Agrochemicals dissipation

Soil, climatic, and hydrogeological conditions vary widely between geographical regions. This variation occurs on a variety of scales and significantly affects agrochemical dissipation rates and fates in the environment. As a result, differences in... 

The need for additional samples to compensate for soil heterogeneity must be reconciled with labor, storage, transportation, analytical, and other constraints that add significantly to study costs. Satisfactory results have been obtained from numerous field studies using three or four treated replications with 5-10 soil cores collected from each replication per sampling period. These replication/repetition numbers strike a reasonable balance between the need for samples sufficient in number to characterize agrochemical dissipation versus financial and logistical constraints associated with sample collection and analysis. 

Plant metabolism studies will provide information on the absorption, translocation, dissipation and degradation of the agrochemical. This information defines the residual analytes of regulatory concern that could include either the parent compound or metabolites in the field crops. Plant metabolism studies should be conducted with at least three crop representatives of three different crop groups listed in Table 1. One of the major objectives is to determine the comparative metabolism of the agrochemicals between animals and plants among different plant species. MAFF approves metabolism studies that are conducted in foreign countries, which should be operated under the certified GLP system. 

The time of year in which a pesticide is applied significantly affects its dissipation rate due to temperature, moisture, and solar-irradiance effects on abiotic and biotic dissipation processes. For example, dissipation rates for agrochemical applications made in the springtime are normally greater than those observed for fall (autumn) applications. Thus, the timing of agrochemical applications made in field soil dissipation studies should closely match those occurring under acmal-use conditions. 

Application rate is generally dictated by the labeled, or anticipated, application rate relevant to the particular use pattern being investigated. To improve analytical detection or to compensate for potentially low zero-time application recoveries, application rates are sometimes increased to 110% of the labeled application rate. An application rate greater than this level would be subject to regulatory scmtiny and may affect the dissipation rates of certain agrochemicals owing to potential short-term effects on sensitive soil microflora. 

Another approach to improving agrochemical detection is to apply more of the active ingredient to increase the initial soil concentration. As mentioned previously, however, one must be careful not to exceed greatly the labeled application rate of the compound as questions may arise as to concentration effects on the observed dissipation. A more common and acceptable approach is to section the upper soil core into smaller depth increments, yielding increased residue concentrations as the total amount of soil mixed with the residues decreases in each processed sample (Table 1). 

J.H. Massey and J.S. LeNoir, Sources and magnitudes of variability in terrestrial field dissipation of agrochemicals, in Terrestrial Field Dissipation Studies Design, Interpretation and Purpose, ed. E.L. Arthur, V.E. Clay, and A. Barefoot, ACS Symposium Series No. 842, American Chemical Society, Washington, DC (2003). 

RICEWQ was the first model developed for agrochemical runoff from paddy fields, incorporating aircraft application, dissipation by drift, adhesion on leaf surfaces, and dissipation from the leaf surface in addition to the processes affecting degradation and transport in sediment and paddy water. An important parameter, desorption from sediment to paddy water, is not considered, although this is not as important as other parameters in paddy fields such as sedimentation rate, behavior of SS, etc. 

G.P. Cobb, L.W. Brewer, E.H.H. Hoi, and C.M. Bens, Diazinon dissipation from vegetation in apple orchards from Pennsylvania and Washington, in ACS Symposium Series Eield Dissipation of Agrochemicals, ed. E. Authur, American Chemical Society, Washington, DC. In press. 

Photochemical reductions and oxidations in aquatic environments provide sinks or sources for halocarbons. Such photoreactions are an important process in the dissipation of low-volatility halocarbons, such as halogenated agrochemicals, in aquatic environments (reference 9 contains lead references). For example, field studies of Crossland and Wolff (10) demonstrated the rapid dissipation of pentachlorophenol residues by its photoreaction in English ponds. Evidence emerged that volatile halocarbons such as 1,1,1-trichloroethane (methylchloroform) may have significant sinks in the aquatic environment... 

Fig. 6.6-37 Description of the action of a microencapsulated two-effect agrochemical product [6.6.3.1] a) microcapsule surrounded by a disinfectant film, b) disinfectant evaporation, c) diffusion of the insecticide through the wall and dissipation of the active substance...

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