The Effect of Natural Organic Ligands on Trace Metal Speciation in San Francisco Bay: Implications for Water Quality Criteria

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Bioavailability, Voltammetry, Organic complexation, San Francisco Bay, Toxicity, Biotic ligand model, Chemical speciation, Organic matter, Trace metals

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Speciation affects bioavailability, the latter of which is a prerequisite for toxic effects. At present, a recognized shortcoming in some speciation models is inadequacies in the specification of interactions between trace metals and natural organic matter in some environmental settings. In particular, naturally occurring organic ligands, which are referred to as strong metal-binding ligands, are often neglected in speciation calculations. This work utilizes a recently developed approach to the determination of ligand concentrations and conditional stability constants for strong metal-binding ligands to characterize strong copper (Cu)-binding ligands in San Francisco Bay, a complex multi-reach estuary with site-specific environmental variability. We find that increased precision in ligand concentrations and conditional stability constant parameters enables the differentiation of ligand signatures, and multivariate analysis shows that ligand signatures vary consistently with expectations from seasonal and geographical changes within the bay. We also use the parameters in speciation calculations to assess what effect site-specific characterization of organic matter might have. While all speciation calculations indicate that the majority of Cu is bound to organic matter, when strong metal-binding ligands are included, for all sites the majority of Cu is bound to strong Cu-binding ligands. This shift in speciation has important implications for bioavailability and toxicity, which we address with the biotic ligand model (BLM), a U.S. EPA speciation-based mechanistic model to develop water quality criteria that is not yet fully extensible to estuarine and marine environments. We find that the toxicity predicted to Mytilus sp. in the presence of strong Cu-binding ligands is significantly less than in BLM default calculations, consistent with a BLM bias previously reported for San Francisco Bay. The approach taken here is generalizable to many metals and natural aqueous systems, most speciation models, and, with respect to the BLM, any biological species for which the model is used.

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Marine Chemistry, v. 173, p. 269-281