Keiya Inao

Pesticide paddy field model (PADDY) for predicting pesticide concentrations in water and soil in paddy fields

To evaluate the fate of pesticides in paddy fields, the pesticide paddy field model (PADDY) has been developed for predicting pesticide concentrations in paddy fields and the run-off 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 fields in Japan. The behavior of pesticides in paddy fields 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 field experiment was carried out on a paddy field and the concentration changes of pesticides in water and soil were measured. These were in reasonably good agreement with those predicted by PADDY.

水収支を考慮した農薬水田圃場モデル (PADDY) による農薬の動態予測

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.

Temperature dependence on the pesticide sampling rate of polar organic chemical integrative samplers (POCIS)

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. Graphic abstract Laboratory experiments were performed to determine the sampling rates of pesticides used in Japan. The sampling rates of 48 pesticides at 18, 24, and 30 °C were obtained, and these temperature dependences were confirmed.

Effect of Time-Dependent Sorption on the Dissipation of Water-Extractable Pesticides in Soils

The dissipation behavior of water-extractable pesticides in soils is important when assessing the phytoavailability of pesticides in soils. This process is less understood than pesticide extraction with organic solvents. To elucidate the dissipation behavior of water-extractable pesticides in soils, we conducted an incubation study using 27 pesticides and five Japanese soils. The rate of decrease of the level of pesticides in water extracts was faster in soils than that of total extracts (water extracts and acetone extracts). This suggests that time-dependent sorption contributed to the difference in the dissipation between the pesticides in water and total extracts from soils. Increased apparent sorption coefficients (Kd,app) with time were positively and significantly correlated with Kd,app values of a 0 day incubation [Kd,app(t0)]. This empirical relationship suggests that Kd,app(t0) values can predict the time-dependent increase in Kd,app and the dissipation of water-extractable pesticides in soils.

An improved PADDY model including uptake by rice roots to predict pesticide behavior in paddy fields under nursery-box and submerged applications

We developed an improved version of the PADDY model for predicting pesticide behavior in paddy fields, which includes pesticide uptake by rice roots. We applied the model to nursery-box and submerged pesticide applications. A paddy field was divided into root-zone and inter-plant areas, and paddy soil containing pesticides was vertically separated into three layers. Pesticide behavior was modeled with mass fractions of the pesticides in paddy water and the soil layers immediately after rice transplanting obtained from field experiments, and uptake by rice roots was described using the transpiration stream concentration factor. The improved model successfully simulated measured concentration changes in a paddy field, including rice plants, under nursery-box and submerged applications. The model evaluated the difference in the concentrations of nursery-box-applied pesticides between root-zone and inter-plant soil samples with several key parameters. Our study provides a useful solution for simulating the uptake of pesticides in soil by rice roots.

Determining the suitability of a polar organic chemical integrated sampler (POCIS) for the detection of pesticide residue in the Ishikawa River and its tributary in Osaka, Japan

The monitoring of pesticide concentrations in Japanese rivers was conducted via a grab sampling method and a passive sampling method using the polar organic chemical integrated sampler (POCIS). The results showed that cumulative detections were 84 with grab sampling and 98 with the POCIS. All of the pesticides detected by grab sampling could be quantified with a POCIS except for one (although its traces were detected). In addition, 15 detections quantified by POCIS were undetected by grab sampling. The average concentrations of pesticides detected by both the POCIS and grab sampling during the investigation period were compared. A good correlation was observed between the two methods with a slope of 1.00 and a coefficient of correlation (r) of 0.897 (n=79). Although high temporal variability was observed in the pesticide concentrations by grab sampling, the average pesticide concentrations obtained by the two methods showed similar values during the investigation period.

Improved PADDY-Large model including lateral seepage loss from paddy fields to predict pesticide behavior in river basins

We developed an improved simulation model for predicting pesticide concentrations in river basins based on PADDY-Large, which includes lateral seepage loss of pesticides from paddy fields. Based on the structure of typical Japanese paddy fields, pesticide transport process due to lateral seepage through bunds was modeled as a compartment system consisting of pore water and soil particle. The model was validated with concentrations measured by monitoring paddy pesticides in a tributary of the Sakura River in Japan. The improved model by including loss of pesticides due to lateral seepage through bunds successfully simulated temporal changes in the pesticide concentrations.