Metal mixture toxicity to aquatic biota in laboratory experiments: application of the WHAM-FTOX model

The WHAM-FTOXmodel describes the combined toxic effects of protons and metal cations towards aquaticorganisms through the toxicity function (FTOX), a linear combination of the products of organism-boundcation and a toxic potency coefficient (˛i) for each cation. Organism-bound, metabolically-active, cation isquantified by the proxy variable, amount bound by humic acid (HA), as predicted by the WHAM chemicalspeciation model. We compared published measured accumulations of metals by living organisms (bacte-ria, algae, invertebrates) in different solutions, with WHAM predictions of metal binding to humic acid inthe same solutions. After adjustment for differences in binding site density, the predictions were in rea-sonable line with observations (for logarithmic variables, r2= 0.89, root mean squared deviation = 0.44),supporting the use of HA binding as a proxy. Calculated loadings of H+, Al, Cu, Zn, Cd, Pb and UO2wereused to fit observed toxic effects in 11 published mixture toxicity experiments involving bacteria, macro-phytes, invertebrates and fish. Overall, WHAM-FTOXgave slightly better fits than a conventional additivemodel based on solution concentrations. From the derived values of ˛i, the toxicity of bound cationscan tentatively be ranked in the order: H < Al < (Zn–Cu–Pb–UO2) < Cd. The WHAM-FTOXanalysis indicatesmuch narrower ranges of differences amongst individual organisms in metal toxicity tests than was pre-viously thought. The model potentially provides a means to encapsulate knowledge contained withinlaboratory data, thereby permitting its application to field situations.