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Federal Register / Vol. 86, No. 52 / Friday, March 19, 2021 / Rules and Regulations
concentrations could exceed criteria values when low flows occur.15
EPA notes that in ambient settings, critical low flow conditions used for NPDES permit limit derivation purposes may not always correspond with conditions of highest aluminum bioavailability and toxicity. EPAs NPDES Permit Writers Manual describes the importance of characterizing effluent and receiving water critical conditions, because if a discharge is controlled so that it does not cause water quality criteria to be exceeded in the receiving water under critical conditions, then water quality criteria should be attained under all other conditions.16
EPAs March 1991 Technical Support Document for Water Quality-based Toxics Control recommends two methods for calculating acceptable critical low flow values: The traditional hydrologically-based method developed by the USGS and a biologically based method developed by EPA.17 The hydrologically-based critical low flow value is determined statistically, using probability and extreme values, while the biologically-based critical low flow is determined empirically using the specific duration and frequency associated with the criterion. For the acute and chronic aluminum criteria, EPA recommends the following critical low flow values, except where modeling demonstrates that the most significant critical conditions occur at other than low flow:

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Acute Aquatic Life CMC: 1Q10 or 1B3
Chronic Aquatic Life CCC: 7Q10 or 4B3
Using the hydrologically-based method, the 1Q10 represents the lowest one-day average flow event expected to occur once every ten years, on average, and the 7Q10 represents the lowest sevenconsecutive-day average flow event expected to occur once every ten years, on average. Using the biologically-based method, 1B3 represents the lowest oneday average flow event expected to occur once every three years, on average, and 4B3 represents the lowest four-consecutive-day average flow event 15 USEPA. 2014. Water Quality Standards Handbook-Chapter 5: General Policies. U.S.
Environmental Protection Agency, Office of Water.
Washington, DC EPA820B14004. http
www.epa.gov/sites/production/files/2014-09/
documents/handbook-chapter5.pdf.
16 The same principle holds for developing a TMDL target, the total load.
17 USEPA. 1991. Technical Support Document For Water Quality-based Toxics Control. U.S.
Environmental Protection Agency, Office of Water, Washington, DC EPA/505/290001. http
www3.epa.gov/npdes/pubs/owm0264.pdf.

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expected to occur once every three years, on average.18
The final criteria must be attained at the point of discharge unless Oregon authorizes a mixing zone. Where Oregon authorizes a mixing zone, the criteria would apply at the locations allowed by the mixing zone i.e., the CMC would apply at the defined boundary of the acute mixing zone and the CCC would apply at the defined boundary of the chronic mixing zone.19 20
V. Endangered Species Act Section 7a2 of the ESA requires that each Federal agency ensure that any action authorized, funded, or carried out by such Agency is not likely to jeopardize the continued existence of any endangered or threatened species or result in the destruction or adverse modification of critical habitat. For this rule, EPA transmitted a Biological Evaluation to NOAA Fisheries Service and USFWS on September 20, 2019.
NOAA Fisheries responded on October 18, 2019, that EPAs Biological Evaluation was insufficient to initiate formal consultation. EPA submitted a revised Biological Evaluation to NOAA
Fisheries on January 2, 2020. On February 13, 2020, EPA received a final Biological Opinion from USFWS that determined that EPAs proposed action is likely to adversely affect, but will not jeopardize the continued existence of bull trout and will not destroy or adversely modify its designated critical habitat. USFWS also concluded that the proposed action may affect, but is not likely to adversely affect, eight other federally-listed species and is not likely to destroy or adversely modify the critical habitat for the other species that were included in the consultation. On July 1, 2020, EPA received a final Biological Opinion from NOAA
Fisheries that determined that EPAs proposed action is likely to adversely affect, but will not jeopardize the continued existence of 18 listed species and will not destroy or adversely modify designated critical habitat for the species that were included in the consultation. The receipt of the NOAA
Biological Opinion concludes the consultation for this rule under ESA
Section 7a2. Documents associated with ESA consultation are available in the docket associated with this rule Docket ID: EPAHQOW20160694.
18 See
USEPA, 2014.
USEPA, 1991.
20 Oregon Administrative Rules, Mixing Zones Chapter 340, Division 41, Section 0053, https
secure.sos.state.or.us/oard/
viewSingleRule.action?ruleVrsnRsn=68770.
19 See
PO 00000

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VI. Under what conditions would Federal standards be withdrawn?
Under the CWA, Congress gave states and authorized tribes primary responsibility for developing and adopting WQS for their navigable waters CWA Sections 303a through c.
Although EPA is finalizing aluminum aquatic life criteria for Oregons fresh waters subject to this rule on the basis of having disapproved Oregons 2004
criteria in February 2013, Oregon retains the option to adopt and submit to the Agency acute and chronic aluminum criteria for the States fresh waters consistent with CWA Section 303c and the Agencys implementing regulation at 40 CFR part 131. Indeed, EPA
encourages Oregon to do so expeditiously. The Agency would approve the States criteria if the criteria meet the requirements of CWA Section 303c and implementing regulation at 40 CFR part 131. If EPAs federally promulgated criteria are more stringent or prescriptive than the States criteria, EPAs federally promulgated criteria are and will be the applicable water quality standard for purposes of the CWA until the Agency withdraws those federally promulgated standards 40 CFR
131.21c. EPA would expeditiously undertake such a rulemaking to withdraw the Federal criteria if and when Oregon adopts, and the Agency approves, corresponding criteria that meet the requirements of CWA Section 303c and implementing regulation at 40 CFR part 131. After such a withdrawal of EPAs federally promulgated criteria, the States EPAapproved criteria would become the applicable criteria for CWA purposes 40 CFR 131.21c.
VII. Alternative Regulatory Approaches and Implementation Mechanisms The Federal WQS regulation at 40
CFR part 131 provides several tools that Oregon has available to use at its discretion when implementing or deciding how to implement these aquatic life criteria, once effective.
Among other things, EPAs WQS
regulation: 1 Specifies how states and authorized tribes establish, modify, or remove designated uses 40 CFR
131.10; 2 specifies the requirements for establishing criteria to protect designated uses, including criteria modified to reflect site-specific conditions 40 CFR 131.11; 3
authorizes and provides regulatory guidelines for states and authorized tribes to adopt WQS variances that provide time to achieve the applicable WQS 40 CFR 131.14; and 4 allows states and authorized tribes to authorize
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