Persistence and Leaching of Two Pesticides in a Paddy Soil in Northern Vietnam

In the present study, persistence and leaching of dimethoate and fenitrothion in a paddy soil were studied under the typical conditions of northern Vietnam. The experiments were conducted on a sandy loam soil (pH 8.1) planted with a local rice variety (Oryza sativa L. var. Nep87) during two consecutive cropping seasons. Concentrations of dimethoate and fenitrothion were monitored in paddy topsoil (5 cm) and in soil water in 20 and 40 cm soil depths. Maximum concentrations were detected in topsoil on day 1 after treatment (DAT 1) or DAT 2 and corresponded to 29-30% (dimethoate) and 2-3% (fenitrothion) of applied mass. Dissipation of both pesticides from topsoil showed strong bi-phasic behavior. Despite differences in their properties and seasonal conditions, calculated dissipation half-lives were always less than one day, indicating strong influence of transport processes. The transport within soil was very rapid: Already 6 h after application dimethoate and fenitrothion were found at soil depths of 20 and 40 cm, suggesting preferential transport acting under submerged conditions. It points to high groundwater vulnerability during pesticide application season on paddy rice in the studied area. To protect groundwater, actions are required both in agricultural and legal fields: Current pesticide application practices should be revised together with a more stringent authorization procedure, market regulation, and an introduction of pesticide groundwater thresholds.