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Soil Science Society of America Journal 64:590-594 (2000)
© 2000 Soil Science Society of America

DIVISION S-2-SOIL CHEMISTRY

Facilitated Transport of Napropamide by Dissolved Organic Matter Through Soil Columns

C.F. Williamsa, M. Agassib, J. Leteya, W.J. Farmera, S.D. Nelsona and M. Ben-Hurb

a Dep. of Soils and Environ. Sci., Univ. of California, Riverside, CA 92521 USA
b Soil Erosion Res. Stn., Rupin Inst. Post, 60960, Israel

williamc{at}cc.usu.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Conclusions
 REFERENCES
 
Contamination of groundwater by pesticide percolation is of great concern. Field observations have revealed that some pesticides move deeper into the soil profile than would be expected from predictions made by solute transport models. The discrepancies have been attributed to preferential flow of water carrying pesticides via macropores in field soils. The same phenomenon may also be explained by transport facilitated by a carrier such as dissolved organic matter (DOM). A laboratory study was conducted to evaluate the transport of napropamide [2-(a-naphthoxy-N,N-diethylpropionamide] through soil via DOM. Soils were sieved, packed in columns, and treated at the surface with 14C-labeled napropamide. Water was applied to the columns by flooding and leachate was collected. It was found that 14C-labeled napropamide was present in the first 0.22 cm of leachate. The 14C and DOM concentrations were highest in the initial leachate and decreased with increasing leachate. Napropamide concentration fell below detection at some depth in all columns and recovery in the soil averaged 95% of the applied napropamide. Gas chromatographic analyses verified that 14C activity in the leachate was associated with napropamide. A dialysis equilibrium technique determined that 17 to 56% of the napropamide in the leachate was retained inside a 500-Da dialysis membrane. The rapid leaching of a small fraction of napropamide was not a result of preferential flow in our experiments but is due to DOM-facilitated transport. Thus, under field conditions rapid pesticide leaching could be the combined effects of preferential flow and facilitated transport.

Abbreviations: DOM, dissolved organic matter • GC, gas chromatography


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Conclusions
 REFERENCES
 
MODELS to describe pesticide transport through the vadose zone generally assume that the pesticide is transported by flowing water with appropriate incorporation of factors to account for solute diffusion, dispersion, and adsorption characteristics. Field observations have revealed that some surface-applied pesticides move deeper into the soil profile than would be expected from predictions of these transport models.

Jury et al. (1986) found that about 20% of the total mass of the napropamide and prometryn [2,4-bis (isopropylamino)-6-(methylthio)-s-triazine] applied to the soil surface moved beyond the depth where existing chemical transport models predicted they would reach by mass flow and adsorption reactions. Ghodrati and Jury (1992) found that under both conservation and no-till management an average of 18.8, 9.4, and 10.4% of the recovered atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], prometryn, and napropamide, respectively, were found in the 30- to 150-cm depth layer while they were expected to be retained in the top 20 cm. Kladivko et al. (1991), investigating six pesticides in a tile-drained soil, found that 0.1 to 1% of the applied pesticides were found in the subsurface drainage flow after <20 mm subsurface drainflow; however, according to model prediction it should have taken {approx}300 mm of drainage for the pesticides to appear. Grochulska and Kladivko (1994) measured the leaching of bromide and two pesticides through intact soil cores in the laboratory and observed chemical concentrations in the effluent that peaked shortly after the effluent drained from the columns. They attributed the phenomenon to transport of pesticide-laden water through the fast flow zone (preferential flow). Undisturbed soil blocks (30 x 30 x 30 cm) were collected in the field by Shipitalo et al. (1990) and brought to the laboratory for pesticide transport studies. They simulated different rainfall events and found that the greatest pesticide leaching occurred when the first rain event created leaching. Low initial rain events reduced pesticide transport by subsequent high rainfall events. They postulated that initial rain events distributed pesticide within the soil microstructure, which protected it from leaching by subsequent rain events.

The results of the above-described studies were attributed to water flow transporting solutes via preferential pathways that exhibit increased flow velocities compared to the average bulk soil. It is evident from the preceding studies that the presence of preferential flow channels results in pesticide transport to deeper levels than present theory predicts. Utermann et al. (1990) speculated that the rapid flow of relatively small amounts of pesticides may be more perilous to groundwater than the slower-moving major mass. They assumed that the travel time of the peak concentration may be long enough to allow nearly complete degradation before reaching groundwater.

Even if preferential flow is considered, theory based strictly on water flow and adsorption reactions will still not accurately predict the distribution of pesticides in the soil profile should the pesticide be transported on a carrier to which the pesticide is adsorbed. Fine mineral or organic particles suspended in and transported by the flowing water and/or dissolved organic matter could serve as carriers.

Colloids have been shown to enhance the transport of pesticides through soil. Vinten et al. (1983) observed that paraquat and DDT (dichlorodiphenyltrichloroethane) would migrate in soil columns if they were adsorbed on suspended Li–montmorillonite. Seta and Karathanasis (1997a, 1997b) reported that water-dispersible colloids fractionated from soil samples with diverse physicochemical characteristics were capable of increasing the transport of atrazine through intact soil columns.

The presence of DOM has been shown to enhance the aqueous solubility of organic pollutants in a manner that could influence their transport through soils. Lee and Farmer (1989) using a dialysis technique found that 14C-labeled napropamide formed a complex capable of overcoming the diffusion gradient across a membrane. The membrane chosen had a molecular weight cutoff of 1000 Da allowing free napropamide to pass through the membrane but preventing any DOM or complex that had a molecular weight >1000 Da from passing through. An amount of 9% of the napropamide was complexed by soil-derived humic acid inside the membrane. Liu et al. (1996) also used the dialysis technique to show that napropamide formed a stable complex with soil-derived humic acid. They also used gel iso-electrofocusing to show that the DOM–napropamide complex was formed with two distinctly different humic acids. Napropamide has also been shown to complex with soluble humic acid according to a Langmuir-type isotherm (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 increase in DOM content in water sediment systems. The mobility of DDT (Ballard, 1971) and of toxaphene (Smith and Willis, 1985) in soil was enhanced by the addition of urea and anhydrous ammonia which raised soil pH, thereby solublizing soil organic matter. Recent efforts have been made to include the effects of macromolecules on the modeling of the transport of organic pollutants in soils (Enfield et al., 1989). Lee and Farmer (1989) demonstrated that for napropamide the association with DOM was not completely reversible, thus increasing the possibility of DOM-enhancing napropamide mobility in soil. Graber et al. (1995) reported the enhanced transport of atrazine under irrigation with secondary treated sewage effluent when compared to irrigation with regular water. The results were explained on the basis of dissolved organic C in the sewage effluent. Nelson et al. (1998) found that a small fraction of napropamide applied to sewage sludge-amended soil was transported through repacked soil columns in a manner consistent with DOM- facilitated transport.

Based on current knowledge, rapid and deep flow of pesticides in the vadose zone can be attributed to preferential flow of water via macropores. The potential for facilitated transport of pesticides associated with DOM has also been reported. The objective of this study was to evaluate the transport of a pesticide through soil via DOM-facilitated transport under controlled laboratory conditions.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Conclusions
 REFERENCES
 
Napropamide was selected as a model compound for the present investigations. It represents a group of polar nonionic herbicides and has an aqueous solubility of 74 mg L-1; vapor pressure of 1.7 x 10-7 mm Hg; KOC equal to 700 L kg-1; and degradation half-life of 70 d (Wauchope et al., 1992). It is a selective herbicide used for controlling several grass and broadleaf weeds in various crops.

Analytical grade chemical was obtained from Chem Service, Inc., West Chester, PA. Carbon-14–labeled napropamide was supplied by Stauffer Chemical (now Zeneca Agrochemicals) with a radiochemical (14C-{alpha}-napthoxy) purity of 99%. Two dimensional TLC was used to confirm that 97% of the impurities was the beta-congener of napropamide, meaning that <0.03% of the 14C activity could be attributed to water soluble components (personal communication, 1989, Stauffer Chemical, Richmond, CA). Napropamide was dissolved in hexane (resulting in a napropamide concentration of 600 mg L-1) to further limit the water-soluble 14C from the solution applied to the soil surface.

Experiments were conducted in the laboratory on sieved soils packed to a depth of 150 mm in acrylic plastic columns 76 mm in diameter. A polypropylene funnel 67 mm in diameter was placed at the bottom of the column such that a very small space was maintained between the edge of the funnel and the inside wall of the column. In initial experiments this gap was to allow for collection of water that might preferentially flow down the column wall. No water was ever collected from the column walls so in later experiments this gap was filled with silicone to ease packing of the columns. A perforated plastic plate was placed 3 mm below the funnel rim with a 0.5-mm mesh fiberglass screen placed over the perforated plate to help support the soil. The leachate was collected through the funnel stem.

Three soils used were Hanford sandy loam (coarse-loamy, mixed, superactive, nonacid, thermic Typic Zerorthents), Domino sandy clay loam (fine-loamy, mixed, thermic Zerollic Paleorthids), and Tujunga loamy sand (mixed, thermic Typic Zeropsamments) (see Table 1 for some properties of these soils). Each soil was collected from 0- to 15-cm layer, air dried and sieved through a 1-mm screen. The sieved soils were packed into columns by consistently tapping the top of the column while the soil was slowly poured in. Columns were packed 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 for Tujunga soil.


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  		Table 1 Some characteristics of the soil studied

 
A total of 3.6 mg of napropamide was applied in 6 mL of hexane to each column. The solution was dripped on the soil surface of each column to create an equivalent napropamide application rate of 8 kg ha-1. The maximum depth of the napropamide–hexane solution penetration into the soil was 3 mm. Hexane was allowed to evaporate in a hood for 48 h. Preliminary investigations based on weight differences found that for all soils <1.0 µg of hexane remained after evaporation for 18 h. Each treatment was replicated three times.

Steps were taken to ensure that particulates were not involved in transport of napropamide. Sodium chloride and CaCl2 were added to deionized water to create a solution having an electrical conductivity of 1 dS m-1 and a sodium adsorption ratio of 2. The saline water was to promote flocculation rather than dispersion of particulates. This water was added to the top of the column and a constant head of 3.5 cm was maintained. Leachate samples were collected for every 15 mL of cumulative leachate. Leachate samples were analyzed for 14C activity and dissolved organic C. Napropamide concentrations in the leachate were determined by liquid scintillation. The leachates were shaken and a 1-mL sample was placed in Liquicent liquid scintillation cocktail (National Diagnostics, Atlanta, GA) and the 14C activity measured in a Beckman LS5000 TD liquid scintillation counter (Beckman Scientific, Fullerton, CA).

The leachate was analyzed for total particulates and sorption of napropamide to particulates. Six milliliters of effluent solution were placed in a centrifuge tube and shaken for 5 min. Immediately after shaking 3 mL of solution were removed of which 1 mL was used for 14C activity by liquid scintillation analysis and the remaining 2 mL of solution was used for particulate analysis by optical transmittance. The remaining 3 mL of solution were then centrifuged for 20 min at 2000 x g. After centrifuging a 1-mL sample was removed for liquid scintillation and the remaining solution was used for optical transmittance. Particulate analysis was performed using optical transmittance at 410 nm in a Beckman Spec-20 spectrophotometer (Beckman Scientific, Fullerton, CA). Solution was placed in a glass spectrophotometer tube, capped, shaken for 30 s, and the transmittance was measured. The presence of particulates would have resulted in an increase in transmittance in the centrifuged samples. Also any napropamide associated with colloids would have resulted in lower 14C activity in the supernatant from the centrifuged samples.

Organic C was measured in a 20-µL sample by ultraviolet promoted persulfate oxidation followed by infrared detection using a Dohrmann DC-80 organic C analyzer (Xertex, Santa Clara, CA). Inorganic C was removed prior to analysis using N2 gas for external sparging and the C contributed by napropamide was subtracted. The term DOM is used in this report since it is the whole organic molecule that participates in the interaction and not just the C.

At the end of the experiment, the soils were allowed to drain for 3 d after which each column was divided into 1.2-cm sections, homogenized, and analyzed for 14C activity. Napropamide was extracted from the soil by placing 0.5 g of soil in 19 mL of scintillation cocktail and shaking on a reciprocating shaker for 6 h. Samples were left over night to allow settling of the soil particulates in solution. Samples were corrected for water content by taking three samples from each depth and drying in an oven at 105°C for 24 h.

Napropamide concentrations as determined by 14C activity were confirmed using gas chromatography (GC). Napropamide concentrations in the leachate were very low and coupled with small sample volumes (15 mL) of effluent it was impossible to sufficiently concentrate the napropamide for GC analysis. Therefore, additional columns were treated as above except the effluent was collected in 45 mL increments and analyzed for napropamide by both GC and liquid scintillation. Napropamide was extracted from 30 mL of effluent using three sequential 1:1 hexane/water extraction procedures followed by roto-evaporating to dryness and redissolving in 3 mL of hexane. Napropamide was analyzed using a Hewlett-Packard 5890 GC (Hewlett-Packard Chemical Analysis Group, Palo Alto, CA) equipped with a N–P detector, auto-injector and on-line integrator. The dB wax column was operated at 210°C with a carrier helium flow of 7 mL min.-1 and a detector temperature of 270°C.

The presence of a napropamide–DOM complex was determined on 27 effluent samples using a modified form of the Lee and Farmer (1989) dialysis equilibrium technique. The first, third, and last 15-mL sample from each column, were selected for analysis. Three milliliters of effluent were placed inside dialysis tubing with a molecular weight cutoff of 500 Da. Dialysis tubing containing effluent samples were then placed in 50-mL Teflon centrifuge tubes and bathed in 30 mL of napropamide free water. Tubes were shaken and the outside solution was analyzed for 14C activity and replaced with fresh deionized water every 2 h. After 8 h no 14C activity was measured in the outside solution and at that time samples from inside were analyzed for 14C activity. As a control napropamide in DOM free water was placed inside the dialysis tubing and in each case the inside and outside solutions were at equilibrium after 2 h.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Conclusions
 REFERENCES
 
The concentration of napropamide in the leachate from each soil is plotted as a function of cumulative leachate in Fig. 1 . All three soils exhibited initially high napropamide concentration that decreased with cumulative leachate. The amount of napropamide leached through all three soils ranged from 1.5 to 1.8% of the total applied within the first 2.4 cm of cumulative leachate (which represents a volume of <60% of one void volume). Figure 2 shows the relationship between 14C activity and napropamide concentration as determined by GC. Note that the regression line between concentration measured by 14C activity and GC has a slope of 0.82. The deviation of the slope from 1 is such that 14C activity under estimates the concentration measured by GC.



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  		Fig. 1 Concentration of napropamide, based on 14C activity, in the initial leachate from Domino, Hanford, and Tujunga soils. Error bars are ± 1 standard error of the mean

 


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  		Fig. 2 Napropamide concentration determined by gas chromatography (GC) and 14C activity. The predicted line is the line where napropamide measured by 14C activity is equal to GC

 
No particulates were measured in the leachate. The optical transmittance of each sample before and after centrifuging was identical. In addition liquid scintillation before and after centrifuging showed no reduction in napropamide concentration, therefore, the napropamide was truly dissolved and not in suspension.

A trend similar to that of napropamide occurred with DOM (Fig. 3) with high initial DOM concentrations which decreased with increasing effluent volume. Except for the initial leachate sample, the highest DOM concentrations were in the Domino effluent. This may be due to the higher clay content in the Domino soil retarding the movement of DOM. Nelson et al. (1990) found that in column-leaching experiments higher soil clay content was responsible for decreased DOM concentrations in the effluent. They attributed the decreased DOM concentrations to higher surface area resulting in increased sorption of DOM to soil minerals.



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  		Fig. 3 Concentration of dissolved organic matter (DOM) in the initial leachate from Domino, Hanford, and Tujunga soils. Error bars are ± 1 standard error of the mean

 
Dissolved organic matter concentrations ranged from a high of 745 to a low of 10 mg C L-1. The initial high DOM concentrations were greater than generally reported in the literature. High values for DOM are expected in our system because the soil was initially dry and then wet from the top, which would result in an accumulation of highly soluble organic matter at or near the wetting front. Ghodrati (1989) under similar conditions also found that an initial DOM concentration of 300 mg L-1 in the initial 1 cm of leachate from the Tujunga soil which is similar to the DOM concentration reported here.

The distribution of napropamide within the soil at the end of the experiment is shown in Fig. 4 . For all three soils the napropamide concentration drops to below detection ({approx}100 ng kg-1) at some point in the profile. Napropamide distribution is as expected on the basis of clay content and organic matter with the napropamide being retained nearer the surface for the Domino sandy clay loam. Napropamide recovery in the soil averaged 95% of the total applied, which gave an average mass balance of 97% for all of the treatments.



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

 
The relationships between napropamide concentration and DOM concentration in the effluent are presented in Fig. 5 . There is an increase in napropamide concentration with increasing DOM concentration which tends to plateau at higher DOM concentrations. The highest napropamide and DOM concentrations were associated with the first collected effluent samples.



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  		Fig. 5 Plot of napropamide concentration vs. dissolved organic matter in the leachate. Error bars are ± 1 standard error of the mean

 
The equilibrium dialysis technique resulted in 17 to 56% of the total activity in the effluent samples remaining inside the dialysis tubing. This result provides evidence that some napropamide formed a stable complex with DOM that could contribute to facilitated napropamide transport. It was also observed that in all cases the percentage of activity that remained inside the dialysis tubing increased for the effluents that came out later.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Conclusions
REFERENCES
 
For facilitated transport to occur the interaction between DOM and napropamide must be strong enough to overcome the partitioning of napropamide to the solid phase. Lee et al. (1990) found that DOM from the Tujunga soil was capable of reducing the amount of napropamide adsorbed to soil minerals. Nelson et al. (1998) also found that the presence of soil-derived DOM could reduce the sorption of napropamide to soil. This might explain how napropamide could leach to the bottom of the soil columns and be found in the effluent but not found in soil at the bottom part of the soil column.

The results obtained in this study are not attributed to preferential flow. Ghodrati (1989) using the Tujunga soil was unable to find preferential flow in repacked soil columns. In our study the columns were repacked in a manner similar to the Ghodrati study. The presence of preferential flow is often verified by the use of a nonreactive tracer, such as Br-. No Br- tracer was used because the soil was initially dry and Br- placed at the surface would appear in the initial leachate regardless of preferential flow. If Br- was introduced continuously then the concentration in the effluent would be a constant since there was no water initially in the column to dilute the Br-.

Results obtained in this study were similar to those obtained by Nelson et al. (1998). In all cases the concentration of napropamide was high in the first drops of effluent and the concentration decreased with increasing cumulative effluent. Preferential flow was eliminated by repacking the columns and there was no preferential flow at the soil column wall interface. Napropamide concentration in the soil was near zero in all cases at the bottom of each column and steps to remove colloidal particulates in the effluent were effective. Gas chromatography confirmed that the 14C activity measured was due to napropamide and not radiolabeled contaminants. Finally the detection of a napropamide–DOM complex by use of the equilibrium dialysis technique provides strong evidence that napropamide was transported through the soil columns complexed to soluble organic matter.


   ACKNOWLEDGMENTS
 
This research was supported by the University of California Kearney Foundation of Soil Science. Carbon-14–labeled napropamide was provided by Stauffer Chemical (now Zeneca Agrochemicals).

Received for publication June 8, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Conclusions
 REFERENCES
 




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