Systematic Evaluation of Chronic Metal-Mixture Toxicity to Three Species and Implications for Risk Assessment

Metal contamination generally occurs
as mixtures. However, it is
yet unresolved how to address metal mixtures in risk assessment. Therefore,
using consistent methodologies, we have set up experiments to identify
which mixture model applies best at low-level effects, i.e., the independent
action (IA) or concentration addition (CA) reference model. The toxicity
of metal mixtures (Ni, Zn, Cu, Cd, and Pb) to Daphnia magna,
Ceriodaphnia dubia, and Hordeum vulgare was
investigated in different waters or soils, totaling 30 different experiments.
Some mixtures of different metals, each individually causing <10%
inhibition, yielded much larger inhibition (up to 66%) when dosed
in combination. In general, IA was most accurate in predicting mixture
toxicity, while CA was the most conservative. At low-effect levels
important in risk assessments, CA overestimated mixture toxicity to
daphnids and H. vulgare, on average, with a factor
1.4 to 3.6. Observed mixture interactions could be related to bioavailability
or by competition interactions, either for binding sites of dissolved
organic carbon or for biotic ligand sites. Our study suggests that
the current metal-by-metal approach in risk evaluations may not be
conservative enough for metal mixtures.