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.

Field aging immobilizes pollutants and reduces their toxicity, but it also boosts their accumulation and holds the risk of future release. We investigated the aging of six insecticides (water solubilities: 0.33 mg L (-1)-completely miscible) applied five times (10-day intervals) to a tropical fruit orchard under natural weather conditions. After sequential extractions of soil samples with 0.01 M CaCl 2, methanol (MeOH), and acetone/ethylacetate/water (AEW), a conventional ( K OC(app) = [ c(MeOH) + c(AEW)]/ c(CaCl 2), normalized to soil organic carbon) and a newly introduced distribution ratio (MAR = MeOH/AEW ratio; c(MeOH)/ c(AEW)) were calculated. Field half-lives of the insecticides correlated with K OC(app) but not with MAR, which might reflect that dissipation was significantly affected by abiotic processes. The extent of aging was related to hydrophobicity of the compounds and most pronounced for endosulfan (3-fold increase in K OC(app) within 84 days). For dimethoate, this increase was even steeper (5- to 10-fold within 10 days), which was, however, mostly caused by dissipation from labile pools rather than by aging. The K OC(app) of chlorpyrifos remained constant, but a significant decrease in MAR ( r = -0.78) revealed that sorption strength increased nevertheless. Results for malathion were ambiguous. Within the time frame of our study, neither K OC(app) nor MAR gave evidence for the aging of mevinphos. The different dynamics of K OC(app) and MAR for the six insecticides studied indicate that different aging mechanisms or rates, or both control the fate of the individual insecticides, which can potentially be revealed by sequential exctraction procedures.

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