Persistence of Pesticides in Soil

The soil is a dynamic biotic and abiotic system. Pesticides deposited in or on the soil have varying capacities to be adsorbed to clay minerals and organic matter. Such adsorption reduces both the movement and the biological activity of the pesticide. In addition to soil adsorption, several other factors are known to influence the behavior and fate of pesticides after the chemicals are in contact with soil. 

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El Beit lOD, Cotton DE, Wheelock V. 1983. Persistence of pesticides in soil leachates Effect of pH, ultraviolet irradiation and temperature. Int J Environ Stud 21 251 -259. 

Kearney, P.C., Nash, R.G., Isensee, A.R. (1969) Persistence of pesticides in soil. In Chemical Fallout Current Research on Persistence Pesticides. Chapter 3, pp. 54—67, Miller, M.W., Berg, C.C., Editors, Charles C. Thomas, Springfield, Illinois. 

Factors affecting the movement and persistence of pesticides in soils can involve leaching, fixation by soil colloids, chemical and microbial decomposition, adsorption, photodecomposition, etc. 

The persistence of pesticides in soil is also influenced by their formulations. Granules are generally the most persistent. Wettable powders and dusts are usually less persistent than emulsifiable concentrates (McEwen and Stephenson, 1979). 

Persistence of pesticides in the environment is controlled by retention, degradation, and transport processes and their interaction. Retention refers to the abihty of the soil to bind a pesticide, preventing its movement either within or outside of the soil matrix. Retention primarily refers to the sorption process, but also includes absorption into the soil matrix and soil organisms, both plants and microorganisms. In contrast to degradation that decreases the absolute amount of the pesticide in the environment, sorption processes do not affect the total amount of pesticide present in the soil but can decrease the amount available for transformation or transport. 

Table 6.4 shows first-order rate coefficients and tx/2 values for degradation of a number of pesticides in soils (Rao and Davidson, 1982). The k and t1/2 values calculated from field data are based on the disappearance of the parent compound (solvent extractable). Table 6.4 also includes k and t1/2 values calculated on mineralization (14C02 evolution) and parent-compound disappearance from laboratory studies. The t1/2 values were smaller for field than for laboratory studies. Rao and Davidson (1980) attribute this to the multitude of factors that can affect pesticide disappearance in the field while only one factor is studied in the laboratory. Rao and Davidson (1982) suggested that pesticides be classified into three groups based on values (Table 6.5) nonpersistent (t1/2 < 20 days), moderately persistent (20 < t1/2 < 100 days), and persistent (/1/2 > 100 days). Most chlorinated hydrocarbons are grouped as persistent, while carboxyl-kanoic acid herbicides are nonpersistent. The s-triazines, substituted ureas, and carbamate pesticides are moderately persistent. 

In summary, an endeavor has been made to cover briefly the effect of several pesticides in our total environment, from the food we consume and the soil in which it is grown to the water we drink and fish in and even to the air we breathe. Of course, with a subject as broad as this, only a small fraction of the actual research conducted in this general area could be cited. Obviously, the fate and persistence of pesticides in our total environment are highly complex and worthy of our continued surveillance. However, we should always keep this problem of pesticide contamination of our environment in its proper perspective. Even though trace quantities of pesticide residues are found in our water, fish, milk, etc., and perhaps somewhat higher concentrations are found in some vegetables and fruit we may consume, there is no reason for unwarranted alarm. When pesticides are used properly, the concentration of residues reported are almost always far below levels judged hazardous to health... 

Any factors that stimulate the growth of soil microorganisms or that increase the availability of pesticides in soil will enhance the degradation of the chemical. Felsot et al. (1981) found that the persistence of carbofuran was inversely correlated with microbial activity in corn-cropped soils. Tu and Miles (1976) reported that 88% of parathion was lost from soil in 7 months diazinon, 92% lost in 20 weeks paraoxon, 100% hydrolyzed in 12 hr mala-thion, 50-90% lost in 24 hr and carbofuran, 50% lost in 3-50 weeks. 

This book focuses on the chemical persistence and ocotoxicological behavior of pesticides In soil, water, and plants. Recent research data are presented on transport, adsorption and absorption, accumulation, degradation, biological effects, aquatic toxicity, air pollution, exposure, and risk estimation. 

Considerable research effort has been paid to the problems of the persistence of chemical means for the plant protection. Many of the studies have been aimed at the persistence of pesticides in the soil. Insecticides based on chlorinated hydrocarbons are particularly persistent, for instance DDT, BHC isomers and so-called polychlorinated cyclodiene compounds, aldrin, dieldrin, andrin, heptachlor. In many countries the use of these pesticides has been either restricted or even prohibited on account of their persistence in the environment. For chemical protection of plants they are gradually being replaced by organophosphate and carbamate substances, which are more toxic, but are less stable in the environment (e.g. parathion, dichlor-vos, carbaryl, propoxur). 

Finally, the real implications and usefulness of data from the literature, when available, must be carefully checked, particularly if they quantify the residence time of pesticides in soil. These data are often produced for agronomic and not ecotoxicological purposes and are related to the complete disappearance of the molecule as a result of leaching, runoff, volatilization and other transport patterns, and not only reaction and transformation. Thus, they are completely useless for the description of persistence in evaluative models. 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

Jury WA, Spencer WF, Farmer WJ. 1983. Use ofmodels for assessing relative volatility, mobility, and persistence of pesticides and other trace organics in soil systems. In SaxenaJ, ed. Hazard assessment of chemicals Current developments. Vol. 2, New York, NY Academic Press, 1-43. 

FIGURE 4.3 Loss of pesticides from soil, (a) Breakdown of herbicides in soil, (b) Disappearance of persistent organochlorine insecticides from soils (from Walker et al. 2000). 

Attention focused on inorganic arsenical pesticides after accumulations of arsenic in soils eventually became toxic to several agricultural crops, especially in former orchards and cotton fields. Once toxicity is observed, it persists for several years even if no additional arsenic treatment is made (Woolson 1975). Poor crop growth was associated with bioavailability of arsenic in soils. For example, alfalfa (Medicago sativa) and barley (Hordeum vulgare) grew poorly in soils con-... 

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