Draft for Public Comment, do not cite (7/31/2019)
United States
Environmental Protection
^1 # %Agency
Office of Water
EPA- 823-D-19-001
July 2019
Draft Technical Support Document:
Implementing the 2018 Recommended Aquatic Life
Water Quality Criteria for Aluminum

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Notice: This draft question and answer document is intended for states and authorized tribes that wish to adopt
and implement the U.S. Environmental Protection Agency's (EPA's) recommended Final Aquatic Life Ambient
Water Quality Criteria for Aluminum. Pursuant to 40 CFR 131.11(b), when establishing numerical criteria
designed to protect designated uses, states and authorized tribes should base those criteria on (i) 304(a)
guidance; (ii) 304(a) guidance modified to reflect site-specific conditions; or (iii) other scientifically defensible
methods. Because the EPA's Section 304(a) aluminum aquatic life criteria are recommendations, and not
requirements, states and authorized tribes should consider the advantages and potential challenges of each
approach to adopting the recommended criteria, as well as other approaches that may not be described in this
document. This document addresses state and tribal adoption of numeric aluminum criteria under
131.11(b)(l)(i) and (ii).
The national 304(a) recommended aluminium criteria are water-chemistry dependent, and criteria values will
vary from site to site based on the values of water chemistry parameters at the site. States and authorized tribes
may choose to adopt these criteria into their water quality standards using a performance-based approach.1
This approach involves a two-step process. First, the state or authorized tribe would adopt criteria and a
methodology for deriving site-specific criteria values (both of which the EPA must first approve). Then, after EPA
approval, the state or authorized tribe would apply the methodology at specific waterbodies to derive site-
specific criteria values for each waterbody. After the EPA approves the state's or authorized tribe's
methodology, additional approval of the site-specific criteria values derived on a waterbody-by-waterbody basis
would not be required. That is, once the state's or authorized tribe's chosen method of adopting the criteria is
approved by the EPA, the state or authorized tribe may use the method to derive site-specific criteria values that
are used for other Clean Water Act purposes2 without additional Agency review. In some cases, more than one
method may be appropriate, as explained in Question 1 of this document.
The EPA could update this document as new information becomes available. While this document cites statutes
and regulations that contain requirements applicable to water quality standards, it does not impose legally
binding requirements on the EPA, states, authorized tribes, other regulatory authorities, or the regulated
community and its content might not be appropriate in a particular situation based upon the circumstances. The
EPA, state, tribal, and other decision makers retain the discretion to adopt approaches on a case-by-case basis
that differ from those provided in this document as appropriate and consistent with statutory and regulatory
requirements. In addition to this document, the EPA has other documents which provide considerations and
recommendations on implementing the aluminum criteria and can be found at the Agency's aluminum website:
https://www.epa.gov/wqc/aquatic-life-criteria-aluminum.
1	The EPA first described the performance-based approach in the preamble to EPA Review and Approval of State and Tribal
Water Quality Standards (65 FR 24641, April 27, 2000). "A performance-based approach relies on adoption of a process
(i.e., a criterion derivation methodology) rather than a specific outcome (i.e., concentration limit for a pollutant) consistent
with 40 CFR 131.11 & 131.13. When such a "performance-based" approach is sufficiently detailed and has suitable
safeguards to ensure predictable, repeatable outcomes, EPA approval of such an approach can also serve as approval of the
outcomes as well. If a particular State or Tribe's approach is not sufficiently detailed or lacks appropriate safeguards, then
EPA review of a specific outcome is still necessary." (65 FR 24648).
2	For example, serving as the basis to derive water quality-based effluent limits for National Pollutant Discharge Elimination
System (NPDES) permits, identifying impaired and threatened waters for waterbody assessments under Section 303(d) of
the Act, and developing total maximum daily loads (TMDLs) for impaired or threatened waters.
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Table of Contents
Background: Adopting and Implementing the 2018 Recommended CWA 304(a) Aquatic Life Water Quality
Criteria for Aluminum	4
1.	What flexibility does a state or authorized tribe have when adopting the EPA's recommended aluminum
criteria into its water quality standards, and what are the advantages and potential challenges of each
approach?	5
2.	How often and over what time period should a state or authorized tribe collect input parameter data?
What if DOC data are insufficient?	8
3.	What methods can be used to reconcile model outputs and derive criteria values that will result in
protection of aquatic life at a site?	9
4.	How can a state or authorized tribe implement the aluminum criteria in its Clean Water Act programs?..10
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Background; Adopting and Implementing the 2018 Recommended Aquatic Life Water
Quality Criteria for Aluminum
On December 21, 2018, the EPA issued the final updated ambient water quality criteria national
recommendations to protect aquatic life from the toxic effects of aluminum.3 The criteria document
recommends two primary methods for deriving instantaneous site-specific acute and chronic concentration
values for aluminum that would be considered protective of aquatic life, given the conditions of pH, total
hardness and dissolved organic carbon (DOC) at the site. For states or authorized tribes that have chosen to
adopt the recommended criteria, the EPA recommends two methods to derive the acute and chronic numeric
criterion values:
(1)	Calculate the criteria values for each waterbody or waterbody segment that has aquatic life as a
designated use by entering the pH, total hardness and DOC values into the EPA's Aluminum Criteria
Calculator V2.0;4 or,
(2)	Use the lookup tables provided in the criteria document to find the values associated with the specific
conditions of pH, total hardness and DOC.
The calculator was derived using a multiple linear regression (MLR) technique to model the interactive effects of
three parameters on the bioavailability and toxicity of aluminum to aquatic life,5 and the lookup tables were
created using results from the calculator. Throughout this document, we use the term input parameters to refer
to site-specific concurrently measured values of pH, total hardness and DOC that a state or authorized tribe may
use to derive numeric values of the criteria magnitude (outputs) that represent local conditions, using the
aluminum criteria calculator or the lookup tables.
Regardless of the method used to derive site-specific criteria values, the state or authorized tribe will need input
parameters for pH, total hardness and dissolved organic carbon at each site of interest.6 These parameters
affect the bioavailability of aluminum and its toxicity to aquatic life; however, the interactive effect of these
three parameters—pH, total hardness and dissolved organic carbon (DOC)—is not linear.7
• For example, if the concentrations of total hardness and DOC are held constant, aluminum is most
bioavailable (and therefore, most toxic) at values of high and low pH; and aluminum is least bioavailable
near values of neutral pH (again, when total hardness and DOC are held constant).
3	In accordance with the provisions of Section 304(a) of the Clean Water Act, the EPA periodically revises ambient water-
quality criteria to reflect the latest scientific knowledge. For information related to EPA's December 2018 recommended
aquatic life criteria for aluminum in freshwater, see: https://www.epa.gov/wqc/2018-final-aquatic-life-criteria-aluminum.
4	For a link to the criteria document (with lookup tables in Appendix K) and the aluminum criteria calculator (v2.0), see
https://www.epa.gOv/wqc/aquatic-life-criteria-aluminum#2018.
5	Two models, one for invertebrates and one for vertebrates, were used to normalize freshwater aluminum toxicity values.
These separate models correspond to effects on invertebrates and vertebrates due to differing effects of pH, total hardness
and DOC on aluminum bioavailability and toxicity, and therefore enable instantaneous criteria magnitude values to be
calculated as a function of the unique chemistry conditions at a given site, at the time at which pH, total hardness and DOC
were measured.
6	Methods using local data are preferred over other methods of deriving site-specific criteria; however, estimated values for
DOC may be used in the absence of local data, as described in Question 2.
7	For more information about the relationships between the pH, total hardness, DOC, and the bioavailability of aluminum to
aquatic life, please see the 2018 aluminum criterion document at: https://www.epa.gov/sites/production/files/2018-
12/documents/aluminu m-final-national-recommended-awqc.pdf.
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• Likewise, as the concentration of DOC increases (and pH and total hardness are held constant),
aluminum becomes less toxic because the aluminum binds to the DOC, making the metal less
bioavailable.
Although pH and DOC are the main factors driving aluminum bioavailability and toxicity, total hardness also has
an effect. By knowing the pH, total hardness and DOC in a waterbody, one may derive the numeric criterion
values for aluminum, for the acute exposure (i.e., the criterion maximum concentration, CMC) and the chronic
exposure (i.e., the criterion continuous concentration, CCC), that will be protective of aquatic life.
1. What flexibility does a state or authorized tribe have when adopting the EPA's
recommended aluminum criteria into its water quality standards, and what are the
advantages and potential challenges of each approach?
A state or authorized tribe may adopt an EPA-recommended method to establish numeric aluminum criteria
protective of aquatic life or may modify the Agency's recommendations to propose an alternative method that
reflects site-specific conditions that are not already incorporated into the MLR models which underpin the
aluminum criteria calculator that the EPA developed. The EPA regulations also allow States and authorized tribes
to adopt scientifically defensible criteria that differ from the EPA's recommendations, if the criteria are
protective of designated uses (in the case of these recommended criteria, the designated use is aquatic life).8
The EPA's Section 304(a) aluminum aquatic life criteria are recommendations, and not requirements. States and
authorized tribes should consider the advantages and potential challenges of each approach, as well as other
approaches that may not be described in this document.
The EPA's national 304(a) recommended aluminum criteria to protect aquatic life in freshwater ecosystems will
vary from site to site based on the values of water chemistry parameters at the site. The derivation of site-
specific criteria values relies on the adoption of both the criteria and implementation of a site-specific criterion
derivation methodology rather than a specific outcome (i.e., a constant concentration or criteria magnitude
value for a pollutant). Using a performance-based approach, the state or authorized tribe would adopt the
water-chemistry dependent criteria and a derivation methodology (both of which EPA must first approve). With
sufficient data inputs for pH, total hardness and DOC, the site-specific criteria magnitude values generated by
following the methodology are predictable and repeatable. Therefore, once a performance-based approach is
approved under CWA Section 303(c), the state or authorized tribe may derive and implement site-specific
criteria values without additional Agency review. See question 3 below for information on adoption of
implementation methods to support these criteria. States or authorized tribes may consider adopting any of the
approaches below as a methodology. More information on performance-based approaches to water quality
standards—including the elements the EPA expects to see in any submission of such an approach—may be
found in EPA Review and Approval of State and Tribal Water Quality Standards (65 FR 24641, April 27, 2000).9
8	In establishing numerical criteria to protect designated uses, states and authorized tribes should base those values on:
304(a) guidance; 304(a) guidance modified to reflect site-specific conditions; or other scientifically defensible methods. (40
CFR 131.11(b)(1)). Additionally, if a waterbody has multiple designated uses with different criteria for the same pollutant,
states and authorized tribes should protect the most sensitive use, in accordance with 40 CFR 131.11(a).
9	For more information, see 40 CFR 131.21(c) State and Tribal Water Quality Standards, "Alaska Rule" (proposed June 1999
and effective May 30, 2000) and EPA Review and Approval of State and Tribal Water Quality Standards, 65 Fed. Reg. 24641
(April 27, 2000) (codified at 45 C.F.R. 131.21(c)).
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If a state or authorized tribe chooses to adopt the recommended aluminum criteria into its water quality
standards, the EPA recommends choosing one, or a combination,10 of the following approaches (see Table 1):
(1)	adopting by reference to the applicable sections of the 304(a) criteria document (e.g., Section 4.1,
Appendix K);
(2)	adopting by reference to the Aluminum Criteria Calculator [V2.0] (note: Future versions of the calculator
may require the state or authorized tribe to update their standards to incorporate the revised calculator
by reference.);
(3)	adopting the criteria value lookup tables11; or,
(4)	adopting relevant ecoregional criteria default values.
10	For example, approaches (2) and (3) can be adopted together (the calculator can be used to derive numeric criteria values
(outputs) when input parameters (i.e., pH, total hardness or DOC measures or estimates) are not displayed in the lookup
tables). Also, ecoregional criteria default values may be adopted in addition to any of the first three listed approaches.
When adopting a combination of approaches, the state or authorized tribe should specify the conditions under which each
method should be applied.
11	If the state or authorized tribe chooses to adopt lookup tables into their water quality standards, they should include
information on interpolating or rounding data when input parameter values for pH, total hardness, or DOC fall between
values listed in the lookup tables.
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Table 1: Comparison of approaches to adopting the recommended aluminum criteria
Approach to adopting
the recommended
criteria
Advantages
Potential Challenges
(1) adopting the
applicable sections of
the 304(a) criteria
document (which
includes the criteria
value calculator and
criteria value lookup
tables).
This approach may provide the greatest
amount of background information and
context for the criteria and may also provide
the greatest flexibility to states and
authorized tribes because it allows multiple
ways to calculate criteria values.
This approach may be difficult to implement
due to individual states' or authorized
tribes' legal and administrative
requirements (e.g., whether a state's or
tribe's regulations allow them to use
incorporation by reference).
(2) adopting the
aluminum criteria
calculator [V2.0] (or
similar method to
calculate outputs based
on the underlying MLR
model equations).
This is likely the most adaptable and concise
approach. Future updates to the criteria
value calculator would likely be accompanied
by guidance.
The calculator may be viewed as a "black
box," compared to the lookup tables that
might be more familiar to some users.
Future versions of the calculator may
require the state or authorized tribe to
update their standards to incorporate the
revised calculator by reference.
(3) adopting the criteria
value lookup tables.
This may be more transparent than adopting
only the criteria value calculator because the
lookup tables do not require access to a
computer. The tables are included in
Appendix K of the publicly-available criteria
document and may be helpful when
communicating to the public about the
criteria implemented at a given site.
The state or authorized tribe may need to
develop a standard procedure to determine
which values for pH, total hardness, or DOC
to use if measured values are between the
range of input parameter values provided in
the lookup table. For example, a state or
authorized tribe may decide to always use
the nearest value in the table for each input
parameter or may decide to use the value
which would yield the most protective
criteria.
(4) adopting relevant
ecoregional12 criteria
default values.
This would allow states and authorized tribes
to apply consistent criteria throughout an
ecoregion. This approach does not require a
state to identify site-specific input
parameters because the criteria values are
the same for all sites within the ecoregion,
calculated based on representative water
quality parameters. Defaults may also help
to increase transparency of criteria for the
public if they are adequately explained. This
approach may be used in combination with
the criteria calculator or lookup tables, for
example as backup criteria for waterbodies
with insufficient input parameters.
This approach may be too general for areas
with complex geology. That is, the
approach, used without the calculator or
lookup tables, does not allow for the use of
site-specific ambient data (input
parameters for a specific site of interest)
even though there may be site-specific
exceptions within a region.
12 For more information on how ecoregions are defined, see https://www.epa.gov/eco-research/ecoregions
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After adopting the recommended aluminum criteria methodology, and obtaining EPA's approval, the state or
authorized tribe may derive site-specific criteria values (i.e., criteria magnitude values) for acute and chronic
criteria that correspond to a given ecoregion or set of site-specific conditions for pH, total hardness and DOC.
The EPA recommends that states and authorized tribes derive criteria values that will protect aquatic life over
the full range of conditions throughout the year (i.e., accounting for the variability of pH, total hardness and
DOC). Question 3 provides more information about how to derive final criteria values that will protect aquatic
life throughout the range of seasonal and flow conditions at a site, including those conditions when aluminum is
most toxic.
To promote transparent and repeatable outcomes, the EPA recommends that states and authorized tribes
consider making the input parameters, along with the calculations and the resulting values for aluminum
criteria, publicly available on-line. A map showing the extent of the site to which the criteria values apply would
also be helpful, if available, to communicate those criteria values to the public. States and authorized tribes may
also include detailed methods in an appendix to their water quality standards or in a Continuing Planning
Process document, as required by 40 CFR 130.5(b)(6). This provision requires that states include a process for
establishing and assuring implementation for new or revised water quality standards.
2. How often and over what time period should a state or authorized tribe collect input
parameter data? What if DOC data are insufficient?
The EPA recommends that states and authorized tribes concurrently collect water chemistry data for pH, total
hardness and DOC throughout the year to ensure that the collected data are representative of the temporal and
spatial variability for each waterbody or waterbody segment. Conditions of pH, total hardness and DOC may vary
within a waterbody throughout the year, thereby affecting the bioavailability of aluminum over time.13 To
ensure the criteria will be protective during the times when aluminum is most bioavailable (and most toxic), the
EPA recommends that the state or authorized tribe collect, if possible, 24 months of monthly sampling data for
the three input parameters. This approach will help to account for both intra- and inter-annual variability of the
input parameters. The EPA recognizes that not all states and authorized tribes will collect this amount of data
prior to calculating site-specific aluminum criteria values. Data may be collected for greater than or less than 24
months, and at a frequency greater than or less than monthly. However, higher quality and more consistently
collected data will provide more information to establish scientifically defensible criteria that are protective of
the designated use. If data collected is insufficient to establish local DOC values, the EPA recommends that
default DOC values may be used with concurrently collected data for pH and total hardness. Lastly, ecoregional
default values may be used in the absence of local data.
A state or authorized tribe may use the collected or default parameter values as inputs to the aluminum criteria
calculator or when using lookup tables to determine the recommended aluminum concentrations for acute and
chronic criteria values which correspond to each set of input parameters. That is, if monthly data are provided as
inputs to the calculator or lookup tables, then corresponding instantaneous criteria values will be generated for
each month data was provided. Then, for permitting or assessment purposes, the state or authorized tribe can
analyze the range of monthly instantaneous criteria values to identify criteria values that will be protective
under conditions when aluminum is most bioavailable and most toxic to aquatic life. A state or authorized tribe
13 For example, as DOC increases and pH is constant, aluminum is less bioavailable because it binds to the DOC instead of
being bioavailable for uptake into aquatic organisms. Similarly, as total hardness increases and pH is constant, aluminum is
less bioavailable because other ions in solution compete with the aluminum ions for uptake into aquatic organisms. That is,
as concentrations of calcium and magnesium (i.e., the components of hardness) increase, the relative bioavailability of
aluminum decreases.
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may find that, depending on the amount and quality of the data, different time periods and conditions
throughout the year are best protected by different criteria values. That is, the state or authorized tribe may
choose a single set of acute and chronic criteria values which are protective throughout the year or may decide
to apply two or more different sets of acute and chronic criteria values appropriate to the different time periods
and conditions for permitting and assessment purposes (e.g., seasonal criteria).
Among the input parameters to the aluminum criteria calculator (or lookup tables), DOC data are least likely to
be available. That is, states and authorized tribes may collect data for pH and total hardness more routinely than
they collect data for DOC. For waterbodies that lack sufficient DOC data, but for which pH and total hardness
data are available, the EPA recommends states and authorized tribes use suitable estimates of DOC
concentrations, in combination with concurrently measured data for pH and total hardness. The estimated DOC
and the measured pH and total hardness values can then be used together to calculate corresponding site-
specific criteria values. Because aluminum is most bioavailable and most toxic at low levels of DOC, the EPA
recommends using conservative estimates for DOC concentrations which will yield corresponding criteria values
that are more likely to ensure protection of aquatic life from the toxic effects of aluminum. States and
authorized tribes may also adopt relevant ecoregional criteria default values, as explained in Question 1, or may
use other scientifically defensible approaches, including those similar to the approach used to develop the EPA's
2016 draft missing parameters document14 to estimate input parameter values or to generate default criteria
values.
3. What methods can be used to reconcile model outputs and derive criteria values that will
result in protection of aquatic life at a site?
The EPA recommends that the final criteria values for each site be derived in a way that will protect aquatic life
throughout the range of seasonal and flow conditions at a site, including those conditions of pH, total hardness
and DOC, when aluminum is most bioavailable and toxic. There are three methods that the EPA recommends
using to derive criteria values that will protect aquatic life throughout the range of seasonal and flow conditions
at a given site. Method 1 needs the greatest amount of input parameter data and Method 3 needs the least.
Method 1: Identify protective criteria values by selecting one or more individual model outputs based
upon spatially and temporally representative site-specific measured values for model inputs. Method 1 can be
used where input datasets are complete and inputs are measured frequently enough to statistically represent
changes in the toxicity of aluminum, including conditions under which aluminum is most toxic. In this case, the
criteria values are determined by selecting one or more individual outputs that will be protective of aquatic life
under the full range of ambient conditions, including conditions of high aluminum toxicity. Method 1 could be
used to also establish criteria values to apply on a seasonal basis where the data are sufficient.
Method 2: Calculate protective criteria values from the lowest 10th percentile of the distribution of
individual model outputs, based upon spatially and temporally representative site-specific measured model
input values. While the 10th percentile of outputs should be protective in a majority of cases, certain
circumstances may warrant use of a more stringent model output (e.g. consideration of listed species). Sufficient
data to characterize the appropriate distribution of model outputs are necessary to derive a protective
percentile so that the site is protected under conditions of high aluminum toxicity.
14 Draft Technical Support Document: Recommended Estimates for Missing Water Quality Parameters for Application in
EPA's Biotic Ligand Model, EPA-820-R-15-106, March 2016 (https://www.epa.gov/sites/production/files/2016-
02/documents/draft-tsd-recommended-blm-parameters.pdf)
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Method 3: Select the lowest model outputs (the lowest CMC and the lowest CCC) calculated from
spatially and temporally representative input datasets that capture the most toxic conditions at a site as the
criteria values. EPA recommends Method 3 be used where ten or fewer individual model outputs are available.
Either Method 2 or 3 is particularly useful when values of acute and chronic criteria need to be protective of the
more toxic of site conditions to be implemented, such as for National Pollutant Discharge Elimination System
(NPDES) permitting (as discussed under Question 4). In order to maximize transparency, defensibility, and
regulatory certainty, states and authorized tribes should consider developing written implementation methods
and make these documents available to the public. To the same end, the EPA recommends states or authorized
tribes make publicly available the following on the state's or authorized tribe's website:
•	Site-specific water chemistry data, including the inputs used in the aluminum criteria value calculations
and resultant criteria values, and
•	The geographic extent of each site.
As mentioned in Question 1, a state or authorized tribe may choose to adopt ecoregional default criteria for all
or some of the waterbodies within the state or tribal jurisdiction. Where a state or authorized tribe chooses to
adopt ecoregional default criteria values, the state or authorized tribe does not need a method for reconciling
criteria calculator outputs (because the ecoregional default criteria values are constants that are independent of
the criteria calculator).
4. How can a state or authorized tribe implement the aluminum criteria in its Clean Water Act
programs?
For NPDES permitting, waterbody assessments and development of total maximum daily loads (TMDLs), states
and authorized tribes can use different methods to derive site-specific criteria values (as discussed in the answer
to Question 3 above). States and authorized tribes should ensure that the methods used are transparent and
predictable, and that they produce repeatable outcomes.
Making information available to the public, the regulated community and other stakeholders is important to
ensuring regulatory certainty and clarity, particularly when a state or authorized tribe adopts a performance-
based approach. For example,
•	states or authorized tribes may wish to describe how they derived the criteria values, including the data
and data source used;
•	the permitting authority may wish to describe in the permit factsheet or statement of basis how it used
the numeric criteria values to determine reasonable potential and derive water quality-based effluent
limits (WQBELs), if needed;
•	states and authorized tribes may wish to describe in TMDL documents how they derived the criteria
values and used them to determine TMDL targets; or,
•	states and authorized tribes may wish to describe how they derived site-specific values for the
aluminum criteria in assessment methodologies and integrated reports for each assessed waterbody.
Pursuant to 40 CFR 131.10(b), states and authorized tribes must take into consideration water quality standards
of downstream waters when designating uses and adopting criteria for instream waters to ensure its water
quality standards provide for the attainment and maintenance of the water quality standards for downstream
waters. The EPA recommends that states and authorized tribes consider water chemistry conditions
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downstream from a point of discharge when deriving aluminum criteria values on a site-specific basis. Because
metals are generally persistent and can travel long distances downstream, calculating a criterion value using
values for pH, total hardness and DOC from a location at or near a discharge point could result in a criterion
value that may not be protective of other areas or downstream waters that have different values of pH, total
hardness and DOC. The EPA recommends that states and authorized tribes also account for tributary sources of
pH, total hardness or DOC that might affect protectiveness downstream for the aluminum discharged at an
upstream point; hence it is recommended that criteria concentration calculations take into account the range of
downstream effects of the discharge.
For NPDES permitting purposes, the EPA recommends that states and authorized tribes collect sufficiently
representative data for pH, total hardness and DOC to ensure that conditions in the waterbody are being
adequately captured downstream from the point of discharge. If a discharge is controlled so that it does not
cause water quality standards to be exceeded in the receiving water under critical conditions, then it is
reasonable to conclude that water quality standards should be attained under all other conditions. Criteria that
will be protective for the more toxic of site conditions should be used to develop WQBELs. Once criteria values
protective for the more toxic conditions are calculated, critical low flows—for the purposes of dilution of the
pollutant concentration in effluent, combined with effluent concentrations of the pollutant—may be used to
establish whether there is reasonable potential for the discharge to cause or contribute to an impairment and
therefore a need to establish WQBELs. The U.S. Environmental Protection Agency's NPDES Permit Writers'
Manual15 describes the importance of characterizing critical conditions for the effluent and the receiving water.
Section 4.5.1 of the Technical Support Document for Water Quality-based Toxics Control explains that, where
adequate data exist, dynamic modeling techniques may be used in lieu of steady-state modeling using critical
conditions.16 Permit writers may also choose to establish tiered effluent limits. The EPA recommends that, in the
context of permit renewals, WQBELs be reevaluated when changes to water chemistry are evident or suspected.
Aluminum toxicity in receiving waters could change as the result of a newly permitted discharge or modification
of an existing discharge, land-use changes or changes to hydrologic conditions; all of which may affect pH, total
hardness and DOC. Additionally, site characterization is important: as the size of a site increases, the spatial and
temporal variability are likely to increase. Thus, more water samples may be needed to adequately characterize
the entire site.17
TMDL and NPDES analysis generally includes considerations for critical conditions. Implementation procedures
should clearly define how permit writers will consider critical conditions related to critical low flows and the
greatest bioavailability and toxicity of aluminum. This should ensure that reasonable potential is properly
assessed and, if needed, appropriate permit limits are established that fully protect aquatic-life beneficial uses
under the full range of environmental conditions.
15	USEPA. 2010. NPDES Permit Writers' Manual. U.S. Environmental Protection Agency, Office of Water, Washington, D.C.
EPA-833-K-10-001. September 2010.
16	USEPA. 1991. Technical Support Document for Water Quality-based Toxics Control. U.S Environmental Protection Agency,
Office of Water, Washington, D.C. EPA/505/2-90-001. March 1991.
17	For information on site characterization, see: USEPA. 1994. Interim Guidance on Determination and Use of Water-Effect
Ratios for Metals. U.S. Environmental Protection Agency, Office of Water, Washington, D.C. EPA-823-B-94-001. February
1994.
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