Structure and Function in Agroecosystem Design and Management - Chapter 13 ppt

CHAPTER 13
Environmental Fate of Pesticides
Masako Ueji and Yuso Kobara
CONTENTS
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
Pesticides in Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
Behavior in Soil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
Residue in Soil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277
Adsorption and Leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
Degradation in Soil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
Pesticides in Aquatic Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
Runoff from Farmland to Aquatic Environment . . . . . . . . . . . . . . . . 280
Degradation in Aqueous Environment. . . . . . . . . . . . . . . . . . . . . . . . 283
Pesticides in the Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
Entry Pathways into the Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . 285
Drift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
Wind Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
Volatilization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286
Behavior of Pesticides in the Atmosphere . . . . . . . . . . . . . . . . . . . . . 287
Deposition of Pesticides with Rainfall and as Dust. . . . . . . . . . . . . . 287
Degradation of Pesticides in the Atmosphere . . . . . . . . . . . . . . . . . . 287
Influences of Pesticides on Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288
Impacts on Nontarget Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289
Bioconcentration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
275
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276 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT

INTRODUCTION
Pesticides play a major role in controlling insect pests and weeds and
have brought about sustained high yields and higher quality of agricultural
produce. They have also helped liberate farmers from the backbreaking task
of weeding. Pesticides, which serve purposes including those of insecticides,
fungicides, and herbicides, are active substances that have some sort of toxi-
city toward living things. Pesticides applied to farmland are put into the
environment on purpose then are dispersed widely throughout the atmos-
phere, soil, and aquatic environment outside farmland. Thus, in an attempt
to solve problems involving toxicity, residual tendency, and selectivity
among organisms, which are drawbacks of pesticides, improvements have
been made in the chemical structures of these chemicals and in the ways they
are formulated and applied. As a result, currently used pesticides are com-
pounds characterized by low toxicity, easy degradability, high selectivity, and
high activity (Takagi and Ueji, 1997). Decreasing the environmental load
caused by pesticides is also needed to further expedite ecological farming
practices.
The environmental fate of pesticides (i.e., their dispersion, movement,
adsorption, desorption and degradation in soil, aquatic environment, and the
atmosphere, as well as their effects on organisms in the environment)
changes greatly depending on environmental factors, such as meteorological
conditions, soil condition, and properties of organisms, in addition to the
physicochemical characteristics of pesticides, the manner of their formula-
tion, and how they are used. In the environment and in the metabolic
processes of organisms, pesticides are generally detoxified, but in some
instances they are transformed into metabolites that are even more toxic. This
makes it vital to ascertain metabolic pathways and the characteristics of
metabolites.
PESTICIDES IN SOIL
Behavior in Soil

After application, pesticides disperse into the atmosphere and aquatic
environment and adhere to plants, but with the passage of time much of the
applied amount settles onto soil surfaces, which is why research into the fate
of these chemicals in soil has had priority.
The pesticides in soil disappear with time and each process is influenced
by various factors. In the first stage, (1) pesticides disperse into the atmos-
phere from the soil surface due to transpiration occurring immediately after
application, resulting in rapid disappearance. Transpiration is governed
largely by the vapor pressure created by the chemicals, the method of use,
meteorological conditions such as temperature and wind velocity, and soil
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ENVIRONMENTAL FATE OF PESTICIDES 277
factors such as soil moisture content and the amount of organic matter. This
is followed by (2) runoff into the aquatic environment, degradation on the
soil surface by sunlight, penetration and leaching into soil, and adsorption by
soil particles. The latter stage processes are influenced by water solubility
and susceptibility to photolysis of pesticides, soil characteristics including
type and structure of soil, clay content and the amount of organic matter, and
meteorological conditions such as rainfall.
The second stage of the disappearance process is degradation reactions,
which proceed at a more leisurely pace than the first stage. While reactions
such as chemical hydrolysis occur here, this stage consists mainly of
biodegradation involving soil microorganisms. Moreover, calculated regres-
sions show good agreement with a logarithmic disappearance of pesticides
(Edwards, 1966).
Pesticide fate in soil can be roughly summarized in the following manner.
1. The greater a chemical’s vapor pressure, the more it disperses into
the atmosphere from the soil surface (Swann et al., 1982).
2. The greater a chemical’s water solubility, the greater its runoff with
surface water and the more it penetrates into the soil (Weber, 1994).

3. The more organic matter in soil, the more readily chemicals are
adsorbed, hence moving with greater difficulty (McEwen and
Stephenson, 1979).
4. Degradation in soil is almost totally biodegradation. Pesticides
thus disappear quickly in soil with high microbial activity (Weed
and Weber, 1974; Scheunert, 1992).
Residue in Soil
The residue of pesticides in soil depends greatly on characteristics of pes-
ticides and the soil, meteorological conditions, and other factors. A look at
pesticides by type shows that the organochlorine chemicals such as DDT,
BHC, and aldrin and dieldrin are especially stable in soil and remain for a
long time. Half-life (the time required for half of the applied pesticide to dis-
appear) is used as an indicator of residual tendency. As shown by the half-
lives of various pesticides determined in laboratory testing and listed in
Table 13.1, many of the chemicals in current use have short residual times
(Hamaker, 1972; Kanazawa,1992). Comparing the disappearance times of dif-
ferent pesticides shows that the organophosphates disappear quickly, while
the carbamates have comparatively long residual times. Fungicides generally
disappear quickly. On the other hand, herbicides generally have long half-
lives, with some persisting as long as several months. The reason for this is
that herbicides must have persistence because weed seeds germinate over a
long period of time.
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278 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Table 13.1 Half-lives of Pesticides in Upland Soil
(A) (B) (C) (D)
alloxydim(H) alachlor(H) benomyl(F) BHC(I)
bensultap(I) captan(F) benthiocarb(H) bromacil(H)
carbaryl(I) carbofuran(I) cypermethrin(I) DDT(I)
chlorfenvinphos(I) cyanofenphos(I) dicofol(I) dieldrin(I)

diazinon(I) dalapon(H) dimethirimol(F) dichlobenil(H)
diflubenzuron(I) dimethoate(I) endosulfan(I) flutolanil(F)
dithianon(F) 2,4-D(H) fenvalerate(I) imazapyr(H)
fenthion(I) ethofenrpox(I) guazatin(F) metribuzin(H)
glufosinate(H) fenothiocarb(I) imazali(F) myclobutanil(F)
ioxynil(H) glyphosate(H) iprodione(F) myclobutanil(F)
malathion(I) meneb(F) lepthophos(I) oxyfluorofen(H)
mecarbam(I) methyl dymron(H) linuron(H) paraquat(H)
methidathion(I) oxamyl(I) matalaxyl(H) simazine(H)
methomyl(I) phosmet(I) metolachlor(H) tebuthiuron(H)
monocrotophos(I) propaphos(I) oxadiazin(H) thiazafluron(H)
permethrin(I) propineb(F) pencycuron(F) triadimeforn(F)
parathion(I) pyridaphenthion(I) prometryne(H)
propanil(H) thiram(F) pyrzophos(F)
tetrachlorvimphos(I) trifluralin(H) terbacil(H)
trichlorfron(I) zineb(F) tetradifon(I)
vamidothion(I) triadimenol(F)
Half-lives in soil: (A) Ͻ14 days; (B) 15–42 days; (C) 43–180 days; (D) Ͼ180 days (I):
Insecticides; (F): Fungicides; (H): Herbicides
Adsorption and Leaching
Most of the pesticides that fall to the ground are adsorbed by the upper
portion of the soil and held there. Subsequently they are desorbed from the
soil particles, move, and disperse through the soil with soil moisture, or they
degrade and disappear. The time needed for pesticides to disappear com-
pletely from soil varies considerably depending on soil conditions and the
physicochemical characteristics of the chemicals. Generally, the more firmly
a chemical adsorbs into soil, the less easily it moves.
Some of the soil adsorption mechanisms of pesticides are by van der
Waals force, hydrogen bonding, covalent bonding, and ion exchange. They
differ depending on the combination of a pesticide’s chemical structure and

the soil’s components. Of these, the primary mechanism of adsorption reac-
tions is cation exchange with the negative charge of the soil surface.
Adsorption is strongest, for example, in the herbicide paraquat and diquat,
which has quaternary amines containing the bipyridinium cation, because
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ENVIRONMENTAL FATE OF PESTICIDES 279
these are absorbed firmly the moment they make contact with the soil
(Hayers et al., 1975). Also readily adsorbed are urea derivatives, the triazines
and the carbamates, etc., whose molecules contain cationic NH groups
(Wauchope and Koskinen, 1983). And as with amino products formed by the
reduction of nitro groups that replace benzene rings, when degradation
products with structures making them easily adsorbed are formed in the soil,
the result is stable bonding (Nash, 1988).
Some soil factors are organic matter content, types of clay minerals, clay
content, and aggregate structure. Many different interactions occur between
these factors and pesticides’ chemical structures. In many instances organic
matter content has the biggest effect. In particular, the higher the soil’s humic
acid content, the stronger its adsorption is; likewise, adsorption is strong in
soils with high clay content and 2:1 clay minerals, such as vermiculite and
montmorillonite. As with chemical substances in general, the strength or
weakness of pesticide soil adsorption is indicated by the soil sorption coeffi-
cient, Kd, (McCall et al., 1976). Kd is the value obtained by agitation mixing
of a chemical substance dissolved in water with soil, noting the concentration
of the chemical in soil when an adsorption equilibrium has been attained,
and dividing that by the concentration in water. Further, adsorption depends
primarily on the amount of organic matter in the soil. For that reason, Kd is
indicated by the soil sorption equilibrium constant (Koc), calculated accord-
ing to the organic carbon content of the soil and used as the mutual soil sorp-
tion of chemical substances (Weber, 1995). With regard to pesticides as well,
the larger Koc is, the more easily a chemical is adsorbed by the soil, and the

less it moves through the soil.
On the other hand, highly water-soluble pesticides could cause ground-
water contamination (discussed below) in sandy soil with little clay or
organic matter, or when soil moisture increases.
Degradation in Soil
Degradation of pesticides in soil consists of nonbiological degradation,
such as photolysis and hydrolysis, and biological degradation by soil
microorganisms and other organisms. The involvement of microorganisms is
especially great (Bollag et al., 1990; Turco and Konopka, 1990). A number of
different microorganisms come into play until a certain chemical is com-
pletely broken down, and sometimes nonbiological chemical reactions pro-
ceed in parallel with biological decomposition. Various decomposing
organisms have been separated out from soil; examples of decomposing
microorganisms isolated from soil are bacteria, actinomycetes, molds, and
yeasts (Goring et al., 1975). Decomposition by microorganisms proceeds dif-
ferently according to the chemical structure of a pesticide and becomes more
difficult as the sizes of molecules, and as their carbon numbers and numbers
of rings, increase. Generally, water-soluble pesticides degrade easily, while
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280 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
fat-soluble chemicals are adsorbed into soil, making it difficult for microor-
ganisms to break them down. Decomposition reactions include thioether oxi-
dation, epoxide and thioether formation, dealkylation, dehalogenation,
reduction, formation of azo compounds, condensation, and isomerization;
ultimately the chemicals are broken down to carbon dioxide (Scheunert,
1992; Kuwatsuka and Yamamoto, 1998). However, it is rare that microorgan-
isms break carbon-chlorine bonds at a benzene ring, which is why represen-
tative organochlorines such as DDT and BHC remain in the environment for
long periods of time and, as a result, are highly bioconcentrated.
Decomposition by microorganisms is greatly affected by the nature of the

soil, and even in the same soil other major factors include temperature, mois-
ture content, and the states of oxidation and reduction. There are very large
differences in breakdown products and the degradation rate depending on
whether soil is aerobic or anaerobic. For example, pesticides like DDT and
BHC, which are decomposed mainly by anaerobic bacteria, degrade quickly
in flooding soil (Lichtenstein and Schulz, 1961). Furthermore, the more
organic matter contained in soil, the larger the number of microorganisms
involved in degradation, which means that pesticide breakdown activity is
greater. And because the decomposing bacteria types for each pesticide have
their optimum pH values, soil pH also influences the degradation rate.
When the same pesticide or chemicals with similar chemical structures
are used continuously, the corresponding decomposing microorganisms accu-
mulate, which sometimes leads to decreased sustainability of a chemical’s
efficacy (Chapman and Harris, 1990). Especially pesticides having carbamate
(N-CO-O), urea (N-CO-N), ester (COO-C), thiocarbamate (N-CO-S), and the
like in their chemical structures undergo cross adaptation, in which degra-
dation is promoted and chemical efficacy is considerably reduced (Roeth et
al., 1990; Somasundaram and Coats, 1990). Measures to address this problem
include the control of microorganisms’ decomposition activity by using
extenders, and rotating the pesticides used (Drost et al., 1990; Harvey, 1990).
PESTICIDES IN AQUATIC ENVIRONMENT
Runoff from Farmland to Aquatic Environment
Water is a chief vehicle for the movement of pesticides in the environ-
ment. Additionally, because water in the environment is used as drinking
water, and it plays a major role in the conservation of aquatic organisms, it is
important to reduce to the greatest possible extent the risk of contaminating
rivers and groundwater with pesticides.
When pesticides are applied to upland fields, the chemicals that fall to
the soil surface enter the aquatic environment, such as a river, lake, or sea,
with rainwater that overflows from the soil surface if heavy rain falls from

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ENVIRONMENTAL FATE OF PESTICIDES 281
the time directly after application to within about two weeks after
(Wauchope, 1978; Leonard, 1988). In particular, the shorter the time elapsed
since application, the greater the amount of pesticide runoff caused by rain-
fall. With the passage of time, the amount of chemical runoff into aquatic
environment lessens because the chemicals move downward from the soil
surface and are more firmly adsorbed into soil particles and absorbed by
crops, in addition to being broken down. The runoff rate into water systems
is about 0.5% of the applied amount if rain falls immediately after applica-
tion, and even a generous estimate puts the total runoff rate two weeks after
application at 1 or 2% (Leonard, 1990). One factor governing runoff is the
water solubility of pesticides, and, in general, the more water soluble a chem-
ical is, the greater its runoff rate. Runoff into aquatic environment is also gov-
erned strongly by environmental factors. Specifically, pesticides with water
solubility of 10 ppm or higher mainly move to the aquatic environment by
dissolving into surface water, while those that dissolve with difficulty or
have high soil adsorption move with soil particles suspended in water or sed-
iment to which the chemicals have been adsorbed (Turco and Kladivko,
1994). Thus, when surface water contains a large amount of minute soil par-
ticles, pesticides tend to be washed off while adsorbed to those particles.
Some characteristics of farmland on which surface water runoff easily occurs
are slopes, hard soil with low water permeability, furrows running
uphill/downhill, and exposed soil (Fujita, 1998). It is important to implement
fully farmland soil erosion control and water management in order to curb
pesticide runoff into the aquatic environment.
Pesticide movement by means of soil moisture percolation into the
ground brings about groundwater pollution. Especially when pesticides are
highly water-soluble, when soil is sandy with little clay or organic matter, or
when soil moisture has increased suddenly, pesticides are detected in

groundwater (Cohen et al., 1990). In western countries, where groundwater
is often used as drinking water, the detection of pesticides in the water of
about 30% of wells in the 1980s became a matter of public concern. Some of
the chemicals detected with especially great frequency were the soil fumigant
ethylene dibromide (0.05–20 ppb), the carbamate insecticide aldicarb (1–50
ppb), and the triazine herbicide atrazine (0.3–3 ppm) (Cohen et al., 1986).
Because of this situation, the U.S. Environmental Protection Agency (EPA)
and agencies of other western countries established pesticide concentration
standards for groundwater and continue strict monitoring of pesticide use
(Kidd and Hartley, 1987; U.S. EPA, 1991).
Factors involved in movement into the groundwater can be categorized
as pesticide characteristics and as environmental conditions such as those of
soil. Because farmland does not necessarily have homogeneous soil struc-
tures, it is hard to discern uniform trends for each field owing to rainfall, soil
conditions, and other factors. Below are some conditions that create a major
potential for groundwater contamination:
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282 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Pesticide characteristics: Water solubility of 30 mg/l or more; Koc
value under 500; Henry’s constant under 10
Ϫ2
atm
.
m
3
/mol; neg-
atively charged at ambient pH; half-life by hydrolysis of 25 weeks
or more; half-life by photolysis of one week or more; and half-life in
soil of three weeks or more.
Farmland conditions: Annual rainfall of 25 cm or more; high possibil-

ity of pesticide contamination in area with high nitric acid ion con-
tent in groundwater; places with porous soil above aquifer; and soil
that has pH providing for high stability of a certain pesticide.
Pesticides applied on paddy field are dispersed into aquatic environment
over broad areas while being diluted as they follow a path from agricultural
water channels to small rivers and then to large rivers. The concentrations of
pesticides in paddy surface water differ according to the amount applied per
unit area, the manner of formulation, physicochemical characteristics, and
environmental conditions, including temperature, rainfall, and soil charac-
teristics. Generally the greater a pesticide’s water solubility, the higher the
concentration. The highest concentration is found between the time immedi-
ately after application and the following day, and many pesticides have short
half-lives of two to five days in paddy surface water (Maru, 1985; Nagafuchi,
1999).
Because most of the pesticide applied to paddy fields directly enters its
surface water, the runoff rate from the fields into the aquatic environment is
larger than that from upland fields. The runoff rate also varies according to
application amount, water management, and other factors, but in particular
one can discern a positive correlation between a pesticide’s runoff rate and its
water solubility (Inao et al., 1999; Maru, 1990). For example, Maru calculated
the pesticide runoff rate on the basis of regular pesticide concentration analy-
sis results for paddy surface water and river water, and the pesticide appli-
cation amounts for the surrounding region (Maru, 1991). Results showed that
the water solubilities of the herbicides chlornitrofen, butachlor, thiobencarb,
and simetrine were 0.25, 23, 30, and 450 ppm, while their runoff rates into
river water were 0.11, 2.32, 1.44, and 5.96%, respectively. The higher the water
solubility is, the higher the runoff rate, with the following regression equa-
tion relating the runoff rate to the logarithmic value of water solubility:
Y ϭ 1.06 ϩ 1.84 log (X), r ϭ 0.75 and n ϭ 10.
where Y is runoff rate and X is water solubility.

The period during which pesticides are detected in the aqueous environ-
ment corresponds to their time of application. Often there is a temporary
peak in the amount detected immediately following application, after which
there is a gradual decline, with the chemicals becoming undetectable after
two or three months. The concentration detected in river water is sometimes
a high value of 100 ppb for a short time with the herbicide molinate, which
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ENVIRONMENTAL FATE OF PESTICIDES 283
has a high water solubility of 900 ppm, but generally the concentration is in
the range of 0.1 to 10 ppb (Nakamura, 1993).
Degradation in Aqueous Environment
Pesticides that have entered into water disappear by adsorption to soil
particles, settling to sediment, atmospheric dispersion by evaporation with
water, or a variety of breakdown reactions. Table 13.2 classifies the persistence
of pesticides in water by their half-lives, which depend largely upon their
chemical structures (McEwen and Stephenson, 1979). Half-lives range from
those less than two weeks for organophosphates and carbamates to the long-
term stability of over six months for chlorinated pesticides. Degradation of
pesticides in water is accomplished by chemical reactions, mainly hydrolysis;
physical reactions caused by photolysis; and biological reactions carried on by
microorganisms. The prevailing breakdown reaction is determined by a pesti-
cide’s chemical structure and conditions in the aqueous environment (Pollard
et al., 1998; Kato, 1998). The author’s measurements of the breakdown rates in
Table 13.2 Half-lives of Pesticides in Aqueous Environment
(A) (B) (C) (D)
Organophosphorous Organochlorines Organochlorines Organochlorines
azinphosmethyl aldrin chlordane dieldrin
chlorpyrifos methoxychlor lindane endrin, BHC
demeton
Organophosphorus Organophosphorus heptachlor

dichlorvos diazinon chlorfenvinphos
Benzimidazoles
fenitrothion disulfoton dimethoate benomyl
malathion phorate fensulfothion
naled
Carbamates Carbamates
phosphamidon chloropropham aldicarb
Carbamates EPTC, swep carbofuran
carbaryl propham,
Triazines
methiocarb vernolate atrazine, simazine
propoxur
Benzoic acids propazine
Pyrethroids chloramben Uracils
pyrethrum Triazoles bromacil, terbacil
Aryloxyalkanoic amitrol Dinitroanilines
acids Ureas
trifluralin
2,4-D fenuron, monuron
Ureas
Aryloxyalkanoic
diuron, linuron
acids
MCPA
Half-lives (A) Ͻ 2 weeks; (B) 2–6 weeks; (C) 6 weeks–6 months; (D) Ͼ 6 months
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284 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
aqueous environments with different qualities showed that these environ-
ments were ranked, from highest to lowest rates, in the order of river water,
lake water, seawater, and groundwater. Thus, pesticides remained for a long

time in groundwater (Hiramatsu, 1990). The main degradation factors under
freshwater conditions of rivers and lakes are breakdown by microorganisms,
and in seawater by weakly alkaline hydrolysis (Kanazawa, 1987).
Sunlight-induced photolysis is an important degradation factor of pesti-
cides in rice paddy water and the surface layer of river water, but at depths
greater than this, where the energy of sunlight is reduced, the contribution of
photolysis is reduced (Oyamada and Kuwatsuka, 1986; Yamaoka et al., 1988).
As photochemical reactions occur in the wavelength region of 290–450 nm,
the more ultraviolet light a pesticide absorbs, the more susceptible it is to
photolysis (Crosby, 1969). Additionally, aqueous environments contain such
photosensitizing substances like chlorophyll, carotenes, quinones, riboflavin,
humic acid, and amino acids, which catalyze light reactions. Roughly, there
are two photosensitizing reactions: (1) energy absorbed by photosensitizing
substances is passed to coexisting substances (pesticides in this case) where
it brings about chemical reactions; (2) oxygen is activated by the action of
photosensitizing substances, thereby forming powerful oxidants such as
hydroxyl radicals, peroxides, and superoxidoanions, which then promote
oxidation reactions (Nakagawa, 1990).
Microbial degradation in water generally proceeds readily under aerobic
conditions, just as in soil, but in the water of a flooded paddy field and sedi-
ment, degradation becomes quite anaerobic, and in some situations a few
chlorinated organic pesticides readily undergo dechlorination reactions
under anaerobic conditions (Marth, 1966; Johnson, 1976; Kanazawa, 1987).
Under whatever conditions, microorganisms use enzymes to completely
degrade pesticides to carbon dioxide by oxidation, reduction, hydrolysis, and
other reactions. As a test for degradation in water by microorganisms, the
Overseas Economic Cooperation Fund (OECD) and the EPA propose a
method that involves adding microorganisms from river sediment or farm-
land soil that has stable microbial flora.
PESTICIDES IN THE ATMOSPHERE

A comprehensive review of existing literature on the occurrence and dis-
tribution of pesticides in the atmosphere showed that the atmosphere is an
important part that acts to distribute and deposit pesticides in areas far
removed from their application sites. A compilation of existing data is that
pesticides have been detected in the atmosphere throughout the world, but
most of the available information is from small-scale, short-term studies, few
of which lasted more than one year. Until the 1960s atmospheric pollution
from pesticide spray drift was generally thought of as a local problem. Long-
range movement of long-lived pesticides through the atmosphere was
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ENVIRONMENTAL FATE OF PESTICIDES 285
believed to be minimal. The detection of DDT and other organochlorine com-
pounds in Arctic and Antarctic fish and animals has changed this notion
(Kurtz, 1990). The atmosphere is now recognized as a major pathway by
which pesticides can be transported and deposited in areas sometimes far
removed from their sources. Long-range transport of pesticides can occur
over hundreds and thousands of kilometers.
Entry Pathways into the Atmosphere
Pesticides can enter the atmosphere as a result of drift during application,
wind erosion of contaminated soil particles, and volatilization from leaf or soil
surfaces (Spencer et al., 1990; Lewis and Lee, 1976; Seiber et al., 1980). Recent
studies have shown that many pesticides readily evaporate into the atmos-
phere. Evaporation is a continuous process that occurs over weeks, months,
and years, until all of the pesticide molecules are degraded. Seventy-five per-
cent or more of what is applied, depending on the pesticide, can ultimately be
lost through evaporation. The atmospheric movement of pesticide residues
may lead to unintentional exposures for humans, animals, and plants.
Drift
The aim of spraying and dressing is to adhere the pesticide to the target
surface (crop or soil) as uniformly as possible. Direct transport into the

atmosphere occurs via the sweeping up and rapid evaporation of smaller
spray droplets. As a result of atmospheric air movements (horizontal and
vertical), a certain proportion of the applied mixture may drift beyond the
target field. This process is generally known as drift. The applied mixture
component that settles beyond the target area up to about 100 m is known as
soil sedimentation. The component that remains suspended in the air and is
capable of being carried in solid, liquid, or gaseous form over larger distances
is defined as atmospheric drift. Drift is highly dependent on the method of
application and on the crop and its stage of development (Ganzelmeier et al.,
1993). Experimental determination of the magnitude of atmospheric drift
poses great difficulties. Drift measurements in a wind tunnel showed that the
proportion contributing to atmospheric drift is 3–7 times higher than the soil
sediment, depending on wind speed. There is a positive linear correlation
between total drift (soil sedimentation plus atmospheric drift) and the pro-
portion by volume of drops with a diameter Յ100
␮
m. For conventional
spraying techniques, atmospheric drift is estimated to be distinctly below 5%
of the applied volume, depending on crop and weather conditions.
Wind Erosion
Soil particles to which traces of pesticides adhere are swept up by strong
wind near the ground and deposited on the field or neighboring region.
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286 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Wind erosion of pesticides, mainly herbicides, is limited to treated fields of
late sprouting crops such as root crops, summer cereals, and maize, with long
vegetation-free periods in autumn and spring. Herbicides are applied mainly
during the early developmental stage of crops, i.e., when soil cover is still
modest. Most of the applied mixture reaches the soil; only a minor portion
wets the growing weeds and the crop directly. Depending on its physico-

chemical properties, a fraction of the applied active ingredient may remain
longer in the upper soil layer and undergo some degree of adsorption onto the
soil particles. Since soil erosion by wind affects principally the soil particles
that are directly at the soil surface, the concentration of pesticides at the soil
surface is the decisive factor that determines the extent of pesticide transport.
The order of magnitude of wind erosion as a path for transport of pesti-
cides into the atmosphere can only be roughly estimated (Glotfelty et al.,
1989; Gaynor and Mac Tavish, 1981), at least for their own locality. Their stud-
ies of the spread and distribution of pesticides cannot be extrapolated
directly to our climatic and geographic conditions. For application of pre-
emergence herbicides in the spring at application rates of about 500 g active
ingredient/ha on highly erodible soils, active ingredient losses of maximum
0.66–3.33 g/ha must be expected as the result of mild erosion events (Ͻ1
t/ha). If losses due to drift, volatilization, and metabolization between herbi-
cidal treatment and the erosion event are taken into account, the actual active
ingredient losses due to soil erosion will be lower than the above-mentioned
values.
Volatilization
After applying pesticides, evaporation may occur from plant or soil sur-
faces in gaseous form. This process is influenced by the following factors: the
prevailing climatic conditions, the target area, the application technique, the
physicochemical properties of the active ingredient, the physical state of
spray deposit, and the formulation.
To investigate behavior, an experimental setup at the Monheime Crop
Protection Center is used (Fritz et. al., 1992) that combines the advantage of
the use of radiolabeled pesticides with the practical conditions of a field
study. The [
14
C] labeling permits quantitative determination of both non-
volatile degradation products and metabolites or active ingredients bound to

the plant or soil matrix, which without tracer-aided analysis would enter the
balance as apparently volatile active ingredients. After application, the
decline in radioactivity and in active ingredient is measured on the target
area — plant or soil. The volatilization rate is calculated from the difference
between the applied radioactivity and the radioactivity recovered after 1, 3,
6, and 24 h. These results of volatilization studies indicated unambiguous
attribution of the measured volatilization rates to physical data such as vapor
pressure, water solubility, or Henry’s Law.
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ENVIRONMENTAL FATE OF PESTICIDES 287
Behavior of Pesticides in the Atmosphere
Pesticides that enter the atmosphere are degraded, carried along with air
masses, and/or returned to the ground by wet deposition (e.g., rain, fog, or
snow) or dry deposition (e.g., dust). Once deposited on the earth’s surface,
the pesticide can revolatilize, reenter the atmosphere, and be transported and
deposited downwind repeatedly, sometimes over decades, until it is finally
degraded. Depending on stability of the active ingredient, the behavior will
tend more in one direction or the other. Widespread horizontal and vertical
distribution by the wind is more relevant for longer-lived pesticides. High
concentrations of locally used pesticides were found to occur seasonally, usu-
ally in conjunction with spring planting of row crops. The more persistent
pesticides were detected at low concentrations throughout the year.
Deposition of Pesticides with Rainfall and as Dust
Although the quantity of pesticides entering the atmosphere is important
in assessing their possible deposition effects, what is decisive is the fraction
that returns to ground with precipitation or dust after passing through the
atmosphere. Precipitation, particularly rain, is the most important route of
deposition of all airborne substances and, hence, also of pesticides from the
atmosphere. In the air layers in which clouds are formed, gaseous or particle-
bound active ingredient molecules become involved in the microphysical

processes of cloud and mist formation that take place there, or they enter the
rain directly when it washes them out of the air beneath the cloud. Locally
high concentrations of pesticides in rain and air are very seasonal and are
correlated to local use. The highest concentrations in air and rain usually
occur in the spring and summer months, coinciding with application times
and warmer temperatures. However, insecticide concentrations in air, rain,
and fog can also be high during autumn and winter in some areas if there is
high use at that time. These off-season occurrences could be due to volatiliza-
tion and wind erosion of previously applied pesticides, or the result of long-
range transport from areas where the growing season started earlier.
Degradation of Pesticides in the Atmosphere
The majority of pesticides degrade relatively rapidly in the atmosphere.
Hydrolysis takes place mainly in the fine liquid droplets of mist and in rain.
Further reaction mechanisms by which organic trace substances—gaseous or
bound to particles—in the atmosphere can be degraded are oxidation by
atmospheric oxygen and photodegradation.
In direct photodegradation, the substance absorbs light and reacts from
the excited state. In sensitized photodegradation–which affects mainly active
ingredients that are absorbed onto the surfaces acting as a sensitizer (e.g., a
920103_CRC20_0904_CH13 1/13/01 11:10 AM Page 287
288 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
formulation component)—the substance absorbs light and undergoes a tran-
sition into the triplet state. Owing to the relatively long lifetime of the triplet
state, energy can be transferred to an acceptor molecule, e.g., the active ingre-
dient molecule, thus exciting the molecule into a state in which it can react in
several ways.
Indirect photodegradation is the name given to reactions with oxi-
dants or other reactive species formed as the result of the action of light.
These are short-lived radicals such as OH
·

, NO
3
·
, HO
2
·
, RO
2
·
, OR, O
3
, or
O(
3
P). Overall it can be assumed that for the vast majority of organic trace
substances, reactions with OH radicals determine degradation behavior.
Indirect photoreactions play a prominent role when a substance in the envi-
ronment is undegradable by direct mechanisms because it does not absorb
UV in the sunlight range or because of low photoreactivity (quantum yield).
In addition to the experimental investigation of photochemical-oxidative
degradation in photoreactors (Parlar et al., 1983; Atkinson, 1985) with all their
methodological limitations (e.g., when the test substance has a low vapor
pressure), there are also computational methods (Atkinson, 1988; Klamt,
1993) available.
The environmental effects of long-term occurrences of low levels of pes-
ticides in the atmosphere are not yet well understood. Even though concen-
trations in the atmosphere are low, historic and continued depositions have
resulted in sufficient loading to cause adverse effects on fish and wildlife due
to food chain accumulation. Only by making the most accurate possible esti-
mate of pesticide deposition from the atmosphere is it possible to draw any

conclusions about the local burden, i.e., the contamination of neighboring
areas or of ecosystems remote from the emission, such as forests or mountain
lakes, or the possible human consequences via direct contact with skin or
inhalation. In addition to agricultural measures, this requires the targeted
development of new active ingredients, focusing on low environmental
impact, particularly on short-lives in soil, water, and air and mild ecotoxico-
logical effect.
INFLUENCES OF PESTICIDES ON ORGANISMS
As pesticides are biologically active chemical substances, there are con-
cerns about their having some kind of effect on organisms other than the tar-
get insects, pathogenic fungi and weeds that users want to arrest. Some of
those effects are long-term, while some are short-lived. Highly fat-soluble
pesticides that degrade with difficulty in the environment are sometimes bio-
concentrated in the food chain. It is important to perform analyses to deter-
mine whether such effects are less than what would cause environmental
changes, and if recovery from any such changes would be possible. A pesti-
cide’s toxicity toward organisms in the environment is roughly classified as
920103_CRC20_0904_CH13 1/13/01 11:10 AM Page 288
ENVIRONMENTAL FATE OF PESTICIDES 289
acute or chronic (Schafer, 1972; Wong, et al., 1995; Perry et al., 1998). Recently
some pesticides have been suspected of being endocrine disruptors. If we
determine in advance how pesticides work, it is possible to a certain extent to
estimate their biological impacts and prevent accidents.
Sometimes pesticides cause damage by contaminating crops that are not
intended targets, and sometimes the continued use of the same pesticide (or
those with similar chemical structures) will promote evolution of resistance
to the chemicals in target species (Brown et al., 1996). Additionally, it is
important to exercise great caution regarding the effects of insecticides on
honey bees, silkworms, and other useful insects because pesticides may,
among other effects, influence pollination or kill silkworms, etc.

Impacts on Nontarget Species
Effects that are manifested on organisms quite soon after pesticide appli-
cation are caused be acute toxicity. Those effects can be put into categories
including (1) direct effects on organisms in the vicinity, (2) secondary effects
through food, (3) decreases in species that provide food and habitat, and (4)
decreases in competing and predatory species. In some situations pesticides
affect the balance of species in an ecosystem; for example, the effects of a pes-
ticide on natural enemies or other organisms might instead increase the num-
bers of harmful insects. It is therefore important to exercise care so as not to
upset equilibria between harmful insects and the natural enemies that prey
on them (Tucker and Haegele, 1971; Kenaga, 1979; Hill, 1985).
There are reports on the deaths of fish and crustaceans in rivers and
lakes, or the occurrence of deformed individuals and other problems. Table
13.3 shows the acute toxicities of pesticides in various aquatic organisms.
There are large differences in toxicity values depending on the type of pesti-
cide and the species. Arthropods are very sensitive to many kinds of pesti-
cides, and a characteristic of pyrethroid insecticides is their strong specific
toxicity to fish (Gasith and Perry, 1980). A good example of herbicide effects
on algae is that the proliferation rate of the green alga Selenastrum declines
when herbicides are applied to paddy fields, and that there is a strong corre-
lation between the herbicide concentration in river water and the decline in
the proliferation rate of green algae (Hatakeyama et al., 1999). As algae have
a position as primary producers in the food chain, the finding that they are
strongly affected by organophosphates, carbamates, and organochlorine pes-
ticides has generated concerns about ecosystem disruption. Moreover, some
examples of chronic toxicity are carcinogenicity and endocrine disruption.
The thinning of egg shells observed in pelicans, European sparrow hawks,
grey herons, and other birds is suspected to be caused by the endocrine-dis-
rupting effect of DDE, a metabolite of the highly persistent pesticide DDT
(Mendelssohn, 1972; Brown, 1978).

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290 STRUCTURE AND FUNCTION IN AGROECOSYSTEMS DESIGN AND MANAGEMENT
Table 13.3 Acute Toxicities of Pesticides in Various Aquatic Organisms
Pesticides Acute toxicities (LC
60
: mg/l) Acute toxicities
mud algae
fish daphnia crab snail (EC
60
.
mg/l)
Insecticides
DDT 0.01–0.3 Ͼ 10 0.2–0.4 2–7 0.0001– Ͻ 1
fenitrothion 3–8 0.05 0.002–0.02 3–94–100
carbaryl 3–13 0.05 0.03
Ͼ 10 1–2
fenvalerate 0.001–0.1 0.001 0.0001
Ͼ 1 Ͼ 1
Fungicides
chlorothalonil 0.1–0.2 8–Ͼ 10 Ͼ 10 9–37 —
captan
Ͼ 10 Ͼ 10 Ͼ 10 Ͼ 10 0.01–Ͼ 50
zineb 0.04–0.3 1–7
Ͼ 10 Ͼ 10 Ͼ 1
Herbicides
alachlor 5–6 Ͼ 10 Ͼ 10 4–5 Ͼ 10
2,4-D
Ͼ 10 Ͼ 10 Ͼ 10 Ͼ 10 Ͼ 10
atrazine
Ͼ 10 Ͼ 10 Ͼ 10 Ͼ 10 0.003–0.1

trifluralin 0.098 –4.2 0.56 ———
Miyamoto et al., 1993
Bioconcentration
In the early 1960s, Rachel Carson observed in her book Silent Spring that
chemical substances that are highly active degrade with difficulty in the envi-
ronment and organisms, have low water solubility bioconcentrate in the
ecosystem and affect living things. Carson’s book had considerable impact
around the world. In bioconcentration, a compound moves from plankton,
algae, and other organisms to crustaceans, insects, fish, and other animals up
the food chain, while being gradually concentrated along the way, and finally
entering mammals where their effects are manifested (Edwards, 1973). As
chlorinated organic pesticides, in particular, are metabolized and broken
down with difficulty in the environment and in plants and animals, once
taken into an organism they are difficult to excrete (Kenaga, 1972; Bevenue,
1976; Ware, 1980). And because many such pesticides are highly fat-soluble,
they have a strong affinity for bodily constitutents. A coefficient for indicat-
ing the extent of bioconcentration is the bioconcentration factor (BCF), which
is the ratio of the concentration in an organism (A) to the concentration in that
organism’s environment (B), i.e., A/B. This is used to compare the biocon-
centration propensities of different pesticides. There is a positive correlation
between the BCF and the n-octanol/water partition coefficient (Kow, a phys-
ical constant showing whether an organic compound dissolves more easily in
water or oil, with n-octanol being used in place of oil), which is one indicator
showing the physicochemical characteristics of pesticides (Johnsen et al.,
920103_CRC20_0904_CH13 1/13/01 11:10 AM Page 290
ENVIRONMENTAL FATE OF PESTICIDES 291
1971; Chiou, 1977; Kanazawa, 1981). Generally pesticides with log Kow Ͼ 5
and chemical structures that make them difficult to metabolize within the
bodies of organisms tend to bioconcentrate highly.
CONCLUSIONS

As pesticides are substances meant to be toxic to organisms, when at least
a certain amount of pesticide is taken into an organism and resides there for
a certain period of time, it has a toxic effect on that organism. Accordingly,
when a pesticide is mistakenly used in terms of quantity, timing, target
species, or some other condition, it can damage human health, affect other
living things, or contaminate the environment. Pesticides now are developed
so as to maximize the benefits from their use while minimizing their risks. A
crucial challenge now before us is producing enough food to accommodate
21st century population growth, and one way of addressing that challenge is
the effective use of pesticides. For pesticides to play that role fully, it will be
necessary, from the perspectives of human health and environmental conser-
vation, to decrease still further the environmental burden of pesticides.
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