Acute Toxicity of Binary Mixtures of Five Cations (Cu2+, Cd2+, Zn2+, Mg2+, and K+) to the Freshwater Amphipod Gammarus lacustris (Sars): Alternative Descriptive Models

Abstract

The combined results of 10 acute toxicity experiments, each testing the joint toxicity of two of the ions Cu2+, Cd2+, Zn2+, Mg2+, and K+, were examined in terms of different response surface models which could be used to make decisions about limiting toxic components in mixtures. The classical probit model for simple similar action described experimental results satisfactorily with a model R2 of 0.282; equations in which probit(p) was described directly by a linear combination of toxicant concentrations fit data significantly better, with an R2 of 0.527. Equations with more complex linear terms and appropriate weighting factors applied to the residual sums of squares yielded R2 values up to 0.931. Predicted LC50 values were midrange compared with published values. Based on the linear description of the probit response, K or Mg in combination with either of Cu, Cd, or Zn had additive effects, the combinations Cu and Cd, Cu and Zn, and Cd and Zn had more-than-additive effects, and Mg and K had less-than-additive effects. The relationships between the response surfaces, other described modes of joint action, the toxic units model, and mixture toxicity indices are discussed.