Persistence of pesticides

For those pesticides that are cometabolized, ie, not utilized as a growth substrate, the assumption of first-order kinetics is appropriate. The more accurate kinetic expression is actually pseudo-first-order kinetics, where the rate is dependent on both the pesticide concentration and the numbers of pesticide-degrading microorganisms. However, because of the difficulties in enumerating pesticide-transforming microorganisms, first-order rate constants, or half-hves, are typically reported. Based on kinetic constants, it is possible to rank the relative persistence of pesticides. Pesticides with half-hves of <10 days are considered to be relatively nonpersistent pesticides with half-hves of >100 days are considered to be relatively persistent. 

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. 

Baker RD, Applegate HG. 1974. Effect of ultraviolet radiation on the persistence of pesticides. TexJ Sci 25 53-59. 

Eichelberger JW, Lichtenberg JJ. 1971. Persistence of pesticides in river water. Environ Sci Technol 5 541-544. 

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. 

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. 

Kazen C, Bloomer A, Welch R, et al. 1974. Persistence of pesticides on the hands of some occupationally exposed people. Arch Environ Health 29 315-318. 

Adapted from Chapman, R.A. and C.M. Cole. 1982. Observations on the influence of water and soil pH on the persistence of pesticides. Jour. 

Gambrell et al. [223] have discussed the recovery of DDT, Kepone and Permethrin added to soil suspensions incubated under controlled redox potential and pH conditions to determine the effect of time on the levels of the insecticides and their degradation products. Samples were analysed by gas chromatography, pH and redox potential affected the persistence of pesticides to different degrees. The recovery of DDT was affected by redox potential but not by pH. 

Different levels of pesticides can be found at harvest on grapes depending on the chemical characteristic of the active ingredients. Moreover, the persistence of pesticides can depend on photodegradation, thermodegradation, codistillation, and enzymatic degradation. 

Redondo, M.J., M.J. Ruiz, R. Boluda, and G. Font (1994). Persistence of pesticide residues in orchard soil. Sci. Total Environ., 156 199-205. 

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. 

To appraise the extent of environmental contamination is a problem in organic pesticide analysis. Analytical methods for such diverse samples as air and human tissue have been developed and are yielding important information. The fate and persistence of pesticides constitutes another complex problem. It includes consideration of environmental and biological effects upon pesticides, how they move in rivers, and how they are translocated from one environmental medium to another. Finally, there is the question of damage control. Can we reduce the introduction of pesticides into air and water If not, can we effectively remove these poisons from the parts we consume ... 

The physical factors (Figure 3) affecting the fate and persistence of pesticides in our environment are primarily climatic parameters such as sunlight, temperature, humidity, rain, wind, and the toxicants vapor pressure (13,42). Other important physical factors involve the plants or animals growth which will dilute and occasionally store pesticides in restricted sites, such as in oil or fat depots. 

Figure 3. Physical factors affecting the fate and persistence of pesticides...

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

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... 

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