Relationship of Aquatic Natural Organic Material Characteristics to the Toxicity of Selected Insecticides

Toxicities of the commercial insecticide formulations of azinphos methyl, chlorpyrifos, fenvalerate, and methyl parathion were evaluated in streamwater samples containing natural organic material (NOM) using a modification of a bacterial bioluminescence assay. Toxicity reduction of azinphos methyl was significantly (P < 0.05) correlated with water sample nonvolatile total solids (NVTS) concentration. Toxicity reductions of fenvalerate and of methyl parathion were significantly correlated to E4/E6 absorbance ratio of NOM, and to the proportion of NOM having molecular weight greater than 3700 Da. Toxicity reductions of fenvalerate and methyl parathion had significant correlations to the concentration of NVTS in water samples. Linear regressions of insecticide EC50 values to these NOM characteristics were used to mathematically model the effect of NOM on insecticide toxicity. Insecticide logKOW values were significantly correlated to the slopes of the EC50-total organic carbon (TOC) concentration regression lines. This confirms that the level of association of some nonionic organic contaminants with NOM is a function of the contaminant′s water solubility. No single NOM characteristic was the best predictor of toxicity for all insecticides examined. However, these findings suggest that the influence of NOM on toxicity results primarily from differences in TOC content and abundance of larger molecular size NOM.