Dicamba Transport in Turfgrass Thatch and Foliage - ACS Symposium

Chapter 13

Dicamba Transport in Turfgrass Thatch and Foliage 1

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M . J . Carroll , R. L . H i l l , S. Raturi , A . E . Herner , and E . Pfeil

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Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: December 29, 1999 | doi: 10.1021/bk-2000-0743.ch013

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Department of Natural Resources Sciences and Landscape Architecture, University of Maryland, College Park, M D 20742 Environmental Chemistry Laboratory, U. S. Department of Agriculture/NRI, BARC-East, Beltsville, M D 20705

The retention of dicamba to turfgrass foliage and thatch, and the transport of dicamba in undisturbed columns of soil and soil+thatch were examined in three separate studies. The use of linear equilibrium (LEM) and two site non-equilibrium (2SNE) models to predict the transport of dicamba in the columns of soil and soil+thatch were also investigated. Rainfall that occurred during the first 8 hours following foliar application removed about 70% of the dicamba present on the foliage. Thereafter, dicamba became increasing resistant to washoff. The high concentration of organic matter present in columns containing a surface layer of thatch reduced dicamba leaching. Although dicamba sorption to thatch was higher than to soil, when normalized for the presence of organic matter (K ), thatch organic matter was a less effective sorbant of dicamba than was soil organic matter. The 2SNE model gave reasonable estimates of dicamba transport when the model retardation factor (R) was calculated using laboratory derived adsorption coefficients. The L E M model satisfactorily described dicamba transport only when R was curve-fitted. Use of column R's based on laboratory derived adsorption coefficients resulted in poor L E M estimates of dicamba transport. OC

Pesticides applied to mature turf move into the soil only after being washed off foliage and moving through turfgrass thatch. Any attempt to predict the movement of pesticides applied to turf requires that the retention characteristics of the pesticide to foliage and thatch be known. Pesticide movement from foliage to underlying porous media layers is usually modeled using foliar washoff algorithms (/). The use of foliar washoff algorithms requires that accurate estimates of the fraction of applied pesticide that is deposited on the foliage, and the fraction of pesticide removed from the foliage as

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© 2000

American Chemical Society

Clark and Kenna; Fate and Management of Turfgrass Chemicals ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


229 a function of rainfall be known. In the case of the latter, the amount of time elapsed between pesticide application and the first rainfall event can significantly affect the fraction of pesticide transferred from foliage to the thatch layer (2). Thatch has a pore space distribution similar to that of a coarse sand and a chemical composition resembling a young organic soil (3). For these reasons it is more analogous to a porous media which supports plant growth than a surface mulch. Pesticide transport in thatch and soil can be modeled using the convectiondispersion equation. Alternate forms of this equation can be used to describe instantaneous or kbetically-driven sorption, or alternately, two domain flow phenomena within the media. Regulatory agencies often use the linear equilibrium model (LEM) that assumes 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 users of the L E M account 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 (4). Since the impact of the thatch layer on pesticide sorption and subsequent transport is not specifically considered when such as approach is used, it is felt the assumption of instantaneous sorption used in L E M ' s may lead to inaccurate predictions of pesticide transport in turfgrass that contains thatch. Sorption of pesticides to a highly organic media such as thatch is presently believed to be a two stage process (5). Thefirststage consists of direct pesticide sorption to the external organic matter surface sites (class 1 sites), whereas the second stage consists of pesticide sorption to sites located within the pores or fissures of organic matter aggregates (class 2 sites). The latter process involves diffusive mass transfer of the pesticide which is highly dependent on the residence time of the solution containing the pesticide. A two-site non-equilibrium model (2SNE) which considers that transfer to some sorption sites is diffusion mediated may be more appropriate than a L E M model when predicting pesticide transport through a soil with a thatch surface layer (6). Dicamba (3,6-dichloro-2-methoxy benzoic acid) is one of the most commonly used herbicides for postemergence control of broadleaf weeds in turf. It has a pKa of 1.95 (7) and is soluble in water (4500 mg L' ). Due to its dissociated anionic character, it is minimally sorbed to most soils (8). Several studies have shown dicamba is readily transported below the rootzone of cool season turfgrass species when high amounts of rainfall or irrigation occur within two days of application (9, 10). The high organic matter content of thatch allows this media to readily sorb non-polar compounds (11,12,13). Less is known, however about the sorptive properties of thatch for ionizable compounds like dicamba. The objectives of our research were to comprehensively address through integrated studies: 1) the effect of residence time on the washoff of dicamba from turfgrass foliage; 2) dicamba sorption to turfgrass thatch and the soil; 3) the effect of thatch on the transport of dicamba through columns containing a surface layer of thatch and columns devoid of thatch and ; 4) the use of L E M and 2SNE models to predict the transport of dicamba through columns containing a surface layer of thatch and columns devoid of thatch. 1


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Materials and Methods Foliar washoff. A 10-month old stand of'Southshore' creeping bentgrass (Agrostis palustris Huds.) was utilized for the foliar washoff study. Prior to the study, the turf was irrigated and fertilized as needed to sustain vigorous growth. The turf was mowed three times a week at 1.6 cm to promote the formation of a dense foliar canopy similar to that found on a golf course fairway. The study area was divided into four strips and each strip subdivided into five 2.1by 1.8-m plots. Individual strips were separated by 1.8-m alleys and the borders of adjacent plots within each strip were placed 2.1-m apart. Dicamba (Banvel 4S) was applied at 0.53 kg ai ha' in 393 L of water on 7 Aug, 1995. The strips were sprayed in a sequential fashion with one strip being sprayed every hour. All plots within a strip were sprayed within 5 minutes of one another. Spray was applied with a C0 -pressurized sprayer equipped with six, 6501 Teejet nozzles. Nozzles were located 0.38 m above the turf surface and were spaced 0.27 m apart. A l l strips were sprayed in calm wind conditions between 800 and 1100 hr with a delivery pressure of 245 kPa. Stationary calibration of the six nozzles prior to spraying indicated individual nozzle delivery volume coefficient of variation was 3.4% for the sprayer operating conditions stated. The entire study area was mowed at 1.6 cm with a triplex mower 1 hr before spraying the first strip. Individual plots were exposed to a 4.9 cm hr" rainstorm for a 40 minute duration at 1, 8,24 and 72 hours after spraying the plots with dicamba. A rainfall simulator that produced drop size distributions and impact velocities similar to natural rainfall was used to apply the precipitation (14). Dicamba washoff was determined by collecting foliage from adjacent strips of turf within plots that were mowed immediately before and shortly after the simulated rainfall. In the case of the latter, the turf was allowed to dry for 1 hr before being mowed. The 'before* and 'after' foliage samples were collected using a 51cm wide front throw mower set at 0.9 cm and samples placed into sealed zip-lock bags. Thefreshweight of the foliage was determined immediately after harvest and the samples quickly placed into a freezer maintained at -20 °C. One plot within each of the four strips sprayed with dicamba was not subjected to simulated rainfall. Separate 'before' and 'after' foliage samples were collected from these plots within 20 minutes of spraying. The 'before' and 'after' samples from these plots were used to evaluate the uniformity of the dicamba spray application and to determine the initial levels of dicamba on the foliage. Foliage samples were analyzed for dicamba using procedures and equipment specifications discussed elsewhere (15). Dicamba extracted from the foliage was analyzed by gradient H P L C - U V analysis. Extraction efficiency was determined by adding known amounts o f analytical grade dicamba to unsprayed foliage samples. Dicamba freezer storage stability was evaluated by adding 1.0 mL of the field dicamba tank mix to 20 gm of unsprayed foliage. Mean foliar extraction recovery and freezer storage recovery levels for dicamba were 118% + 4% (n =13) and 76 + 14% (n = 4), respectively.


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Thatch and soil sample collection. Soil and turfgrass thatch samples were collected from a three-year old stand of Meyer zoysiagrass (Zoysia japonica Steud.) at the University of Maryland Turfgrass Research and Education Facility. The zoysiagrass was established and maintained without the use of dicamba and contained a 1.5 to 2.0-cm


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thick thatch layer (16). The upper half of the thatch layer consisted of non-decomposed and partially decomposed plant biomass. The rest of the layer was comprised of well composed plant biomass intermixed with the underlying soil. The soil beneath the thatch layer was classified as a Sassafras loamy sand (fine loamy, mixed, mesic, Typic Hapludult). The soil contained 82% sand, 10 % silt, 8% clay and had a cation exchange capacity of 4.2 cmol kg". Thatch and soil pH and organic matter content are presented in Table 1. The thatch and soil used in the isotherm study were removed from thefieldusing a 10-cm diameter golf green cup cutter. Ten centimeter deep cores containing a surface layer of thatch and mat were brought into the laboratory and the thatch and mat separated from the underlying soil using hand shears. The thatch+mat and soil were sieved by hand to pass through a 4-mm screen to homogenize each material. The thatch+mat and soil were then placed into separate plastic bags and the sealed samples stored at 4 °C until needed. In the leaching study, undisturbed soil, and soil plus thatch columns, 12-cm Igth. by 10-cm diam., were extracted from the surface 12 cm of the zoysiagrass site using a specially designed drop hammer-sleeve assembly. The columns containing soil only were obtained after removing all above ground thatch and foliage. The columns were transported to the laboratory immediately after collection.


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Sorption isotherm. Sorption isotherms were determined using a mechanical vacuum extractor. This device controls the rate a which a solution moves through a column of thatch or soil. The columns are created by packing known amounts of media into syringe tube barrels. Since the sample is not shaken during the procedure little disruption of the media aggregates and organic matter occurs.Moreover, theflowingconditions used in this modified batch/flow technique better represent the physiochemical interactions that occur in thefield.A detailed description of this technique is presented elsewhere (17). In this study,field-moistsamples having oven dry weights equivalent to 6.7-g thatch, or 13-g soil, were added to syringe tube barrels after placing a 2.0-cm length by 2.5-cm diameter foam plug (Baxter Healthcare Corp., McGaw Park, IL, DiSPo PLUGS# Τ1385) into the bottom of the barrel. The thatch was gently tamped to a bulk density of 0.67 Mg m", and the soil to a bulk density of 1.30 Mg m*, before placing a second foam plug over the top of the sample to ensure even distribution of the solution as it entered each sample. The presence of the foam plugs also prevented channeling between the syringe tube wall and the sample. The syringe tube barrels were 2.5-cm in diameter and held 60 mL of solution. A combination of commercially formulated dicamba and ring-labeled C dicamba were used in this study. The ring-labeled C dicamba (98% purity) was obtained from Sigma Chemical Co., St. Louis, MO., and had a specific activity of 16.65 χ 10 Bq mmol" . The commercial product of dicamba (Banvel) was formulated with 480 g L' dicamba. Separate stock solutions of commercially formulated dicamba and C dicamba were created by dissolving each material in distilled-deionized water. Dicamba solution concentrations of 1,4,8,10, and 20 mg L* , each of which contained 0.3 mg L" , or 2.31 χ 10 Bq L" , of C dicamba, were made by adding appropriate amounts of each stock solution to known amounts of distilled-deionized water. Sorption isotherms were determined by leaching 30 ml of 1, 4, 8, 10, and 20 mg dicamba L" through samples of thatch and soil for 24 hrs. One milliliter of the leachate 3

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from each sample was added to a 20-mL scintillation vial that contained 5 mL of Scintiverse II (Fisher Scientific Co., Fair Lawn, NJ) scintillation cocktail. The radioactivity of the resulting sample was determined by liquid scintillation counting using a Beckman LS 5800 series (Beckman Instruments, Inc., Fullerton, CA) liquid scintillation counter. Dicamba sorption to any material other than thatch or soil was taken into account by including syringe tube blanks. The blanks were identical to the syringe tubes containing thatch and soil except that they contained no thatch or soil. The amount of dicamba sorbed at 24 hrs, x/m, (mg kg* ) was determined from the formula ( C C ) V / M , where C i is the initial concentration of the solution added to a sample (mg L* ); C the dicamba concentration of the leachate (mg L" ); V , the volume of solution added to thatch or soil; and M , the thatch or soil dry weight (kg). Dicamba sorption was fitted to the linear form of the Freundlich equation; 1

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log (x/m) = log (K ) + 1/n log (c),

(1)

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where x/m is the surface concentration of the sorbate per unit amount of sorbent (mg kg" ), c is the sorbate equilibrium solution concentration (mg L" ) and K and 1/n are constants that characterize the dicamba sorption capacity. Regression analyses were used to calculate 1/n and K . 1

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Leaching. Columns collected in the field were brought to the laboratory and saturated from the bottom. The bottom end of each saturated column was trimmed and placed into a funnel containing a 12-μηι pore diameter saturated, porous, stainless steel plate which was made vacuum tight. The column and funnel were then inserted into one port of a multi-port vacuum chamber. A null balance vacuum regulator was used to maintain a pressure of -10 Kpa within the vacuum chamber at the base of each column. A 0.001 M CaCl solution was applied to each column using a specially designed drop emitter (J 8) that uniformly distributed solution to the surface of each column. Leachate was collected in 400 mL sterile polyethylene cups located beneath the funnel of each column within the vacuum chamber. After steady-state flow conditions were achieved (0.85 cm hr* ), 10 mL of 300 mg bromide L" was surface-applied to each column. Leachate was then collected every half an hour for the next 12 hours and the bromide concentration of the leachate determined by ion chromatography. After this initial leaching period, 10 mL of 40 mg dicamba L" was uniformly added to the surface of each column. The dicamba solution consisted of 2.31 χ 10 Bq L" , of C ring-labeled dicamba and 39.7 mg L" of commercially formulated dicamba. After adding dicamba, leaching solution inputs and the vacuum applied to the base of each column were discontinued for 24 hr to permit sorption of dicamba to the thatch and soil. During this time all columns were covered with plastic wrap to prevent volatile dicamba losses. After the 24 hr sorption period, the leaching process resumed. Sampling continued every half hour for 6 hr and then, once every one hour for the next 12 hours with dicamba in the leachate being determined using the liquid scintillation techniques described previously. After collecting the last leachate sample, thatch layer thickness was measured before being separated from the soil, and the moisture contents of the thatch and soil layers gravimetrically determined. Sub-samples were stored in the freezer until the amount 2

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of dicamba remaining in each sample could be determined. Thawed thatch and soil subsamples were first shaken for 2 hr in a 50:50 water and methanol solution. The resulting slurry was then subjected to vacuum filtration and the filtrate analyzed for C . The amount of C remaining in the sample was determined through combustion using a biological material oxidizer with the amount of C evolved measured by liquid scintillation. The amount of C removed by the water and methanol solution was considered to represent the easily extractable fraction of dicamba and dicamba metabolites present in the sample at the end of the leaching event. The amount removed by combustion was considered to be the tightly bound fraction of dicamba and dicamba metabolites remaining in the sample. 14

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Estimating Transport Parameters from Breakthrough Curves (BTC's) Theoretical background. The one dimensional convection dispersion equation (CDE) for steady-state transport of a solute through homogeneous soil is (19): 2

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R*ôC/ôt - D*(ô C/ôt ) - ν * (ôC/δχ)

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Where C is solution-phase solute concentration ^ g cm" ), t is time (h), D is the hydrodynamic dispersion coefficient (cm h" ), R is the retardation factor (dimensionless), χ is distance from solute origin (cm), and ν is the average pore water velocity (cm h" ). The R term reduces to one for non-reactive solutes and is greater than 1 when solute retention occurs. The retardation factor is defined as (20): 2

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R=l+[pK (l/n)c

(l/n4)

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/e]

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where ρ is the soil bulk density (g cm* ) θ is the volumetric water content (cm cm* ), and Kf, c and 1/n are as previously defined. 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 pesticide adsorbed. This mathematical approach is called linear equilibrium sorption. Where bimodal porosity leads to two-region flow, or situations where the sorption process is controlled by two-site kinetic non-equilibrium sorption processes, nonequilibrium models may more accurately describe the transport of pesticides through soil. Chemical non-equilibrium models consider adsorption on some, of the sorption sites to be instantaneous, while sorption on the remaining sites is governed by first-order kinetics (21). The two-site chemical non-equilibrium model (2SNE) conceptually divides the porous medium into two sorption sites: type-1 sites assume equilibrium sorption and type2 sites assume sorption processes as afirst-orderkinetic reaction (6). In contrast, physical non-equilibrium is often modeled by using a two-region dual-porosity type formulation. 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 non-equilibrium CDE, however, they can be put into the same dimensionless form (2SNE) for conditions of linear adsorption and steady-state water flow (22):


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p*R*6C /ôt = l/P*(ô C /ôx ) - 6C,/ôx - c ^ Q - C ^

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(l-p)»R*ô(Vôt « (ùiC, - Q - μ Α + γ ( χ )

(5)

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Where the subscripts 1 and 2 refer to equilibrium and non-equilibrium sites, respectively, β is a partitioning coefficient, ω is a dimensionless mass transfer coefficient, Ρ is the Peclet number; and μ (h* ) and γ (ug h" ) define first-order decay and zero-order production terms, respectively, each represented in component contributions of both the liquid and solid phases. Customarily β and ω are obtained by fitting solute B T C ' s to the non-equilibrium model using a non-linear least squares minimization technique (23). The values of β and ω obtained from the BTC's of non-interacting solutes can be used to evaluate the potential contributions from two-region flow. In the absence of two-region flow, β and ω may be used to evaluate the contributions from two-site kinetic non-equilibrium sorption (24). For interacting solutes, β represents the fraction of instantaneous solute retardation in two-site non-equilibrium model and ω the ratio of hydrodynamic residence time to characteristic time for sorption. They are equivalent to:


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β = ( θ + / Κ ) / ( θ + ρΚ)

(6)

Dicamba Transport in Turfgrass Thatch and Foliage - ACS Symposium

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