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8 Fate and Transport of Pesticides
Pages 313-336

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From page 313...
... . Before World War II, pesticides consisted of products from natural sources such as nicotine, pyrethrum, petroleum and oils, and rotenone, as well as inorganic chemicals such as sulfur, arsenic, lead, copper, and 313
From page 314...
... Pesticides can also be immobilized through sorption onto soil organic matter and clay minerals. Pesticides can also be lost to the atmosphere through volatilization.
From page 315...
... The major loss pathways of pesticides to the environment are volatilization into the atmosphere and aerial drift, runoff to surface water bodies in dissolved and particulate forms, and leaching into groundwater basins. The fate and transfer pathways of pesticides applied to croplands are complex, requiring some knowledge of their chemical properties, their transformations (breakdown', and the physical transport process.
From page 316...
... 316 o ._ II: tt5~ a)
From page 317...
... Pesticides may be sorbed to soil particles, particularly the clays and soil organic matter. Strongly sorbed pesticides do not readily leach through the soil profile but may be bound to the sediments discharged from croplands.
From page 318...
... 318 I Soil and Water Quality: An Agenda for Agriculture TABLE 8-1 Pardtion Coefficients and Half-Lives of Pesticides Used ~n Flor~da PesticideSorption Coefficient Half-Life (common name) (ml/g of organic chemical)
From page 319...
... . Those pesticides that are strongly sorbed to soil clays and organic matter may be subject to removal by surface runoff.
From page 320...
... Under particular water application rate conditions, both water and chemicals in the dissolved and particulate forms tend to preferentially move through the macropores and cracks and reach the water table in a shorter period of time. Site Conditions Other site conditions affect runoff and leaching of pesticides (Porter and Stimman, 1988~.
From page 321...
... Such impermeable layers may, however, contribute to the lateral flow of shallow groundwaters and to the eventual discharge of groundwaters and its contaminants into surface waters. On the other hand, the presence of high-permeability earth materials such as sands and gravel may greatly accelerate the vertical and horizontal flows of contaminants.
From page 322...
... , such as hydrolysis, microbial degradation, and transport through the vadose region for cases in which the water table is many tens of meters below the land surface (the vadose region is that part of the soil above the permanent groundwater level)
From page 323...
... With regard to the efficacy of pesticide applications, losses to soil and peripheral nontarget foliage may be as high as 60 to 80 percent for most sprays (Cheng, 1990~. In contrast, pesticide losses from soil-incorporated application methods are much lower.
From page 324...
... ~ = 300/0 Drift and misapplication \ Volatilization, leaching, and surface transport \ Off target area 5% Off target Ground, other crop ~ nontarget surfaces in target area 41% Off target insect Absorbed by insect through contact, inhalation, and ingestion <1 % 1 Site of toxic action inside insect <<1 % _ No contact <1% Not at site of action . Residue on · treated crop FIGURE 8-3 Mass balance of a hypothetical aerial foliar-spray application of an insecticide.
From page 325...
... For the most part, these runoff models are more useful for evaluation of alternative management strategies and are less useful for predictive purposes. Pesticide Leaching Models More recently, a concerted effort has been made to model pesticide leaching.
From page 326...
... They point out that the convection-dispersion models appear to be unable to predict pesticide transport in the vadose zone. The reasons contributing to this dilemma include the spatial variability in the hydraulic properties usually encountered in field soils, the potential nonequilibrium sorption in the field, the depth dependency of biodegradation, and preferential flow through macropores.
From page 327...
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From page 328...
... A number of models simulate surface runoff and the resultant pesticide loading of surface waters (Adams and Kurisu, 1976; Bruce et al., 1975; Donigian and Crawford, 1976; Donigian et al., 1977; Frere, 1978; Haith, 1980, 1986; Leonard and Wauchope, 1980; Wauchope and Leonard, 1980~. In almost all cases, the models represent a compromise between the available data, which are often quite sparse and variable, and the need for a predictive tool that can be used across different soils, climates, and pesticides.
From page 329...
... REDUCTION OF PESTICIDE POLLUTION The management practices that can be used to reduce environmental pollution from pesticide use in agroecosystems can be broadly categorized into / · selection of proper pesticides and formulations; · timing of and improvement in pesticide application methods to minimize drift and volatile losses; · use of erosion and runoff control measures to reduce losses through runoff and leaching; · use of nonchemical pest control measures such as crop rotations and management; and · integrated pest management, which embodies most of the recom mended practices cited earlier.
From page 330...
... If a potential pesticide user can choose among a number of available pesticides, the user should select those that will be least harmful to the environment. For many conditions, the characteristics of a selected pesticide should include low water solubility, high sorptive capacity, low vapor pressure, higher potential for chemical and microbial degradation, and shorter overall half-life in the field.
From page 331...
... In reducing surface water runoff, however, some pesticides may be subjected to greater losses through leaching. Increasing the soil organic matter content may reduce the erosion hazard, but it would increase sorption, making the chemical less bioavailable, or it would increase the rate of microbial degradation of pesticides.
From page 332...
... Subsurface drainage, and hence, excessive leaching, may be reduced with improved water distribution uniformities, irrigation scheduling by using agroclimatic data, and use or management of the shallow groundwater. Nonchemical Control Measures Nonchemical pest control methods may involve such crop management practices as crop rotation, intercropping, and manipulation of planting and harvesting dates to aid in controlling pest populations.
From page 333...
... On the basis of chemical-specific properties and vulnerable site conditions, investigators should be able to assess whether a given pesticide will be a leacher that contaminates the underlying groundwater body. As monitoring of groundwaters for pesticides is aggressively pursued and a larger data base is accumulated, investigators may be able to confirm candidate leachers.
From page 334...
... Given the difficulty of predicting the fate and transport of pesticides with certainty, efforts to reduce pesticide losses by reducing the total mass of pesticides used, reducing pesticide losses through runoff and erosion, improving the efficacies of pesticide applications, and matching the pesticide selection to site conditions must go forward at the same time that investigators improve their understanding of pesticide behavior in the environment. Currently available technologies, farming systems, and farming practices allow significant reductions in pesticide losses while sustaining profitability.
From page 335...
... Fate and Transport of Pesticides / 335 day regulations covering toxic compounds. Although the benefits derived from the use of pesticides are considerable, increasing numbers of them are expected to be regulated for only restrictive use or banned outright as the public becomes increasingly aware of the risks to humans and the ecological environment.


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