Transport of Imazethapyr in Undisturbed Soil ColumnsJ. David O'Dell,* Jeff D. Wolt, and Philip M. Jardine

ABSTRACTThe disappearance of imazethapyr [(±)-2-[4,5-dihydro-4-methyl-4-

(l-methylethyl)-5-oxo-lH-unidazole-2-yl]-5-ethyl-3-pyridinecarbojg1ic acid]from soil solution was investigated to evaluate the transport of ima-zethapyr in undisturbed soil columns. Soil treated with imazethapyrwas incubated for 0.15,1,2,4,8, and 16 d, at which times soil solutionwas obtained by vacuum displacement. Bromide and imazethapyr werethen applied to the surface of undisturbed soil columns followed byapplication of deionized water at approximately 3 cm d~'. The re-moval of imazethapyr from solution (i.e., sorption) exhibited biphasickinetics and was well described (R2 = 0.99) by two simultaneous first-order reactions. The initial reaction was essentially instantaneous andthe secondary reaction was time dependent. The classical convective-dispersive (CD) equation was used to describe solute transport throughundisturbed soil columns. Bromide and imazethapyr breakthroughcurves (BTCs) were displaced to the left of one pore volume andshowed considerable tailing, with imazethapyr BTCs retarded in re-lation to Br~. This was indicative of preferential solute mobility inthe soil. Imazethapyr BTCs were similar to model simulations usingtransport parameters determined from Br~ BTCs, kinetic parametersfrom the imazethapyr solution concentration decay curves, and re-tardation factors calculated from the distribution coefficient at the 1-d incubation time. Model simulations using retardation factors fromlater times predicted increasingly delayed imazethapyr breakthroughwith lower peak concentrations than measured. Preferential flow pathsapparently reduced attenuation of imazethapyr as predicted fromequilibrium adsorption measurements.

THE POTENTIAL MOBILITY of herbicides in soil hasbeen evaluated by several methods. Field meth-

ods often involve sampling the soil or soil water atvarious points laterally and below the point of herbi-cide application (Basham et al., 1987). Laboratorymethods include soil thin- and thick-layer chromatog-raphy (Wu and Santelmann, 1975), soil column leach-ing (Weber and Peeper, 1982; White et al., 1986),and adsorption isotherms (Goetz et al., 1986; Bashamet al., 1987).

Batch equilibration methods have been used to de-termine pesticide distribution between the soil solidand liquid phases, but they may overestimate adsorp-tion in undisturbed soils since exposure to adsorptionsites may be limited during transport through the solum.Batch equilibration methods are also sensitive to thesolution-to-solid ratio (Murali and Aylmore, 1983) andthe solution matrix composition. Soil solution dis-placement has been used to measure herbicide ad-sorption (Goetz et al., 1986; Wolt et al., 1989), andmay be performed at soil water contents that moreclosely approximate those found in the field. Soil so-lution methods also allow flexibility in the moisturecontent of the soil at the time of pesticide application,which may affect pesticide mobility (White et al., 1986).

J.D. O'Dell, Dep. of Plant and Soil Science, Univ. of Tennessee,P.O. Box 1071, Knoxville, TN 37901; J.D. Wolt, DowElanco,9550 North Zionsville Rd., Indianapolis, IN 46268; and P.M.Jardine, Oak Ridge National Lab., P.O. Box 2008, Oak Ridge,TN 37831. Received 8 May 1991. * Corresponding author.

Published in Soil Sci. Soc. Am. J. 56:1711-1715 (1992).

Columns of sieved, hand-packed soil have the ad-vantage of being easy to obtain, but the relativelyuniform pore-size distribution and lack of structure inthese soils may lead to leaching patterns that differgreatly from those that occur in the field. Differencesbetween solute BTCs in disturbed and undisturbed soilcolumns have been well documented (Elrick and French,1966; Cassel et al., 1974; McMahon and Thomas,1974), with BTCs from undisturbed columns typicallycharacterized by a more rapid appearance of the solutein the leachate. The effects of preferential flow havebeen shown to be greater in structured soils (Tyler andThomas, 1981) and in soils having vertical wormchannels (Bouma and Wosten, 1979).

Imazethapyr is an imidazolinone herbicide devel-oped for the control of broadleaf weeds in soybean[Glycine max (L.) Merr.]. It has a water solubility of1400 mg L"1 and a dissociation constant (pKa) of 3.9,which suggests it is potentially mobile in soil. Studieswith disturbed soil columns, however, have indicatedlimited mobility (Flint et al., 1989). The objectivesof this study were to determine the rate of disappear-ance of imazethapyr from soil solution, and to deter-mine the ability of the CD equation to predict thedegree of mobility of imazethapyr in undisturbed soilcolumns based on soil solution imazethapyr decaycurves.

MATERIALS AND METHODSSoil Solution Study

Bulk soil was collected from the Ap horizon of a clayey,kaolinitic, thermic Typic Hapludult and passed through a2-mm screen. Imazethapyr (99.4% purity) was applied tothe soil as an aqueous solution at rates of 0, 31, 62, and124 /ig kg-1. Six sets (to allow for six sampling times) ofquadruplicate samples of the treated soil were packed into50-mL polypropylene syringes to a bulk density of 1.2 gcm-3, and brought to a water content of 0.35 kg kg-1.Samples were incubated at 298 K for 0.15, 1, 2, 4, 8, and16 d after treatment, and soil solution obtained by displace-ment with saturated CaSO4 (Wolt and Graveel, 1986) usinga procedure similar to that employed in previous investi-gations with imazaquin[2-(4,5-dihydro-Y-methyl-Y-(l-methylethyl)-5-oxo-lH-imidazol-2-yl)-3-quinoline-carbox-ylic acid] (Wolt et al., 1989). The displacement solutionalso contained FeCl3 so that contamination of the soil so-lution by the displacement solution could be ruled out byadding a drop of KCNS and verifying that there was nocolor change. The solution was immediately analyzed todetermine pH and electrical conductivity, and the remainingsolution (~5 mL) was stored in polypropylene vials at 262K until analyzed for imazethapyr concentration.

Apparent distribution coefficients (Kd) were calculatedby plotting the concentration of imazethapyr in soil solutionagainst the concentration on the solid phase for each in-cubation time. Cantwell et al. (1989) found that only 35%of applied imazethapyr was microbially degraded in a sim-ilar soil after a 12-wk incubation, so it was assumed thatimazethapyr was not appreciably degraded via microbialactivity during the 16-d sampling period. Therefore the solid-

Abbreviations: CD, convective-dispersive; BTC, breakthroughcurve; UV, ultraviolet; PVC, polyvinyl chloride; DI, deionized.

1711

1712 SOIL SCI. SOC. AM. J., VOL. 56, NOVEMBER-DECEMBER 1992

phase concentration was calculated as the difference be-tween total applied imazethapyr and measured soil solutionimazethapyr. No measurable interaction of imazethapyr withthe labware occurred during the experiments.

Solution imazethapyr concentrations were determined byhigh-performance liquid chromatography. The mobile phaseconsisted of 65:35 (v/v) 4% aqueous acetic acid/acetonitrileat a flow rate of 1 mL min-1, and the stationary phaseconsisted of a 25 cm by 4.6 mm C-8-DB column (Supelco,Bellefonte, PA) with a 5-cm guard column. Imazethapyrwas detected by UV absorbance at 254 nm, and a 200 /uLinjection yielded a detection limit of 5 /iig L~'.

Column Leaching StudyTwo undisturbed soil columns 30 cm in diameter and 68

cm in length were collected in PVC pipe with a tractor-mounted sampler (Walker et al., 1990). Plant material wasremoved from the surface, and the top 5 cm of soil passedthrough a 2-mm screen and replaced on the surface of thecolumn. The columns were initially saturated with DI H2Oand a pressure of - 2 kPa established on the bottom of onecolumn and —0.5 kPa on the other. Each column had abulk density of 1.44 Mg m~3, with the -0.5 and -2 kPacolumns having a volumetric water content of 0.42 and0.41, respectively. The pore volume was calculated as theproduct of the column volume and the volumetric watercontent. Deionized water was uniformly supplied to thecolumn inlet at approximately 3 cm d-1 using a peristalticpump. After a 5-d equilibration period, a 25-mL pulse of100 mM Br- (as KBr) and 0.518 mM imazethapyr (tech-nical material) in DI H2O was instantaneously applied toeach column by a pipette, and leaching with DI H2O wascontinued for a period of 23 d at 3 cm d-1. Leachate sam-ples were analyzed for Br~ using ion chromatography. Themobile phase consisted of 0.75 mM NaHCO3-2 mM Na2CO3at a flow rate of 2 mL min-1, and the stationary phaseconsisted of a 25 cm by 4 mm i.d. AS4A column with a5-cm guard column. Bromide was detected by chemicallysuppressed conductivity. Imazethapyr was determined byhigh-performance liquid chromatography as described above.

THEORYImazethapyr transport was described using a two-site,

nonequilibrium, CD transport model. The model assumesthat adsorption sites may be divided into two fractions whereType-1 sites are governed by instantaneous equilibrium ad-sorption, and adsorption on Type-2 sites is controlled bynonequilibrium first-order kinetics. The governing equa-tions that describe this model are as follows (Parker andvan Genuchten, 1984; van Genuchten and Wagenet, 1989):

(1 + F^JG) (dc/dt) + pje(dS2ldt)

= D(d2c/dx2) - v(dc/dx) [1]

(dsjdt) = a[(l - F)KAc - s2] [2]

where F is the fraction of Type-1 sites, pb is the bulk density(M L-3), Kd is the distribution coefficient (L3 M-1), 0 isthe volumetric water content (L3 L~3), c is solute concen-tration in solution (M L~3), t is time (T), s is adsorbedsolute concentration per mass of solid phase(M M"1), D isthe dispersion coefficient (L2 T-1), x is distance (L), anda is a first-order rate coefficient (T-1). Degradation coef-ficients for imazethapyr transport were assumed to be zero.

The time-dependent sorption of imazethapyr with the soilwas modeled using the two simultaneous general-order re-action expressions described by Jardine and Zelazny (1987):

C - C =^o w(1

(» -1) +1^1m-\

[3]- 1) +

where C0 is the initial solution concentration of imazetha-pyr, C, is the solution concentration remaining at time t, nand m are reaction orders, A and B are the amounts ofimazethapyr removed from solution and sorbed onto Type-1 and Type-2 sites, respectively, and a, and «2 are sorptionrate coefficients for imazethapyr interactions with Type-1and Type-2 sites, respectively. Two simultaneous first-or-der reactions were found to best describe the data, thus nand m were fixed at 1.01 (Jardine and Zelazny, 1987) andC0 — C, was regressed as a function of ( using nonlinearleast-squares inversion to determine als a2, A, and B. Thefraction of Type-1 sites was then calculated as F = A/(A+ B). If only first-order reactions are considered, Eq. [7]of Jardine and Zelazny (1986) may be used, providing iden-tical results to Eq. [3] when n = m = 1.01. The use ofEq. [3], however, allows other reaction orders to be inves-tigated.

Bromide ETC were analyzed using the classical CDequation of the computer program CXTFIT (Parker and vanGenuchten, 1984) to fit the pulse duration and dispersioncoefficient. These parameters were then used with the ki-netic parameters determined from Eq. [3] to predict ima-zethapyr ETC.

RESULTS AND DISCUSSIONSoil Solution Study

The removal of imazethapyr from soil solution wasindicative of a biphasic, time-dependent reaction (Fig.1). The observed data were modeled as two simulta-neous first-order reactions with optimization of pa-rameters «i, a2, and F (Table 1). The initial reactionwas rapid and essentially instantaneous, while the sec-

400

3162ngkg-1

A 124ngkg-1— Model lilted

Time (d)Fig. 1. Soil solution imazethapyr concentration as a function

of time at three application rates.

O'DELL ET AL.: TRANSPORT OF IMAZETHAPYR 1713

Table 1. Kinetic and physical parameters! determined fromsoil solution imazethapyr decay curves. Values in parenthesesare 95% confidence limits.

Imazethapyrrate

Mgkg'13162

124

«i

1.35 (0.46)2.88 (1.25)27.4 (92.9)

«2

- d-1

0.079 (0.029)0.107 (0.014)0.074 (0.014)

F

0.56 (0.08)0.31 (0.04)0.29 (0.04)

Table 3. Distribution coefficients (Ka) determined from soilsolution displacement at different equilibration times.

Time K. /F

t a, is the rate constant for Type-1 sites; «2 is the rate constant for Type-2 sites; and Fis the fraction of Type-1 sites.

Table 2. Mean percentage of imazethapyr in soil solution aftersix different equilibration times relative to total amountapplied to soil.

Imazethapyrrate

Mg kg"1

3162

124

Imazethapyr in soil solutionO.lSd

967359

Id

465555

2d

364653

4d

253745

8d

192431

16d

111120

ondary reaction was much slower and independent ofthe initial imazethapyr concentration. The consistencyof a2 for the different imazethapyr application ratesfurther suggests that the removal of this pesticide fromsolution follows a first-order reaction. Parameter Fincreased with decreasing initial concentration of im-azethapyr, which suggests a larger percentage of rap-idly reacting imazethapyr with lower application rates.If one assumes that the removal of imazethapyr fromsolution is primarily due to sorption on the soil, in-creasing F values with decreasing pesticide applica-tion rates is probably related to the extent of sorptionsite saturation. At low initial concentrations, the de-gree of sorption site saturation is low, with most im-azethapyr interacting with the rapidly reacting, moreaccessible sites (i.e., sites governed by ar). Highermodel-fitted F values at lower initial concentrationsreflect this condition. Nevertheless, values of at, a2,

CO

50 100 150 200 250 300

Solution (ug L-1)D 0.15d V 1d O 4d O 8d A 16d

Fig. 2. Imazethapyr adsorption isotherms at five equilibrationtimes.

0.151248

16

Lkg-1

0.1280.2050.2400.3430.6551.25

0.780.970.940.900.960.93

and F at the various imazethapyr application rates canbe used to parameterize transport models used to as-sess imazethapyr mobility in undisturbed soil columns(see section on column leaching studies).

The percentage of applied imazethapyr in soil so-lution ranged from 96 to 11% during the 16-d sam-pling period (Table 2), with the solution concentrationsdeclining exponentially as a function of time (Fig. 1).Using the same soil, Wolt et al. (1989) observed alinear decline of imazaquin in solution, ranging from80 to 96% at the time of application to 57 to 60%after 4 d. The apparently greater adsorptivity of im-azethapyr than imazaquin is interesting in that ima-zethapyr has a much greater water solubility. Usingbatch equilibration methods, Renner et al. (1988) andStougaard et al. (1990) also showed imazethapyr tobe more strongly adsorbed than imazaquin. This sug-gests that the type and arrangement of functional groupsplays a significant role in the distribution of theseherbicides. This is consistent with the results of An-derson et al. (1968), who showed varying mobilityamong three herbicides that had the same water sol-ubility and only slight structural differences.

Apparent distribution coefficients (Kd) for imazeth-apyr sorption on soil may be calculated at differentreaction times as equilibrium is approached. Becausethese Kd values are time-dependent, they are desig-nated as apparent values. The plots of solution- andsolid-phase imazethapyr concentrations were linear (Fig.2), suggesting a unique Kd value existed at each in-cubation time. Values of Kd consistently increased from0.128 to 1.25 L kg-1 during the 16-d sampling period(Table 3), suggesting a slow approach to equilibriumbetween solution- and solid-phase distribution of im-azethapyr. The poor fit at the 0.15-d sampling timewas probably due to the packing and displacement ofthe soil solution immediately after treatment, causingmore heterogeneity within the samples. The nonequi-librium conditions that exist between solution- andsolid-phase imazethapyr are related to the realistic watercontents used with the vacuum displacement tech-nique. Traditional batch techniques that use high so-lution-to-solid ratios and agitation to obtain equilibriummay not obtain the essential kinetic information nec-essary to model solute mobility in unsaturated, undis-turbed soils.

The more rapid disappearance of imazethapyr fromsolution during the first 4 d of the sampling periodmay be due to adsorption onto sites surrounding thelarger pores in the soil, with the slow declines in so-lution concentration at longer times the result of dif-fusion of the herbicide into water held in smaller poreswhere it encounters additional sorptive sites. Although

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1714 SOIL SCI. SOC. AM. J., VOL. 56, NOVEMBER-DECEMBER 1992

0.004• Observed bromide

— Fitted bromideo Observed imazethapyrh . -0.5 kPa

• Observed bromide— Fitted bromideo Observed imazethapyrh = -2 kPa

0.0025

0.0020

0.5 1 1.5

Pore volumeFig. 3. Bromide and imazethapyr breakthrough curves (BTCs)

from two undisturbed soil columns; h = potential.

we assume imazethapyr sorption to the soil, however,it is difficult to ascertain that complete adsorption hastaken place after diffusion occurs. It is possible thatthe herbicide is still in solution that is held at energylevels too great to be recovered by the displacementprocedure (i.e., bypassing of the displacement solu-tion). Although imazethapyr that is in solution closeto the soil surfaces may be available for uptake byplant roots, it is unlikely that it would be susceptibleto leaching unless diffusion back into the mobile por-tion of soil water occurs.

Column Leaching StudyBreakthrough curves of Br~ and imazethapyr from

undisturbed soil columns at — 0.5 and — 2.0 kPa wereasymmetrical and displaced to the left of one porevolume (Fig. 3). Displacement of one pore volume ofeffluent requires =30 h with one pore volume equiv-alent to -20 L. Both chemicals were detected in thefirst 100 mL of leachate collected from the - 2 kPacolumn, while 300 mL of leachate had been collectedfrom the -0.5 kPa column before the solutes weredetected. Bromide and imazethapyr BTC were similarat short times, followed by retardation of the ima-zethapyr BTC as the chemical was removed from thesoil solution (Fig. 3). Since a reduced concentration

Table 4. Percentage of applied Br~ and imazethapyr recoveredin column effluent.

Soil waterpressure

kPa-0.5

-2.0

Porevolume

0.51.02.00.51.02.0

Br-

648093567283

Imazethapyr

435462385364

O

0.5 1 1.5 2Pore volume

Fig. 4. Model simulations of imazethapyr breakthrough curves(BTCs) using parameters determined from Br~ BTC andimazethapyr decay curves; h = potential.

scale is used (C/C0), Br~ and imazethapyr BTC aredirectly comparable, with lower concentrations of thelatter solute reflecting retardation.

The retention of imazethapyr in the soil columnswas apparently limited by preferential transport of waterand solutes through the profile. Preferential flow issuggested by the rapid appearance of the solutes inthe leachate followed by extended tailing to long times.The asymmetric characteristics of the BTCs suggest avast distribution of pore class sizes in this soil, evenunder conditions of unsaturated flow. The occurrenceof preferential flow is also supported by the physicalcharacteristics of the soil, such as moderate suban-gular blocky structure and vertical worm channels thatare continuous through the profile. These preferentialflow paths limit the interaction of the herbicide withsoil particles, thus decreasing the probability of reten-tion. These results demonstrate the importance ofmaintaining soil physical properties when evaluatingthe potential mobility of reactive solutes.

Observed Br~ effluent concentrations were well de-scribed (R2 = 0.93 and 0.97 for -0.5 and -2 kPacolumns, respectively) by the classical CD equationby fitting the dispersion coefficient (£>) and pulse du-ration (T) (Fig. 3). The mobile-immobile CD modelmay describe the observed Br~ data better; however,its use is not justified in this case since the fractionof mobile water and the diffusive mass transfer coef-ficient are not known.

Values of D for Br~ transport at soil water pressuresof -0.5 and -2.0 kPa were 27.9 and 26.6 cm2 h-1,respectively, which represent Peclet numbers (P) of0.67 for each column. The dispersion observed wasprobably due mainly to unequal pore water velocitieswithin the column, which resulted from the varyingpore sizes in the structured soil. In the presence oflarger pores, flow through micropores may be negli-gible. Since only an additional 11 to 13% of the ap-

O'DELL ET AL.: TRANSPORT OF IMAZETHAPYR 1715

plied Br~ had been recovered with the elution of thesecond pore volume of leachate (Table 4), it appearsthat relatively little transport of the solutes was due todiffuse flow in small soil pores.

Imazethapyr transport was modeled well using atwo-site, nonequilibrium CD model that assumes thatsorption can be partitioned onto two different types ofsites. Adsorption on Type-1 sites is assumed instan-taneous and reversible, while adsorption on Type-2sites is controlled by reversible first-order kinetics.The model was parameterized using (i) batch al5 a2»and F values (Table 1), with a^ assumed to be in-stantaneous, (ii) batch Kd values at various equil-ibration times, and (iii) model-fitted D and T valuesobtained from the observed Br~ effluent concentra-tions of each column. Model-predicted curves weregenerated by simultaneously solving Eq. [1] and [2](Parker and van Genuchten, 1984) and compared withthe observed data (Fig. 4). Transport model simula-tions were run using ccj at the model-determined valueand at increasing values to verify the assumption thatthe reaction could be approximated by an instanta-neous reaction rate. Increasing the parameter a^ didnot significantly change the predicted imazethapyrbreakthrough, therefore ax values could be approxi-mated by instantaneous reaction rates.

The best description of imazethapyr breakthroughwas obtained using the kinetic and physical parame-ters determined from the soil solution decay curves(Table 1) and the Kd value at the 1-d displacement(Fig. 1). Model simulations using the Kd value cal-culated from the 0.15-d displacement overpredictedthe peak imazethapyr concentration, while values cal-culated from later soil solution displacements pre-dicted increasingly delayed imazethapyr breakthroughwith lower peak concentrations than observed (Fig.4). Preferential flow paths apparently limited the in-teraction of the solute with the soil, with nonequili-brium conditions prevailing during transport.

CONCLUSIONPreferential flow paths are an important mechanism

for the transport of reactive tracers in this structuredsoil. Because the solute may come into contact withrelatively little of the soil as it is transported, conven-tional methods of measuring equilibrium adsorptionmay underestimate transport of reactive solutes throughstructured soils. When adsorption is not completelyinstantaneous, adsorption kinetics may be useful inpredicting the potential mobility of reactive solutes.

The extensive heterogeneity of undisturbed soil col-umns, however, makes it difficult to determine correctreaction times to use when determining transport pa-rameters.