Modelling multiple metal toxic effects in the field – evaluation of the Toxicity Binding Model (TBM)

Summary • Understanding metal and proton toxicity under field conditions requires consideration of the complex nature of chemicals in mixtures. Here, we demonstrate a novel method for relating the stream and river water concentrations of cationic species to a field ecological variable. The model WHAM-FTOX postulates that non-specific binding sites on or in aquatic macroinvertebrates can be represented by the functional groups of natural organic matter (humic acid), as described by the Windermere Humic Aqueous Model (WHAM6). • Using quantile regression on published data from over 400 sites across three continents, complex water chemistries were condensed into a single linear function that relates the combined toxicities of metals and H+ to the species richness of Ephemeroptera, Trichoptera and Plecoptera. The toxicity function (FTOX) is the sum of the products of the bound concentration and a toxicity coefficient (M), for each metal or the proton. A lower threshold of FTOX is defined, below which toxic effects are absent, and an upper threshold above which organisms are absent. • The available field data, from waters affected by acid deposition and abandoned mines, permitted the derivation of parameters for four cations, with values of M following the sequence Cu > Zn > Al > H+. For waters affected mainly by H+ and Al, FTOX shows a steady decline with increasing pH, crossing the lower threshold near to pH 7. Competition effects mean that toxicity due to Cu and Zn is most significant between pH 6 and pH 8. • It should be clearly recognised that our results do not deny the environmental toxicity of heavy metals other than Cu and Zn (e.g. Ni, Cd, Hg, Pb) – it is simply that their toxicities are not expressed at the study sites used in this work. • WHAM-FTOX is a plausible model describing the toxicity of mixtures of metals and protons, based on chemical speciation concepts, and as such is a significant forward step. The results are consistent with mixture dose-response relationships in the field, supporting and extending previous conclusions based on a much smaller data set. • Calculations with the parameterised model for different streamwater discharges suggest that in some streams, changes in the concentration of the important competing cation Ca can at least sometimes compensate for changes in heavy metal concentration. However, in systems that are only acidified (not impacted by heavy metals) low pH conditions brought about by high discharge are substantially more toxic than low-discharge conditions. • The analysis presented here is incomplete, limited by the available published data, and further research is clearly desirable. This could include further field work, laboratory work, or the analysis of existing toxicity data. With respect to field studies, it would be especially helpful to work on systems with fewer unknown factors, or with well-known factors, so there is less reliance on quantile regression. • Several applications of the model can be envisaged. Scientifically WHAM-FTOX provides a conceptual and quantitative framework within which to evaluate mixture toxicity effects, the contributions of individual components, and the role played by “protective” cationic metals such as Mg and Ca. The model provides a means to estimate and predict actual toxicity effects in the field, as well as site-specific Environmental Quality Standards. It could be useful to guide remediation activities, including cost-benefits, which might be especially valuable for abandoned mines.