.26(b)(8)(i) (see above) to read: 

“Owned or operated by the United States, a State, city, town, borough, county....”

Storm Water Phase II

Proposed Rule 

Fact Sheet Series

Overview

1.0 – Storm Water Phase II

Proposed Rule Overview

Small MS4 Program

2.0 – Small MS4 Storm Water

Program Overview

2.1 – Who’s Covered? Designation

and Waivers of Regulated Small

MS4s

2.2 – Urbanized Areas: Definition

and Description

Minimum Control Measures

2.3 – Public Education and

Outreach Minimum Control

Measure

2.4 – Public Participation/

Involvement Minimum Control

Measure

2.5 – Illicit Discharge Detection and

Elimination Minimum Control

Measure

2.6 – Construction Site Runoff

Control Minimum Control Measure

2.7 – Post-Construction Runoff

Control Minimum Control Measure

2.8 – Pollution Prevention/Good

Housekeeping Minimum Control

Measure

2.9 – Permitting and Reporting:

The Process and Requirements

2.10 – Federal and State-Owned

MS4s: Program Implementation

Construction Program

3.0 – Construction Program

Overview

Industrial “No Exposure”

4.0 – Conditional No Exposure

Exemption for Industrial Activity
Fact Sheet 2.1 – Who’s Covered? Designation and Waivers of Regulated Small MS4s Page 2

What is a Small, Medium, or Large MS4?

 EPA’s national storm water program labels MS4s as either

“small,” “ medium,” or “large” for the purposes of

regulation.

 A small MS4 is any MS4 that is not already covered by the

existing Phase I storm water program.

 The Phase I storm water program covers medium and large

MS4s. Phase I MS4s were automatically designated

nationwide as medium MS4s if the systems served a

population between 100,000 - 249,999 people or as large

MS4s if the systems served a population of 250,000 or

more. Many MS4s that serve a population below 100,000,

however, have been individually brought into the Phase I

program by NPDES permitting authorities and are

considered medium or large MS4s, and not small MS4s,

regardless of the size of the population served.

Would All Small MS4s Be Covered by the Phase II

Proposed Rule?

No. The universe of small MS4s is quite large since it includes

every MS4 except for the approximately 900 medium and

large MS4s already regulated under the Phase I storm water

program. Only a select sub-set of small MS4s, referred to as

regulated small MS4s, would be covered by the Phase II

Proposed Rule, either through automatic nationwide designation

or designation on a case-by-case basis by the NPDES permitting

authority.

How Would A Small MS4 Be Designated as a

Regulated Small MS4?

Asmall MS4 could be designated as a regulated small MS4 in

one of three ways:

 Automatic Nationwide Designation

The Phase II Proposed Rule would require nationwide

coverage of all owners or operators of small MS4s that are

located within the boundaries of a Bureau of the Censusdefined “urbanized area” (UA) based on the latest decennial

Census. Once a small MS4 is designated into the program

based on the UA boundaries, it would not be waived from the

program if in a subsequent UA calculation the small MS4 is

no longer within the UA boundaries. An automatically

designated small MS4 would remain regulated unless, or

until, it meets the criteria for a waiver.

 Urbanized Areas

At the time of permit issuance (not to exceed 3 years and

90 days after publication of the final Phase II rule), UA

calculations based on the 2000 Census are scheduled to

be completed and the regulated universe then would be

based on these new calculations. For more information

on UAs, see Fact Sheet 2.2.

 Appendix 6 to the Preamble of the Phase II

Proposed Rule

Appendix 6 is a listing of all incorporated places and

counties that are fully or partially located within a UA

according to the 1990 Census. The list is meant to serve

as a general geographic reference for owners or operators

of small MS4s that are trying to determine their status in

regard to the Phase II criterion (located within a UA) for

automatic designation. Appendix 6 can be obtained from

the EPA Office of Wastewater Management or

downloaded from the OWM web site. See Fact Sheet 2.2

for more information on UAs and how to determine

potential coverage under the Phase II program.

It is important to note that Appendix 6 is not a listing of

all the Phase II regulated small MS4s. Because a

Phase II regulated small MS4 would be any small MS4

located within the boundaries of a UA, providing an

accurate nationwide list of these entities is not feasible at

this time.

In addition, Appendix 6 does not include incorporated

places and counties already permitted under Phase I. 

Any small MS4s located in a Phase I incorporated place

or in the UA portion of a Phase I county (e.g., a Federal

prison), however, would be covered under the Phase II

program. This is particularly important to note in the

case of Federally owned or operated MS4s since they

were not covered nationally under Phase I and may exist,

unpermitted, in Phase I areas. A list of Phase I counties

and incorporated places can be obtained from the EPA

Office of Wastewater Management or downloaded from

the OWM web site.

An urbanized area (UA) is a land area comprising

one or more places – central place(s) – and the

adjacent densely settled surrounding area – urban

fringe – that together have a residential population

of at least 50,000 and an overall population density

of at least 1,000 people per square mile. 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.
Fact Sheet 2.1 – Who’s Covered? Designation and Waivers of Regulated Small MS4s Page 3

Physically interconnected means that one MS4 is

connected to a second MS4 in such a way that it allows

for direct discharges into the second system.

TMDLs are water quality assessments that determine

the source or sources of pollutants of concern for a

particular waterbody, consider the maximum amount

of pollutants the waterbody can assimilate, and then

allocate to each source a set level of pollutants that it

is allowed to discharge (i.e., a “wasteload allocation”).

Small MS4s that are not given a wasteload allocation

would meet the third criterion above. 

Appendix 6 also does not include minor civil divisions

(towns and townships). A list of the minor civil divisions

that are functioning governmental entities and are entirely

or partially located within a UA can be obtained from the

EPA Office of Wastewater Management or downloaded

from the OWM web site.

Potential Designation by the NPDES Permitting

Authority (Required Evaluation)

Owners or operators of small MS4s located outside of a UA

could be designated as regulated small MS4s if the NPDES

permitting authority determines that the discharges from the

small MS4 cause, or have the potential to cause, an adverse

impact on water quality. The Phase II Proposed Rule would

require the NPDES permitting authority to develop a set of

designation criteria and apply them, at a minimum, to all

small MS4s located outside of a UA serving an area with a

population of at least 10,000 and a population density of at

least 1,000 people per square mile.

 Designation Criteria

EPA recommends that the NPDES permitting authority

use a balanced consideration of the following

designation criteria on a watershed or other local basis:

Discharge to sensitive waters;

High population density;

High growth or growth potential;

Contiguity to a UA;

Significant contributor of pollutants to 

waters of the United States; and

Ineffective control of water quality 

concerns by other programs.

 Appendix 7 to the Preamble of the Phase II

Proposed Rule

Appendix 7 is a listing of all incorporated places and

counties, located outside of a UA, that have a population

of at least 10,000 and a population density of at least

1,000 people per square mile. As with Appendix 6, the

list is meant to serve as a geographic reference only. 

Owners or operators of small MS4s located within a

listed area should be aware that they could be examined

by their NPDES permitting authority for potential

designation into the Phase II program. Appendix 7 can

be obtained from the EPA Office of Wastewater

Management or downloaded from the OWM web site. 

It is important to note that Appendix 7 is not a complete

listing of all potentially designated small MS4s because

the NPDES permitting authorities may require any small

MS4 that is contributing pollutants to waters of the U.S.

to be permitted.

 Deadline for Designation

The NPDES permitting authority would be required to

designate small MS4s meeting the designation criteria

within 3 years and 90 days of publication of the final

rule, or within 5 years if a watershed plan is in place.

 Potential Designation by the NPDES Permitting

Authority

Under the proposed rule, the NPDES permitting authority

would be required to designate any small MS4 located outside

of a UA that contributes substantially to the storm water

pollutant loadings of a physically interconnected MS4

regulated by the NPDES storm water program. The proposed

rule does not set a deadline for designation of small MS4s

meeting this criterion.

Does the Proposal Provide a Waiver from the

Phase II Permit/Program Requirements?

Yes, if a regulated small MS4's discharges have been

determined to not cause, or have the potential to cause, water

quality impairment. A waiver option would be available to

owners or operators of automatically designated small MS4s

where:

(1) the jurisdiction served by the system is less than 1,000

people;

(2) the system is not contributing substantially to the storm

water pollutant loadings of a physically interconnected

regulated MS4; and

(3) the owner or operator can certify that storm water

controls are not needed based on “total maximum daily

loads” (TMDLs) assessments or a comprehensive

watershed plan that includes the equivalents of TMDLs.
Fact Sheet 2.1 – Who’s Covered? Designation and Waivers of Regulated Small MS4s Page 4

To obtain a waiver, the owner or operator would be responsible

for certifying on a form, provided by the NPDES permitting

authority, that the three waiver criteria have been met

and therefore implementation of storm water controls is not

necessary. Since the waiver is indefinite, the owner or operator

would not need to recertify every 5 years.

Could Multiple Jurisdictions in the Same

Urbanized Area Be Designated?

Yes. Since the proposed rule would provide automatic

coverage of all small MS4s within a UA, the result would

likely be coverage of several governments and agencies with

multiple, perhaps overlapping, jurisdictions. For example, a city

that is located within a UA and operates its own small MS4

would be designated alongside the State’s department of

transportation (DOT) and the county’s DOT if the State and

county own or operate roads that are within the borders of the

city. All three entities would be responsible for developing a

storm water management program for the portion of their

respective MS4s within the city limits. In such a case, the

permittees would be strongly encouraged to work together to form

a unified storm water management program.

Who Would Be Responsible if the Small MS4

Owner/Operator Lacks the Necessary Legal

Authority?

Some regulated small MS4s may lack the necessary legal

authority to implement one or more of the required minimum

control measures that comprise the proposed Phase II storm water

management program. For example, a local government that is a

small MS4 operator may be in a State that does not have an

enabling statute that allows local regulatory control of

construction site runoff into the sewer system. In such cases, the

NPDES permitting authority would be responsible for

implementation of the particular minimum control measure. 

Another example is a State DOT that may not have the legal

authority to require and enforce controls on illicit discharges into

its system. Since the regulated portion of a DOT system would

likely run through the area of, or connect to, another regulated

small MS4, the DOT would be encouraged to work with the

neighboring regulated small MS4. As co-permittees, they could

form a shared storm water management program in which each

permittee is responsible for activities that are within their

individual legal authorities and abilities.

For Additional Information

Contact

 U.S. EPA Office of Wastewater Management

• Phone : 202 260-5816

• E-mail: SW2@epa.gov 

• Internet: www.epa.gov/owm/sw2.htm

Reference Documents

 Storm Water Phase II Proposed Rule Fact Sheet

Series. 

• Contact the U.S. EPA Water Resource Center at

202 260-7786 or at waterpubs@epa.gov 

• Internet: www.epa.gov/owm/sw2.htm

 Storm Water Phase II Proposed Rule, published on

Jan. 9, 1998 in the Federal Register (63 FR 1536).

• Internet: www.epa.gov/owm/sw2.htm