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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D C 20460 

AUG 6 1996 

OFFICE OF 

WATER 

MEMORANDUM 

SUBJECT: EPA Guidance on Application of State Mixing Zone Policies in EPA-Issued 

NPDES Permits 

FROM: 

TO: 

Robert Perciasepe 

Assistant Administrator 

Water Program Directors 

Regions I-X 

The purpose of this memorandum is to transmit the Office of Water’s “Guidance on 

Application of State Mixing Zone Policies in EPA-Issued NPDES Permits.” The attached 

guidance discusses the circumstances under which the Environmental Protection Agency (EPA), 

when it is the National Pollutant Discharge Elimination System (NPDES) permitting authority,- 

may include mixing zones in NPDES permits. The guidance also provides legal analyses of its key 

provisions. 

EPA's Water Quality Standards (WQS) regulation allows dates to adopt provisions 

authorizing mixing zones. Thus, individual state law and policy determine whether or not a 

mixing zone is permitted. EPA recommends that states make a definitive statement in their WQS 

or implementing regulations on whether mixing zones are allowed and how they will be defined. 

State regulations addressing mixing zones generally fall into one of two categories. Some states 

have regulations that generically authorize mixing zones without specifying who may exercise that 

authority. Other states’ regulations specifically confer discretionary authority to allow mixing 

zones only on the state agency. The guidance explains the legal authority and procedures for 

inclusion of mixing zones under both types of state regulations. The key provisions of the 

guidance are summarized as follows: 

1. If mixing zones are not authorized by state WQS or implementing 

regulations 

EPA is not authorized to include mixing zones in NPDES permits. 
2 

2. If mixing zones are authorized by state WQS or implementing regulations 

and the state approves EPA’s decision to include a mixing zone in the 

NPDES permit in question through a Clean Water Act (CWA) §401 

certification 

EPA is authorized to include a mixing zone in that specific NPDES permit. 

3. If mixing zones are authorized by state WQS or implementing regulations, 

but the state does not provide a CWA §401 certification lot the NPDES 

permit in question 

EPA is authorized to include mixing zones in NPDES permits only if such action is 

a “reasonable” interpretation of state WQS or implementing regulations. 

3a. If state WQS or implementing regulations generically authorize mixing 

zones without specifying who may exercise that authority, it is a reasonable 

interpretation of state WQS to include mixing zones in EPA-issued permits 

at EPA’s discretion. 

3b. If state WQS confer authority to include mixing zones specifically on the 

state, it is reasonable to include mixing zones in EPA-issued permits only 

when there is a written interpretation of WQS or implementing-regulations 

by the state confirming that EPA may exercise that discretionary authority 

as well. The state’s written interpretation may be in the form of a 

memorandum of understanding between EPA and the state, an Attorney 

General statement from the state, an exchange of letters between the state 

and EPA, or through other appropriate supporting materials. 

Please note that the same approach outlined in the attached guidance for mixing zones 

would apply to schedules of compliance for water quality-based effluent limits. Also, you should 

be aware that the guidance is prospective only. To the extent that EPA may have issued permits 

with mixing zones in the past in states where the authority to grant a mixing zone remains with the 

state, these permits should remain in effect as written until expiration. 

If you have any questions regarding the attached guidance, please call James Pendergast, 

Acting Director, Permits Division at (202) 260-9545 or Elizabeth Southerland, Acting Director, 

Standards and Applied Science Division at (202) 260-7301. 

Attachment 
Guidance on Application of State Mixing Zone Policies 

in EPA-Issued NPDES Permits 

August 1996 

This guidance discusses the circumstances under which EPA when it is the National 

Pollutant Discharge Elimination System (NPDES) permitting authority, may, in its discretion, 

specify mixing zones in NPDES permits. Specifically, this guidance addresses the ability to 

include mixing zones in EPA-issued permits in the absence of a permit-specific authorization from 

the state through the Clean Water Act (CWA) § 401 certification process. The guidance is 

divided into five sections. The first section provides an overview of the importance of mixing 

zones in establishing water qualify-based effluent limits and the role of EPA and states in setting 

mixing zone policy. The second section discusses the types of mixing zone provisions commonly 

found in state water quality standards. Section three discusses the legal authority for EPA to 

establish a mixing zone in a permit based upon such provisions. Section four provides guidance 

regarding when to include a mixing zone in an NPDES permit where an EPA Region is the 

permitting authority. The final section discusses implications of this guidance on inclusion of 

schedules of compliance in EPA-issued permits. 

Background 

In developing water quality-based effluent limits in NPDES permits, states and EPA 

Regions must consider an array of factors including, for example, effluent variability, critical 

receiving water flows, downstream uses, appropriate water quality models, and mixing zones. 

Briefly stated, a mixing zone is an allocated impact zone in the receiving water which may include 

a small area or volume where acute criteria can be exceeded provided there is no lethality (zone of 

initial dilution), and a larger area or volume where chronic water quality criteria can be exceeded 

if the designated use of the water segment as a whole is not impaired is a result of the mixing 

zone. Mixing zones are sized to cover areas where effluent undergoes initial dilution and may be 

extended to cover secondary mixing in the ambient water body. The decision on mixing zones 

(e.g., whether to conduct a mixing zone analysis, to assume rapid and complete mixing, or to 

require that a point source discharge meet water quality criteria at the end of the pipe) is a key 

factor in setting water quality-based effluent limits. 

EPA’s Water Quality Standards regulation allows states to adopt provisions authorizing 

mixing zones. 40 CFR § 131.13. Thus, individual state law and policy determine whether or not 

a mixing zone is permitted. EPA has recommended that states make a definitive statement in their 

water quality standards or implementing regulations on whether or not mixing zones are allowed 

and how they will be defined. EPA has provided guidance on when to conduct a mixing zone 

analysis and how to determine the boundaries and size of a mixing zone. See EPA’s Water 

Quality Standards Handbook (2nd Edition, 1994) and Technical Support Document for Water 

Quality-based Toxics Control ( 1991). 
-2- 

State Water Quality Standards and Mixing Zones 

A review of the water quality standards regulations of most states across the country 

reveals two major categories of mixing zone authorizations. First, some states, such as New 

Mexico, have regulations and policies that generically authorize a mixing zone without specifying 

who may exercise that authority. For example, New Mexico’s water quality standards allow a 

" . . limited mixing tone, contiguous to a point source wastewater discharge . . in any stream 

receiving such a discharge.” 20 New Mexico Regulations 6-1-1105.D 

Other states’ regulations and policies confer discretionary authority to allow mixing zones 

on the state agency. For example, in Massachusetts, ". . . the Division may recognize a limited 

area or volume of a waterbody as a mixing zone. . ." (emphasis added). 314 Code of 

Massachusetts Regulations 4.03(2). Alaska’s regulations are even more restrictive; they specifying 

that’... in applying the water quality criteria set out in this chapter, the Department will, upon 

application and in its diction, prescribe in its permits or certifications a volume of dilution for 

an effluent or substance within a receiving water unless [the environmental impact would be 

adverse] . . ” (emphasis added). 18 Alaska Administrative Code 70.032.1 

Neither of the two major categories of state mixing zone regulations explicitly confers 

authority on EPA to include a mixing zone in NPDES permits where EPA is the permitting 

authority. In some instances, the state may approve EPA’s decision to include a mixing zone in a 

specific permit through the CWA § 401 certification process. But, for other permits, the state 

might not provide permit-specific approval of a mixing zone. This circumstance raises two 

important questions: 

1) Where state water quality standards (or implementing regulations) authorize 

mixing zones, may EPA exercise the discretion to include a mixing zone in an 

NPDES permit in the absence of a specific state authorization of a mixing zone for 

that permit? 

2) If the answer to question one is “yes,” how and when should EPA exercise that 

discretion? 

The remainder of this guidance answers these two questions. 

1 States statutes and regulations also vary in the level of detail defining how a mixing zone 

is to be determined, regardless of whether the provisions specify which entity will determine 

whether to authorize a mixing zone. Some states provide very specific narrative or numeric 

criteria and standards for determining the size of a mixing zone. Other states’ statutes and 

regulations, by contrast, are very general. For instance, Georgia allows for "a reasonable and 

limited mixing zone” if it is demonstrated that a mixing zone “is necessary and. . . will not create 

an objectionable or damaging pollution condition.‘* 391 Rules and Regulations of the State of 

Georgia 3-6-03( 10). 
Legal Authority to Include .Mixiog Zones in EPA-issued NPDES Permits 

Section 30 1 (b)(l)(C) of the CWA requires that SPDES permits include “any more 

stringent (et&rent] limitation . . . necessary to meet water quality standards[.)” The EnvironmentaJ 

Appeals Board has interpreted this language to mean that. in the absence of a state certification 

under CWA 5 401 (i.e., where certification is waived), EPA’s interpretation of what constitutes a 

limitation necessary to meet the state’s water quality standard wiU be upheld ifit is “rcasonahle.” 

ncan Cyanmd Co. Santa Rosa Plant. et al., 4 E.AD. 790, 801 (EAB i993). Ifthe 

state does cenify a permit under CWA $401, its interpretation of its own water quality standards 

generally is controlling. Lf the state determines that a more stringent etIluent limitation is 

necessary (e.g., by deleting a mixing zone) and so specifies in its CWA 5 401 ceh&ation, EPA 

m include the more stringent limitation. 40 CFR 124.55(e). In addition, ifthc state informs 

EPA in its CWA 

5 401 cenification that a h stringent eauent limitation is all that is necessary to meet its water 

quality standards (e.g., a mixing zone should be included), EPA must defer to the state’s 

. . interpretation unless it is clearly wrong. -reIna CoCouw NPDES Appeal 84- 12 (Nov. 6, 1985) at 3; gee a&, w v. 

w 996 F.2d 346,352 (D.C. Cir. 1993). 

As discussed above, states may include provisions for mixing zones as part of their water 

quality standards or implementing regulations. Thus, if the state provides for mixing zones in its 

water quaky standards or implementing regulations, then inclusion of a mixing zone @ an EPAissued permit would be fully consistent with, and therefore “meet” the state’s water quality - 

standards, as required by C WA 4 3 0 1 (b)( l)(C), even in the absence of a state &cation under 

CWA 6 401, provided that such action is a “reasonable” interpretation of state water quality 

standards. (See section entitled “Proper Procedures for Inclusion of Mixing Zones in EPA-Issued 

Permits” beiow.) (JI & re: Star-I&t Caribe. II1E, 3 E.AD. 172, 175 (Adm’r 1990) (inclusion of 

a schedule of compliance for a water quality-based efnuent limit consistent with CWA 

0 30 l(b)(l)(C) only if the state’s water quality standards or implementing regulations provide for 

such a schedule). In such cases, the state has made a legislative or administrative dereknation 

that mixing zones are consistent with the state’s water quality standards as a whole, and EPA is 

simply developing water quality-based efnuent limits that arc consistent with those standards. 

rf state water quality standards do not provide for mixing zones, then EPA lacks any 

authority under CWA 8 301(b)(l)(C) to include a mixing zone in an NPDES permit. s 

3 E.AD. at 182 (“whether limited forms of reliefsuch as . . . m&g zones . . . should be granted are 

purely matters of state law, which EPA has no authority to override”). 

Most states do provide for some form of mixing zone authority in their state water quality 

standards. Nonetheless, EPA’s inclusion of a mixing zone in an NPDES permit constitutes an 

interpretatibn of the state water quality standards, which must therefore be “reasonable” if the 

state does not certi.Q to the pennit under CWA 6 401. American Cvanamid. sunra. . 
For state laws or regulations such as New .Mexico’s, which do not specify on their face 

that a particular entity will determine whether to grant a mixing zone, it is certainly reasonable to 

interpret such language as authoridng EPA to include a mixing zone in a permit to “meet” the 

state’s water quality standards. 

The more difficult legal question involves the extent of EPA’s authority to grant a mixing 

zone when the state statute or regulation, on its face, reserves the power to determine whether to 

grant a mixing zone to the state its& Such laws. like the more general mixing zone provisions 

discussed in the previous paragraph do reflect a state legislative or administrative policy judgment 

that mixing zones generally are consistent with the state’s water quality standards as a whole. 

Yet, such laws could be interpreted to limit the authority to establish mixing zones to the state. 

The Clean Water Act reseNes primary authority to determine appropriate water quality 

requirements to the states, and explicitly authorizes states to be more stringent than federai 

standards. CWA $3 10 l(b). 5 10 Respect for the Rate role under the Act to determine the 

appropriate water quality standards and necessary implementing regulations suggests that EPA 

should not assume that provisions specifically author&g the state to grant a mixing zone also 

give EPA the authority to grant a mixing zone without some exuinsic evidence that the state 

intends EPA to exercise such authority. Therefore, as discussed below, EPA policy dictates tha! 

EPA will not grant a mixing zone in such states unless the state interprets its water quality . 

standards or implementing regulations to provide EPA with this discretionary authority and 

cotdrms its interpretation in writing. Absent such a statement, and without 2 permit-specik 

authorizuion through the CWA 6 401 &cation process, it would not be reasonable, and 

therefore would not be within EPA’s discretion under B for EPA-to grant a 

mixing zone. 

Proper Procedures for Inclusion of Mixing Zones in EPA-issued Permits 

As noted above, under water quality standards such as New Mexico’s, which do not 

specify exactly which entity may determine whether to grant a mixing zone, EPA Regioxu may 

exercise their discretion to grant a mixing zone even without a permit-specific approval &om the 

state (e.g., through CWA 8 401 certification or other procedures). The Region should document 

in the fact sheet for the permit how the mixing zone reasonably satisfies the technical criteria in 

the state’s standards or implementing regulations for dcmmining mixing zones. 

Under state laws such as Massachusetts’, which authorize the state agency to grant mixing 

zones, EPA’s discretion to act properly in the state’s capacity is more circumrrcribcd EPA 

Regions may exercise their d&r&on to grant a mixing zone even in the absence of a pamitspecific state authorizat.iorS but only ifthe state interprets its water quality standards or 

implementing regulations to provide EPA with this discretionary authority and conhu its 

interpretation in writing. The state’s interpretation that its water quality standards give EPA this 

discretiow authority to include mixing zone provisions in Wure permits may be documented 

through a memorandum of understanding between EPA and the state, an Attorney General