Although it is well known that increasing water hardness and dissolved organic carbon (DOC) concentrations mitigate the toxicity of aluminum (Al) to freshwater organisms in acidic water (i.e., pH < 6), these effects are less well characterized in natural waters at circumneutral pHs for which most aquatic life regulatory protection criteria apply (i.e., pH 6-8). The evaluation of Al toxicity under varying pH conditions may also be confounded by the presence of Al hydroxides and freshly precipitated Al in newly prepared test solutions. Aging and filtration of test solutions were found to greatly reduce toxicity, suggesting that toxicity from transient forms of Al could be minimized and that precipitated Al hydroxides contribute significantly to Al toxicity under circumneutral conditions, rather than dissolved or monomeric forms. Increasing pH, hardness, and DOC were found to have a protective effect against Al toxicity for fish (Pimephales promelas) and invertebrates (Ceriodaphnia dubia, Daphnia magna). For algae (Pseudokirchneriella subcapitata), the protective effects of increased hardness were only apparent at pH 6, less so at pH 7, and at pH 8, increased hardness appeared to increase the sensitivity of algae to Al. The results support the need for water quality-based aquatic life protection criteria for Al, rather than fixed value criteria, as being a more accurate predictor of Al toxicity in natural waters. Environ Toxicol Chem 2018;37:49-60. C 2017 SETAC

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Acceptance of the biotic ligand model (BLM) to derive aquatic life criteria, for metals in general and copper (Cu) in particular, is growing among regulatory agencies worldwide. Thus, it is important to ensure that water quality data are used appropriately and consistently in deriving such criteria. Here we present a suggested BLM implementation framework (hereafter referred to as "the Framework") to help guide the decision-making process when designing sampling and analysis programs for use of the BLM to derive water quality criteria applied on a site-specific basis. Such a framework will help inform stakeholders on the requirements needed to derive BLM-based criteria, and thus ensure that the appropriate types and amount of data are being collected and interpreted. The Framework was developed for calculating BLM-based criteria when data are available from multiple sampling locations on a stream. The Framework aspires to promote consistency when applying the BLM across data sets of disparate water quality, data quantity, and spatial and temporal representativeness and is meant to be flexible to maximize applicability over a wide range of scenarios. Therefore, the Framework allows for a certain level of interpretation and adjustment to address the issues unique to each data set. Integr Environ Assess Manag 2018;14:736-749. © 2018 SETAC.