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Environmental problems - Chemical approaches
RESEARCH ARTICLE

Evaluating the concentration addition approach for describing expected toxicity of a ternary metal mixture (Ni, Cu, Cd) using metal speciation and response surface regression

Yamini Gopalapillai A B and Beverley Hale A
+ Author Affiliations
- Author Affiliations

A School of Environmental Sciences, University of Guelph, Guelph, ON, N1G 2W1, Canada.

B Corresponding author. Email: ygopalap@uoguelph.ca

Environmental Chemistry 13(3) 447-456 https://doi.org/10.1071/EN15124
Submitted: 16 June 2015  Accepted: 31 July 2015   Published: 8 October 2015

Environmental context. Environmental quality guidelines are often based on an ‘additive’ approach using single metal toxicity values. We evaluated the ‘additive’ approach by testing it on three priority pollutant metals (Ni, Cu, Cd), and found that the toxicity of the metal mixture was less than additive when dose was expressed as total metal concentration, but it was additive when dose was expressed as bioavailable metal. We suggest that for metal mixtures, a more realistic indicator of risk is provided by calculations based on the bioavailable form of metals.

Abstract. Aquatic environments containing elevated metal concentrations, such as natural waters receiving mining effluents, are often a mixture of metals because mineral deposits are commonly an association of multiple metals. Water quality guidelines for protection of aquatic life are not designed for multiple toxicants but are overwhelmingly based on dose–response studies of a single toxicant and assuming additivity. Resolving the uncertainty in risk assessment for metal mixtures in waters surrounding Canada’s many current and legacy extractive mining sites is a high priority for both government and base metal mining companies. Our study evaluated the ‘concentration addition’ approach to predicting the chronic toxicity of a ternary metal mixture (Ni, Cu, Cd) to Lemna minor (a free-floating macrophyte used in biomonitoring of mining effluents) using either total metal concentration as dose or free-ion activity. The aim was to fill several data gaps in mixture toxicity studies, such as: inclusion of water chemistry to calculate metal speciation, test species other than the commonly studied rainbow trout and cladocerans, and test mixture effects on chronic toxicity. Results indicate that toxicity of Ni, Cu plus Cd to L. minor was less than additive (overestimated toxicity) when expressed as total metal concentration but was additive when expressed as free ion (the bioavailable form). We suggest that applying single-element quality guidelines ‘additively’ for plants is likely to overestimate risk to the ecosystem from metal mixtures, and that the use of a concentration addition approach based on the bioavailable form of metals provides a more realistic indicator of risk.


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