The improved version of the pesticide paddy field model (PADDY) was evaluated for the prediction of pesticide concentrations in water and soil considering water balance on the water management. Field studies under the two different conditions on the water management practices, continuous irrigation and drainage, and water holding management that regulate drainage during the experimental periods, were performed to validate the improved model using molinate and simetryne. For both pesticides, concentrations in paddy water were the same level during the first day after the application on both fields. After second day, pesticide concentrations on the field for continuous irrigation and drainage were lower than those on the field of water holding management. Good fits between model predictions and field observations were obtained for the two pesticides by considering the water management and hydrological condition.

Thirty-nine paddy pesticides and 11 of their metabolites were monitored in the Sakura River during the rice cultivation season in 2007 and 2008. Pesticide concentrations in the river water depended on the timing of pesticide application. Herbicides that were shipped to Ibaraki Prefecture in large amounts or had high water solubility, a low soil adsorption constant value, or large usage rates were detected at high peak concentrations. The concentrations of nursery-box-applied fungicides and insecticides peaked immediately after transplanting. The concentrations of pesticide metabolites, such as bromobutide-desbromo, cafenstrol-descarbamoyl, clomeprop-propionic acid, carbofuran, and fenthion-sulfoxide depended on the degradation rates and metabolic pathways of the parent compounds and on the stability of the metabolites in water and soil. Clomeprop-propionic acid, carbofuran, and fenthion-sulfoxide, which were formed from rapidly degradable pesticides, were detected at much higher peak concentrations than the parent compounds. © Pesticide Science Society of Japan Keywords: paddy pesticide, application timing, river water, physicochemical property, metabolic pathway, environmental fate. Original Articlefield area is under rice cultivation. After soil puddling and land leveling in late April, rice seedlings are transplanted in early May. Intermittent irrigation starts in mid-June and continues throughout the growing period, except during the midsummer drainage period from late June to mid-July. Generally, heading occurs from late July to early August, and harvest starts in early September, after drainage of the paddy water in late August. 15)Early-mid-season herbicides are applied within 2 weeks after transplanting (early to mid-May), and mid-season herbicides are applied 2-3 weeks after transplanting (late May to early June). Depending on the emergence of weeds, a lateseason herbicide might be used in June.16) Fungicides and insecticides are used on rice seedlings in nursery boxes before transplanting and for direct application to paddy fields before heading or harvest. Recently, nursery-box application has become increasing popular in the Sakura River basin. Aerial spraying of paddy pesticides was not conducted in the basin in 2007 and 2008 (Japan Agricultural Aviation Association: private communication; unpublished data). Analyzed compoundsTwenty-nine herbicides, 2 fungicides, and 8 insecticides were selected for analysis (Table 1), in consideration of the amounts of paddy pesticides shipped to Ibaraki Prefecture recently. We selected 11 metabolites of the herbicides and insecticides in accord with previous studies 11,14,17,18) as a reference. All analytical standards were purchased from Wako Pure Chemicals Industries (Japan), Hayashi Pure Chemicals Industries (Japan), or Kanto Chemicals (Japan). Sample collection and analysisThe sampling site (Kimijima Bridge) is located midway along the course of the Sakura River (Fig. 1). Water samples (approximately 3000 ml per sample) were collected once a week on Monday fr...

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To evaluate the fate of pesticides in paddy üelds, the pesticide paddy üeld model (PADDY) has been developed for predicting pesticide concentrations in paddy üelds and the run-oþ amount of pesticides to the aquatic environment. This model focused particularly on granule formulation because these formulations have been used widely as herbicides on paddy üelds in Japan. The behavior of pesticides in paddy üelds was assessed by considering the main processes on the basis of a compartment system and the mass-balance equations of pesticides in the compartments were derived from kinetic data. The mathematical model, PADDY, was constructed by numerical solution techniques. A method for measuring the pesticide parameters for this model was also developed. To validate the model, a üeld experiment was carried out on a paddy üeld and the concentration changes of pesticides in water and soil were measured. These were in reasonably good agreement with those predicted by PADDY.1999 Society of Chemical Industry (

Laboratory experiments were performed to determine the sampling rates of pesticides for the polar organic chemical integrative samplers (POCIS) used in Japan. The concentrations of pesticides in aquatic environments were estimated from the accumulated amounts of pesticide on POCIS, and the effect of water temperature on the pesticide sampling rates was evaluated. The sampling rates of 48 pesticides at 18, 24, and 30 °C were obtained, and this study confirmed that increasing trend of sampling rates was resulted with increasing water temperature for many pesticides.

Paddy herbicides contribute to the reduction of weeding labor, however, there are concerns about their effects on the environment and ecosystems.The environmental burden of applied herbicides is heaviest in water systems such as irrigation channels and rivers. Herbicides are generally detected in rivers in concentrations in levels of ng/L for only 2 to 3 months after use. It is to be regretted that herbicides have been implicated in accidents involving fish, the impeded propagation of algae and other non-target organisms. Therefore, it is necessary to assess the ecological risk, and the Environment Agency in Japan compiled an interim report on how pesticides' ecological effects should be assessed. Pesticides are separately examined for their toxicity (hazard assessment) and exposure (exposure analysis). However, to the environment and ecosystems there are many problems in assessing the ecological risk of pesticides, such as selection of geographic locations, methodology of assessing the impacts on ecosystems and monitoring the effect of pesticides. New herbicides are expected to have high selectivity and low toxicity. Decreasing herbicide toxicity requires high selectivity to distinguish target weeds from crops and non-target organisms. New groups of compounds will be developed based on a biorational approach. Moreover, it is necessary to develop an environmentally low-impact application method such as the use of granular types and sustainedrelease formulation among others. It is important that integrated methods be used to control paddy weeds by combining ecological/agronomical, mechanical and biological control methods, instead of relying solely on chemical herbicides.

: To evaluate the fate of pesticides in paddy üelds, the pesticide paddy üeld model (PADDY) has been developed for predicting pesticide concentrations in paddy üelds and the run-oþ amount of pesticides to the aquatic environment. This model focused particularly on granule formulation because these formulations have been used widely as herbicides on paddy üelds in Japan. The behavior of pesticides in paddy üelds was assessed by considering the main processes on the basis of a compartment system and the mass-balance equations of pesticides in the compartments were derived from kinetic data. The mathematical model, PADDY, was constructed by numerical solution techniques. A method for measuring the pesticide parameters for this model was also developed. To validate the model, a üeld experiment was carried out on a paddy üeld and the concentration changes of pesticides in water and soil were measured. These were in reasonably good agreement with those predicted by PADDY.1999 Society of Chemical Industry (

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