title: Insecticide dissipation after repeated field application to a Northern Thailand Ultisol.
abstract: Nontarget effects of pesticides may occur when the active ingredients have a long persistence in the environment. The half-lives of six insecticides commonly used in Thai fruit orchards under tropical field conditions were determined. A mixture of endosulfan-alpha and -beta, chlorpyrifos, malathion, dimethoate, and mevinphos was applied five times in 10-day intervals onto an Ultisol (lychee plantation ground-covered with grass vegetation, northwestern Thailand). On days 1, 3, 5, 7, and 10 after each application, composite samples of the topsoil (0-10 cm) were collected and exhaustively extracted. Fitting a first-order model to the datasets revealed rapid initial dissipation [half-lives from 2.2 +/- 0.4 (malathion) to 5.4 +/- 1.3 days (chlorpyrifos)]. Volatilization appeared to be a major process of pesticide dissipation, especially for malathion and mevinphos. Because 8% of the applied endosulfan-alpha and -beta had been converted to the sulfate metabolite within 1 day after the first application, also microbial degradation contributed significantly to pesticide dissipation. Nevertheless, no trend in half-lives over the five application cycles could be observed, indicating that accelerated microbial degradation did not occur for these insecticides following the five applications. Precipitation and soil moisture were key parameters of dissipation, but dissipation processes were too complex to be generalized for all substances studied. Despite their short half-lives, all pesticides except mevinphos accumulated in soil (up to 656%; endosulfan-alpha), and this accumulation correlated significantly with the hydrophobicity of the substances (r = 0.88). This was interpreted as an aging process, and it was concluded that pesticide aging must be considered relevant also in tropical environments, where it has received very limited attention so far.
authors.affiliation: Department of Soil Science, Berlin University of Technology, Salzufer 11-12, 10587 Berlin, Germany. holger@ciglasch.de
authors.forename: Holger
authors.initials: H
authors.lastname: Ciglasch
authors.forename: Julia
authors.initials: J
authors.lastname: Busche
authors.forename: Wulf
authors.initials: W
authors.lastname: Amelung
authors.forename: Suphot
authors.initials: S
authors.lastname: Totrakool
authors.forename: Martin
authors.initials: M
authors.lastname: Kaupenjohann
mesh_terms: D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003198:Computer Simulation; D006207:Half-Life; D057927:Hydrophobic and Hydrophilic Interactions; D007306:Insecticides / Q000032:analysis* / Q000737:chemistry*; D011891:Rain; D012989:Soil Pollutants / Q000032:analysis* / Q000737:chemistry*; D013785:Thailand; D014835:Volatilization
chemical_list: D007306:Insecticides; D012989:Soil Pollutants