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 copper, zinc, cadmium and lead, 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 CaCl2- 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 cadmium and zinc to the soil decreased in the presence of other metals, whereas the sorption of copper and lead 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 cadmium. 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 cadmium to the mixture was the most important factor associated with antagonism. Dose ratio-dependent deviations related to cadmium 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 to leached soils suggest that chloride contributed to the toxicity of the metal salts and may explain the dose level-dependent deviations.