Ecotoxicity of toxicant mixtures in soils. Recommendations for application in the Dutch regulatory context, as derived from a scientific review on approaches, models and data.

This report proposes an approach to address the ecotoxicological risks of mixtures in soils that can be applied in the context of various practical risk assessment problems. The approach is derived from an evaluation of recent scientific developments and validation studies. An explicit distinction is made between the possibilities to interpret experimental data on mixture effects in detail and the extrapolation of these findings to the field of risk assessment. The proposed approach consists of three steps, through which environmental, toxicological and ecological interactions are addressed separately. In the first step exposure is addressed. In the second step, a toxicological 'mixed-model' approach is applied. That is: the total toxic pressure of groups of compounds with the same toxic mode of action is predicted by assuming concentration additivity for the biologically active fractions within such groups, while the overall risks over all such groups and remaining compounds (with a unique toxic mode of action in such a mixture) is predicted by assuming response addition for the biologically active fractions. Third, ecological interactions need to be addressed. However, concerning the last step, theory development is weak, and data are hardly available - if at all. Options for implementation of the proposed approach in risk assessment practice are discussed, by listing advantages and disadvantages of implementing the proposed approach