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Flexibilities for States Applying 

EPA’s Ammonia Criteria Recommendations

Background

The U.S. Environmental Protection Agency (EPA) is updating its 1999 Clean Water Act (CWA) 

§ 304(a) national ambient water quality criteria recommendations for ammonia to account for the 

sensitivity of freshwater mussels and snails to ammonia toxicity. The updated criteria 

recommendations reflect new science on juvenile mussels and gill-bearing, non-pulmonate 

snails. Through extensive peer review processes, reviewers agreed on the quality and 

acceptability of the new data EPA included in the quantitative derivations of the updated

recommendations. The criteria recommendations for ammonia apply to all freshwaters for the 

protection of the aquatic community, including both freshwater mussels and snails.

Freshwater mussels are highly sensitive to ammonia toxicity and represent the most sensitive 

species in the dataset for the criteria recommendations. New science has demonstrated that 

freshwater snails are also sensitive to ammonia toxicity. Both mussels and snails are important to 

the environment because they serve as food sources for other organisms in the food web and 

provide vital services in improving and maintaining water quality. Specifically, mussels are filter 

feeders and can filter nutrients, toxics, and other pollutants out of the water, thereby helping to 

control the levels of these pollutants and reduce exposure to humans and other aquatic 

organisms. Snails feed on organic debris including algae, which helps to reduce the effects of 

eutrophication and keeps bottom substrates clean for other benthic organisms.

Flexibilities for Applying EPA’s Ammonia Criteria Recommendations

This section describes some of the flexibilities that states1 may want to consider in adoption and 

application of EPA’s ammonia criteria recommendations. These flexibilities include the 

Recalculation Procedure for site-specific criteria derivation, variances, revisions to designated 

uses, dilution allowances, and compliance schedules.

1. Recalculation Procedure for Site-specific Criteria Derivation

The water quality standards (WQS) regulation at 40 CFR § 131.11(b)(1)(ii) provides states with 

the opportunity to adopt water quality criteria that are “...modified to reflect site-specific 

conditions.” As with any criteria, site-specific criteria must be based on a sound scientific 

rationale in order to protect the designated use and are subject to review and approval or 

disapproval by EPA.

 

1

Throughout this document, the term “states” refers to authorized tribes and U.S. territories in addition to states.

United States

Environmental Protection Agency

Office of Water

Mail Code 4305T

EPA-820-F-13-001

 April 2013
2

The Recalculation Procedure for site-specific criteria derivation is intended to allow site-specific 

criteria that differ from national criteria recommendations (i.e., concentrations that are higher or 

lower than national recommendations) where there are demonstrated differences in sensitivity 

between the aquatic species that occur at the site and those that were used to derive the national 

criteria recommendations. The national dataset may contain aquatic species that are sensitive to a 

particular pollutant, but these or comparably sensitive species might not occur at the site (e.g., 

freshwater mussels are included in the national ammonia dataset but may not be present at a 

particular site). On the other hand, a species that is critical at the site might be sensitive to the 

pollutant and require site-specific criteria that are lower than the national recommended criteria.

In the case of ammonia, where a state can demonstrate that mussels are not present on a sitespecific basis, the Recalculation Procedure may be used to remove the mussel species from the 

national criteria dataset to better represent the species present at the site. 

For example, many of the commonly occurring freshwater bivalves (e.g., pea clam) are more 

closely related to the non-unionid fingernail clam Musculium (which is the fourth most sensitive 

genus in the national dataset for the chronic criterion) than to the unionid mussels Lampsilis and 

Villosa (which are the two most sensitive genera in the national dataset for the chronic criterion). 

At sites where all bivalves present are more closely related to Musculium than to Lampsilis and 

Villosa (i.e., where unionid mussels are not present at the site), the Recalculation Procedure may 

be used to remove Lampsilis and Villosa from the dataset because they would not be 

representative of the species present at the site.2The retention of Musculium in the dataset would 

represent the other non-unionid bivalves present at the site, so the non-unionid bivalves would 

still be protected if Lampsilis and Villosa were removed from the dataset. However, at sites 

where both unionid and non-unionid bivalves are present, all three bivalves in the national 

dataset (i.e., Lampsilis, Villosa, and Musculium) would be retained because they would represent 

the species present at the site. The Recalculation Procedure describes how to compare the 

taxonomy of species present at the site with the taxonomy of species in the national dataset. 

The number of tested genera (N) in the criteria calculations must be updated where genera such 

as Lampsilis and Villosa are removed from the dataset. For example, if only the two unionid 

mussels are removed from the dataset for the national chronic ammonia criterion, N would be 

reduced from 16 genera in the national dataset to 14 genera in the site-specific dataset.

Freshwater snails are another sensitive species used in the criteria derivation. However, they are 

ubiquitous in the environment and, therefore, not likely to be deleted from the dataset in a criteria 

recalculation.

As with any criteria, states choosing to utilize the Recalculation Procedure should ensure that 

their site-specific criteria “...provide for the attainment and maintenance of the water quality 

standards of downstream waters.” 40 CFR § 131.10(b). In addition, states should consider how 

they will demonstrate that mussels are not present at the site before selecting this approach. For 

 

2 With removal of Lampsilis and Villosa from the national dataset, the Recalculation Procedure would result in 

criteria (and associated water quality-based effluent limits (WQBELs) based on such criteria) with higher 

concentrations than EPA’s recommendations but that are still protective of the designated use.
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additional information on the Recalculation Procedure, see EPA’s Revised Deletion Process for 

the Site-Specific Recalculation Procedure for Aquatic Life Criteria at

http://water.epa.gov/scitech/swguidance/standards/criteria/aqlife/ammonia/index.cfm. For 

additional information on site-specific water quality criteria, see EPA’s Water Quality Standards 

Handbook at http://www.epa.gov/wqshandbook.

2. Variances

The WQS regulation at 40 CFR § 131.13 authorizes states, at their discretion, to “...include in 

their [s]tate standards, policies generally affecting their application and implementation, such as 

mixing zones, low flows and variances [emphasis added]. Such policies are subject to EPA 

review and approval.” 

A variance may be described as a time-limited designated use and criteria that target a specific 

pollutant(s), source(s), waterbody(ies) and/or waterbody segment(s). As first articulated in 1977, 

a state may adopt a variance and applicable criteria where the state can satisfy the same 

substantive and procedural requirements as a designated use removal.3Therefore a variance must 

be supported by one of the factors at § 131.10(g) demonstrating that the current designated use 

and criteria are unattainable at the present time. In addition, because a variance is a revision to 

WQS, it requires review and approval or disapproval by EPA. Variances are different from 

revisions to designated uses (described below) in that variances are time-limited and intended to 

be a mechanism to provide time for states, dischargers, and other stakeholders to implement 

adaptive management approaches that are aimed at improving water quality and ultimately 

attaining the designated use.

A discharger may be interested in a variance where 1) the National Pollutant Discharge 

Elimination System (NPDES) permitting authority has determined that there is reasonable 

potential for the discharger to cause or contribute to an excursion above a newly adopted 

ammonia criterion and 2) the state and discharger can show, based on §131.10(g), that the 

designated use and criteria for the particular waterbody or segment are unattainable immediately 

or within a limited period of time because the discharger cannot meet its new ammonia 

WQBELs. In such a case, the state may adopt a discharger-specific variance as long as the 

variance is consistent with the CWA and implementing regulations. The practical effect of such a 

variance is that the discharger’s NPDES permit may be written to comply with a less stringent 

designated use and criteria.

In the case of ammonia, the variance demonstration would likely be based on § 131.10(g)(6), 

which allows a use to be removed where water quality-based controls “...would result in 

substantial and widespread economic and social impact.” If a state successfully demonstrates that 

§ 131.10(g)(6) precludes attainment of the use and ammonia criteria immediately or within a 

limited period of time, the state may establish interim ammonia criteria that would allow the 

 

3

US EPA. March 29, 1977. Office of General Counsel on Matters of Law Pursuant to 40 CFR Section 125.36(m). 

No. 58. U.S. Environmental Protection Agency. Washington, D.C. Available at: 

http://water.epa.gov/scitech/swguidance/standards/upload/2008_08_04_standards_section40cfr3.pdf; and 

EPA’s Water Quality Standards Handbook. Available at: http://www.epa.gov/wqshandbook. 
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discharger’s new WQBELs to be adjusted for ammonia only. See EPA’s Interim Economic 

Guidance for Water Quality Standards: Workbook (March 1995, EPA-823-B-95-002, 

http://water.epa.gov/scitech/swguidance/standards/economics/index.cfm) for guidance on the 

types of information that a state should consider and include in its record to support a variance 

based on § 131.10(g)(6).

Typically, a state adopts a variance for an individual discharger for a specific pollutant in a 

specific waterbody. However, where multiple dischargers have similar attainment challenges, a 

state may streamline its variance process by adopting a multiple discharger variance. Such a 

variance would apply to several dischargers and may be supported by a single technical rationale 

justifying the need for a variance. EPA has previously published information on individual and 

multiple discharger variances in Water Quality Guidance for the Great Lakes System (60 FR 56 

(March 23, 1995) p. 15366, 40 CFR § 132). For additional information on variances, also see 

Discharger-Specific Variances on a Broader Scale: Developing Credible Rationales for 

Variances that Apply to Multiple Dischargers (March 2013, EPA-820-F-13-012, 

http://water.epa.gov/scitech/swguidance/standards/library/) and EPA’s Water Quality Standards 

Handbook at http://www.epa.gov/wqshandbook.

3. Revisions to Designated Uses

The WQS regulation at 40 CFR § 131.10(g) provides that “[s]tates may remove a designated 

use... or establish sub-categories of a use if the [s]tate can demonstrate that attaining the 

designated use is not feasible...” because of at least one of the six specified factors. The state 

would make such a demonstration through a use attainability analysis (UAA). A UAA is defined 

under § 131.3(g) as “...a structured scientific assessment of the factors affecting the attainment 

of the use which may include physical, chemical, biological, and economic factors as described 

in § 131.10(g).” A UAA must be conducted where the state designates or has designated uses 

that do not include a CWA § 101(a)(2) use, where the state wants to remove a designated use 

that is also a § 101(a)(2) use (e.g., aquatic life use), or where the state wants to adopt 

subcategories of a § 101(a)(2) use that require less stringent criteria. 40 CFR § 131.10(j). 

In the case of ammonia, a UAA would likely be based on the factor described in § 131.10(g)(6), 

which allows a use to be removed where water quality based controls “...would result in 

substantial and widespread economic and social impact.” For guidance on the types of 

information that a state should consider and include in its record to support a use revision based 

on § 131.10(g)(6), see EPA’s Interim Economic Guidance for Water Quality Standards: 

Workbook (March 1995, EPA-823-B-95-002, 

http://water.epa.gov/scitech/swguidance/standards/economics/index.cfm). 

While the WQS regulation allows states to remove uses that are not feasible to attain, it does not 

allow states to remove uses that are feasible to attain. Therefore, states must retain/adopt such 

feasible uses (i.e., the highest attainable use) in their state WQS. 40 CFR 131.10(g) and (h). 

States have the flexibility to determine how to articulate attainable uses in their WQS. For 

example, to the extent allowed by state law, the state could refine its designated use from 

“aquatic life” to “ammonia-limited aquatic life” where ammonia is the parameter preventing 
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attainment of the full aquatic life use. In this situation, the state could then revise its ammonia 

criteria to protect the ammonia-limited aquatic life use while retaining the other pollutant criteria 

designed to protect the full aquatic life use.

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For additional information on use changes and UAAs, see EPA’s Water Quality Standards 

Handbook at http://www.epa.gov/wqshandbook. Additional information is also available at 

http://water.epa.gov/scitech/swguidance/standards/uses/uaa/.

4. Dilution Allowances

The NPDES permitting regulation at 40 CFR § 122.44(d)(1)(ii) states that “[w]hen determining 

whether a discharge causes, has the reasonable potential to cause, or contributes to an in-stream 

excursion above a narrative or numeric criterion within a [s]tate water quality standard, the 

permitting authority shall use procedures which account for...where appropriate, the dilution

[emphasis added] of the effluent in the receiving water.”

Many state WQS have general provisions allowing some consideration of dilution when 

determining the need for and calculating WQBELs. A dilution allowance is typically expressed 

as the flow of a river or stream, or a portion thereof, that is allowed to mix with and dilute 

effluent before water quality criteria must be met. Under incomplete mixing situations, the 

dilution allowance is typically expressed as a dilution factor determined from a regulatory 

mixing zone. State WQS and implementation policies often describe the conditions under which 

dilution is allowed and might also indicate specific locations or conditions (e.g., breeding 

grounds for aquatic species or bathing beaches) or specific water quality criteria (e.g., pathogens, 

bioaccumulative pollutants, or carcinogenic pollutants) for which consideration of a dilution 

allowance is not allowed or is otherwise considered inappropriate. For additional information on 

dilution, see sections 6.2.2 and 6.2.5 of EPA’s NPDES Permit Writers’ Manual (September 

2010, EPA-833-K-10-001, http://www.epa.gov/npdes/pwmanual), sections 2.2.2, 4.4, and 4.5 of 

Technical Support Document for Water Quality-based Toxics Control (March 1991, EPA/505/2-

90-001, http://www.epa.gov/npdes/pubs/owm0264.pdf) , and Water Quality Standards 

Handbook at http://www.epa.gov/wqshandbook.

The amount of dilution a particular discharger has available for calculating WQBELs depends on 

the applicable water quality criteria as well as the instream ambient concentration of ammonia. 

Taking into consideration flow conditions and ambient ammonia concentrations throughout the 

watershed may be useful to identify and address all sources of ammonia to a waterbody. For 

more information on this watershed approach, see EPA’s watershed approach resources at 

http://water.epa.gov/type/watersheds/approach.cfm.

 

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See 40 CFR § 131.20(a) for review and revision requirements when new information indicates that the previously unattainable 

uses specified in § 101(a)(2) of the CWA are now attainable.
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5. Compliance Schedules

Permit compliance schedules, along with variances, are the most common regulatory tools used 

by states to provide dischargers with time to meet regulatory requirements. Which tool is 

appropriate depends upon the circumstances. Variances can be appropriate to address situations 

where it is known that the designated use and criteria are unattainable immediately or within a 

limited period of time, but feasible progress could be made toward attaining the designated use 

and criteria. In contrast, a permit compliance schedule may be appropriate when 1) the 

designated use is attainable, 2) the discharger’s WQBELs are achievable, and 3) the discharger

needs time to modify or upgrade treatment facilities in order to meet its WQBELs such that a 

schedule and resulting milestones will lead to compliance with the WQBELs (which are based 

on the currently applicable WQS) “as soon as possible.” See § 502(17) of the CWA for a 

definition of “schedules of compliance” and 40 CFR § 122.47.

Compliance schedules in NPDES permits can be used to meet WQBELs based on WQS adopted 

after July 1, 1977, if the state has clearly indicated in its WQS or implementing regulations that it 

intends to allow the use of permit compliance schedules for the pollutant of concern. A 

compliance schedule may be included in a permit with WQBELs for ammonia where (1) the 

discharger requires time to install treatment technology or implement other controls necessary to 

meet the new WQBELs and (2) the permitting authority (which is the state in most cases) 

determines that a compliance schedule is “appropriate” in light of all the circumstances and that 

the discharger can ultimately meet its new ammonia WQBELs by an “as soon as possible” date 

certain in the future. To ensure that the compliance schedule is enforceable by the permitting 

authority, the schedule must consist of a sequence of interim requirements, actions, or operations 

leading to compliance with the CWA and its implementing regulations. 40 CFR § 122.2. If the 

schedule is longer than one year, the schedule must include annual interim requirements and 

dates for their achievement. 40 CFR § 122.47(a)(3). Additionally, the permit must include a final 

WQBEL and a date for its achievement. The decision to include a compliance schedule as well 

as all of the dates and interim requirements must be supported by the administrative record. The 

permit fact sheet should include a justification explaining why the compliance schedule was 

determined to be “appropriate” and why the chosen time frame was deemed “as soon as 

possible.” For additional information on compliance schedules, see section 9.1.3 of EPA’s

NPDES Permit Writers’ Manual (September 2010, EPA-833-K-10-001, 

http://www.epa.gov/npdes/pwmanual) and Compliance Schedules for Water Quality-based 

Effluent Limitations in NPDES Permits (May 2007, 

http://www.epa.gov/npdes/pubs/memo_complianceschedules_may07.pdf).

Flexibility Framework for States Applying EPA’s Ammonia Criteria 

Recommendations 

The above flexibilities are available to assist states in considering the adoption and application of 

EPA’s ammonia criteria recommendations. In a given situation, a state may want to employ 

multiple flexibilities depending on the particular circumstances. For example, a state may decide 

that only a variance is appropriate in one situation while the Recalculation Procedure, a dilution 
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allowance, and a compliance schedule are appropriate in a different situation. Figures 1 and 2 

display the components of the Framework for States Applying EPA’s Ammonia Criteria 

Recommendations (Framework) and help to guide states in determining which flexibility or 

combination of flexibilities is available in a particular situation. Figure 1 focuses on the WQS 

adoption process, and Figure 2 focuses on WQS application.

Utilizing Flexibilities in the Water Quality Standards Adoption Process

The first section of the Framework displays the process by which states may evaluate the 

ammonia criteria recommendations and under what circumstances a state may want to utilize the 

Recalculation Procedure, variances, or revisions to designated uses: 

EPA publishes final ammonia criteria recommendations.

State evaluates the recommended criteria during its current or next WQS triennial 

review process: Are freshwater mussels present in waters throughout the state?

Evaluate criteria applicability 

and use attainability. If given 

time, is the designated use 

ultimately attainable?

Utilize Recalculation 

Procedure to derive sitespecific criteria, where 

appropriate.

State considers 

adopting a variance.

Figure 1: State Water Quality Standards Adoption Process

State submits new or revised WQS to EPA for review.

Assess whether to 

utilize existing 

mussel data and/or 

collect new data.

State proposes and finalizes new or revised WQS for ammonia.

State considers 

revising the use.

Unknown

No Yes Unknown

No Yes
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CWA § 303(c)(1) and EPA’s WQS regulations at 40 CFR § 131.20(a) provide that states review 

their WQS every three years and modify and adopt standards, as appropriate. With the

publication of EPA’s final ammonia criteria recommendations, states should consider the 

availability of these new data and criteria recommendations during their current or next WQS 

triennial review.5States should consider adopting the recommendations to protect the aquatic life 

uses in their waters. 

One main consideration in adoption of ammonia criteria is the presence of mussels throughout 

state waters. If mussels are present throughout the state, states may want to consider whether 

aquatic life designated uses can be attained in their waterbodies as part of the state WQS 

adoption process. That way, if a variance or revision to a designated use is warranted, the state 

can include those actions as part of the WQS revisions to better facilitate application of the 

criteria. If the state concludes that the designated use is unattainable, the state should then 

consider whether the use is unattainable within a limited period of time or if the use is ultimately 

unattainable in the long term. Where the use is unattainable within a limited period of time or 

where attainability is unknown, the state may want to consider adopting a variance. Where the 

use is ultimately unattainable, the state may want to consider revising the designated use and 

criteria necessary to protect the revised use. 

Where mussels are not present throughout the state, states may want to consider utilizing the 

Recalculation Procedure on a site-specific or watershed basis where the state can demonstrate 

that freshwater mussels are not present at a particular site or in a particular watershed. Before 

choosing this approach, the state should consider how it will demonstrate that mussels are not 

present at the site. If the state is unsure whether mussels are present at a particular site or in a 

particular watershed, the state should assess whether to utilize existing mussel distribution data 

and/or collect new data to determine whether mussels are present and adopt criteria that are 

protective of the use. As with any criteria, states choosing to utilize the Recalculation Procedure 

should ensure that their site-specific criteria “...provide for the attainment and maintenance of 

the water quality standards of downstream waters.” 40 CFR § 131.10(b).

Utilizing Flexibilities in Water Quality Standards Application

The second section of the Framework outlines the process by which the ammonia criteria, as 

adopted into state WQS, may be applied in the NPDES permitting, monitoring, assessment, 

listing, and Total Maximum Daily Load (TMDL) programs:

 

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In addition to ammonia criteria, if a state is also considering adoption of criteria for total nitrogen (TN), it is important to note 

that the two parameters and their treatment processes are linked. A full biological nutrient removal (BNR) process of 

nitrification and denitrification will provide treatment for both parameters, as ammonia is one component of TN (TN is the sum 

of nitrate, nitrite, ammonia, and organic nitrogen). BNR systems that are designed to remove TN can be used to remove 

some ammonia, but ammonia treatment alone is not sufficient to remove TN. Where the state plans to adopt criteria 

for both ammonia and TN, dischargers within the state may be required to meet WQBELs for both parameters and may 

see long-term cost savings by planning treatment upgrades for both parameters up front.
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If given time to install necessary controls, can 

the discharger achieve the new WQBELs?

New state WQS for ammonia are approved by EPA for CWA purposes.

If state provisions allow, determine if the 

discharge has reasonable potential (RP) to cause 

or contribute to an excursion above the criterion 

using a dilution allowance, as appropriate.

If the discharger has RP and if state provisions 

allow, calculate new WQBELs with a dilution 

allowance, as appropriate. If a TMDL is 

available, calculate new WQBELs that are 

consistent with the assumptions and 

requirements of the wasteload allocations.

During the state’s regular 

assessment cycle, monitor and 

assess waters using the new 

criteria.

Impaired waterbodies are placed 

on the state’s list of impaired 

waters (i.e., the state’s CWA 

§ 303(d) list). 

State considers 

whether the 

designated use 

is ultimately 

attainable.

Is the 

designated 

use 

ultimately 

attainable?

State develops 

a TMDL.

Permitting 

authority 

calculates 

WQBELs that 

are consistent 

with the 

assumptions 

and 

requirements of 

the available 

wasteload 

allocations.

State 

considers 

adopting a 

variance.

Figure 2: State Water Quality Standards Application Processes

Unknown

If EPA-approved 

state provisions 

allow, permitting 

authority may 

include a 

compliance 

schedule in the 

permit, as 

appropriate.

State considers 

adopting a 

variance.

State considers 

adopting a 

variance.

State considers 

revising the 

designated use.

Yes Unknown

No Yes

No

If the use is 

ultimately 

unattainable, 

state considers 

revising the 

designated 

use.
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Water Quality Standards Application in NPDES Programs

Once a state’s new ammonia criteria have been submitted to and approved by EPA, the 

permitting authority (which is the state in most cases) can begin to apply the new criteria in its 

NPDES permits. Permitting authorities typically revise permits based on new criteria at the time 

of permit reissuance.6The permitting authority should determine whether the state WQS allow 

for consideration of dilution and, if so, whether dilution is available in the discharger’s receiving 

water. If the state allows for dilution and dilution is available in the receiving water for a specific 

discharger, the permitting authority may determine the need for ammonia WQBELs (i.e., a 

“reasonable potential” or RP analysis) and calculate WQBELs, if necessary, using a dilution 

allowance.7 However, if the state does not allow for consideration of dilution and/or dilution is 

not available in the discharger’s receiving water, the permitting authority should determine RP 

and calculate WQBELs, if necessary, without consideration of a dilution allowance (i.e., at the 

end-of-pipe). If a state does not currently have a provision allowing for dilution, the state may 

want to consider adopting such a provision.

The permitting authority should also determine whether the state WQS allow for the use of 

compliance schedules in NPDES permits. The permitting authority may consider including a 

compliance schedule in the discharger’s permit if 1) the state has an EPA-approved authorizing 

provision for compliance schedules in its WQS or implementing regulations and 2) the 

discharger can meet its new WQBELs for ammonia but requires time to install treatment 

technology or implement other controls necessary to meet its new WQBELs.8 However, the 

permitting authority may not include a compliance schedule in the permit if the state does not 

have an EPA-approved authorizing provision for compliance schedules in its WQS or 

implementing regulations or where the discharger is already capable of meeting its new ammonia 

WQBELs without additional time to install treatment technology or implement other controls. 

The state may want to consider adopting a provision to authorize the use of compliance 

schedules if the state does not currently have such a provision. Any such provision must be 

submitted to EPA for review and approval or disapproval under § 303(c) of the CWA.

As previously explained in the WQS adoption process, where the state concludes that the 

designated use is unattainable within a limited period of time or attainability is unknown, the 

state may consider adopting a variance. Where the current designated use is ultimately 

unattainable, the state may want to consider revising the designated use and associated criteria

necessary to protect the new designated use. 

 

6

The maximum permit term for an NPDES permit is five years. 

7

The NPDES permitting regulation at 40 CFR § 122.44(d)(1)(i) requires that permit limits control pollutants that “...are or may 

be discharged at a level which will cause, have the reasonable potential [emphasis added] to cause, or contribute to an 

excursion above any [s]tate water quality standard...”

8 Where a state adopts ammonia criteria into its WQS and a reasonable potential analysis shows that WQBELs are necessary for 

a particular discharger, treatment technology upgrades would reduce the ammonia levels in the effluent and, combined with a 

permit compliance schedule where appropriate, would assist the discharger in achieving new ammonia WQBELs. The treatment 

technology to remove ammonia may be a combination of biological, physical, or chemical processes. The biological process of 

nitrification present in BNR technology is a primary method used to remove ammonia to certain levels by converting it to 

nitrate.
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Water Quality Standards Application in Monitoring, Assessment, and Listing Programs

On a parallel track, once a state’s new ammonia criteria have been submitted to and approved by 

EPA, the state can begin to monitor and assess waters using its new criteria during its regular 

assessment cycle. If a waterbody is determined to be impaired and placed on the state’s list of 

impaired waters (i.e., the state’s CWA § 303(d) list), the state must develop a TMDL for that 

waterbody. The permitting authority can then calculate new ammonia WQBELs that are 

consistent with the assumptions and requirements of the wasteload allocations established in the 

TMDL. Concurrent with the TMDL development process, the state may also want to assess 

whether the designated use is ultimately attainable, as the TMDL development process may 

inform a UAA. If the use is ultimately unattainable, the state may want to consider revising the 

designated use and associated criteria necessary to protect the new designated use. 

Summary

EPA is updating its 1999 CWA § 304(a) national ambient water quality criteria 

recommendations for ammonia to account for the sensitivity of freshwater mussels and snails to 

ammonia toxicity. In applying these updated ammonia criteria recommendations, a number of 

flexibilities are available for state consideration including the Recalculation Procedure for sitespecific criteria derivation, WQS variances, revisions to designated uses, dilution allowances, 

and compliance schedules. Flexibilities may be utilized alone or in combination with one another 

depending on the specific situation. EPA has developed the Flexibility Framework for States 

Applying EPA’s Ammonia Criteria Recommendations to help guide states in determining which 

flexibility or combination of flexibilities is available to the state in a particular situation as the 

state considers the new information provided by EPA’s updated criteria recommendations. 

Flexibilities are available throughout the WQS adoption and application processes.