Surface residues pesticides

Raju J, Gupta VK. 1989. A new extractive spectrophotometric method using malonyl dihydrazide for the determination of organophosphoms pesticides in surface residues. Microchem J 39 166-171. 

Chlordecone has been primarily released to surface waters in waste waters from a manufacturing plant in Hopewell, Virginia, and may be released in activities associated with the disposal of residual pesticide stocks, and as a result of the direct use of mirex. Chlordecone has been released directly as a contaminant of mirex and indirectly from the degradation of mirex. 

Evidence was obtained recently that pesticide vapors may enter the air by still another mechanism, involving plant circulation and water loss (57). Rice plants were found to efficiently transport root-zone applied systemic carbamate insecticides via xylem flow to the leaves, eventually to the leaf surface by the processes of guttation and/or stomatal transpiration, and finally to the air by surface volatilization. Results from a model chamber showed that 4.2, 5.8, and 5.7% of the residues of carbaryl, carbofuran, and aldicarb, respectively, present in rice plants after root soaking vaporized within 10 days after treatment. The major process was evaporation of surface residues deposited by guttation fluid. 

It is therefore reasonable to assume that the evaporation rate of residue is proportional to the gross area of spray deposit for a pesticide remaining exposed on a leaf surface. The pesticide will be lost at a constant rate, under constant conditions of ventilation, until the contaminated area decreases significantly. 

Photolysis of pesticides can occur on soil surfaces when pesticides are directly applied to soil. However, it is restricted to residues on or very near the surface (e.g., a few millimeters deep) because penetration of ultraviolet light into soil is limited. This is probably true for plant surfaces, especially leaf surfaces. On the other hand, pesticides in water are very vulnerable to photolysis because ultraviolet light can penetrate relatively thick layers of clear water. 

Attenuated total reflectance Eourier-transform infrared (ATR-ETIR) spectroscopy is another technique recently reported to have been applied to the determination of pesticide residues on human skin and residential surfaces (Doran et al., 2000). While this technique gave good results when evaluated in the laboratory for three pesticides at 0.5 to 5 Rg/m skin loadings, field use would be very limited by the size and transportability of the instrument and the liquid-nitrogen coolant for the detector. Whatever the method, surface residues are also difficult to measure quantitatively in situ, especially on the skin. 

The many uses of an indoor pesticide require fhaf exposure estimates should be based on fhe mosf likely application dial will lead fo fhe highesf probabilify of dermal and inhalation confacf. For insfance, a broadcasf carpel Ireatmenl is generally presumed fo resull in more pesticide surface residue being accessible to individuals fhan fhe amounf or accessibility of residue when the pesticide is placed inside an insect bait station. 

Approaches for aggregating exposure for simple scenarios have been proposed in the literature (Shurdut et al., 1998 Zartarian et al., 2000). The USEPA s National Exposure Research Laboratory has developed the Stochastic Human Exposure and Dose Simulation (SHEDS) model for pesticides, which can be characterized as a first-generation aggregation model and the developers conclude that to refine and evaluate the model for use as a regulatory decision-making tool for residential scenarios, more robust data sets are needed for human activity patterns, surface residues for the most relevant snrface types, and cohort-specific exposure factors (Zartarian et al, 2000). The SHEDS framework was used by the USEPA to conduct a probabilistic exposure assessment for the specific exposure scenario of children contacting chromated copper arsenate (CCA)-treated playsets and decks (Zartarian et al, 2003). 

Pesticides can move away from the release site when they are on or in objects or organisms that move (or are moved) offsite. Pesticides may stick to shoes or clothing, to animal fur, or to blowing dust and be transferred to other surfaces. When pesticide handlers, applicators, and users bring home or wear home contaminated personal protective equipment, work clothing, or other items, residues can rub off on carpeting, furniture, and laundry items and onto pets and people. 

Pesticide formulation and volatility of the chemical greatly affects the ratio of airborne to surface residues. Wettable powder formulation of permethrin resulted In an airborne concentration of 0.39 ug/1 during the first 2 hours after application, whereas the emulslflable concentrate of the same pesticide resulted In a concentration of 2.82 ug/1. The surface deposition of benomyl averaged approximately 750 ng/cm from a HV application and 632 from an LV application (Figure 2) and airborne residues were not measurable 1 hour after application. Approximately 33 percent of the benomyl deposition from the HV application was on the under surface of the glass plate, whereas only 1 percent from the LV application was so deposited. 

Figure 2. Pesticide surface residues (ng/cm ) measured at 20 designated locations in the greenhouse after HV and LV applications of permethrin or benomyl.

The documented occurrence of pesticides in surface water is indicative that mnoff is an important pathway for transport of pesticide away from the site of appHcation. An estimated 160 t of atra2ine, 71 t of sima2ine, 56 t of metolachlor, and 18 t of alachlor enter the Gulf of Mexico from the Mississippi River annually as the result of mnoff (47). Field appHcation of pesticides inevitably leads to pesticide contamination of surface mnoff water unless mnoff does not occur while pesticide residues remain on the surface of the soil. The amount of pesticides transported in a field in mnoff varies from site to site. It is controUed by the timing of mnoff events, pesticide formulation, physical—chemical properties of the pesticide, and properties of the soil surface (48). Under worst-case conditions, 10% or more of the appHed pesticide can leave the edge of the field where it was appHed. 

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

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