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Home , Leaching (agriculture), Pesticide degradation

Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

SL 40

Behavior of in and 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 and mankind. . Certainly, pesticides have improved longevity and the quality of , 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 . Florida's temperate to pesticides may reach surface and ground - subtropical climate favors growth of many harmful through runoff and . 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 . Leaching crop management. is a process whereby pollutants are flushed through Unfortunately, pesticides are poisons and can be the soil by or 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 or ingested by animals, insects, worms, in order to avoid environmental contamination is or 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 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 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. 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 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 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 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 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 dissolve and mix with the water between soil such as which are also not adsorbed by particles, called "." 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. an overall pest management strategy which includes oc biological controls, cultural methods, pest ESTIMATING PESTICIDE LOSSES monitoring and other applicable practices, referred to altogether as Integrated Pest Management or IPM. In estimating pesticide losses from soils and their When a pesticide is needed its selection should be potential to contaminate groundwater or surface based on effectiveness, toxicity to non-target species, water, it is essential to consider simultaneously both cost, and site characteristics, as well as its solubility persistence and sorption. Quantitative estimations of and persistence. pesticide losses via runoff or leaching requires complex mathematical models. These models are Half- and partition coefficients are solved using mainframe or micro-computers and particularly important when the application site of a utilize site-specific soil, crop, management, and pesticide is near surface waters or is underlain with climatological information. This would include soil permeable and a shallow . Short type and its physical-chemical properties; the date, half-lives and intermediate to large K are best in oc amount, and method of ; the this situation. amount, duration, and frequency of rainfall or following pesticide application; and crop Many areas of Florida have impermeable growth characteristics. In the absence of such sub-soils which impede deep leaching of soluble information, however, a qualitative assessment of a pesticides. On such land, soluble pesticides with low K and moderate-to-long half-lives require cautious pesticide's potential to contaminate or oc groundwater is possible using K and T values application to prevent rapid transport in oc 1/2 as indices of pesticide sorption and persistence. water to a nearby lake or stream. Nonerosive soils are common to much of Florida and pesticides with large Strongly-sorbed and persistent pesticides (that is, K remain on the application site for a long time. oc compounds with large K and large T ) are likely However, the user should be cautious of pesticides oc 1/2 to remain near the ground surface, increasing the with long half-lives as they are likely to build up in chances of being carried to a stream or a lake via the soil. runoff. In contrast, weakly-sorbed but persistent pesticides (that is, compounds with small K and In addition to the pesticide solubility and soil oc large T ) may be readily leached through the soil permeability it is important that the pesticide's 1/2 and are more likely to contaminate groundwater. For toxicity to non-target species be considered. Some of nonpersistent pesticides with small T , the the pesticides listed in Table 1 have severely 1/2 possibility of surface water or groundwater restricted use due to acute toxicity or long half-life. contamination depends primarily on whether heavy An important purpose of the pesticide container label (or irrigation) occur soon after pesticide is to instruct users to apply the pesticide safely and application. Without water to move them downward, with minimum threat to non-target species, both on pesticides with short half-lives remain within the and off the application site. Pesticide users assume biologically - active crop root zone and may be responsibility to follow label instructions. It is unsafe degraded readily. In terms of then, and unlawful not to do so. pesticides with intermediate K values and short oc RELATED EXTENSION T values may be considered low risk because they 1/2 are not readily leached and are degraded fairly PUBLICATIONS rapidly. These general concepts are summarized in 1. Hornsby, A. G. 1986. Ground water: The Table 2. hidden . Fact Sheet SL48, Institute of Food and Agricultural Sciences,, University of Florida, Gainesville, FL. Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

Behavior of Pesticides in Soils and Water 5

2. Rao, P. S. C., A. G. Hornsby, and O. N. Nesheim. 1988. Regulation of pesticide use. Soil Science Fact Sheet SL53, University of Florida, Gainesville, FL.

3. Rao, M. P., and P. S. C. Rao. 1988. Organic pollutants in groundwater: 1. Health effects. Soil Science Fact Sheet SL54, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL.

4. Rao, P. S. C., M. P. Rao, and B. S. Anderson. 1988. Organic pollutants in groundwater: 2. Risk assessment. Soil Science Fact Sheet SL55, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL.

5. Rao, P. S. C. 1988. Criteria for monitoring organic pollutants in groundwater. Soil Science Fact Sheet SL56, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL.

6. Skrotch, W.A. and Sheet, T.J. (Eds.) 1981. Injury and Diagnosis. North Carolina Agricultural Extension Service, AG-85.

7. Haman, D. Z., and D. B. Bottcher. 1986. Home water quality and safety. Extension Circular 703, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL. Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

Behavior of Pesticides in Soils and Water 6

Table 1. Grouping of selected pesticides by persistence and sorption in soils.

Common Name Trade Name(s) K (ml/gOC) T (days) oc 1/2 NON-PERSISTENT (half-life 30 days or less) dalapon Basfapon, Dowpon 1 30 dicamba Banvel 2 14 chloramben Amiben 15 14 oxamyl Vydate 25 4 aldicarb Temik 30 30 metsulfuron-methyl Ally, Escort 35 30 ethoprop Mocap 70 25 2,4,5-T Dacamine 4T, Trioxone 80 30 alachlor Alanex 170 15 cyanazine Bladex 190 14 captan Orthocide, Captanex 200 3 propham Ban-Hoe, Chem-Hoe 200 10 carbaryl Sevin 300 10 iprodione Rovral 700 14 azinphos-methyl Guthion 1000 10 fluridone Sonar 1000 21 malathion Cythion 1800 1 parathion Thiophos, Bladen 5000 14 methyl parathion Penncap-M, Metacide 5100 5 chlorpyrifos Lorsban, Dursban 6070 30 fluvalinate Mavrik, Spur 1000000 7 MODERATELY-PERSISTENT (half-life greater than 30 but less than 100 days) picloram Tordon 16 90 carbofuran Furadan, Curaterr 22 50 bromacil Hyvar, Bromax 32 60 metalaxyl Ridomil 50 70 fluometuron Cotoran, Lanex 100 85 Attrex 100 60 chlorimuron-ethyl Classic 110 40 simazine Princep 130 60 metolachlor Bicep 200 90 monolinuron Aresin, Afesin 200 60 ametryne Evik 300 60 dichlobenil Casoron 400 60 linuron Lorox, Aflon 400 60 prometryn Caparol, Primatol Q 400 60 diuron Karmex 480 90 chlorbromuron Maloran 500 40 fonofos Dyfonate 870 40 phorate Thimet 1000 60 diazinon Basudin, Spectracide 1000 40 benomyl Benlate 1900 67 chloroxuron Tenoran, Norex 3000 60 cyhexatin Plictran 4000 50 Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office.

Behavior of Pesticides in Soils and Water 7

Table 1. Grouping of selected pesticides by persistence and sorption in soils.

Common Name Trade Name(s) K (ml/gOC) T (days) oc 1/2 ethafluralin Sonalan, EL-161 4000 60 ethalfluralin Solanan 4000 60 esfenvalerate Asana 5300 35 fenvalerate Extrin, Sumitox 5300 35 trifluralin Treflan 8000 60 cacodylic Bolate, Bolls-Eye 10000 50 endosulfan Thiodan, Endosan 12400 50 Roundup 24000 47 PERSISTENT (half-life greater than 100 days) terbacil Sinbar 55 120 fomesafen Flex 60 100 prometon Pramitol 150 500 propazine Milogard, Primatol P 154 135 isofenphos Oftanol 600 150 lindane Isotox 1100 400 chloroneb Terraneb 1650 130 DCPA Dacthal 5000 100 ethion Ethion 10000 150 Difenzoquate Methylsulfate Avenge 54,500 100 Formetanate Hydrochloride Carzol, Dicarzol 1,000,000 100

Table 2. Pesticide persistence and sorption: its potential impact on groundwater.

PERSISTENCE SORPTION POTENTIAL IMPACT Groundwater Surface Water Nonpersistent Low-moderate Low Low Nonpersistent Moderate-high Low Moderate Moderately persistent Moderate-high Moderate Moderate Moderately persistent Low-moderate High High Persistent Moderate-high Moderate High Moderately persistent and Low-high Site-specific conditions determine groundwater or surface persistent water impacts