Fate of Pesticides in Combined Paddy Rice-Fish Pond Farming Systems in Northern Vietnam

During the last decades, high population growth and exportoriented economics in Vietnam have led to a tremendous intensifi cation of rice production, which in turn has signifi cantly increased the amount of pesticides applied in rice cropping systems. Since pesticides are toxic by design, there is a natural concern on the impact of their presence in the environment on human health and environmental quality. Th e present study was designed to examine the water regime and fate of pesticides (fenitrothion, dimethoate) during two consecutive rice crop seasons in combined paddy rice-fi sh pond farming systems in northern Vietnam. Major results revealed that 5 and 41% (dimethoate), and 1 and 17% (fenitrothion) of the applied mass of pesticides were lost from the paddy fi eld to the adjacent fi sh pond during spring and summer crop seasons, respectively. Th e decrease of pesticide concentration in paddy surface water was very rapid with dissipation half-life values of 0.3 to 0.8 and 0.2 d for dimethoate and fenitrothion, respectively. Key factors controlling the transport of pesticides were water solubility and paddy water management parameters, such as hydraulic residence time and water holding period. Risk assessment indicates that the exposure to toxic levels of pesticides for aquaculture (Cyprinus carpio, Daphnia magna) is signifi cant, at least shortly after pesticide application.