Toxicity of quaternary mixtures of phenolic compounds and formulated glyphosate to microbial community of river water

The toxicity of mixtures of formulated glyphosate, phenol, 4-chlorophenol and 2,4-dichlorophenol to river water microbial community was investigated using non specific dehydrogenase activity as endpoint. The microbial community was exposed to individual toxicant and quaternary mixture ratios at concentrations ranging from 50 mg L-1 to 3000 mg L-1 over 24 h. The ecological doses (EC50) as estimated from logistic or hormesis dose-effect models were 1317.946 ± 51.460 mg L-1, 1046.414 ± 65.534 mg L-1, 85.080 ± 4.468 mg L-1 and 60.897 ± 5.199 mg L-1 for formulated glyphosate, phenol, 4-chlorophenol and 2,4-dichlorophenol respectively. The toxicity and interaction of the mixtures were evaluated using concentration addition and toxic index models. The model deviation ratios (MDR) ranged from 0.553 ± 0.024 to 1.096 ± 0.021 while the toxic index (TI) varied between 0.912 ± 0.017 and 1.810 ± 0.078. However, the MDR and TI for all mixtures lie between 0.5 and 2.0. Thus, the joint action of the mixtures were considered additive.