2,4-Dichlorophenoxyacetic acid (2,4-D), a phenoxyalkanoic acid herbicide, is among the most widely distributed pollutants in the environment. 2,4-Dichlorophenol (2,4-DCP), as the main metabolite of 2,4-D, always accompanies 2,4-D. In this paper, we did research on the combined toxicities of 2,4-D and 2,4-DCP to Vibrio qinghaiensis sp.-Q67 (Q67) and Caenorhabditis elegans . It was found that the toxicity of 2,4-DCP is more severe than that of its parent 2,4-D at any concentration levels whether to Q67 or to C. elegans . Furthermore, 2,4-DCP to Q67 has the time-dependent toxicity. The toxicity of the mixture of 2,4-D and 2,4-DCP to Q67 is increasing with the exposure time, but that to C. elegans does not change over time. There is a good linear relationship between the pEC 50 /pLC 50 value of binary mixture ray of 2,4-D and 2,4-DCP and the mixture ratio of 2,4-DCP, which implies the predictability of mixture toxicity of 2,4-D and 2,4-DCP. The toxicological interactions of the binary mixtures to Q67 are basically additive actions whether at 0.25 or at 12 h. However, most mixtures have antagonistic interactions against C. elegans .

Pesticides and their metabolites often coexist in the real environment. The combined toxicity (synergism or antagonism) between pesticide and metabolites directly affects the environment risk assessment of pesticide. Dichlorvos (A) has three main metabolites, 2,2-dichloroethanol (B), 2,2-dichloroacetic acid (C) and dimethyl phosphate (D), in water and soil environment. Under different environmental conditions, metabolites with various concentration compositions form a variety of mixtures with dichlorvos. In this paper, five mixture rays with different mixture ratios were selected by optimal experimental design method. A typical aquatic (Vibrio qinghaiensis sp.-Q67) and a soil organisms (Caenorhabditis elegans) were selected as the tested organisms. The photoluminescence inhibitory toxicity (IT) of parent A and its metabolites B, C and D as well as their mixtures to Q67 and the lethal toxicity (LT) to C. elegans at different exposure time and concentration levels were determined by microplate toxicity analysis. The combination index with 95% observation-based confidence intervals was used to evaluate the change of combined toxicity of each mixture ray under different exposure times and the concentration levels. The results showed that the ITs of parent A and two metabolites C and D to Q67 do not change with the exposure time, but the IT of metabolite B at 12 h is significantly larger than that at 0.25 h. However, at two exposure times, the IT of parent A is greater than that of any of metabolites. The LTs of A and B, C and D to C. elegans do not change with the exposure time. The LTs of A, C and D to C. elegans are basically the same and significantly greater than that of B. The ITs of five mixture rays to Q67 at 12 h are significantly greater than those at 0.25 h at various concentration levels. The combined toxicities of the mixture rays to Q67 are concentration additive at low concentration levels and antagonistic at high concentration levels whether at 0.25 h or 12 h. For C. elegans, the LTs of five mixture rays at various concentration levels do not basically change with the exposure time. At two exposure times (12 h and 24 h), the combined toxicities of mixture rays are concentration additive except for the slight antagonism in the rays of R2 and R5. Keywords pesticide; multicomponent mixture; uniform design ray; basic concentration composition; time-dependent toxicity