Three-year comprehensive experiments were conducted to compare the dissipation patterns of a total of 16 pesticides, including 3 metabolites, as granular formulations applied in lysimeters and paddy fields with two soil types. Analytical concentrations of the target pesticides in paddy water were analyzed using a granular kinetic model consisting of the following parameters: release rate, decrease rate, and dissolved concentration. Results of parameter grouping analyses of the kinetic models showed that 56% of data reproducibility (entire grouping) was obtained between replicates for the lysimeters. In comparisons between the lysimeters and paddy fields, 48% of decrease rates and 34% of release rates were grouped, although significant differences were observed with a nearly 90% difference for dissolved concentrations. These differences might be attributed to the hydrological components such as water management and meteorological covariates in paddy fields, the daily percolation in lysimeters and the adsorption-desorption kinetics between paddy water and soil.

BACKGROUND In Japan, while experimental data for the dissipation behavior of paddy pesticides under a standardized test system are available, the application of a mathematical model is limited. This paper proposes a new model calibration procedure for inversely deriving the model parameters from the experimental data. This procedure is tested in the open software R by running an improved Pesticide Concentration in Paddy Field‐1 (PCPF‐1) model with R packages to analyze the dissipation of simetryn and molinate in flooded lysimeters and paddy fields. RESULTS The model fitting was performed by a random minimization routine. Furthermore, the uncertainties of the model parameters envisioned by the global sensitivity analysis were successfully reduced using the Markov chain Monte Carlo technique. The calibrated simulation was validated at each test plot by confirming multiple statistical indices (i.e. Nash–Sutcliffe efficiency 0.88–1.00, percent bias <±5%). The dissipation pathways of two herbicides were quantitatively clarified by the mass balance of calibrated simulations and the effect of the unexpected herbicide runoff was quantified. The case study showed that the adjustment of daily percolation rate in the lysimeter experiment is the key to simulate the actual paddy field condition more accurately, especially in a case where pesticides show higher water solubility and soil mobility. CONCLUSION The developed procedure can analyze the experimental data with acceptable accuracy and extract the unobservable information quantitatively. Our approach is applicable to the optimization of not only the model but also future experimental design. © 2018 Society of Chemical Industry

Comparative experiments investigating the dissipation of four nursery-box-applied pesticides and three foliar-applied pesticides were conducted using lysimeters and in actual paddy fields. In the lysimeter experiments, there were test plots for submerged application for both application types. Analytical concentrations of the pesticides in paddy water were evaluated using appropriate kinetic models. The detection levels of pesticides in the paddy water for the nursery-box and foliar applications were 10-77% and 42-79% of the submerged application, respectively. The times required for 50% dissipation (DT 50 s) in case of the nursery-box and foliar applications were 0.8-10.4 days and 0.5-2.7 days, respectively. Although overall dissipations were affected by the physicochemical properties of the pesticide and the experimental design in the test plots, the initial detection levels in the lysimeters, governed by the runoff at transplanting and the deposition at spraying, were comparable with those in the actual paddy fields.

BackgroundExtraction of environmental fate parameters for pesticides by inverse modeling in laboratory experiments has evolved to become a common practice in higher tier exposure modeling. This study focuses on flooded paddy soil conditions using a simple container test system. Four active ingredients of paddy herbicide were tested. The results were parameterized and transferred to analyze the effect of formulation types on the outdoor experimental data via inverse analyses of two structurally‐compatible mathematical models, namely: pesticide concentration in paddy field for laboratory (PCPF‐LR) and PCPF for outdoors (PCPF‐1Rv1.1).ResultsAfter in‐laboratory calibration, the PCPF‐LR model revealed statistically acceptable or ideal simulations of pesticide concentrations in both the aqueous and soil phases (e.g. Nash–Sutcliffe efficiency > 0.7), in addition to determining the apparent sorption from the laboratory data. The extracted persistence indicators (degradation half‐life, DegT50) in the aqueous phase were 1.4–38.7 times higher than those of the dissipation (DT50) due to the exclusion of partitioning and phase transfer processes (diffusion and sorption). In the outdoor experiment, 72% of the outdoor‐calibrated simulations of the PCPF‐1Rv1.1 model, showed statistically acceptable representations of the concentrations in paddy water. Furthermore, the DegT50 as ‘bulk’ degradation in paddy water was statistically insignificant between the formulation types; however, the DT50 demonstrated statistically different results.ConclusionThe laboratory/outdoor data interconnections using proposed modeling approach facilitate the data‐specific model calibration and analysis. These can be useful in the exposure modeling of paddy pesticide by manipulating the parameter uncertainties associated with the experimental constraints. © 2020 Society of Chemical Industry