EVALUATION OF HUMIC PESTICIDE INTERACTIONS ON THE ACUTE TOXICITY OF SELECTED ORGANOPHOSPHATE AND CARBAMATE INSECTICIDES

The ability of water resource managers to accurately predict the toxicity of agricultural chemicals in aquatic ecosystems is essential for the development of reliable water quality standards. At present, accurate predictions are difficult due to the paucity of data concerning the interaction of these chemicals with organic constituents of the freshwater chemical matrix. Dissolved humic materials are ubiquitous components of freshwater ecosystems and can affect the availability and toxicity of chemicals in the environment. In the present investigation, the influence of humic acid (HA) on the acute toxicity of selected organophosphate and carbamate insecticides was examined using a bacterial bioluminescence inhibition assay. EC50 values for the pesticides were determined in the presence of 0, 0.5, 5.0, 50, and 100 mg/liter HA. A combination of antagonistic and synergistic effects was observed with the selected organophosphate and carbamate insecticides examined. HA significantly reduced the toxicity of azinophos-methyl, chlorpyrifos, and carbofuran, while enhanced toxicity was observed with methyl parathion and carbaryl. These results indicate that humic-pesticide interactions can alter the toxicity of agricultural chemicals. Moreover, the influence of humic materials on the toxicity of these chemicals is dependent on temporal relationships and HA concentration. © 1991.