Effects of temperature on the dissipation of total- and water-extractable pesticides in Japanese soils.

We investigated the dissipation of 27 pesticides in five Japanese soils at three temperatures and the variability of activation energies (E a). The dissipation of total pesticides extracted sequentially using water and acetone was fitted to a single first-order (SFO) model. The E a values calculated from the dissipation rate constants of the SFO model showed a normal distribution with a median of 61.1 kJ mol-1. The dissipation of water-extractable pesticides (i.e., phytoavailable pesticides) was fitted to a double first-order in parallel model with two dissipation rate constants: k 1 and k 2. The E a values calculated from k 1 and k 2 showed normal or lognormal distribution, and the medians of the normal distribution calculated from k 1 and k 2 were 62.8 and 45.2 kJ mol-1, respectively. Furthermore, the method for estimating the biphasic dissipation of phytoavailable pesticides at different temperatures by using the median E a values of the laboratory experiment was demonstrated in a field experiment.

Comparison of soil sorption parameters of pesticides measured by batch and centrifugation methods using an andosol.

We compared the soil sorption coefficient (K d) measured by batch and centrifugation methods using a Japanese andosol and ten pesticides. Although the K d values measured by both methods increased with time, those obtained via the batch method tended to be higher during the test period. The difference in K d values between the two methods affected pesticide concentrations estimated in the soil solution, and the results estimated using K d values obtained via the batch method underestimated the observed trends.

Studies on the behavior and ecotoxicity of pesticides and their transformation products in a river.

To clarify the properties of pesticide transformation products (TPs) for which the risk to aquatic organisms should be evaluated, I monitored the concentrations of paddy pesticides and their TPs in the Sakura River, Japan, during the rice-growing season in 2007-2010. I also conducted algal growth inhibition tests of herbicides and their TPs using a diatom and a green alga and acute toxicity tests of insecticides and their TPs using a caddisfly and a daphnid. Moreover, on the basis of the results of pesticide monitoring and toxicity tests, I attempted to evaluate the risk of these compounds to the riverine organisms as well as the risk of mixtures of insecticides and their TPs for caddisflies and cladocerans. The TPs were detected in the river water depending on the half-lives of the parent compounds and of the TPs in water and soil. The toxicities of the parent compound and its TPs may be related to their hydrophobicities and chemical structures. Some toxic and persistent TPs that formed rapidly in water and soil posed a risk to the organisms over a long period. The physicochemical properties and chemical structures of a parent compound and its TPs can be key factors in evaluating the pesticide TP risk to aquatic organisms in rivers.

Differential uptake and translocation of organic chemicals by several plant species from soil.

We performed uptake experiments with 12 different organic chemicals using 16 plant species and determined differences in the ability of plant species to take up and translocate these chemicals. There were differences among the plant species in the shoot and root concentrations of each organic chemical. The root concentration factor values increased with an increasing log of the n-octanol-water partition coefficient (log K OW) of organic chemicals. Thus, the concentrations in roots may be predicted to a certain extent because the root concentration factor values were related to the log K OW. The root-to-shoot translocation was related to the log K OW because the shoot-to-root concentration ratio decreased with an increasing log K OW; however, there was no clear relationship between the shoot concentration factor value and the log K OW, and this differed among plant species.

Behavior of isoprothiolane and fipronil in paddy water, soil, and rice plants after nursery-box or submerged applications.

We investigated the behavior of isoprothiolane and fipronil after nursery-box application and that of isoprothiolane after submerged application in an experimental paddy field. The concentrations of the pesticides and their metabolites were monitored in paddy water, soil, and rice plants. The distribution profile for isoprothiolane mass in the field differed greatly between the nursery-box and submerged applications. The nursery-box-applied pesticides were mostly distributed in soil near the transplanted rice seedlings (root zone), versus little distribution in paddy water and rice plants (<1.1 and <0.3% of the applied mass, respectively). The residual levels in rice plants were similar to those in the root-zone soil. To estimate the soil pesticide mass, we defined a key parameter: the ratio of the root-zone area to the total area of the paddy field estimated to be 0.1 to 0.15. This parameter is important when evaluating the concentrations of nursery-box-applied pesticides in soil and rice plants.

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.

Improvement and application of the PCPF-1@SWAT2012 model for predicting pesticide transport: a case study of the Sakura River watershed.

BACKGROUND: The Soil and Water Assessment Tool combined with Pesticide Concentration in Paddy Field (PCPF-1@SWAT) model was previously developed to simulate the fate and transport of rice pesticides in watersheds. However, the current model is deficient in characterizing the rice paddy area and is incompatible with the ArcSWAT2012 program. In this study, we modified the original PCPF-1@SWAT model to develop a new PCPF-1@SWAT2012 model to address the deficiency in the rice paddy area and utilizing the ArcSWAT2012 program. Next, the new model was applied to the Sakura River watershed, Ibaraki, Japan in order to simulate the transport of four herbicides: mefenacet, pretilachlor, bensulfuron-methyl and imazosulfuron. RESULTS: The results showed that the water flow rate simulated by PCPF1@SWAT2012 was similar with the observed data. The calculated Nash-Sutcliffe efficiency coefficient (NSE) (0.73) and percent bias (PBIAS) (-20.38) suggested satisfactory performance of the model. In addition, the concentrations of herbicides simulated by the PCPF-1@SWAT2012 model were in good agreement with the observed data. The statistical indices NSE and root mean square error (RMSE) estimated for mefenacet (0.69 and 0.18, respectively), pretilachlor (0.86 and 0.18, respectively), bensulfuronmethyl (0.46 and 0.21, respectively) and imazosulfuron (0.64 and 0.28, respectively) indicated satisfactory predictions. CONCLUSION: The PCPF-1@SWAT2012 model is capable of simulating well the water flow rate and transport of herbicides in this watershed, comprising different land use types, including a rice paddy area. © 2018 Society of Chemical Industry.

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.

Water-based extraction and liquid chromatography-tandem mass spectrometry analysis of neonicotinoid insecticides and their metabolites in green pepper/tomato samples.

This paper proposes an environmentally friendly method involving water-based extraction of the samples, cleanup of the extracts by solid-phase extraction, and subsequent liquid chromatography coupled with tandem mass spectrometry, which was used for simultaneous determination of seven hydrophilic neonicotinoid insecticides as well as their metabolites in agricultural samples. The effects of sample matrix on detection of the target compounds were negligibly small. Mean recoveries obtained at spiked concentrations between 0.01 and 1.00 mg/kg were 71.2-122.3% with relative standard deviations of ≤ 7.5%. When the method was applied to crop samples sprayed with commercial formulations of the target compounds, the residual concentrations of the compounds determined by the proposed method (0.015-0.27 mg/kg in green peppers and 0.017-0.31 mg/kg in tomatoes) were equivalent to those determined by the official Japanese method (0.017-0.26 mg/kg in green peppers and 0.013-0.30 mg/kg in tomatoes).

Investigation of concentrations of paddy herbicides and their transformation products in the Sakura River, Japan, and toxicity of the compounds to a diatom and a green alga.

Four paddy herbicides and their transformation products (TPs) were monitored in the Sakura River, Japan, during the rice growing seasons of 2009 and 2010. Toxicity tests to an attached diatom, Mayamaea atomus, and a green alga, Pseudokirchneriella subcapitata, were also conducted. Clomeprop propionic acid, which forms from the degrading herbicide, was detected in the river water at much higher concentrations than the parent compound (the maximum concentration of the TP and the parent compound; 0.829-0.925 µg/L and 0.039-0.073 µg/L, respectively). The toxicity of the TPs to the diatom and green alga was relatively low; the 72-h median effective concentration (EC(50)) value > 1,470 µg/L; for each compound, the maximum concentration in the river did not exceed the EC(50) value.

Evaluation of the risk of mixtures of paddy insecticides and their transformation products to aquatic organisms in the Sakura River, Japan.

To assess the risk of mixtures of six paddy insecticides and their transformation products (TPs) to aquatic organisms in the Sakura River, Japan, their concentrations in the river water were monitored during the rice cultivation season in 2008 and 2009, and acute toxicity tests for Cheumatopsyche brevilineata (caddisflies) and Daphnia magna (daphnids), surrogate test species for caddisflies and cladocerans, respectively, were conducted. The mixture of fipronil, applied in the rice nursery box, and its desulfinyl, sulfide, and sulfone TPs were detected in the river for several months after transplanting, and they were more toxic to C. brevilineata than the other tested compounds. The toxicities of the parent compound and its TPs, such as fipronil and its TPs, may be related to their hydrophobicities. Risk quotients for mixtures (RQ(mix)) of only parent compounds did not exceed 1, but, in mid-June 2009, the RQ(mix) of parent compounds and TPs for caddisflies exceeded 1. Diazinon, fenitrothion, and fenthion sprayed on the rice crop and their TPs posed a sporadic risk for cladocerans, depending on the application timing, whereas fipronil TPs contributed to the RQ(mix) for caddisflies for several months after transplanting. The risk of mixtures of insecticides and their TPs differed seasonally between caddisflies and cladocerans, depending on insecticide application timing and the persistence and toxicity of TPs.

Effects of four rice paddy herbicides on algal cell viability and the relationship with population recovery.

Paddy herbicides are a high-risk concern for aquatic plants, including algae, because they easily flow out from paddy fields into rivers, with toxic effects. The effect on algal population dynamics, including population recovery after timed exposure, must be assessed. Therefore, we demonstrated concentration-response relationships of four paddy herbicides for algal growth inhibition and mortality, and the relationship between the effect on algal cell viability and population recovery following exposure. We used SYTOX Green dye assay and flow cytometry to assess cell viability of the alga Pseudokirchneriella subcapitata. Live cells could be clearly distinguished from dead cells during herbicide exposure. Our results showed that pretilachlor and quinoclamine had both algicidal and algistatic effects, whereas bensulfuron-methyl only had an algistatic effect, and pentoxazone only had an algicidal effect. Then, a population recovery test following a 72-h exposure was conducted. The algal population recovered in all tests, but the periods required for recovery differed among exposure concentrations and herbicides. The periods required for recovery were inconsistent with the dead cell ratio at the beginning of the recovery test; that is, population recovery could not be described only by cell viability. Consequently, the temporal effect of herbicides and subsequent recovery of the algal population could be described not only by the toxicity characteristics but also by toxicokinetics, such as rate of uptake, transport to the target site, and elimination of the substance from algal cells.

Simulating the dissipation of two herbicides using micro paddy lysimeters.

A set of packed micro paddy lysimeters, placed in a greenhouse, was used to simulate the dissipation of two herbicides, simetryn and thiobencarb, in a controlled environment. Data from a field monitoring study in 2003, including the soil condition and water balances, were used in the simulation. The herbicides were applied and monitored over a period of 21 d. The water balances under two water management scenarios, intermittent irrigation management (AI) and continuous irrigation management (CI), were simulated. In the AI scenario, the pattern of herbicide dissipation in the surface water of the field were simulated, following the first-order kinetics. In the CI scenario, similarity was observed in most lysimeter and field concentrations, but there were differences in some data points. Dissipation curves of both herbicides in the surface water of the two simulated scenarios were not significantly different (P>0.05) from the field data except for intercept of the thiobencarb curve in the CI scenario. The distribution of simetryn and thiobencarb in the soil profile after simulation were also similar to the field data. The highest concentrations of both herbicides were found on the topsoil layer at 0-2.5 cm depth. Only a small amount of herbicides moved down to the deeper soil layers. Micro paddy lysimeters are thus a good alternative for the dissipation study of pesticides in the paddy environment.