AIRBORNE RESIDUES RESULTING FROM USE OF METHYL PARATHION, MOLINATE AND THIOBENCARB ON RICE IN THE SACRAMENTO VALLEY, CALIFORNIA

Abstract

In connection with requirements of California's Toxic Air Contaminant Act, 24-h ambient air samples were collected using an XAD-4 resin trap technique capable of simultaneously collecting methyl parathion, methyl paraoxon, molinate and thiobencarb. Sampling was conducted on rooftops of public buildings located in four towns in two counties where methyl parathion, molinate and thiobencarb were used in significant quantities, and at a background site located in a county where no use occurred, for four intervals each week for 5 weeks during the springtime application period of 1986. Satisfactory recoveries (greater than 66%) were obtained from dynamic spiking experiments, and the precision from field replicates was consistently less than 50% relative standard deviation using a protocol designed to accommodate a large number of samples. Daily maximum average concentrations (and the range in averages for all sites over the 19 and 20 sampling days in the two-county use area) were 25.7 ng/m3 (0.2-6.2 ng/m3) for methyl parathion, 3.1 ng/m3 (<0.5-0.8 ng/m3) for methyl paraoxon, 1,720 ng/m3 (60-650 ng/m3) for molinate and 250 ng/m3 (12.9-67.8 ng/m3) for thiobencarb. Concentrations correlated well with reported uses of methyl parathion and molinate in the general vicinity of the sampling sites. The likely sources of observed residue levels, based upon the method of application and pesticide physicochemical properties, were spray drift during application for methyl parathion, vapor-phase oxidation of parent thion for methyl paraoxon, and postapplication volatilization from field water for molinate and thiobencarb.