[Federal Register Volume 89, Number 69 (Tuesday, April 9, 2024)]
[Proposed Rules]
[Pages 24758-24775]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2024-07450]


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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 131

[EPA-HQ-OW-2023-0325; FRL 11009-03-OW]
RIN 2040-AG35


Mercury Criterion To Protect Aquatic Life in Idaho

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule; notice of public hearing.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing to 
promulgate a Federal Clean Water Act (CWA) chronic aquatic life ambient 
water quality criterion for waters under the state of Idaho's 
jurisdiction to protect aquatic life from the effects of

[[Page 24759]]

exposure to harmful concentrations or levels of total mercury (i.e., 
including methylmercury and inorganic mercury). In 2008, the EPA 
disapproved the state's revision of its mercury aquatic life criteria. 
The state has not adopted and submitted revised mercury aquatic life 
criteria to the EPA to address the EPA's 2008 disapproval. Therefore, 
the EPA is proposing a Federal mercury criterion to protect aquatic 
life uses in Idaho.

DATES: Comments must be received on or before June 10, 2024. Public 
Hearing: The EPA will hold two public hearings during the public 
comment period. Please refer to the SUPPLEMENTARY INFORMATION section 
for additional information on the public hearings.

ADDRESSES: You may send comments, identified by Docket ID No. EPA-HQ-
OW-2023-0325, by any of the following methods:
     Federal eRulemaking Portal: https://www.regulations.gov/ 
(our preferred method). Follow the online instructions for submitting 
comments.
     Mail: U.S. Environmental Protection Agency, EPA Docket 
Center, Office of Water Docket, Mail Code 28221T, 1200 Pennsylvania 
Avenue NW, Washington, DC 20460.
     Hand Delivery or Courier: EPA Docket Center, WJC West 
Building, Room 3334, 1301 Constitution Avenue NW, Washington, DC 20004. 
The Docket Center's hours of operations are 8:30 a.m. to 4:30 p.m., 
Monday through Friday (except Federal Holidays).
    Instructions: All submissions received must include the Docket ID 
No. for this rulemaking. Comments received may be posted without change 
to https://www.regulations.gov/, including any personal information 
provided. For detailed instructions on sending comments and additional 
information on the rulemaking process, see the ``Public Participation'' 
heading of the SUPPLEMENTARY INFORMATION section of this document. The 
EPA is offering two public hearings on this proposed rulemaking. Refer 
to the SUPPLEMENTARY INFORMATION section below for additional 
information.

FOR FURTHER INFORMATION CONTACT: Kelly Gravuer, Office of Water, 
Standards and Health Protection Division (4305T), Environmental 
Protection Agency, 1200 Pennsylvania Avenue NW, Washington, DC 20460; 
telephone number: (202) 566-2946; email address: [email protected].

SUPPLEMENTARY INFORMATION: 
    This proposed rulemaking preamble is organized as follows:

I. Public Participation
    A. Written Comments
    B. Participation in Public Hearings
II. General Information
    A. Does this action apply to me?
III. Background
    A. Statutory and Regulatory Authority
    B. Sources of Mercury and Effects on Aquatic Life
    C. History of Mercury Aquatic Life Criteria in Idaho
    D. General Recommended Approach for Deriving Aquatic Life 
Criteria
IV. Proposed Mercury Aquatic Life Criterion for Idaho
    A. Scope of the EPA's Proposed Rule
    B. Proposed Mercury Criterion
    C. Implementation
V. Endangered Species Act
VI. Applicability of EPA-Promulgated Water Quality Standards When 
Final
VII. Implementation and Alternative Regulatory Approaches
    A. NPDES Permit Compliance Schedules
    B. Site-Specific Criteria
    C. WQS Variances
    D. Designated Uses
VIII. Economic Analysis
IX. Statutory and Executive Orders Reviews
    A. Executive Order 12866 Regulatory Planning and Review and 
Executive Order 14094 Modernizing Regulatory Review
    B. Paperwork Reduction Act (PRA)
    C. Regulatory Flexibility Act (RFA)
    D. Unfunded Mandates Reform Act (UMRA)
    E. Executive Order 13132 (Federalism)
    F. Executive Order 13175 (Consultation and Coordination With 
Indian Tribal Governments)
    G. Executive Order 13045 (Protection of Children From 
Environmental Health and Safety Risks)
    H. Executive Order 13211 (Actions That Significantly Affect 
Energy Supply, Distribution, or Use)
    I. National Technology Transfer and Advancement Act of 1995
    J. Executive Order 12898: Federal Actions To Address 
Environmental Justice in Minority Populations and Low-Income 
Populations and Executive Order 14096: Revitalizing Our Nation's 
Commitment to Environmental Justice for All

I. Public Participation

A. Written Comments

    Submit your comments, identified by Docket ID No. EPA-HQ-OW-2023-
0325, at https://www.regulations.gov (our preferred method), or the 
other methods identified in the ADDRESSES section. Once submitted, 
comments cannot be edited or removed from the docket. The EPA may 
publish any comment received to its public docket. Do not submit to the 
EPA's docket at https://www.regulations.gov any information you 
consider to be Confidential Business Information (CBI), Proprietary 
Business Information (PBI), or other information whose disclosure is 
restricted by statute. Multimedia submissions (audio, video, etc.) must 
be accompanied by a written comment. The written comment is considered 
the official comment and should include discussion of all points you 
wish to make. The EPA will generally not consider comments or comment 
contents located outside of the primary submission (i.e., on the web, 
cloud, or other file sharing system). Please visit https://www.epa.gov/dockets/commenting-epa-dockets for additional submission methods; the 
full EPA public comment policy; information about CBI, PBI, or 
multimedia submissions; and general guidance on making effective 
comments.

B. Participation in Public Hearings

    The EPA is offering two online public hearings so that interested 
parties may provide oral comments on this proposed rulemaking. For more 
details on the online public hearings and to register to attend the 
hearings, please visit https://www.epa.gov/wqs-tech/mercury-criterion-protect-aquatic-life-idaho.

II. General Information

A. Does this action apply to me?

    Entities that discharge mercury to waters under Idaho's 
jurisdiction \1\ that are subject to relevant aquatic life designated 
uses--such as industrial facilities and municipalities that manage 
stormwater, separate sanitary, or combined sewer systems--could be 
indirectly affected by this rulemaking because Federal water quality 
standards (WQS) promulgated by the EPA would be the applicable WQS for 
Clean Water Act (CWA) purposes. Specifically, these WQS would be the 
applicable standards that must be used in CWA regulatory programs, such 
as permitting under the National Pollutant Discharge Elimination System 
(NPDES) (CWA section 402) \2\ and identifying impaired waters under CWA 
section 303(d). Categories and entities that could be affected include 
the following:
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    \1\ Throughout this preamble, the phrase ``waters under Idaho's 
jurisdiction'' refers to waters of the United States under Idaho's 
jurisdiction, since the Clean Water Act applies to waters of the 
United States.
    \2\ Before any water quality-based effluent limit would be 
included in an NPDES permit, the permitting authority (here, the 
Idaho Department of Environmental Quality [IDEQ]), must first 
determine whether a discharge ``will cause or has the reasonable 
potential to cause, or contribute to an excursion above any WQS.'' 
40 CFR 122.44 (d)(1)(i) and (ii).

[[Page 24760]]



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                                      Examples of potentially affected
             Category                             entities
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Industry..........................  Industrial point sources discharging
                                     mercury to waters under Idaho's
                                     jurisdiction.
Municipalities, including those     Publicly owned treatment works or
 with stormwater or combined sewer   similar facilities responsible for
 system outfalls.                    managing stormwater, separate
                                     sanitary, or combined sewer systems
                                     that discharge mercury to waters
                                     under Idaho's jurisdiction.
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    This table is not intended to be exhaustive, but rather provides a 
guide for readers regarding entities that could be indirectly affected 
by this action. If you have questions regarding the applicability of 
this action to a particular entity, consult the person listed in the 
FOR FURTHER INFORMATION CONTACT section above.

III. Background

A. Statutory and Regulatory Authority

    CWA section 101(a)(2) establishes a national goal of ``water 
quality which provides for the protection and propagation of fish, 
shellfish, and wildlife, and provides for recreation in and on the 
water'' (hereafter, collectively referred to as ``101(a)(2) uses''), 
wherever attainable. The EPA's regulation at 40 CFR 131.10(g) and (h) 
implements this statutory provision by requiring that WQS protect 
101(a)(2) uses unless those uses are shown to be unattainable.
    Under the CWA, states have the primary responsibility for 
establishing, reviewing, and revising WQS applicable to their waters 
(CWA section 303(c)). WQS define the desired condition of a water body, 
in part, by designating the use or uses to be made of the water and by 
setting the numeric or narrative water quality criteria to protect 
those uses (40 CFR 131.2, 131.10, and 131.11). There are two primary 
categories of water quality criteria: human health criteria and aquatic 
life criteria. Human health criteria protect designated uses such as 
public water supply, recreation, and fish and shellfish consumption. 
Aquatic life criteria protect designated uses such as survival, growth, 
and reproduction of fish, invertebrates, and other aquatic species. 
Regardless of their category, water quality criteria ``must be based on 
sound scientific rationale and must contain sufficient parameters or 
constituents to protect the designated use. For waters with multiple 
use designations, the criteria shall support the most sensitive use'' 
(40 CFR 131.11(a)(1)).
    Section 304(a) of the CWA directs the EPA to periodically develop 
and publish recommended water quality criteria ``accurately reflecting 
the latest scientific knowledge'' on the effects of pollutants on human 
health and welfare, including effects on aquatic life, as well as 
information on those pollutants, including their concentration and 
dispersal and how pollutants affect receiving waters (CWA section 
304(a)(1)). Those recommendations are available to states for use in 
developing their own water quality criteria (CWA section 304(a)(3)). 
When states establish criteria, the EPA's regulation at 40 CFR 
131.11(b)(1) specifies that they should establish numeric criteria 
based on: (1) the EPA's CWA section 304(a) recommended criteria, (2) 
modified 304(a) recommended criteria that reflect site-specific 
conditions, or (3) other scientifically defensible methods.
    CWA section 303(c)(2)(B), added to the CWA in the 1987 amendments 
to the Act,\3\ requires states to adopt numeric criteria, where 
available, for all toxic pollutants listed pursuant to CWA section 
307(a)(1) (i.e., priority toxic pollutants \4\) for which the EPA has 
published CWA section 304(a) recommended criteria, the discharge or 
presence of which could reasonably be expected to interfere with the 
states' designated uses.
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    \3\ Water Quality Act Amendments of 1987, Public Law 100-4, 101 
Stat. 7.
    \4\ See 40 CFR part 423, Appendix A--126 Priority Pollutants.
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    States are required to hold a public hearing to review applicable 
WQS at least once every three years and, if appropriate, revise or 
adopt new standards (CWA section 303(c)(1); 40 CFR 131.20(a)). Any new 
or revised WQS must be submitted to the EPA for review and approval or 
disapproval (CWA section 303(c)(2)(A) and (c)(3)). If the EPA 
disapproves a new or revised WQS because it is inconsistent with the 
requirements of the CWA, the EPA must notify the state within 90 days 
and ``specify the changes to meet such requirements'' (CWA section 
303(c)(3)). If the state does not adopt changes to comply with the Act 
within 90 days of notification, the EPA must promptly propose a new or 
revised WQS for the waters involved (CWA section 303(c)(3) and (4)).

B. Sources of Mercury and Effects on Aquatic Life

    Mercury is a naturally occurring metal that can be enriched in some 
mineral deposits (e.g., cinnabar) and is often present as an impurity 
in coal. In Idaho, there are several areas with geologically enriched 
mercury deposits.
    Human activities can result in the release and transport of mercury 
to the aquatic environment primarily through the deposition of mercury 
that was released to the atmosphere, discharges to water, and leaching 
from mercury-bearing strata exposed due to mining or other activities. 
Historically, mercury was both mined directly and used in hardrock and 
placer gold mining in Idaho, resulting in a legacy of elevated mercury 
levels in several parts of the state. Industrial processes (e.g., 
chemical manufacture and metals processing) are the predominant sources 
of current mercury emissions to air in Idaho and nationally. Globally, 
natural sources of mercury are less significant than anthropogenic 
sources and include the weathering of mercury-containing rocks, 
volcanoes, and geothermal activity.\5\ In Idaho, hot springs throughout 
the state are a natural mercury source.\6\ Because atmospheric releases 
of mercury, whether natural or human-caused, can ultimately be 
deposited in waterways far from their point of emission, some of the 
mercury in Idaho's environment originated outside the state.
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    \5\ UN Environment, 2019. Global Mercury Assessment 2018. UN 
Environment Programme, Chemicals and Health Branch. Geneva, 
Switzerland. https://www.unep.org/resources/publication/global-mercury-assessment-2018.
    \6\ U.S. Geological Survey. 1985. Geochemistry and hydrology of 
thermal springs in the Idaho Batholith and adjacent areas, Central 
Idaho. Water Resources Investigations Report 85-4172. H.W. Young, 
Boise, Idaho.
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    In water, mercury can occur in a dissolved form or bound to 
particles. The main forms of dissolved mercury in the aquatic 
environment are inorganic mercury and methylmercury. Aquatic organisms 
can take up both forms of mercury through dietary exposure and through 
direct water column exposure. Aquatic organisms tend to take up mercury 
more rapidly than they eliminate it, causing mercury (especially 
methylmercury) to bioaccumulate. Methylmercury can also biomagnify 
(i.e., increase in concentration at successively higher trophic levels) 
within aquatic food webs, whereas inorganic mercury does not. Because 
of methylmercury's potential for

[[Page 24761]]

biomagnification, dietary exposure is of greater concern than direct 
water column exposure for mercury toxicity.
    Mercury is a potent neurotoxin that causes neurological damage, 
which can result in behavioral changes and ultimately in reduced growth 
and reproduction in aquatic organisms. Dietary exposure to 
methylmercury has been shown to impair reproduction in fish. Aquatic 
invertebrates are typically more tolerant to both inorganic and 
methylmercury exposures than vertebrates, with larval stages tending to 
be the most sensitive. However, there are exceptions to this general 
pattern. For example, the red swamp crayfish \7\ was found to be the 
fourth most sensitive (out of 19 mostly vertebrate) species for which 
data were available to derive this mercury criterion (see section IV.B. 
in this preamble below).
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    \7\ Although the red swamp crayfish (Procambarus clarkii) is not 
native to Idaho, it serves as a surrogate for similar native 
invertebrate species for which toxicity data were not available.
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    In general, mercury cycling in the aquatic environment is affected 
by pH, temperature, oxidation-reduction (redox) potential, and the 
availability of nutrients, humic acids, and complexing agents. The 
conversion of inorganic mercury to the more toxic methylmercury occurs 
in anoxic environments, such as wetlands. Higher mercury methylation 
rates tend to occur in areas with higher anerobic microbial activity 
and when inorganic mercury is in a form that is bioavailable to the 
microbial community.\8\ Mercury has a high affinity for sorbing to 
sediments as well as dissolved and particulate matter suspended in the 
water column. This sorption to sediments can allow sediments to serve 
as a source of mercury to the water column long after mercury-releasing 
activities have ceased.
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    \8\ USEPA. 2023. Technical Support Document: Aquatic Life Water 
Quality Criterion for Mercury in Idaho. U.S. Environmental 
Protection Agency, Office of Water, Washington, DC. https://www.epa.gov/wqs-tech/mercury-criterion-protect-aquatic-life-idaho.
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C. History of Mercury Aquatic Life Criteria in Idaho

    On June 25, 1996, the EPA approved Idaho's numeric aquatic life 
mercury criteria (0.012 [mu]g/L chronic and 2.1 [mu]g/L acute) under 
CWA section 303(c). In 2003, the Idaho Department of Environmental 
Quality (``IDEQ'') began a negotiated rulemaking in response to a 
petition from the Idaho Mining Association to update Idaho's mercury 
criteria. As a result of that negotiated rulemaking, Idaho adopted and, 
on August 8, 2005, submitted revised standards to the EPA for review 
under CWA section 303(c). IDEQ's revised standards removed the acute 
and chronic numeric aquatic life criteria for mercury and added a 
footnote ``g'' to the state's toxic criteria table. Footnote ``g'' 
stated that Idaho's existing narrative criteria for toxics would apply 
instead of the numeric criteria and that the existing human health 
criterion for methylmercury would be protective of aquatic life in most 
situations.
    On December 12, 2008, the EPA disapproved Idaho's removal of 
numeric acute and chronic aquatic life criteria for mercury and their 
replacement with footnote ``g,'' stating that these revisions were 
inconsistent with CWA section 303(c) and 40 CFR 131.11.\9\ The EPA 
noted that ``the supporting documentation that Idaho had submitted 
[did] not provide specific information which would demonstrate that the 
designated aquatic life uses in Idaho are assured protection from 
discharges of mercury that would adversely affect water quality and/or 
the attainment of the aquatic life uses.'' The EPA further stated that 
Idaho's Implementation Guidance for the Mercury Water Quality Criteria 
\10\ (which primarily pertains to Idaho's human health criteria for 
mercury) did not ``contain definitive information on how the State 
would translate the fish tissue criterion developed to protect human 
health to a value which could be used to protect aquatic life.''
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    \9\ Letter from Michael F. Gearheard, Director, EPA Region 10 
Office of Water and Watersheds to Barry Burnell, Water Quality 
Program Administrator, Idaho Department of Environmental Quality, 
Re: EPA's Disapproval of Idaho's Removal of Mercury Acute and 
Chronic Freshwater Aquatic Life Criteria, Docket No. 58-0102-0302 
(December 12, 2008).
    \10\ Idaho Department of Environmental Quality. 2005. 
Implementation Guidance for the Idaho Mercury Water Quality 
Criteria. Boise, ID. https://www2.deq.idaho.gov/admin/LEIA/api/document/download/4836.
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    To remedy this disapproval, the EPA specified ``several options 
Idaho could consider in establishing mercury criteria that are based on 
scientifically defensible methods and protect Idaho's designated 
aquatic life uses.'' These options included (1) evaluating the 
protectiveness of the EPA's existing recommended 304(a) numeric acute 
aquatic life criterion for mercury (1.4 [micro]g/L); (2) evaluating the 
protectiveness of Idaho's previous numeric chronic aquatic life 
criterion for mercury (0.012 [micro]g/L); (3) evaluating development of 
Idaho-specific numeric acute and chronic aquatic life criteria for 
mercury; and (4) evaluating the use of a combination of protective 
numeric water column values and numeric wildlife criteria appropriate 
for Idaho species. The EPA also pointed out that it was not 
recommending Idaho use the EPA's existing 304(a) numeric chronic 
aquatic life criterion for mercury (0.77 [micro]g/L) as one of the 
options. The EPA explained that information arising after the 
derivation of that 304(a) criterion had indicated that it may not 
adequately protect certain fish species that are present in Idaho.
    The EPA concluded that ``[u]ntil Idaho develops and adopts and EPA 
approves revisions to [the] numeric acute and chronic aquatic life 
criteria for mercury, the numeric aquatic life mercury criteria 
applicable to the designated aquatic life uses in Idaho that are 
effective for Clean Water Act [p]urposes are the previously adopted 
acute (2.1 [mu]g/L) and chronic (0.012 [mu]g/L) mercury criteria which 
EPA approved'' in 1996. No revisions to Idaho's aquatic life mercury 
criteria have been made since the EPA's December 2008 disapproval. 
Idaho's WQS acknowledge the EPA's 2008 disapproval and state that the 
mercury aquatic life criteria that were published in the 2004 Idaho 
Administrative Code (prior to adoption of the disapproved standards) 
still apply and are effective for CWA purposes.\11\ Those criteria are 
currently being implemented for CWA purposes including NPDES permitting 
in the state.
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    \11\ IDAPA 58--Department of Environmental Quality, Surface and 
Wastewater Division, 58.01.02--Water Quality Standards. https://adminrules.idaho.gov/rules/current/58/580102.pdf.
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    On June 14, 2013, Northwest Environmental Advocates filed suit in 
the Federal district court for the District of Idaho against the 
National Marine Fisheries Service and the Fish and Wildlife Service 
(the Services).\12\ The complaint alleged that the Services 
unreasonably delayed or unlawfully withheld completion of Endangered 
Species Act (ESA) consultation with the EPA regarding new and revised 
WQS that Idaho submitted in 1996 and/or 1997. On September 24, 2013, 
Northwest Environmental Advocates were joined by the Idaho Conservation 
League (collectively, the plaintiffs) in filing an amended complaint 
adding various CWA and ESA claims against the EPA regarding dozens of 
Idaho WQS submissions dating back to 1994.
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    \12\ Nw. Env't Advocs. v. United States Env't Prot. Agency, No. 
1:13-cv-263 (D. Idaho filed June 14, 2013).
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    By 2020, all claims against the EPA except one had either been 
dismissed on statute of limitations grounds or included in a stipulated 
dismissal agreed upon by the parties. The remaining claim alleged that 
the EPA failed to act under section 303(c)(4) of the CWA to promulgate 
aquatic life

[[Page 24762]]

mercury criteria for Idaho following the EPA's December 12, 2008 
disapproval of the state's revisions to its mercury criteria. On July 
19, 2021, the Court issued a decision on that claim in favor of the 
plaintiffs, concluding that, as a result of its disapproval, the EPA 
was subject to a mandatory duty to promulgate new criteria for the 
state.\13\ The Court directed the parties to file briefs regarding an 
appropriate remedy. The parties negotiated a settlement and entered 
into a Stipulated Order on Remedy on October 4, 2022.\14\ The Order 
states that the EPA will sign for publication in the Federal Register 
proposed aquatic life mercury criteria for the state of Idaho within 18 
months of its entry with the Court (i.e., by April 4, 2024).
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    \13\ Nw. Env't Advocs. v. United States Env't Prot. Agency, 549 
F. Supp. 3d 1218 (D. Idaho 2021).
    \14\ Stipulated Order on Remedy, Nw. Env't Advocs. v. United 
States Env't Prot. Agency, No. 1:13-cv-263 (D. Idaho October 4, 
2022).
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    With regard to the form of the proposed criteria, the Stipulated 
Order on Remedy states that ``[i]n recognition of the comparative ease 
of translating water column concentrations and values into permit 
effluent limitations and wasteload allocations, EPA commits to 
developing proposed Mercury Criteria that include water column 
concentrations, or default water column values that can be modified on 
a case-by-case basis, if EPA determines there are sufficient data 
available to support this form of criteria.''

D. General Recommended Approach for Deriving Aquatic Life Criteria

    The EPA developed the mercury criterion for Idaho in this proposed 
rulemaking consistent with the EPA's Guidelines for Deriving Numerical 
National Water Quality Criteria for the Protection of Aquatic Organisms 
and Their Uses (referred to as the ``Aquatic Life Guidelines'').\15\ 
The EPA's Aquatic Life Guidelines describe a method to estimate the 
highest concentration (magnitude) of a substance in water--averaged 
over a given time period (duration) and that should not be exceeded 
more than the allowable number of times during a specified time period 
(frequency)--that will not present a significant risk to the aquatic 
organisms in the water. The Aquatic Life Guidelines recommend using 
toxicity test data from a minimum of eight taxa of aquatic organisms to 
derive criteria. These taxa are intended to be representative of a wide 
spectrum of aquatic life, and act as surrogates for untested species. 
Therefore, the specific test organisms do not need to be present in the 
water(s) where the criteria will apply.
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    \15\ USEPA. 1985. Guidelines for Deriving Numerical National 
Water Quality Criteria for the Protection of Aquatic Organisms and 
Their Uses. U.S. Environmental Protection Agency, Office of Research 
and Development, Duluth, MN, Narragansett, RI, Corvallis, OR. PB85-
227049. https://www.epa.gov/sites/production/files/2016-02/documents/guidelines-water-quality-criteria.pdf.
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    Aquatic life criteria are typically represented as concentrations 
of a pollutant in the water column with two magnitudes: one associated 
with a shorter-term (acute) duration and another associated with a 
longer-term (chronic) duration. However, depending on the mode of 
toxicity, for some pollutants, an acute-only or chronic-only water 
column criterion is appropriate.\16\ For example, for pollutants where 
toxicity to aquatic life is primarily driven by diet (i.e., the 
consumption of contaminated prey) rather than by direct exposure to 
dissolved contaminants in the water column, longer-term water column 
measurements that capture the degree of likely pollutant uptake via 
dietary exposure--such as measurements with a 30-day average (chronic) 
duration--are often the most appropriate water column-based measure of 
their toxicity to aquatic life. Furthermore, for some pollutants, 
measurements of pollutant concentrations within the tissues of aquatic 
organisms provide a more direct measure of toxicity (to both the 
organisms themselves, and to humans consuming those organisms) than 
water column measurements. For bioaccumulative pollutants such as 
mercury, where exposure is primarily through diet, both of these 
rationales apply, with tissue measurements and longer-term water column 
measurements providing more appropriate measures of toxicity than the 
1-hour and 4-day water column measurements that capture the toxic 
effects of many other pollutant types.
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    \16\ https://www.epa.gov/wqc/national-recommended-water-quality-criteria-aquatic-life-criteria-table#table.
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    Because tissue measurements provide a more direct measure of 
toxicity for bioaccumulative pollutants such as mercury, the EPA has 
considered it appropriate to establish tissue criteria for these 
pollutants. However, criteria expressed as organism tissue 
concentrations can prove challenging to implement in CWA programs such 
as NPDES permitting and Total Maximum Daily Loads (TMDLs) because these 
programs typically demonstrate that water quality standards are met by 
using a water column concentration to calculate a load-based effluent 
limit or daily load, respectively. In recent years, the EPA has 
developed tissue-based national criteria recommendations for certain 
bioaccumulative pollutants and then assessed the degree to which 
available knowledge and data support translating those tissue criteria 
to water column criteria at the site, state, or national level.
    For exceedance frequency, most water column aquatic life criteria 
developed by the EPA include a recommended exceedance frequency of no 
more than once in three years. The EPA based this maximum exceedance 
frequency recommendation of once every three years on the time aquatic 
ecosystems require to recover from the exceedances. For water column 
criteria, an exceedance occurs when the average concentration over the 
duration of the averaging period is above the criterion. Because fish 
tissue concentrations of bioaccumulative pollutants reflect longer-term 
uptake and elimination dynamics and tend to change slowly over time, 
their frequency and duration components tend to be different than those 
of water column criteria. Specifically, for fish tissue criteria, the 
EPA recommends for bioaccumulative pollutants 17 18 that the 
criteria be expressed with an ``instantaneous measurement'' duration 
and be considered exceeded if a fish tissue sample measurement from a 
single sampling event (defined as a composited tissue sample from each 
fish species or a central tendency estimate of individual tissue 
samples from each fish species, collected from a given site or 
waterbody in a discrete sampling period) exceeds the criterion 
value.\19\
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    \17\ USEPA. 2021. 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.
    \18\ USEPA. 2022. Draft Aquatic Life Ambient Water Quality 
Criteria for Perfluorooctanoic Acid (PFOA). EPA-842-D-22-001. U.S. 
Environmental Protection Agency, Office of Water, Washington, DC. 
https://www.epa.gov/system/files/documents/2022-04/pfoa-report-2022.pdf; USEPA. 2022. Draft Aquatic Life Ambient Water Quality 
Criteria for Perfluorooctane Sulfonate (PFOS). EPA-842-D-22-002. 
U.S. Environmental Protection Agency, Office of Water, Washington, 
DC. https://www.epa.gov/system/files/documents/2022-04/pfos-report-2022.pdf.
    \19\ As previously stated, since fish tissue concentrations of 
bioaccumulative pollutants tend to change slowly over time, any 
exceedance indicates that waterbody conditions may not be protective 
of aquatic life.
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IV. Proposed Mercury Aquatic Life Criterion for Idaho

A. Scope of the EPA's Proposed Rule

    The final criterion resulting from this proposed rulemaking would 
establish

[[Page 24763]]

levels of mercury appropriate for the protection and maintenance of a 
viable aquatic life community in waters under Idaho's jurisdiction that 
are designated for aquatic life uses. The criterion would apply to all 
of Idaho's aquatic life use designations and would replace the current 
CWA-effective acute and chronic mercury criteria.

B. Proposed Mercury Criterion

    Since mercury is significantly more toxic through chronic dietary 
exposure than through water-based exposure, the EPA developed a 
proposed chronic criterion that is based on dietary exposures. The EPA 
did not develop a separate acute or chronic criterion from the results 
of toxicity tests with only water-based exposure. Because the most 
harmful effects of mercury on aquatic organisms are due to its 
bioaccumulative properties and because the resulting chronic effects 
are observed at lower mercury concentrations than acute effects, this 
chronic criterion based on dietary exposure is expected to additionally 
protect aquatic communities from any potential acute effects of 
mercury. For reasons described below, the EPA concluded that this 
chronic mercury criterion should integrate consideration of both 
relative organismal sensitivity (i.e., inherent toxicity) and relative 
exposure potential (i.e., bioaccumulation) across the aquatic species 
for which data are available. A summary of the EPA's approach is 
described below; for more details, please see the Technical Support 
Document included in the docket for this rulemaking.\20\
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    \20\ USEPA. 2023. Technical Support Document: Aquatic Life Water 
Quality Criterion for Mercury in Idaho. U.S. Environmental 
Protection Agency, Office of Water, Washington, DC. https://www.epa.gov/wqs-tech/mercury-criterion-protect-aquatic-life-idaho.
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1. Inherent Toxicity Data
    To account for inherent toxicity, the EPA evaluated toxicity 
studies in which the authors fed food spiked with methylmercury and/or 
inorganic mercury to aquatic organisms for an appropriate chronic 
duration (based on the taxon and the endpoint of interest, ranging up 
to 249 days in this data set \21\). The EPA then assessed each study 
that measured the organisms' resulting tissue mercury levels and 
associated toxicity effects. The tissue mercury levels in these studies 
were measured as methylmercury or total mercury. Although the toxicity 
reported in most of these studies was primarily due to methylmercury, 
the toxicity observed in at least some aquatic taxa was likely due to 
the combined effects of inorganic and methylmercury.
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    \21\ The chronic studies used in the derivation of the mercury 
criterion followed taxa-specific exposure duration requirements from 
various test guidelines (i.e., EPA's 1985 Aquatic Life Criteria 
Guidelines: https://www.epa.gov/sites/production/files/2016-02/documents/guidelines-water-quality-criteria.pdf and EPA OCSPP's 2016 
Ecological Effects Test Guidelines: https://www.epa.gov/test-guidelines-pesticides-and-toxic-substances/series-850-ecological-effects-test-guidelines) when available. Thus, most studies 
consisted of partial life-cycle tests of sufficient length to 
ascertain whether dietary exposure to mercury had a deleterious 
effect on the endpoint of interest. For studies involving amphibian 
taxa, only dietary exposure studies using fully aquatic life stages 
(larvae, tadpoles, and metamorphs) of these species were considered.
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    Idaho's aquatic life uses call for water quality appropriate for 
the protection and maintenance of a viable aquatic life community, 
including active self-propagating populations of salmonid fishes where 
appropriate habitat is available and the salmonid spawning use is 
designated. To protect these aquatic life designated uses, the EPA 
seeks to protect aquatic life and health of the aquatic community by 
minimizing adverse effects on the assessment endpoints of survival, 
growth, and reproduction in the taxa present in the aquatic community. 
Measures of effect (such as increased mortality, reduction in organism 
weight, or the number of eggs laid per female fish) reported in each 
study were used to quantify changes in the assessment endpoints of 
survival, growth, and reproduction. As with recent national recommended 
bioaccumulative pollutant criteria, the EPA selected the 
EC10--the concentration that results in a 10% difference in 
a measure of effect (e.g., a 10% decrease in number of eggs laid per 
female) in the test population--as the numeric metric for the measures 
of effect, wherever possible. The EC10 estimates a low level 
of effect that is different from controls but is not expected to cause 
severe effects at the population level for a bioaccumulative 
contaminant. For studies with experimental designs that did not provide 
sufficient test concentrations to calculate an EC10, the EPA 
generally used an estimate of the No Observed Effect Concentration 
(NOEC) as a surrogate for the EC10.\22\
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    \22\ USEPA. 2023. Technical Support Document: Aquatic Life Water 
Quality Criterion for Mercury in Idaho. U.S. Environmental 
Protection Agency, Office of Water, Washington, DC. https://www.epa.gov/wqs-tech/mercury-criterion-protect-aquatic-life-idaho.
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    The EPA collected chronic dietary toxicity test data of sufficient 
quality across the eight diverse taxonomic groups (including 
vertebrates and invertebrates) recommended in the Aquatic Life 
Guidelines. Quantitative data were available for 19 species within 18 
genera. For each toxicity study, the EPA recorded the type of tissue in 
which the mercury concentration had been measured (muscle or whole-
body) and then used conversion factors derived from the literature to 
create two equivalent data sets: one in terms of muscle tissue 
concentrations and the other in terms of whole-body tissue 
concentrations. This approach allowed the EPA to develop two tissue 
criterion elements (one for muscle tissue and one for whole-body 
tissue).
2. Bioaccumulation Data
    The EPA estimated bioaccumulation using the bioaccumulation factor 
(BAF) approach; a BAF is the ratio of the concentration of a chemical 
in the tissue of an aquatic organism to the concentration of the 
chemical dissolved in ambient water at the site of sampling. Because 
mercury bioaccumulation, and thus BAFs, can be affected by multiple 
site-specific factors (see section III.B. in this preamble above), it 
is desirable to base BAFs on field-collected data from the location(s) 
to which the criterion will be applied. Consequently, the EPA assembled 
a data set of paired (i.e., collected in the same waterbody within one 
year) aquatic organism tissue and water samples from Idaho. The data 
set contained data from 30 fish species and one crayfish species. 
Although no paired tissue and water data from Idaho were found for 
amphibians, the EPA conducted a literature search and identified paired 
tissue and water data for the wood frog (resident in Northern Idaho) 
that had been collected in Maine and Vermont; these data were added to 
the data set to ensure consideration and protection of Idaho 
amphibians.
    From this data set, the EPA calculated species-level BAFs by first 
taking the median for a species at a site in a particular year, then 
the median across years within a site, then the median across sites for 
a species to get one median BAF per species.
3. Development of Fish Tissue Criterion Elements: Magnitude
    Having assembled data on both toxicity and bioaccumulation for a 
suite of aquatic species relevant to protection and maintenance of a 
viable aquatic life community in Idaho, the EPA proceeded to develop 
the muscle and whole-body tissue criterion elements. The EPA noted that 
there were large ranges of toxicological sensitivity and 
bioaccumulation potential across taxa. Two specific issues were 
apparent

[[Page 24764]]

related to differing bioaccumulation rates among species for mercury.
    First, the two amphibians in the toxicity data set were the two 
most sensitive species based on dietary exposure (inherent toxicity), 
but also have by far the lowest mercury bioaccumulation potential. 
Fish, on the other hand, are comparatively more tolerant to inherent 
(direct) toxicity, but generally more vulnerable to mercury pollution 
due to their higher mercury bioaccumulation potential. Therefore, 
establishing a criterion based solely on inherent toxicity data, i.e., 
without considering bioaccumulation differences, would be 
inappropriate. The EPA also aimed to develop a criterion that was 
practical and implementable, recognizing that Idaho typically samples 
fish (rather than amphibians) for CWA implementation purposes. 
Therefore, in consideration of the bioaccumulation data, the EPA is 
proposing a chronic criterion for mercury based on fish and aquatic 
invertebrate inherent toxicity data, which also protects amphibians.
    Second, mercury bioaccumulation potential among fish species varies 
widely (up to 20-fold differences) due primarily to their diets: as 
trophic level increases so does mercury bioaccumulation. In order to 
protect higher trophic level fish, such as salmonids, which are 
commercially, recreationally, and ecologically important in Idaho, the 
EPA made adjustments to account for known bioaccumulation differences 
among fish species. Doing so ensures that higher trophic level fish 
species are protected when evaluating sampling data from lower trophic 
level species (e.g., bluegill, suckers, pumpkinseed) for implementation 
purposes.
    To address these two issues, the EPA used a modified approach based 
on the ``good science'' clause in the Aquatic Life Guidelines \23\ to 
integrate inherent toxicity and bioaccumulation. Briefly, to address 
the first issue (the most sensitive organisms having by far the lowest 
bioaccumulation potential), the EPA calculated both tissue criterion 
elements using the fish and aquatic invertebrate data (i.e., excluding 
amphibians) and then analyzed whether the resulting criterion elements 
would be protective of all aquatic species in the data set in light of 
their inherent toxicity and bioaccumulation differences (see further 
details below).
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    \23\ The Aquatic Life Guidelines note that a modified approach 
may be needed in some situations, directing users to: ``On the basis 
of all available pertinent laboratory and field information, 
determine if the criterion is consistent with sound scientific 
evidence. If it is not, another criterion, either higher or lower, 
should be derived using appropriate modifications of these 
Guidelines.'' (pg. 30).
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    To address the second issue, the EPA evaluated the differences in 
bioaccumulation between fish species in the data set and developed 
adjustment factors that can be used when sampling fish for 
implementation. If a high trophic level adult fish (e.g., trophic level 
4) is sampled and found to have mercury tissue concentrations at (or 
below) the criterion level, it would be reasonable to assume that all 
aquatic species in that water body are protected (i.e., because lower 
trophic level species are expected to have lower levels of mercury 
bioaccumulation). However, if a lower trophic level fish is sampled and 
found to be below the criterion level, it does not necessarily mean 
that higher trophic level fish are protected. To resolve this issue, 
the EPA developed a method to estimate the tissue mercury levels of 
higher trophic level adult fish resident in that water body to 
determine whether all aquatic species in that water body are protected.
    To make these estimates, the EPA developed Bioaccumulation Trophic 
Adjustment Factors (BTAFs). The BTAF is an adjustment factor applied to 
the tissue sample data from a lower trophic level fish and is based on 
the relative relationship of bioaccumulation rates of the highest 
trophic level fish species as compared to lower trophic level fish 
species. The EPA first assigned all the fish in the bioaccumulation 
data set to one of three trophic categories: low (trophic level 2 or 
TL2), medium (trophic level 3 or TL3), or high (trophic level 4 or 
TL4).\24\ The EPA then developed two BTAFs by calculating the ratio 
between the trophic level BAFs: one to be used if a TL2 species is 
sampled (representative TL4 BAF/representative TL2 BAF) and another to 
be used if a TL3 species is sampled (representative TL4 BAF/
representative TL3 BAF). To calculate representative BAFs, the EPA used 
the median of BAFs for species at that trophic level from the species-
level BAF data set for TL3 (TL3 BAF = 108,418 L/kg, n = 21) and TL4 
(TL4 BAF = 378,150 L/kg, n = 6) fish. For the representative TL2 BAF, 
due to the paucity of TL2 fish species in the data set (n = 3), the EPA 
used the 20th centile of the full distribution of the species-level 
median BAFs (TL2 BAF = 67,203 L/kg, n = 30). The EPA's use of the 20th 
centile ensures appropriate protection for aquatic species in Idaho 
(i.e., providing water quality appropriate for the protection and 
maintenance of a viable aquatic life community as specified by Idaho's 
aquatic life uses) and is consistent with previous EPA approaches for 
bioaccumulative chemicals.25 26
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    \24\ Fish species were binned into three trophic magnitude 
categories largely corresponding to trophic levels designated in 
Essig 2010 (Arsenic, mercury, and selenium in fish tissue and water 
from Idaho's major rivers: A statewide assessment. Idaho Department 
of Environmental Quality, Boise, ID. https://www2.deq.idaho.gov/admin/LEIA/api/document/download/3472) based on Zaroban et al. 1999 
(Classification of species attributes for Pacific Northwest 
freshwater fishes. Northwest Sci. 73(2): 81-93). In some instances, 
additional information regarding trophic ecology and other 
attributes of Pacific Northwest fish species resident in Idaho were 
also incorporated into the trophic level categorization 
determination (Brown, C.J.D. 1971. Fishes of Montana. Bozeman, MT: 
Big Sky Books/Montana State University. 207 p.; Zaroban et al. 1999. 
Classification of species attributes for Pacific Northwest 
freshwater fishes. Northwest Sci. 73(2): 81-93; Froese, R. and D. 
Pauly. Editors. 2022. FishBase. World Wide Web electronic 
publication. www.fishbase.org).
    \25\ USEPA. 2022. Draft Aquatic Life Ambient Water Quality 
Criteria for Perfluorooctanoic Acid (PFOA). EPA-842-D-22-001. U.S. 
Environmental Protection Agency, Office of Water, Washington, DC. 
https://www.epa.gov/system/files/documents/2022-04/pfoa-report-2022.pdf; USEPA. 2022. Draft Aquatic Life Ambient Water Quality 
Criteria for Perfluorooctane Sulfonate (PFOS). EPA-842-D-22-002. 
U.S. Environmental Protection Agency, Office of Water, Washington, 
DC. https://www.epa.gov/system/files/documents/2022-04/pfos-report-2022.pdf.
    \26\ USEPA. 2021. 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.
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    Therefore, the EPA is proposing that if a TL2 fish is sampled, its 
muscle tissue mercury concentration (converted from whole-body tissue 
concentration where appropriate, as discussed below) must be multiplied 
by 5.6 (378,150 L kg-1/67,203 L kg-1) to estimate 
the muscle tissue mercury concentration of a TL4 fish in the same water 
body, and that estimate must be compared to the muscle tissue criterion 
element (225 ng total mercury (THg)/g wet weight (ww)) to determine 
whether the criterion is met. Similarly, if a TL3 fish is sampled, its 
muscle tissue mercury concentration must be multiplied by 3.5 (378,150 
L kg-1/108,418 L kg-1) and the resulting value 
compared to the muscle tissue criterion element. If an adult TL4 fish 
species is sampled, its muscle tissue mercury concentration must be 
compared directly to the muscle tissue criterion element. Because the 
BAFs in this data set were calculated using muscle tissue 
concentrations, it is most appropriate to use the BTAFs to adjust 
muscle (rather than whole-body) tissue concentration measurements. If 
whole-body tissue samples are taken from TL2 or TL3 fish, the EPA is 
proposing that those measurements must be converted

[[Page 24765]]

to a muscle tissue equivalent (by dividing by 0.72, a conversion factor 
derived from the literature \27\) before multiplying by the appropriate 
BTAF and comparing the result to the muscle tissue criterion element.
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    \27\ USEPA. 2023. Technical Support Document: Aquatic Life Water 
Quality Criterion for Mercury in Idaho. U.S. Environmental 
Protection Agency, Office of Water, Washington, DC. https://www.epa.gov/wqs-tech/mercury-criterion-protect-aquatic-life-idaho.
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    Trophic level assignments for fish species found in Idaho are 
included in the Technical Support Document \28\ and should be used 
where available. Additional sources for trophic level assignment cited 
in the Technical Support Document should be consulted to assign trophic 
levels for other species. In some cases, consultation with state 
fisheries experts may be necessary. At this time, the EPA has developed 
BTAFs for fish based on Idaho species with available BAF data. The EPA 
requests comment on whether there is interest in sampling species other 
than fish to determine compliance with the criterion, and if so, 
whether any data exist to develop appropriate BTAFs for those other 
species.
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    \28\ Ibid.
---------------------------------------------------------------------------

    Having confirmed that the most bioaccumulative species (i.e., those 
at the highest trophic level) would be protected by the tissue 
criterion with BTAF adjustments applied as appropriate, the EPA 
analyzed whether a tissue criterion derived based solely on fish and 
aquatic invertebrates (excluding the two amphibian species) would be 
protective of all aquatic species in the data set. Comparing the 
amphibian BAF (8,222 L/kg) to the median TL4 fish BAF (378,150 L/kg), 
the EPA found that amphibians would be expected to bioaccumulate 
approximately 46 times less mercury than the median TL4 fish when 
exposed to the same mercury levels. Therefore, if a TL4 fish is sampled 
and found to have a mercury level equivalent to the muscle tissue 
criterion value (225 ng THg/g ww), amphibians in that same water body 
would be expected to have muscle tissue concentrations of approximately 
4.9 ng THg/g ww, well below the EC10 of the most sensitive 
amphibian species (33.7 ng THg/g ww). Similar reasoning would apply if 
TL2 or TL3 fish species were sampled and adjusted with the BTAFs to an 
estimated TL4 muscle tissue concentration at or below 225 ng THg/g ww; 
in all cases, estimated amphibian muscle tissue concentrations in that 
water body would be below the most sensitive amphibian's 
EC10. Therefore, the EPA concluded that the tissue criterion 
elements protect the full suite of aquatic species (including 
amphibians) without being unnecessarily stringent.
    The EPA's proposed tissue criterion elements are expressed as total 
mercury (THg) (i.e., including methylmercury and inorganic mercury). As 
noted above, both forms of mercury can bioaccumulate and have toxic 
effects, although only methylmercury biomagnifies. Furthermore, the 
analysis of total mercury incorporates the measurement of 
methylmercury, but costs less and uses less complex analytical methods 
than the measurement of methylmercury alone. Additionally, measurement 
of total mercury in fish tissue has served as the basis for quantifying 
mercury concentrations in fish tissue monitoring programs implemented 
by the EPA and many states, including Idaho.
4. Development of the Water Column Criterion Element: Magnitude
    To develop the water column criterion element, the EPA first needed 
to assign a BAF to each species in the toxicity data set to facilitate 
the translation from tissue to water, since not all species in the 
toxicity data set were also present in the bioaccumulation data set. To 
determine appropriate BAFs for the fish species without species-
specific BAFs, the EPA calculated TL-specific BAFs by taking the 80th 
centile of the median species-level BAFs for all fish within that TL. 
The EPA's use of the 80th centile here is consistent with the process 
for deriving water column criteria for other bioaccumulative 
pollutants.29 30 The EPA then assigned the most 
representative BAF (i.e., species- or genus-level where available, 
otherwise trophic-level) to each fish species in the toxicity data 
set.\31\ Nearly all BAFs were derived from field-collected Idaho tissue 
and water data, representing a diverse range of site-specific 
relationships between mercury in tissue and water across the state of 
Idaho (see TSD section 3.5 for more details). The EPA then translated 
the tissue-based toxicity value for each species in the toxicity data 
set to a water column-based toxicity value by dividing the species' 
tissue-based toxicity value by its assigned BAF.
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    \29\ USEPA. 2021. 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.
    \30\ USEPA. 2022. Draft Aquatic Life Ambient Water Quality 
Criteria for Perfluorooctanoic Acid (PFOA). EPA-842-D-22-001. U.S. 
Environmental Protection Agency, Office of Water, Washington, DC. 
https://www.epa.gov/system/files/documents/2022-04/pfoa-report-2022.pdf; USEPA. 2022. Draft Aquatic Life Ambient Water Quality 
Criteria for Perfluorooctane Sulfonate (PFOS). EPA-842-D-22-002. 
U.S. Environmental Protection Agency, Office of Water, Washington, 
DC. https://www.epa.gov/system/files/documents/2022-04/pfos-report-2022.pdf.
    \31\ For invertebrates, the EPA assigned the crayfish BAF to the 
other invertebrates in the data set (daphnid, mayfly, and clam). For 
amphibians, the EPA assigned the wood frog BAF.
---------------------------------------------------------------------------

    The EPA ranked the translated water column-based toxicity values by 
sensitivity and calculated the water column criterion element per the 
Aquatic Life Guidelines calculation method to arrive at a final water 
column value of 2.1 ng/L (see Table 1 to proposed 40 CFR 131.XX(b)). No 
exclusions or adjustments to this criterion element were needed to 
account for bioaccumulation differences because in this case both 
mercury toxicity and bioaccumulation in aquatic species were directly 
incorporated into the water column criterion element derivation. The 
EPA is proposing to express the water column criterion element as total 
mercury in whole water (not dissolved or filtered)--i.e., including 
methylmercury and inorganic mercury measured from an unfiltered water 
sample. The EPA chose this unit rather than dissolved mercury for the 
following reasons. First, the water column data used to derive the BAFs 
were from unfiltered water samples. Second, NPDES regulations (40 CFR 
122.45(c)) require that permit effluent limits be expressed as total 
recoverable metal (with limited exceptions), so most point source 
discharge monitoring data for mercury (in Idaho and elsewhere) is from 
unfiltered samples. Third, because the primary route of mercury 
toxicity is through dietary exposure, particulate mercury may 
contribute to toxicity (in contrast to some other metals for which the 
primary route of toxicity is absorption from water, and for which 
measurements of the dissolved fraction may therefore be more 
appropriate).
    For most of the paired aquatic organism tissue and water samples 
that were available for the calculation of Idaho BAFs, the unfiltered 
water samples were collected during the July to October period. In 
Idaho flowing waters, discharge rates and turbidity tend to be highest 
in the spring due to snowmelt, whereas they tend to be lower during the 
July to October time period (i.e., under baseflow conditions). In an 
analysis of time series data from several Idaho rivers, the EPA found 
that there are higher total mercury concentrations during high flow 
periods (see Technical Support Document

[[Page 24766]]

section 3.1.2 for more details \32\). The EPA calculated BAFs using 
unfiltered water samples collected primarily during baseflow 
conditions, and then used those BAFs to calculate the water column 
criterion element. Therefore, water samples collected during baseflow 
conditions would be most representative of the data used to derive this 
criterion element.
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    \32\ USEPA. 2023. Technical Support Document: Aquatic Life Water 
Quality Criterion for Mercury in Idaho. U.S. Environmental 
Protection Agency, Office of Water, Washington, DC. https://www.epa.gov/wqs-tech/mercury-criterion-protect-aquatic-life-idaho.
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5. Frequency and Duration of Water Column and Fish Tissue Criterion 
Elements
    The EPA also determined appropriate frequencies and durations for 
the tissue and water column criterion elements. For the tissue 
criterion elements, because fish tissue mercury concentrations change 
slowly (e.g., changing on the order of 2-3% per year), fish tissue 
collected from a site can be assumed to integrate and represent the 
mercury bioaccumulation dynamics at that site over several years. 
Therefore, the EPA is proposing an ``instantaneous measurement'' 
duration for the fish tissue criterion elements (Table 1 to proposed 40 
CFR 131.XX(b)) because fish tissue measurements already reflect longer-
term bioaccumulation dynamics. For similar reasons and considering that 
fish tissue mercury concentrations are relatively slow to respond to a 
decrease in mercury inputs, the EPA is proposing a frequency of ``not 
to exceed'' for the fish tissue criterion elements (Table 1 to proposed 
40 CFR 131.XX(b)).
    For the water column criterion element, the EPA considered observed 
durations of mercury methylation processes affecting trophic transfer 
and of mercury bioaccumulation and elimination processes in aquatic 
organisms and, consistent with the duration components of other 
bioaccumulative contaminants,33 34 set the duration at 30 
days (Table 1 to proposed 40 CFR 131.XX(b)). For the frequency aspect, 
the EPA considered the number of times mercury concentrations in water 
could exceed the criterion over time without negatively affecting the 
aquatic community and determined that a once-in-three years exceedance 
frequency is appropriate (Table 1 to proposed 40 CFR 131.XX(b)), based 
on the ability of an aquatic ecosystem to recover from stress caused by 
a toxic pollutant such as mercury.35 36
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    \33\ USEPA. 2021. 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.
    \34\ USEPA. 2022. Draft Aquatic Life Ambient Water Quality 
Criteria for Perfluorooctanoic Acid (PFOA). EPA-842-D-22-001. U.S. 
Environmental Protection Agency, Office of Water, Washington, DC. 
https://www.epa.gov/system/files/documents/2022-04/pfoa-report-2022.pdf; USEPA. 2022. Draft Aquatic Life Ambient Water Quality 
Criteria for Perfluorooctane Sulfonate (PFOS). EPA-842-D-22-002. 
U.S. Environmental Protection Agency, Office of Water, Washington, 
DC. https://www.epa.gov/system/files/documents/2022-04/pfos-report-2022.pdf.
    \35\ USEPA. 1985. Guidelines for Deriving Numerical National 
Water Quality Criteria for the Protection of Aquatic Organisms and 
Their Uses. U.S. Environmental Protection Agency, Office of Research 
and Development, Duluth, MN, Narragansett, RI, Corvallis, OR. PB85-
227049. https://www.epa.gov/sites/production/files/2016-02/documents/guidelines-water-quality-criteria.pdf.
    \36\ USEPA. 2023. Proceedings from the EPA Frequency and 
Duration Experts Workshop September 11-12, 2019. U.S. Environmental 
Protection Agency, Office of Water, Washington, DC. https://www.epa.gov/system/files/documents/2023-02/proceedings-frequency-duration-workshop.pdf.
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6. Structure of Criterion
    The EPA requests comment on two alternatives for the relationship 
of the fish tissue and water column elements. The first alternative, 
preferred by the EPA, is for the fish tissue criterion elements to 
supersede the water column criterion element in a hierarchical 
structure (Table 1 to proposed 40 CFR 131.XX(b)). Because the tissue 
criterion elements were estimated directly from toxicity studies, 
whereas the water column criterion element required the use of BAFs to 
translate those tissue values, the water column element is a step 
removed from the toxicity values. These translations introduced some 
uncertainty into the water column values since species-specific BAFs 
from Idaho were not available for every species. In other words, the 
EPA has greater confidence in the tissue criterion elements, and 
therefore greater confidence in implementation decisions made using 
these criterion elements. If the EPA were to finalize this hierarchical 
structure, a water body would be attaining its aquatic life designated 
use if a tissue criterion element was met, even if its water column 
criterion element was exceeded.
    The second alternative is for the fish tissue and water column 
criterion elements to be independently applicable. Because major 
sources of mercury to aquatic systems in Idaho are legacy mining 
contamination and atmospheric deposition, water column measurements of 
mercury from a waterbody are expected to be relatively stable over 
time. In contrast, pollutants with new and increasing direct sources 
tend to have more variable measurements over time, depending on the 
anthropogenic source of the pollutant. This expected relative stability 
of water column concentrations over time suggests that, while the EPA 
has relatively greater confidence in the fish tissue elements, as noted 
above, it would also be reasonable to conclude that a water body that 
is not meeting the water column element may be worthy of further 
evaluation, even if the fish tissue elements are being met. If the EPA 
were to finalize an independently applicable criterion structure, a 
water body would not be attaining its aquatic life designated use if 
either a tissue criterion element or the water column criterion element 
was exceeded. The EPA requests comment on the most appropriate 
relationship (hierarchical or independently applicable) of the fish 
tissue and water column elements.
    Within the fish tissue elements, the EPA is proposing that sample 
data from TL4 fish supersede sample data from TL3 or TL2 fish. Where 
possible, TL4 fish should be sampled to determine whether a fish tissue 
criterion element is met, because these data provide a direct 
assessment of whether highly bioaccumulative species in the water body 
are experiencing tissue mercury levels associated with adverse effects. 
This direct assessment is more certain than an assessment based on an 
estimated TL4 fish tissue concentration generated by applying the 
appropriate BTAF to TL3 or TL2 fish tissue sample data, so if tissue 
sample data from fish at multiple trophic levels are available, the TL4 
fish sample data would supersede.
    The EPA requests comment on two alternatives for the relationship 
between TL3 fish sample data and TL2 fish sample data. The first 
alternative, preferred by the EPA, is for sample data from TL3 fish to 
supersede sample data from TL2 fish (with both still being superseded 
by sample data from TL4 fish), for two reasons. First, the trophic 
ecology of TL4 fish is closer to that of TL3 fish than TL2 fish. 
Second, more data were available to establish the relationship between 
TL3 and TL4 fish than between TL2 and TL4 fish.\37\ The second 
alternative is for sample data from TL3 fish and sample data from TL2 
fish to be independently applicable (with both still being superseded 
by sample data from TL4 fish). A rationale

[[Page 24767]]

for this structure would be that TL3 and TL2 sample data are equally 
uncertain, relative to TL4 sample data, because BTAFs must be applied 
to both. The EPA requests comment on the most appropriate relationship 
(hierarchical or independently applicable) of the TL3 fish sample data 
and TL2 fish sample data.
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    \37\ USEPA. 2023. Technical Support Document: Aquatic Life Water 
Quality Criterion for Mercury in Idaho. U.S. Environmental 
Protection Agency, Office of Water, Washington, DC. https://www.epa.gov/wqs-tech/mercury-criterion-protect-aquatic-life-idaho.
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    In addition to the criterion structure alternatives described 
above, the EPA invites public comment on all aspects of the process 
used to derive the proposed mercury criterion, including but not 
limited to the compilation of toxicity and bioaccumulation data, the 
derivation of the proposed tissue criterion element magnitudes and the 
water column criterion element magnitude from these data, the 
derivation and proposed application of the BTAFs, and the proposed 
frequency and duration of the criterion elements.

C. Implementation

    The EPA understands that states have certain flexibility with how 
they implement WQS. The EPA is recommending possible approaches below 
to facilitate consistent implementation of the mercury aquatic life 
criterion resulting from this proposed rulemaking for the state's 
consideration and for public comment. The EPA recommends that Idaho 
develop implementation guidance, potentially building on its existing 
implementation guidance for the methylmercury fish tissue human health 
criterion,\38\ adding information to clarify how implementation should 
proceed given the presence of a water column element and fish tissue 
elements as presented in this proposed mercury aquatic life criterion.
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    \38\ Idaho Department of Environmental Quality. 2005. 
Implementation Guidance for the Idaho Mercury Water Quality 
Criteria. Boise, ID. https://www2.deq.idaho.gov/admin/LEIA/api/document/download/4836.
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1. Identification of Impaired Waters and TMDL Development
    Section 303(d) of the CWA and the EPA's supporting regulations in 
40 CFR 130.7 require states to develop biennial lists of waters 
impaired (i.e., not meeting one or more applicable water quality 
standards) or threatened by a pollutant and needing a TMDL (i.e., the 
Section 303(d) list). States are required to establish a prioritized 
schedule for waters on the lists and develop TMDLs for the identified 
waters based on the severity of the pollution and the sensitivity of 
their uses, among other factors (40 CFR 130.7(b)(4)). A TMDL is a 
calculation of the maximum amount of a pollutant that a waterbody can 
receive and still safely meet water quality standards, and an 
allocation of that load among the various point and/or nonpoint sources 
of the pollutant.
    The state is required to assemble and evaluate all existing and 
readily available water-quality related data and information when 
determining which waterbodies belong on the CWA section 303(d) list (40 
CFR 130.7(b)(5)). If multiple types of data and information are 
collected at a site, they must be assembled and evaluated consistent 
with the final structure of the mercury criterion. If the final 
criterion has a hierarchical structure as proposed, the fish tissue 
criterion elements would supersede the water column criterion element. 
If only water column data are available, assessment decisions can be 
made by comparing those data to the water column criterion element. If 
the final criterion does not have a hierarchical structure, each 
element would be its own criterion, and the waterbody would be listed 
if any criterion is exceeded. The water column criterion element 
proposed here would apply unless site-specific water column criterion 
elements were adopted by Idaho and approved by the EPA pursuant to CWA 
section 303(c) and the EPA's implementing regulation. Regardless of the 
structure of the fish tissue vs. water column elements (hierarchical or 
independent criteria), the trophic level hierarchy applies within the 
fish tissue criterion element. As noted above (section IV.B.6. in this 
preamble), the EPA is proposing that data from TL4 fish would supersede 
data from TL3 or TL2 fish, and data from TL3 fish would supersede data 
from TL2 fish.
    Idaho has flexibility to determine how to evaluate individual and 
composite samples for fish tissue. Tissue data provide instantaneous 
point measurements that reflect integrative accumulation of mercury 
over time and space in fish at a given site. The proposed mercury 
criterion provides Idaho with flexibility in how the state can 
interpret a discrete fish tissue sample to represent a given species' 
population at a site. Generally, fish tissue samples collected to 
calculate average tissue concentrations (often in composites) for a 
species at a site are collected during one sampling event, or over a 
short interval due to logistical constraints and the cost for obtaining 
samples. Consistent with the EPA's \39\ and Idaho's \40\ current 
recommendations for implementation of selenium fish tissue criterion 
elements, a central tendency of fish tissue data may be calculated, or 
a composite of fish tissue samples may be analyzed, within a fish 
species but should not be calculated or analyzed across species to 
determine whether a fish tissue element of this proposed mercury 
criterion is met. The EPA recommends that the state clearly describe 
its decision-making process in its assessment methodology.
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    \39\ USEPA. 2021. Frequently Asked Questions: Implementing EPA's 
2016 Selenium Criterion in Clean Water Act Sections 303(d) and 
305(b) Assessment, Listing, and Total Maximum Daily Load Programs: 
Draft. U.S. Environmental Protection Agency, Office of Water, 
Washington, DC. https://www.epa.gov/system/files/documents/2021-10/selenium-faq-cwa305-draft-2021.pdf.
    \40\ Idaho Department of Environmental Quality. 2022. 
Implementation Guidance for the Idaho Selenium Criteria for Aquatic 
Life. Boise, ID. https://www2.deq.idaho.gov/admin/LEIA/api/document/download/16846.
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    Although the frequency component is expressed as ``The average 
tissue concentration must not be exceeded,'' not meeting a fish tissue 
criterion element does not mean that fish populations cannot recover. 
As such, if Idaho determines that a fish tissue criterion element is 
not met and identifies the water as impaired on their CWA section 
303(d) list, Idaho may determine in the future that the criterion is 
met based on readily available data and information and remove the 
waterbody-pollutant combination from the list. The EPA recommends that 
Idaho include in their assessment methodology a discussion of how the 
fish tissue criterion elements will be implemented, including 
information on how the criterion will be determined to be met after an 
exceedance of the fish tissue criterion elements.
2. NPDES Permitting
    Under the CWA, WQS are used to derive Water Quality-Based Effluent 
Limits (WQBELs) in NPDES permits for point source discharges, thereby 
limiting the concentrations or levels of pollutants that may be 
discharged into a waterbody to attain and maintain its designated uses. 
The EPA is proposing a water column criterion element, which can be 
used for NPDES permitting as well as other aspects of implementation. 
To account for the 30-day duration of the proposed water column 
criterion element, adjustments can be made to WQBEL calculation methods 
that assume a 4-day averaging period \41\ as the EPA described in its 
Notice of Availability for the 1999 ambient water quality criteria for

[[Page 24768]]

ammonia,\42\ which also included a 30-day duration. However, this water 
column criterion element would not prevent Idaho from using the fish 
tissue criterion elements for monitoring and regulating pollutant 
discharges at the state's discretion.
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    \41\ USEPA. 1991. Technical Support Document For Water Quality-
based Toxics Control. EPA/505/2-90-001. U.S. Environmental 
Protection Agency, Office of Water, Washington, DC. https://www3.epa.gov/npdes/pubs/owm0264.pdf.
    \42\ USEPA. 1999. Water Quality Criteria; Notice of 
Availability; 1999 Update of Ambient Water Quality Criteria for 
Ammonia. 64 FR 71974-71980 (December 22, 1999). U.S. Environmental 
Protection Agency, Office of Water, Washington, DC. https://www.govinfo.gov/content/pkg/FR-1999-12-22/pdf/99-33152.pdf.
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    Determination of critical low flows and mixing zones for any 
criterion that results from this proposed rulemaking should proceed in 
the same manner as for other aquatic life criteria for toxic pollutants 
in Idaho, with appropriate adjustments to account for the 30-day 
duration of the water column element.

V. Endangered Species Act

    On May 7, 2014, the National Marine Fisheries Service (NMFS) 
finalized a Biological Opinion \43\ which evaluated whether the EPA's 
1996 approval of Idaho's mercury aquatic life criteria--along with EPA 
actions in Idaho related to several other pollutants--would jeopardize 
the continued existence of threatened and endangered species in Idaho 
for which NMFS is responsible. NMFS concluded that the EPA's approval 
of the chronic mercury criterion (0.012 [mu]g/L) would jeopardize Snake 
River spring/summer Chinook salmon, Snake River fall Chinook salmon, 
Snake River sockeye salmon and Snake River Basin steelhead--as well as 
adversely modify designated critical habitat for rearing Snake River 
salmon and steelhead--due to potential bioaccumulation occurring from 
exposure to mercury in the diet. In contrast, NMFS concluded that 
exposure of listed salmon and steelhead to mercury at the acute 
criterion (2.1 [mu]g/L) was unlikely to result in death or sub-lethal 
effects that would result in injury or reduced survival.
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    \43\ National Marine Fisheries Service (NMFS). 2014. Endangered 
Species Act Section 7(a)(2) Biological Opinion and Magnuson-Stevens 
Fishery Conservation and Management Act Essential Fish Habitat (EFH) 
Consultation: Idaho Water Quality Standards for Toxic Substances. 
Biological Opinion. NMFS Consultation Number: 2000-1484.
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    The NMFS biological opinion contained Reasonable and Prudent 
Alternatives (RPAs) for the chronic criterion that would avoid the 
likelihood of jeopardy to the species. The RPAs directed the EPA to 
promulgate a new chronic mercury criterion that would be protective of 
aquatic life in Idaho, unless the EPA was able to approve such a 
criterion promulgated by the state. NMFS also specified an RPA for 
interim protection until this criterion was effective, stating that 
``until a new chronic criterion is adopted EPA will use the 2001 EPA/
2005 Idaho human health fish tissue criterion of 0.3 mg/kg wet weight 
for WQBELs and reasonable potential to exceed criterion calculations 
using the current methodology for developing WQBELs to protect human 
health.'' The biological opinion also stated that ``implementation of 
the Idaho methylmercury criterion shall be guided by EPA's 
methylmercury water quality criteria implementation guidance \44\ or 
IDEQ's methylmercury water quality criteria implementation 
guidance,\45\ '' and that ``for water bodies for which appropriate fish 
tissue data are not available, if the geometric mean of measured 
concentrations of total mercury in water is less than 2 ng/L, then the 
water body will be presumed to meet the fish tissue criterion of 0.3 
mg/kg wet weight. If the water column concentration is greater than 2 
ng/L, fish tissue data shall be collected.'' In the biological opinion, 
NMFS also opined that one significant digit was the appropriate level 
of precision for the total mercury water column value included in their 
RPA in light of the limitations of the data set from which it had been 
derived. The U.S. Fish and Wildlife Service reached the same conclusion 
for bull trout and Kootenai River white sturgeon and their associated 
critical habitats in its 2015 Biological Opinion evaluating the EPA's 
1996 approval of Idaho's mercury aquatic life criteria and included the 
same RPAs for mercury.
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    \44\ USEPA. 2010. Guidance for Implementing the January 2001 
Methylmercury Water Quality Criterion. EPA 823-R-10-001. U.S. 
Environmental Protection Agency, Office of Water, Washington, DC. 
https://www.epa.gov/sites/default/files/2019-02/documents/guidance-implement-methylmercury-2001.pdf.
    \45\ Idaho Department of Environmental Quality. 2005. 
Implementation Guidance for the Idaho Mercury Water Quality 
Criteria. Boise, ID. https://www2.deq.idaho.gov/admin/LEIA/api/document/download/4836.
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    The EPA's proposed chronic mercury criterion is consistent with the 
Services' RPAs, with the proposed muscle tissue criterion element being 
more stringent than the human health criterion (0.225 vs. 0.3 mg/kg 
\46\ wet weight) and the proposed water column element being comparable 
to the RPA water column value (both 2 ng/L using one significant 
digit). The EPA will continue to work closely with the Services to 
ensure that the mercury criterion that the EPA ultimately finalizes is 
protective of federally listed species in Idaho.
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    \46\ Idaho's framework for implementing their mercury human 
health criterion in their TMDL and NPDES programs uses a mercury 
tissue concentration of 0.24 mg/kg, which represents a 20 percent 
margin of safety below the 0.3 mg/kg; Idaho Department of 
Environmental Quality. 2005. Implementation Guidance for the Idaho 
Mercury Water Quality Criteria. Boise, ID. https://www2.deq.idaho.gov/admin/LEIA/api/document/download/4836.
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VI. Applicability of EPA-Promulgated Water Quality Standards When Final

    Under the CWA, Congress gave states primary responsibility for 
developing and adopting WQS for their waters (CWA section 303(a) 
through (c)). Although the EPA is proposing a mercury criterion for the 
protection of aquatic life in Idaho, Idaho continues to have the option 
to adopt and submit to the EPA mercury criteria for the state's waters 
consistent with CWA section 303(c) and the EPA's implementing 
regulation at 40 CFR part 131. The EPA encourages Idaho to consider 
adoption of mercury criteria protective of aquatic life uses. 
Consistent with CWA section 303(c)(4) and the Stipulated Order on 
Remedy, if Idaho adopts and submits mercury criteria for the protection 
of aquatic life, and the EPA approves such criteria before finalizing 
this proposed rulemaking, the EPA will not proceed with the 
promulgation for those waters for which the EPA approves Idaho's 
criteria. Under those circumstances, Federal promulgation would no 
longer be necessary to meet the requirements of the Act.
    If the EPA finalizes this proposed rulemaking and Idaho 
subsequently adopts and submits mercury criteria for the protection of 
aquatic life in Idaho, the EPA would review Idaho's criteria to 
determine whether the criteria meet the requirements of section 303(c) 
of the CWA and the EPA's implementing regulation at 40 CFR part 131 and 
if so, the EPA would approve such criteria. If the EPA's federally 
promulgated criterion is more stringent than the EPA-approved state's 
criteria, the EPA's federally promulgated criterion would remain the 
applicable WQS for purposes of the CWA until the Agency withdraws that 
federally promulgated standard. The EPA would expeditiously undertake 
such a rulemaking to withdraw the Federal criterion if and when Idaho 
adopts and the EPA approves corresponding criteria. After the EPA's 
withdrawal of the federally promulgated criterion, the state's EPA-
approved criteria would become the applicable criteria for CWA 
purposes. If the EPA-approved state's criteria are as stringent or more 
stringent than the federally promulgated criterion, then the state's 
criteria would become the CWA applicable WQS upon the EPA's approval of 
such criteria (40 CFR 131.21(c)).

[[Page 24769]]

VII. Implementation and Alternative Regulatory Approaches

    The Federal WQS regulation at 40 CFR part 131 provides several 
approaches that Idaho may utilize, at its discretion, when implementing 
or deciding how to implement the final aquatic life criterion resulting 
from this proposed rulemaking. Among other things, the EPA's WQS 
regulation: (1) allows states and authorized Tribes to authorize the 
use of compliance schedules in NPDES permits to meet water quality-
based effluent limits (WQBELs) derived from the applicable WQS (40 CFR 
131.15); (2) specifies the requirements for adopting criteria to 
protect designated uses, including criteria modified to reflect site-
specific conditions (40 CFR 131.11); (3) authorizes and provides a 
regulatory framework for states and authorized Tribes to adopt WQS 
variances where it is not feasible to attain the applicable designated 
use and criterion for a period of time (40 CFR 131.14); and (4) 
specifies how states and authorized Tribes adopt, revise, or remove 
designated uses (40 CFR 131.10). Each of these approaches is discussed 
in more detail in the next sections.

A. NPDES Permit Compliance Schedules

    The EPA's NPDES regulations at 40 CFR 122.47 address how a 
permitting authority can use compliance schedules in a permit if a 
discharger needs additional time to undertake actions like facility 
upgrades or operation changes that will lead to compliance with a WQBEL 
based on an applicable WQS that was issued or revised after July 1, 
1977. See In The Matter of Star-Kist Caribe, 3 E.A.D. 172, 175, 177 
(1990). 40 CFR 122.47 allows a permitting authority to include a 
compliance schedule in an NPDES permit, when appropriate, and the 
schedule must require compliance with the final WQBEL as soon as 
possible. Schedules longer than 1 year must include interim 
requirements and dates for their achievement. The EPA's Office of 
Wastewater Management 2007 Memorandum, Compliance Schedules for Water 
Quality-Based Effluent Limitations in NPDES Permits,\47\ provides 
additional information about implementing 40 CFR 122.47 compliance 
schedule requirements. The EPA's WQS program regulation at 40 CFR 
131.15 requires that a state that intends to allow the use of NPDES 
permit compliance schedules adopt specific provisions authorizing their 
use and obtain EPA approval under CWA section 303(c) to ensure that a 
decision to allow permit compliance schedules is transparent and allows 
for public input.\48\ Consistent with 40 CFR 131.15, Idaho has an EPA-
approved WQS for compliance schedules. This WQS allows IDEQ to include 
compliance schedules in NPDES permits to meet WQBELs that are 
established to ensure that the discharge does not cause or contribute 
to an exceedance of the final Federal mercury criterion. In Idaho, 
compliance schedules can only be included in permits for new WQBELs 
that are more stringent than the WQBEL in a facility's previous NPDES 
permit.
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    \47\ USEPA. 2007. Compliance Schedules for Water Quality-Based 
Effluent Limitations in NPDES Permits. Memo from James A. Hanlon, 
Director, Office of Wastewater Management to Alexis Strauss, 
Director, Water Division, EPA Region 9. 10 May 2007. https://www3.epa.gov/npdes/pubs/memo_complianceschedules_may07.pdf.
    \48\ 80 FR 51022, August 21, 2015.
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B. Site-Specific Criteria

    The regulation at 40 CFR 131.11 specifies requirements for 
modifying water quality criteria to reflect site-specific conditions. 
In the context of this rulemaking, a site-specific criterion (SSC) is 
an alternative value to the Federal mercury criterion that would be 
applied on an area-wide or water body-specific basis that meets the 
regulatory standard of protecting the designated uses, being based on 
sound science, and ensuring the protection and maintenance of 
downstream WQS. A SSC may be more or less stringent than the otherwise 
applicable Federal criterion. A SSC may be called for when further 
scientific data and analyses indicate that a different mercury 
concentration (e.g., a different fish tissue element) may be needed to 
protect the aquatic life designated uses in a particular water body or 
portion of a water body. A SSC may also be called for when the 
relationship between fish tissue and water column mercury 
concentrations at a site differs significantly from the relationship 
between fish tissue and water column mercury concentrations in the 
Idaho-specific dataset that the EPA used to derive the statewide water 
column criterion element.

C. WQS Variances

    Idaho could adopt and submit WQS variances for the EPA's approval, 
consistent with 40 CFR 131.14, to aid in implementation of this 
federally promulgated criterion. The Federal regulation at 40 CFR 
131.3(o) defines a WQS variance as a time-limited designated use and 
criterion, for a specific pollutant or water quality parameter, that 
reflects the highest attainable condition (HAC) during the term of the 
WQS variance. A WQS variance may be appropriate if attaining the use 
and criterion would not be feasible during a given time period because 
of one of the seven factors specified in 40 CFR 131.14(b)(2)(i)(A) but 
may be attainable in the future. These factors include where complying 
with NPDES permit limits more stringent than technology-based effluent 
limits would result in substantial and widespread economic and social 
impact. When adopting a WQS variance, states and authorized Tribes 
specify the interim requirements by identifying a quantifiable 
expression that reflects the HAC during the term of the WQS variance, 
establishing the term of the WQS variance, and justifying the term by 
describing the pollutant control activities expected to occur over the 
specified term of the WQS variance. WQS variances provide a legal 
avenue by which NPDES permit limits can be written to comply with the 
WQS variance rather than the underlying WQS for the term of the WQS 
variance. WQS variances adopted in accordance with 40 CFR 131.14 
(including a public hearing consistent with 40 CFR 25.5) provide a 
flexible but defined pathway for states and authorized Tribes to issue 
NPDES permits with limits that are based on the HAC during the term of 
the WQS variance, thus allowing dischargers to make incremental water 
quality improvements. If dischargers are still unable to meet the 
WQBELs derived from the applicable designated use and criterion once a 
WQS variance term ends, the regulation allows the state to adopt a 
subsequent WQS variance if it is adopted consistent with 40 CFR 131.14.

D. Designated Uses

    The EPA's proposed mercury criterion, once finalized, would apply 
to Idaho waters where the protection of aquatic life is a designated 
use. The Federal regulation at 40 CFR 131.10 provides requirements for 
adopting, revising, and removing designated uses related to aquatic 
life and recreation when attaining the use is not feasible based on one 
of the six factors specified in the regulation. If Idaho removes the 
aquatic life designated use from any of the waters to which the EPA is 
proposing to apply this mercury criterion (i.e., from any water 
designated for an aquatic life use at the time this criterion is 
finalized), the state must adopt the highest attainable aquatic life

[[Page 24770]]

use \49\ and criteria, including a mercury criterion, to protect the 
newly designated highest attainable use consistent with 40 CFR 131.11 
for those waters. It is possible that criteria other than the federally 
promulgated criteria would protect the highest attainable use. If the 
EPA were to find Idaho's designated use revision to be consistent with 
CWA section 303(c) and the implementing regulation at 40 CFR part 131, 
the Agency would approve the revised WQS. The mercury criterion 
proposed here, once finalized, would not apply to those waters to which 
the aquatic life use no longer applies upon the EPA's approval.
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    \49\ If a state or authorized Tribe adopts a new or revised WQS 
based on a required use attainability analysis, then it must also 
adopt the highest attainable use (40 CFR 131.10(g)). Highest 
attainable use is the modified aquatic life, wildlife, or recreation 
use that is both closest to the uses specified in section 101(a)(2) 
of the Act and attainable, based on the evaluation of the factor(s) 
in 40 CFR 131.10(g) that preclude(s) attainment of the use and any 
other information or analyses that were used to evaluate 
attainability. There is no required highest attainable use where the 
state demonstrates the relevant use specified in section 101(a)(2) 
of the Act and sub-categories of such a use are not attainable (see 
40 CFR 131.3(m)).
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VIII. Economic Analysis

    The complete economic analysis for this proposed rulemaking is 
documented in ``Economic Analysis for Proposed Mercury Criterion to 
Protect Aquatic Life in Idaho,'' which can be found in the docket for 
this rulemaking. For the economic analysis, the EPA assumed the 
baseline to be full implementation of Idaho's existing water quality 
criteria (i.e., ``baseline criteria''), and then estimated the 
incremental impacts for compliance with the mercury criterion in this 
proposed rulemaking. Specifically, the EPA assumed full implementation 
of Idaho's existing 2.1 [micro]g/L acute (1-hour) and 0.012 [micro]g/L 
chronic (4-day) aquatic life water column total mercury criteria and 
Idaho's existing 0.3 mg/kg human health fish tissue methylmercury 
criterion. To estimate the incremental impacts of compliance, the EPA 
focused its economic analysis on two types of costs. First, the EPA 
estimated the potential cost impacts to current holders of NPDES 
permits. Second, the EPA estimated costs the state of Idaho may bear to 
develop Total Maximum Daily Loads (TMDLs) for waters newly identified 
as impaired under CWA section 303(d) using the proposed criterion.
    Costs might also arise to sectors with operations that include 
nonpoint sources of mercury through implementation of TMDLs or through 
other voluntary, incentivized, or state-imposed controls. However, 
these costs were not included in this economic analysis for several 
reasons. First, the CWA, and therefore this proposed rulemaking, does 
not regulate nonpoint sources. The EPA recognizes that controls for 
nonpoint sources may be part of implementing future TMDLs, but those 
decisions would be at the state's discretion. Furthermore, to 
reasonably estimate those decisions, the EPA would need to have today 
the detailed water quality data that Idaho would have in hand in the 
future when they reach those decision points. Second, nonpoint sources 
are intermittent, variable, and occur under hydrologic or climatic 
conditions associated with precipitation events. As such, any estimate 
of these costs would be associated with significant uncertainty.
    The EPA seeks public comment on all aspects of the economic 
analysis including, but not limited to, its assumptions relating to the 
baseline, affected entities, implementation, and compliance costs.

A. Identifying Affected Entities

    The proposed criterion would serve as a basis for development of 
new or revised NPDES permit conditions for point source dischargers. 
The EPA cannot be certain of whether a particular discharger would 
change their operations if this proposed criterion were finalized and 
the discharger were found to have reasonable potential to cause or 
contribute to an exceedance of the criterion. Moreover, the EPA cannot 
anticipate how Idaho would implement the criterion. Idaho is authorized 
to administer the NPDES program and retains discretion in implementing 
WQS. Despite this discretion, if Idaho determines that a permit is 
necessary, such permit would need to comply with the EPA's regulations 
at 40 CFR 122.44(d)(1)(i). Still, to best inform the public of the 
potential impacts of this proposed rulemaking, the EPA made some 
assumptions to evaluate the potential costs associated with state 
implementation of the EPA's proposed criterion.
    Any NPDES permitted facility discharging mercury could potentially 
incur incremental compliance costs. The EPA identified 146 facilities 
in Idaho with effective or administratively continued individual 
permits (for any discharge, not just permits with mercury limits). The 
types of affected facilities include sewage treatment facilities and 
industrial facilities discharging wastewater to surface waters. In its 
analysis of point sources, the EPA did not include facilities on Tribal 
lands with permits issued by the EPA because the proposed rulemaking 
would not cover Tribal lands.
    Of the 146 facilities with individual permits, 17 are stormwater 
discharges. The EPA excluded facilities with individual permits for 
stormwater discharges (e.g., large or medium municipal separate storm 
sewer systems) and facilities covered under general permits for 
stormwater discharges because of limited data for such facilities and 
permit requirements that typically focus on best management practices 
(BMPs). This left 129 point source facilities with individual permits. 
In addition, the EPA identified one facility covered under an NPDES 
general permit that could be affected by the proposed rulemaking based 
on the general permit requirements and available effluent data, 
bringing the total number of potentially affected facilities to 130. Of 
these, 38 are major dischargers and 92 are minor dischargers.
    The EPA reviewed Discharge Monitoring Report (DMR) data for the 130 
facilities to identify facilities with effluent limitations and/or 
monitoring requirements for mercury in their NPDES permits. The EPA's 
review of DMR data indicates that 31 facilities with individual permits 
(24 majors, 7 minors) have effluent limitations and/or monitoring 
requirements for mercury. Of these, 20 (18 majors, 2 minors) are 
publicly owned treatment works (POTWs) categorized under North American 
Industry Classification System (NAICS) Industry 221320 (Sewage 
Treatment Facilities) and 11 (6 majors, 5 minors) are facilities 
categorized under other NAICS Industries. The one facility covered 
under a non-stormwater general permit with mercury data reported on 
DMRs operates under an EPA-issued general permit for Groundwater 
Remediation Discharge Facilities in Idaho, which includes mercury 
limits applicable to the facility. Table 1 in this preamble summarizes 
the potentially affected facilities by type (major or minor) and 
category (NAICS Industry 221320 or other NAICS Industries). Table 1 in 
this preamble also shows the number of facilities for which DMRs 
indicate there are effluent limits and/or monitoring requirements for 
mercury, including the facility covered by a general permit for 
groundwater remediation discharges.

[[Page 24771]]



Table 1--Potentially Affected Facilities, With Facilities Having Mercury
Effluent Limitations and/or Monitoring Requirements for Mercury Shown in
                               Parentheses
------------------------------------------------------------------------
                                               Major           Minor
                Category                    facilities      facilities
------------------------------------------------------------------------
Sewage Treatment Facilities (NAICS               30 (18)          74 (2)
 Industry 221320).......................
Industrial (Other NAICS Industries).....           8 (6)          18 (6)
                                         -------------------------------
    Total...............................         38 (24)          92 (8)
------------------------------------------------------------------------

B. Method for Estimating Costs

    The EPA grouped facilities with individual permits by major or 
minor status and further grouped major facilities in NAICS Industry 
221320 by design flow range. The EPA identified the facilities in each 
grouping with effluent concentration data for mercury. The EPA reviewed 
data for these facilities reported on DMRs accessed through the EPA's 
Enforcement and Compliance History Online (ECHO) site and the 
facilities' NPDES permits and fact sheets. The EPA used this 
information to characterize baseline conditions; determine whether a 
discharge would cause, have the reasonable potential to cause, or 
contribute to an exceedance of baseline or proposed mercury criteria; 
and assess whether the discharge is likely to exceed water quality-
based effluent limitations (WQBELs) derived from baseline and proposed 
mercury criteria. Based on this analysis, the EPA identified facilities 
that may need to implement additional actions to achieve compliance 
with the proposed mercury criterion.
    The EPA assumed that dischargers would pursue the least cost means 
of compliance with WQBELs derived from the proposed mercury criterion. 
Only the costs of compliance actions above the level of controls needed 
to comply with baseline criteria are attributable to the proposed 
rulemaking. To determine these incremental compliance costs, the EPA 
considered potential one-time costs (e.g., costs for developing or 
revising a pollutant minimization program (PMP), or applying for a WQS 
variance) and annual costs (e.g., costs for implementing a new PMP or 
for additional treatment).
    For purposes of the analysis, the EPA assumed that major facilities 
in NAICS Industry 221320 with no mercury data reported in DMRs for the 
past five years would still likely discharge quantifiable 
concentrations of mercury, though not at high enough concentrations for 
mercury to be a pollutant of concern under the baseline Idaho mercury 
criteria (i.e., the facilities currently have no mercury effluent 
limits or monitoring requirements). The EPA also assumed that mercury 
may become a pollutant of concern at these facilities under the 
proposed mercury criterion. Based on these assumptions, the EPA 
extrapolated estimated one-time and annual incremental compliance costs 
for major facilities in NAICS Industry 221320 for which effluent data 
for mercury are available to major facilities in NAICS Industry 221320 
with no available effluent data for mercury. Specifically, the EPA 
extrapolated cost within each facility flow rate range grouping 
proportionally by number of facilities for one-time costs and annual 
costs that are not flow-dependent (e.g., if 25% of the facilities with 
mercury data would incur one-time costs that do not depend on effluent 
flow rate, then the EPA assumed that 25% of facilities not reporting 
mercury data would also incur such costs). For flow-dependent annual 
costs, the EPA extrapolated based on design flow rate.
    The EPA did not extrapolate costs for minor facilities in NAICS 
Industry 221320 or for facilities categorized in other NAICS Industries 
(major and minor industrial facilities). The EPA assumed that minor 
POTWs (NAICS Industry 221320) are less likely than major POTWs to 
receive influent from industrial and commercial sources of mercury, 
which reduces the likelihood of mercury being a pollutant of concern 
for those facilities where it has not already been identified as such. 
The EPA also assumed that facilities in other NAICS Industries 
(industrial discharges) for which mercury is a potential pollutant of 
concern based on the proposed criterion typically would already have 
effluent limits or monitoring requirements based on Idaho's baseline 
mercury criteria.
    The EPA also evaluated potential administrative costs to the state 
for developing additional TMDLs under CWA section 303(d) for waters 
that may be newly identified as impaired as a result of the proposed 
mercury criterion, as well as potential costs for revising existing 
TMDLs. Idaho assesses water bodies by assessment units (AUs). AUs are 
subdivisions of water body units (WBIDs) which are subdivisions of 8-
digit hydrologic unit codes (HUCs). Using available fish tissue and 
ambient water column monitoring data, the EPA compared mercury 
concentrations to baseline Idaho mercury criteria and the proposed 
mercury criterion, and identified AUs that may be incrementally 
impaired (i.e., impaired under the proposed criterion but not under the 
baseline criteria). For waters impaired under the baseline criteria, 
the EPA assumes that the state will develop TMDLs and implementation 
plans to bring all these waters into compliance with baseline criteria. 
Therefore, only incremental costs identified to comply with the 
proposed criterion above and beyond the baseline are attributable to 
this proposed rulemaking.

C. Results

    Based on the results for the 32 major and minor facilities (31 with 
individual permits and 1 covered under a general permit) with available 
effluent monitoring data for mercury, and extrapolation within each 
design flow rate range to the 12 additional major NAICS Industry 221320 
facilities without mercury data, the EPA estimated a range of total 
one-time and total annual costs as shown in Table 2 in this preamble.

[[Page 24772]]



                                              Table 2--Estimated One-Time and Annual Costs to Point Sources
                                                                     [2022 Dollars]
--------------------------------------------------------------------------------------------------------------------------------------------------------
                               Total estimated one-time cost                                 Total estimated annual cost (capital costs annualized over
-------------------------------------------------------------------------------------------                        20 years at 2%)
                                                                                           -------------------------------------------------------------
                            Low                                           High                           Low                            High
--------------------------------------------------------------------------------------------------------------------------------------------------------
$253,000...................................................                    $1,220,000                       $120,000                    $16,800,000
--------------------------------------------------------------------------------------------------------------------------------------------------------

    The low end of the one-time cost range reflects an assumption that 
most facilities potentially impacted would be able to comply with 
revised effluent limitations or would revise an existing PMP to achieve 
compliance. The high end of the one-time cost range assumes that 
facilities would revise or develop a new PMP and, in some cases, 
conduct the studies needed to apply for a WQS variance.
    The low end of the annual cost range reflects an assumption that, 
for most facilities, one-time actions, if needed, would result in 
compliance with revised effluent limitations. The low end annual cost 
estimate includes the costs for a limited number of facilities to 
implement a new PMP and assumes that facilities implementing a revised 
PMP plan do not incur incremental annual costs. The high end of the 
annual cost range assumes that some facilities would incur the cost of 
implementing a new PMP plan and some facilities would incur capital and 
operation and maintenance costs associated with installing and 
operating new or additional treatment, in this case non-membrane 
filtration for mercury removal.
    Based on available fish tissue data, the EPA identified four 
instances of lake or reservoir AUs and two instances of river or stream 
AUs that may be considered incrementally impaired under the proposed 
criterion. In addition, based on ambient water quality data for 
mercury, the EPA identified an additional 7 AUs that may be considered 
incrementally impaired under the proposed criterion. The EPA estimated 
a range for the total cost to develop TMDLs for the 13 AUs potentially 
placed on Idaho's CWA section 303(d) list for mercury as a result of 
the proposed criterion. These costs were based on single-cause single-
waterbody TMDL development costs. Actual costs may be lower if the 
state develops multi-cause or multi-waterbody TMDLs. In addition, Idaho 
currently has one approved TMDL for mercury for ID17040213SK007L_0L: 
Salmon Falls Creek Reservoir. This TMDL may need to be revised based on 
the proposed criterion and any new information that has become 
available since the TMDL was approved. Based on administrative costs 
associated with TMDL development for the 13 AUs identified as 
incrementally impaired and for potential revision of 1 TMDL, the EPA 
estimated total costs associated with incremental impairments to be 
$586,000 to $629,000.

IX. Statutory and Executive Orders Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive 
Order 14094: Modernizing Regulatory Review

    This action is not a significant regulatory action as defined in 
Executive Order 12866, as amended by Executive Order 14094, and was 
therefore not subject to a requirement for Executive Order 12866 
review.

B. Paperwork Reduction Act (PRA)

    This action does not impose any new information collection burden 
under the PRA. OMB has previously approved the information collection 
requirements activities contained in the existing regulation and has 
assigned OMB control number 2040-0049. This action does not directly 
contain any information collection, reporting, or record-keeping 
requirements.

C. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic 
impact on a substantial number of small entities under the RFA. This 
action will not impose any requirements on small entities.
    EPA-promulgated WQS are implemented through various water quality 
control programs, including the NPDES program, which limits discharges 
to navigable waters except in compliance with a NPDES permit. CWA 
section 301(b)(1)(C) \50\ and the EPA's implementing regulation at 40 
CFR 122.44(d)(1) provide that all NPDES permits shall include any 
limits on discharges that are necessary to meet applicable WQS. Thus, 
under the CWA, the EPA's promulgation of WQS establishes standards that 
the state implements through the NPDES permit process.
---------------------------------------------------------------------------

    \50\ 301(b) Timetable for achievement of objectives. In order to 
carry out the objective of this chapter there shall be achieved--
(1)(C): not later than July 1, 1977, any more stringent limitation, 
including those necessary to meet water quality standards, treatment 
standards, or schedules of compliance, established pursuant to any 
State law or regulations (under authority preserved by section 1370 
of this title) or any other Federal law or regulation, or required 
to implement any applicable water quality standard established 
pursuant to this chapter.
---------------------------------------------------------------------------

    After the EPA promulgates a final mercury criterion, the state of 
Idaho must ensure that NPDES permits it issues include any limitations 
on discharges necessary to comply with the WQS established in the final 
rule. While Idaho's implementation of the rule may ultimately result in 
new or revised permit conditions for some dischargers, including small 
entities, the EPA's action, by itself, does not impose any of these 
requirements on small entities; that is, these requirements are not 
self-implementing.

D. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandates as described in 
UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely affect 
small governments. The action imposes no enforceable duty on any state, 
local or Tribal governments or the private sector.

E. Executive Order 13132 (Federalism)

    This action does not have federalism implications. The EPA 
believes, however, that this action may be of significant interest to 
state governments. Consistent with the EPA's policy to promote 
communications between the EPA and state and local governments, the EPA 
consulted with Idaho officials early in the process of developing this 
rulemaking to permit them to have meaningful and timely input into its 
development.
    On several occasions starting on July 12, 2023, the EPA discussed 
the development of this rulemaking with the Idaho Department of 
Environmental Quality. Early in this process, the EPA clarified that if 
and when the state decides to revise its own mercury aquatic life 
criteria, the EPA would assist the state in its process. During these 
discussions, the EPA also explained: the scientific basis for the

[[Page 24773]]

fish tissue and water column elements of the mercury criterion; the 
external peer review process and the comments the EPA received on the 
derivation of the criterion; the EPA's consideration of those comments 
and responses; the assumptions and data being used in the economic 
analysis associated with the rulemaking; and the overall timing of the 
Federal rulemaking effort. The EPA took these discussions with the 
state into account during the drafting of this proposed rulemaking.

F. Executive Order 13175 (Consultation and Coordination With Indian 
Tribal Governments)

    This action does not have Tribal implications as specified in 
Executive Order 13175. This rule does not impose substantial direct 
compliance costs on federally recognized Tribal governments, nor does 
it substantially affect the relationship between the Federal government 
and Tribes, or the distribution of power and responsibilities between 
the Federal government and Tribes. Thus, Executive Order 13175 does not 
apply to this action.
    Consistent with the EPA Policy on Consultation and Coordination 
with Indian Tribes, the EPA consulted with Tribal officials during the 
development of this action. A Summary of Consultation, Coordination and 
Outreach with Federally Recognized Tribes on the EPA's Proposed Federal 
Promulgation of a Mercury Criterion to Protect Aquatic Life in Idaho is 
available in the docket.

G. Executive Order 13045 (Protection of Children From Environmental 
Health and Safety Risks)

    The EPA interprets Executive Order 13045 as applying only to those 
regulatory actions that concern environmental health or safety risks 
that the EPA has reason to believe may disproportionately affect 
children, per the definition of ``covered regulatory action'' in 
section 2-202 of the Executive Order. Therefore, this action is not 
subject to Executive Order 13045 because it does not concern an 
environmental health risk or safety risk. Since this action does not 
concern human health, the EPA's Policy on Children's Health also does 
not apply.

H. Executive Order 13211 (Actions That Significantly Affect Energy 
Supply, Distribution, or Use)

    This action is not a ``significant energy action'' because it is 
not likely to have a significant adverse effect on the supply, 
distribution, or use of energy.

I. National Technology Transfer and Advancement Act of 1995

    This rulemaking does not involve technical standards.

J. Executive Order 12898: Federal Actions To Address Environmental 
Justice in Minority Populations and Low-Income Populations and 
Executive Order 14096: Revitalizing Our Nation's Commitment to 
Environmental Justice for All

    The EPA believes that the human health and environmental conditions 
that exist prior to this action do not result in disproportionate and 
adverse effects on communities with environmental justice (EJ) 
concerns. In the EPA's Economic Analysis for Proposed Mercury Criterion 
to Protect Aquatic Life in Idaho (economic analysis), which can be 
found in the docket for this rulemaking, Exhibit 5-3 illustrates the 
geographic distribution of waters where available data indicate levels 
of mercury that exceed Idaho's existing mercury criteria. These waters 
are located throughout the state, and waters with the highest levels of 
exceedance are similarly found in multiple parts of the state. Given 
the widespread nature of these impaired waters across the entire state, 
it is unlikely that impaired waters are disproportionately located in 
proximity to communities with potential EJ concerns.
    The EPA believes that this action is not likely to result in new 
disproportionate and adverse effects on communities with EJ concerns. 
The EPA's proposed criterion for mercury in Idaho applies to aquatic 
life uses and does not directly address human health impacts. However, 
this rulemaking, if finalized and implemented, would support the health 
and abundance of aquatic life in Idaho and would, therefore, not only 
benefit those aquatic species but also benefit human communities that 
rely on or use these ecosystems. Compared to higher-income populations, 
low-income populations tend to rely more on fishing as a food 
source,\51\ and therefore, this rulemaking may especially benefit low-
income communities.
---------------------------------------------------------------------------

    \51\ Von Stackelberg, K., et al. (2017). Results of a national 
survey of high-frequency fish consumers in the United States. 
Environmental Research 158, 126-136. https://bgc.seas.harvard.edu/assets/vonstackelberg2017.pdf.
---------------------------------------------------------------------------

    To achieve the benefits associated with a final rule, the EPA 
recognizes that some facilities may need to add pollution control 
measures and incur additional compliance costs over time to meet any 
new permit conditions or limits resulting from the mercury criterion, 
once finalized. The EPA's economic analysis identified three wastewater 
treatment plants and one mine that may need to install additional 
treatment technologies (e.g., non-membrane filtration) if the criterion 
is finalized as proposed. For the wastewater treatment plants, the EPA 
analyzed the compliance costs that might be passed on to residential 
households alongside the socioeconomic characteristics of those 
households.
    For the West Boise Water Renewal Facility, the high end of the 
estimated annual cost range from the economic analysis is $6.7M. For 
the Nampa Wastewater Treatment Facility, the high end of the estimated 
annual cost range is $5.1M. For the City of Caldwell Wastewater 
Treatment Plant, the high end of the estimated annual cost range is 
$2.4M. Based on the estimated number of households served by each 
facility \52\ and conservatively assuming that 100% of the additional 
treatment costs are borne by residential ratepayers, these costs would 
translate to monthly household sewer bill increases of approximately 
$7.93, $11.78, and $10.16 for households served by the West Boise, 
Nampa, and Caldwell facilities, respectively. These amounts would 
represent approximately a 20-30% increase relative to current sewer 
bills in these areas.\53\ After this increase, household sewer bills 
would represent approximately 0.85%, 1.17%, and 1.05% of the median 
household income \54\ in Boise, Nampa, and Caldwell, respectively.
---------------------------------------------------------------------------

    \52\ The EPA estimated the number of households served by the 
West Boise Water Renewal Facility from the 2022 IPDES Permit Fact 
Sheet. The EPA estimated the number of households served by the 
Nampa Wastewater Treatment Facility and the City of Caldwell 
Wastewater Treatment Plant from 2018-2022 American Community Survey 
5-year data, since the most recent Permit Fact Sheets for these 
facilities were from 2015 and their service areas could be 
approximated by U.S. Census Places (Nampa City and Caldwell City).
    \53\ https://www.idahopress.com/news/local/boise-voters-overwhelmingly-pass-sewer-bond/article_a72230a4-6875-5708-a41b-c7a9fbce8e6e.html; https://www.cityofnampa.us/1397/2021-Rate-
Increase#:~:text=Sewer%20Rate%20Increase%20Approved%20as%20Part%20of%
20Bond%20Repayment%20Plan&text=Beginning%20October%201%2C%20the%20ave
rage,per%20month%20for%20residential%20customers.
    \54\ 2018-2022 American Community Survey 5-year data. https://www.census.gov/data/developers/data-sets/acs-5year.html.
---------------------------------------------------------------------------

    Using EJScreen, the EPA performed a screening-level analysis of the 
socioeconomic characteristics of these communities, focusing on 
EJScreen's

[[Page 24774]]

individual socioeconomic indicators.\55\ To interpret EJScreen results, 
the EPA used an 80th percentile filter for each indicator,\56\ using 
percentiles reflecting comparison to the Idaho population and to the 
entire U.S. population. The percentile indicates what percent of the 
comparison population (here, Idaho or entire U.S.) has an equal or more 
favorable value.
---------------------------------------------------------------------------

    \55\ People of color, low income, unemployment rate, limited 
English speaking households, less than high school education, under 
age 5, over age 64, and low life expectancy. See EJScreen Technical 
Documentation for Version 2.2 for indicator definitions (https://www.epa.gov/system/files/documents/2023-06/ejscreen-tech-doc-version-2-2.pdf).
    \56\ https://www.epa.gov/ejscreen/how-interpret-ejscreen-data.
---------------------------------------------------------------------------

    When comparing each of the three communities to the entire U.S. 
population, the EPA found limited indication of potential EJ concern 
that would warrant further analysis; only one indicator in one 
community just reached the 80th percentile threshold (the percentage of 
people under age 5 in Caldwell, ID was at the 80th percentile). At the 
same time, comparing each of the communities to the Idaho population 
highlighted some differences in their socioeconomic situations. While 
Boise did not exceed the 80th percentile (relative to the Idaho 
population) for any of the eight socioeconomic indicators, Nampa 
exceeded for two indicators (people of color and limited English 
speaking households) and Caldwell exceeded for three indicators (people 
of color, limited English speaking households, and less than high 
school education) and had another two indicators (under age 5 and 
unemployment rate) at the 77th percentile. Therefore, due to the 
potentially greater socioeconomic vulnerability as indicated by this 
screening-level analysis, these potential (albeit relatively modest) 
sewer rate increases may have disproportionate economic impacts in 
Caldwell relative to Boise, Nampa, and other Idaho communities.
    However, actual impacts would depend on a number of factors, 
including how the state implements the criterion, how costs are 
financed, and how costs are distributed among ratepayers. States have 
wide latitude in how they implement criteria, including the authority 
to adopt variances for those facilities for which meeting WQS would 
cause substantial and widespread economic and social impact. 
Communities can apply for various grants to finance wastewater 
treatment upgrades or the state may share part of the cost burden. In 
addition, the Bipartisan Infrastructure Law included $50 billion in 
funding for infrastructure improvements to the Nation's wastewater and 
drinking water systems. Moreover, municipalities may implement customer 
assistance or progressive rate structures that reduce the cost burden 
on low income households.\57\ Finally, the costs of wastewater 
treatment upgrades must be balanced against the potential benefits of 
having access to cleaner water. The EPA seeks comment on all potential 
EJ impacts of the rulemaking.
---------------------------------------------------------------------------

    \57\ USEPA. 2023. Clean Water Act Financial Capability 
Assessment Guidance. 800b21001. U.S. Environmental Protection 
Agency, Office of Water, Washington, DC. https://www.epa.gov/system/files/documents/2023-01/cwa-financial-capability-assessment-guidance.pdf.
---------------------------------------------------------------------------

    In addition to Executive Order 12898, and in accordance with Title 
VI of the Civil Rights Act of 1964, each Federal agency shall ensure 
that all programs or activities receiving Federal financial assistance 
that affect human health or the environment do not directly, or through 
contractual or other arrangements, use criteria, methods, or practices 
that discriminate on the basis of race, color, or national origin. With 
that directive in mind, in August 2011 the Environmental Justice 
Interagency Working Group established a Title VI Committee to address 
the intersection of agencies' EJ efforts with their Title VI 
enforcement and compliance responsibilities. While the EPA only has an 
oversight role for CWA implementation, if Idaho receives Federal funds 
for CWA implementation, the state is legally prohibited from 
discriminating on the basis of race, color, or national origin under 
Title VI when engaging in CWA implementation activities. Additionally, 
and in compliance with Executive Order 12898, the EPA expects that 
Idaho will consider disproportionately high adverse human health and 
environmental effects on communities with EJ concerns when implementing 
this rulemaking under the CWA.
    The information supporting this Executive Order review is contained 
in the EPA's Economic Analysis for Proposed Mercury Criterion to 
Protect Aquatic Life in Idaho.

List of Subjects in 40 CFR Part 131

    Environmental protection, Indians--lands, Intergovernmental 
relations, Reporting and recordkeeping requirements, Water pollution 
control.

Michael S. Regan,
Administrator.

    For the reasons set forth in the preamble, the EPA proposes to 
amend 40 CFR part 131 as follows:

PART 131--WATER QUALITY STANDARDS

0
1. The authority citation for part 131 continues to read as follows:

    Authority:  33 U.S.C. 1251 et seq.

Subpart D--Federally Promulgated Water Quality Standards

0
2. Add Sec.  131.XX to read as follows:


Sec.  131.XX  Mercury criterion to protect aquatic life in Idaho.

    (a) Scope. This section promulgates an aquatic life criterion for 
mercury in Idaho.
    (b) Criterion for mercury in Idaho. The applicable aquatic life 
criterion for mercury is shown in Table 1 to Paragraph (b).

Table 1 to Paragraph (b)--Proposed Chronic Mercury Ambient Water Quality Criterion for the Protection of Aquatic
                                                  Life in Idaho
----------------------------------------------------------------------------------------------------------------
                                        Fish muscle tissue 1 2   Fish whole body tissue   Water column 1 4 total
              Media type               3 total mercury (ng THg/  1 2 total mercury (ng   mercury (ng/L) in whole
                                            g wet weight)          THg/g wet weight)              water
----------------------------------------------------------------------------------------------------------------
Magnitude............................  225....................  162....................  2.1.
                                      --------------------------------------------------
Duration.............................  Instantaneous measurement \5\.                    30 day average.
Frequency............................  The average tissue concentration must not be      Not more than once in
                                        exceeded.                                         three years on
                                                                                          average.
----------------------------------------------------------------------------------------------------------------
\1\ The proposed criterion elements are hierarchical, with both tissue elements superseding the water column
  element. The fish muscle tissue and fish whole body tissue criterion elements are independently applicable.

[[Page 24775]]

 
\2\ Tissue sample measurements must be based on measurement(s) of the total mercury concentration (in a
  composited tissue sample from each fish species or a central tendency estimate of individual tissue samples
  from each fish species) collected from a given site or waterbody in a discrete sampling period. These
  criterion elements support Idaho's aquatic life uses. Only samples of adult life stage trophic level (TL) 4
  fish can be directly compared to the muscle or whole-body criterion elements.
\3\ If adult life stage TL2 or TL3 fish are sampled, a Bioaccumulation Trophic Adjustment Factor (BTAF) must be
  applied to the muscle concentrations of those fish. If whole-body tissue from TL2 or TL3 fish is sampled, the
  fish whole body--muscle conversion factor of 0.72 must be applied to generate a translated muscle value before
  a BTAF is applied to the sample concentration. A TL2 sampled fish concentration must be multiplied by the TL2
  BTAF of 5.6 and the resultant value compared to the muscle tissue criterion element. A TL3 sampled fish
  concentration must be multiplied by the TL3 BTAF of 3.5 and the resultant value compared to the muscle tissue
  criterion element. If multiple adults of different TLs are sampled, the TL4 fish result would supersede TL3
  BTAF-applied or TL2 BTAF-applied value outcomes. If TL3 and TL2 fish are sampled, the TL3 BTAF-applied values
  supersede the TL2 BTAF-applied values.
\4\ Water column values are based on total mercury in unfiltered or ``whole water'' samples. Total mercury
  includes all inorganic and organic species of mercury in the water column. Water samples collected during
  baseflow conditions would be most representative of the data used to derive this criterion element. This
  criterion element supports Idaho's aquatic life uses.
\5\ Fish tissue data provide integrative measurements that reflect accumulation of mercury over time and space
  in aquatic organisms from a given site or waterbody in a discrete sampling period.

    (c) Applicability. (1) The criterion in paragraph (b) of this 
section applies to all of Idaho's aquatic life use designations and 
applies concurrently with other applicable water quality criteria.
    (2) The criterion established in this section is subject to Idaho's 
general rules of applicability in the same way and to the same extent 
as are other federally promulgated and state-adopted numeric criteria 
when applied to waters in Idaho designated to protect aquatic life 
uses.
    (3) For all waters with mixing zone regulations or implementation 
procedures, the criterion applies at the appropriate locations within 
or at the boundary of the mixing zones and outside of the mixing zones; 
otherwise the criterion applies throughout the water body including at 
the end of any discharge pipe, conveyance or other discharge point 
within the water body.

[FR Doc. 2024-07450 Filed 4-8-24; 8:45 am]
BILLING CODE 6560-50-P