Single metal and metal mixture toxicity of five metals to Oppia nitens in five different Canadian soils.

Metal mixture toxicity across soil types is a daunting challenge to risk assessment. Here, we evaluated metal mixture toxicity in Oppia nitens, using ten fixed metal mixture ratios in five Canadian soils that closely matched some of the EU PNEC reference soils. Soils were dosed with five metals (Cu, Zn, Pb, Co, Ni) as single metals (ten concentrations) and as mixtures (eight concentrations). Synchronized adult mites were exposed to metals, with survival and reproduction assessed after 28 days. We found out that (i) the differences among soils in mite sensitivity and single metals were not consistent when mites were exposed to metal mixtures, (ii) assuming concentration addition, the mixture interaction factor (MIF) showed that single metal low effect levels excessively underestimated low level metal mixture effects (iii) Zn emerged as a protective metal in most mixtures, and (iv) Soil properties such as CEC, independent of effects on metal speciation, explained more of the variation than measured metals. This study suggests that metal risk assessment should be done on a case by case basis. Further work is needed to ensure that by protecting soil-dwelling organisms from single metals, the risk from metal mixtures is appropriately protected for.