Pesticides applied to farmland are put into theenvironment on purpose then are dispersed widely throughout the atmos-phere, soil, and aquatic environment outside farmland.. The environme
Trang 1CHAPTER 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 0-8493-0904-2/01/$0.00+$.50
Trang 2Pesticides play a major role in controlling insect pests and weeds andhave brought about sustained high yields and higher quality of agriculturalproduce 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 theenvironment 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 selectivityamong organisms, which are drawbacks of pesticides, improvements havebeen made in the chemical structures of these chemicals and in the ways theyare formulated and applied As a result, currently used pesticides are com-pounds characterized by low toxicity, easy degradability, high selectivity, andhigh activity (Takagi and Ueji, 1997) Decreasing the environmental loadcaused by pesticides is also needed to further expedite ecological farmingpractices
The environmental fate of pesticides (i.e., their dispersion, movement,adsorption, desorption and degradation in soil, aquatic environment, and theatmosphere, as well as their effects on organisms in the environment)changes greatly depending on environmental factors, such as meteorologicalconditions, soil condition, and properties of organisms, in addition to thephysicochemical characteristics of pesticides, the manner of their formula-tion, and how they are used In the environment and in the metabolicprocesses of organisms, pesticides are generally detoxified, but in someinstances they are transformed into metabolites that are even more toxic Thismakes it vital to ascertain metabolic pathways and the characteristics ofmetabolites
PESTICIDES IN SOIL Behavior in Soil
After application, pesticides disperse into the atmosphere and aquaticenvironment and adhere to plants, but with the passage of time much of theapplied 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 phere from the soil surface due to transpiration occurring immediately afterapplication, resulting in rapid disappearance Transpiration is governedlargely by the vapor pressure created by the chemicals, the method of use,meteorological conditions such as temperature and wind velocity, and soil
Trang 3atmos-factors such as soil moisture content and the amount of organic matter This
is followed by (2) runoff into the aquatic environment, degradation on thesoil surface by sunlight, penetration and leaching into soil, and adsorption bysoil particles The latter stage processes are influenced by water solubilityand susceptibility to photolysis of pesticides, soil characteristics includingtype and structure of soil, clay content and the amount of organic matter, andmeteorological 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 reactionssuch as chemical hydrolysis occur here, this stage consists mainly ofbiodegradation 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 intothe atmosphere from the soil surface (Swann et al., 1982)
2 The greater a chemical’s water solubility, the greater its runoff withsurface water and the more it penetrates into the soil (Weber, 1994)
3 The more organic matter in soil, the more readily chemicals areadsorbed, hence moving with greater difficulty (McEwen andStephenson, 1979)
4 Degradation in soil is almost totally biodegradation Pesticidesthus disappear quickly in soil with high microbial activity (Weedand Weber, 1974; Scheunert, 1992)
Residue in Soil
The residue of pesticides in soil depends greatly on characteristics of ticides and the soil, meteorological conditions, and other factors A look atpesticides by type shows that the organochlorine chemicals such as DDT,BHC, and aldrin and dieldrin are especially stable in soil and remain for a
pes-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 lives of various pesticides determined in laboratory testing and listed inTable 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, whilethe carbamates have comparatively long residual times Fungicides generallydisappear quickly On the other hand, herbicides generally have long half-lives, with some persisting as long as several months The reason for this isthat herbicides must have persistence because weed seeds germinate over along period of time
Trang 4half-Table 13.1 Half-lives of Pesticides in Upland Soil
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)
Half-lives in soil: (A) 14 days; (B) 15–42 days; (C) 43–180 days; (D) 180 days (I):
Insecticides; (F): Fungicides; (H): Herbicides
Adsorption and Leaching
Most of the pesticides that fall to the ground are adsorbed by the upperportion of the soil and held there Subsequently they are desorbed from thesoil particles, move, and disperse through the soil with soil moisture, or theydegrade and disappear The time needed for pesticides to disappear com-pletely from soil varies considerably depending on soil conditions and thephysicochemical 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 derWaals force, hydrogen bonding, covalent bonding, and ion exchange Theydiffer depending on the combination of a pesticide’s chemical structure andthe 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
Trang 5these are absorbed firmly the moment they make contact with the soil(Hayers et al., 1975) Also readily adsorbed are urea derivatives, the triazinesand the carbamates, etc., whose molecules contain cationic NH groups(Wauchope and Koskinen, 1983) And as with amino products formed by thereduction of nitro groups that replace benzene rings, when degradationproducts 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, claycontent, and aggregate structure Many different interactions occur betweenthese factors and pesticides’ chemical structures In many instances organicmatter content has the biggest effect In particular, the higher the soil’s humicacid content, the stronger its adsorption is; likewise, adsorption is strong insoils with high clay content and 2:1 clay minerals, such as vermiculite andmontmorillonite As with chemical substances in general, the strength orweakness 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 dependsprimarily on the amount of organic matter in the soil For that reason, Kd isindicated 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 theless it moves through the soil
On the other hand, highly water-soluble pesticides could cause water contamination (discussed below) in sandy soil with little clay ororganic matter, or when soil moisture increases
ground-Degradation in Soil
Degradation of pesticides in soil consists of nonbiological degradation,such as photolysis and hydrolysis, and biological degradation by soilmicroorganisms and other organisms The involvement of microorganisms isespecially great (Bollag et al., 1990; Turco and Konopka, 1990) A number ofdifferent 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 decomposingorganisms have been separated out from soil; examples of decomposingmicroorganisms isolated from soil are bacteria, actinomycetes, molds, andyeasts (Goring et al., 1975) Decomposition by microorganisms proceeds dif-ferently according to the chemical structure of a pesticide and becomes moredifficult as the sizes of molecules, and as their carbon numbers and numbers
of rings, increase Generally, water-soluble pesticides degrade easily, while
Trang 6fat-soluble chemicals are adsorbed into soil, making it difficult for 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 forlong periods of time and, as a result, are highly bioconcentrated.Decomposition by microorganisms is greatly affected by the nature of thesoil, and even in the same soil other major factors include temperature, mois-ture content, and the states of oxidation and reduction There are very largedifferences in breakdown products and the degradation rate depending onwhether soil is aerobic or anaerobic For example, pesticides like DDT andBHC, which are decomposed mainly by anaerobic bacteria, degrade quickly
microor-in floodmicroor-ing soil (Lichtenstemicroor-in and Schulz, 1961) Furthermore, the moreorganic matter contained in soil, the larger the number of microorganismsinvolved in degradation, which means that pesticide breakdown activity isgreater And because the decomposing bacteria types for each pesticide havetheir optimum pH values, soil pH also influences the degradation rate.When the same pesticide or chemicals with similar chemical structuresare used continuously, the corresponding decomposing microorganisms accu-mulate, which sometimes leads to decreased sustainability of a chemical’sefficacy (Chapman and Harris, 1990) Especially pesticides having carbamate(N-CO-O), urea (N-CO-N), ester (COO-C), thiocarbamate (N-CO-S), and thelike in their chemical structures undergo cross adaptation, in which degra-dation is promoted and chemical efficacy is considerably reduced (Roeth etal., 1990; Somasundaram and Coats, 1990) Measures to address this probleminclude the control of microorganisms’ decomposition activity by usingextenders, 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 ment Additionally, because water in the environment is used as drinkingwater, and it plays a major role in the conservation of aquatic organisms, it isimportant to reduce to the greatest possible extent the risk of contaminatingrivers and groundwater with pesticides
environ-When pesticides are applied to upland fields, the chemicals that fall tothe 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
Trang 7the time directly after application to within about two weeks after(Wauchope, 1978; Leonard, 1988) In particular, the shorter the time elapsedsince application, the greater the amount of pesticide runoff caused by rain-fall With the passage of time, the amount of chemical runoff into aquaticenvironment lessens because the chemicals move downward from the soilsurface and are more firmly adsorbed into soil particles and absorbed bycrops, 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 tion, and even a generous estimate puts the total runoff rate two weeks afterapplication at 1 or 2% (Leonard, 1990) One factor governing runoff is thewater 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 watersolubility of 10 ppm or higher mainly move to the aquatic environment bydissolving into surface water, while those that dissolve with difficulty orhave 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 occursare slopes, hard soil with low water permeability, furrows runninguphill/downhill, and exposed soil (Fujita, 1998) It is important to implementfully farmland soil erosion control and water management in order to curbpesticide runoff into the aquatic environment
applica-Pesticide movement by means of soil moisture percolation into theground brings about groundwater pollution Especially when pesticides arehighly water-soluble, when soil is sandy with little clay or organic matter, orwhen soil moisture has increased suddenly, pesticides are detected ingroundwater (Cohen et al., 1990) In western countries, where groundwater
is often used as drinking water, the detection of pesticides in the water ofabout 30% of wells in the 1980s became a matter of public concern Some ofthe chemicals detected with especially great frequency were the soil fumigantethylene dibromide (0.05–20 ppb), the carbamate insecticide aldicarb (1–50ppb), 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 concentrationstandards 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 ofsoil Because farmland does not necessarily have homogeneous soil struc-tures, it is hard to discern uniform trends for each field owing to rainfall, soilconditions, and other factors Below are some conditions that create a majorpotential for groundwater contamination:
Trang 8Pesticide characteristics: Water solubility of 30 mg/l or more; Kocvalue under 500; Henry’s constant under 102atm m3/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 insoil of three weeks or more
Farmland conditions:Annual rainfall of 25 cm or more; high ity of pesticide contamination in area with high nitric acid ion con-tent in groundwater; places with porous soil above aquifer; and soilthat has pH providing for high stability of a certain pesticide
possibil-Pesticides applied on paddy field are dispersed into aquatic environmentover broad areas while being diluted as they follow a path from agriculturalwater channels to small rivers and then to large rivers The concentrations ofpesticides in paddy surface water differ according to the amount applied perunit area, the manner of formulation, physicochemical characteristics, andenvironmental conditions, including temperature, rainfall, and soil charac-teristics Generally the greater a pesticide’s water solubility, the higher theconcentration The highest concentration is found between the time immedi-ately after application and the following day, and many pesticides have shorthalf-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 itssurface water, the runoff rate from the fields into the aquatic environment islarger than that from upland fields The runoff rate also varies according toapplication amount, water management, and other factors, but in particularone can discern a positive correlation between a pesticide’s runoff rate and itswater solubility (Inao et al., 1999; Maru, 1990) For example, Maru calculatedthe 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 thatthe water solubilities of the herbicides chlornitrofen, butachlor, thiobencarb,and simetrine were 0.25, 23, 30, and 450 ppm, while their runoff rates intoriver water were 0.11, 2.32, 1.44, and 5.96%, respectively The higher the watersolubility is, the higher the runoff rate, with the following regression equa-tion relating the runoff rate to the logarithmic value of water solubility:
Y 1.06 1.84 log (X), r 0.75 and n 10.
where Y is runoff rate and X is water solubility.
The period during which pesticides are detected in the aqueous ment corresponds to their time of application Often there is a temporarypeak in the amount detected immediately following application, after whichthere is a gradual decline, with the chemicals becoming undetectable aftertwo or three months The concentration detected in river water is sometimes
environ-a high venviron-alue of 100 ppb for environ-a short time with the herbicide molinenviron-ate, which
Trang 9has a high water solubility of 900 ppm, but generally the concentration is inthe range of 0.1 to 10 ppb (Nakamura, 1993).
Degradation in Aqueous Environment
Pesticides that have entered into water disappear by adsorption to soilparticles, settling to sediment, atmospheric dispersion by evaporation withwater, or a variety of breakdown reactions Table 13.2 classifies the persistence
of pesticides in water by their half-lives, which depend largely upon theirchemical structures (McEwen and Stephenson, 1979) Half-lives range fromthose less than two weeks for organophosphates and carbamates to the long-term stability of over six months for chlorinated pesticides Degradation ofpesticides in water is accomplished by chemical reactions, mainly hydrolysis;physical reactions caused by photolysis; and biological reactions carried on bymicroorganisms 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
Organophosphorous Organochlorines Organochlorines Organochlorines
chlorpyrifos methoxychlor lindane endrin, BHC demeton Organophosphorus Organophosphorus heptachlor dichlorvos diazinon chlorfenvinphos Benzimidazoles
fenitrothion disulfoton dimethoate benomyl
phosphamidon chloropropham aldicarb
Carbamates EPTC, swep carbofuran
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) 2 weeks; (B) 2–6 weeks; (C) 6 weeks–6 months; (D) 6 months
Trang 10aqueous environments with different qualities showed that these ments were ranked, from highest to lowest rates, in the order of river water,lake water, seawater, and groundwater Thus, pesticides remained for a longtime in groundwater (Hiramatsu, 1990) The main degradation factors underfreshwater conditions of rivers and lakes are breakdown by microorganisms,and in seawater by weakly alkaline hydrolysis (Kanazawa, 1987).
environ-Sunlight-induced photolysis is an important degradation factor of cides in rice paddy water and the surface layer of river water, but at depthsgreater than this, where the energy of sunlight is reduced, the contribution ofphotolysis is reduced (Oyamada and Kuwatsuka, 1986; Yamaoka et al., 1988)
pesti-As photochemical reactions occur in the wavelength region of 290–450 nm,the more ultraviolet light a pesticide absorbs, the more susceptible it is tophotolysis (Crosby, 1969) Additionally, aqueous environments contain suchphotosensitizing substances like chlorophyll, carotenes, quinones, riboflavin,humic acid, and amino acids, which catalyze light reactions Roughly, thereare two photosensitizing reactions: (1) energy absorbed by photosensitizingsubstances is passed to coexisting substances (pesticides in this case) where
it brings about chemical reactions; (2) oxygen is activated by the action ofphotosensitizing substances, thereby forming powerful oxidants such ashydroxyl radicals, peroxides, and superoxidoanions, which then promoteoxidation reactions (Nakagawa, 1990)
Microbial degradation in water generally proceeds readily under aerobicconditions, 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 fewchlorinated organic pesticides readily undergo dechlorination reactionsunder anaerobic conditions (Marth, 1966; Johnson, 1976; Kanazawa, 1987).Under whatever conditions, microorganisms use enzymes to completelydegrade pesticides to carbon dioxide by oxidation, reduction, hydrolysis, andother reactions As a test for degradation in water by microorganisms, theOverseas Economic Cooperation Fund (OECD) and the EPA propose amethod 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 tribution of pesticides in the atmosphere showed that the atmosphere is animportant part that acts to distribute and deposit pesticides in areas farremoved from their application sites A compilation of existing data is thatpesticides have been detected in the atmosphere throughout the world, butmost of the available information is from small-scale, short-term studies, few
dis-of which lasted more than one year Until the 1960s atmospheric pollutionfrom pesticide spray drift was generally thought of as a local problem Long-range movement of long-lived pesticides through the atmosphere was