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was not conducted in the basin in 2007 and 2008 (Japan Agricultural Aviation Association: private communication; unpublished data).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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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.

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.

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.