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APPROACHES TO THE PROBLEM ON PESTICIDE RESIDUESIN CROPS AND SOILS IN JAPAN

Yasuhiro YogoOrganochemicals Division,

National Institute for Agro-Environmental Sciences,3-1-3, Kannondai, Tsukuba, Ibaraki, 305-8604, Japan

ABSTRACT

Pesticide residues adversely affect human health and the natural environment. In Japan, severalpesticides were detected to exceed Maximum Residue Level (MRL) of foods. Its current pesticideregistration system requires performing a set of acute toxicity examinations in three aquaticorganisms. Persistent Organic Pollutants (POPs), most of which are “past pesticides,” have nowbecome a global issue. The objective of this paper is to provide basic knowledge on pesticideexposure and what are the approaches used to deal with residue issues in Japan. Each pesticidehas its own physicochemical/biochemical properties and management purposes, including targetdiseases/insects/weeds and target crop. The characters of each target pest and crop are alsolargely diverse. In addition there are a lot of pesticide formulation and application methods. Thus,various routes exist in pesticide exposure to crop and soil. First is the direct exposure of crop topesticides, which includes the application of target/applicable crop and drift to non-target crop.The second is indirect exposure of non-target plants via variety of routes. Soil, water and air ortheir combinations are major media for pesticide behavior. Pesticides intentionally or accidentallyapplied pollute soil in arable land. It persists in soil by several kinds of intermolecularinteractions. “Aging” is an important idea to deal with pesticide persistence in soil. Thebioavailability and/or the extractability from bound residues are getting more difficult with time. Itlargely affects length of persistence of pesticides in soil. Pesticides largely move with water in soilto ground water and on soil to surface water. Dissolved organic matter and suspended solidshould be taken into account on pesticide contamination in water. POPs high bioaccumulation,and long distance transport have long persistence in the environment. Methyl bromide had beenused intensively worldwide. These were transported to the atmosphere causing problems both tohuman and the environment. There are a lot of approaches to the problem on pesticide residuesin crop and soils. In the viewpoint of air pollution, the risk of spray drift is assessed in EU andUSA by computer simulation models. However, it is very difficult to introduce them in Japan dueto the limitation of buffer zone. Therefore, application methods and drift guards as alternativesprevent spray drift, which fits small-scale farming. In addition, vapor drift, which has two states(gas and/or particle-adsorption), also have very important issues to be considered. Multimediamodel for global behavior of organic chemicals were based on “fugacity capacity” developed justrecently in Japan. There are three types of on-site remediation methods in soil. Those werephytoremediation, bioremediation, and chemical remediation. Grafting method can be used incucumber/pumpkin combination in phytoremediation of drins. Bioremediation is effective for on-siteremediation of pesticides and POPs. The practically effective bacterial consortia, which degradetriazines, were found in our institute. Chemical remediation is useful with single or incombination with the above remediation methods. Computer simulation models for pesticidebehavior in paddy-river system were developed in Japan and in Europe. In diatom, goodcorrelation was observed in the 50% effective concentration (EC50) values of dimethametrynbetween the native colonies and the cumulative values of isolated single species. Large differencein susceptibility was observed between the upper and middle reaches of stream. Crop residue isnot simple, due to the following reasons, 1) many exposure ways to existing pesticides, 2)pesticide transport to edible parts depend upon crop and pesticide combination, and 3) pesticidesare metabolized not only in soil but also in crop. Pesticide registration in minor crops is a socialproblem in Japan. There are more than 200 crops, which could not be categorized in 11 minor

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INTRODUCTION

The import/export of agricultural productsqualitatively and quantitatively had beenincreasing in recent years, due to internationalfood diversification. On May 29, 2006 in Japan,the Ministry of Health Labour and Welfareintroduced the “Positive List System” foragricultural chemicals (pesticides, feed additivesand veterinary drugs) in order to improve foodsafety.

Paracelsus (1493-1541) mentioned: “Allsubstances are poisons; there is none which isnot a poison. The right dose differentiates apoison and a remedy. The dose makes thepoison.” It means that risk by chemicalscannot be determined by either hazardproperties (toxicity) or the amount of exposure.They must be combined for risk assessment.

Pesticide residues adversely affecthuman health and the natural environment(Baker et al. 1994, Freemark and Boutin, 1994,Clark and Ohkawa 2003). For issues concerninghuman health, several pesticides were detectedto exceed Maximum Residue Level (MRL) inimported vegetables such as spinach,mushroom and shellfish including Corbiculajaponica Prime (brackish-water clam) inbrackish-water region in Japan in recent years.Social problems arose on dermal/skin exposureof organophosphorus pesticide to children andpersons with chemical sensitivity. In case ofnatural environment, Japan’s current pesticideregistration system requires performing a set ofacute toxicity examinations in three organisms— fish, water fleas, and green algae (Nyholmand Källqvist 1989) — to assess the ecotoxicityof a pesticide in inland water ecosystem, basedon the OECD guideline (Campbell 1999).However, they live in lakes and marshes, notin rivers directly linked from paddy fields. Nochronic and/or ecological study was requiredfor pesticide registration (Newman et al. 2006).

In addition, POPs are now considered asglobal issues (Barra et al. 2004). Future POPsare proposed, and will be approved in POPROCin a few years time. Some pesticides areincluded such as chlordecone,hexachlorocyclohexane (lindane, ã-HCH),endosulfan, as well as current POPs.

The objective of this paper is to providebasic knowledge on pesticide exposure andhow to approach the residue issues in Japan.Pesticides here refer to synthetic organicpesticides. Biological pesticides such asmicrobe-origin pesticides are not included.

PESTICIDE EXPOSURE

Each pesticide has its own physicochemical/biochemical properties and managementpurposes, including target diseases/insects/weeds and target crop. The characters of eachtarget pest and crop are largely diverse. Thereare a lot of pesticide formulation andapplication methods. As a result, variousroutes exist in pesticide exposure to crop andsoils (Fig. 1).

First, there are two types of directexposure when pesticide with dilution water issprayed. One is to apply target/applicable crop,and the other is “spray drift” of sprayedpesticide from target crop to non-target crop.What is a “drift”? There are spray drift andvapor drift. Spray drift occurs at spraying, andis composed by preliminary droplet, whichdirectly occur from spray nozzle, and secondarydroplet, which occur after hitting/bounding tocrop and soils at application. Vapor drift alsooccurs from crop and soils. The driving forceis the vapor pressure of pesticide. Pesticidesare lost in the atmosphere with gas and/orparticle-adsorption state for long period,depending on pesticides, soils and themeteorology.

crop groups. However, research results showed that the good correlation was observed inpesticide residues within the family level. In the future the following matters should be considered:1) integrated risk assessment to human and natural ecosystem; 2) ecological risk assessment inarid and semiarid ecosystem; 3) ecological risk assessment on long-term effect; and 4) cost benefitanalysis. Further cooperation in exchange of information and technology is needed to solve theproblem effectively.

Key words: pesticide residue, POPs, pesticide exposure, persistence, on-site remediation, riskassessment, cost and benefit analysis

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Second, non-target plants, includingcrops, are indirectly exposed to pesticide via avariety of routes. Soil, water and air are majormedia for pesticide behavior, and are sometimescombined (Jury et al. 1987, Ferenc 2001). Mostdistinctive phenomena of organic pesticidebehavior are metabolism and degradation.Pesticides change their structures biotically(Wolt et al. 2001) and abiotically in theenvironment (Choudhry 1985; Katagi 2002;Katagi 2004; Kobara et al. 2002; Kobara et al.2003).

Via Soil

Pesticides that are intentionally or accidentallyapplied pollute soils in arable land and persistin soil by several kinds of intermolecularinteractions, such as van der Waals-London,hydrophobic bond and ligand or ion exchange(Kovacs 1986). “Aging” is an important idea todeal with pesticide persistence in soil. Thebioavailability and/or the extractability frombound residues are getting more difficult withtime (Kovacs 1986). It largely affects length ofpersistence of pesticides in soil.

Pesticides equilibrate between solid (soil)and liquid (water) phase, by adsorption to soiland desorption from soil. Pesticides in soilwere carried over to succeeding crop, when

they have long duration of activity or thecropping interval was short. Especially ingreenhouse were soil fumigant is applied inmany cases, plastic film largely cut UV, androtation of vegetables is very short.Consequently, risk of phytotoxicity andpesticide residue in crop will increase.

Pesticides move in soil via water.However, soil particles with pesticides alsomove in the soil by cultivation, on the soilsurface by excess water, and in the air bywind. They cannot be neglected environmentalcontamination of pesticides.

Via Water

Pesticides largely move with water in soil toground water and on soil to surface water. Themovement in soil is leaching and percolation(penetration). Leaked water to ground water isused for irrigation or drinking water (Honeycuttand Schabacker 1994), and percolated watercame to the next field. The movement on soilis runoff and drainage, which are also used aswater flow (Figs. 1 and 2). About half ofJapanese arable land is paddy field, whichdirectly link to river via drainage canal. Thesecontaminated water cause phytotoxicity to cropor bring excess crop residue, and adverselyaffect human health.

Fig. 1. Scheme of pesticide exposure to crop and soils.

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Dissolved organic matter and suspendedsolid should be taken into account onpesticide contamination, since they are verymobile and most of pesticides withhydrophobic moiety strongly adsorb to them(Krop et al. 2001).

Via Air

Twelve compounds are currently listed as POPsat Stockholm Convention (May 17, 2004), andnine of them are pesticides, such as mirex,drins (aldrin, dieldrin, endrin), toxaphene, DDT,

chlordane, heptachlor, hexachlorobenzene.Dioxins are their byproducts. They have hazard(highly toxic) and exposure (long persistence inthe environment, high bioaccumulation, longdistance transport) characteristics. Results werefound in the polar region. POPs are monitoredall over the world (Hebert and Miller, 2004).Passive and active air sampler is introduced tocorrect them other than collecting precipitation.Passive air sampler is fit for covering numberof sampling sites in wide areas. Active airsampler is fit for relatively precise and detailedexperiment to clarify trend of pesticide transfer

Fig. 2. Scheme of pesticide behavior in paddy field.

Source: Inao and Kitamura, 1999Note: The scheme was rearranged from above literature.

Fig. 3. Geometric meanconcentrations of sum PCBs,DDTs, and PBDEs and HCBin Asia.Source: Jaward et al. 2005

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from soil to the atmosphere. Jaward et al.(2005a/2005b) monitored POPs and the relatedcompounds in Asian region (Fig. 3). There aresignificant difference in amount and kinds ofPOPs among countries.

Methyl bromide as soil fumigant hadbeen intensively used in the world but itcauses ozone layer depletion. Global efforts arerequired to repair the world’s protective ozonelayer. Methyl bromide was then bannedstarting in 2005. Many researchers has studiedthe environmental fate (Yates et al. 2003).Kobara (Kobara 2005, Kobara et al. 2001)estimated atmospheric concentrations of methylbromide alternatives in the Kanto area byAIST-ADMER (Atmospheric Dispersion Modelfor Exposure and Risk Assessment) developedby National Institute of Advanced IndustrialScience and Technology.

Other studies on pesticide exposure arealso conducted in silage corn, (Uegaki et al.2004), spinach (Eun et al. 2001) and domesticanimals (Gurge et al. 2004, Gurge et al. 2005)

APPROACHES TO PESTICIDE RESIDUE

There are a lot of approaches to the problemof pesticide residues in crop and soils. Thefollowing approaches are arranged for riskassessment and risk management (mitigation)including some example in our institute, theNational Institute for Agro-EnvironmentalSciences (NIAES), Japan.

Air Pollution

Spray drift

Risk assessment of spray drift is moreadvanced in EU and USA in comparison toJapan. Computer simulation model were used topredict the spray drift in aerial and groundspray. These simulation models are used toelucidate the buffer zone to the neighboringcrop and residential area. Physicalcharacteristics of spray droplet, sprayingapparatus and meteorological parameters areinput to these simulation models, but activeingredient and formulation of pesticides are notconsidered.

� Spray Drift Task Force (DRIFT):Website(http://www.agdrift.com/index.htm)

� DRIFTSIM:Website(http://ohioline.osu.edu/b923/pdf/b923.pdf)

In Japan it is very difficult to place thebuffer zone. Therefore, spray drift wereprevented by application method and driftguard. Drift less nozzle, air curtain in boomspray, inter-row application, and electrostaticspray deposition (ESD) technique weredeveloped and practically used in some cases.Small pore size net and crop such as sorghumare used as drift guards by surrounding targetfield.

Attention should be made on smalldroplet size particularly those smaller than fineparticulate matter (PM2.5). PM2.5 is very small,has droplets in the air (size is less than 2.5�min width) and able to go deeply into the lungs.Long-term exposure to PM2.5 is associatedwith increased chronic bronchitis, reduced lungfunction and increased mortality from lungcancer and heart disease. People with breathingand heart problems, children and the elderlymay be particularly sensitive to PM2.5.

Kobara designed and developed thedetection apparatus for fine droplet size inspraying pesticides. Spray pressure andtemperature showed positive relation to numberof droplets (Fig. 4).

Vapor drift

As mentioned before, vapor drift transfer fromcrop and soils to the atmosphere with gasand/or particle-adsorption state. Balancebetween gas and particle is greatly differentamong pesticides (herbicides). Pendimethalinand trifluralin were in gas state. Alachlor andmetolachlor were in particle state. Thiobencarbwas detected in both states.

Several types of Multimedia Model(MMM) for global behavior of organicchemicals, such as pesticides and POPs, weredeveloped in the world (Mackay 2001). Kobaraand Nishimori are recently developing MMMfor pesticides and POPs (Kobara et al. 2005;Kobara et al. 2006; and Nishimori et al. 2006).Most of MMM are fugacity model, which isclosely related to the principle of pressure. Thedifferentiations of this model in comparison tothe conventional models are as follows:

� Target chemicals are expanded to polarand ion chemicals.

� Temperature is included as a parameter,this largely affect half-life and movement.

� Application method of pesticides isconsidered.

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� Nine media are considered, includingpaddy field, rice growth and saltconcentration in the oceans

� Relative assessment� Quantitative Structure-Activity and

Structure- Property Relationship (QSAR/QSPR) secure accuracy of parameters.

� The target status is Level IV, non-equilibrium and non-stationary.Kobara et al. (1999) developed the

plastic film to prevent methyl bromide emissionto the atmosphere. The film is composed ofgas-barrier layer, TiO2 photocatalytic layer andsupport layer (Fig. 5). Then, Methyl bromidewas dramatically reduced with this file withoutany vapor.

Soil Residue

Dioxin concentrations were analyzed inpreserved arable land. Soils were periodicallycollected from 1960 to 2002. Its concentrationincreased in the 1960s till 1970s and eventuallydecreased. The results of principal componentanalysis and chemical mass balance based onfunctional relationship analysis indicated thatdioxin concentrations were increased due to theherbicide use. The examples arepentachlorophenol (PCP) and chlornitrofen(CNP), which included dioxin as impurities (Fig.6). The half-lives of dioxins in the paddy soilswere estimated to be 10 to 20 years and thedissipation trends are “hockey-stick” curve, i.e.

Fig. 4. Effect of spray conditions to spray drift.Source: Koraba et al. 2007 (In Japanese)

Fig. 5. Component and performance of the multilayersheet containing a Photocatalytic Layer.

Source: Kobara et al. 1999

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quick reduction in the first 10 years, and slowreduction in the later years (Seike and Otani,2004; Seike et al. 2005; and Seike et al. 2007).Therefore, Monitored Natural Attenuation(MNA) is difficult to be introduced in POPscontaminated soils.

Enantiomeric ratio of chiral pesticides iseffective in analyzing the environmentalbehavior (Hegeman et al. 2002). Sincephysicochemical and/or biological properties aredifferent among optical isomers. Thus,

contamination source can be assumed byanalyzing enantiomeric ratio in soils amongdifferent depth and sites.

There are three types of on-siteremediation methods; 1) phytoremediation byusing plants, 2) bioremediation by usingmicrobe and 3) chemical remediation. Theuptake of drins were particularly high inCucurbita genera, and no other crops canabsorb them by roots (Otani et al. 2006,; andOtani et al. 2007). Otani and Seike (2006)

Fig. 6. Principal component plots of loading on PC1 and PC2 usingdioxins’ congener and homolog profiles in PCP (�), CNP (�),atmospheric deposition (�), and Japanese paddy soils (O).Source: Seike et al. 2007

Fig. 7. Effect of stock varieties on dildrin concentration in cucumber fruitsSource: Otani and Seike 2006Note: The chart was arranged based on the paper of Otani and Seike (2006). “A”, “C” and “J”

are stock (pumpkin) varieties, and “d” and “u” are scion (cucumber) varieties.

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examined the effect of grafting by pumpkin asrootstock on drins (dieldrin and endrin) uptakefrom soil into cucumber plants. The resultsindicated that drin concentrations in the graftedplants were influenced mainly by rootstockvarieties (Fig. 7, Otani and Seike 2006). Thisremediation method can be applied not only todrins but also to other POPs (dioxins andheptachlor).

Bioremediation is effective for on-siteremediation of pesticides and POPs. Takagi etal. found the bacteria, which degraded triazineherbicides. They produced bacterial consortiathat would completely degrade methylthio- andchloro-triazines. Then they constructed acharcoal material containing these consortia(Fig. 8). Consequently, triazine contamination ofthe subsoil, river water, and groundwater couldbe minimized (Fujii et al. 2006; Harada et al.2006; and Iwasaki et al. 2006). This idea canbe applied to the other pesticides includingPOPs.

Chemical remediation is useful in singleor in combination with the above remediationmethods. However, details are not mentionedhere.

Water Contamination

Computer simulation models for pesticidebehavior in paddy-river system were developed

in Japan. They were PADDY (Inao andKitamura 1999; Inao et al. 2001; Inao et al.2003), PCPF (Tournebize et al. 2006; Vu et al.2006; Watanabe and Takagi, 2000a/2000b;Watanabe et al. 2006), and RICEWQ (Williamset al. 1996) in Europe. They were useful toolsto prevent pesticide contamination in drainagecanal and river. PADDY model accuratelysimulated the pesticide behavior in paddy field(Fig. 9, Inao and Kitamura 1999). It will enablethe prediction of the required water holdingperiods to avoid excess amount of pesticiderunoff from the paddy outlet. They are nowdeveloping the model, which can simulate notonly parent ingredient but also the metabolitesand the optical isomers. Watanabe et al. alsoimproved their model in order to simulate therequired foot path height by using originallydeveloped compact lysimeter.

In the viewpoint of ecotoxicology,Ishihara et al. developed an easy detectionmethod of herbicide susceptibility of nativecolonies of diatoms. These are the dominantproducer organisms in rivers of Japan. Diatomgrowth was measured by absorbance ofchlorophyll a, and growth inhibition wascalculated between 24 and 96 h afterapplication of the herbicide.

A good correlation was observed in the50% effective concentration (EC50) values ofdimethametryn between the native colonies and

Fig. 8. Estimated metabolic pathway of chloro- and methylthio-triazines by bacterial consortia.Source: NIAES Annual Report 2006

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the cumulative values of isolated single species(Fig. 10). Large difference in susceptibility wasobserved between upper and middle reaches ofstream, where paddy fields either exist or notin nearby fields.

Crop Residue

Approaches for crop residue problem werecomplicated. The reasons are first there aremany different ways of pesticide exposure asmentioned above. For example, cucumberabsorbs dioxins via soil while other plants are

exposed in a different way. Gramineous plantswere contaminated due to high concentrationof dioxin in the atmosphere and its isomerprofile (Fig. 11, Uegaki et al. 2005; and Uegakiet al. 2006).

Second, pesticide transport to the edibleparts depends upon crop and pesticidecombination. Even though certain amounts ofpesticides were absorbed from roots, almost noresidues were detected in seeds when there isa strong barrier between roots and shoots.

Third reason was because pesticideswere metabolized not only in soil but also in

Fig. 9. Comparison between calculated and measured molinateconcentration in (a) paddy water and (b) soil.Source: Inao and Kitamura, 1999Note: (a) calculated (Solid line) and measured ( � ) value, (b) calculated(Solid line = 0-2 cm, broken line = 2-4cm) and measured (� = 0-2 cm, � =2-4 cm)

Fig. 10. Comparison of acute toxicity of dimethametryn to nativediatom colonies in upper and middle reaches of streams.Source: NIAES Annual Report 2006

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crop. Metabolism depends upon crop andpesticide combination as well as transportation.Pesticides were metabolized in plants in twomajor steps, the first phase reactions andsecond phase reactions, including mammals.The first phase reactions are oxidation,reduction and hydrolysis by P450, esterase andso forth. The second phase reactions arecomposed of several types of conjugations byglutathione-S-transferase (GST) and so forth.Some pesticide combinations antagonisticallyinhibit these enzymatic activities, such aspiperonyl butioxide (PBO) against P450, andorganochlorines or carbamates against GST oresterase.

Pesticide registration in minor crops is asocial problem in Japan. There are more than400 minor crops in Japan, although it was only0.9% of total diet. It is difficult to register eachpesticide to each minor crop. The conduct ofcrop residue trials is time and labor consumingand the pesticide companies cannot afford theregistration costs. About 120 minor crops werecategorized to 11 groups in order to make theregistration efficient. It was required that foreach pesticide residue trial it should be with

two crops. However, there are more than 200minor crops, which could not be categorized inthese 11 groups. In trying to estimate pesticideresidue by harvesting crops based on initiallyattached pesticide to the crop, a goodcorrelation was observed in pesticide residuesamong tested minor crops within the familylevel (Fig. 12).

� Cucurbitaceae: cucumber, Momordicacharantia L. (“Nigauri”), zucchini

� Umbelliferae (“Seri”): 15 crops, such ascelery, parsley

� Lamiaceae (“Shiso”): 17 crops, such asperilla, basil, rosemary etc.

� Minor gramineae: foxtail millet, barnyardgrass and millet (Panicum miliaceum L.)

FUTURE PROSPECTS

The 11th IUPAC International Congress ofPesticide Chemistry was held in August 6-11 atKobe, Japan (Ohkawa et al. 2007). Here areseveral hints and approaches which maybeused as reference for future prospects.

Integrated risk assessment to human andnatural ecosystem is the most remarkable issue

Fig. 11. Dioxin homologue concentrations in the atmosphericparticle phase in rice plants.Source: Uegaki et al., 2006

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(Sekizawa and Tanabe, 2005; and Sutter et al.2003). There are many interactions not only inliving things but also in the environment suchas genetically, biochemically, biologically,physiologically and chemically. They negativelyand positively affect human and environment.These interactions are too complex to clarify.The idea of grouping chemicals based on theirproperties is rising now (Fig. 13).

Ecological risk assessment in arid andsemiarid ecosystem will come soon forpesticide registration (Markwiese et al. 2001).This ecosystem is called “upland” againstpaddy field and/or inland water. OECD hadrecently set several test guidelines for acutetoxicity of living things in upland. However,upland conditions varied more than paddy orinland water, due to the change of soilmoisture and wide range of living thingsinhabiting there (birds in the sky andearthworm in the soil). Therefore riskassessment is quite difficult to perform for theecosystems with upland condition.

Ecological risk assessment on long-termeffect is also important to clearly understandthe adverse effect of pesticides to the

ecosystem (Newman et al. 2006). Recovery isthe most important key word, and the testguideline should be newly developed tocompensate conventional acute toxicity tests.Another important keyword is time. It meansthat each living thing has their own life cycleand sensitivity to pesticides are largelydifferent among ages. Pesticides temporally (notcontinuously) contaminate the environment.Therefore, timing between their life cycles andpesticide contamination will affect the intensityof adverse effect to the population. Theyenable to reduce safety factors for theecological risk assessment.

Cost benefit analysis should also bedone. The relationship between lasting effectand persistence of pesticides in soil had beenstudied in the past (Rajagopal 1984).Furthermore, pesticides are essential tools inagriculture. However, they have their own“pros (merit)” and “cons (demerit)”, forbiology, ecology and economy. Therefore,these points should be taken into considerationto understand and assess the pesticide use.

Approaches to the problem on pesticideresidues in crop and soils are very complex it

Fig. 12. Chlorotharonil dissipation in cucurubitaceae crops.Source: Eun et al. (In Japanese)

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involves piles of research for issues to betackled. A single researcher or one countrycannot address these issues. Thus, furthercooperation among countries and individualsby exchanging information and technology isneeded to solve them effectively.

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