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Behavior of Pesticides in Soils and Water 1 Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. SL 40 Behavior of Pesticides in Soils and Water 1 P.S.C. Rao and A.G. Hornsby2 During the past twenty years, concern has arisen applied, and to show how this information can be as to the presence of pesticides in the environment used, along with other factors, to select the proper and the threat they pose to wildlife and mankind. pesticide. Certainly, pesticides have improved longevity and the quality of life, chiefly in the area of public health. PATHWAYS OF PESTICIDE LOSS The use of pesticides also constitutes an important There are basically two ways properly-applied aspect of modern agriculture. Florida's temperate to pesticides may reach surface and ground waters- subtropical climate favors growth of many harmful through runoff and leaching. Runoff is the physical insects, weeds and diseases, thus making this state transport of pollutants over the ground surface by particularly dependent on pesticides for economical rainwater which does not penetrate the soil. Leaching crop management. is a process whereby pollutants are flushed through Unfortunately, pesticides are poisons and can be the soil by rain or irrigation water as it moves particularly dangerous when misused. Fish-kills, downward. In many areas of Florida, soils are sandy reproductive failure in birds, and acute illnesses in and permeable and leaching is likely to be a more people have all been attributed to exposure to or serious problem than runoff. We now have ingestion of pesticides - usually as a result of technology to help estimate the potential misapplication or careless disposal of unused contamination of water from a given pesticide. To pesticides and containers. Pesticide losses from areas understand this technology, it is necessary to know of application and contamination of non-target sites how a pesticide behaves in soil and water. such as surface and ground water represent a Once applied to cropland, a number of things monetary loss to the farmer as well as a threat to the may happen to a pesticide (Figure 1) . It may be taken environment. Thus, careful management of pesticides up by plants or ingested by animals, insects, worms, in order to avoid environmental contamination is or microorganisms in the soil. It may move desired by both farmers and the general public. downward in the soil and either adhere to particles or The purpose of this fact sheet is to explain how dissolve. The pesticide may vaporize and enter the pesticides can move from the area in which they are atmosphere, or break down via microbial and chemical pathways into other, less toxic compounds. 1. This document is Fact Sheet SL40, one of a series of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published: May 1993. Revised: December 2001. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. 2. P.S.C. Rao, Professor Emeritis; A.G. Hornsby, professor, Soil and Water Science Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611. The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean. Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. Behavior of Pesticides in Soils and Water 2 Pesticides may be leached out of the root zone or PERSISTENCE washed off the surface of land by rain or irrigation water. Evaporation of water at the ground surface Persistence defines the "lasting-power" of a can lead to upward flow of water and pesticides. In pesticide. Most pesticides break down or "degrade" most Florida soils this process is likely to be not as over time as a result of several chemical and important as downward leaching in response to microbiological reactions in soils. Sunlight breaks irrigation and or rainfall. The fate of a pesticide down some pesticides. Generally, chemical pathways applied to soil depends largely on two of its result in only partial deactivation of pesticides, properties: persistence and sorption. whereas soil microorganisms can completely breakdown many pesticides to carbon dioxide, water Note: Two other pathways of pesticide loss are and other inorganic constituents. Some pesticides through removal in the harvested plant and by produce intermediate substances, called metabolites vaporization (volatilization) into the atmosphere. as they degrade. The biological activity of these Occurrence of pesticides released into the atmosphere substances may also have environmental significance. have an impact on air quality and create problems Because populations of microbes decrease rapidly when agricultural workers enter the treated areas. below the root zone, pesticides leached beyond this High concentrations of pesticide residues in harvested depth are less likely to be degraded. However, some produce can have adverse health effects on pesticides will continue to degrade by chemical consumers. While these two pathways are important, reactions after they have left the root zone. they will not be covered in this factsheet, which is devoted to pesticide behavior in soil and water. Figure 1. Pathways of pesticide loss. P=pesticide. [Adapted from Skrotch and Sheet. 1981.] Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. Behavior of Pesticides in Soils and Water 3 likely to be leached compared to those with large K oc values. Degradation time is measured in half-life. Partition coefficients of several chemicals are Half-life (T ) is a measure of the amount of time it 1/2 shown in Table 1 . Note the wide range of partition takes for one-half the original amount of a pesticide coefficients. Values of partition coefficients listed in in soil to be deactivated. Half-life is sometimes Table 1 are independent of soil type and are defined as the time required for half the amount of characteristic of each pesticide. The partition applied pesticide to be completely degraded and coefficient is determined by a pesticide's chemical released as carbon dioxide. Usually, the half-life of a pesticide measured by the latter basis is longer than properties such as solubility and melting point. that based on deactivation only. This is especially The partition coefficient makes it possible to true if toxic or non-toxic metabolites accumulate in estimate a particular pesticide's chance of being lost the soil during the degradation. Table 1 groups some via runoff or leaching in a specific soil, and is of the pesticides used in Florida by persistency, or calculated via the formula: length of half-life, on the basis of their deactivation in surface soils. T values in subsoils and in ground K = (K )(%OM)(0.0058) 1/2 p oc water are usually much larger. Thus, as pesticides • K = index of sorption for a given pesticide on are leached to lower depths, their persistency p increases. Values for pesticide degradation, T , in a particular soil 1/2 subsoils and groundwater are scarce. • %OM = is the percent organic matter in the soil, SORPTION as determined by chemical analysis • K = partition coefficient of the pesticide Probably the single most important property oc influencing a pesticide's movement with water is its normalized to organic carbon as listed in Table1 . solubility. Soil is a complex mixture of solids, liquids Four important aspects of pesticide sorption by and gases that provides the life support system for soils should be recognized: roots of growing plants and microorganisms such as bacteria. When a pesticide enters soil, some of it will • For pesticides not adsorbed by soils, K is oc stick to soil particles, particularly organic matter, equal to zero (or K = 0). Thus, such pesticides p through a process called sorption and some will will leach in a manner similar to inorganic ions dissolve and mix with the water between soil such as nitrate which are also not adsorbed by particles, called "soil water." As more water enters most soils. the soil through rain or irrigation, the sorbed pesticide molecules may be detached from soil particles • For a given pesticide, sorption is greater in soils through a process called desorption. The solubility of with larger organic matter content (% OM). a pesticide and its sorption on soil are inversely Thus, pesticide leaching in soils with higher related; that is, increased solubility results in less %OM is expected to be slower compared to soils sorption. low in %OM. One of the most useful indices for quantifying • In most soils %OM decreases rapidly with pesticide sorption on soils is the partition coefficient increasing depth, this is particularly so beyond (K ). The K value is defined as the ratio of the crop root zone in sandy soils of Florida's oc oc pesticide concentration in the sorbed-state (that is, central ridge. Thus, pesticide sorption decreases bound to soil particles) and the solution-phase (that with increasing soil depth, and, as a is, dissolved in the soil-water). Thus, for a given consequence, leaching is likely to be more rapid amount of pesticide applied, the smaller the K in subsoils. oc value, the greater the concentration of pesticide in solution. Pesticides with small K values are more oc Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. Behavior of Pesticides in Soils and Water 4 • For a given soil, pesticides with smaller K oc PESTICIDE SELECTION AND USE values are sorbed to a lesser extent, and are therefore more likely to be leached when Agricultural use of pesticides should be part of compared to pesticides having larger K values.
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