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Journal of Environmental Quality 32:215-223 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Organic Compounds in the Environment

Turfgrass Thatch Effects on Pesticide Leaching

A Laboratory and Modeling Study

S. Raturia, M. J. Carroll*,b and R. L. Hillb

a Dep. of Environmental Sciences, 14 College Farm Road, Rutgers, The State University of New Jersey, New Brunswick, NJ 08901
b Dep. of Natural Resource Sciences and Landscape Architecture, 1112 H.J. Patterson Hall, Univ. of Maryland, College Park, MD 20742

* Corresponding author (mc92{at}umail.umd.edu)

Received for publication November 26, 2001.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Process-based models are frequently used to assess the water quality impacts of turfgrass management emanating from proposed or existing golf courses. Thatch complicates the prediction of pesticide transport because surface-applied pesticides must pass through an organic-rich layer before entering the soil. This study was conducted to (i) compare the use of a linear equilibrium model (LEM) and two-site nonequilibrium (2SNE) model to predict pesticide transport through soil and thatch + soil columns, and (ii) evaluate thatch effects on pesticide transport through soil columns with a volume-averaging approach. Pesticide breakthrough curves were obtained for soil and thatch + soil columns from a 1 cm h-1 flux applied one day after applying triclopyr (3,5,6-trichloro-2-pyridinyloxyacetic acid) and carbaryl (1-napthyl-methyl carbamate). Pesticide and bromide transport parameters indicated that nonequilibrium processes were affecting pesticide transport. Columns containing zoysiagrass (Zoysia japonica Steud.) thatch had lower triclopyr and carbaryl leaching losses than did soil-only columns, although total reductions attributable to thatch did not exceed 15% of the applied pesticide. When laboratory-based retardation factors were used, the 2SNE model explained 88 to 93% of the variability for triclopyr and 70 to 94% of the variability for carbaryl. Laboratory-based retardation factors performed well in a 2SNE model to predict the peak concentration and tailing behavior of triclopyr and carbaryl with a volume-averaging approach. These results suggest that separate representation of the thatch layer in process-based models is not a prerequisite to obtain reasonable estimates of pesticide transport under steady state flow conditions.

Abbreviations: BTC, breakthrough curve • LEM, linear equilibrium model • R, retardation factor • 2SNE, two-site nonequilibrium


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PROCESS-BASED SIMULATION MODELS are often used in the development of water quality risk assessments for proposed golf course sites. Accurate prediction of pesticide movement using these models requires proper representation of the transport mechanisms responsible for pesticide movement in the field. Mature turfgrass swards often contain a surface layer of living and decaying plant material called thatch. This layer complicates the prediction of pesticide transport in turf systems because surface-applied pesticides must pass an organic-rich layer before entering the soil.

Most process-based models use the convection–dispersion equation to simulate pesticide transport in porous media. Alternate forms of this equation can be used to describe instantaneous or kinetically driven sorption, or alternately, two-domain flow phenomena within the media. Regulatory agencies and private consultants oftentimes use linear equilibrium-based models (LEM) to assess the effect of turfgrass management practices on the quality of water emanating from proposed or existing golf course sites. Linear equilibrium-based models assume that all pesticide sorption sites are identical and that sorption equilibrium occurs instantaneously between the pesticide in the bulk soil solution and the sorbed pesticide. Some model users have accounted for the presence of thatch by simply increasing the overall organic matter content of the soil profile to account for the volume-averaged contribution of the thatch layer (Primi et al., 1994). Since the effect of the thatch layer on pesticide sorption and subsequent transport is not specifically considered when such an approach is used, it is felt the assumption of instantaneous sorption used in equilibrium-based models may lead to inaccurate predictions of pesticide transport in turfgrass that contains thatch.

Sorption of pesticides to highly organic media such as thatch is believed to be a two-stage process (Brusseau and Rao, 1989). The first stage consists of direct pesticide sorption to the external organic matter sites, whereas the second stage consists of pesticide sorption to sites located within the pores or fissures of the organic matter aggregates. The latter process involves diffusive mass transfer of the pesticide and is highly dependent on the residence time of the solution containing the pesticide. Precipitation that washes pesticides off turfgrass foliage readily moves through thatch so that pesticide interception and subsequent movement within the layer is likely to be dependent on the instantaneous thatch pesticide sorptive behavior. A two-site nonequilibrium model (2SNE), which considers that transfer to some sorption sites is diffusion mediated, may be more appropriate than an LEM model when predicting pesticide transport through a soil with a thatch surface layer.

The choice of the appropriate process-based model depends on the equilibrium and nonequilibrium conditions, which characterize the transport process in the media of interest. A process-based model should accurately simulate the processes involved in solute transport when as many factors as possible involved in solute transport are independently determined. Use of laboratory-based determinations is the most prevalent way of obtaining independent estimates of the sorption parameters needed in solute transport modeling. Thatch would generally be expected to have a much higher retardation factor than the underlying soil. Selim et al. (1977) have demonstrated that the use of volume-averaged (or length-weighted) model retardation factors in process-based simulation models will allow for the successful prediction of pesticide movement in layered mineral soils leached at a constant rate. Although some modelers have used an averaging approach to account for the retention properties of thatch (Primi et al., 1994), we are not aware of any published data that has demonstrated the utility of using a volume-averaged retardation factor to predict the transport of pesticides in soils that contain a surface layer of thatch.

Two pesticides widely used on highly managed turfgrass are triclopyr and carbaryl. Triclopyr is a nonphenoxy herbicide used to control broadleaf weeds in cool-season turf (Olson and Mackasey, 1989), whereas carbaryl is a carbamate insecticide used to control surface-feeding insects such as chinchbugs, billbugs, and leaf-eating caterpillars in turf (Christians, 1998). Triclopyr is moderately persistent (30–90 d half-life) (Wauchope et al., 1992) but is highly mobile in most soils (Koc = 1–160 L kg-1). Carbaryl is less persistent (7 d half life) and less mobile (Koc = 104–423 L kg-1) (Balogh and Anderson, 1992) in soils than triclopyr. The high organic carbon content of thatch allows this media to readily retain pesticides having high normalized sorption coefficients (Koc) (Horst et al., 1996; Cisar and Snyder, 1996). Less is known, however, about attenuating effects of thatch on the mobility of pesticides having low or moderate Koc values. Snyder and Cisar (1997) measured the dissipation of 2,4-D (2,4-dichlorophenoxyacetic acid) and dicamba (3,6-dichloro-2-methoxy benzoic acid) in clippings, thatch, soil, and percolate and found that most of the applied herbicide was located in thatch. They found that 40 to 90% of the dicamba and 2,4-D recovered from the thatch and soil of a golf green was located in the thatch. Conversely, Gardner and Branham (2001) reported that varying levels of thatch and turfgrass cover had little effect on the soil mobility of mefenoxam [N-(2,6-dimethylphenyl)-N-(methoxyacetyl)-D-alanine methyl ester]. Mefenoxam is a stereoisomer of metalaxyl mefenoxam [N-(2,6-dimethylphenyl)-N-(methoxyacetyl)-DL-alanine methyl ester] and is considered to have a soil mobility similar to that of metalaxyl (Koc = 29–287 L kg-1) (Gardner and Branham, 2001). Primi et al. (1994) used the USEPA's Pesticide Root Zone Model (PRZM) to simulate the effect of thatch on the transport of seven pesticides. The presence of thatch was simulated by increasing the organic matter content of the surface layer from 2.6 to 8.0%. The PRZM model predicted that increasing the organic matter content within the uppermost 5 cm of surface soil would reduce the maximum dissolved concentrations of all seven pesticides at the 150-cm soil depth. The pesticides commonly identified as being the most likely to leach were least affected by the addition of the surface organic matter layer.

The objectives of this study were to (i) compare the use of LEM and 2SNE models using laboratory-based retardation factors to predict triclopyr and carbaryl transport through soil columns containing a surface layer of thatch and columns devoid of thatch, and (ii) evaluate the effect of thatch on the transport of triclopyr and carbaryl through undisturbed soil columns using a volume-averaging approach.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sample Collection Sites
Columns of soil and soil plus thatch were collected from two sites at the University of Maryland Turfgrass Research and Education Facility in Silver Spring, Maryland. One site was a 5-yr-old stand of ‘Southshore’ creeping bentgrass [Agrostis stolonifera L. var. palustris (Huds.) Farw.] with a 2.0- to 2.5-cm-thick thatch layer. The second site was an 8-yr-old stand of ‘Meyer’ zoysiagrass (Zoysia japonica Steud.) with a 3.0- to 3.5-cm-thick thatch layer. The zoysiagrass site was established from sod ribbons and debris without the use of triclopyr or carbaryl, as described by Dernoeden and Carroll (1992). The bentgrass site was established from seeds without the use of triclopyr or carbaryl. Visual inspection of the bentgrass site revealed the presence of a finely granulated, well-decomposed thatch layer. The zoysiagrass thatch layer consisted primarily of nondecomposed and partially decomposed rhizomes, stolons, and tillers. The soil at both sites was classified as a Sassafras series soil (fine-loamy, siliceous, semiactive, mesic Typic Hapludult) with the bentgrass site having a sandy loam surface phase and the zoysiagrass site having a loamy sand surface phase.

Disturbed samples of thatch and soil were also collected from each site to determine triclopyr and carbaryl sorption isotherms. Thatch samples were collected by removing the thatch and mat layer of the turfgrass sward at each site with a sod cutter. Before using the sod cutter, all verdure was removed from the sward with a walk-behind greens mower. The intact rolls of the turfgrass thatch and mat were shredded with a modified wood chipper, and the shredded field-moist material passed through a 4-mm screen. The soil directly beneath the thatch and mat layer (0- to 2-cm depth) of each turfgrass species was collected with a shovel, and the field-moist soil passed through a 4-mm screen. The soil and combined thatch and mat material from each site were stored in separate plastic bags at 4°C until analysis. The chemical properties of the thatch and soil are presented in Table 1.


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  		Table 1. Chemical properties of bentgrass thatch, zoysiagrass thatch, and the soil below each thatch layer.

 
Sorption Isotherms
Sorption isotherms were determined with a mechanical vacuum extractor (Centurion, Lincoln, NE). This technique uses a column of three vertically oriented plastic syringes, connected in series, to evaluate pesticide sorption. Solution placed into the uppermost syringe moves through a column of thatch or soil located in the middle syringe. Flow occurs in response to displacement of a plunger located in the bottom syringe. The vacuum created by plunger displacement causes leachate from the middle syringe to flow into the bottom syringe, where it is collected for analysis. Solution flow through the sample is regulated by varying the rate of plunger displacement. Unlike conventional batch techniques, this modified batch–flow technique causes little disruption of the sample aggregates and organic matter, hence sorption is investigated under conditions similar to those found in the field. A detailed description of this technique is presented by Raturi et al. (1997).

Columns of thatch or soil were created by packing known amounts thatch or soil into 2.5-cm-diameter plastic syringes. Field-moist samples having oven-dry weights equivalent to 10 g thatch or soil were added to syringes after placing a single sheet of 2.5-cm-diameter glass fiber filter paper into the bottom of the syringe. The samples were then gently tamped to create a 2- to 3-cm-deep column of thatch or soil. Sorption was determined by leaching 1.25 mL h-1 of a known pesticide solution through columns of thatch or soil for 24 h. Pesticide sorption to any material other than thatch or soil was accounted for by including blank syringes. The blanks were identical to the syringes containing thatch or soil, except they contained no thatch or soil. The difference between the mean concentration leached from the two blanks included in each sorption run and the concentration of pesticide in the leachate from the sample was used to determine the amount of pesticide that was sorbed to each media.

Triclopyr sorption to thatch and soil was evaluated using solution concentrations of 0.1, 1.0, 10, and 100 mg triclopyr L-1. The concentration of triclopyr in leachate was measured with a magnetic particle-based enzyme immunoassay technique developed by Ohmicron Environmental Diagnostics (Newtown, PA). Quality control procedures were used to ensure the accuracy of the methodology. Carbaryl sorption was determined for solution concentrations of 1, 10, 100, and 300 mg carbaryl L-1. Each solution contained 2.31 x 105 Bq L-1 of ring-labeled 14C carbaryl. Carbaryl in the leachate was determined with liquid scintillation counting (LSC) techniques.

Triclopyr and carbaryl sorption data were fitted to the linear form of the Freundlich equation. Regression analysis were used to calculate the Freundlich constants (Kf and n) that characterize pesticide sorption. Student's t tests were used to test for homogeneity of slopes and to compare equation intercepts.

Pesticide Transport
To evaluate pesticide transport through the soil profile, soil and soil plus thatch columns, approximately 10.7 cm long by 10 cm in diameter, were extracted from the upper 12 cm of the surface layer at each site with a specially designed drop hammer–sleeve assembly. The columns containing soil only were obtained after removing all aboveground thatch and foliage. The columns were brought to the laboratory and saturated from the base. The bottom end of each column was trimmed and placed into a funnel containing a 12-µm-pore-diameter, saturated, porous, stainless steel plate that was made vacuum tight. The column and funnel were then inserted into one port of a multiport vacuum chamber. A null balance vacuum regulator was used to maintain a suction pressure of -10 kPa at the base of each column.

A 0.001 M CaCl2 solution was continuously applied to each column with a specially designed drop emitter that uniformly distributed solution to the surface of each column (modified design of Ogden et al., 1997). The emitters were calibrated to deliver approximately 1 cm h-1 by adjusting the pressure head of a Marriote tube located within the emitter. Leachate was collected in 400-mL sterile plastic cups located beneath the funnel of each column within the vacuum chamber. The emitter–vacuum chamber system permitted sampling of leachate to take place under steady state unsaturated flow conditions. After steady state flow conditions were achieved in all columns, the emitters were removed, the vacuum applied to the base of each column was discontinued, and 20 mL of a 600 mg bromide L-1 was uniformly surface-applied to each column with a pipette. The emitters were then placed back atop of each column and the vacuum chamber system turned on. Leachate was then collected every 30 min for the next 18 h. The volume of the leachate was determined by removing plastic cups located beneath each column and weighing the cups. The leachate was then stored in a refrigerator maintained at 4°C until bromide analysis was conducted.

After the initial 18-h leaching period, the emitters were removed from each column and the vacuum applied to the base of each column was discontinued. Immediately after discontinuing the vacuum, 10.5 mL of 200 mg triclopyr L-1 was uniformly surface-applied to each column with a pipette. Leaching activity ceased for 24 h to permit adsorption of triclopyr to thatch and soil. During this time all columns were covered with plastic wrap to prevent surface evaporation and volatile losses of triclopyr. After the 24 h adsorption period, the plastic wrap was removed. The leaching process was resumed by placing the emitters back atop of each column, and the vacuum chamber system turned on. Leachate samples were collected every 45 min for the next 24 h. The volume of the triclopyr leachate from each column was measured and 20 mL of the leachate was transferred into a scintillation vial and stored at 4°C for future analysis of triclopyr. Triclopyr in the leachate was determined with the procedure previously described.

At the conclusion of collecting leachate samples for the triclopyr segment of this study, the emitters were removed from each column and 20 mL of 136.36 mg carbaryl L-1 surface-applied to each column. The carbaryl solution consisted of 231 Bq L-1 of 14C ring-labeled carbaryl and 136.21 mg L-1 of technical-grade carbaryl. Using procedures identical to those just described for triclopyr, leaching activities were ceased for 24 h to permit adsorption of the carbaryl to thatch and soil. Once leaching activities were resumed leachate samples were collected every 2 h until most of the carbaryl had leached out of the columns. Immediately after determining the volume of leachate collected from a column, the radioactivity of the resulting sample was measured by liquid scintillation counting. At periodic intervals, 1 mL of leachate from select columns was transferred into 1.8-mL glass vials that contained 0.1 mL of ChlorAc buffer (p/n1700-0132; Pickering Labs), and the samples placed into freezer maintained at -4°C until analysis. After collecting the last leachate sample, columns were removed from the vacuum chamber and gently tapped along their length to loosen the soil from the edge of the PVC exterior. Each column was then gently pushed from the top and removed intact with minimal compaction. In the columns containing thatch, thatch layer thickness was measured before being separated from the soil, and the moisture contents of the thatch and soil layers were determined.

The 1.8-mL carbaryl leachate samples that had been frozen immediately after collection were analyzed by high-performance liquid chromatography (HPLC) for the presence of carbaryl and carbaryl's first intermediate metabolite, 1-naphthol. This procedure was done to confirm that the radioactivity measured by the liquid scintillation counter was 14C carbaryl and not its first intermediate metabolite. Bromide concentration in the leachate was measured by halide electrode (Model 94-35; Orion Research, Boston, MA). Calibration was performed for a series of standards and then, the concentration of the sample was determined by comparison with the standards. Ionic strength adjustor (ISA) was added to all solutions to ensure that sample and standards had similar ionic strengths.

Total leachate losses of triclopyr or carbaryl from the bentgrass and zoysiagrass site columns were compared using the Student's t test to separate treatment means. Columns that experienced ponding while being leached were removed from the study. Using this process of elimination, the leachate data for four replications of the bentgrass thatch plus soil columns, two replications of the bentgrass soil columns, and three replications of the zoysiagrass site thatch and thatch plus soil columns were examined in this study.

Estimating Transport Parameters from Breakthrough Curves
Theoretical Background
The one-dimensional convection–dispersion equation (CDE) for steady state transport of a solute through homogeneous soil is (Lapidus and Amundson, 1952):

[1]
where C is solution-phase solute concentration (µg cm-3), t is time (h), D is the hydrodynamic dispersion coefficient (cm2 h-1), R is the retardation factor (dimensionless), x is distance from solute origin (cm), and v is the average pore water velocity (cm h-1). The R term is generally assumed to reduce to a value of 1 for nonreactive solutes and is greater than 1 when solute retention occurs. The retardation factor is defined as (Hashimoto et al., 1964):

[2]
where {rho} is the soil bulk density (g cm-3), {theta} is the volumetric water content (cm3 cm-3), and Kf and n are the Freundlich empirical distribution coefficient constants that characterize sorption.

The simplest approach is to assume that all pesticide sorption sites are identical and that equilibrium occurs instantaneously between the pesticide in the bulk soil solution and the adsorbed pesticide. This mathematical approach is called linear equilibrium sorption. Where bimodal porosity leads to two-region flow, or situations exist where the sorption process is controlled by either rate-limiting diffusive mass transfer processes and/or kinetically driven adsorption processes, nonequilibrium models may more accurately simulate the processes that have occurred and result in improved soil pesticide transport predictions. Chemical nonequilibrium models consider adsorption on some of the sorption sites to be instantaneous, while sorption on the remaining sites is governed by first-order kinetics (Selim et al., 1976). The two-site chemical nonequilibrium model conceptually divides the porous medium into two sorption sites: Type 1 sites that assume equilibrium sorption and Type 2 sites that assume sorption processes as a first-order kinetic reaction (van Genuchten and Wagenet, 1989). In contrast, physical nonequilibrium is often modeled by using a two-region dual-porosity type formulation of the nonequilibrium model. The two-region transport model assumes the liquid phase can be partitioned into mobile (flowing) and immobile (stagnant) regions. Solute exchange between the two liquid regions is modeled as a first-order process. The concepts are different for both chemical and physical nonequilibrium CDE, however, the processes can be described by the same dimensionless form of the CDE for conditions of linear adsorption and steady state water flow (Nkedi-Kizza et al., 1984):

[3]

[4]
where the subscripts 1 and 2 refer to equilibrium and nonequilibrium sites, respectively; ß is a partitioning coefficient; {omega} is a dimensionless mass transfer coefficient; P is the Peclet number; and µ (h-1) and {gamma} (µg h-1) define first-order decay and zero-order production terms, respectively, each represented in component contributions of both the liquid and solid phases.

Customarily, ß and {omega} are obtained by fitting solute breakthrough curves (BTCs) to the nonequilibrium model with a nonlinear least squares minimization technique (Toride et al., 1995). The values of ß and {omega} obtained from the BTCs of noninteracting solutes can be used to evaluate the potential contributions from two-region flow. In the absence of two-region flow, ß and {omega} may be used to evaluate the contributions from two-site kinetic nonequilibrium sorption (Gaber et al., 1995).

Model Evaluation
Solute BTCs were plotted for bromide, triclopyr, and carbaryl using the relative concentrations (C/Co) versus the pore volumes of leachate. The computer program CXTFIT (Parker and van Genuchten, 1984) was used to provide a nonlinear least squares fit of observed BTCs to the convection–dispersion model for a steady state one-dimensional homogenous system. The LEM and 2SNE models for pulse injection with first-order decay and zero-order production assumed to be zero for flux-averaged concentrations were used to interpret the bromide, triclopyr, and carbaryl BTCs as relative concentration versus pore volume.

Convective transport parameters were estimated by a least squares minimization procedure (CXTFIT; Parker and van Genuchten, 1984) using the bromide breakthrough data. Actual mean pore water velocities (v) were used and the retardation factor was assumed to be equal to 1. One- and two-domain flow forms of the convective–dispersive equation were curve-fit to the bromide leachate data. The dispersion coefficient (D) was the only fitting parameter determined for the one-domain flow model while D, ß, and {omega} were fitting parameters for the two-domain flow models.

The 2SNE model was fitted to the triclopyr and carbaryl transport data for all columns. The LEM was fitted to the triclopyr and carbaryl transport data for all columns that did not display significant two-domain flow during the bromide leaching portion of the study. All models used calculated mean v values and the bromide-fitted D values. The R values were calculated based on the column-measured values of {theta}, {rho}, maximum pesticide breakthrough concentration, and the triclopyr and carbaryl Kf values. Pesticide R values for individual columns were calculated using thatch and soil Kf values in a volume-averaged approach where the relative length of the thatch and soil layers were used as weighing factors in calculating a mean R for each column. Simulations were repeated for all columns a second time with retardation factors being fitted so comparisons could be made of model fits using measured and fitted retardation factors. In both instances ß and {omega} were fitted for the 2SNE model.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Sorption
There were no statistical differences in the intensity of sorption (n) or in the sorption capacity (Kf) of the thatch of the two turfgrass species, thus the data were pooled and a single Freundlich isotherm was used to describe the sorption of triclopyr and carbaryl to thatch of both turfgrass species (Table 2). Triclopyr sorption was also averaged over the two soils since there were no statistically significant differences in the intensity or capacity of sorption to the two soils. A pooling procedure was not used for the carbaryl soil isotherms as the capacity and intensity factors differed for the two soils.


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  		Table 2. Freundlich sorption parameters for triclopyr and carbaryl in thatch and soil.

 
Similar sorption capacities were observed for the thatch of the two turfgrass species even though the zoysiagrass thatch organic carbon content was 31% lower than the organic carbon content of the bentgrass thatch. These differences in organic carbon content can probably be attributed to the differences in the age of the organic matter in thatch for each turfgrass species. The zoysiagrass site was established three years before the bentgrass site. Although the creation and degradation of organic matter in the thatch layer is a constantly evolving process, a more mature thatch layer would likely have a broader range and a larger content of organic carbon materials that are resistant to decay. The more degraded organic matter in the zoysiagrass thatch would also likely be a more effective sorbent than the less-degraded organic material in the bentgrass thatch (Hurto and Turgeon, 1979). Normalized sorption coefficients for each pesticide can be calculated by dividing the measured Kf values by the fraction of organic carbon present in the media. The average soil normalized sorption coefficients for triclopyr and carbaryl were 40 and 350, respectively. These values are within the range reported for each in the literature (Wauchope et al., 1992; Balogh and Anderson, 1992; Nkedi-Kizza and Brown, 1998).

Transport
Triclopyr and carbaryl leaching losses in columns containing soil and a surface layer of thatch and columns containing soil only are presented in Table 3. It should be noted that triclopyr and carbaryl losses are the result of 260 and 800 mm of simulated rainfall being applied to the columns, respectively, and represent a "worst-case" scenario in terms of the timing and amount of the hydraulic load applied. Total leaching losses of triclopyr and carbaryl were high in all columns. Columns containing a surface layer of zoysiagrass thatch had lower triclopyr and carbaryl leaching losses than the zoysiagrass site columns consisting of soil only. Similar reductions in triclopyr and carbaryl leaching losses were not observed when the bentgrass thatch plus soil and bentgrass site soil-only columns were compared. Reductions in triclopyr and carbaryl leaching losses attributable to presence of zoysiagrass thatch did not exceed 15% of the applied pesticide amount. Reduced pesticide leaching in turfs containing thatch has been attributed to the high sorptive capacity of thatch (Dell et al., 1994; Lickfeldt and Branham, 1995) and to the higher population of microorganisms found in thatch compared with soil (Mancino et al., 1993). The initiation of leaching 24 h after the application of each pesticide severely limited the role of microbial degradation in reducing the amount of triclopyr and carbaryl in the columns. Partial confirmation of this was obtained from the carbaryl samples that were analyzed by the liquid scintillation and high-performance liquid chromatography methodologies. There was good agreement between the quantity of carbaryl determined by the two methodologies (r2 > 0.94, data not shown), indicating there was little degradation of 14C carbaryl within the columns during this study. The zoysiagrass site data indicate that thatch will not substantially reduce triclopyr or carbaryl leaching when either pesticide is applied to turf shortly before a precipitation event that causes a sustained period of leaching.


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  		Table 3. Percent of triclopyr and carbaryl leached from columns containing a surface layer of thatch and columns devoid of thatch layers.

 
Channels created by earthworm burrowing were readily visible in all bentgrass site columns when the columns were sectioned at the end of the leaching phase of the study. Asymmetric distribution of the bromide breakthrough curves indicated preferential flow in all bentgrass columns (Fig. 1) . Preferential flow in the bentgrass columns was likely the result of the earthworm channels functioning as bypass flow conduits, which may explain why there were no differences observed in the amount of triclopyr and carbaryl leached from the bentgrass thatch + soil and bentgrass soil-only columns. Because physical nonequilibrium transport existed in the bentgrass columns, comparisons of the LEM and 2SNE models to predict triclopyr and carbaryl transport were limited to the zoysiagrass thatch plus soil and zoysiagrass site soil-only columns.



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  		Fig. 1. Bromide breakthrough curves for one- and two-domain flow models for individual soil columns that contained a surface layer of thatch or were devoid of thatch.

 
Peak bromide concentrations in all zoysiagrass site soil-only columns occurred near one pore volume (Fig. 1). In addition, the one domain flow model predicted bromide transport very well in zoysiagrass soil-only columns, indicating that flow was relatively homogenous within these columns. In contrast, peak bromide concentrations occurred before one pore volume in the zoysiagrass thatch plus soil columns, indicating that preferential solute transport had occurred in these columns. Consequently, improved estimates of bromide transport were obtained using the two-domain flow model for these columns.

The presence of thatch lowered the mean bulk density and increased the volumetric water content of the thatch plus soil columns when compared with the zoysiagrass soil-only columns. Higher values of {theta} and v have been shown to be correlated with increased fractions of immobile water in intact columns and may lead to conditions that favor nonequilibrium solute transport (Gaber et al., 1995). Mean pore water velocity was higher in zoysiagrass soil-only columns than in the zoysiagrass thatch plus soil columns (Table 4). Conversely, {theta} was higher and {rho} was lower in the zoysiagrass thatch plus soil columns than in the zoysiagrass soil-only columns. The contribution of hydrodynamic dispersion to solute transport is often described using Peclet numbers (P), where P = (vL/D) (Brusseau et al., 1989). Peclet numbers for the zoysiagrass thatch + soil and zoysiagrass soil-only columns were all less than 35, indicating that convective flow was the predominant avenue of water and solute transport in all columns.


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  		Table 4. Mean pore water velocity (v), Darcy flux, mean soil water content ({theta}), and mean bulk density ({rho}) of zoysiagrass site columns.

 
A graphical comparison of model estimations and measured values of triclopyr and carbaryl transport for representative zoysiagrass thatch + soil and zoysiagrass site soil-only cores is presented in Fig. 2 . The lower amplitude of the zoysiagrass thatch plus soil column BTCs compared with the BTCs for columns devoid of thatch may be attributed to the greater triclopyr and carbaryl sorptive capacities of the zoysiagrass thatch compared with soil. Observed BTCs for triclopyr and carbaryl in all zoysiagrass columns were asymmetrical and exhibited significant tailing compared with the BTCs for the nonsorbing solute, bromide (Fig. 1 and 2). These observed differences suggest that nonequilibrium processes other than two-domain flow for some columns may have been operative during triclopyr and carbaryl transport. Curve-fitted parameters for triclopyr and carbaryl transport in the zoysiagrass columns are presented in Tables 5 and 6. Because the bromide BTC data indicated two-domain flow in the zoysiagrass thatch + soil columns, only the 2SNE model was used to estimate triclopyr and carbaryl transport through these columns. If model evaluation is based solely on the coefficient of determination, the curve-fits of the 2SNE model explained 88 to 93% of the variability for triclopyr and 70 to 94% of the variability for carbaryl when laboratory-measured sorption coefficients were used to calculate the column retardation factors. Values of {omega} > 100 are generally indicative of a minimal amount of nonequilibrium transport (Gaber et al., 1995, Brusseau et al., 1989). The fact that all {omega} values were substantially less than 100 provided additional evidence that nonequilibrium processes were governing triclopyr and carbaryl transport in all columns.



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  		Fig. 2. Triclopyr and carbaryl breakthrough curves for the two-site nonequilibrium model using measured and fitted retardation factors for a soil column containing a surface layer of thatch, and for a soil column devoid of thatch.

 

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  		Table 5. Estimated transport parameters for triclopyr breakthrough curves using measured and fitted retardation factors for zoysiagrass site columns.

 

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  		Table 6. Estimated transport parameters for carbaryl breakthrough curves using measured and fitted retardation factors for zoysiagrass site columns.

 
In zoysiagrass soil columns, the predicted triclopyr and carbaryl BTCs based on the LEM model did not adequately describe the observed triclopyr and carbaryl BTCs when measured retardation factors were used (Tables 5 and 6). The LEM model gave acceptable estimates when R was fitted although fitted values of R were substantially lower than the measured R. The 2SNE model gave reasonable estimates using measured retardation factors, indicating the 2SNE model may have more accurately accounted for the processes occurring during solute transport. When R was fitted for the 2SNE model, the fitted retardation factors were also substantially lower than measured R. The fact that fitted retardation factors were always less (substantially less for carbaryl) than measured retardation factors suggests that curve-fitting retardation factors may result in values that do not agree with observed physically based phenomena. Our sorption studies were designed to mimic the sorption of triclopyr and carbaryl to thatch and soil for the experimental conditions of our transport study. Brusseau (1998) has warned against using curve-fitted values of retardation factors to describe solute transport processes in porous media. He believes the use of such an approach can lead to misinterpretation of the factors controlling solute transport.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The results of this study support the hypothesis that thatch may have significant impacts on solute transport that differ from the underlying soil through thatch's effects on hydraulic properties and/or increased solute retention. Preferential flow was observed in columns containing a surface layer of zoysiagrass thatch, but not in zoysiagrass columns without thatch. The increased porosity and higher water contents in the zoysiagrass columns containing thatch are partially responsible for the preferential flow observed in these columns. The relatively high rate of water application (1 cm h-1) used in this study also likely enhanced the contribution that preferential flow processes had on solute transport. Similar comparisons of bentgrass thatch plus soil and bentgrass site soil-only columns were not possible due to the presence of extensive earthworm burrows in all bentgrass site columns. Any sorption-based nonequilibrium process that might be occurring would be confounded with the observed physical nonequilibrium flow behavior.

The presence of thatch reduced the leaching potential of triclopyr and carbaryl applied to the zoysiagrass site columns. The {omega} values obtained when the 2SNE model was fitted for triclopyr and carbaryl BTC data indicated nonequilibrium processes were affecting the transport of triclopyr and carbaryl. Consequently, the equilibrium assumptions normally associated with use of the LEM were not valid for the description of pesticide transport that occurred in this study.

The 2SNE model gave reasonable to very good estimates of triclopyr and carbaryl transport using measured values of R. The 2SNE model predicted triclopyr transport equally well in columns containing a surface layer of zoysiagrass thatch and in zoysiagrass site columns without thatch. Coefficient of determination values for model predictions of carbaryl breakthrough were lower for the zoysiagrass thatch + soil columns than for the zoysiagrass soil-only columns. Weaker fits in the columns containing thatch were attributed to lower carbaryl levels in the leachate, which magnified sampling errors associated with the analytical technique used to measure carbaryl. Overall, the 2SNE model correctly predicted the peak concentration and tailing behavior of triclopyr and carbaryl when a volumetric averaging approach was used to account for the presence of thatch in a column. Relatively good agreement between actual and model-predicted triclopyr and carbaryl leaching losses indicates that volume-averaging thatch and laboratory-based R values can be used to satisfactorily predict the transport of pesticides applied to soils that contain thatch. These results suggest that separate representation of the thatch layer in process-based models is not a prerequisite to obtain reasonable estimates of pesticide transport under steady state flow conditions in soils that contain a surface layer of thatch.


   ACKNOWLEDGMENTS
 
The authors wish to express their thanks to the United States Golf Association Greens Section Research for providing financial assistance to conduct this research. We also wish to thank Ms. Emy Pfeil for performing the HPLC carbaryl analysis.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Part of a thesis by the senior author in partial fulfillment of the requirements for the Ph.D. degree at The University of Maryland.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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