Effect of simulated sunlight on atrazine and metolachlor toxicity of surface waters

Atrazine and metolachlor are the two most widely used herbicides in the United States; through non-point-source runoff both herbicides may cause toxicity to aquatic organisms. Toxicity changes were measured for atrazine and metolachlor in surface waters after exposure to simulated sunlight (0, 20, and 40 kJ/m2) using a Xenon Weather-Ometer. A Microtox toxicity test, using the marine luminescent bacterium Vibrio fischeri, was conducted on deionized, river, and bay water samples mixed with atrazine or metolachlor herbicide (12 mg/liter) after exposure to simulated sunlight. Microtox test (EC50%) results demonstrated that the toxicity decreased with increasing light intensity for both herbicides in river and bay water. These results also indicate that the toxicity of the bay water, with high concentrations of organic and suspended matter, was reduced, for both herbicides, compared with the toxicity of the river water, possibly through photodegradation of pesticides.