The effect of leaching on the joint toxicity of a complex metal mixture to Folsomia candida in relation to bioavailability in soil.

Metal-contaminated soils generally contain mixtures rather than single metals. Laboratory toxicity tests often focus on single metals in soils freshly spiked with soluble metal salts, potentially overestimating bioavailability in field soils. This study determined the toxicity of mixtures of Cu, Zn, Cd, and Pb dosed as chloride salts to the springtail Folsomia candida in LUFA 2.2 soil that was either left as is after spiking or leached to remove the chloride counterion. Effects on survival, growth, and reproduction were related to total, 0.01 M calcium chloride(CaCl2)-extractable, and water-extractable metal concentrations in the soil and to internal concentrations in the springtails. Leaching the spiked soil adequately removed the counterion with relatively small metal losses. The sorption of Cd and Zn to the soil decreased in the presence of other metals, whereas the sorption of Cu and Pb was not changed. Metal uptake by the springtails was not affected by the other metals but decreased at high chloride concentrations. Leaching did not change metal uptake in the springtails, suggesting no direct influence of chloride competition. However, leaching reduced metal toxicity, except for Cd. Mixture toxicity showed overall antagonism for all metal fractions and all endpoints, with dose ratio- and dose level-dependent deviations from concentration addition. The relative contribution of Cd to the mixture was the most important factor associated with antagonism. Dose ratio-dependent deviations related to Cd may be explained by its high toxicity combined with the large effect of other metals on its sorption. Changes in ecotoxicological effects and metal uptake at high mixture concentrations in unleached compared with leached soils suggest that chloride contributed to the toxicity of the metal salts and may explain the dose level-dependent deviations.