Facilitated Transport of Napropamide by Dissolved Organic MatterThrough Soil Columns

C. F. Williams,* M. Agassi, J. Letey, W. J. Farmer, S. D. Nelson, and M. Ben-Hur

ABSTRACTContamination of groundwater by pesticide percolation is of great

concern. Field observations have revealed that some pesticides movedeeper into the soil profile than would be expected from predictionsmade by solute transport models. The discrepancies have been attrib-uted to preferential flow of water carrying pesticides via macropores infield soils. The same phenomenon may also be explained by transportfacilitated by a carrier such as dissolved organic matter (DOM). Alaboratory study was conducted to evaluate the transport of napro-pamide [2-<a-naphthoxy-A'./V-diethylpropionaniide] through soil viaDOM. Soils were sieved, packed in columns, and treated at the surfacewith 14C-labeled napropamide. Water was applied to the columns byflooding and leachate was collected. It was found that "C-labelednapropamide was present in the first 0.22 cm of leachate. The "C andDOM concentrations were highest in the initial leachate and decreasedwith increasing leachate. Napropamide concentration fell below detec-tion at some depth in all columns and recovery in the soil averaged95% of the applied napropamide. Gas chromatographic analyses veri-fied that "C activity in the leachate was associated with napropamide.A dialysis equilibrium technique determined that 17 to 56% of thenapropamide in the leachate was retained inside a 500-Da dialysismembrane. The rapid leaching of a small fraction of napropamidewas not a result of preferential flow in our experiments but is due toDOM-facilitated transport. Thus, under field conditions rapid pesti-cide leaching could be the combined effects of preferential flow andfacilitated transport.

MODELS to describe pesticide transport through thevadose zone generally assume that the pesticide

is transported by flowing water with appropriate incor-poration of factors to account for solute diffusion, dis-persion, and adsorption characteristics. Field observa-tions have revealed that some surface-applied pesticidesmove deeper into the soil profile than would be expectedfrom predictions of these transport models.

Jury et al. (1986) found that about 20% of the totalmass of the napropamide and prometryn [2,4-bis (iso-propylamino)-6-(methylthio)-5-triazine] applied to thesoil surface moved beyond the depth where existingchemical transport models predicted they would reachby mass flow and adsorption reactions. Ghodrati andJury (1992) found that under both conservation and no-till management an average of 18.8, 9.4, and 10.4% ofthe recovered atrazine [2-chloro-4-(ethylamino)-6-(iso-propylamino)-5-triazine], prometryn, and napropamide,respectively, were found in the 30- to 150-cm depth layerwhile they were expected to be retained in the top 20 cm.Kladivko et al. (1991), investigating six pesticides in atile-drained soil, found that 0.1 to 1% of the applied

C.F. Williams, J. Letey, WJ. Farmer, and S.D. Nelson, Dep. of Soilsand Environ. Sci., Univ. of California, Riverside, CA 92521; and M,Agassi, Soil Erosion Res. Stn., Rupin Inst. Post, 60960, Israel. Re-ceived 8 June 1998. *Corresponding author (williamc@cc.usu.edu).

Published in Soil Sci. Soc. Am. J. 64:590-594 (2000).

pesticides were found in the subsurface drainage flowafter <20 mm subsurface drainflow; however, accordingto model prediction it should have taken -300 mm ofdrainage for the pesticides to appear. Grochulska andKladivko (1994) measured the leaching of bromide andtwo pesticides through intact soil cores in the laboratoryand observed chemical concentrations in the effluentthat peaked shortly after the effluent drained from thecolumns. They attributed the phenomenon to transportof pesticide-laden water through the fast flow zone(preferential flow). Undisturbed soil blocks (30 X 30 X30 cm) were collected in the field by Shipitalo et al.(1990) and brought to the laboratory for pesticide trans-port studies. They simulated different rainfall eventsand found that the greatest pesticide leaching occurredwhen the first rain event created leaching. Low initialrain events reduced pesticide transport by subsequenthigh rainfall events. They postulated that initial rainevents distributed pesticide within the soil microstruc-ture, which protected it from leaching by subsequentrain events.

The results of the above-described studies were attrib-uted to water flow transporting solutes via preferentialpathways that exhibit increased flow velocities com-pared to the average bulk soil. It is evident from thepreceding studies that the presence of preferential flowchannels results in pesticide transport to deeper levelsthan present theory predicts. Utermann et al. (1990)speculated that the rapid flow of relatively smallamounts of pesticides may be more perilous to ground-water than the slower-moving major mass. They as-sumed that the travel time of the peak concentrationmay be long enough to allow nearly complete degrada-tion before reaching groundwater.

Even if preferential flow is considered, theory basedstrictly on water flow and adsorption reactions will stillnot accurately predict the distribution of pesticides inthe soil profile should the pesticide be transported ona carrier to which the pesticide is adsorbed. Fine mineralor organic particles suspended in and transported bythe flowing water and/or dissolved organic matter couldserve as carriers.

Colloids have been shown to enhance the transportof pesticides through soil. Vinten et al. (1983) observedthat paraquat and DDT (dichlorodiphenyltrichloro-ethane) would migrate in soil columns if they were ad-sorbed on suspended Li-montmorillonite. Seta and Kar-athanasis (1997a, 1997b) reported that water-dispersiblecolloids fractionated from soil samples with diversephysicochemical characteristics were capable of increas-ing the transport of atrazine through intact soil columns.

Abbreviations: DOM, dissolved organic matter; GC, gas chroma-tography.

590

WILLIAMS ET AL.: TRANSPORT OF NAPROPAMIDE BY DISSOLVED ORGANIC MATTER 591

The presence of DOM has been shown to enhancethe aqueous solubility of organic pollutants in a mannerthat could influence their transport through soils. Leeand Farmer (1989) using a dialysis technique found that14C-labeled napropamide formed a complex capable ofovercoming the diffusion gradient across a membrane.The membrane chosen had a molecular weight cutoffof 1000 Da allowing free napropamide to pass throughthe membrane but preventing any DOM or complexthat had a molecular weight >1000 Da from passingthrough. An amount of 9% of the napropamide wascomplexed by soil-derived humic acid inside the mem-brane. Liu et al. (1996) also used the dialysis techniqueto show that napropamide formed a stable complexwith soil-derived humic acid. They also used gel iso-electrofocusing to show that the DOM-napropamidecomplex was formed with two distinctly different humicacids. Napropamide has also been shown to complexwith soluble humic acid according to a Langmuir-typeisotherm (Clapp et al., 1997)

The solubility and therefore the mobility of DDT(Caron et al., 1985) and of polychlorinated biphenyl's(Gschwend and Wu, 1985) increased with an increasein DOM content in water sediment systems. The mobil-ity of DDT (Ballard, 1971) and of toxaphene (Smithand Willis, 1985) in soil was enhanced by the additionof urea and anhydrous ammonia which raised soil pH,thereby solublizing soil organic matter. Recent effortshave been made to include the effects of macromole-cules on the modeling of the transport of organic pollut-ants in soils (Enfield et al., 1989). Lee and Farmer (1989)demonstrated that for napropamide the association withDOM was not completely reversible, thus increasing thepossibility of DOM-enhancing napropamide mobility insoil. Graber et al. (1995) reported the enhanced trans-port of atrazine under irrigation with secondary treatedsewage effluent when compared to irrigation with regu-lar water. The results were explained on the basis ofdissolved organic C in the sewage effluent. Nelson etal. (1998) found that a small fraction of napropamideapplied to sewage sludge-amended soil was transportedthrough repacked soil columns in a manner consistentwith DOM- facilitated transport.

Based on current knowledge, rapid and deep flowof pesticides in the vadose zone can be attributed topreferential flow of water via macropores. The potentialfor facilitated transport of pesticides associated withDOM has also been reported. The objective of this studywas to evaluate the transport of a pesticide through soilvia DOM-facilitated transport under controlled labora-tory conditions.

MATERIALS AND METHODSNapropamide was selected as a model compound for the

present investigations. It represents a group of polar nonionicherbicides and has an aqueous solubility of 74 mg L"1; vaporpressure of 1.7 X 10~7 mm Hg; Koc equal to 700 L kg"1; anddegradation half-life of 70 d (Wauchope et al., 1992). It isa selective herbicide used for controlling several grass andbroadleaf weeds in various crops.

Analytical grade chemical was obtained from Chem Service,

Inc., West Chester, PA. Carbon-14-labeled napropamide wassupplied by Stauffer Chemical (now Zeneca Agrochemicals)with a radiochemical (14C-a-napthoxy) purity of 99%. Twodimensional TLC was used to confirm that 97% of the impuri-ties was the beta-congener of napropamide, meaning that<0.03% of the 14C activity could be attributed to water solublecomponents (personal communication, 1989, Stauffer Chemi-cal, Richmond, CA). Napropamide was dissolved in hexane(resulting in a napropamide concentration of 600 mg L"1) tofurther limit the water-soluble 14C from the solution appliedto the soil surface.

Experiments were conducted in the laboratory on sievedsoils packed to a depth of 150 mm in acrylic plastic columns76 mm in diameter. A polypropylene funnel 67 mm in diameterwas placed at the bottom of the column such that a very smallspace was maintained between the edge of the funnel and theinside wall of the column. In initial experiments this gap wasto allow for collection of water that might preferentially flowdown the column wall. No water was ever collected from thecolumn walls so in later experiments this gap was filled withsilicone to ease packing of the columns. A perforated plasticplate was placed 3 mm below the funnel rim with a 0.5-mmmesh fiberglass screen placed over the perforated plate tohelp support the soil. The leachate was collected through thefunnel stem.

Three soils used were Hanford sandy loam (coarse-loamy,mixed, superactive, nonacid, thermic Typic Zerorthents),Domino sandy clay loam (fine-loamy, mixed, thermic ZerollicPaleorthids), and Tujunga loamy sand (mixed, thermic TypicZeropsamments) (see Table 1 for some properties of thesesoils). Each soil was collected from 0- to 15-cm layer, air driedand sieved through a 1-mm screen. The sieved soils werepacked into columns by consistently tapping the top of thecolumn while the soil was slowly poured in. Columns werepacked such that the resulting bulk densities were 1.5 Mg m~3

for Hanford, 1.4 Mg m~3 for Domino, and 1.3 Mg m~3 forTujunga soil.

A total of 3.6 mg of napropamide was applied in 6 mL ofhexane to each column. The solution was dripped on the soilsurface of each column to create an equivalent napropamideapplication rate of 8 kg ha"1. The maximum depth of thenapropamide-hexane solution penetration into the soil was3 mm. Hexane was allowed to evaporate in a hood for 48 h.Preliminary investigations based on weight differences foundthat for all soils <1.0 ^g of hexane remained after evaporationfor 18 h. Each treatment was replicated three times.

Steps were taken to ensure that particulates were not in-volved in transport of napropamide. Sodium chloride andCaCl2 were added to deionized water to create a solutionhaving an electrical conductivity of 1 dS m"1 and a sodiumadsorption ratio of 2. The saline water was to promote floccula-tion rather than dispersion of particulates. This water wasadded to the top of the column and a constant head of 3.5 cmwas maintained. Leachate samples were collected for every15 mL of cumulative leachate. Leachate samples were ana-lyzed for 14C activity and dissolved organic C. Napropamideconcentrations in the leachate were determined by liquid scin-tillation. The leachates were shaken and a 1-mL sample wasplaced in Liquicent liquid scintillation cocktail (National Diag-nostics, Atlanta, GA) and the 14C activity measured in a Beck-man LS5000 TD liquid scintillation counter (Beckman Scien-tific, Fullerton, CA).

The leachate was analyzed for total particulates and sorp-tion of napropamide to particulates. Six milliliters of effluentsolution were placed in a centrifuge tube and shaken for 5 min.Immediately after shaking 3 mL of solution were removed ofwhich 1 mL was used for 14C activity by liquid scintillation

592 SOIL SCI. SOC. AM. J., VOL. 64, MARCH-APRIL 2000

Table 1. Some characteristics of the soil studied.Soil type series

Hanford sandy loamDomino sandy clay loamTujunga loamy sand

Classification

Typic XerothentXerollic CalciorthidTypic Xeropsamment

Sand

67.150.582.4

Silt

25.824.313.9

Clay

7.125.23.7

CECt

cmolc kg"1 soil4.5

15.42.3

Organic carbon^

%0.430.520.71

PH

6.18.07.9

f Sodium saturation procedure used (Bower et al., 1952); CEC is cation-exchange capacity.I Walkley-Black procedure used (Nelson and Sommers, 1982).

analysis and the remaining 2 mL of solution was used forparticulate analysis by optical transmittance. The remaining3 mL of solution were then centrifuged for 20 min at 2000 X g.After centrifuging a 1-mL sample was removed for liquidscintillation and the remaining solution was used for opticaltransmittance. Particulate analysis was performed using opti-cal transmittance at 410 nm in a Beckman Spec-20 spectropho-tometer (Beckman Scientific, Fullerton, CA). Solution wasplaced in a glass spectrophotometer tube, capped, shaken for30 s, and the transmittance was measured. The presence ofparticulates would have resulted in an increase in transmit-tance in the centrifuged samples. Also any napropamide asso-ciated with colloids would have resulted in lower 14C activityin the supernatant from the centrifuged samples.

Organic C was measured in a 20-|j,L sample by ultravioletpromoted persulfate oxidation followed by infrared detectionusing a Dohrmann DC-80 organic C analyzer (Xertex, SantaClara, CA). Inorganic C was removed prior to analysis usingN2 gas for external sparging and the C contributed by napro-pamide was subtracted. The term DOM is used in this reportsince it is the whole organic molecule that participates in theinteraction and not just the C.

At the end of the experiment, the soils were allowed todrain for 3 d after which each column was divided into 1.2-cmsections, homogenized, and analyzed for 14C activity. Napro-pamide was extracted from the soil by placing 0.5 g of soil in19 mL of scintillation cocktail and shaking on a reciprocatingshaker for 6 h. Samples were left over night to allow settlingof the soil particulates in solution. Samples were correctedfor water content by taking three samples from each depthand drying in an oven at 105°C for 24 h.

Napropamide concentrations as determined by 14C activitywere confirmed using gas chromatography (GC). Napropam-ide concentrations in the leachate were very low and coupledwith small sample volumes (15 mL) of effluent it was impossi-ble to sufficiently concentrate the napropamide for GC analy-sis. Therefore, additional columns were treated as above ex-

1000

800

600

o•oI-s -l!<S 400

200

—e—Domino—a -Hanford— A- -Tujunga

0.0 2.0 2.50.50 1.0 1.5Cumulative Leachate

(cm)Fig. 1. Concentration of napropamide, based on 14C activity, in the

initial leachate from Domino, Hanford, and Tujunga soils. Errorbars are ± 1 standard error of the mean.

cept the effluent was collected in 45 mL increments andanalyzed for napropamide by both GC and liquid scintillation.Napropamide was extracted from 30 mL of effluent using threesequential 1:1 hexane/water extraction procedures followed byroto-evaporating to dryness and redissolving in 3 mL of hex-ane. Napropamide was analyzed using a Hewlett-Packard 5890GC (Hewlett-Packard Chemical Analysis Group, Palo Alto,CA) equipped with a N-P detector, auto-injector and on-lineintegrator. The dB wax column was operated at 210°C witha carrier helium flow of 7 mL min."1 and a detector tempera-ture of 270°C.

The presence of a napropamide-DOM complex was deter-mined on 27 effluent samples using a modified form of theLee and Farmer (1989) dialysis equilibrium technique. Thefirst, third, and last 15-mL sample from each column, wereselected for analysis. Three milliliters of effluent were placedinside dialysis tubing with a molecular weight cutoff of 500Da. Dialysis tubing containing effluent samples were thenplaced in 50-mL Teflon centrifuge tubes and bathed in 30 mLof napropamide free water. Tubes were shaken and the outsidesolution was analyzed for 14C activity and replaced with freshdeionized water every 2 h. After 8 h no 14C activity was mea-sured in the outside solution and at that time samples frominside were analyzed for 14C activity. As a control napropamidein DOM free water was placed inside the dialysis tubing andin each case the inside and outside solutions were at equilib-rium after 2 h.

RESULTSThe concentration of napropamide in the leachate

from each soil is plotted as a function of cumulativeleachate in Fig. 1. All three soils exhibited initially highnapropamide concentration that decreased with cumu-lative leachate. The amount of napropamide leachedthrough all three soils ranged from 1.5 to 1.8% of thetotal applied within the first 2.4 cm of cumulative lea-chate (which represents a volume of <60% of one voidvolume). Figure 2 shows the relationship between 14Cactivity and napropamide concentration as determinedby GC. Note that the regression line between concentra-tion measured by 14C activity and GC has a slope of0.82. The deviation of the slope from 1 is such that14C activity under estimates the concentration measuredby GC.

No particulates were measured in the leachate. Theoptical transmittance of each sample before and aftercentrifuging was identical. In addition liquid scintillationbefore and after centrifuging showed no reduction innapropamide concentration, therefore, the napropam-ide was truly dissolved and not in suspension.

A trend similar to that of napropamide occurred withDOM (Fig. 3) with high initial DOM concentrationswhich decreased with increasing effluent volume. Ex-

WILLIAMS ET AL.: TRANSPORT OF NAPROPAMIDE BY DISSOLVED ORGANIC MATTER 593

100

;- 80

D)a.

O TJ6 0

a 40

2 0

—•—Data Fit— Predicted

y = 0.011407 + 0.8251 x R2= 0.98113

20 80 1004 0 6 0G C

( Napropamide ug L"1 )Fig. 2. Napropamide concentration determined by gas chrontatogra-

phy (GC) and MC activity. The predicted line is the line wherenapropamide measured by 14C activity is equal to GC.

cept for the initial leachate sample, the highest DOMconcentrations were in the Domino effluent. This maybe due to the higher clay content in the Domino soilretarding the movement of DOM. Nelson et al. (1990)found that in column-leaching experiments higher soilclay content was responsible for decreased DOM con-centrations in the effluent. They attributed the de-creased DOM concentrations to higher surface area re-sulting in increased sorption of DOM to soil minerals.

Dissolved organic matter concentrations ranged froma high of 745 to a low of 10 mg C L"1. The initialhigh DOM concentrations were greater than generallyreported in the literature. High values for DOM areexpected in our system because the soil was initially dryand then wet from the top, which would result in anaccumulation of highly soluble organic matter at or nearthe wetting front. Ghodrati (1989) under similar condi-tions also found that an initial DOM concentration of300 mg L"1 in the initial 1 cm of leachate from theTujunga soil which is similar to the DOM concentrationreported here.

The distribution of napropamide within the soil atthe end of the experiment is shown in Fig. 4. For all three

800

700

— 600

'-1 5005 oO € 400Q i?

C, 200

100

0

—e—Domino DOM—B -Hanford DOM—A- -Tujunga DOM

0.0 2.50.50 1.0 1.5 2.0Cumulative Leachate

(cm)Fig. 3. Concentration of dissolved organic matter (DOM) in the

initial leachate from Domino, Hanford, and Tujunga soils. Errorbars are ± 1 standard error of the mean.

Napropamide(mg kg 1)

o

a.<uQ

—e—Domino—B -Hanford— A- -Tujunga

Fig. 4. Distribution of napropamide in soil columns after terminationof the leaching experiment. Error bars are ± 1 standard error ofthe mean.

soils the napropamide concentration drops to belowdetection (=100 ng kg"1) at some point in the profile.Napropamide distribution is as expected on the basis ofclay content and organic matter with the napropamidebeing retained nearer the surface for the Domino sandyclay loam. Napropamide recovery in the soil averaged95% of the total applied, which gave an average massbalance of 97% for all of the treatments.

The relationships between napropamide concentra-tion and DOM concentration in the effluent are pre-sented in Fig. 5. There is an increase in napropamideconcentration with increasing DOM concentration whichtends to plateau at higher DOM concentrations. Thehighest napropamide and DOM concentrations wereassociated with the first collected effluent samples.

The equilibrium dialysis technique resulted in 17 to56% of the total activity in the effluent samples re-maining inside the dialysis tubing. This result providesevidence that some napropamide formed a stable com-plex with DOM that could contribute to facilitated na-propamide transport. It was also observed that in allcases the percentage of activity that remained inside thedialysis tubing increased for the effluents that cameout later.

1000

900

800- 700

(D•o1 -

£ g 6°°

jj — 500z

400

300200

DominoHanfordTujunga

0 100 200 300 400 500 600 700 800Dissolved Organic Matter

(mg carbon L"1)Fig. 5. Plot of napropamide concentration vs. dissolved organic mat-

ter in the leachate. Error bars are ± 1 standard error of the mean.

594 SOIL SCI. SOC. AM. J., VOL. 64, MARCH-APRIL 2000

CONCLUSIONSFor facilitated transport to occur the interaction be-

tween DOM and napropamide must be strong enoughto overcome the partitioning of napropamide to thesolid phase. Lee et al. (1990) found that DOM from theTujunga soil was capable of reducing the amount ofnapropamide adsorbed to soil minerals. Nelson et al.(1998) also found that the presence of soil-derived DOMcould reduce the sorption of napropamide to soil. Thismight explain how napropamide could leach to the bot-tom of the soil columns and be found in the effluent butnot found in soil at the bottom part of the soil column.

The results obtained in this study are not attributedto preferential flow. Ghodrati (1989) using the Tujungasoil was unable to find preferential flow in repackedsoil columns. In our study the columns were repackedin a manner similar to the Ghodrati study. The presenceof preferential flow is often verified by the use of anonreactive tracer, such as Br~. No Br~ tracer was usedbecause the soil was initially dry and Br~ placed at thesurface would appear in the initial leachate regardlessof preferential flow. If Br~ was introduced continuouslythen the concentration in the effluent would be a con-stant since there was no water initially in the columnto dilute the Br~.

Results obtained in this study were similar to thoseobtained by Nelson et al. (1998). In all cases the concen-tration of napropamide was high in the first drops ofeffluent and the concentration decreased with increas-ing cumulative effluent. Preferential flow was elimi-nated by repacking the columns and there was no prefer-ential flow at the soil column wall interface. Napro-pamide concentration in the soil was near zero in allcases at the bottom of each column and steps to removecolloidal particulates in the effluent were effective. Gaschromatography confirmed that the 14C activity mea-sured was due to napropamide and not radiolabeledcontaminants. Finally the detection of a napropamide-DOM complex by use of the equilibrium dialysis tech-nique provides strong evidence that napropamide wastransported through the soil columns complexed to solu-ble organic matter.

ACKNOWLEDGMENTSThis research was supported by the University of California

Kearney Foundation of Soil Science. Carbon-14-labeled na-propamide was provided by Stauffer Chemical (now Zen-eca Agrochemicals).