United States
Environmental Protection
\*^l m %Agency
Office of Water
EPA 820-F-16-009
September 2016
Frequently Asked Questions (FAQs): Implementing
WQS that Include Elements Similar or Identical to
EPA's 2016 Selenium Criterion in Clean Water Act
Section 402 NPDES Programs
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EPA Draft for Public Comment
Frequently Asked Questions (FAQs): Implementing WQS that
Include Elements Similar or Identical to EPA's 2016 Selenium
Criterion in Clean Water Act Section 402 NPDES Programs
This document supports Clean Water Act (CWA) regulators in states, authorized tribes, and
territories in implementing EPA's four-part selenium water quality criterion (WQC)1 in EPA's
Aquatic Life Ambient Water Quality Criterion for Selenium-Freshwater 20162 as it relates to the
CWA section 402 National Pollutant Discharge Elimination System (NPDES) Programs. A
summary of the recommended criterion is provided in Attachment 1.
This set of FAOsprovides 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 its content might not apply to a particular situation based
upon the circumstances. EPA, states, authorized tribes, and other decision makers retain
the discretion to adopt approaches on a case-by-case basis that differ from those
provided in this document as appropriate and consistent with statutory and regulatory
requirements. EPA could update this document as new information becomes available. In
addition to this document, EPA has other documents which provide considerations and
recommendations on implementing the selenium criterion and can be found at EPA's
selenium website: https://www.epa.gov/wac/aauatic-life-criterion-selenium. Note: For
state or authorized tribal WQS that do not include elements similar or identical to EPA's
2016 recommended selenium criterion, EPA will assess the needfor alternative state- or
authorized tribal-specific implementation permitting procedures to ensure consistency
with the NPDES permitting requirements at Title 40 of the Code of Federal Regulations
(CFR) section 122.44.
Table of Contents
1. Identifying the Applicable Selenium WQS to Implement in the NPDES Permit	3
Ql-1: Which selenium criterion should the permit writer use for conducting a reasonable potential
(RP) determination or developing effluent limits in an NPDES permit?	3
Ql-2: Which elements of a four-part criterion for selenium should be used when implementing
WQS through the NPDES permits program?	4
Ql-3: Does state or authorized tribal adoption of a site-specific egg-ovary element that differs from
EPA's 2016 selenium criterion require EPA approval under CWA 303(c) before using the site-
specific element in NPDES permitting?	4
Ql-4: May permit writers use mixing zones, initial zones of dilution, or dilution factors in NPDES
permitting for selenium?	4
1	Reference to a four-part criterion or four-part selenium criterion assumes adoption and EPA approval of a four-
part aquatic life WQS for selenium that includes elements similar or identical to EPA's 2016 selenium criterion.
2	USEPA (U.S. Environmental Protection Agency). 2016. Aquatic Life Ambient Water Quality Criterion for
Selenium-Freshwater 2016. EPA 822-R-16-006. U.S. Environmental Protection Agency, Office of Water,
Washington DC. https://www.epa.gov/wqc/aquatic-life-criterion-selenium-documents.
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Ql-5: If a state or authorized tribe adopts a new WQS before the end of the 5-year permit term, can
the permit be modified to ensure that the effluent limit reflects the new standard?	5
2.	NPDES Reasonable Potential (RP) Determinations (Basis for NPDES Permit WQBELs)	5
Q2-1: Which elements of a four-part criterion for selenium should be used for conducting RP
analyses?	5
Q2-2: How should a regulatory authority determine the appropriate water column element based on
the four-part criterion (e.g., a 30-day chronic element versus an intermittent element) for use in a
given NPDES permit?	6
Q2-3: What effluent data would be necessary to assess RP using a four-part criterion?	6
Q2-4: If a state or authorized tribe has sufficient and representative fish tissue data available to
demonstrate that selenium levels in fish tissue do not indicate an excursion of the fish tissue
elements of the four-part selenium criterion, can that information be used to determine that there is
no RP and, thus, no need for a WQBEL?	7
Q2-5: In cases in which a discharge occurs at a lotic location and downstream waters are lentic
waterbody types (e.g., lakes, impoundments), which selenium criterion water column element
should be used in RP analysis?	7
Q2-6: In some cases, it might not be clear whether a receiving waterbody is lentic or lotic in terms
of an appropriate selenium water column element (e.g., run-of-the-river reservoirs). Does EPA have
recommendations that address whether a receiving waterbody should be considered lentic or lotic
for purposes of NPDES RP analysis and permit WQBEL determinations?	7
3.	NPDES Permit WQBEL Calculations	8
Q3-1: Can an NPDES permitting authority using a state- or authorized tribal-adopted standard based
on or similar to EPA's recommended four-part selenium criterion derive selenium NPDES permit
WQBELs for a noncontinuous or intermittent effluent discharge containing selenium? How?	8
Q3-2: Can an NPDES permitting authority develop permit limits (WQBELs) for a four-part
selenium criterion using the fish tissue element(s) of a four-part criterion (i.e., egg-ovary or whole
body and/or muscle) rather than the water column element?	8
Q3-3: EPA's 2016 selenium criterion does not include an acute expression. Must permits contain
both short- and long-term limit expressions?	9
Q3-4: NPDES regulations require that permit limits for metals be expressed as total recoverable;
however selenium is a nonmetal or metalloid. Should selenium be expressed as total recoverable in
WQBELs and, if so, how should an NPDES permitting authority translate the dissolved selenium water
column element of EPA" s 2016 selenium criterion to a total recoverable selenium NPDES permit limit? .. 9
4.	NPDES Monitoring (Basis for RP and WQBELs, Compliance Determination)	10
Q4-1: What EPA analytical test methods are recommended for use in analyzing water and fish
tissue samples for selenium under EPA's NPDES program?	10
5.	Compliance and Enforcement of NPDES Permit Requirements	10
Q5-1: Would a compliance schedule be allowed in an NPDES renewal permit in which a WQBEL
is being replaced with a more stringent permit limit based on EPA's 2016 selenium criterion?	10
Attachment 1: EPA's Aquatic Life Ambient Water Quality Criterion for Selenium—
Freshwater 2016	11
Attachment 2: EPA's Recommended Flowchart for Implementing EPA's 2016 Selenium
Criterion in NPDES Permits Program	12
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This document addresses questions about National Pollutant Discharge Elimination System
(NPDES) permitting based on state or authorized tribal water quality standards (WQS) that
include elements similar or identical to EPA's 2016 water quality criterion (WQC) for selenium.3
The four-part criterion—as summarized in Attachment 1—replaces EPA's 1987 recommended
selenium WQC. Attachment 1 includes a reference to a performance-based approach that is
based on recommendations for implementing selenium WQC in regulatory programs. The
performance-based approach is defined in EPA's Technical Support for Adopting and
Implementing EPA's 2016 Selenium Criterion in Water Quality Standards at Section 1.2.2 as: an
approach to developing site-specific criterion in which, "States and authorized tribes can choose
to adopt into their WOS and submit to EPA a set ofprocedures to facilitate the translation of the
fish tissue criterion concentration elements into site-specific water concentration values. "
This document is intended to help EPA, state, authorized tribes, and territorial NPDES permit
writers understand how permitting procedures might be refined where states adopt EPA's 2016
selenium criterion (as summarized in Attachment 1), which includes the following elements of
the four-part criterion4: (1) a fish egg-ovary element; (2) a fish whole-body and/or muscle
element; (3) a water column element (one value for lentic and one value for lotic aquatic
systems); and (4) a water column intermittent element to account for potential chronic effects
from short-term exposures (one value for lentic and one value for lotic aquatic systems).
1. Identifying the Applicable Selenium WQS to Implement in the NPDES Permit
Q1-1: Which selenium criterion should the permit writer use for conducting a reasonable
potential (RP) determination or developing effluent limits in an NPDES permit?
Al-1: A state or authorized tribe might have adopted a selenium aquatic life criterion as a
component of their WQS based on EPA's 2016 selenium criterion; or EPA's 1987 selenium
criterion, WQC promulgated pursuant to 40 CFR Part 132 (60 FR 15387, March 23, 1995); or
other scientifically defensible criterion. The state's or authorized tribe's selenium criterion might
have been adopted either on a site-specific basis or on a statewide or tribe-wide basis and
generally will be part of the state's or authorized tribe's EPA-approved WQS to protect
applicable designated uses of the receiving water. Permit writers must develop WQBELs, where
RP has been demonstrated, to meet the state's or authorized tribe's EPA-approved standards as
required in Clean Water Act (CWA) section 301(b)(1)(C) and NPDES regulations at 40 CFR
122.44(d). In addition, a state or authorized tribal permit writer might have authority under state
statute or regulation or tribal law to include WQBELs necessary to meet more stringent state or
authorized tribal WQS that have not yet been approved by EPA. EPA permit writers also must
include more stringent conditions necessary to meet other appropriate requirement(s) under state
or tribal law if a state or authorized tribe specified such conditions in a CWA 401 certification.
3	USEPA (U.S. Environmental Protection Agency). 2016. Aquatic Life Ambient Water Quality Criterion for
Selenium-Freshwater 2016. EPA 822-R-16-006. U.S. Environmental Protection Agency, Office of Water,
Washington DC. https://www.epa.gov/wac/aauatic-life-criterion-selenium-documents.
4	Reference to a four-part criterion or four-part selenium criterion assumes state, authorized tribe, and territorial
adoption of a four-part aquatic life WQS for selenium that includes elements similar or identical to EPA's 2016
selenium criterion.
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In addition, there may be EPA-promulgated WQS for certain waters. For waters covered by
EPA's federally promulgated WQS, the permit writer must develop WQBELs to meet the WQS
found in 40 CFR Part 131, Subpart D.
EPA's approved state and authorized tribal WQS are available at
http://water.epa. gov/ scitech/ swguidance/ standards/wqslibrary/index. cfm.
Ql-2: Which elements of a four-part criterion for selenium should be used when
implementing WQS through the NPDES permits program?
Al-2: Any of the elements of a four-part criterion for selenium can be used to assess the need
for and to establish effluent limits as stringent as necessary to meet the applicable selenium
criterion. The flowchart provided in Attachment 2 presents an 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 provided in sections 3 and 4 of this
document.
Ql-3: Does state or authorized tribal adoption of a site-specific egg-ovary element that differs
from EPA '.s 2016 selenium criterion require EPA approval under CWA 303(c) before using
the site-specific element in NPDES permitting?
Al-3: Site-specific elements that differ from the egg-ovary element of EPA's 2016 selenium
criterion would require EPA approval under CWA 303(c). EPA review and approval are
important because WQS are not applicable for CWA purposes, including NPDES permitting,
until EPA has approved them (40 CFR 131.21(c)). [NOTE: As indicated in Ql-1, a state or
authorized tribal permit writer might have authority under state statute or regulation or tribal law
to include a WQBEL necessary to meet more stringent state or authorized tribal WQS that have
not yet been approved by EPA.] Since all of the other elements in EPA's 2016 selenium criterion
are derived from the egg-ovary element, any state or authorized tribe that adopts a site-specific
egg-ovary element would need to derive the related whole body/muscle and water column
elements from that element and submit them to EPA for review and approval under CWA 303(c)
as well. See EPA's Technical Support for Adopting and Implementing EPA's 2016 Selenium
Criterion in Water Quality Standards for additional discussion on site-specific criterion
development.5
Ql-4: May permit writers use mixing zones, initial zones of dilution, or dilution factors in
NPDES permitting for selenium?
Al-4: EPA recommends in section 5.1.2 of its online Water Quality Standards Handbook
(http://www.epa.gov/wqshandbook) that state and authorized tribal mixing zone policies should
carefully consider whether mixing zones are appropriate for bioaccumulative pollutants.6 When
the state or authorized tribal WQS prohibit or do not authorize mixing zones or consideration of
5	USEPA (U.S. Environmental Protection Agency). 2016. Technical Support for Adopting and Implementing EPA's
2016 Selenium Criterion in Water Quality Standards. U.S. Environmental Protection Agency, Office of Water,
Office of Science and Technology, Washington DC.
6	USEPA (U.S. Enviromnental Protection Agency). 2014. General Policies. Chapter 5 in Water Quality Standards
Handbook. EPA 820-B-14-004. U.S. Enviromnental Protection Agency, Office of Water, Washington DC.
http ://www. epa. gov/wqshandbook.
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dilution for bioaccumulative pollutants such as selenium, and because WQBELs must be as
stringent as necessary to meet EPA-approved state or authorized tribal WQS, the permit writer
must develop WQBELs that meet those standards at the point of discharge.
If 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, standards must be
met during critical conditions 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.
Any allowance for dilution or mixing should be made on a case-by-case basis that accounts for
site-specific conditions, including effluent flow, stream flow, and ambient pollutant
concentrations. In-stream dilution that is often provided in a mixing zone will not be available in
cases such as discharges to headwaters of a stream where the discharge makes up the entire
stream flow. Additionally, if the ambient concentration of selenium is greater than the criterion,
there will be no in-stream dilution available.
Ql-5: If a state or authorized tribe adopts a new WQS before the end of the 5-year permit
term, can the permit be modified to ensure that the effluent limit reflects the new standard?
Al-5: EPA regulations at 40 CFR 122.62(a) specify that permits can be modified before the end
of the 5-year permit term for cause. Cause can include permit reopeners (40 CFR 122.62(a)(7))
and new regulations, including EPA-approved or -promulgated WQS, if requested by the
permittee (40 CFR 122.62(a)(3)). Most typically, however, permits are revised to incorporate
WQS based on a new criterion at the time of renewal. If the permit writer anticipates the need for
new data in advance of reissuing the permit at the end of the 5-year term, CWA section 308
allows EPA to require the permittee to collect information that will be needed for development
of the revised permit.
2. NPDES Reasonable Potential (RP) Determinations (Basis for NPDES Permit WQBELs)
Q2-1: Which elements of a four-part criterion for selenium should be usedfor conducting RP
analyses?
A2-1: Because a four-part criterion includes multiple elements, any of the elements of the
criterion can be used by the permitting authority to determine whether the discharge causes, has
the RP to cause or contributes to an in-stream excursion above the state or authorized tribe's
EPA approved WQC. RP is demonstrated for a point source discharge where:
•	Fish tissue data collected from a waterbody show an excursion of any of the fish tissue
elements of the state's or authorized tribe's EPA-approved WQC (i.e., fish egg-ovary,
fish whole-body and/or muscle tissue) and a permitting authority determines that a point
source discharge to the waterbody contains selenium; or
•	The permitting authority determines that a point source discharge containing selenium
causes, has the RP to cause or contributes to an in-stream excursion above any of the
water column elements for the state's or authorized tribe's EPA approved WQC.
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Note: The absence of fish tissue data or instances in which available fish tissue data do not
indicate an excursion of the fish tissue elements of the state's or authorized tribe's EPA-
approved selenium WQC does not mean there is no RP. The RP analysis is by design predictive
of a potential excursion (presumably before an excursion occurs); thus, even if fish tissue levels
are below the applicable fish tissue elements of the state or authorized tribe WQC, the
determination should generally evaluate whether there is RP with respect to the water column
elements of the standard. [Also see Q2-4 below.]
The flowchart in Attachment 2 summarizes EPA's recommended procedures for conducting RP
analyses based on a four-part selenium criterion.
Q2-2: How should a regulatory authority determine the appropriate water column element
based on the four-part criterion (e.g., a 30-day chronic element versus an intermittent element)
for use in a given NPDES permit?
A2-2: For NPDES-permitted facilities that continuously discharge effluent containing selenium,
the water column elements of a four-part criterion—which represent a 30-day average
concentration that corresponds to the concentration of selenium in fish tissue estimated to result
in a 10 percent effect in fish for a specific waterbody type (lentic or lotic)—should be used in RP
analysis and WQBEL development (see also chapter 5 of EPA's Technical Support Document
for Water Quality-based Toxics Control,7 [TSD], March 1991, EPA/505/2-90-001).
For NPDES-permitted discharges that discharge effluent noncontinuously (e.g., some mining
discharges), the intermittent element should be used. The intermittent element is calculated using
the equations provided in EPA's Aquatic Life Ambient Water Quality Criterion for Selenium—
Freshwater 2016% (see Attachment 1).
Q2-3: What effluent data would be necessary to assess RP using a four-part criterion?
A2-3: To assess whether the discharge causes, has the RP to cause, or contributes to an in-stream
excursion above the water column element of a four-part selenium criterion, effluent data
requirements recommended for toxics in section 3.3 of EPA's 1991 TSD (March 1991,
EPA/505/2-90-001),9 and other NPDES monitoring guidance should be used to identify
appropriate monitoring requirements for RP determinations and NPDES permitting. The
flowchart presented in Attachment 2 summarizes the options to assess RP based on the different
elements of EPA's 2016 selenium criterion and data availability.
7	USEPA (U.S. Environmental Protection Agency). 1991. Technical Support Document for Water Oualitv-based
Toxics Control. EPA/505/2-90-001. U.S. Environmental Protection Agency, Office of Water, Washington, DC.
http://www.epa.gov/npdes/pubs/owm0264.pdf.
8	USEPA (U.S. Enviromnental Protection Agency). 2016. Aquatic Life Ambient Water Quality Criterion for
Selenium-Freshwater 2016. EPA 822-R-16-006. U.S. Enviromnental Protection Agency, Office of Water, Office of
Science and Technology, Washington, DC. https://www.epa.gov/wac/aauatic-life-criterion-selenium-documents.
9	USEPA (U.S. Enviromnental Protection Agency). 1991. Technical Support Document for Water Oualitv-based
Toxics Control. EPA/505/2-90-001. U.S. Enviromnental Protection Agency, Office of Water, Washington, DC.
http://www.epa.gov/npdes/pubs/owm0264.pdf.
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Q2-4: If a state or authorized tribe has sufficient and representative fish tissue data available
to demonstrate that selenium levels in fish tissue do not indicate an excursion of the fish tissue
elements of the four-part selenium criterion, can that information be used to determine that
there is no RP and, thus, no need for a WQBEL?
A2-4: EPA considers that its 2016 selenium criterion's water column element is the most
appropriate predictor for whether the discharge causes, has the RP to cause or contributes to an
in-stream excursion above the applicable WQC. However, the state or authorized tribe might be
able to make the determination that there is no RP if the following conditions exist
simultaneously: (1) the sources of selenium are already present in a waterbody and are not
expected to increase; (2) the population of fish have been exposed to the existing levels of
selenium and the system is determined to be in steady-state (based on scientifically defensible
documented data); and (3) relevant and timely collection of fish tissue data, accounting for the
duration of the discharge relative to collected fish tissue, demonstrate that the fish tissue levels
are significantly below the criterion's fish tissue element.
Q2-5: In cases in which a discharge occurs at a lotic location and downstream waters are
lentic waterbody types (e.g., lakes, impoundments), which selenium criterion water column
element should be used in RP analysis?
A2-5: EPA's 2016 selenium criterion water column element for lentic waters is more stringent
than it is for lotic waters. Therefore, if an NPDES discharge is located in lotic waters upstream of
lentic waters, the permit writer would need to ensure that the lentic downstream WQC are
protected when conducting RP analysis and in developing WQBELs for selenium (see 40 CFR
122.4(d)).
Water quality and hydrological modeling, as discussed in chapter 4 of EPA's 1991 TSD (March
1991, EPA/505/2-90-001),10 might be needed to determine potential water column
concentrations of selenium in lentic waters downstream and whether there is RP for an excursion
above the water column element in the lentic downstream waters. If the RP analyses indicate that
the discharge causes, has the RP to cause, or contributes to an in-stream excursion of the lentic
water column element downstream, then that water column element should be used as the basis
of selenium WQBELs even though the discharge is to the lotic waterbody upstream.
Q2-6: In some cases, it might not be clear whether a receiving waterbody is lentic or lotic in
terms of an appropriate selenium water column element (e.g., run-of-the-river reservoirs).
Does EPA have recommendations that address whether a receiving waterbody should be
considered lentic or lotic for purposes of NPDES RP analysis and permit WQBEL
determinations ?
A2-6: Generally, classification of fresh waters into lotic or lentic categories are made by the state
or authorized tribe. States and authorized tribes should ensure that they use transparent,
scientifically defensible methods to classify receiving waterbodies appropriately. 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 EPA's Aquatic Life
Ambient Water Quality Criterion for Selenium—Freshwater 2016 provides further discussion of
10 Ibid.
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categories of fresh water.11 A site-specific study or use of available hydrology information might
be necessary to determine the residence time of the waterbody so that the application of the lentic
or lotic water column value can be applied in a defensible manner for RP analyses.
3. NPDES Permit WQBEL Calculations
Q3-1: Can an NPDES permitting authority using a state- or authorized tribal-adopted
standard based on or similar to EPA '.s recommendedfour-part selenium criterion derive
selenium NPDES permit WQBELs for a noncontinuous or intermittent effluent discharge
containing selenium? How?
A3-1: The intermittent element is intended to provide protection from long-term chronic effects
on aquatic life caused by short-term elevation of selenium in aquatic systems (i.e., noncontinuous
exposures). It should be calculated based on the number of days a discharge that contains
selenium occurs and the equations provided in EPA's Aquatic Life Ambient Water Quality
Criterion for Selenium—Freshwater 2016 (Attachment 1) that address chronic effects associated
with noncontinuous discharges.12 Permit writers should use those recommendations when
determining whether RP exists and to calculate NPDES permit limits when RP has been
demonstrated. RP analysis and WQBEL determination should be based on the intermittent value
calculated for the potential effects of the discharge to the receiving water.
Q3-2: Can an NPDES permitting authority develop permit limits (WQBELs) for a four-part
selenium criterion using the fish tissue element(s) of a four-part criterion (i. e., egg-ovary or
whole body and/or muscle) rather than the water column element?
A3-2: States and authorized tribes should use the water column element to develop and establish
WQBELs in NPDES permits. As described in EPA's Technical Support for Adopting and
Implementing EPA's 2016 Selenium Criterion in Water Quality Standards,13 if a state or
authorized tribe believes that the default national water column element is not appropriate for a
waterbody, it can develop a site-specific 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, in EPA's Aquatic Life Ambient Water Quality Criterion for Selenium—
Freshwater 2016.14 Any development or use of a site-specific water column value must be
consistent with EPA-approved translation procedures in the state's or authorized tribe's WQS.
EPA has not developed guidance for calculating or expressing WQBELs directly from a fish
tissue-based criterion expression. If a permitting authority were to develop a procedure that
11	USEPA (U.S. Environmental Protection Agency). 2016. Aquatic Life Ambient Water Quality Criterion for
Selenium-Freshwater 2016. EPA 822-R-16-006. U.S. Environmental Protection Agency, Office of Water, Office of
Science and Technology, Washington DC. https://www.epa.gov/wac/aauatic-life-criterion-selenium-documents.
12	Ibid.
13	USEPA (U.S. Enviromnental Protection Agency). 2016. Technical Support for Adopting and Implementing EPA's
2016 Selenium Criterion in Water Quality Standards. U.S. Enviromnental Protection Agency, Office of Water,
Washington, DC.
14	USEPA (U.S. Enviromnental Protection Agency). 2016. Aquatic Life Ambient Water Quality Criterion for
Selenium-Freshwater 2016. EPA 822-R-16-006. U.S. Enviromnental Protection Agency, Office of Water, Office of
Science and Technology, Washington DC. https://www.epa.gov/wqc/aquatic-life-criterion-selenium-documents.
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derived the WQBEL directly from the fish tissue element(s) of the criterion, any WQBEL
developed would have to be consistent with the requirements of CWA 301(b)(1)(C) as well as
40 CFR 122.44(d) and 122.45.
Q3-3: EPA's 2016 selenium criterion does not include an acute expression. Must permits
contain both short- and long-term limit expressions?
A3-3: 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), and
average weekly and average monthly limitations for POTWs.
Many of EPA's WQC and many EPA-approved state or authorized tribal WQC include only a
single criterion expression. The availability of a single criterion value does not restrict or
preclude the development of both short- and long-term WQBELs. WQBELs establish end-of-
pipe effluent quality requirements that ensure compliance with the applicable in-stream ambient
criteria. WQBEL calculations typically rely on physical and statistical models that anticipate
how the effluent and receiving water comingle and account for factors such as effluent variability
and pollutant fate and transport characteristics. Chapter 5 of EPA's TSD provides methods to
derive both short- and long-term WQBELs from one or more ambient criteria values with a
range of duration components.15
Q3-4: NPDES regulations require that permit limits for metals be expressed as total
recoverable; however selenium is a nonmetal or metalloid. Should selenium be expressed as
total recoverable in WQBELs and, if so, how should an NPDES permitting authority translate
the dissolved selenium water column element of EPA '.s 2016 selenium criterion to a total
recoverable selenium NPDES permit limit?
A3-4: WQBELs for selenium should be expressed as total recoverable selenium; however, the
selenium criterion water column elements are expressed as dissolved selenium. Given that
particulate as well as dissolved selenium can have deleterious effects on aquatic life—as
discussed in EPA's Aquatic Life Ambient Water Quality Criterion for Selenium—Freshwater
201616—in the absence of site-specific data, it may be appropriate to use a total recoverable-to-
dissolved selenium ratio of 1.00 in determining RP and calculating permit WQBELs for
selenium. EPA has provided previous guidance on procedures for translating a dissolved
criterion to total recoverable permit limits that also includes recommendations for sampling and
analysis (see EPA's document 823-B-96-007, 1996).17
15	USEPA (U.S. Environmental Protection Agency). 1991. Technical Support Document for Water Oualitv-based
Toxics Control. EPA/505/2-90-001. U.S. Environmental Protection Agency, Office of Water, Washington, DC.
http://www.epa.gov/npdes/pubs/owm0264.pdf.
16	USEPA (U.S. Enviromnental Protection Agency). 2016. Aquatic Life Ambient Water Quality Criterion for
Selenium-Freshwater 2016. EPA 822-R-16-006. U.S. Enviromnental Protection Agency, Office of Water, Office of
Science and Technology, Washington DC. https://www.epa.gov/wqc/aquatic-life-criterion-selenium-documents.
17	USEPA (U.S. Enviromnental Protection Agency). 1996. The Metals Translator: Guidance For Calculating a
Total Recoverable Permit Limit from a Dissolved Criterion. EPA 823-B-96-007. U.S. Enviromnental Protection
Agency, Office of Water, Washington, DC. https://www3.epa.gov/npdes/pubs/metals translator.pdf.
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4.	NPDES Monitoring (Basis for RP and WQBELs, Compliance Determination)
Q4-1: What EPA analytical test methods are recommended for use in analyzing water and fish
tissue samples for selenium under EPA '.s NPDES program?
A4-1: EPA provides analytical test methods approved for measuring selenium for wastewater
effluent and sewage sludge at 40 CFR Part 136.3 (also see Appendix L, Analytical Methods for
Measuring Selenium, in EPA's Aquatic Life Ambient Water Quality Criterion for Selenium—
Freshwater 2016).18
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 Selenium Criterion, which describes fish tissue
sampling methods and sample collection considerations.19
5.	Compliance and Enforcement of NPDES Permit Requirements
Q5-1: Would a compliance schedule be allowed in an NPDES renewal permit in which a
WQBEL is being replaced with a more stringent permit limit based on EPA '.s 2016 selenium
criterion?
A5-1: Possibly. EPA's water quality standards regulations at 40 CFR 131.15 specify that, if a
state or authorized tribe intends to authorize the use of schedules of compliance for WQBELs in
NPDES permits, that state or authorized tribe must adopt a permit compliance schedule-
authorizing provision, and any schedule developed must be consistent with the requirements
established for compliance schedules in 40 CFR 122.2 and 122.47.
18	USEPA (U.S. Environmental Protection Agency). 2016. Aquatic Life Ambient Water Quality Criterion for
Selenium-Freshwater 2016. EPA 822-R-16-006. U.S. Environmental Protection Agency, Office of Water, Office of
Science and Technology, Washington DC. https://www.epa.gov/wac/aauatic-life-criterion-selenium-documents.
19	USEPA (U.S. Enviromnental Protection Agency). 2016. Technical Support for Fish Tissue Monitoring for
Implementation of EPA's 2016 Selenium Criterion. U.S. Enviromnental Protection Agency, Office of Water, Office
of Science and Technology, Washington DC.
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EPA Draft for Public Comment
Attachment 1:
EPA's Aquatic Life Ambient Water Quality Criterion for
Selenium—Freshwater 2016
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 dw
8.5 mg/kg dw
whole-body
or
11.3 mg/kg dw
muscle (skinless,
boneless filet)
1.5 (xg/L in lentic
aquatic systems
3.1 (xg/L in lotic
aquatic systems
WQCint =
WQC^q—day Cbkgrnd(\ fint)
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.
3.	Fish whole-body or muscle tissue supersedes water column element when both fish tissue and water concentrations are
measured.
4.	Water column values are based on dissolved total selenium in water and are derived from fish tissue values via
bioaccumulation modeling. Water column values are the applicable criterion element in the absence of steady-state condition
fish tissue data.
5.	Where WQC3o-dayis the water column monthly element for either lentic or lotic waters; Cbkgmdis the average background
selenium concentration; and fint is the fraction of any 30-day period during which elevated selenium concentrations occur, with
fint 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.
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EPA Draft for Public Comment
Attachment 2:
EPA's Recommended Flowchart for Implementing EPA's 2016 Selenium Criterion in
NPDES Permits Program
^^New^^evise^tat^o^uthorize^^
tribe WQS for Selenium are approved
by EPA for CWA purposes (all 4
components have been adopted)
Could include site-specific WC element
in WQS if approved by EPA
Permitting
Potential discharger of
Selenium applies for
permit
Collect effluent
data priorto
permit issuance
or develop
estimate for new
discharger.
Does the
discharger have
detectable
levels of Se in
it's effluent?
Yes
Are there fish tissue
(egg/ovary, whole body,
or fillet) data available?
Are effluent
available:"
Reassess next
permit cycle
Are fish tissue
valuesator
abovethe FT
element?
Are WC
data
available?
WQBEL
Required
-1~
Consider collecting FT data to
determine whether FT element
is exceeded; once FT data are
availablego toD.:
/ Are WC
—f data
\available?
Calculate
."Ud:-
usingWCE
Calculate WQBEL
based on site-
specific FT element
if approved by EPA
in WQS.
/—s—V
( IsWCE r
\ exceeded? /
/	\Yes
/ Is WCE \	
\ exceeded? J
Calculate site-specific
WC element. If using
EPA translator
proceed to RP below.
If i n corporati ng WCE
in WQS go to top of
flowchart
< Conduct RP \
analysis: Isthere >
	/
Yes
NO Goto
"C"

No


No WQBEL
Consider
effluent
monitoring
requirement.
Yes
Calculate

WCE EOP.
Calculate
WQBEL
using WCE
EOP.
Yes No
)
No
Conduct RP
Analysis: Is
there RP?
I Yes
No .
Calculate
WQBEL using
WCE with
consideration
of dilution.
No WQBEL.
Consider
effluent
monitoring
requirement.
Three Options:
1)	Collect ambient data priorto
permit issuance: Go to "A"
2)	Assume water body has no
assimilative capacity (EOP): Go
to "B"
3) Assume background
concentration present in
waterbody that is below
criterion: Goto "C"
Conduct RP analysis
with consideration of
k dilution: Isthere RP? ,
No WQBEL.
Consider
effluent
monitoring
equirement.
1- RP analysis should not be delayed pending collection offish tissue data.
... ^ ^ i r-, .		 RP has been determined
•	WCE: Water Column Element
•	WC: Water Column Selenium	Transition box
Concentration
•	RP: Reasonable Potential	NoRP
•	EOP: End of Pipe
•	WQBEL: Water Quality Based Effluent Limit
•	FT = Fish Tissue
^^^Question^^^^
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