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Federal Register / Vol. 86, No. 52 / Friday, March 19, 2021 / Rules and Regulations
freshwater aluminum criteria for Oregon. To support the State, the proposed rulemaking identified a range of acceptable approaches for the State and the commenting public to consider.
The State may elect to utilize one or more of the approaches or to implement the final aluminum criteria in other ways that are consistent with 40 CFR
part 131.
For CWA implementation purposes, the State will need to identify one or more outputs from the calculator or a value derived from a scientifically defensible percentile of the distribution of the output values as the magnitudes of the criteria, to be applied together with duration and frequency, to protect the water body under the range of water chemistry conditions at a site. In practice, EPA expects the State to collect sufficient data to characterize the most toxic conditions at a site. The State could collect samples for the input parameters concurrently or as close to the same time as possible while representing the same environmental condition, and could use default values if appropriate where data are unavailable or insufficient to capture the variability in conditions. The ways by which the State may evaluate sufficiency are described in more detail below.
The proposal preamble described three example approaches that the State could use to calculate criteria values when multiple calculator outputs, representing different ambient conditions over time, are available i.e., how to reconcile multiple calculator outputs. EPA agrees with commenters suggestions that further development and implementation of these approaches should be left to the States discretion, and that the term used to identify one or more protective values, reconcile, was not appropriate to describe how the State should manage multiple calculator outputs. The appropriate approach for each circumstance will depend primarily on data availability and on the programmatic purposes for which criteria values are being calculated. For purposes which require forecasting a protective loading allocation under varying ambient conditions, for example, the State could calculate a single set of numeric criteria values CMC and CCC by choosing the lowest output or a low percentile of the outputs of multiple calculator runs, or use conservative default values.
Oregon should ensure that sufficiently representative data are collected for the calculators input parameters pH, DOC, and total hardness to have confidence that the most toxic conditions are adequately characterized. To
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accomplish this, Oregon may evaluate the input parameter data and resultant criteria values that are calculated over time for different flows and seasons through the use of appropriate statistical methods, such as Monte Carlo 11
simulation. One consideration when defining a site to which to apply criteria for aluminum is whether the concentration of metals are generally consistent throughout the area. As the size of a site increases, the spatial and temporal variability is likely to increase;
thus, more water samples may be required to adequately characterize the entire site. Implementation materials that outline the States approaches will help provide transparency for the public and predictable, repeatable outcomes.
Additional transparency and public accountability will be achieved if Oregon makes publicly available each sites ambient water chemistry data, including the inputs used in the aluminum criteria value calculations, resultant criteria values, and the geographic extent of the site.
Similarly, NPDES permit effluent limits that are derived from the aluminum criteria calculator should be sufficiently explained in Fact Sheets or Statements of Basis. This includes providing an explanation of how the aluminum criteria values were calculated; the input data or summary of input data and source of data; and how criteria values were used to determine whether the discharge would cause or have the reasonable potential to cause or contribute to an excursion above the aluminum criteria, and if so, how the values were used to derive WQBELs for aluminum. The States assessment methodology and any TMDLs developed for waters impaired for aluminum criteria developed using the calculator should also be adequately explained for transparency and public accountability in TMDL documents and Integrated Reports, as appropriate.
Substantial changes in a water bodys ambient input parameter concentrations will likely affect aluminum toxicity at that site. In addition, as a robust, sitespecific dataset is developed with regular monitoring, criteria values previously calculated by the State can be updated to more accurately reflect site conditions. The State may wish to revisit calculated aluminum criteria values periodically for example, with each CWA Section 303d listing cycle or WQS triennial review or when
changes in water chemistry are evident or suspected at a site and as additional monitoring data become available. This will ensure that the criteria values used for implementing CWA programs accurately reflect the toxicity of aluminum and remain protective of the aquatic life designated uses including when aluminum is most toxic.

11 Given sufficient data, Monte Carlo simulation or equivalent analysis can be used to determine the probability of identifying the most toxic time period for a series of monitoring scenarios. From such an analysis, the State can select an appropriate monitoring regime.

12 Rodriguez PH, Arbildua JJ, Villavicencio G, Urrestarazu P, Opazo M, Cardwell AS, Stubblefield W, Nordheim E, Adams W. 2019. Determination of bioavailable aluminum in natural waters in the presence of suspended solids. Environmental Toxicology and Chemistry. 388:16681681.

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Analytical Methods Considerations As discussed earlier, the forms of aluminum introduced into the laboratory toxicity tests upon which EPA relied for criteria development do not include suspended solids or clays where aluminum may be bound.
Aluminum bound in suspended solids or clays would be extracted when using total recoverable methods that have a strong acid pH <2 digestion step, but these forms of aluminum would not be biologically available to aquatic species in ambient waters. Empirical laboratory chronic 7-day testing with Ceriodaphnia dubia investigating survival and reproduction endpoints indicates that total recoverable pH
0.05 to +0.7 digestion and bioavailable measurements of aluminum in lab waters are essentially equal up to approximately 1 mg/L of aluminum.12
Studies are currently being conducted at Oregon State University with test solutions with greater than 1 mg/L of aluminum to better understand the relationship between the total recoverable and bioavailable analytical methods at concentrations above 1 mg/
L. Initial studies indicate there is little variability between total recoverable and bioavailable aluminum above 1 mg/
L in lab waters because the laboratory waters do not include clays or suspended solids.
It is not necessary to apply a conversion or translation factor to compare field measurements using a bioavailable method against the promulgated aluminum total recoverable criteria. This is because both bioavailable and total recoverable analytical methods quantify the toxic fraction of aluminum equivalently in laboratory test waters given that standard toxicity test waters do not include suspended solids or clays per test protocols. For NPDES compliance monitoring and reporting, total recoverable measurements for metals are required. By comparison, for ambient water measurements, analytical methods that measure bioavailable
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