Whole Document Navigator (Click Here)    ---------------------------------- Top of Document PATHWAYS OF PESTICIDE LOSS PERSISTENCE SORPTION ESTIMATING PESTICIDE LOSSES PESTICIDE SELECTION AND USE RELATED EXTENSION PUBLICATIONS Footnotes Disclaimer Copyright Infomation

Behavior of Pesticides in Soils and Water ¹

P.S.C. Rao and A.G. Hornsby²

During the past twenty years, concern has arisen as to the presence of pesticides in the environment and the threat they pose to wildlife and mankind. Certainly, pesticides have improved longevity and the quality of life, chiefly in the area of public health. The use of pesticides also constitutes an important aspect of modern agriculture. Florida's temperate to subtropical climate favors growth of many harmful insects, weeds and diseases, thus making this state particularly dependent on pesticides for economical crop management.

Unfortunately, pesticides are poisons and can be particularly dangerous when misused. Fish-kills, reproductive failure in birds, and acute illnesses in people have all been attributed to exposure to or ingestion of pesticides - usually as a result of misapplication or careless disposal of unused pesticides and containers. Pesticide losses from areas of application and contamination of non-target sites such as surface and ground water represent a monetary loss to the farmer as well as a threat to the environment. Thus, careful management of pesticides in order to avoid environmental contamination is desired by both farmers and the general public.

The purpose of this fact sheet is to explain how pesticides can move from the area in which they are applied, and to show how this information can be used, along with other factors, to select the proper pesticide.

PATHWAYS OF PESTICIDE LOSS

There are basically two ways properly-applied pesticides may reach surface and ground waters- through runoff and leaching. Runoff is the physical transport of pollutants over the ground surface by rainwater which does not penetrate the soil. Leaching is a process whereby pollutants are flushed through the soil by rain or irrigation water as it moves downward. In many areas of Florida, soils are sandy and permeable and leaching is likely to be a more serious problem than runoff. We now have technology to help estimate the potential contamination of water from a given pesticide. To understand this technology, it is necessary to know how a pesticide behaves in soil and water.

Once applied to cropland, a number of things may happen to a pesticide (Figure 1) . It may be taken up by plants or ingested by animals, insects, worms, or microorganisms in the soil. It may move downward in the soil and either adhere to particles or dissolve. The pesticide may vaporize and enter the atmosphere, or break down via microbial and chemical pathways into other, less toxic compounds. Pesticides may be leached out of the root zone or washed off the surface of land by rain or irrigation water. Evaporation of water at the ground surface can lead to upward flow of water and pesticides. In most Florida soils this process is likely to be not as important as downward leaching in response to irrigation and or rainfall. The fate of a pesticide applied to soil depends largely on two of its properties: persistence and sorption.

Note: Two other pathways of pesticide loss are through removal in the harvested plant and by vaporization (volatilization) into the atmosphere. Occurrence of pesticides released into the atmosphere have an impact on air quality and create problems when agricultural workers enter the treated areas. High concentrations of pesticide residues in harvested produce can have adverse health effects on consumers. While these two pathways are important, 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.]

PERSISTENCE

Persistence defines the "lasting-power" of a pesticide. Most pesticides break down or "degrade" over time as a result of several chemical and microbiological reactions in soils. Sunlight breaks down some pesticides. Generally, chemical pathways result in only partial deactivation of pesticides, whereas soil microorganisms can completely breakdown many pesticides to carbon dioxide, water and other inorganic constituents. Some pesticides produce intermediate substances, called metabolites as they degrade. The biological activity of these substances may also have environmental significance. Because populations of microbes decrease rapidly below the root zone, pesticides leached beyond this depth are less likely to be degraded. However, some pesticides will continue to degrade by chemical reactions after they have left the root zone.

Degradation time is measured in half-life. Half-life (T_1/2 ) is a measure of the amount of time it takes for one-half the original amount of a pesticide in soil to be deactivated. Half-life is sometimes defined as the time required for half the amount of applied pesticide to be completely degraded and released as carbon dioxide. Usually, the half-life of a pesticide measured by the latter basis is longer than that based on deactivation only. This is especially true if toxic or non-toxic metabolites accumulate in the soil during the degradation. Table 1 groups some of the pesticides used in Florida by persistency, or length of half-life, on the basis of their deactivation in surface soils. T_1/2 values in subsoils and in ground water are usually much larger. Thus, as pesticides are leached to lower depths, their persistency increases. Values for pesticide degradation, T_1/2 , in subsoils and groundwater are scarce.

SORPTION

Probably the single most important property influencing a pesticide's movement with water is its solubility. Soil is a complex mixture of solids, liquids and gases that provides the life support system for roots of growing plants and microorganisms such as bacteria. When a pesticide enters soil, some of it will stick to soil particles, particularly organic matter, through a process called sorption and some will dissolve and mix with the water between soil particles, called "soil water." As more water enters the soil through rain or irrigation, the sorbed pesticide molecules may be detached from soil particles through a process called desorption. The solubility of a pesticide and its sorption on soil are inversely related; that is, increased solubility results in less sorption.

One of the most useful indices for quantifying pesticide sorption on soils is the partition coefficient (K_oc). The K_oc value is defined as the ratio of pesticide concentration in the sorbed-state (that is, bound to soil particles) and the solution-phase (that is, dissolved in the soil-water). Thus, for a given amount of pesticide applied, the smaller the K_oc value, the greater the concentration of pesticide in solution. Pesticides with small K_oc values are more likely to be leached compared to those with large K_oc values.

Partition coefficients of several chemicals are shown in Table 1 . Note the wide range of partition coefficients. Values of partition coefficients listed in Table 1 are independent of soil type and are characteristic of each pesticide. The partition coefficient is determined by a pesticide's chemical properties such as solubility and melting point.

The partition coefficient makes it possible to estimate a particular pesticide's chance of being lost via runoff or leaching in a specific soil, and is calculated via the formula:

K_p = (K_oc)(%OM)(0.0058)

• K_p = index of sorption for a given pesticide on a particular soil

• %OM = is the percent organic matter in the soil, as determined by chemical analysis

• K_oc = partition coefficient of the pesticide normalized to organic carbon as listed in Table1 .

Four important aspects of pesticide sorption by soils should be recognized:

• For pesticides not adsorbed by soils, K_oc is equal to zero (or K_p = 0). Thus, such pesticides will leach in a manner similar to inorganic ions such as nitrate which are also not adsorbed by most soils.

• For a given pesticide, sorption is greater in soils with larger organic matter content (% OM). Thus, pesticide leaching in soils with higher %OM is expected to be slower compared to soils low in %OM.

• In most soils %OM decreases rapidly with increasing depth, this is particularly so beyond the crop root zone in sandy soils of Florida's central ridge. Thus, pesticide sorption decreases with increasing soil depth, and, as a consequence, leaching is likely to be more rapid in subsoils.

• For a given soil, pesticides with smaller K_oc values are sorbed to a lesser extent, and are therefore more likely to be leached when compared to pesticides having larger K_oc values.

ESTIMATING PESTICIDE LOSSES

In estimating pesticide losses from soils and their potential to contaminate groundwater or surface water, it is essential to consider simultaneously both persistence and sorption. Quantitative estimations of pesticide losses via runoff or leaching requires complex mathematical models. These models are solved using mainframe or micro-computers and utilize site-specific soil, crop, management, and climatological information. This would include soil type and its physical-chemical properties; the date, amount, and method of pesticide application; the amount, duration, and frequency of rainfall or irrigations following pesticide application; and crop growth characteristics. In the absence of such information, however, a qualitative assessment of a pesticide's potential to contaminate surface water or groundwater is possible using K_oc and T_1/2 values as indices of pesticide sorption and persistence.

Strongly-sorbed and persistent pesticides (that is, compounds with large K_oc and large T_1/2 ) are likely to remain near the ground surface, increasing the chances of being carried to a stream or a lake via runoff. In contrast, weakly-sorbed but persistent pesticides (that is, compounds with small K_oc and large T_1/2 ) may be readily leached through the soil and are more likely to contaminate groundwater. For nonpersistent pesticides with small T_1/2 , the possibility of surface water or groundwater contamination depends primarily on whether heavy rains (or irrigation) occur soon after pesticide application. Without water to move them downward, pesticides with short half-lives remain within the biologically - active crop root zone and may be degraded readily. In terms of water quality then, pesticides with intermediate K_oc values and short T_1/2 values may be considered low risk because they are not readily leached and are degraded fairly rapidly. These general concepts are summarized in Table 2.

PESTICIDE SELECTION AND USE

Agricultural use of pesticides should be part of an overall pest management strategy which includes biological controls, cultural methods, pest monitoring and other applicable practices, referred to altogether as Integrated Pest Management or IPM. When a pesticide is needed its selection should be based on effectiveness, toxicity to non-target species, cost, and site characteristics, as well as its solubility and persistence.

Half-lives and partition coefficients are particularly important when the application site of a pesticide is near surface waters or is underlain with permeable subsoil and a shallow aquifer. Short half-lives and intermediate to large K_oc are best in this situation.

Many areas of Florida have impermeable sub-soils which impede deep leaching of soluble pesticides. On such land, soluble pesticides with low K_oc and moderate-to-long half-lives require cautious application to prevent rapid transport in drainage water to a nearby lake or stream. Nonerosive soils are common to much of Florida and pesticides with large K_oc remain on the application site for a long time. However, the user should be cautious of pesticides with long half-lives as they are likely to build up in the soil.

In addition to the pesticide solubility and soil permeability it is important that the pesticide's toxicity to non-target species be considered. Some of the pesticides listed in Table 1 have severely restricted use due to acute toxicity or long half-life. An important purpose of the pesticide container label is to instruct users to apply the pesticide safely and with minimum threat to non-target species, both on and off the application site. Pesticide users assume responsibility to follow label instructions. It is unsafe and unlawful not to do so.

RELATED EXTENSION PUBLICATIONS

1. Hornsby, A. G. 1986. Ground water: The hidden resource. Soil Science Fact Sheet SL48, Institute of Food and Agricultural Sciences,, University of Florida, Gainesville, FL.

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. Herbicide 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.

Tables

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

Common Name	Trade Name(s)	Koc(ml/gOC)	T1/2 (days)
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
atrazine	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
ethafluralin	Sonalan, EL-161	4000	60
ethalfluralin	Solanan	4000	60
esfenvalerate	Asana	5300	35
fenvalerate	Extrin, Sumitox	5300	35
trifluralin	Treflan	8000	60
cacodylic acid	Bolate, Bolls-Eye	10000	50
endosulfan	Thiodan, Endosan	12400	50
glyphosate	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 persistent	Low-high	Site-specific conditions determine groundwater or surface water impacts

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Footnotes

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.

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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.

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Copyright Information

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