United States
Environmental Protection Agency
ice of Water
Mail Code 4305T
EPA-820-F-13-001
April 2013
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 §131.1 l(b)(l)(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.
1
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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 site-
specific 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.2 The 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.3 Therefore 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 EP A'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.4
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)(l)(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.
4 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, EP A-83 3-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?
No
Yes
Utilize Recalculation
Procedure to derive site-
specific criteria, where
appropriate.
Evaluate criteria applicability
and use attainability. If given
time, is the designated use
ultimately attainable?
No
State considers
revising the use.
Unknown
Assess whether to
utilize existing
mussel data and/or
collect new data.
Yes
Unknown
State considers
adopting a variance.
State proposes and finalizes new or revised WQS for ammonia.
State submits new or revised WQS to EPA for review.
Figure 1: State Water Quality Standards Adoption Process
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CWA § 303(c)(l) 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.5 States 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:
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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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.
I
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.
I
If given time to install necessary controls, can
the discharger achieve the new WQBELs?
± No
Is the
designated
use
ultimately
attainable?
,Yes ^ Unknc*
If EP A-approved
state provisions
allow, permitting
authority may
include a
compliance
schedule in the
permit, as
appropriate.
State
considers
adopting a
variance.
During the state's regular
assessment cycle, monitor and
assess waters using the new
criteria.
I
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.
State develops
a TMDL.
If the use is
ultimately
unattainable,
state considers
revising the
designated
use.
Unknown
State considers
revising the
designated use.
State considers
adopting a
variance.
State considers
adopting a
variance.
Permitting
authority
calculates
WQBELs that
are consistent
with the
assumptions
and
requirements of
the available
wasteload
allocations.
Figure 2: State Water Quality Standards Application Processes
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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.6 The 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.
The NPDES permitting regulation at 40 CFR § 122.44(d)(l)(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 site-
specific 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.
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