PARENT SESSION

5F - Probabilistic approaches
Poster Hall
8:30 AM - Monday, 28 April 2003

(MOP/201) Probabilistic assessment of synergism in pesticides under field conditions.

Solomon, Keith¹, ¹ University of Guelph, Guelph, ON, Canada

ABSTRACT- Recent reports of studies with Chironomus tentans have demonstrated synergistic interactions between triazine herbicides and organophosphorus pesticides and concentrations that are representative of those found in the field. Synergism of occurred at concentrations of atrazine (> 10 g/L) and with organophosphorus pesticides that are activated by MFO enzymes. To assess the probability of this occurring in the field, atrazine and chlorpyrifos concentrations in surface waters obtained from the NAWQA database were sorted to retain only samples analyzed for atrazine and chlorpyrifos at the same location, date and time. Of 13,094 data pairs, 98.7% had concentrations of atrazine <10 g/L. There was a low correlation between atrazine and chlorpyrifos (r² = 0.031). Only three of the combinations of chlorpyrifos and atrazine concentrations (0.02%) were at concentrations great enough to be synergistic. This suggests that the likelihood of co-occurrence of atrazine and chlorpyrifos in surface waters of the US at concentrations that will result in significant increases in toxicity through synergism in C. tentans is very small. Methyl parathion and diazinon are less toxic and show lower synergistic ratios, suggesting that the likelihood of synergism would be even lower.

Key words: Atrazine, Mixtures, Organophosphorus, Surface water