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FATE AND TRANSPORT OF PESTICIDES 313 8 Fate and Transport of Pesticides The agricultural production systems of the United States are capable of producing a bountiful supply of food and fiber, but at some cost to the nation's water, soil, and air resources. As agricultural production intensified, the natural pest-predator relationship that keeps many crop pests in check was disturbed. This contributed to the increasing use of pesticides. Chemical control of pests and diseases escalated in the mid-1950s with the discovery of new synthetic organic compounds (for example, dichlorodiphenyltrichloroethane [DDT] and 2,4-dichlorophenoxyacetic acid [2,4-D]). The consumer began to expect attractive-looking food products without blemishes or insects. However, Rachel Carson's book Silent Spring (Carson, 1962) gave rise to public concern about the threat of pesticide contamination of the environment. By 1980, agriculture used 72 percent of all pesticides applied in the United States, and herbicides and insecticides made up 89 percent of the pesticides used by agriculture. About 50,000 pesticide products are now registered for use with the U.S. Environmental Protection Agency (EPA), but the number of those used extensively is smaller. These pesticides are commonly classified according to their intended target organism (for example, insecticides, herbicides, fungicides, nematicides, rodenticides, and miticides) and according to their intended use (for example, defoliants, desiccants, fumigants, and plant growth regulators). 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
FATE AND TRANSPORT OF PESTICIDES 314 lime. During and then after World War II, phenoxy herbicides and organochlorine insecticides were widely used. In the mid-1960s, their use declined because they were replaced by triazine and amide herbicides and carbamate and organophosphate insecticides. Some pesticides (for example, DDT and dibromochloropropane [DBCP]) have been banned from use mainly because of their toxicities. In the past, agriculture was mostly concerned with on-site measures that could be used to enhance crop and livestock production. In the 1960s, investigators became more aware of the off-site effects of farming operations, such as the degradation of surface water quality. In the 1980s, investigators became acutely aware of groundwater contamination. Water pollution, for instance, was initially a local problem created mainly by identifiable and easily regulated point sources of contamination. However, with widespread pesticide applications the problem has spread regionally, nationally, and globally. Recent assessments (Garner et al., 1986; Holden, 1986; National Research Council, 1989s; U.S. Congress, Office of Technology Assessment, 1990; U.S. Environmental Protection Agency, 1990b) of pesticide contamination of waters indicate that contamination is widespread, although at low concentrations. This chapter evaluates the fate and transport of pesticides in agroecosystems, opportunities for the prevention of water pollution from such systems, and assessment of the knowledge base relative to policy implications. FATE AND TRANSPORT PROCESSES Figure 8-1 (Sawhney and Brown, 1989) shows the interactions and loss pathways of organic chemicals in soils. Figure 8-2 (Cheng, 1990) shows similar and additional features of the environmental fates of pesticides applied to croplands. Pesticides are formulated in a variety of ways (as liquids, gases, and solids) and are applied by a number of methods (aerial or canopy spraying, incorporation or injection into the soil, and with water). Pesticides applied to cropping systems can be degraded by microbial action and chemical reactions in the soil. Pesticides can also be immobilized through sorption onto soil organic matter and clay minerals. Pesticides can also be lost to the atmosphere through volatilization. Pesticides that are taken up by pests or crop plants either can be transformed to degradation products (which are often less toxic than the original compound) or, in some cases, can accumulate in plant or animal tissues. A certain portion of the pesticides applied are also removed when the crop is harvested. Pesticides that are not degraded, immobilized, detoxified, or removed with the harvested crop are subject to movement away from the point of
FATE AND TRANSPORT OF PESTICIDES 315 application. 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. FIGURE 8-1 Interactions and loss pathways of organic chemicals (OCs) in soils. Source: B. L. Sawhney and K. Brown. 1989. Reactions and Movement of Organic Chemicals in Soils. Special Publication No. 22. Madison, Wis.: Soil Science Society of America. Reprinted with permission from Â© American Society for Agronomy, Crop Science Society of America, and Soil Science Society of America. 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. Transformations and transport are strongly influenced by site-specific conditions and management practices. Pesticide Properties Chemical-specific properties influence the reactivities of pesticides (Porter and Stimman, 1988). Pesticides that dissolve readily in water are
FATE AND TRANSPORT OF PESTICIDES FIGURE 8-2 Pesticide transport and transformation in the soil-plant environment and the vadose zone. Source: H. H. Cheng, ed. 1990. Pesticides in the Soil Environment: Processes, Impacts, and Modeling. Soil Science Society of America Book Series No. 2. Madison, Wis.: Soil Science Society of America. Reprinted with permission from Â© American Society for Agronomy, Crop Science Society of America, and Soil Science Society of America. 316
FATE AND TRANSPORT OF PESTICIDES 317 considered to be highly soluble. These chemicals have a tendency to be leached through the soil to groundwater and to be lost as surface water runoff from rainfall events or irrigation practices. Pesticides with high vapor pressures are easily lost to the atmosphere during application, and shortly thereafter they are lost from the soil through gaseous diffusion. Some highly volatile pesticides, however, may also move downward into aquifers. 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. Pesticides may be degraded (transformed) by chemical and biological processes. Chemical degradation occurs through such reactions as photolysis (photochemical degradation), hydrolysis (reaction with water), oxidation, and reduction. Biological degradation may also occur as soil microbes consume or breakdown pesticides. These microbes are most prevalent in the top several centimeters of soil. The extent of degradation may range from the formation of metabolites (daughter products) to the formation of inorganic decomposition products. Once a pesticide enters the soil, its fate is largely dependent on sorption and persistence (Rao and Hornsby, 1989). Sorption is commonly evaluated by use of a sorption (partition) coefficient (Koc) based on the organic carbon content of soils. Persistence is commonly evaluated in terms of half-life, which is the time that it takes for 50 percent of a chemical to be degraded or transformed. Pesticides with low sorption coefficients are likely to leach. Pesticides with long half-lives could be persistent. In Table 8-1 Rao and Hornsby (1989) provide a list of the pesticides as well as their sorption coefficients and half-lives. Pesticides are classified as nonpersistent if they have half-lives of 30 days or less, moderately persistent if they have half-lives longer than 30 days but less than 100 days, and persistent if they have half-lives longer than 100 days. Within these persistence classes, the pesticides are listed in ascending order of their sorption coefficients. Threshold values indicating the potential of a chemical for groundwater contamination have been proposed by the EPA (1986a). A pesticide is likely to contaminate groundwater (leach) if its sorption coefficient is low, its half-life is long, and its water solubility is high. It should be noted that it is difficult to predict the half- life of a chemical in the field because of dependent variables such as soil temperature and moisture, microbial populations, and soil types. The pesticide residues most commonly found in U.S. groundwaters include alachlor, aldicarb, atrazine, bromacil, carbofuran, cyanazine,
FATE AND TRANSPORT OF PESTICIDES 318 TABLE 8-1 Partition Coefficients and Half-Lives of Pesticides Used in Florida Pesticide (common name) Sorption Coefficient (ml/g of organic Half-Life (days) chemical) Nonpersistent Dalapon 1 30 Dicamba 2 14 Chloramben 15 15 Metalaxyl 16 21 Aldicarb 20 30 Oxamyl 25 4 Propham 60 10 2,4,5-T 80 24 Captan 100 3 Fluometuron 100 11 Alachlor 170 15 Cyanazine 190 14 Carbaryl 200 10 Iprodione 1,000 14 Malathion 1,800 1 Methyl parathion 5,100 5 Chlorpyrifos 6,070 30 Parathion 7,161 14 Fluvalinate 100,000 30 Moderately Persistent Picloram 16 90 Chlormuron-ethyl 20 40 Carbofuran 22 50 Bromacil 32 60 Diphenamid 67 32 Ethoprop 70 50 Fensulfothion 89 33 Atrazine 100 60 Simazine 138 75 Dichlorbenil 224 60 Linuron 370 60 Ametryne 388 60 Diuron 480 90 Diazinon 500 40 Prometryn 500 60 Fonofos 532 45 Chlorbromuron 996 45 Azinphos-methyl 1,000 40 Cacodylic acid 1,000 50 Chlorpropham 1,150 35 Phorate 2,000 90 Ethalfluralin 4,000 60
FATE AND TRANSPORT OF PESTICIDES 319 Pesticide (common name) Sorption Coefficient (ml/g of organic Half-Life (days) chemical) Chloroxuron 4,343 60 Fenvalerate 5,300 35 Esfenvalerate 5,300 35 Trifluralin 7,000 60 Glyphosphate 24,000 47 Persistent Fomesafen 50 180 Terbacil 55 120 Metsulfuron-methyl 61 120 Propazine 154 135 Benomyl 190 240 Monolinuron 284 321 Prometon 300 120 Isofenphos 408 150 Fluridone 450 350 Lindane 1,100 400 Cyhexatin 1,380 180 Procymidone 1,650 120 Chloroneb 1,653 180 Endosulfan 2,040 120 Ethion 8,890 350 Metolachlor 85,000 120 SOURCE: P. S. C. Rao and A. G. Hornsby. 1989. Behavior of Pesticides in Soils and Waters. Soil Science Fact Sheet SL 40 (revised). Gainesville: University of Florida. DBCP, dimethyltetrachloroterephthalate (DCPA), 1,2-dichloropropane, dinoseb, dyfonate, ethylenedibromide (EDB), metolachlor, metribuzon, oxamyl, simazine, and 1,2,3-trichloropropane (U.S. Environmental Protection Agency, 1986a). Those pesticides that are strongly sorbed to soil clays and organic matter may be subject to removal by surface runoff. Pesticides that exhibit such behavior and that are present in surface waters include the organochlorine DDT and its metabolites dichlorodiphenyldichloroethane) (DDD) and dichlorodiphenyldichloroethylene (DDE), dieldrin, endosulfan, toxaphene, lindane, heptachlor, chlordane, and difocol. Other pesticides that are weakly sorbed and have high water solubilities may be lost in the dissolved state. Erosion control practices will have little effect on such losses (Wauchope, 1978). Examples of pesticides found in the water phase of agricultural runoffs include the herbicides 2,4-D, dicamba, dinoseb, (4-chloro-2-methylphenoxy) acetic acid (MCPA), and molinate (Wauchope, 1978).
FATE AND TRANSPORT OF PESTICIDES 320 Soil Properties Soil properties have significant influences on the fate and transport of pesticides in croplands (Porter and Stimman, 1988). In general, the infiltration rate and hydraulic conductivity (soil permeability) of coarser-textured soils are greater than those of finer-textured soils. A chemical that readily infiltrates into the soil is less likely to be lost in surface runoff but is more likely to be leached into groundwater. The travel time of soil water and its associated dissolved pesticide is shorter in coarser-than in finer-textured soils. Soil permeability may have some influence on the rate at which volatile gases are lost. Moreover, the sorptive capacity of fine-textured soils is greater than that of coarse-textured soils because of the higher clay and organic matter contents of fine-textured soils; hence, pesticides are less vulnerable to leaching. pH is an important soil property for those pesticides participating in hydrolysis reactions. For instance, DBCP is chemically degraded into its metabolites by the substitution of chloride and bromide at the halogenated sites by hydrogen ions. The hydrolysis or dehalogenation of DBCP occurs in the soil at a faster rate in the alkaline pH range. Soil structure is another property that reflects the manner in which soil particles are aggregated and cemented. A soil with a weak structure is more likely to be eroded and have lower infiltration rates, and hence, sorbed pesticides are more likely to be discharged through runoff. Recent evidence indicates that at times soil macropores and cracks have a major effect on the movement of pesticides in soils. Macropores are formed by earthworms and decayed root systems, while cracks are formed by soil shrinkage. 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). In general, the groundwater table is shallower in humid regions than in more arid regions. A shallow depth to the groundwater offers less opportunities for pesticide sorption and degradation. The travel time of the pesticide to the water table may range from days to a week if the depth to the water table is shallow, and soil is permeable, and the amount of rainfall exceeds the water-holding capacity of the soil. In contrast, the travel time may be on the order of decades
FATE AND TRANSPORT OF PESTICIDES 321 in arid regions where the water table is tens of meters below the land surface. Hydrogeologic conditions (underground plumbing) beneath the soil profile may dictate the direction and rate of chemical movement. The presence of impermeable lenses or layers in the soil profile and underlying strata may limit the vertical movement of pesticides. 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. Of particular concern is the presence of karsts (limestone) and fractured geologic materials that generally transmit water and chemicals rapidly to the groundwater body. Climatic and weather conditions other than rainfall may also influence the fate of pesticides. Warmer temperatures tend to accelerate physical, chemical, and biological processes such as volatility, water solubility, and microbial degradation, respectively. High winds and high evaporation rates may accelerate volatilization and other processes that contribute to gaseous losses of pesticides. Management Practices Management practices such as the rate and timing of pesticide applications and the mode of pesticide application also affect pesticide transport processes. The recommended practices (Porter and Stimman, 1988; U.S. Congress, Office of Technology Assessment, 1990) include pesticide use only when and where it is necessary and in amounts adequate to control pests. Those who use pesticides should carefully follow the directions on the label to minimize harmful effects to the applicator as well as potential losses to the environment. Pesticide users should select pesticides that are less likely to leach. Irrigation should be avoided shortly after pesticide application, to reduce losses through runoff and leaching. The best-management practices for pesticide use are highly specific to crops and locations. Following is a list of variables that affect and conditions that increase the likelihood of pesticides leaching into groundwater: â¢ pesticide properties â¢ high solubility â¢ low adsorption â¢ persistence
FATE AND TRANSPORT OF PESTICIDES 322 â¢ soil characteristics â¢ coarse texture, high permeability â¢ low organic matter content â¢ presence of macropores â¢ site conditions â¢ high permeability of vadose region â¢ shallow depth to groundwater â¢ wet climate or heavy irrigation â¢ low soil temperatures â¢ management practices â¢ pesticide injection or incorporation into soil â¢ poor timing of chemical application with rainfall or irrigation A number of universities and agencies in various locations (for example, Hawaii, New York, Pennsylvania, Minnesota, and Florida) are using geographic information systems to identify aquifers vulnerable to contamination by pesticides. The management practices that can be used to reduce pesticide pollution of surface water and groundwater are discussed below in greater detail. Mass Balance Figure 8-2 presents a comprehensive scheme of the fates of pesticides applied in agroecosystems. Despite the vast knowledge base for the reactivity and transport of pesticides, a complete mass balance of the fate of any field- applied pesticide does not exist in the literature. Investigators have difficulty obtaining mass balances of the fates of pesticides for a number of reasons. Pesticides include a broad class of agrichemicals with widely ranging properties and behaviors that defy generalizations. There are technical difficulties and high costs associated with measuring over time the fraction of pesticides present in the various multimedia compartments and subcompartments in Figure 8-2. Some processes, such as volatilization, sorption, photolysis, foliar washouts, and surface runoffs, occur over short time intervals (for example, hours and days), whereas others occur over long time intervals (for example, months and decades), 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). Recognizing this dilemma of acquiring an adequate mass balance, a concerted effort to obtain mass balances is being made through modeling (see below).
FATE AND TRANSPORT OF PESTICIDES 323 Some researchers have estimated that only 1 to 2 percent of insecticides applied to foliage is absorbed by the target pest. They base this estimate on a synthesis of conceptual mass balance. Figure 8-3, for example, is a mass balance for a typical aerial spray-foliar application of an insecticide. The hypothetical mass balance in Figure 8-3 indicates problem areas where the efficacies of pesticide applications can be improved. Even though a complete mass balance may not be available for a specific field case study, a few examples of the measured fates of pesticides from numerous literature sources would give some perspective. During the application stage of pesticide use, considerable losses may occur through spray drift and volatilization. Spray drift constitutes about 3 to 5 percent of the loss under quiescent wind conditions, but it is typically much greater (40 to 60 percent for many insecticides). Loss from volatilization ranges from 3 to 25 percent, but it may be as great as 20 to 90 percent for methylparathion, for example, depending on weather conditions. 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. Field measurements of pesticides applied by such practices reveal that the portion of pesticide volatilized is 2 to 12 percent for most pesticides but could be as high as 50 to 90 percent for volatile chemicals such as trifluralin. Seasonal losses of pesticides in surface runoffs are typically in the range of less than 1 to 5 percent (Wauchope, 1978); the lower losses are for foliar-applied organochlorines like toxaphene, and the higher losses are for wettable powders such as triazine. Pesticide loss through leaching into groundwater is another major component in the mass balance. Factors contributing to the vulnerability of groundwater contamination were discussed above. The mass flux for leaching is sometimes taken at some prescribed soil depth, like the bottom of the root zone or the surface of the groundwater table. It should be noted that within the crop root zone and in the vadose region the pesticide is subject to numerous degradation and immobilization mechanisms. A study of the fate of DBCP applied to cropland in California's San Joaquin Valley provides an example of the impact of these multiple processes. The peak concentration of DBCP at the time of application was about 1,500 mg/ L (1,500 ppm), but at the 30-cm depth in the surface soil its peak concentration was only about 1.5 mg/L (1.5 ppm). Based on a transport model that considered diffusive and mass flows as well as sorption and decay (Tanji, 1991a), the peak concentration is expected to be about 0.03 mg/L (0.03 ppm) as the DBCP reached the water table after passing through 30 m of the vadose zone. That peak concentration of
FATE AND TRANSPORT OF PESTICIDES 324 FIGURE 8-3 Mass balance of a hypothetical aerial foliar-spray application of an insecticide. Source: U.S. Congress, Office of Technology Assessment. 1990. Beneath the Bottom Line: Agricultural Approaches to Reduce Agrichemical Contamination of Groundwater. Report No. OTA-4-418. Washington, D.C.: U.S. Government Printing Office.
FATE AND TRANSPORT OF PESTICIDES 325 DBCP at the bottom of the vadose region was expected to be diluted to about 0.009 mg/L (0.009 ppm) in the surface of the horizontally flowing groundwater. The cumulative loss of DBCP from the soil surface to the groundwater, therefore, was about 99.4 percent of the 1.5 mg/L (1.5 ppm) at the 30-cm soil depth and nearly 100 percent of the 1,500 mg/L (1,500 ppm) at the soil's surface. These calculations, which involved 31 years of travel time, indicate that even a persistent chemical like DBCP is subject to huge sink losses, but because the maximum contaminant level set by EPA is so low 0.02 Âµg/L (0.02 ppb) DBCP in drinking water in California, the minuscule leaching loss to the groundwater system is of growing concern. Given the difficulty of obtaining mass balances on the fates of pesticides applied to croplands and the fact that minuscule losses to particular portions of the environment may be hazardous, a concerted effort must be made to increase the efficiency of pesticide use. Since everything must go somewhere, source control is the best approach. Modeling Efforts Computer simulation models are used as tools to evaluate more fully the fate and transport of pesticides in agricultural systems. Spray Pesticide Models Spray pesticide models (Cheng, 1990) based on diffusion or ballistics are available for the design and evaluation of spray application systems to minimize aerial drift and volatile losses as well as to elevate accurate targeting of the spray. Pesticide Runoff Models Pesticide runoff models (Cheng, 1990) from the small plot to the watershed-scales are being used to develop best-management practices. Considerable efforts were initially made toward determining the hydrologic processes that contribute to runoff and erosion. The reactivities of pesticides were then coupled to runoff models. 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. Modeling of pesticide behavior and the leachabilities of pesticides
FATE AND TRANSPORT OF PESTICIDES 326 has taken two general approaches. Conceptual screening models for pesticides typically consider solubility, sorption, persistence, volatility, and mobility. Such models rank the behavior of the pesticide and its potential movement in soil. One example is that prepared by Rao and Hornsby (1989) (see Table 8-1). A more comprehensive approach is that of Jury and colleagues (1984), in which chemicals are screened under idealized, standardized scenarios. However, screening models are not environmental fate prediction models and are inappropriate outside the idealized conditions that lead to their derivation. Process-Based Simulation Modeling Process-based simulation modeling for pesticide reactivity and transport has received a more intensive effort. These research-oriented models require extensive input data and have mainly been tested in laboratory soil columns and small-scale research plots. Recently, they have been extended and/or applied for management of larger-scale field environments, for example, the erosion/ productivity impact calculator (EPIC) (Sharpley and Williams, 1990), groundwater loading effects of agricultural management systems (GLEAMS) (Leonard et al., 1987), pesticide root zone model (PRZM) (Carsel et al., 1984), and leaching estimation and chemistry modelâpesticides (LEACHM-P) (Wagenet and Hutson, 1989). Model Performance Jury and colleagues (1988) and Green and colleagues (1986) have pointed out some difficulties in predicting groundwater contamination by chemicals even with state-of-the-art simulation models. 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. Such considerations need to be incorporated into pesticide leaching models for improved model performances. Most recently, Pennell and colleagues (1990) compared the performances of five simulation models for simulating the behaviors of aldicarb and bromide from a given field study. The models tested were the chemical movement in layered soils model (CMLS) (Nofziger and Hornsby, 1986), the method of saturated zone solute estimation (MOUSE) (Steehuis et al., 1987), the pesticide root zone model (PRZM)
FATE AND TRANSPORT OF PESTICIDES 327 Groundwater in artesian aquifers have enough pressure to flow all the way to the surface. The detection of pesticides in aquifers that are hundreds of feet deep has increased concern about the eventual fate of pesticides applied to croplands. Credit: Agricultural Research Service, USDA.
FATE AND TRANSPORT OF PESTICIDES 328 (Carsel et al., 1984), GLEAMS (Leonard et al., 1987), and LEACHM-P (Wagenet and Hutson, 1989). GLEAMS and MOUSE underestimated bromide and aldicarb dissipations, whereas the other models proved satisfactory in predicting both the depth of the solute's center of mass and the amount of pesticide degradation. None of the models, however, accurately predicted the pesticide concentrations measured throughout the soil profile. In addition to possible deficiencies in the model, the investigators pointed out the potentially large sampling error in the field because of spatial variability. Appraisals of Models Mathematical models of surface runoff and leaching of pesticides have been constructed, tested, and used with varying degrees of success. The formulation of each model varies according to the objectives of the modeling exercises and the professional training and biases of the model developer. The result has been a collection of approaches applicable to descriptions of surface runoff processes and a second body of efforts that have focused on leaching processes. Investigators have not often attempted to make comprehensive simultaneous descriptions, and when they have, the results have been complex, data-intensive models that cannot easily be used by anyone other than the developer (Wagenet and Rao, 1990). 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. Investigators have obtained mixed results with these models. To date there is apparently no increased predictive capability obtained by using models that are more mechanistic and data-intensive than using models that provide less of an understanding of the field-scale processes related to pesticide loss including surface hydrological processes. Soil leaching models of pesticide fates contain similar problems, although the basic physical, chemical, and biological processes in the soil are perhaps better-defined than surface hydrology processes. Useful field-scale models exist in both mechanistic (Carsel et al., 1984; Wagenet and Hutson, 1989) and nonmechanistic (Nofziger and Hornsby, 1986; Rao et al., 1976) forms, although care must be used in choosing the
FATE AND TRANSPORT OF PESTICIDES 329 situations to which these models are applied. Neither mechanistic nor empirical models have been widely tested under field conditions. The empirical versions are generally intended for qualitative educational purposes rather than quantitative regulatory purposes. A number of solute transport models that are intermediate between the mechanistic and nonmechanistic extremes have been proposed (reviewed by Addiscott and Wagenet, 1985), but they have yet to be applied to pesticide leaching by water. The spatial variability of soil processes also has generated interest in stochastic or probabilistic approaches to describing chemical leaching in soil (Jury et al., 1988) or surface loss of pesticides (Mills and Leonard, 1984). These approaches may prove to be the most useful because they show promise as descriptors of spatially variable processes, yet they are neither as mathematically cumbersome nor as computationally demanding as current mechanistic models (Wagenet and Rao, 1990). Stochastic or probabilistic approaches can also account for the stochastic nature of precipitation and its effect on leaching or runoff (Hornsby, 1988). One excellent source of advancements in groundwater modeling software is the International Ground Water Modeling Center, Colorado School of Mines. Although not all of these groundwater models would be suitable for simulating pesticide transport, some should be directly applicable. These new modeling efforts, however, are not typically incorporated into pesticide leaching models; hence, incorporation will require considerable effort on the part of modelers researching the transformations and transport of pesticides in agricultural systems. 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 â¢ selection of proper pesticides and formulations; â¢ timing of and improvement in pesticide application methods to minimize drift and volatile losses; â¢ use of erosion and runoff control measures to reduce losses through runoff and leaching; â¢ use of nonchemical pest control measures such as crop rotations and management; and â¢ integrated pest management, which embodies most of the recommended practices cited earlier.
FATE AND TRANSPORT OF PESTICIDES 330 Source control or reducing the amounts of pesticides used should be the first line of action. Selection and Formulation of Pesticides Pesticides should be used only when and where they are necessary and only in amounts adequate to control the target pest. 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. Although pesticides are formulated mainly for ease of application, the natures of the formulations do have some impacts on potential losses to the environment. For instance, use of pesticides in the granular, pelleted, or emulsified form results in less drift and volatile losses during application. Pesticides in the form of dusts, wettable powders, or fine liquid sprays are more subject to drift losses. Pesticides applied as liquid mixtures or concentrated solutions have greater potential for loss through volatilization. Those pesticides in wettable powders are more susceptible to runoff losses. Timing and Pesticide Application Methods For maximum efficacy, pesticides should be applied at the right time. Irrigation shortly after application may result in excessive runoff losses. On the other hand, some pesticides, especially those for soilborne pests, are irrigated into the soil for more uniform application or deeper placement. At times, repeated applications of a given pesticide may become ineffective, perhaps because of an increase in the transformation rate. Pesticide users should follow the directions on the pesticide label. Pesticides should be carefully measured, and the application equipment should be properly calibrated and maintained. Pesticides are applied by aerial and ground methods and through irrigation systems. During application, the pesticide should be directed only to the target site or pest. Aerial application methods generally result in higher drift losses than those from ground application. If conversion from aerial to ground spraying is not possible, aerial applications should be accomplished when the potential for drift, volatile losses, and runoffs are the least, that is, under calm conditions and cooler temperatures and not when rain is likely to occur.
FATE AND TRANSPORT OF PESTICIDES 331 Ground application of pesticides is favored, but substantial losses can also occur by use of this application method. Losses may be reduced with improved application technologies such as controlled droplet applicators, drift-shielded applicators, ultra-low-volume equipment, electrostatic sprayers, and computer- controlled equipment. Other means of controlling losses include the use of formulations that thicken the spray, such as oil emulsions and foliage-wetting agents. Pesticides may be also introduced into surface irrigation streams or pressurized systems such as sprinklers and drip/trickle irrigation devices. Surface water applications, such as furrow and basin methods, tend to be less uniform in distributing water to the field and, therefore, often affect the uniformity of distribution of water and chemicals. Use of overhead sprinklers can result in the same losses that occur with aerial spray applications. Erosion and Runoff Control Practices Environmental losses of pesticides through surface runoff, leaching, and volatilization may be reduced by erosion and runoff control practices (Wagenet and Rao, 1990; Wauchope, 1978), essentially the same practices recommended for the control of other agricultural nonpoint source pollutants. In some instances, however, practices that result in lower losses by one pathway (for example, runoff) may result in greater losses through another pathway (for example, leaching). Conservation practices can reduce surface runoff and soil losses through tillage practices, including conservation tillage and no-till practices, contouring and strip-cropping, and use of cover crops, grassed waterways, and filter strips. Structural practices include the use of land leveling, terraces, subsurface drainage, improved application systems, and sediment retention ponds. There are some differences of opinion about the potential benefits of conservation and no-tillage practices with regard to pesticides. For instance, conservation tillage practices have great potential for reducing erosion and sediment production. Such reductions, in turn, would reduce the discharge of sediment-bound pesticides. 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. Herbicide application rates are generally greater in no-tillage systems. In some conservation tillage practices, incorporation of pesticides into soil may be more difficult.
FATE AND TRANSPORT OF PESTICIDES 332 There is great potential for reducing runoff and leaching losses in irrigated agricultural systems. Improved furrow irrigation systems that recycle drainage water may eliminate runoff losses. 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. For the foreseeable future, a mixture of chemical and nonchemical practices cannot be avoided. Future developments in breeding of pest-resistant crop species or genetic engineering of organisms that prey on current pests will play a large role in determining the extent to which nonchemical control measures can be adopted. For the foreseeable future, nonchemical control measures that complement crop management practices intended for pest control will need to be developed. This approach is the general strategy of integrated pest management programs. Integrated Pest Management Integrated pest management (IPM) has the potential to reduce the need for pesticides and reduce the use of pesticides that might become pollutants. IPM involves understanding the pest in question, its host crop, and its natural predators so that ecologically and economically sound pest control techniques can be realized (Flint, 1989; Holden, 1986; U.S. Congress, Office of Technology Assessment, 1990). An IPM strategy involves a number of guidelines. â¢ Determine the economic threshold of damaging pests. This economic threshold is defined as the point at which the cost of pest control equals the value of the crop lost because of pest damage. â¢ Lower the equilibrium position of the pest below the economic threshold. The equilibrium position is the average pest density in a field as determined over many years. Lowering the equilibrium position may be achieved by encouraging or establishing the pest's natural enemies such as parasites and predators, using pest-resistant or pest-free plant varieties, and modifying the pest's environment by using crop rotations and eliminating overwintering sites. â¢ Use the least environmentally damaging pesticide.
FATE AND TRANSPORT OF PESTICIDES 333 â¢ Monitor the pest populations to decide when to apply pesticides or when to adjust integrated pest management strategies. There is a need for integrated management of not only pesticides but also fertilizers. There must also be proper soil, water, and crop management. ASSESSMENTS OF THE KNOWLEDGE BASE There is considerable laboratory-based knowledge about the physical and chemical properties of pesticides, their chemical and biological degradation mechanisms and persistence, their tendency to be sorbed by soil particles, and their movement in the dissolved and gaseous states in soils. In contrast, investigators' abilities to predict the behavior and transport of pesticides under field conditions appear to be weak. Part of this weakness may be attributed to the spatial and temporal variabilities that are part of every field soil, introducing much uncertainty into the interpretation of sampling and monitoring studies. This unpredictability applies to all of the agricultural nonpoint source pollutants, but it is especially so for pesticides because of the diversity of pesticides in use as well as their different behaviors in physical, chemical, and biological processes. Nevertheless, it seems that the existing knowledge base from research and practical field experiences is not being fully disseminated or used to protect the environment. 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. The same applies to pesticide losses via surface runoff. Wauchope and colleagues (1992) developed an extensive data base that provides referenced data on sorption, degradation vapor pressure, and aqueous solubilities. This data base can be used to select pesticides that are less vulnerable to leaching or runoff. It is of interest to examine DDT and DBCP. DDT is an organochlorine insecticide that is strongly sorbed by soils and has a low water solubility and a long half-life. Yet, it is found in some California wells, even though investigators predicted that it would take thousands of years for DDT to reach the water tables of those wells. The mode of entry into these groundwaters was probably not from passage through the soil profile and substrata but, perhaps, through the well casing. In contrast,
FATE AND TRANSPORT OF PESTICIDES 334 DDT and its metabolites are still found in suspended sediments eroded from furrow irrigated lands in some portions of California. With a half-life of about 3,800 years and a sorption coefficient of about 2,400, it is understandable that residues of DDT and its metabolites still exist in surface soils and sediments. DBCP is a nematicide that was banned from use in 1979 because of its toxicity to humans. This compound has a high water solubility, moderate vapor pressure, low sorption coefficient, and short photolysis half-life, but it has a long hydrolysis half-life in the vadose zone. Although much of this volatile pesticide is dissipated shortly after application, it is highly subject to leaching losses. The presence of DBCP was detected in California and Florida wells in 1979. After 15 years of well sampling in California, 2,500 of 4,500 wells showed detectable concentrations of DBCP. The maximum contaminant level of 1 Âµg/liter (1 ppb) for DBCP in California was lowered to 0.02 Âµg/liter (0.02 ppb) in 1989, and numerous drinking water wells have been shut down recently. The behavior and accumulation of DBCP in well waters are now more clearly understood because of the gain in the knowledge base of this pesticide since the 1960s. PROPER USE OF PESTICIDES There appears to be little chance of discovering a perfect pesticideâone that is precise enough to attack the target pest and then suddenly dissipate and accurate enough to reach the target pest and not move past the root zone. 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. Aggressive efforts to adopt and adapt these available technologies, systems, and practices must be pursued. The research required to develop alternative pest control strategies and to develop farming systems based on alternative pest control practices should be accelerated. Long- term efforts to reduce the need for environmentally damaging pesticides is the most promising approach to reducing environmental damages from pesticides. Pesticides are perhaps the only toxic substances that are purposefully applied to the environment, a rather unique permit given the present-day
FATE AND TRANSPORT OF PESTICIDES 335 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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