Measurements and computations on the behaviour of the insecticides azinphosmethyl and dimethoate in ditches

Abstract

The unintentional pollution of surface water was studied during spraying of the insecticides azinphos-methyl and dimethoate on two fruit farms. Spray drift depended closely on the local situations at the fruit farms (windbreaks, distance from trees to ditches, paths) and on way of application.During application, the concentrations of both insecticides in water and in ditch bottoms were measured. Methods were adapted or developed for sampling, extraction, clean-up and gas-chromatography. Shortly after spraying, concentrations were several hundreds of mg m -3. The half-lives of azinphos- methyl ranged from 3 to 4 d; those for dimethoate ranged from 4 to 13 d.Water flow from and to ditch sections was estimated on both fruit farms during application. Flow through the ditch bottom was estimated as a closing item in a balance equation. All items of water balance were introduced into computation models of the behaviour of pesticides in surface water and bottom material. The set of differential equations was solved numerically after programming in the computer language CSMP III. Simulation of a trial with low discharge from a siphon-linked ditch indicated that conversion of both compounds in water was 70-90% of the material balances. Penetration into the ditch bottom was slow. During water flow through the ditches, convective transport and dispersion were predominant.Decline of azinphos-methyl and dimethoate was also measured in outdoor tank systems with a bottom layer. Fluctuations in pH and variations in light penetration influenced decline rates. Computations for the tank system indicated that conversion in the water compartment was the major item in material balance. The computed and measured masses of the insecticides in the bottom layer were less than 10% of the amount added.Conversion rates in surface water and in systems with anaerobic bottom material were measured in the laboratory at 10 and 20 °C. Conversion in water in the dark was slow, with half-lives of both compounds at about 100 d at 20 °C. The conversion rates of azinphos-methyl in anaerobic bottom material at 20 °C. was about ten times those in surface water. Copper ions were catalytic in the conversion of both insecticides in water.</p