Metal mixtures - Is 'concentration addition' the correct model for terrestrial ecological risk assessment?

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Canada's mining industry an important economic driver - it employs over 300,000 Canadians and is the largest private employer of aboriginal Canadians, contributing 3.5% of Canada's GDP. In Canada, the risk assessment for a site with multiple contaminants is based on the concentration of each contaminant of concern (CoC) relative to its toxicological reference value (TRV); if this ratio for each metal, termed a hazard quotient (HQ), is less than 1, then no adverse effect is expected. The expected toxicity of metal mixtures is not considered. The concentration addition (CA) theory of mixtures toxicity is that the effect of individual CoC is assumed to sum. The proposed research will evaluate if CA is a valid theoretical model for the risk assessment of metal mixtures in soils when the concentration of metals in the mixture is elevated relative to the CCME's soil quality guidelines (SQGs). We will test the CA theory by: (a) developing toxicity thresholds for spiked metal mixtures for three different phyla: microbial processes, invertebrates and plants across a variety of landuses, (b) comparing this response to metal mixtures to single dose responses calculated through the European Union's Predicated No Effect Concentration (PNEC) model, and (c) comparing this response to spiked mixtures from a suite of field-contaminated soils. In brief, the experimental approach is to field-collect reference and metal-mixture-contaminated soils from a variety of locations in Canada, spike the reference soils with single metals and metal mixtures of Co, Cu, Ni, Pb and Zn, from which dose-toxicity relationships will be determined for soil organisms, and use the field-contaminated soils for the 'whole mixture' validation. Multiple toxicity endpoints will be determined for a number of phyla, and true, realized dose of soil metals to receptors will be determined using biomimetics. Achieving these objectives will test the robustness of the CA assumption for soil risk assessment and offer predictive tools for sites with complex mixtures. If successful, this project will reduce the need for determination of site specific remediation objectives by providing a robust process for determining site specific estimates of metal mixture toxicity.