Pesticide degradation in a ‘biobed’ composting substrate

Pest Management Science Pest Manag Sci 59:527–537 (online: 2003)DOI: 10.1002/ps.685

Pesticide degradation in a ‘biobed’composting substratePaul Fogg,1∗ Alistair BA Boxall,1 Allan Walker2 and Andrew A Jukes2

1Cranfield Centre for EcoChemistry, Shardlow Hall, Shardlow, Derby, DE72 2GN, UK2Horticulture Research International, Wellesbourne, Warwickshire, CV35 9EF, UK

Abstract: Pesticides play an important role in the success of modern farming and food production.However, the release of pesticides to the environment arising from non-approved use, poor practice, illegaloperations or misuse is increasingly recognised as contributing to water contamination. Biobeds appearto offer a cost-effective method for treating pesticide-contaminated waste. This study was performed todetermine whether biobeds can degrade relatively complex pesticide mixtures when applied repeatedly. Apesticide mixture containing isoproturon, pendimethalin, chlorpyrifos, chlorothalonil, epoxiconazole anddimethoate was incubated in biomix and topsoil at concentrations to simulate pesticide disposal. Althoughthe data suggest that interactions between pesticides are possible, the effects were of less significance inbiomix than in topsoil. The same mixture was applied on three occasions at 30-day intervals. Degradationwas significantly quicker in biomix than in topsoil. The rate of degradation, however, decreased with eachadditional treatment, possibly due to the toxicity of the pesticide mixture to the microbial community.Incubations with chlorothalonil and pendimethalin carried out in sterile and non-sterile biomix indicatedthat degradation, rather than irreversible adsorption to the matrix, was the main mechanism responsiblefor the reduction in recovered residues. Results from these experiments suggest that biobeds offer a viablemeans of treating pesticide waste. 2003 Society of Chemical Industry

Keywords: biobeds; pesticide; waste treatment; degradation; mixtures; repeat applications

1 INTRODUCTIONPesticide contamination of surface waters can arisefrom a number of sources, including releases fromfields during and after the application process, leakagefrom equipment, spillages and incorrect disposal ofwaste pesticide and washings.1 While the movementof pesticides to surface waters from treated fieldshas been extensively investigated,2,3 only recently hasthe contamination arising from the other sourcesbeen considered.4–6 These studies indicate thatsuch sources may make a significant contributionto pesticide contamination of surface waters in theUK.4

Releases due to incorrect disposal, leakages andspillages can be better controlled through training ofsprayer operators and good machinery maintenance.7

Moreover, if the operator follows the Code of Practicefor the Safe use of Pesticides on Farms and Holdings(1998, currently under review)8 and the Groundwaterregulations (1998),9 releases from tank washings willbe minimised. However, due to the practicalities andcosts associated with the recommended procedures

and the fact that spillages can never be totally avoided,it would be beneficial to employ additional methodsof control.

A number of possible approaches are available,including: (1) washing of spray equipment in thefield10,11 thus reducing the requirements for decon-tamination at the farmyard and the disposal of anyassociated waste; (2) better design of the farmyardto minimise release of pesticides to nearby surfacewaters; or (3) treatment systems that are installed onthe farmyard to treat any waste arising from sprayequipment and during the filling process. Possibletreatment systems include the Sentinel,12,13 whichcombines a chemical treatment process with filtra-tion to remove organic substances from water, orthe use of biobeds. Whilst the Sentinel system effec-tively treats waste and washings, it is costly to installand to maintain.14,15 In contrast, the biobed is alow-cost and low-maintenance system. In its sim-plest form it is a hole in the ground filled with amixture of topsoil, peat and straw.16,17 The biobedis covered with grass and equipped with a ramp

∗ Correspondence to: Paul Fogg, Cranfield Centre for EcoChemistry, Shardlow Hall, Shardlow, Derby, DE72 2GN, UKE-mail: [email protected]/grant sponsor: Department for the Environment, Food and Rural AffairsContract/grant sponsor: Environment AgencyContract/grant sponsor: Crop Protection AssociationContract/grant sponsor: Monsanto Agricultural Company(Received 19 July 2002; revised version received 13 September 2002; accepted 14 November 2002)

2003 Society of Chemical Industry. Pest Manag Sci 1526–498X/2003/$30.00 527

P Fogg et al

enabling the tractor and sprayer to be driven overthe bed.

Studies in Sweden have demonstrated that biobedscan effectively retain and degrade pesticide wastearising from accidental spillages of concentrate andprepared pesticides.18 However, the suitability of abiobed to treat tank and sprayer washings has not yetbeen established. This study was therefore performedto assess the suitability of biobeds for treatingpesticides arising from tank and sprayer cleaningprocesses and spillages. The specific objectives of thestudy were to: (1) determine the degradation ratesof a wide range of pesticides in the biobed mixtureat concentrations that might be expected in the realworld; (2) investigate the degradability of mixtures ofpesticide in the biobed mixture; and (3) explore theeffects of repeated applications on the performance ofa biobed.

2 MATERIALS AND METHODS2.1 Test matrices and chemicalsThe biobed matrix (Biomix) was prepared by mixingtopsoil (69% sand, 13% silt, 18% clay), peat-freecompost (Levington Peat Free Universal) and winterwheat straw in the volumetric proportions of 1:1:2respectively. Peat-free compost was selected as it isa more ‘environmentally friendly’ alternative to thepeat mould that has been used in biobeds in the past.The mixture was composted outside for 80–100 days,then macerated using a food processor, air-dried toapproximately 30–40% w/w and refrigerated at a0–10 ◦C prior to use. A sample of the same topsoil wasair-dried, passed through a 5.4-mm mesh sieve andrefrigerated with the biomix prior to use. Disturbedsub-samples of topsoil and biomix were re-packedinto 222-cm3 volumetric tins and the maximum waterholding capacity determined by capillary rise.19

The following test chemicals were used: isoproturon500 g litre−1 SC (Alpha Isoproturon 500, Aventis),chlorothalonil 500 g litre−1 SC (Cropgard, Syngenta),pendimethalin 400 g litre−1 SC (Stomp, BASF),chlorpyrifos 480 g litre−1 EC (Dursban 4, Dow),epoxiconazole 125 g litre−1SC (Opus, BASF) anddimethoate 400 g litre−1 EC (Rogor 40, Isagro). Thesewere selected to give a range of physico-chemicalproperties and reported degradation rates in soil, andto represent compounds that were of relatively wideannual usage20 (Table 1).

2.2 Degradation of pesticides in biomixand topsoilThe degradation of each of the test chemicals in soiland biomix was determined over time. Samples oftopsoil and biomix were weighed in glass jars (125 ml)to give 24 samples of topsoil (25 g) and 24 samplesof biomix (25 g) for each chemical treatment. Thetopsoil and biomix samples were then treated with1.9 ml of suspensions made up in tap water containingisoproturon 1233, pendimethalin 986.5, chlorpyrifos

Table 1. Study compounds and their reported physico-chemical

characteristicsa

Active substanceKoc

(ml g−1)DT50

(days)Water solubility

(mg litre−1)

Isoproturon 100 6–28 65Pendimethalin 5000 90–120 0.3Chlorpyrifos 6000 60–120 1.4Chlorothalonil 1600–14 000 6–43 0.81Epoxiconazole 957–2647 60–90 6.6Dimethoate 16–52 7–16 22 300

a Values taken from Wauchope et al35 and Tomlin.36

354.9, chlorothalonil 739.8, epoxiconazole 246.6 ordimethoate 167.5 mg AI litre−1. This resulted inconcentrations of isoproturon 94, pendimethalin 75,chlorpyrifos 27, chlorothalonil 56, epoxiconazole 19or dimethoate 13 mg AI kg−1 fresh soil or fresh biomix.A further eight samples of soil and eight samples ofbiomix were prepared to act as untreated controls.

Immediately following treatment, three treatedsamples and one control sample of topsoil and biomixfor each active ingredient were removed and storedat −20 ◦C prior to analysis. The remaining sampleswere loosely capped and incubated in the dark at20 ◦C. A moisture content of 40% of the maximumwater holding capacity was maintained throughoutthe experiment. Three soil and three biomix sampleswere removed for each chemical treatment at 3, 10,20, 30, 60, 90 and 120 days after treatment (DAT)with a single sample from the untreated controls.The samples were stored at −20 ◦C prior to chemicalanalysis.

2.3 Effect of pesticide mixtures on degradationrateSamples (24) of topsoil and biomix were pre-pared as described in Section 2.1 and treatedwith 1.9 ml of a suspension containing isopro-turon 1233, pendimethalin 986.5, chlorpyrifos354.9, chlorothalonil 739.8, epoxiconazole 246.6 ordimethoate 167.5 mg AI litre−1, giving final concentra-tions of 94, 75, 27, 56, 19, 13 mg AI kg−1, respectively,in fresh soil or fresh biomix. The control samples weretreated with the sample volume of tap water. Thedegradation study was then performed using the samesampling time-points and methodology as describedin Section 2.2 for the single compound studies.

2.4 Effect of repeat application on degradationrateSamples (25 g, 63 each of topsoil and biomix) wereprepared as described in Section 2.1, and split intothree batches (A, B, C) of topsoil and biomix. Afurther 21 samples each of biomix and topsoil wereused as controls. Batches A, B and C were each treatedwith 2.75 ml of a suspension in tap water containingisoproturon 874.3, pendimethalin 702.5, chlorpyrifos252.8, chlorothalonil 526.8, epoxiconazole 43.9or dimethoate 119.4 mg AI litre−1, giving final

528 Pest Manag Sci 59:527–537 (online: 2003)

Pesticide degradation in a ‘biobed’ composting substrate

concentrations of 96, 77, 28, 58, 5 or 13 mg AIkg−1, respectively, in fresh soil or fresh biomix. Threesamples from batch A and one control sample weretaken immediately after pesticide application andstored at −20 ◦C prior to analysis. All remainingsamples (batches A, B and C) were allowed to standfor approximately 30 min before being gently shaken.The bottles were loosely capped, weighed and thenincubated in the dark at 20 ◦C. The topsoil and biomixmoisture contents were made up to 15 and 139%w/w respectively. These represented 40 and 110%of the maximum water holding capacity for soil andbiomix respectively, although visually the biomix wasnot saturated. Samples (three treated and one control)were then taken from batch A at 3, 10, 30, 60, 90 and120 days following this first treatment. These werestored at −20 ◦C prior to analysis.

After 36 days of incubation, lids were removed frombatches B and C to allow evaporation of water from thesamples. The samples were taken from the incubator3 days later, weighed, and the weight lost since the firstapplication calculated. A second treatment of 2.75 mlof the pesticide suspension used for treatment 1 wasthen applied. Tap water was used to make up themoisture balance. Control samples were treated withwater only. Immediately after the second treatment,three treated samples were taken from Batch B andstored prior to chemical analysis. The remainingbottles were capped, weighed and then returned to20 ◦C storage. Samples (three treated and one control)were then taken from Batch B at 3, 10, 30, 60, 90 and120 days after treatment.

After 36 days of incubation following treatment 2,lids were removed from the Batch C samples. Thesereceived a third treatment 37 days after treatment 2with 1.38 ml of a pesticide suspension to give thesame total application as used for treatments 1 and 2.Any remaining moisture deficit was corrected with tapwater. Untreated controls were treated with wateronly. Immediately after treatment, samples (threetreated and one control) were taken and stored priorto analysis. The remaining samples were then returnedto 20 ◦C storage. Samples were then taken from BatchC at 3, 10, 20, 30, 60, 90 and 120 days after the thirdtreatment.

2.5 Bound residuesSamples (25 g) of biomix were weighed into 125-ml clear glass bottles. Half of the samples were thentreated with aliquots (2 ml) of ethanol-free chloroform,sealed, and incubated at 30 ◦C for 7 days to fumigatethe samples. Following incubation the fumigatedsamples were evacuated in a vacuum desiccator sixto eight times to remove all traces of chloroform.

Aqueous suspensions of pendimethalin and chloro-thalonil were prepared in distilled water to giveconcentrations of 1102 and 827 mg AI litre−1

respectively. Aliquots (1.3 ml) of each solution werethen applied to the chloroform-treated and untreatedbiomix samples to give final concentrations of 57

and 43 mg kg−1 fresh biomix for pendimethalinand chlorothalonil, respectively. The biomix moisturecontent was then made up to 50% w/w (40% ofthe maximum water holding capacity). Samples wereallowed to stand for approximately 30 min before lidswere attached and the samples were then incubated at20 ◦C. Three treated samples for each pesticide and forboth sterile and non-sterile treatments were removedat 0, 3, 10, 20, 30, 60, 90 and 120 DAT. A singleuntreated sample for each substrate was taken as acontrol at each time-point.

A further experiment was conducted with chloro-thalonil using a more vigorous sterilisation method.Biomix samples (19 g and 20 g) were weighed into100-ml Duran bottles. Samples were then autoclavedat 121 ◦C for 1 h. Bacterial and fungal sterility wasconfirmed by spreading a sub-sample (0.1 g freshweight) of the autoclaved material over plates of R2Aand malt extract agar (MEA). Plates were maintainedat 20 ◦C and checked regularly over a 20-day period forgrowth of bacterial colonies on R2A and fungal hyphaeon MEA. A single sample of the autoclaved biomix wasextracted with acetonitrile (50 ml) and analysed usingHPLC to check for background interference. The19 g samples were then re-inoculated with 1 g of non-autoclaved biomix. A 400 mg AI litre−1 suspension ofchlorothalonil was prepared in sterile distilled waterand both un-inoculated and inoculated samples weretreated with 3 ml of this to achieve a final concentrationof 60 mg kg−1 (fresh weight) chlorothalonil and amoisture content of 50% w/w. Both sterile and non-sterile samples were incubated at 20 ◦C with threetreated and one untreated sample from each removedat 0, 5, 10, 20 and 30 DAT.

2.6 AnalysisConcentrations of isoproturon and chlorothalonilin samples obtained from the single substancesdegradation studies were determined by HPLC.Samples were extracted with methanol (50 ml) byshaking for 1 h on an end-over-end shaker. Extractswere then analysed by HPLC using a Spectra PhysicsSP8810 pump linked to a Cecil 1200 UV detector.Samples (20 µl) were injected using a Spectra PhysicsSP8775 autosampler. Separation was achieved usinga Spherisorb C8 column (150 mm × 4.6 mm). Forisoproturon determinations the mobile phase usedwas acetonitrile + water (40 + 60 by volume) with aflow rate of 1.45 ml min−1 to give a retention timeof 4.5 min. For chlorothalonil the mobile phase usedwas acetonitrile + water (60 + 40) with a flow rate of1.3 ml min−1 to give a retention time of 3.3 min. Thedetection wavelength was 230 nm for both substances.

Concentrations of pendimethalin and chlorpyri-fos, epoxiconazole and dimethoate from the sam-ples from the single substance degradation studieswere analysed by GC. Each sample was mixedwith anhydrous sodium sulfate (40 g) and extractedwith dichloromethane + methanol (90 + 10 by vol-ume). Soil samples were extracted with 50 ml of

Pest Manag Sci 59:527–537 (online: 2003) 529

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solvent, whilst 75 ml was used for biomix samples.Concentrations of each pesticide in the resultingextracts were then determined by GC. GC analy-sis was performed using a Hewlett-Packard HP5890gas chromatograph fitted with a split/splitless injector,12 m × 0.53 mm BPX5 column (SGE), and a nitro-gen–phosphorus detector. The carrier gas (helium)flow rate was 7 ml min−1 and detector gas flow rateswere 100 ml min−1 (air) and 4 ml min−1 (hydrogen).The oven temperature was raised from 90 ◦C to190 ◦C (40 ◦C min−1) and then to 220 ◦C (10 ◦Cmin−1) and finally to 245 ◦C (15 ◦C min−1). Sam-ples (2 µl) were injected using a Hewlett-PackardHP7673 autosampler. Under these conditions allfour pesticides were baseline separated with reten-tion times of 3.1 (dimethoate), 4.2 (chlorpyrifos), 4.7(pendimethalin) and 7.2 min (epoxiconazole). Detec-tor response was linear for all four compounds in therange 0.2–10 µg ml−1. Quantification was achieved bycomparison of peak areas with results from externalstandards.

Samples from the mixture study involving sixcompounds were analysed by GC as described above,the only difference being that 75 ml of solvent wasused in the soil extractions and 100 ml in the biomixextractions. Concentrations of all six pesticides inthe extracts were then determined using the GCconditions described above. All six pesticides wereresolved, and isoproturon and chlorothalonil hadretention times of 3.9 and 3.5 min respectively.

Concentrations of pendimethalin and chlorothalonilin samples obtained from the bound residues studywere determined after extraction by HPLC using thesame method as used described above for isoproturonand chlorothalonil in the single substance studies. Thewavelength for determinations of pendimethalin was250 nm, and the retention time was 6.4 min.

2.7 Data analysisDegradation data were fitted to either first-orderkinetics or bi-exponential curves where the pattern ofresidue decline was bi-phasic. Data were summarisedby calculating DT50 and DT90 values from the fittedcurves.

3 RESULTS3.1 Degradation of pesticides in topsoil andbiomixWith the exception of epoxiconazole in both topsoiland biomix, the degradation data for all compoundsapproximated to first-order kinetics (Fig 1) andappropriate DT50 and DT90 values were computedfrom lines of best fit. With epoxiconazole DT50

and DT90 were estimated by interpolation betweendata points. Degradation data for all pesticides andtreatments are summarised in Table 2. With theexception of chlorpyrifos and epoxiconazole, DT50

values for the substances in biomix were lower than intopsoil by a factor of between 1.7 (dimethoate) and 5.6

(chlorothalonil). With the exception of epoxiconazole,DT50 values in biomix were all less than 50 dayswhereas the maximum DT50 in soil was 225 days(chlorothalonil).

3.2 Effect of pesticide mixtures on degradationWith the exception of chlorothalonil, where the valueswere similar, DT50 and DT90 values of the testcompounds when applied in mixture to biomix werehigher than those obtained where substances wereapplied individually (Table 2, Fig 1). Generally, DT50

and DT90 values for the chemicals applied as a mixtureto biomix were lower than those when applied as amixture to topsoil. The exceptions to this were withchlorpyrifos and epoxiconazole.

3.3 Effect of repeated applications ondegradation rateWhen the mixture of pesticides was added repeatedlyto topsoil and biomix, degradation of the studycompounds was significantly more rapid in biomixthan in topsoil (Figs 2 and 3; F value = 627, P <

0.001, df = 1). In both matrices, there was a significant(F value 758; P < 0.001, df = 2) decrease in the rateof degradation following each additional applicationof the study compounds (Tables 3 and 4).

3.4 Bound residuesDegradation was significantly (P < 0.05) quicker innon-sterile biomix than in material fumigated withchloroform, (Fig 4). Calculated DT50 values forpedimethalin were 81.4 and 124.5 days (Fig 4a)in non-sterile and sterile biomix, respectively, andDT50 values for chlorpyrifos were 25.3 and 41.0 days,respectively (Fig 4b). Chlorothalonil degradation inautoclaved biomix that had been re-inoculatedwith non-sterile biomix was significantly (P < 0.001)quicker than in autoclaved sterile biomix. Degradationin both matrices followed first-order kinetics (Fig 5)with calculated DT50 values of 23.4 and 77.7 days fornon-sterile and sterile biomix, respectively.

4 DISCUSSIONBiobeds are capable of retaining and degrading smallvolumes of pesticide waste17 and have been in opera-tional use in Sweden since 1993.18 Concentrations ofup to 60 mg kg−1 have been measured in a number offield biobeds but, in general, residue levels are similarto those that would be measured in a field soil fol-lowing normal agricultural applications.17 However,if such a system is to treat dilute pesticide waste andequipment washings in the UK, it must cope with highconcentrations of complex mixtures of pesticide, oftenapplied repeatedly and in large volumes. This studywas therefore performed to determine whether biomixis able to degrade the loadings of pesticide that couldbe applied to a biobed in the UK, and to determinethe effects of pesticide mixtures and repeat applicationon pesticide degradation.



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Figure 1. Degradation of (a) isoproturon, (b) pendimethalin, (c) chlorpyrifos, (d) chlorothalonil, (e) epoxiconazole and (f) dimethoate when applied to(�) biomix as an individual treatment, (ž) topsoil as an individual treatment, (�) biomix applied with each of the remaining five pesticides and (°)topsoil with each of the remaining five pesticides.

DT50 values in biomix for pesticides covering arange of physico-chemical properties and stabilitiesand, based on recent research,6 applied at concentra-tions likely to arise from UK application rates, weregenerally substantially shorter than the DT50 valuesmeasured in a topsoil sample. With the exception ofepoxiconazole, which was not degraded during thetest duration, DT50 values in biomix ranged from5 to 50 days. For a treatment system of this type,however, the DT90 measurement may be of moresignificance in order to determine whether or not com-pounds are likely to accumulate. DT90 values >1 yearindicate that accumulation may be a problem partic-ularly when regular treatments are made.21 With theexception of epoxiconazole, all substances tested hadDT90 values of less than 6 months. Other researchershave obtained similar results. For example, Henriksen

et al22 demonstrated that isoproturon applied at con-centrations from 0.0005 to 25 000 mg kg−1 degradedfaster in biomix than in topsoil at all concentrationswith no sign of toxic effects on the micro-organisms.In the same study, mecoprop was applied to biomixand topsoil, and at concentrations below 5000 mg kg−1

there was no significant difference between biomix andtopsoil. However, above this concentration degrada-tion was only measured in biomix. The results of thecurrent study and previous work therefore indicatethat a biobed can degrade pesticides and that accu-mulation is unlikely to be a problem. There may besubstances that cannot be treated by the biobed (egepoxiconazole) and it may be necessary to controlreleases of these substances.

Most studies of the environmental fate of pesticidesare done with single applications of one compound.


P Fogg et al

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DT 5

0an

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retr

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Tops

oil

Bio

mix

Pes

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T 50

(day

s)D

T 90

(day

s)K

deg

(day

s−1)

r2D

T 50

(day

s)D

T 90

(day

s)K

deg

(day

s−1)

r2D

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(day

s)D

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s)K

deg

(day

s−1)

r2D

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deg

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110

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560.

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7998

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Figure 2. Degradation of (a) isoproturon, (b) pendimethalin, (c) chlorpyrifos, (d) chlorothalonil, (e) epoxiconazole and (f) dimethoate following (�)one, (�) two and (°) three applications to biomix of a mixture containing all six pesticides.

Table 3. DT50 and DT90 degradation rates and determination coefficients (r2) for isoproturon, pendimethalin, chlorpyrifos, chlorothalonil,

epoxiconazole and dimethoate following three repeat treatments to biomix of a mixture containing all six pesticides

Application 1 Application 2 Application 3

PesticideDT50

(days)DT90

(days) r2DT50

(days)DT90

(days) r2DT50

(days)DT90

(days) r2

Isoproturon 14.5 48.1 0.98 22.7 122.6 0.96 101.7 118.5 0.97Pendimethalin 23.5 78.1 0.94 33.5 198.8 0.84 149.8 497.6 0.64Chlorpyrifos 26.5 115.3 0.99 34.9 297.3 0.89 314.3 1044.0 0.80Chlorothalonil 2.9 9.8 1.0 2.7 9.1 0.99 17.7 45.5 0.94Epoxiconazole 61.0 —a 0.89 24.4 — 0.79 — — —Dimethoate 8.6 28.4 1.0 19.3 64.1 0.97 25.9 86.0 0.95

a Value could not be calculated.

However, in practice, repeated applications of tankmixes containing herbicides, fungicides and insec-ticides are made.23–26 Biobeds are likely to receivecomplex mixtures of more than one active substance,often applied repeatedly at concentrations far higherthan would occur in soil following normal field treat-ments. Experiments involving a mixture of six activesubstances showed that, in general, degradation wasfaster in biomix than in topsoil; the exceptions to

this were chlorpyrifos and epoxiconazole. With theexception of chlorothalonil, degradation of the com-pounds applied to the biomix as a mixture was slowerthan when the compounds were applied individually.However, DT50 values measured in the mixture weregenerally less than 5 months and the majority of DT90

values were less than 1 year. The biomix thereforeappears to be able to degrade a complex mixture ofpesticides better than soil could and, as with single


P Fogg et al

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Figure 3. Degradation of (a) isoproturon, (b) pendimethalin, (c) chlorpyrifos, (d) chlorothalonil, (e) epoxiconazole and (f) dimethoate following (�)one, (�) two and (°) three applications to topsoil of a mixture containing all six pesticides.

Table 4. DT50 and DT90 degradation rates and determination coefficients (r2) for isoproturon, pendimethalin, chlorpyrifos, chlorothalonil,

epoxiconazole and dimethoate following three repeat treatments to topsoil of a mixture containing all six pesticides

Application 1 Application 2 Application 3

Pesticide DT50 DT90 r2 DT50 DT90 r2 DT50 DT90 r2

Isoproturon 136.7 453.9 0.71 142.3 472.6 0.86 — — —Pendimethalin —a — — — — — — — —Chlorpyrifos 68.6 228.0 0.90 186.1 618.1 0.66 237.3 788.3 0.83Chlorothalonil 46.9 155.7 0.92 111.2 147.2 0.66 74.1 246.0 0.89Epoxiconazole 545.7 — 0.76 187.4 622.6 0.61 — — —Dimethoate 40.3 133.8 0.97 62.7 208.4 0.96 66.3 220.3 0.97

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Figure 4. Degradation of (a) pendimethalin and (b) chlorothalonil in (�) chloroform fumigated and (�) non-fumigated biomix.



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Figure 5. Degradation of chlorothalonil in (�) autoclaved ‘sterile’biomix and (�) biomix that has been autoclaved and re-inoculatedwith non-sterile biomix.

applications, accumulation of pesticides in the biomixover time is unlikely to be a problem.

Repeated use of certain compounds over a numberof seasons can result in enhanced rates of degradationdue to adaptation of specific microbial communitieswhich utilise the compound as an energy sourceand thus degrade the compound very rapidly.27–29

In the field, such enhanced degradation can resultin reduction or loss of efficacy of a pesticide,30 butin a biobed, enhanced degradation could improveperformance. The degradability of three applications,made at 30-day intervals, was therefore investigated.Whilst degradation was quicker in biomix than intopsoil, the rate of degradation decreased with eachadditional application. Whilst many agricultural soilspossess the necessary ingredients to cause enhanceddegradation of a susceptible pesticide, the lack ofenhancement in some soils may be due to theabsence of responsive microbes or essential cofactors,unsuitable environmental conditions, presence ofinhibitory factors or faster reversion to normality.31

The present experiments were performed usinga mixture of six active substances applied atconcentrations four times higher than the maximumrecommended dose. Whilst the timing and numberof pesticide treatments can effect the rate ofpesticide degradation,29 it is likely that the negativeeffects of high concentrations and the interactionbetween the different active substances masked anyincrease in microbial activity. Whilst no increase indegradation was observed in these studies, repeatedexposure of an agricultural soil to a susceptiblepesticide increases the chances that adaptation andenhancement will occur.31 The present experimentsused a 30-day interval between treatments, butthis may not represent real-world use conditions.Analysis of pesticide usage data, in particular that forautumn-applied herbicides, shows that applicationsare typically made over continuous 5- to 10-dayperiods. Apart from other occasional days, it is likelythat the same compounds will not be used again forfurther 12 months. Experiments performed over thistime-frame may show results that are different fromthose reported here.

Organic compounds entering the environment aresubject to several fate processes, with the net resultbeing a decline in residual concentrations. However, asignificant proportion of organic compounds or theirdegradation products can remain within the soil in theform of bound residues. It is generally observed thatthe available portion of a compound remaining in thesoil decreases with time and the bound residue fractionincreases.32 In order for biobeds to gain approvalfor use in the UK, it is essential that the pesticideresidues that are retained within the biomix aredegraded and not simply retained within the organicmatrix of the system. Experiments were thereforemade using chlorothalonil, a compound known todegrade rapidly in biomix, and pendimethalin, a morepersistent herbicide, in order to determine whetherthe decline in residues observed in the individual andmixture studies resulted from degradation or sorptionto the matrix. Whilst a statistical difference wasmeasured in degradation rates for both pendimethalinand chlorothalonil in biomix sterilised by chloroformfumigation, the data suggested that there was adecrease in the extractable concentration in thesterile matrix. Possible reasons for this may beincomplete sterilisation of the biomix by chloroformfumigation33 or that a microbial community becamere-established in the biomix during the course ofthe study. Concentrations of pendimethalin remainedrelatively static for approximately 10 days before adecline was observed. Ingham and Horton34 reportedthat, whilst bacterial and fungal populations werereduced to 37–79% of their original populationsby chloroform fumigation, the populations recoveredto their original numbers after 2 days. In a secondexperiment using biomix sterilised by autoclave,degradation was minimal in the sterile relative tothe non-sterile biomix. These data therefore suggestthat degradation was the main process responsiblefor the reduction in chlorothalonil residues and notirreversible binding to the biobed matrix.

5 CONCLUSIONSThis study was performed to investigate the suitabilityof biomix for treating pesticide waste and washings.Degradation was generally faster in biomix than intopsoil, both when a pesticide was applied on its ownand in a mixture. Whilst degradation of pesticidesapplied in mixture to biomix was slower than whenapplied alone, DT90 values indicate that, even in amixture, pesticides will be degraded within 12 months.Multiple treatments of a mixture containing six activesubstances were made to biomix and topsoil atapplication rates four times higher than the maximumrecommended for field use. Whilst degradation wassignificantly quicker in biomix relative to topsoil, therate of degradation decreased with each additionaltreatment, possibly due to the toxicity of the pesticidemixture to the microbial community or to a higherproportion of pesticide being available for extraction at


P Fogg et al

higher concentrations. The results suggest that biomixmay be capable of treating waste containing a complexmixture of pesticides often applied repeatedly at highconcentrations, although control measures may needto be introduced to ensure that certain pesticidesare not released to a biobed. Clearly, degradationis only one factor that needs to be consideredwhen assessing the suitability of a biobed system.We have also examined the leaching potential ofpesticides in biobeds, and other aspects of biobedmanagement. The results of these studies will bepresented elsewhere.

ACKNOWLEDGEMENTSThe authors acknowledge financial support from thefollowing: Department for the Environment Food andRural Affairs, Environment Agency, Crop ProtectionAssociation, Monsanto Agricultural Company.

Opinions expressed within this paper are those of theauthors and do not necessarily reflect the opinion ofthe sponsoring organisations. No comments should betaken as an endorsement or criticism of any compoundor product.

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Pesticide degradation in a ‘biobed’ composting substrate

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