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Journal of Environmental Quality 31:619-627 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Organic Compounds in the Environment

Molecular Weight of Dissolved Organic Matter–Napropamide Complex Transported through Soil Columns

C. F. Williamsa, J. Letey*,b and W. J. Farmerb

a Dep. of Plants, Soils, and Biometeorology, Utah State Univ., Logan, UT 84322-4820
b Dep. of Environmental Science, Univ. of California, Riverside, CA 92521-0424

* Corresponding author (john.letey{at}ucr.edu)

Received for publication February 9, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Soil-derived dissolved organic matter (DOM) has been shown to form stable complexes with the herbicide napropamide [2-({alpha}-naphthoxy-N,N-diethylpropionamide] capable of enhancing the transport of napropamide through soil columns. Two soils, one containing sewage sludge–derived organic matter (SS) and the other having only natural organic matter (NoSS) were treated with napropamide and allowed to dry to promote complex formation. Soil columns were prepared by packing a 10-cm layer of untreated, dry, sieved soil followed by an overlying 5-cm layer of napropamide-treated soil. Columns were irrigated and the effluent collected and placed in dialysis chambers. After equilibration napropamide concentrations were determined on both sides of the membrane and complex and quantified based on the amount of napropamide unable to cross the membrane. It was found that for the SS soil 7% and for the NoSS 2.4% of the applied napropamide underwent facilitated transport. In addition, most of the complex transported through the columns had a molecular weight between 500 and 1000 Daltons (Da). The solutions from the SS soil were also found to have formed at least two distinct complexes that were resolved after passing through the untreated soil layer. The results obtained were in agreement with other published results and the techniques used offer a way to separate and concentrate DOM complexes from column effluents for further characterization.

Abbreviations: DOM, dissolved organic matter • EC, electrical conductivity • NoSS, soil without sewage sludge applied • SS, soil with sewage sludge applied


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
PESTICIDE TRANSPORT through the vadose zone is governed by soil properties including organic mater content and the thermodynamic properties of the pesticide such as water solubility and volatility. Movement of organic chemicals through soil is usually retarded by the presence of adsorptive soil constituents. Soil organic matter provides most of the sites for organic sorption in natural soils; therefore, the addition of organic wastes to soils is viewed as very beneficial in retarding pesticide movement through agricultural soil while providing an economical method of disposal. One potential untoward consequence of these practices is the potential for enhanced chemical transport of organics if dissolved organic matter (DOM) from these amendments complexes with applied pesticides.

Recently, mathematical models have become useful tools in predicting the fate and transport of organic chemicals in the vadose zone. Generally, these models assume that the pesticide is transported by flowing water with incorporation of appropriate soil and pesticide properties. Generally, these models are capable of accurately predicting the movement of the majority of the pesticides mass. However, field observations have revealed that a fraction of many surface-applied pesticides moves deeper into the soil profile than would be expected from results obtained from models.

This rapid, sometimes unpredictable transport of organic chemicals in soil is most often attributed to preferential flow pathways. Jury et al. (1986) found that about 20% of the total mass of 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. In addition, it was 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 (Ghodrati and Jury, 1992). A review by Flury (1996) of the experimental literature on pesticide transport in field soils revealed that there is still a lack of knowledge about the mechanisms responsible for rapid chemical transport. Mechanisms such as soil-induced changes in the sorptive behavior of the pesticide, due to complexation with DOM, can also lead to increased transport of a small portion of applied compound through soil in the absence and/or presence of preferential flow. Uterman et al. (1990) proposed that a small quantity of highly mobile pesticide posed a greater threat to ground water contamination than the slower-moving majority of the pesticide. Therefore, understanding the mechanisms that allow even a small portion of the applied pesticide to be transported rapidly through the soil profile is critical in protecting ground water from potential contamination.

Various researchers have reported increased movement of pesticides through soil in the absence of preferential flow. Vinten et al. (1983) found that DDT [1,1'-(2,2,2-trichloroethylidene)bis(4-chlorobenzene)] and paraquat (1,1'-dimethyl-4,4'-bipyridinium) were much more mobile in soil when bound to Li–montmorillonite colloids. Water-dispersible colloids have also been found to increase the transport of atrazine through intact soil columns (Seta and Karathanasis, 1997a, b). In addition to colloids, dissolved organic matter (DOM) has also been shown to increase the mobility of agricultural chemicals through soil. Ballard (1971) found that soil-derived, water-soluble, humic substances complexed with DDT and increased its mobility through the organic layers of forest soils. It has also been shown that water-soluble organic materials were capable of binding to several herbicides and postulated that the resulting complex would be more mobile in soils than the distribution coefficient (KD) of the pesticide would indicate (Madhun et al., 1986). Baskaran et al. (1995) found that soil columns leached with solutions containing dissolved organic carbon caused enhanced leaching of two herbicides. Williams et al. (2000) showed that soil-derived DOM facilitated the transport of napropamide in three soils having very different physical and chemical properties.

Dissolved organic matter derived from soils amended with sewage sludge has also been shown to increase the transport of herbicides through soil. Graber et al. (1995) reported the enhanced transport of atrazine in field soils under irrigation with secondary treated sewage effluent when compared to irrigation with regular water. The results were explained on the basis of dissolved organic carbon in the sewage effluent. Nelson et al. (1998) also reported the rapid movement of a small fraction of napropamide through sewage sludge–amended soil and attributed it to facilitated transport due to the presence of DOM in the leachate. They found that in sewage sludge–amended soils there was a significant increase in DOM-facilitated transport of napropamide when compared with unamended soils. Nelson et al. (2000a) were further able to show that the complex formed between napropamide and DOM derived from sewage sludge–amended soils was very stable. They found that when a napropamide-treated soil was placed over an untreated layer that the DOM–napropamide complex was able to pass largely unaffected through the underlying layer.

Techniques to elucidate the nature and strength of the complex formed between pesticides and DOM usually involve measuring an increased solution-phase concentration of the pesticide compared with its water solubility. One often-used method involves determining the extraction efficiency of water-immiscible organic solvents for the organic compound. Hassett and Anderson (1979)(1982) used liquid–liquid extraction with carbon tetrachloride and dichloromethane and found increased solution-phase concentration of cholesterol and polychlorinated biphenyls (PCB) in natural waters, and in the presence of sewage sludge. Dichloromethane was also used to show that DOM could increase the solution-phase concentration of C12 to C26 n-alkanes (Maguire et al., 1993). Hexane extraction was used by Driscoll et al. (1991) to measure the increased solution-phase concentration of DDT and DDE [1,1'-(dichloro-ethenylidene) bis(4-chlorobenzene)] in the presence of DOM. Hexane extraction was also used to determine enhanced solution-phase concentration of a number of organic compounds with solubilities ranging from 10-4 to 10-10 mol L-1 (Maguire et al., 1995). Williams et al. (1999c) also used hexane extraction to measure the enhanced aqueous-phase concentration of napropamide in the presence of soil-derived humic acids.

Equilibrium dialysis is another method to determine increased aqueous-phase concentration. Equilibrium dialysis employs a membrane with size-dependant permeability. The sample is placed on one side of the membrane and allowed to come to equilibrium with compound-free water on the other side. If the concentration on the sample side of the membrane is higher than on the compound-free side of the membrane then the result is due to lowered chemical potential, increased aqueous-phase concentration, or the presence of a stable DOM–organic complex. 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 greater than 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 isoelectric focusing to show that the DOM–napropamide complex was formed with two distinctly different humic acids. Clapp et al. (1997) used the dialysis technique with atrazine and napropamide to measure increased aqueous-phase concentration.

In the studies mentioned above dialysis membranes with a molecular weight cutoff of 1000 Da were used. However, Williams et al. (1999a)( b) have developed a new dialysis technique that allows for the use of multiple membranes with various molecular weight cutoffs to determine the size fraction of DOM participating in the complex. This technique was used in batch equilibrium systems to determine that five times more complex was formed between DOM and napropamide from soils amended with sewage sludge than unamended soils. It was also found that the largest complex formed was smaller than 25000 Da, and that most of the complex formed was between 500 and 1000 Da.

Currently the published literature related to DOM-facilitated transport of pesticides through soil has focused on providing a foundation to establish the presence of facilitated transport. The objective of this research was to use the equilibrium dialysis technique from Williams et al. (1999a)(b) on the leachate from soil columns with and without sewage sludge amendments to better understand the nature of the complex capable of being transported through soil so that preventative management practices might be developed. The results were used to determine the size of the complex formed between napropamide and soil-derived DOM that is capable of being transported through soil. In addition, the technique was used on column effluent to determine if DOM from sewage sludge–amended soils forms a complex, distinguishable from a similar unamended soil, based on molecular weight.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
The surface 18 cm of an Airport silt loam (fine-silty, mixed, active, mesic Aquic Natrixeroll) was collected from two adjacent sites in Davis County, Utah. One soil (SS) had received annual amendments of 137 Mg (dry wt) sewage sludge ha-1 for three consecutive years. The second soil (NoSS) did not receive sewage sludge application. Soils were collected two years following the last sludge application. Soils were air-dried and sieved to 2.0 mm, and some of their physical properties were measured by standard techniques (Table 1).


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  		Table 1. Soil physical properties of Airport silt loam (fine-silty, mixed, active, mesic Aquic Natrixerolls).

 
Napropamide (MW = 271) was selected as a model compound for elucidating the complexes that can form between trace organics and DOM. It represents a group of polar nonionic herbicides and has an aqueous solubility of 74 mg L-1, vapor pressure of 2.3 x 10-2 mPa, KOC equal to 700 L kg-1, KOW equal to 2300, 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 napropamide was obtained from Chem Service, Inc., West Chester, PA. Sigma Chemical, St. Louis, MO supplied 14C-labeled napropamide. Radiolabled napropamide had a radiochemical (14C-{alpha}-napthoxy) purity of 99% (specific activity 3.68 x 108 Bq mol-1) napropamide with approximately 1% being other 14C-labeled chemicals. High pressure liquid chromatography followed by radiochemical detection showed that all of the 14C impurities were associated with a single retention time corresponding to a compound having a lower water solubility than napropamide.

Column Preparation and Leaching
Soil columns were constructed by placing a 5-cm layer of napropamide-treated soil over a similar 10-cm layer of untreated soil. Treatments consisted of both the SS and NoSS soil. Each 5 cm of soil consisted of 440 g air-dried soil that was continuously poured into columns and then lightly tapped until a bulk density of 1.2 Mg m-3 was achieved. Therefore, 880 g of untreated soil was added followed by 440 g of napropamide-treated soil and then the column was lightly tapped until the proper bulk density was achieved. The total pore space for each 5 cm of soil was approximately 192.5 cm3. Each treatment was replicated three times.

A 50 mg L-1 herbicide stock solution consisting of 1 part 14C-labeled and 4.5 parts nonlabeled (w/w) napropamide in double deionized water was made. A total of 44 mL of the aqueous napropamide solution was applied dropwise by pipette to 440 g of soil. During application the soil was continuously stirred in an aluminum pan to give a final napropamide concentration of 5 mg kg-1 soil. The treated soil was allowed to dry in the dark at 23°C for 8 d in a laminar flow hood before being packed in the columns. The water was allowed to evaporate to facilitate the formation of the DOM–napropamide complex (Nelson et al., 2000b; Williams et al., 1999a).

Soil columns 35 cm in length were constructed of 9.49-cm (i.d.) acrylic plastic pipe. A perforated Teflon disk was fastened to the bottom of the column to support the soil and allow effluent drainage. Soil columns were leached with napropamide-free water with sufficient NaCl and CaCl2 added to create a solution with an electrical conductivity (EC) of 1 dS m-1 and a sodium adsorption ratio (SAR) of 2 to prevent soil dispersion. A constant flow rate of 3.3 cm3 min-1 was maintained through all soil columns and a uniform wetting front was observed, as the soil column was initially wetted from the top. Soil leachate was collected from the bottom of each column through borosilicate glass funnels and Teflon tubing in 55-mL increments and stored in glass bottles. A constant flow was maintained until a total of 275 mL of leachate was collected, then the columns were allowed to drain under a falling head until a total of 440 mL of leachate was collected from the bottom of the column. This represented approximately 1.3 pore volumes if columns had been saturated at the initiation of leaching. Leachates were analyzed for pH, EC, 14C activity, and DOM content, and then placed in dialysis chambers. Samples were also analyzed for particulate content with none of the samples containing particulates.

Following leaching the soil in each column was sectioned and analyzed for 14C activity to determine the final distribution of napropamide remaining in the soil. The napropamide-treated layer was separated from the untreated layer to determine the amount of applied napropamide remaining in the treated layer. The untreated layer was then sectioned into 1-cm lengths. All soil was then allowed to air-dry in the dark, mixed thoroughly, and analyzed for 14C activity by shaking 5 g of soil in 20 mL of pesticide-grade methanol for 12 h in 50-mL Teflon centrifuge tubes. The tubes were then centrifuged for 10 min at 4000 x g and 1 mL of the supernatant analyzed for 14C activity in 19 mL of scintillation cocktail. Soil napropamide concentrations were corrected for air-dry water content by placing subsamples in an oven at 105°C for 24 h and using the weight difference for water content measurements.

Dialysis and Sample Analysis
Equilibrium dialysis was used to characterize the DOM–napropamide complex in the column effluents. A modification to the equilibrium dialysis technique reported by Lee and Farmer (1989) was used and is described in detail by Williams et al. (1999b). Dialysis membranes were placed between two Teflon chambers. Sample solution was added to one side (sample side) and napropamide-free water was placed in the opposite chamber (free side) and allowed to equilibrate. In all cases the napropamide-free water was deionized water where the pH and EC were adjusted to match the solution added to the sample side.

Effluent from each of the columns was placed on one side of the dialysis chambers with one of six different Spectra/Por cellulose ester dialysis membranes (Spectrum Laboratories, Rancho Dominguez, CA). The membranes had molecular weight cutoffs of 500, 1000, 5000, 10000, 25000, and 50000 Da. After 8 d the solution from each side of the membrane was analyzed for 14C activity and DOM. Preliminary studies showed that equilibrium was reached within 5 d with all membranes. Napropamide has a molecular weight of 271.4 Da and free napropamide is able to freely cross any of the membranes used. Hence, any 14C activity in the sample side greater than in the free side is a result of not being able to cross the barrier due to increased size. In addition, adding sodium azide as an antimicrobial agent eliminated napropamide degradation during dialysis.

Solutions were analyzed for 14C activity by placing 1 mL of sample in 19 mL of Liquiscint liquid scintillation cocktail (National Diagnostics, Atlanta, GA) and the activity determined using a Beckmann LS 5000TD liquid scintillation counter (Beckmann Scientific, Fullerton, CA). Scintillation counts were converted to napropamide concentration after correcting for quenching and background interference.

Before DOM analysis, samples were passed through a 0.25-µm disposable membrane filter to ensure removal of any large colloidal particles that may interfere with the analysis. Dissolved organic matter adsorption to the filter was determined by passing dialysis solution that had initially passed through a 500-Da membrane through a nest of four filters. Dissolved organic matter was measured in all filtrates and no reduction in DOM was observed, indicating no adsorption to the filter membrane. In addition, 14C activity was measured directly from the filter membranes to ensure that no large particles were responsible for the observed napropamide complex and no 14C activity was ever measured from the membranes. Dissolved organic matter was determined using ultraviolet promoted persulfate oxidation followed by infrared detection with a Dohrmann DC-80 organic carbon analyzer (Xertex, Santa Clara, CA). Inorganic carbon was removed prior to analysis using N2 gas for external sparging. All DOM values reported have been corrected for carbon contributed by napropamide. The concentration of DOM reported in these studies is the total organic carbon content in solution (mg C L-1).

Data Analysis
Complexed napropamide is reported as FC, the fraction of total napropamide in solution complexed, and is calculated according to the following equation:

[1]
where FC is the fraction of total solution-phase napropamide that was complexed, CS is the napropamide concentration on the sample side, CF is the concentration on the free side, and Ctot is the total mass of napropamide in solution divided by the total volume of solution. Any napropamide that passed through the 500-Da membrane was assumed to be uncomplexed and the FC for this membrane was taken as the total complexed napropamide. With successively larger membrane pore sizes, a portion of the complexed napropamide passed through the membrane. Therefore, the calculated FC for a given membrane size represents the fraction of complex with a molecular size larger than the membrane molecular weight cutoff. The difference in FC between two membrane sizes represents the fraction of complexed napropamide that has a molecular weight between the two membrane sizes.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
The average concentration of DOM from all three replicates was highest in the initial effluent and decreased with cumulative leachate (Fig. 1) . In addition, the total amount of DOM in the effluent was similar for both the SS and NoSS soil. Figure 2 shows the total napropamide concentration in the column effluent from both soils. The initial concentration of napropamide in the leachate is much higher in the SS soil compared with the NoSS soil. The napropamide concentration in the effluent from both soils then decreased with cumulative leachate. In the absence of facilitated transport no napropamide should have been detected in any of the samples collected, thus these results are consistent with facilitated transport. Figure 2 also shows that there was nearly three times more total napropamide transported through the SS soil than the NoSS soil.



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  		Fig. 1. Concentration (reported as total carbon) of dissolved organic matter (DOM) plotted as a function of cumulative leachate eluted from the bottom of the columns containing soil with sewage sludge applied (SS) and soil without sewage sludge applied (NoSS). Values reported are the average of the three replications. Error bars are ±1 standard error of the mean.

 


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  		Fig. 2. Napropamide concentration versus cumulative leachate eluted from the bottom of columns containing soil with sewage sludge applied (SS) and soil without sewage sludge applied (NoSS). Values reported are the average of the three replicates. Error bars are ±1 standard error of the mean.

 
A total of 7% of the applied napropamide was transported through the SS soil and less than 2.4% of the applied napropamide was transported through the NoSS soil. These results are expected based on previous research. Using batch equilibrium techniques it was found that the SS soil led to the formation of more DOM–napropamide complex than the NoSS soil (Williams et al., 1999a). Also, Nelson et al. (1998) found that there was significantly more facilitated transport of napropamide through the SS than the NoSS soil. Data in Fig. 2 as well as data previously reported have shown that DOM is capable of facilitating the transport of a small fraction of applied (<7%) napropamide through soil in a manner inconsistent with current transport models based on retardation factors (Nelson et al., 1998; Williams et al., 2000). The retardation factor is dependant on soil sorption properties and is defined as:

[2]
where R is the retardation factor, {rho}b is the soil bulk density, KD is the linear distribution coefficient between soil and water, and {theta}v is the volumetric water content of the soil. When R is equal to unity then the pesticide will travel as a nonreactive solute (KD = 0) but as the KD increases then R increases, resulting in increased retention and reduced mobility. It has been shown that the KD and therefore the retardation factor for napropamide are dependent on the method of pesticide application (Williams et al., 1999a,c; Nelson et al., 2000b). It was found that allowing the pesticide to dry on the soil before water application led to the formation of a napropamide–DOM complex that reduced the KD, which led to decreased napropamide sorption and potentially increased napropamide movement. In addition, it was found that the drying event was critical in the formation of a DOM–napropamide complex capable of being transported through soil columns.

Soil amended with sewage sludge has also been shown to form more total and stronger complexes with napropamide than non-amended soil. Nelson et al. (1998)(2000b) found that the SS soil had increased facilitated transport of napropamide when compared with the NoSS soil and that the complex formed with the organic matter from the SS soil was capable of being transported further through untreated soil. Using equilibrium dialysis, Williams et al. (1999a) found that when napropamide was applied to the SS soil, allowed to dry, extracted with napropamide-free water, and placed in dialysis chambers that more complex was formed in comparison with the NoSS soil. These results are consistent with the data presented in Fig. 2.

The distribution of napropamide remaining within the soil profile is shown in Fig. 3 . The concentration of napropamide is reported as a ratio of the measured napropamide at each depth divided by the total napropamide applied to the surface layer of the column. The total napropamide recovery in the effluent and from the soil matrix was 97.2 and 97.9% of the total applied for the SS and NoSS soils, respectively. The final napropamide distribution in the soil column can be explained according to the differences in the type of organic matter between the SS and NoSS soil. In addition, the differences in amount of organic matter can also affect the KD and therefore the retardation factor as expressed in Eq. [2]. Published KD values for the SS (12.1) and NoSS (8.3) soil are also consistent with the results shown in Fig. 3 (Nelson et al., 2000a). Another way to explain the results from Fig. 2 and 3 is that the transport of napropamide is at least a two-component system with Fig. 2 representing the napropamide–DOM complex that is capable of overcoming the sorptive capacity of the soil and Fig. 3 representing the noncomplexed napropamide that undergoes normal sorption represented by the KD. The complex has one KD that allowed for rapid transport of the napropamide found in the initial effluent, and the free napropamide has a distinct KD represented by the napropamide remaining in the soil that is being transported according to R as predicted by the KD reported in the literature. Also, it can be seen that for both soils there is an absence of sorbed napropamide below about 13 cm. This is also consistent with the presence of a napropamide–DOM complex being responsible for the napropamide found in solution since noncomplexed napropamide would have been readsorbed to the soil below 13 cm.



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  		Fig. 3. Ratio of napropamide mass measured (M) in a particular soil layer divided by the total mass (Mt) of napropamide applied to the surface layer of the column after passing 440 mL of water (approximately 1.3 pore volumes if started at saturation) through columns containing soil with sewage sludge applied (SS) and soil without sewage sludge applied (NoSS). Data points of the curve are located at the bottom of each soil sample section and are the average of the three replications. Error bars are ±1 standard error of the mean.

 
Figures 4 and 5 show the results of the equilibrium dialysis. Both figures show that the majority of the napropamide undergoing facilitated transport through the columns, at all cumulative leachates, was between 500 and 1000 Da. Using the SS and NoSS soils, Williams et al. (1999a) found that batch sorption techniques led to the formation of a napropamide–DOM complex and that the majority of the resulting complex that could potentially undergo transport was smaller than 5000 Da. Results shown in Fig. 4 and 5 verify that the majority of the complexes that undergo transport are indeed the smallest complexes. Nelson et al. (2000a) showed that as the length of underlying soil layer was increased the amount of napropamide–DOM complex in the effluent was reduced. Therefore, it can be inferred that a large portion of the complexes formed larger than 1000 Da, which Williams et al. (1999a) predicted would undergo transport, were retarded through sorption or physical processes by the untreated soil underlying the treated soil.



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  		Fig. 4. The average fraction (FC) of complexed napropamide within the specified range of molecular weights is plotted for each effluent sample from the soil without sewage sludge applied (NoSS). The fraction of complexed napropamide is 1 for each effluent sample but the total amount of complexed napropamide is different for each sample. Therefore, the fraction of napropamide complexed with a molecular weight between 500 and 1000 Daltons is similar for the 55- and 275-mL samples but the total amount of complex is much lower for the 275-mL sample than the 55-mL sample.

 


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  		Fig. 5. The average fraction (FC) of complexed napropamide within the specified range of molecular weights is plotted for each effluent sample from the soil with sewage sludge applied (SS). The fraction of complexed napropamide is 1 for each effluent sample but the total amount of complexed napropamide is different for each sample. Therefore, the fraction of napropamide complexed with a molecular weight between 500 and 1000 Daltons is similar for the 165- and 225-mL samples but the total amount of complex is lower for the 225-mL sample than the 165-mL sample.

 
Figure 5 also shows that for the SS soil it appears that there are at least two distinctly different complexes formed due to the bimodal size distribution of the complex. With the exception of the first effluent sample collected no complex with a size between 1000 and 5000 Da was found. Additionally, the first effluent sample had two replications with no complex larger than 1000 and smaller than 5000 Da and one replication had no complex between 500 and 1000 Da. This leads to the conclusion that the size of the complex formed from the SS soil and transported through the column was very near the cutoff of the 1000-Da membrane and that there were no complexes formed between just greater than 1000 and 5000 Da. No conclusions can be drawn about the chemical nature of these different complexes but it does provide a starting point for future research into a better understanding of what types of soil organic matter participate in complex formation.

Figures 4 and 5 also show the size distribution for the napropamide–DOM complex for each effluent sample. By plotting the FC for each membrane size from each effluent sample it is very clear that the size distribution of the complexes that underwent transport is very different for the NoSS and SS soils. The NoSS soil had less total complex formed with a molecular weight greater than 5000 Da than the SS soil. Both soils exhibited a somewhat bimodal distribution in the size of the complex transported. The data from the SS soil are also consistent with the possibility that there are distinctly different types of DOM participating in the complex. In Fig. 5 at a cumulative leachate of 110 mL there is a significant amount of complex formed with a size greater than 5000 Da. However, at 165 and 220 mL the presence of these complexes is greatly reduced, but by the 275-mL sample there is another flush of this larger complex. If the untreated portion of the soil column is viewed as a simple chromatography column then complexes with differences in affinity for the solid phase would be resolved from one another. These results are in agreement with the findings of Liu et al. (1996), who used gel isoelectric focusing to find that two distinct complexes were formed between napropamide and DOM. In contrast to Fig. 5 it appears that the complex formed in the NoSS soil (Fig. 4) is much more homogeneous than the SS soil as evidenced by the lack of any similar "resolution" of napropamide–DOM complex peaks in the column effluent.

Results presented here show that soil-derived DOM can facilitate the transport of napropamide through soil. Also, it has been shown that the use of equilibrium dialysis can provide a means by which the complex formed can be functionally classified based on molecular weight. The techniques can also be used to separate and concentrate the complex such that further analysis is possible. Data presented here also show that sewage sludge amendments lead to increased complex formation and transport of napropamide, which would indicate a difference in the composition of soil-derived DOM from sewage sludge–amended soils. This difference in complex formation also indicates that prediction of potential facilitated transport may be possible based on specific component analysis of the soil organic matter.


   ACKNOWLEDGMENTS
 
This research was funded by the University of California Kearney Foundation of Soil Science.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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