A rnA United States
Environmental Protection
Agency October 2021
Frequently Asked Questions: Implementing Water Quality
Standards Based on EPA's 2016 Recommended Selenium
Criterion in Clean Water Act Section 402 NPDES Permits
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Frequently Asked Questions (FAQs): Implementing
Water Quality Standards Based on EPA's 2016
Recommended Selenium Criterion in Clean Water
Act Section 402 NPDES Permits
This document supports Clean Water Act (CWA) regulators in states, territories, and authorized
tribes1 in implementing state water quality standards based on the U.S. Environmental
Protection Agency (EPA) recommendedfour-part selenium water quality criterion (WOC)2 in
EPA's 2021 Revision to: Aquatic Life Ambient Water Quality Criterion for Selenium-Freshwater
2016 (USEPA 2021a; here after referred to as Aquatic Life Ambient Water Quality Criteria for
Selenium - Freshwater 2016) as it relates to the CWA section 402 National Pollutant Discharge
Elimination System (NPDES) permits. A summary of the recommended criterion is provided in
Table 1. This set of frequently asked questions (FAQs) provides information on how to establish
water quality-based effluent limits (WOBELs) in NPDES permits.
While this document cites statutes and regulations that contain requirements applicable to
NPDES permitting, it does not impose legally binding requirements on EPA, states, authorized
tribes, other regulatory authorities, or the regulated community and may not apply to a
particular situation based upon the circumstances. EPA, state, tribal, and other decision makers
retain the discretion to adopt approaches on a case-by-case basis that differ from those provided
in this technical support document as appropriate and consistent with statutory and regidatory
requirements. EPA may update this document as new information becomes available. In addition
to this document, EPA has related documents that provide considerations and recommendations
on implementing the national CWA section 304(a) recommended selenium criterion for
freshwater, which are available at EPA's selenium website: https://www. epa.sov wqc aquatic-
life-criterion-selenium.
Note: For applicable state or authorized tribal water quality standards (WOS) that are not based
on the national CWA section 304(a) recommended selenium criterion, EPA will assess the need
for alternative state or authorized tribal specific implementation permitting guidance to ensure
consistency with the NPDES permitting requirements at Title 40 of the Code of Federal
Regulations (CFR) section 122.44.
1 Throughout this document and in the CWA. the term "states" means a state, the District of Columbia, the
Commonwealth of Puerto Rico, the United States Virgin Islands, Guam, American Samoa, Commonwealth of the
Northern Mariana Islands, and the Trust Territory of the Pacific Islands. CWA section 502(3). The term "authorized
tribe" means those federally recognized Indian tribes with authority to administer a CWA WQS program.
2 These FAQs apply specifically with respect to an EPA-approved state and tribal water quality criterion for
selenium that is based on EPA's national CWA section 304(a) recommended selenium criterion - that is, one that
includes elements substantially identical to the recommended selenium criterion. Where the applicable state or tribal
water quality criterion for selenium differs from EPA's recommended selenium criterion these FAQs may not be
relevant or applicable. As required by CWA section 301(b)(1)(C), permit writers must include limits necessary to
meet the applicable state or tribal water quality criterion.
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Table of Contents
Introduction 1
EPA's National CWA section 304(a) Recommended Chronic Aquatic Life Selenium
Criterion in Freshwater 1
Selenium Technical Support Materials 3
1. Identifying the Applicable Selenium WQS to Implement in the NPDES Permit 4
Ql-1: What is the applicable selenium criterion to use for conducting a reasonable
potential (RP) determination or for developing effluent limits in an NPDES permit? 4
Ql-2: Which criterion elements of EPA's CWA section 304(a) recommended four-part
criterion for selenium should be used when implementing the criterion through the
NPDES permits program? 5
Ql-3: Does state or authorized tribal adoption of a site-specific value for either egg-ovary
or whole body/muscle elements require EPA approval under CWA section 303(c) before
using the site-specific value in NPDES permitting? 6
Ql-4: When using the water column criterion element, can permit writers use mixing
zones, initial zones of dilution, or dilution factors in NPDES permits? 7
2. NPDES Reasonable Potential Determinations 7
Q2-1: Which elements of the four-part criterion included in state or authorized tribal
WQS for selenium should be used for conducting RP analyses? 7
Q2-2: How should a regulatory authority determine the appropriate water column
criterion element based on the four-part criterion (e.g., a 30-day chronic criterion element
versus an intermittent criterion element) for use in an NPDES permit? 9
Q2-3: What data should be used to assess RP using the four-part criterion? 10
Q2-4: In cases where a discharge occurs to a lotic (e.g. a flowing stream) water, but
downstream waters are lentic (e.g., lakes, impoundments), which selenium criterion water
column element should be used in the RP analysis? 10
Q2-5: In some cases, it might be unclear whether a receiving or downstream water body
is lentic or lotic with regard to an appropriate selenium water column criterion element
(e.g., run-of-the-river reservoirs). Does EPA have recommendations that address whether
a receiving water body should be considered lentic or lotic for purposes of NPDES RP
analysis and permit WQBEL determinations? 10
3. NPDES Permit WQBEL Calculations 11
Q3-1: Can an NPDES permitting authority derive selenium WQBELs using an EPA-
approved state or authorized tribal WQS based on EPA's recommended four-part
selenium criterion for a noncontinuous or intermittent effluent discharge containing
selenium? If so, how? 11
Q3-2: Can an NPDES permitting authority develop WQBELs using the fish tissue
criterion element(s) of the four-part selenium criterion (i.e., egg-ovary or whole body
and/or muscle) rather than the water column elements? 12
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Q3-3: Can the selenium WQBEL be expressed in units of mg/kg selenium in ambient fish
tissue, measured by dry weight? 12
Q3-4: The national CWA section 304(a) recommended selenium criterion does not
include an acute expression. Must permits contain both short- and long-term limit
expressions? 13
Q3-5: Should the selenium WQBEL be expressed as total recoverable selenium? If so,
how should an NPDES permitting authority translate the dissolved selenium water
column criterion element of the national CWA section 304(a) recommended selenium
criterion to a total recoverable selenium concentration for a NPDES permit limit? 14
4. NPDES Monitoring (Basis for RP and WQBELs, Compliance Monitoring) 15
Q4-1: What EPA analytical test methods are recommended for analyzing water and fish
tissue samples for selenium under EPA's NPDES permits program? 15
5. Compliance Schedules in NPDES Permits 15
Q5-1: Would a compliance schedule be allowed in a renewed, modified, or reissued
NPDES permit in which an existing WQBEL is being replaced with a more stringent
NPDES permit limit or a new NPDES permit limit based on an EPA-approved state or
tribal selenium criterion reflecting EPA CWA section 304(a) recommended selenium
criterion? 15
References 16
Attachment 1: EPA's Recommended Flowcharts for Implementing the National
CWA Section 304(a) Recommended Selenium Criterion in NPDES Permits Program .... 18
in
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List of Acronyms
jig/L
micrograms per liter
AML
Average monthly limit
CFR
Code of Federal Regulations
CV
Coefficient of variation
CWA
Clean Water Act
EPA
Environmental Protection Agency
FAQ
Frequently asked question
FT
Fish tissue
FTE
Fish tissue element
LTA
Long-term average
MDL
Maximum daily limit
mg/kg
milligrams per kilogram
NPDES
National Pollutant Discharge Elimination System
POTW
Publicly owned treatment works
RP
Reasonable potential
TMDL
Total maximum daily load
TSD
Technical support document
USEPA
United States Environmental Protection Agency
WC
Water column
WCE
Water column element
WLA
Wasteload allocation
WQBELs
Water quality-based effluent limits
WQC
Water quality criterion
WQS
Water quality standards
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Definitions
bioaccumulation
The uptake and retention of a chemical by an aquatic organism from all surrounding media (e.g.,
water, food, sediment) (USEPA 2003).
chronic
A stimulus that lingers or continues for a relatively long period of time, often one-tenth of the
life span or more. Chronic should be considered a relative term depending on the life span of an
organism. The measurement of a chronic effect can be growth, reduced reproduction, etc., in
addition to lethality (USEPA 1991).
coefficient of variation
A standard statistical measure of the relative variation of a distribution or set of data, defined as
the standard deviation divided by the mean (USEPA 1991).
frequency
How often criteria can be exceeded without unacceptably affecting the community (USEPA
1991).
lentic
Characterized by largely standing water, such as lakes and ponds (Jones 1997).
lotic
Characterized by flowing water, such as rivers and streams (Jones 1997).
magnitude
How much of a pollutant (or pollutant parameter such as toxicity), expressed as a concentration
or toxic unit is allowable (USEPA 1991).
reasonable potential (RP)
Where an effluent is projected or calculated to cause an excursion above a water quality standard
based on a number of factors including, as a minimum, the four factors listed in 40 CFR
122.44(d)(l)(ii).
steady state
An organism is in steady state when the rates of chemical uptake and depuration are equal and
tissue concentrations remain constant over time (USEPA 2003). For the purposes of the national
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CWA section 304(a) recommended selenium criterion, steady state refers to conditions where
sufficient time has passed after the introduction of a new or increased discharge of selenium into
a water body so that fish tissue concentrations of selenium are no longer increasing (USEPA
2016).
total maximum daily load (TMDL)
The sum of the individual wasteload allocations and load allocations. A margin of safety is
included with the two types of allocations so that any additional loading, regardless of source,
would not produce a violation of water quality standards (USEPA 1991).
wasteload allocation (WLA)
The portion of a receiving water's total maximum daily load that is allocated to one of its
existing or future point sources of pollution (USEPA 1991).
water quality criteria (WQC)
Comprised of numeric and narrative criteria. Numeric criteria are scientifically derived ambient
concentrations developed by EPA or States for various pollutants of concern to protect human
health and aquatic life. Narrative criteria are statements that describe the desired water quality
goal (USEPA 1991).
water quality standard (WQS)
A law or regulation that consists of beneficial designated use or uses of a water body, the
numeric and narrative water quality criteria that are necessary to protect the use or uses of that
particular waterbody, and an antidegradation statement (USEPA 1991).
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Introduction
EPA's National CWA section 304(a) Recommended Chronic Aquatic Life
Selenium Criterion in Freshwater
In 2016, EPA updated its national CWA section 304(a) recommended chronic aquatic life
criterion for selenium in freshwater systems to reflect the latest scientific information. This
information indicates that toxicity to aquatic life is driven by dietary exposures, and that the
reproductive life-stages of egg-laying vertebrates are the most sensitive to the toxic effects of
selenium. The recommended criterion has four criterion elements: (1) a fish egg-ovary criterion
element; (2) a fish whole-body and/or muscle criterion element; (3) a water column criterion
element (one value for lentic and one value for lotic aquatic systems); and (4) a water column
intermittent criterion element (to account for potential chronic effects from short-term exposures
to high concentrations in lentic and lotic aquatic systems (see Table 1)). Under EPA's 2016
CWA section 304(a) recommended selenium criterion, the fish tissue criterion elements have
primacy over water column elements, except where there are no fish, where fish tissue data do
not meet state or tribal quality assurance procedures, or for water bodies with new discharges and
hence selenium concentrations in fish tissue might not have stabilized (USEPA 2018a). EPA also
recommends that the egg-ovary tissue criterion element has primacy over the whole-body and/or
muscle tissue criterion element.
Toxicity data indicate that the selenium concentration in fish eggs and ovaries is the most robust
and consistent measurement endpoint directly linked to adverse reproductive effects in aquatic
organisms. Likewise, toxicity to developing embryos and larvae is directly linked to egg
selenium concentration (USEPA 2021a). EPA derived the whole-body, muscle tissue, and water
column elements from the egg-ovary criterion element so that states and authorized tribes could
more readily implement WQC based on EPA's national CWA section 304(a) recommended
selenium criterion. The assessment of the available data on chronic selenium exposure for fish,
invertebrates, and amphibians indicated that a criterion element derived from fish is expected to
be protective of the aquatic community, because other taxa appear to be less sensitive to
selenium than fish.
EPA did not develop an acute criterion for selenium when it updated the chronic criterion
because, although selenium may cause acute toxicity at high concentrations, the most deleterious
effects on aquatic organisms are due to selenium's bioaccumulative properties. The
bioaccumulative effects, coupled with chronic effects, occur at lower concentrations than acute
effects. In the case of bioaccumulative compounds like selenium, acute toxicity studies do not
address risks that result from chronic exposure to chemicals via the diet (i.e., through the food
web pathway). Such studies also do not account for the accumulation kinetics of many
bioaccumulative compounds, such as selenium, and may underestimate effects from long-term
accumulation in some types of aquatic systems. As described in EPA's Aquatic Life Ambient
Water Quality Criterion for Selenium-Freshwater 2016, EPA also included an intermittent
exposure criterion element to provide protection from the most significant selenium toxicity
effects, reproductive toxicity, by protecting against selenium bioaccumulation in the aquatic
ecosystem resulting from short-term, high concentration exposure events (USEPA 2016a).
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EPA recommends, as stated in the Aquatic Life Ambient Water Quality Criterion for Selenium-
Freshwater 2016, that states and authorized tribes3 adopt into their WQS a selenium criterion
that includes all four criterion elements (USEPA 2021a). For more information see EPA's
Aquatic Life Ambient Water Quality Criterion for Selenium-Freshwater 2016, which can be
found at:
https://www.epa.gov/svstem/files/documents/2021 -08/selenium-freshwater2016-2021 -revision.pdf
Table 1: Summary of the Recommended Freshwater Selenium Ambient Chronic Water
Quality Criterion for Protection of Aquatic Life.
Media Type
Fish Tissue1
Water Column4
Criterion
Element
Egg-ovary 2
Fish Whole-body
or Muscle3
Monthly
Average
Exposure
Intermittent Exposure5
Magnitude
15.1 mg/kg dry
weight
8.5 mg/kg dry
weight whole-
body
or
11.3 mg/kg dry
weight muscle
(skinless, boneless
fillet)
1.5 pg/L in lentic
aquatic systems
3.1 pg/L in lotic
aquatic systems
WQClnt =
WQC^o-duy Cbkgrndi.^- f int.
f int
Duration
Instantaneous
measurement6
Instantaneous
measurement6
30 days
Number of days/month with an
elevated concentration
Frequency
Not to be
exceeded
Not to be exceeded
Not more than
once in three
years on average
Not more than once in three
years on average
1. Fish tissue elements are expressed as steady state.
2. Egg-ovary supersedes any whole-body, muscle, or water column element when fish egg-ovary concentrations are measured,
except as noted in footnote 4 below.
3. Fish whole-body or muscle tissue supersedes water column element when both fish tissue and water concentrations are
measured, except as noted in footnote 4 below.
4. Water column values are based on dissolved total selenium in water and are derived from fish tissue values via
bioaccumulation modeling. When selenium inputs are increasing, water column values are the applicable criterion element in
the absence of steady state condition fish tissue data.
5. Where WQC3o-dayis the monthly average exposure from previous column for either lentic or lotic waters; Cbkgmdis the average
background selenium concentration; and fmt is the fraction of any 30-day period during which elevated selenium
concentrations occur, with fmt assigned a value >0.033 (corresponding to 1 day).
6. Fish tissue data provide instantaneous point measurements that reflect integrative accumulation of selenium over time and
space in fish population(s) at a given site.
3 Throughout this document and in the CWA. the term "states" means the fifty states, the District of Columbia, the
Commonwealth of Puerto Rico, the United States Virgin Islands, Guam, American Samoa, the Commonwealth of
the Northern Mariana Islands, and the Trust Territory of the Pacific Islands. CWA section 502(3): The term
"authorized tribe" means those federally recognized Indian tribes with authority to administer a CWA WQS
program.
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Selenium Technical Support Materials
EPA has prepared a four-volume set of documents to provide recommendations to states,
authorized tribes, and other agencies for implementing the WQC based on the national CWA
section 304(a) recommended selenium criterion (USEPA 2016a). These four documents
constitute the Technical Support Materials for EPA's Aquatic Life Ambient Water Quality
Criterion for Selenium-Freshwater 2016 (USEPA 2021a). Each document focuses on a specific
aspect of implementation of the national CWA section 304(a) recommended selenium criterion.
Together, these four EPA documents provide information to assist states and authorized tribes
with adopting the WQC based on EPA's CWA section 304(a) recommended selenium criterion
and implementing it in various CWA programs.
1) Technical Support for Adopting and Implementing EPA's Selenium 2016 Criterion in
Water Quality Standards, Draft (USEPA 2021b): provides recommendations for the
adoption and implementation of the national CWA section 304(a) recommended selenium
criterion, including the various flexibilities available to states and tribes using WQS tools.
2) Technical Support for Fish Tissue Monitoring for Implementation of EPA's 2016 Selenium
Criterion, Draft (USEPA 2021c): provides an overview on how to establish or enhance
existing fish tissue monitoring programs to facilitate implementation of the fish tissue-
based criterion elements in the national CWA section 304(a) recommended selenium
criterion.
3) Frequently Asked Questions: Implementing Water Quality Standards Based on EPA 's
2016 Recommended Selenium Criterion in Clean Water Act Section 402 NPDES Permits,
Draft: is intended to help NPDES permit writers understand what permitting guidance
(i.e., state or tribal implementation procedures) may be appropriate to implement state and
authorized tribal WQS based on EPA's CWA section 304(a) recommended selenium
criterion. This set of FAQs also provides recommendations on how to establish water
quality-based effluent limits (WQBELs) in NPDES permits.
4) Frequently Asked Questions (FAQs): Implementing the 2016 Selenium Criterion in Clean
Water Act Sections 303(d) and 305(b) Assessment, Listing, and Total Maximum Daily
Load (TMDL) Programs, Draft (USEPA 2021d): provides information on how to
complete assessments, list impaired waters, and develop TMDLs based on EPA-approved
WQS that adhere to EPA's national CWA section 304(a) recommended selenium
criterion, including all four elements.
In addition to the four-part selenium criterion, states or authorized tribes may want to adopt a
method to derive site-specific water column criterion elements. Referred to by EPA as a
"performance-based approach," EPA first formalized this concept for a WQS in the preamble of
the rule, EPA Review and Approval of State and Tribal Water Quality Standards (USEPA 2000).
In the preamble, EPA describes this approach as a WQS that is a transparent process, rather than
a specific outcome. The state or authorized tribe can adopt a process, such as a criterion
derivation methodology, rather than a specific outcome, such as a concentration of a pollutant. If
a state or authorized tribe adopts a process or methodology that is sufficiently detailed and has
suitable safeguards that ensure predictable, repeatable outcomes, EPA can approve that process
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as a WQS. This approval would also serve for CWA purposes as the approval of each outcome
generated from following that process or method. The performance-based approach is defined in
section 1.2.2 of EPA's Technical Support for Adopting and Implementing EPA's Selenium 2016
Criterion in Water Quality Standards, Draft as a recommended approach to developing a site-
specific water column criterion element in which, "States and authorized tribes can choose to
adopt into their WOS and submit to EPA [for review and approval under CWA section 303(c)] a
set ofprocedures to facilitate the translation of the fish tissue criterion concentration elements
into site-specific water concentration values. " Currently, the performance-based approach for
development of WQS for selenium is based on the availability of fish tissue data and therefore is
not appropriate for Ashless waters.
This document addresses questions about NPDES permitting based on state or authorized tribal
WQS that include elements based on EPA's recommended selenium criterion (USEPA 2021a). It
is intended to help NPDES permit writers understand what permitting guidance (i.e., state or
tribal implementation procedures) may be appropriate to implement EPA-approved state and
authorized tribal WQS based on EPA's CWA section 304(a) recommended selenium criterion
(USEPA 2021a). The four-part recommended criterion, as summarized in Table 1, replaces
EPA's 1987 recommended selenium acute and chronic water quality criteria
1. Identifying the Applicable Selenium WQS to Implement in the
NPDES Permit
Ql-1: What is the applicable selenium criterion to use for conducting a
reasonable potential (RP) determination or for developing effluent limits
in an NPDES permit?
Al-l: Permits must include limits necessary to meet the applicable state or tribal water quality
standards, as required by CWA section 301(b)(1)(C). The applicable selenium criterion for CWA
purposes is a state or authorized tribal criterion that has been approved by EPA as protective of
the water body's designated uses. Accordingly, permit writers must develop WQBELs, where
RP has been demonstrated, to meet a state's or authorized tribe's applicable EPA-approved
selenium criterion (see CWA 301(b)(1)(C) and NPDES regulations at 40 CFR Part
122.44(d)(1)). In addition, a state or authorized tribal permit writer may have authority under a
state statute or regulation or tribal law to include WQBELs necessary to meet a more stringent
state or authorized tribal WQC for selenium that has not yet been approved by EPA. Permit
writers must also include more stringent conditions necessary to meet other appropriate water-
quality related requirement(s) under state or tribal law if a state or authorized tribe specified such
conditions in a CWA section 401 certification.
As a component of their EPA-approved WQS, a state or authorized tribe's selenium aquatic life
criterion, could be based on: (1) the national CWA section 304(a) recommended selenium
criterion; (2) EPA's 1987 recommended selenium criteria; (3) Water Quality Guidance for the
Great Lakes System, 40 CFR Part 132.6, (60 FR 15387, March 23, 1995); or (4) other
scientifically defensible methods. A state or authorized tribe's selenium criterion might be
adopted either on a site-specific basis, or throughout the state or tribe's jurisdiction, and used to
protect applicable designated uses of the receiving water.
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In addition, there may be EPA-promulgated WQS for selenium for certain waters. For waters
covered by EPA's federally promulgated WQS, the permit writer must conduct a RP
determination (40 CFRPart 122.44(d)(1)) and if RP is demonstrated develop a WQBEL that is
protective of the state or tribal WQS (CWA 301(b)(1)(C)).
EPA's approved state and authorized tribal WQS are available at
http ://water. epa. gov/ scitech/ swguidance/ standards/wq slibrary/index. cfm.
Ql-2: Which criterion elements of EPA's CWA section 304(a)
recommended four-part criterion for selenium should be used when
implementing the criterion through the NPDES permits program?
Al-2: Where a state has adopted a selenium aquatic life criterion based on EPA's recommended
four-part criterion (see previous FAQ Al-1), the NPDES permitting authority can use any of the
criterion elements to assess the need for and to establish effluent limits as stringent as necessary
to meet the applicable selenium criterion. While all four elements of EPA's 2016 selenium
criterion could be used to evaluate reasonable potential and develop permit limits for a permittee,
EPA recommends primacy in the use of fish tissue elements (i.e., egg-ovary, or whole
body/muscle) over the water column elements for receiving waters where the designated use
includes protecting fish and where steady state fish tissue concentrations have been achieved.
This recommendation is because selenium toxicity is closely associated with fish reproductive
impairment due to maternal transfer, resulting in embryotoxicity and teratogenicity (USEPA
2021a). For the purposes of the national CWA section 304(a) recommended selenium criterion,
steady state4 refers to conditions where sufficient time has passed after the introduction of a new
or increased discharge of selenium into a water body, such that fish tissue concentrations of
selenium are no longer increasing. The flowcharts provided in Attachment 1 present a
recommended approach that considers both the fish tissue and water column elements of the
criterion. Specific questions about using the four-part criterion to assess RP and WQBEL
development are discussed in sections two through four of this document.
Considerations when using the water column criterion element include the following:
1. The relationship between the concentration of selenium in the tissues of fish and the
concentration of selenium in the water column can vary substantially across aquatic
systems.
2. The species of fish, species and proportion of prey, and a variety of site-specific
biogeochemical factors affect selenium bioaccumulation. As a result, these factors
determine the allowable concentration of selenium in ambient water that is protective of
aquatic life.
3. Water column element (WCE) is the recommended part of the criterion to use in fishless
waters or for new discharges and other scenarios where fish are not at steady state in
terms of selenium bioaccumulation.
4 This definition of steady state applies to the national CWA section 304(a) recommended selenium criterion. It does
not apply to the discussion of steady state modeling or conditions in USEPA 1991. For more information on the
definition of steady state, see USEPA 2018a.
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Due to the site-specific nature of this relationship, EPA provides two recommended approaches
for translating a selenium fish tissue criterion element to a site-specific water column value. The
first approach is the mechanistic model used by EPA to derive its default water column criterion
elements. The second modeling approach uses an empirical bioaccumulation factor approach.
Both approaches are described in detail in Appendix K of Aquatic Life Ambient Water Quality
Criterion for Selenium- Freshwater 2016 (USEPA 2021a). They can be used to derive either
site-specific values for the water column elements individually adopted into a state's WQS and
submitted to EPA for review, adopted into a state's WQS as part of a performance-based
approach used to derive site-specific water column values, or used to develop site-specific water
column values that NPDES permit writers use to implement fish tissue based WQC to perform
RP analyses and calculate WQBELs. An exception is in the case of fishless waters, where a
performance-based approach is not appropriate (see Appendix K of Aquatic Life Ambient Water
Quality Criterion for Selenium- Freshwater 2016) (USEPA 2021a).
Within this NPDES FAQ document, references to the "water column criterion element" could
mean the national default water column criterion element, a state- or site-specific water column
value approved by EPA, a site-specific water column value based on an invertebrate toxicity
threshold approved by EPA, or a site-specific water column value developed using a
performance-based approach where appropriate and approved by EPA. Please see: Technical
Support for Adopting and Implementing EPA's 2016 Recommended Selenium Criterion in
Water Quality Standards, Draft (EPA 820-F-16-010, USEPA 2018a) for additional discussion
regarding the flexibilities available to states in the adoption of water column criterion
elements.
Ql-3: Does state or authorized tribal adoption of a site-specific value for
either egg-ovary or whole body/muscle elements require EPA approval
under CWA section 303(c) before using the site-specific value in
NPDES permitting?
Al-3: Site-specific values for fish tissue criterion elements would require EPA approval under
CWA section 303(c) where such criteria are applicable for CWA purposes, including NPDES
permitting (40 CFR 131.21(c)). However, as indicated in Al-1, a state or authorized tribal permit
writer may have authority under a state statute or regulation or tribal law to include a WQBEL
necessary to meet a more stringent state or authorized tribal criterion that has not yet been
approved by EPA, or to include such conditions as part of their CWA section 401 certification.
The whole-body fish and fish muscle element as well as the water column elements in the
national CWA section 304(a) recommended selenium criterion are derived from the egg-ovary
criterion element. As a result, any state or authorized tribe that adopts a site-specific value for the
egg-ovary criterion element and intends to also use other CWA section 304(a) selenium criterion
elements in their WQS, would need to derive the related whole body/muscle and water column
criterion elements from the site-specific egg-ovary criterion element. They would then need to
submit those related criterion elements to EPA for review and approval under CWA section
303(c) as well. See EPA's Technical Support for Adopting and Implementing EPA's 2016
Recommended Selenium Criterion in Water Quality Standards, Draft for additional discussion on
development of site-specific criterion values for the elements of the four-part criterion (USEPA
2018a).
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Ql-4: When using the water column criterion element, can permit
writers use mixing zones, initial zones of dilution, or dilution factors in
NPDES permits?
Al-4: If applicable state or authorized tribal WQS allow mixing zones or consideration of
dilution for bioaccumulative pollutants such as selenium, the permit writer may use mixing zones
or dilution allowances in assessing the need for and deriving WQBELs. In such cases, WQS
should be met during critical conditions5 after accounting for allowable dilution or at the edge of
the regulatory mixing zone. The permit or fact sheet should describe the geographic extent of the
mixing zone or dilution volume allowed under the permit. The NPDES permitting authority
should make any allowance for dilution or mixing on a case-by-case basis that accounts for site-
specific conditions, including effluent flow, stream flow, and ambient pollutant concentrations.
EPA recommends in section 5.1.2 of its online Water Quality Standards Handbook that states
and tribes should carefully consider whether mixing zones are appropriate where a discharge
contains bioaccumulative pollutants (USEPA 2014). When the applicable state or authorized
tribal WQS prohibit mixing zones or consideration of dilution for bioaccumulative pollutants
such as selenium, the permit writer must develop WQBELs that meet EPA-approved state or
authorized tribal WQS at the point of discharge.
As stated in Section 2.2.2 of EPA's Technical Support Document for Water Onality-based
Toxics Control (TSD) (USEPA 1991), mixing zones for bioaccumulative pollutants should be
restricted such that they do not affect areas often used for fish harvesting, particularly of
stationary species such as shellfish. In addition, mixing zones for bioaccumulative pollutants
might be denied to account for uncertainties in the assimilative capacity of the water body. As
stated in Section 4.3.4 of the TSD, where evidence indicates a lack of assimilative capacity for a
bioaccumulative pollutant, mixing zones may not be appropriate.
2. NPDES Reasonable Potential Determinations
Q2-1: Which elements of the four-part criterion included in state or
authorized tribal WQS for selenium should be used for conducting RP
analyses?
A2-1: Any of the elements from the recommended four-part selenium criterion that have been
approved by EPA and adopted as part of the state or authorized tribal water quality standards can
be used by the permitting authority to determine whether the discharge causes, has the
reasonable potential to cause, or contributes to an in-stream excursion above the applicable
WQS. If a state or authorized tribe has steady state, site-specific fish tissue data available (that
were collected under and met a quality-assurance plan according to EPA's guidance Technical
Support for Fish Tissue Monitoring for Implementation of EPA's 2016 Selenium Criterion
(USEPA 2021c), that information should be used directly to assess RP consistent with the state
5 With respect to "critical conditions" for fish tissue sampling. Section 2.1 of EPA's Technical Support for Fish
Tissue Monitoring for Implementation of EPA's 2016 Selenium Criterion outlines a monitoring strategy that could
be used to properly collect adequate fish tissue samples including a recommendation on fish species and timing for
collection of different tissue types.
7
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or tribe's approved WQS on a case-by-case basis (USEPA 2021c)6. In Ashless waters or where
steady state conditions are not met, the water column elements can be used to conduct RP
analyses. Since the water column element was derived to be protective of fish, the state or
authorized tribe can use the water column element to conduct RP analyses for any discharge to
freshwater. Given the complexities of collecting fish tissue data, documenting steady state
conditions in the water body with respect to selenium tissue concentrations over time, and the
current lack of an approved analytical method for selenium in fish tissue in 40 CFR Part 136,
states and authorized tribes may choose to use the water column element in RP analyses for
selenium at the present time. In any case, such determinations should ensure that state and
authorized tribal WQS are protected. The flowcharts in Attachment 1 summarize a recommended
decision tree for conducting RP analyses assuming the state or tribe has adopted all four
elements of the selenium criterion. The information outlined below and illustrated in Attachment
1 considers both the fish tissue and water column elements of the selenium criterion when
conducting an RP analysis where fish tissue data are representative of steady state conditions in
the water body. See the "Permitting for New or Increased Dischargers" flowchart (Attachment
1). Permit writers may want to consider requesting fish tissue monitoring to demonstrate whether
fish are at steady state and for use in assessing reasonable potential. See the "Permitting for
Existing Dischargers" and "Permitting for New or Increased Dischargers" flowcharts
(Attachment 1).
a. Fish Tissue Data Available (With Criterion Excursion in Fish at Steady State): If
site-specific fish tissue concentrations indicate an excursion of the applicable fish
tissue criterion elements (as explained in Question 1 of Frequently Asked Questions
(FAOs): Implementing the 2016 Recommended Selenium Criterion in Clean Water
Act Section 303(d) and 305(b) Assessment, Listing, and Total Maximum Daily Load
(TMDL) Programs, Draft (USEPA 2021d)7) and a facility discharges detectable
quantities of selenium to the water body, then the facility would have RP and the
permit must contain a WQBEL limiting selenium discharge. See the "Permitting for
Existing Dischargers" flowchart (Attachment 1).
b. Fish Tissue Data Available (With No Criterion Excursion in Fish at Steady
State):8 If a state or authorized tribe has site-specific fish tissue data available that do
not indicate an excursion of the fish tissue criterion elements of the four-part
selenium criterion, these fish tissue data may be used by the permitting authority to
determine whether there is the "reasonable potential to cause or contribute" to an
excursion of the WQS. When conducting an RP determination, the permitting
authority shall take into account existing controls on point and nonpoint sources of
6 USEPA 2021c addresses fish tissue monitoring for assessment of the fish tissue elements of the selenium criterion
and for the development of site-specific water column criterion elements. Elements of this document will be useful
for consideration when sampling fish for RP determinations, such as target species and tissue types, however some
elements may differ such as temporal and spatial considerations (e.g. distance from the facility, age of data) that may
be specific to RP determinations.
7 See USEPA 2021d for additional information on assessing fish tissue data for the national CWA section 304(a)
recommended selenium criterion (see FAQs 7-10).
8 EPA recommends that WQBELs be developed from fish tissue criterion elements where selenium loads discharged
to the water body (including but not limited to the permitted load) are consistent over time and fish tissue
measurements are representative of steady state conditions. See Section 3.4 and Appendix K of USEPA 2016.
8
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pollution, the variability of the pollutant or pollutant parameter in the effluent, and,
where appropriate, the dilution of the effluent in the receiving water (40 CFR Part
122.44(d)(l)(ii)). The permitting authority satisfies this requirement by evaluating
whether the discharged selenium concentration (taking into account variability of
these concentrations and available dilution in the receiving water if allowed in State
or Tribal Water Quality Standards) cause, have the reasonable potential to cause, or
contribute to excursions of the WQS that may have been calculated using the method
in Appendix K in USEPA 2016. Other environmental factors may also affect the use
of fish tissue data when determining RP. See the "Permitting for Existing
Dischargers" flow chart (Attachment 1) and FAQ 2-3.
c. Fish Tissue Data Available (With No Criterion Excursion in Fish Not Yet at
Steady State): If a state or authorized tribe has site-specific fish tissue data available
that do not indicate an excursion of the fish tissue criterion elements of the four-part
selenium criterion, but the permitting authority determines that the observed fish
tissue concentrations do not correspond to current or anticipated water column
selenium concentrations (e.g., fish collected have not been exposed to or come to
steady state4 with the anticipated water column selenium concentration), the
permitting authority should use the water column criterion element to determine RP.
See the "Permitting for New or Increased Dischargers" flowchart (Attachment 1).
d. No Fish Tissue Data Available: If a state or authorized tribe does not have site-
specific fish tissue data, or the receiving water body is fishless or not at steady state
with respect to selenium in fish tissue, the permitting authority should use the water
column criterion element for conducting RP analysis.
Q2-2: How should a regulatory authority determine the appropriate
water column criterion element based on the four-part criterion (e.g., a
30-day chronic criterion element versus an intermittent criterion
element) for use in an NPDES permit?
A2-2: The national CWA section 304(a) recommended selenium criterion (USEPA 2016a) is
based on a 30-day average concentration and having less than a 30-day average concentration
requires use of the intermittent water column criterion element. NPDES-permitted facilities that
continuously discharge effluent over 30-days can use the water column criterion element, which
is well suited in conducting RP analysis and developing WQBELs (see also chapter 5 of EPA's
TSD (USEPA 1991). The permitting authority should use the state adopted and EPA-approved
water column criterion element, which could be EPA's default recommended value, an EPA-
approved state-specific value, or an EPA-approved site-specific water column element (see FAQ
1-2).
For NPDES permittees that discharge effluent non-continuously (<30 days per month), the
intermittent water column criterion element provides an appropriate measure of aquatic life
protection. The intermittent water column element can be calculated using ambient spikes in the
receiving water, with the equations provided in EPA's Aquatic Life Ambient Water Quality
Criterion for Selenium—Freshwater 2016 (Table 1) (USEPA 2021a). Note that both the
9
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continuous (30-day) and the intermittent water column elements are designed to be protective of
EPA's criterion for fish tissue over a 30-day period.
Q2-3: What data should be used to assess RP using the four-part
criterion?
A2-3: Permitting authorities may use any available relevant and representative fish tissue or
effluent data to assess RP. See sections 2.1 - 2.3 of EPA's Draft Technical Support for Fish
Tissue Monitoring for Implementation of EPA's 2016 Selenium Criterion for a review of relevant
and representative fish tissue monitoring strategies (USEPA 2021c). For new dischargers,
permitting authorities may use estimates of effluent characteristics to assess RP. See sections 3.2
and 3.3 of EPA's TSD (USEPA 1991). For existing discharges and steady state conditions, and
where RP analysis are based on a fish tissue-based criterion, the permit writer uses site-specific
fish tissue data to calculate the site-specific water column value and actual or estimated effluent
selenium concentrations, in addition to information about effluent variation and, where
appropriate, available dilution in the receiving stream, to determine whether the permittee's
selenium discharge causes, has the reasonable potential to cause, or contributes to exceedances
of the site-specific water column value. If so, then the discharge has reasonable potential to cause
or contribute to exceedance of the fish-tissue based criterion. See Section 3.4 and Appendix K in
USEPA 2021a. The permitting authority may take a more stringent approach by presuming
reasonable potential based on other information provided by the NPDES permit applicant, such
as the known presence of selenium in minerals extracted at the permitted facility. Determinations
of reasonable potential should be based on analysis of receiving stream assimilative capacity and
if RP is demonstrated, then the permit requires incorporation of a WQBEL.
Q2-4: In cases where a discharge occurs to a lotic (e.g., a flowing
stream) water, but downstream waters are lentic (e.g., lakes,
impoundments), which selenium criterion water column element should
be used in the RP analysis?
A2-4: The national CWA section 304(a) recommended selenium criterion water column element
for lentic waters is more stringent than the criterion element for lotic waters. Therefore, if an
NPDES discharge is located in lotic waters upstream of lentic waters, the permit writer should
ensure that both the lotic WQC and the downstream lentic WQC, incorporated into a state's
WQS, are protected when conducting the RP analysis and when developing WQBELs for
selenium (see 40 CFR 122.4(d)). If a permitting authority bases RP on fish tissue elements of the
selenium criterion, it is then important to document that discharge concentrations of selenium do
not cause, have the reasonable potential to cause, or contribute to an excursion of the fish tissue
criterion elements in the downstream lentic environment.
Q2-5: In some cases, it might be unclear whether a receiving or
downstream water body is lentic or lotic with regard to an appropriate
selenium water column criterion element (e.g., run-of-the-river
reservoirs). Does EPA have recommendations that address whether a
10
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receiving water body should be considered lentic or lotic for purposes of
NPDES RP analysis and permit WQBEL determinations?
A2-5: Generally, the classification of fresh waters into lotic or lentic categories is made by the
state or authorized tribe, based on site-specific information. States and authorized tribes should
ensure that they use transparent, scientifically defensible methods to appropriately classify
receiving waterbodies. Reservoirs with extended residence times are more likely to demonstrate
bioaccumulation dynamics that are lentic in nature. Section 3.2.4, Classifying Categories of
Aquatic System, in USEPA (2021a) provides further discussion of categories of fresh water.
Lotic systems such as rivers and streams are characterized by flowing water. Lentic systems,
such as lakes and ponds, are characterized by largely standing water. Water residence time is
generally shorter in lotic systems than in lentic systems, and subsequently, aquatic organisms
living in lentic systems tend to bioaccumulate more selenium than organisms living in lotic
systems for a given dissolved selenium concentration. A site-specific study or use of available
hydrologic information may be necessary to determine the residence time of the water body so
that the application of the lentic or lotic water column value can be applied in a defensible
manner for RP analyses consistent with EPA-approved WQS. Permitting authorities should
determine whether to use the lentic or lotic water column element for a particular water body on
a case-by-case basis, factoring in downstream water body characteristics (e.g., a lake or other
lentic system downstream of a lotic system (e.g., stream) with a point source discharge).
3. NPDES Permit WQBEL Calculations
Q3-1: Can an NPDES permitting authority derive selenium WQBELs
using an EPA-approved state or authorized tribal WQS based on EPA's
recommended four-part selenium criterion for a noncontinuous or
intermittent effluent discharge containing selenium? If so, how?
A3-1: Yes, the permitting authority can derive WQBELs for noncontinuous or intermittent
discharges by using the intermittent water column element of the selenium criterion. Under
EPA's CWA section 304(a) recommended selenium criterion, the intermittent water column
element is applicable to any receiving water where the long-term instream background
concentration is less than the 30-day average criterion value, and where there are instream spikes
of selenium concentrations (above the background value) with a duration of less than 30 days.
However, in some cases additional data should be collected to determine the duration of instream
spikes (e.g., collection of more frequent instream selenium samples, such as - daily samples
instead of weekly samples).
Permit writers who use the intermittent water column criterion element should, at a minimum,
assess the long-term average background concentration of selenium in the portion of the stream
not impacted by a discharge, as well as the maximum number of days out of any 30-day period
that the instream concentration is expected to exceed the long-term average stream background
concentration. With respect to a point source discharge of selenium, these in-stream "pulses" are
likely to occur on days when the intermittent discharge is occurring, thus, the number of days a
discharge is expected to occur may be used as part of the intermittent criterion element as the
fraction of 30 days in which an elevated selenium concentration may occur. The number of days
a discharge is expected to occur can be determined from historical effluent data. If sufficient
11
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effluent data are not available, data from other sources such as local precipitation maps or data
from discharges at other similar facilities may be used. In cases where selenium concentration
data are not available, such as new facilities, permit writers should consider using data from
other sources such as similar facilities. With that information, the permit writer can use the
equation in Table 1 to estimate the calculated value for the intermittent water column element
that would be protective on the days when a discharge would occur and could then calculate a
WQBEL for the intermittent discharge. The permit writer would determine RP and establish
selenium limits using the applicable implementation guidance (i.e., state or tribal implementation
procedures) that is used for analysis of other water quality criteria, and the projected WQBEL
would be calculated using the intermittent water column criterion element.
Q3-2: Can anNPDES permitting authority develop WQBELs using the
fish tissue criterion element(s) of the four-part selenium criterion (i.e.,
egg-ovary or whole body and/or muscle) rather than the water column
elements?
A3-2: Any of the criterion elements in EPA's four-part selenium criterion can be used for
deriving WQBELs. However, implementation guidance (i.e. state or tribal implementation
procedures) for the water column criterion element, as described in this document, are currently
available to develop and establish WQBELs in NPDES permits.9 As described by EPA (2021c),
if a state or authorized tribe believes that the default national water column criterion element is
not appropriate for a water body capable of supporting fish, it can develop a site-specific value
for a water column element using the procedures provided in Appendix K (Translation of a
Selenium Fish Tissue Criterion Element to a Site-Specific Water Column Value) by EPA
(2021a). Any site-specific WQC that is used for the NPDES permits program must be either
approved by EPA or derived through an EPA-approved water column translation procedure (e.g.,
performance-based approach). The procedures outlined in Appendix K of USEPA 2021a do not
apply to Ashless waters.
While the water column criterion element is one approach for WQBEL calculations, the unique
nature of the four-part selenium criterion and the recommended primacy of the egg-ovary
criterion element (see FAQ Al-3) may lead state permitting authorities to develop innovative
implementation guidance (i.e., state or tribal implementation procedures) for the expression of
WQBELs. EPA supports the development of such innovative approaches to implement water
quality criteria based on EPA's CWA section 304(a) recommended selenium criterion; however,
as with any WQBEL expression, the limit included in the NPDES permit must be consistent with
the requirements of CWA section 301(b)(1)(C) as well as 40 CFR 122.44(d) and 122.45. See
FAQ Al-4.
Q3-3: Can the selenium WQBEL be expressed in units of mg/kg
selenium in ambient fish tissue, measured by dry weight?
A3-3: No. The WQBEL must limit the amount of selenium discharged by the NPDES permittee.
40 CFR § 122.44(d)(l)(i) requires that WQBELs control pollutants when the permitting
9 Note that many states have not established implementation procedures for use in deriving WQBELs from fish
tissue data, so the water column element may be the only available choice that is consistent with the state's
established procedures.
12
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authority determines that a pollutant may be discharged at a level which will cause, have the
reasonable potential to cause, or contribute to an excursion above the WQS. The limit must be
expressed as a maximum effluent pollutant concentration or maximum effluent pollutant mass
load or both, as necessary to meet applicable water quality standards. See 40 CFR 122.45(f).
Q3-4: The national CWA section 304(a) recommended selenium
criterion does not include an acute expression. Must permits contain
both short- and long-term limit expressions?
A3-4: EPA's NPDES permit regulations at 40 CFR 122.45(d)(1) require that, for continuous
discharges, all permit limits shall, unless impracticable, be stated as maximum daily and average
monthly limitations for all dischargers other than publicly owned treatment works (POTWs). For
POTWs, NPDES permit limits should be stated as average weekly or 7-day average and average
monthly or 30-day average limitations.
Consistent with 40 CFR 122.45(d)(1), where permitting authorities determine the need for
WQBELs for selenium, permits must establish both short- and long-term limit expressions, or
the record must describe why such limits are impracticable. Where the permitting authority
derives WQBELs from the water-column criterion element of the chronic WQC, EPA's 1991
TSD provides a detailed statistical methodology to calculate both short- and long-term WQBELs
from a single criterion value.
A common misperception is that short-term WQBELs (e.g., maximum daily limits [MDLs])
implement acute water quality criteria and long-term limits (e.g., average monthly limits
[AMLs]) implement chronic water quality criteria. This is inconsistent with the procedures
described in EPA's TSD (USEPA 1991). Chapter 5 of EPA's TSD (USEPA 1991) provides
methods to derive both short- and long-term WQBELs from one or more ambient criteria values
with a range of duration components. EPA's TSD procedure quantifies the observed variability
of the pollutant of concern in the effluent (expressed as the coefficient of variation [C V])
(USEPA 1991). EPA's TSD (USEPA 1991) procedure then calculates a long-term average
(LTA) effluent performance concentration necessary to achieve the desired wasteload allocation
(WLA), taking into account the duration component of the water column criterion element.
Multiple WQBELs can then be statistically derived from the LTA value and can be set at any
desired averaging period (e.g., daily, weekly, or monthly). Thus, the WQBELs derive from the
underlying water quality criteria, but account for the observed effluent variability to establish the
daily, weekly, or monthly effluent quality that will ensure WQC are achieved.
Many of EPA's WQC and many EPA-approved state or authorized tribal WQC include only a
single criterion expression, yet state permitting authorities routinely establish both short- and
long-term effluent limits. For example, most EPA human health criteria are expressed as a single
ambient concentration not to be exceeded. For this situation, EPA's 1991 TSD uses the statistical
procedure described above to take a single long-term criterion expression and develop WQBELs
expressed as MDL and AML values (see EPA's 1991 TSD section 5.4.4).
While EPA's 1991 TSD guidance provides a recommended approach to develop both short- and
long-term WQBELs, the regulations provide an exception where the permitting authority
determines that such an approach is "impracticable." If the permitting authority establishes
selenium criteria that differ from EPA's 304(a) criteria, or where the state's or authorized tribe's
13
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implementation guidance (i.e., state or tribal implementation procedures) differ from EPA's 1991
TSD, the permitting authority could provide the "impracticability" justification in the permit
record. The justification would need to explain why the limit expressions required in 40 CFR
122.45(d)(1) were impracticable and demonstrate that the WQBELs included in the permit
derive from and ensure attainment with the applicable WQS.
Q3-5: Should the selenium WQBEL be expressed as total recoverable
selenium? If so, how should an NPDES permitting authority translate the
dissolved selenium water column criterion element of the national CWA
section 304(a) recommended selenium criterion to a total recoverable
selenium concentration for a NPDES permit limit?
A3-5: Yes, WQBELs for selenium should be expressed as total recoverable selenium. While
EPA's CWA section 304(a) recommended selenium criterion water column elements are
expressed as dissolved selenium, the particulate, as well as dissolved selenium, can have
deleterious effects on aquatic life. Total recoverable selenium is further discussed in EPA's
Aquatic Life Ambient Water Quality Criterion for Selenium—Freshwater 2016 (USEPA 2021a).
In the absence of site-specific data, EPA recommends that it may be appropriate to use a total
recoverable-to-dissolved selenium ratio of 1.00 to determine RP and calculate NPDES permit
WQBELs for selenium. A ratio of 1.00 means that the concentration of total recoverable
selenium is equal to the concentration of dissolved selenium, which may be a valid assumption
for some surface waters, especially certain lotic waters, based on geochemistry.
The geochemical conditions governing solubility of selenium will affect the ratio of total-to-
dissolved selenium in a water body. Selenate (SeC>42") and selenite (SeC>32") oxyanions are
typically the dominant species under the pH and redox conditions of most surficial aquatic
environments (Presser and Luoma 2010). Selenate is highly mobile due to the solubility of its
salts, whereas selenite is more likely to be immobilized by adsorption onto particulates,
particularly iron oxyhydroxides (Presser and Luoma 2010). An oxygenated water with neutral to
slightly alkaline pH favors selenate (Presser and Luoma 2010). Under such conditions, selenate
will be the dominant form and the ratio of dissolved-to-total for selenium in the water column
would be 1.00 and this is the reason why EPA recommended the use of a ratio of 1.00.
EPA is aware of alternative total:dissolved selenium translators that have been developed by
states for specific waterbodies. Water quality data collected by a state or tribe, U.S. Geological
Survey, EPA, and others should be consulted when considering such translators for a given
NPDES permit. Additionally, EPA has provided previous guidance on procedures for translating
a criterion based on the concentrations of dissolved constituent(s) to total recoverable permit
limits; EPA's guidance also includes recommendations for sampling and analysis (USEPA
1996).
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4. NPDES Monitoring (Basis for RP and WQBELs, Compliance
Monitoring)
Q4-1: What EPA analytical test methods are recommended for analyzing
water and fish tissue samples for selenium under EPA's NPDES permits
program?
A4-1: EPA provides analytical test methods approved for measuring selenium in wastewater
effluent at 40 CFR Part 136.3 (also see Appendix L in USEPA 2021a). Use of any other method
for permit compliance monitoring must first be approved through the alternate test procedures
approval process (see 40 CFR 136.4 and 136.5). For selenium fish tissue sampling, EPA has
developed Technical Support for Fish Tissue Monitoring for Implementation of EPA's 2016
Recommended Selenium Criterion, Draft, which describes analytical methods that may be used
for measuring selenium in fish tissue samples (USEPA 2021c).
5. Compliance Schedules in NPDES Permits
Q5-1: Would a compliance schedule be allowed in a renewed, modified,
or reissued NPDES permit in which an existing WQBEL is being
replaced with a more stringent NPDES permit limit or a new NPDES
permit limit based on an EPA-approved state or tribal selenium criterion
reflecting EPA CWA section 304(a) recommended selenium criterion?
A5-1: A compliance schedule may be allowed in cases where the NPDES permitting authority
determines that one is appropriate and the applicable state or authorized tribal WQS authorize the
use of compliance schedules for meeting NPDES permit limits. Decisions to allow compliance
schedules are made on a case-by-case basis, taking into account the facility and site-specific
conditions. EPA's WQS regulations at 40 CFR 131.15 specify that if a state or authorized tribe
intends to allow the use of schedules of compliance for WQBELs in NPDES permits, that state
or authorized tribe must adopt a permit compliance schedule authorizing provision in its WQS.
Additionally, any schedule developed must be consistent with the requirements established for
compliance schedules in 40 CFR 122.2 and 122.47.
15
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References
Jones, A. M. 1997. Environmental Biology (Routledge Introductions to Environment:
Environmental Science). Routledge. New York, NY. 216 pp.
Presser, T.S. and S.N. Luoma. 2010. AMethodology for Ecosystem-Scale Modeling of Selenium.
Integrated Env. Ass. and Man. Vol 6(4): 685 - 710.
USEPA (U.S. Environmental Protection Agency). 1991 .Technical Support Document for Water
Ouality-based Toxics Control (EPA/505/2-90-001, March 1991). U.S. Environmental Protection
Agency, Office of Water, Washington, DC. http://www.epa.gov/npdes/pubs/owm0264.pdf.
USEPA. 1996. The Metals Translator: Guidance for Calculating a Total Recoverable Permit
Limitfi'om a Dissolved Criterion. EPA 823-B-96-007. U.S. Environmental Protection Agency,
Office of Water, Washington, DC. https://www3.epa.gov/npdes/pubs/metals translator.pdf.
USEPA. 2000. USEPA Review and Approval of State and Tribal Water Quality Standards 65
Fed. Reg. 24641 (Apr. 27, 2000).
USEPA. 2003. Methodology for Deriving Ambient Water Quality Criteria for the Protection of
Human Health (2000) Volume 2: Development of National'Bioaccumulation Factors. EPA-882-
R-03-030. U.S. Environmental Protection Agency. Office of Water. Washington. DC. pp. 1-4.
https://nepis.epa.gov/Exe/ZvPDF.cgi/P1005EZQ.PDF?Dockev=P 1005EZQ.PDF
USEPA. 2014. General Policies. Chapter 5 in Water Quality Standards Handbook. EPA 820-B-
14-004. U.S. Environmental Protection Agency, Office of Water, Washington, DC.
http://www.epa.gov/wqshandbook.
USEPA. 2021a. 2021 Revision to: Aquatic Life Ambient Water Quality Criterion for Selenium-
Freshwater 2016. EPA 822-R-21-006. U.S. Environmental Protection Agency, Office of Water,
Washington, DC.
https ://www. epa. gov/ system/files/documents/2021 -08/selenium-freshwater2016-2021 -
revision.pdf
USEPA. 2021b. Technical Support for Adopting and Implementing EPA's 2016 Selenium
Criterion in Water Quality Standards. Draft. EPA-823-D-21-001. U.S. Environmental Protection
Agency, Office of Water, Office of Science and Technology, Washington, DC.
https://www.epa.gov/wac/aauatic-life-criterion-selenium.
USEPA. 2021c. Technical Support for Fish Tissue Monitoring for Implementation of EPA 's
2016 Selenium Criterion. Draft. EPA-823-D-21-002. U.S. Environmental Protection Agency,
Office of Water, Office of Science and Technology, Washington, DC.
https://www.epa.gov/wqc/aquatic-life-criterion-selenium.
USEPA. 2021d. Frequently Asked Questions (FAQs): Implementing the 2016 Recommended
Selenium Criterion in Clean Water Act Section 303(d) and 305(b) Assessment, Listing, and Total
Maximum Daily Load (TMDL) Programs. Draft. EPA-823-D-21-004 U.S. Environmental
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Protection Agency, Office of Water, Office of Science and Technology, Washington, DC.
https://www.epa.gov/wac/aquatic-life-criterion-selenium.
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Attachment 1:
EPA's Recommended Flowcharts for Implementing the National CWA
Section 304(a) Recommended Selenium Criterion in NPDES Permits
Program
18
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New or revised state or authorized
tribe WQS for Selenium are approved
by EPA for CWA purposes (all 4
components have been adopted)
Potential discharger of
Selenium applies for
permit
Permitting for
Existing Dischargers
See FAQ 2-1 for
additionaldetail
Collect effluent
data prior to
permit issuance
or estimate
effluent
concentration
I
No
Are effluent
data
available?
Reassess next
permit cycle
Yes
Does the
discharger have
detectable levels
of Selenium in its
effluent?
No
Yes
Do state procedures allow
use of fish tissue for RP
analysis?
Yes
Are there steady-state
fish tissue (egg/ovary,
whole body, or fillet)
data available?
No
Con s i d e r co 11 e cting ste a dy- sta te FT
data to determine whether FTE is
exceeded; once steady-state FT
data are available proceed to next
step.1
Yes
Three Options:
1) Collect ambient data priorto
permit issuance
2) Assume water body has no
assimilative capacity
3) Assume background
concentration present in water
body that is below criterion
Conduct RP Analysis
using FTE:3
Is there RP?
^ Yes
Yes
f "
Calculate WQBEL
* \
Conduct RP Analysis
using WCE:2
Is there RP?
No
No WQBEL. Consider
effluentmonitoring
requirement.
No
- RP analysis should not be unreasonably delayed pending collection offish tissue data.
- RP analysis may consider dilution consistent with state WQS and procedures.
- RP analysis usingfish tissue must considerthe effects of effluent variability and other
environmental factors to determine the "potential" for possible excursions of the FTE.
WCE: Water Column Criterion Element*
WC: Water Column (Ambient) Selenium
Concentration
RP: Reasonable Potential
WQBEL: Water Quality Based Effluent Limit
FT: Fish Tissue
FTE: Fish Tissue Criterion Element
RP has been determined
Transition box
No RP
Optional pathway
^ Question ^
* "Water Column Criterion Element" referstoeither:
1. the nationaldefaultwatercolumncriterion
element;
2. a state- or site-specific water column value
approved by EPA;
3. a site-specific water column value developed
from performance-based approach approved
by EPA.
19
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New or revised state or authorized
tribe WQS for Selenium are approved
by EPA for CWA purposes [all 4
components have been adopted)
Permitting for
New Dischargers
Potential discharger of
Selenium appliesfor
permit
See FAQ 2-1 for
additionaldetail
Develop estimate of
Selenium
concentrations in
proposed effluent
discharge
/AreambientX
{ WC data V
\ available? /
No
Yes
Three Options:
1) Collect ambient data priorto
permit issuance
2) Assume water body has no
assimilative capacity
3) Assume background
concentration present in water
body that is below criterion
/ Conduct RP Analysis \
/ using WCE:1
\ Is there RP? /
Yes
I
No
Calculate
WQBEL
No WQBEL.
Consider
effluent
monitoring
requirement.
Do state procedures allow
use of fish tissue for RP
analysis?
No
No further
t
action
Yes
Considera permit condition to require
collection of FT data at steady-state to
determine whether FTE is exceeded;
once steady-state FT data are available
proceed to next step.
Are FT values at or
abovethe FTE?
No
ConductRP Analysis
using FTE:2
Is there RP?
Yes
Yes: RP exists
No
WQBEL remains in
place; consider
updatingWCE
based on FT data
Consider modifying
permitto remove
WQBEL, and ongoing
effluent monitoring
requirement.
- RP analysis may consider dilution consistent with state WQS and procedures.
- RP analysis usingfish tissue must considerthe effects of effluent variability and other
environmental factors to determine the "potential" for possible excursions of the FTE.
WCE: Water Column Criterion Element*
WC: Water Column (Ambient) Selenium
Concentration
RP: Reasonable Potential
WQBEL: Water Quality Based Effluent Limit
FT: Fish Tissue
FTE: Fish Tissue Criterion Element
RP has been determined
Transition box
No RP
Optional pathway
^ Question ^
* "Water Column Criterion Element" refersto either:
1. the national default water column criterion
element;
2. a state- or site-specific water column value
approved by EPA;
3. a site-specific water column value developed
from performance-based approach approved
by EPA.
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