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Published online 5 April 2007
Published in J Environ Qual 36:613-620 (2007)
DOI: 10.2134/jeq2006.0258
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Measuring Flux of Soil Fumigants Using the Aerodynamic and Dynamic Flux Chamber Methods

I. J. van Wesenbeeck*, J. A. Knuteson, D. E. Barnekow and A. M. Phillips

Dow AgroSciences, 9330 Zionsville Rd., Building 306/A2, Indianapolis, IN 46268

* Corresponding author (ijvanwesenbeeck{at}dow.com)

Received for publication July 2, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS AND MATERIALS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Methods for measuring and estimating flux density of soil fumigants under field conditions are important for the purpose of providing inputs to air dispersion models and for comparing the effects of management practices on emission reduction. The objective of this study was to measure the flux of 1,3-dichloropropene (1,3-D) and chloropicrin at a site in Georgia (GA) using the aerodynamic method and the dynamic flux chamber (FC) method. A secondary objective was to compare the effects of high density polyethylene (HDPE), and virtually impermeable film (VIF) tarps on fumigant flux at a site in Florida (FL). Chloropicrin and 1,3-D were applied by surface drip application of In-Line soil fumigant on vegetable beds covered by low density polyethylene (LDPE), HDPE, or VIF. The surface drip fumigation using In-Line and LDPE tarp employed in this study resulted in volatilization of 26.5% of applied 1,3-D and 11.2% of the applied chloropicrin at the GA site, as determined using the aerodynamic method. Estimates of mass loss obtained from dynamic FCs were 23.6% for 1,3-D and 18.0% for chloropicrin at the GA site. Flux chamber trials at the FL site indicate significant additional reduction in flux density, and cumulative mass loss when VIF tarp is used. This study supports the use of dynamic FCs as a valuable tool for estimating gas flux density from agricultural soils, and evaluating best management practices for reducing fumigant emissions to the atmosphere.

Abbreviations: COB, center of bed • DAT, days after treatment • EOB, edge of bed • FC, flux chamber • HDPE, high density polyethylene film • LDPE, low density polyethylene film • LOD, limit of detection • LOQ, limit of quantification • VIF, virtually impermeable film


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS AND MATERIALS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
THE characterization of the fate of volatile agricultural chemicals from field soils is important to determine the potential for air pollution, and for attaining mass balance in large-scale field dissipation studies of volatile agricultural chemicals such as soil fumigants. Field studies of gas flux from soils have included N2O–N flux (Valente et al., 1995), methyl bromide (Gao et al., 1997; Yates et al., 1997; Wang et al., 1998a), and 1,3-dichloropropene (Wang et al., 1999, 2000). Methods used to characterize the mass loss and flux of a gas from a field soil include passive or static (closed) flux chambers (FCs) (Rolston, 1986; Yagi et al., 1993), dynamic (flow-through) FCs (Woodrow and Seiber, 1991; Gao et al., 1997), and larger scale measurements using micrometeorological measurements such as the aerodynamic method (Parmele et al., 1972; Majewski et al., 1989; Ross et al., 1990).

Extensive work has been conducted comparing passive and active FCs (Valente et al., 1995) and comparing FC data to model predicted results of gas flux (Gao et al., 1997; Wang et al., 2000). Currently there is little data comparing gas fluxes from dynamic FCs and larger scale measurements of gas flux using the aerodynamic method or other field scale methods, although studies by Christensen et al. (1996) and Yates et al. (1997) showed good agreement between the methods. Another study by Yates (2006) attributed underprediction of herbicide volatilization by FCs, compared with larger scale field methods, to the effect of the chamber on temperature, water evaporation, and fumigant gas concentration gradients near the soil surface.

The purpose of this study is to (i) compare dynamic FC measurements of soil fumigant gas flux density (field volatilization) to field scale measurements of gas flux density and volatilization made using the aerodynamic method on the same field and (ii) obtain information on the flux density of surface drip applied 1,3-dichloropropene and chloropicrin under low density polyethylene (LDPE) tarp, high density polyethylene (HDPE) tarp, and under virtually impermeable film (VIF). 1,3-dichloropropene (1,3-D) and chloropicrin are the active ingredients in the soil fumigant In-Line which contains about 60% 1,3-D and 33% chloropicrin as an emulsion.


   METHODS AND MATERIALS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS AND MATERIALS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Test Sites
The Georgia (GA) test site was located in Coffee County, approximately 8 km northeast of Douglas, GA (31°34.07' N and 82°45.45' W). The test plot is located on a Fuquay series loamy sand soil (loamy, kaolinitic, thermic Arenic Plinthic Kandiudults) in an area with gentle (1–5%) slopes. The Fuquay series consists of well-drained soils that have moderate permeability in the upper part of the subsoil and slow permeability in the lower part. These soils are formed in the sandy and loamy marine sediments of the Southern Coastal Plain (Rigdon and Green, 1988). Soil characterization of the test plot showed that the soil was a sand from 0 to 45 cm, and a sandy loam to loamy sand from 45 to 90 cm. Soil organic matter ranged from 0.5 to 0.6% near the surface to <0.2% below 45 cm. Soil pH ranged from 6.9 to 7.1 at all depths. A 4.33-ha fallow field was prepared by forming 81.3-cm beds on 152.4-cm centers and placing drip tape on the soil surface on the center of the beds and then covering the beds and drip tape with 1.25 mil black LDPE tarp. The area of the treated beds and furrow areas was 3.29 ha when access roadways between each group of six beds were taken into account. The actual area of treated beds was 1.75 ha. Flux chambers were placed on tarped beds and in furrows near the north end of the plot.

The Florida (FL) test site was located in Lee County, in southern FL, approximately 32 km southeast of Fort Myers. This field is in Section 11, of Township 47 S., Range 26 E., and has an approximate elevation of 5.5 m above mean sea level. The major soil unit found in the area of the study site is a Pineda series fine sand (loamy, siliceous, hyperthermic Arenic Glossaqualfs). The Pineda series are deep, poorly drained, slowly permeable soils that formed in thick beds of sandy and loamy marine sediments. Under normal conditions, the water table is within 25 cm of the surface for 2 to 4 mo, and from 25 to 100 cm for more than 6 mo (Henderson, 1984). A 2.6-ha field was prepared with 1.8-m center-to-center beds approximately 30 cm high, and tapered from a base width of 91 cm to a top width of 76 cm. The drip tape was attached to the permanent irrigation line buried in the centerline of the field, and the row ends of the tape were tied and buried to prevent surface leakage. Black 0.75 mil HDPE tarp was stretched over the beds as they were formed. An approximately 10-m length of two adjacent beds were manually tarped with Hytibarrier VIF, a 35-µm-thick three-layer, LDPE-polyamide-LDPE tarp, for comparison using FCs. The FCs were placed on the HDPE-tarped beds and VIF-tarped beds in one corner of the treated field.

Properties and Environmental Chemistry of 1,3-Dichloropropene
The soil fumigant 1,3-D is a liquid at ambient temperatures, and consists of two isomers, cis- and trans-1,3-D. Basic physical/chemical properties are shown in Table 1. Degradation of 1,3-D is by hydrolysis, soil microbial metabolism, and other processes in the soil environment and thus dissipates rapidly in soil. Van Dijk (1974, 1980) studied the soil metabolism of 1,3-D under aerobic incubated conditions for soils of different texture classes. Average half-lives of 6 and 17 d were reported for clay and sand textures, respectively, when incubated at 20°C. The degradation of 1,3-D in aerobic soils has been reported with half-lives ranging from 2 d on silty clay to about 6 d on clay and 17 d on sand at 20°C (Ruzo, 2006). Batzer et al. (1997) observed aerobic soil laboratory half-lives of 11.5 and 53.9 d for 1,3-D on a Catlin silt loam and a Fuquay loamy sand, respectively, the latter being a very dry soil. McCall (1987) reported average half-lives of 2.4 and 8.4 d at 25 and 15°C, respectively, for two anaerobic soils. The hydrolysis half-life of 1,3-D is 11 d at 20°C in sterilized systems; the rate of reaction was independent of pH within the range 5 to 9, but dependent on temperature (Ruzo, 2006). Van Dijk (1974) found an 11-d half-life at 15°C in a study of unsterilized buffer systems. Although 1,3-D does not adsorb solar radiation at wavelengths greater than 290 nm, naturally occurring atmospheric activators can rapidly degrade this material. Tuazon et al. (1984) estimated 1,3-dichloropropene half-life of cis and trans isomers to be 7 to 12 hr, respectively, when exposed to ambient daytime OH radical concentrations of about 2 x 106 molecules cm–3.


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  		Table 1. Physical/chemical properties of 1,3-dichloropropene and chloropicrin.

 
Properties and Environmental Chemistry of Chloropicrin
Chloropicrin is used as a soil fumigant by itself, or in combination with other soil fumigants such as 1,3-D or methyl bromide. The physical/chemical properties of chloropicrin are shown in Table 1. Chloropicrin does not hydrolyze significantly at neutral pH, but in the presence of light undergoes photohydrolysis (Castro and Belser, 1981). In the vapor phase, chloropicrin photolyzes rapidly to phosgene, which under field conditions ultimately converts to carbon dioxide in the presence of moisture. When exposed to a xenon light source neutral aqueous solutions of chloropicrin degraded quantitatively to carbon dioxide with a half-life of 31 h (Wilhelm et al., 1996). Chloropicrin undergoes rapid aerobic metabolism in soils, with half-lives of 0.2 to 4.5 d (Ruzo, 2006, Gan et al., 2000). A detailed review of the chemical and environmental fate properties of chloropicrin is reported by Ruzo (2006).

Meteorological Equipment Installation
On-site monitoring of weather at the GA site began on 6 Dec. 1999 using a Campbell 21X datalogger (Campbell Scientific, Logan UT) and sensors for soil temperature at 2.5-, 10-, and 20-cm depths, air temperature (Campbell 107 thermistor probes) and barometric pressure at 1.5-m height (SBP270 sensor, Setra Electronics, Plainville, CT), precipitation, total solar radiation (LI-200S Silicon Pyranometer, Li-Cor, Inc., Lincoln, NE), and wind speed and direction (03001-5 Wind Sentry, R.M. Young, Inc., Traverse City, MI) at the 2.5-m height. An additional datalogger was placed on the center of the test plot on a bed, and collected wind speed and temperature at the 15-, 33-, 55-, and 90-cm heights above ground level and relative humidity (HMP35C probe, Vaisala, Inc., San Jose, CA) at 33 and 90 cm. Data measurements were made once each second, and data was summarized for output each hour and downloaded automatically to a removable solid-state data-storage module.

Air Sampling for Aerodynamic Method
At the GA site, 10 air samplers were located on the mast in the center of the treated plot, at heights of 15, 33, 55, 90, and 150 cm above the land surface for measuring both 1,3-D and chloropicrin (five each). Ambient air was pulled at relatively low flow rates (~1.5 L min–1 for 1,3-D and 0.05 L min–1 for chloropicrin) through sampling tubes (containing charcoal for trapping 1,3-D) or XAD-4 (containing polystyrene resin for trapping chloropicrin). In a typical period, a fresh sample tube was inserted into the flexible tubing receptacle and start times were recorded and the flow rate was checked. At the end of the sample period, the pump flow rate was again checked with a flow meter, and the value and time recorded, and the sample tube was removed and placed into a portable cooler. Samples were placed in a freezer near the site and subsequently shipped with dry ice via air express to Dow AgroSciences, IN, for analysis.

Pre-application air monitoring was conducted a day before the application for a period of approximately 6 h, indicating that there was no measurable background 1,3-D or chloropicrin in the area. Samplers were changed at about 0700, 1200, and at about 1700 h. The first sample period for air monitors started at about 1200 h on 6 Dec. 1999. These operated on a 5 h-5 h-14 h schedule for the first 7 d of the study, then on a 24-h schedule until the last sample was removed at approximately 0700 h on 20 Dec. 1999. The period from 1200 to 1700 h on 6 December was designated as Period 2. Sampling periods were numbered sequentially from that period until the end of the study. The In-Line application spanned all of Period 2. The 24-h schedule for the off-site samplers after 7 d after treatment (DAT) was implemented to expose the sample tubes for a longer period of time and hence reduce the effective limit of detection (LOD) for cis- and trans-1,3-D and chloropicrin.

Air Sampling for Flux Chamber Method
Dynamic FCs with a base area of 0.41 x 0.41 m (0.168 m2) were constructed from galvanized sheet metal as shown in Fig. 1 and similar to the design of Gao et al. (1997) but modified by the addition of automated sample collection, and on-board datalogging of air flow rates, temperature, and pressure. The FCs were about 8 cm high and were fitted with two fans to draw air at a constant flow rate of approximately 20 L min–1 through the FC. The FC had an environmental enclosure mounted on the top to house a Campbell CR21X datalogger, an SKC personal pump (SKC, Eighty Four, PA), a solenoid valve and sample tube manifold, three air flow meters, and two thermocouples. One of the flow meters measured air volume flowing through the FC, one measured flow through the SKC pump used for chloropicrin sampling, and another measured flow through the SKC pump used for the 1,3-D sampling.


Figure 1
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  		Fig. 1. Cross-section of flux chambers placed on the bed and in the furrow.

 
At the GA field site, FCs (FC#2 and #5) were placed over the center of beds on top of the LDPE tarp, two were placed over the center of furrows on the soil (FC#3 and #4), and one was placed on the center of a bed where the LDPE tarp had been removed (FC #1) so that the chamber was placed directly on the soil. At the FL site, FC#1, FC#3, and FC#5 were placed over the center of beds covered with the HDPE tarp. Flux chambers FC#2 and FC#4 were placed over the center of beds covered with VIF film. Flux from furrows was not measured in FL. The base plates of FCs placed on the tarped beds were sealed to the tarp with silicone caulk and duct tape to ensure no ambient air would enter the FC. The base plate of the FCs in the furrows had a 2.5-cm flange that was inserted into the soil to form a good seal. The inlet end of the FCs were attached to 5-cm diam. smooth plastic tubing, the other end of which was attached to a 3-m stack to draw air relatively free of 1,3-D or chloropicrin through the FC.

The SKC pump was connected to a sample tube manifold containing electric solenoid driven gas valves that were controlled by the CR21X datalogger. The manifold could hold three sample tubes for each of 1,3-D and chloropicrin and the datalogger was programmed to switch from one set of tubes (1,3-D and chloropicrin) to another following the same 5/5/14-h intervals that were used for the aerodynamic method at the GA site.

At the FL site, FC air sampling was split into three separate sampling periods; 0700 to 1300 h, 1300 to 2000 h, and 2000 to 0700 h, or a 6/7/11-h sampling scheme. These time periods best represented the periods between sunrise, solar noon, and sunset during the study. Flux chamber sampling began at approximately 1300 h on 25 August, and ended at 0700 h on 1 September, for a total of 20 sample periods.

Theory of Flux Density Measurement
Aerodynamic Method
The field scale flux measurement technique employed was the aerodynamic (AD) method (Parmele et al., 1972; Majewski et al., 1989; Majewski et al., 1990; Ross et al., 1990; Majewski et al., 1991). The AD method is a modified form of the Thornthwaite-Holzman (TH) equation (Thornthwaite and Holzman, 1939), a gradient method that is based on the log-law of the wind speed profile. The TH equation is valid only during adiabatic atmospheric conditions (i.e., neutral stability). Practically, that is when the near surface air temperature is the same at several heights above ground level. Neutral stability normally occurs for periods during sunrise and sunset. In flux studies, the lower boundary layer generally refers to the lower 2 m of the atmosphere, although the meteorological boundary layer concepts being used are valid to 20 m or more. A basic assumption of the AD method is that the flux density is the same throughout the boundary layer, so that one may estimate a flux through a discrete horizontal plane above the surface (assuming adequate fetch) and the estimated value is also valid at the surface-air interface.

The modified TH equation used here is described by Majewski et al. (1991) to compensate for conditions that cause the atmosphere to be stable or unstable. The modifications are two coefficients, {phi}m and {phi}c, in the denominator of the AD flux equation:

Formula 1[1]
where P{uparrow} is the vertical pesticide flux (mg m–2 h–1), k is von Kármán's constant (dimensionless, {approx} 0.4), {Delta}c is the difference of average air concentration of the analyte (mg m–3) at heights z1 and z2, and {Delta}u is the difference of average horizontal wind speed (m s–1) at heights z2 and z1. The coefficient {phi}m arises from the fact that the horizontal movement of wind over a stationary surface results in a downward transfer of momentum (mass x velocity) to the surface. Empirical functions, {phi}m and {phi}c, have been developed to correct the TH equation for non-neutral conditions through use of the relationship between momentum and wind speed profiles (Majewski et al., 1991). They assumed that {phi}c is equal to the correction factor for water vapor. For unstable conditions (Ri < 0),

Formula 2[2]
For stable conditions (Ri > 0),

Formula 3[3]
where Ri is the Richardson gradient number and relates the counteracting effects of air buoyancy effects from temperature and wind speed gradients that generate turbulence, in a dimensionless form, given by the equation:

Formula 4[4]
where g is the acceleration due to gravity; {Delta}T, {Delta}z, and {Delta}u are the change in temperature, height, and wind speed, respectively; and Tave is the geometric mean of temperatures (°K) at the heights for {Delta}T. Flux density, in mg m–2 h–1, was calculated by the aerodynamic flux equation.

The mass of 1,3-D or chloropicrin volatilized (mg) for each period was calculated by multiplying the area of the treated field by the flux density for each measurement period.

Flow-through Flux Chamber Method
Dynamic or flow-through FCs are widely used, convenient tools for field measurements of gas fluxes, including soil fumigants, from soils to the atmosphere (Rolston, 1986; Gao et al., 1997; Reichman and Rolston, 2002; Yates, 2006). The method involves placing an open-bottom chamber on the soil surface, introducing an air flow through the chamber from the inlet to the outlet, and measuring the target gas concentrations in the incoming air and the outgoing air (Fig. 1). When it is assumed that the chamber is operating under steady state conditions (i.e., constant air flow), the gas flux is uniform over the entire covered surface and relatively constant during the sampling interval, the incoming stream and the outgoing stream are well mixed (i.e., the inlet and outlet concentrations, Cin and Cout are representative) and the diffusive flux is dominant, and the advective mass flow is negligible, then the mass flux can be calculated as:

Formula 5[5]
where J is the mass flux of the target gas (mg m–2 h–1), Q is the constant flow rate of air through the chamber (m3 h–1), and A is the enclosed soil surface area (m2). Concentrations Cin and Cout are measured in a time interval (t2 – t1). A weighted average of the flux density measured by the two FCs placed on the tarped beds (bed sidewall flux assumed to equal top of bed flux) and the two placed in the furrows was determined by weighting the measured flux by the relative width of the bed (including sidewalls) and furrows.

Application of In-Line
At the GA site In-Line (60.2% 1,3-D, 33% chloropicrin) was injected through the drip irrigation system at a rate of 230 L ha–1 for the treated bed, or 93.5 L ha–1 of field area on 6 Dec. 1999, at a concentration of 1.36 g L–1 of In-Line in water. The drip irrigation system was charged with water for approximately 3 h before the injection of In-Line. In-Line injection was completed after approximately 4.5 h and water was pumped for an additional 30 min to flush remaining In-Line from the system. Assuming a density of 1.34 kg L–1 for In-Line, the mass of 1,3-D applied to the test plot was 326.3 kg and the mass of chloropicrin applied was 178.9 kg.

At the FL site, application of the test material occurred on 25 Aug. 2000. The application started at 1326 h, and continued until 2015 h. A total of 547.9 kg, or 408.9 L of test material was used. The treated bed area was a measured 2.2 ha, compared with a total field area of 2.67 ha. Thus, the application rate was 188.7 L ha–1 treated bed, or 153.1 L ha–1 based on total field area.

Uniformity of 1,3-Dichloropropene and Chloropicrin Application
Spatial and temporal uniformity of the applications were determined by taking samples from the ends of eight randomly selected emitter lines (four from each end of the field) and analyzing for the presence of 1,3-D and chloropicrin. At the GA site, samples of the mixture of irrigation water and In-Line were taken from the ends of four randomly selected drip lines in each half of the treated field at approximately 100 and 200 min after the start of irrigation and were analyzed for 1,3-D and chloropicrin (n = 16). At the FL site, eight samples of the test material and irrigation water mixture were collected from the ends of selected drip irrigation lines at approximately 120 and 240 min after application began.

Soil Temperature, Gas, and Water Content
Soil temperature was measured using copper-constantan thermocouples installed at depths of 15, 30, 45, 60, and 75 cm below the center of bed (COB) and at depth of 15, 30, and 45 cm below the edge of bed (EOB). The probes were monitored every 15 min and the data stored on a datalogger.

Soil gas samples were taken using dedicated soil gas probes at depths of 15 and 60 cm below the COB and at depths of 15 and 30 cm below the center of the furrow, resulting in a total of four soil gas measurements for each time point. The samples were taken daily from 1 through 13 DAT.

Analytical Methods and Quality Control
Charcoal tubes were analyzed for 1,3-D according to Dow AgroSciences Method GRM 97.09 and 97.09.S1. Sample extracts were analyzed by gas chromatography (GC) using mass selective detection. The LOD and limit of quantification (LOQ) for both cis- and trans-1,3-D were 0.03 and 0.1 µg tube–1, respectively. The XAD-4 tubes were analyzed for residues of chloropicrin using A&L Great Lakes Laboratories, Inc. method RAM-10-050 with a modification of using a mass selective detector instead of an electron capture detector. The LOD and LOQ for chloropicrin were 0.03 and 0.1 µg tube–1, respectively. The residue levels of cis- and trans-1,3-D and chloropicrin in water samples were analyzed using Dow AgroSciences method GRM 94.11. The LOD and LOQ for both 1,3-D and chloropicrin in water was 0.015 and 0.05 ng mL–1, respectively.

Two sets of triplicate spikes were prepared at each of 0, 10, 100, and 2000 µg tube–1 of 1,3-D or chloropicrin for both travel spikes and field-exposed samples. The 1,3-D field-exposed spikes were exposed to ambient air for 12 h at 1.5 L min–1, while the chloropicrin field-exposed spikes were exposed to ambient air for 12 h at 0.05 L min–1.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS AND MATERIALS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Approximately 0.3 cm of rain fell early in the morning on the day of application (6 Dec. 1999), before the start of In-Line injection at the GA site. About 1.7 cm of rainfall fell between 13 and 19 Dec. 1999. There was a dramatic decrease in average daily temperature from the day before application to the day of application, with the average daily temperature dipping from 17.7 to 13.8°C and then to 6.7°C on the day after application. The air temperature dipped slightly below freezing early on the morning of the day after the application. The maximum average daily wind speed occurred on the day of application, with a range of 1.2 to 2.9 m s–1 during the 14-d study. Wind direction during the application, as measured by the weather station at the north end of the plot was from the west and northwest. Wind direction was from the north, northeast, and east in the days following application. At the FL site, the average daily air temperature during the study was 26.2°C, with a range of 21.2 to 35.7°C. A total of 7.54 cm of rainfall was measured during the period of the study. The majority of this precipitation (6.15 cm) occurred in the middle of the study, on 30 to 31 Aug. 2000. Average wind speeds were relatively light during the course of the study (1.7 m s–1), with a maximum speed of 11.2 m s–1 occurring around 1800 h on 29 August.

Uniformity of 1,3-Dichloropropene and Chloropicrin Application
The application of In-Line was spatially and temporally uniform, with an overall coefficient of variation (CV) of 11.5 and 12.1% for 1,3-D and 12.0 and 18.0% for chloropicrin (n = 16) at the GA and FL sites, respectively. The average measured 1,3-D and chloropicrin concentrations were within 15% of the target concentration at both sites.

Travel Spike and Field-Exposed Sample Results
Recoveries of cis- and trans-1,3-D from all nine of the travel spikes averaged 112 and 110%, respectively. Recovery from the nine field-exposed spikes averaged 108 and 105% for cis- and trans-1,3-D respectively after 12 h of exposure to ambient air. Recoveries of chloropicrin averaged 98% from both the nine travel spikes and the nine field-exposed spikes. Results of the travel spikes and field-exposed spikes suggest that 1,3-D and chloropicrin were stable during typical storage and shipping conditions encountered during this study, and were not desorbed from the sample media during sampling.

Mass Loss of 1,3-Dichloropropene and Chloropicrin
Aerodynamic Method
The total volatilization of 1,3-D to the atmosphere during the entire study was 86.5 kg, or 26.5% of the applied 1,3-D. Figure 2 shows that approximately 50% of the 1,3-D volatilization occurred by 2 DAT, 90% by 8 DAT, and 99% by 13 DAT. Concentrations of 1,3-D were nearing the LOQ by the last sampling period, and mass losses were very small. The peak flux density of 1,3-D was 65.7 mg m–2 h–1 and occurred during application (Period 2).


Figure 2
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  		Fig. 2. Flux density and cumulative mass volatilization of 1,3-Dichloropropene (1,3-D) calculated using the aerodynamic (AD) method.

 
The total volatilization of chloropicrin to the atmosphere during the entire study was 20.0 kg, or 11.2% of the applied chloropicrin. Figure 3 shows that chloropicrin volatilized sooner than 1,3-D with over 50% of the volatilization occurring by 1 DAT, 90% by 4 DAT, and 99% by 6 DAT. No chloropicrin was detected above the treated field after 7 DAT, and samples taken after 8 DAT were not analyzed for chloropicrin. The peak flux density of chloropicrin was 23.9 mg m–2 h–1 and occurred during application. Chloropicrin flux decreased more quickly than 1,3-D, likely due to its more rapid degradation in soil. Both 1,3-D and chloropicrin flux patterns are characterized by very low flux during evening periods with most of the flux occurring during the daytime.


Figure 3
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  		Fig. 3. Flux density and cumulative mass volatilization of chloropicrin volatilized as calculated using the aerodynamic (AD) method.

 
Flux Chamber Method
The total mass loss of 1,3-D determined using the dynamic FC method was 77.1 kg, or 23.6% of the applied 1,3-D. The overall pattern of 1,3-D mass loss as determined using the FC method is similar to the AD method (Fig. 4). However diurnal variations in measured flux are dampened by the FCs. Reasons for the lower 1,3-D mass flux determined by the chamber method could be due to the effect of the FC itself on the incident solar radiation and subsequent temperature and moisture fluxes in the soil below the chamber. Air temperatures measured inside and outside the FC showed a slight damping effect within the chamber. However, the largest absolute temperature difference was always less than 2°C, which is consistent with the findings of Reichman and Rolston (2002) who found a daily average temperature perturbation of 1.5°C due to the chamber.


Figure 4
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  		Fig. 4. Comparison of 1,3-Dichloropropene (1,3-D) flux density predictions for the aerodynamic (AD) method and the flux chamber (FC) method.

 
The total mass loss of chloropicrin determined using the FC method was 32.2 kg, or 18.0% of the applied chloropicrin. The overall pattern of chloropicrin mass loss as determined using the FC method is similar to the AD method with the same damping effect that was observed in the 1,3-D FC measurements (Fig. 5).


Figure 5
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  		Fig. 5. Comparison of chloropicrin flux density predictions for the aerodynamic (AD) method and the flux chamber (FC) method (first data point is Period 2, afternoon).

 
Overall the estimates of total mass flux and the temporal patterns of mass flux predicted by the FC method are similar to the AD method. Thus, the FC method shows potential for estimating mass loss from different application methodologies in smaller plot-scale studies. Systematic temporal differences between the AD and FC flux estimates did occur, with the AD method predicting higher flux than the FC method during daytime periods and lower flux during nighttime periods (Fig. 6). This could be due to the constant air flow over the soil in the FC, which results in underestimation during the day when wind speeds may be higher and overestimation at night when ambient wind speed is zero or negligible.


Figure 6
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  		Fig. 6. Difference between 1,3-Dichloropropene (1,3-D) flux measured by aerodynamic (AD) method and flux chamber (FC) method. Solid symbols (•) are nighttime sample periods, open symbols ({circ}) are daytime periods.

 
The FCs are also useful for determining where the majority of the mass flux is originating from within the field. In this study, it appeared that the majority of the 1,3-D and chloropicrin mass flux came from the bed area (FC #2, FC #5) in GA, with minimal mass flux from the furrow area (FC #3, FC #4) (Table 2).


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  		Table 2. Flux chamber volatilization of 1,3-dichloropropene (1,3-D) and chloropicrin from Georgia (GA) study under low density polyethylene (LDPE) tarp.

 
Soil Gas Concentration of 1,3-Dichloropropene and Chloropicrin
Soil gas concentrations of 1,3-D peaked 1 d after treatment at the 15-cm depth in the bed (4668 mg m–3) and decreased over the 14 DAT (Fig. 7). The 1,3-D concentration at the 60-cm depth reached 697 mg m–3 at 3 DAT and then decreased slowly over the next 11 d. Some 1,3-D was observed at the 15- and 30-cm depths under the furrow during the first 6 d after treatment; however, the levels are generally an order of magnitude lower than those observed in the COB area. This corroborates the negligible flux of 1,3-D and chloropicrin measured by the FCs placed in the furrow area at the GA site. Concentrations of chloropicrin in soil gas followed a similar pattern to 1,3-D, peaking at 5222 mg m–3 on 1 DAT at the 15-cm depth and then decreasing monotonically until the end of the study. Chloropicrin dissipated more rapidly from the soil than 1,3-D which supports the observation of more rapid decline of chloropicrin flux measured by the AD and FC methods.


Figure 7
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  		Fig. 7. 1,3-Dichloropropene (1,3-D) and chloropicrin concentrations in soil gas at (a) the 15- and 60-cm depth below the center of the bed (COB) and (b) the 15- and 30-cm depth below the center of the furrow.

 
Effect of Tarp
Numerous studies have reported significant effects of tarp materials in reducing flux of fumigants (Wang et al., 1999; Nelson et al., 2000; Desaeger et al., 2004; Papiernik et al., 2004; Thomas et al., 2004; Ou et al., 2006). Figure 8 shows that VIF film reduced the peak flux density of 1,3-D by threefold and the peak flux of chloropicrin by sevenfold during the first sampling period compared with HDPE film. The reduction in flux density through VIF tarp reduced total 1,3-D mass loss from 20.5 to 6.5% and total chloropicrin mass loss from 31.9 to 3.9% compared with HDPE film (Table 3).


Figure 8
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  		Fig. 8. Florida (FL) site comparison of flux chamber estimates of 1,3-Dichloropropene (1,3-D) and chloropicrin flux density for beds tarped with high density polyethylene (HDPE) and virtually impermeable (VIF) film. A = afternoon, N = night, M = morning sampling period.

 

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  		Table 3. Flux chamber volatilization of 1,3-dichloropropene (1,3-D) and chloropicrin from Florida (FL) study under high density polyethylene (HDPE) and virtually impermeable film (VIF) tarp.

 
The reduced 1,3-D and chloropicrin emissions at night suggest that tarp permeability may decrease during night time when temperatures are cooler. Temperature dependence of tarp permeability has been reported by Wang et al. (1998a, 1999) and Papiernik and Yates (2002).


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS AND MATERIALS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
The surface drip fumigation using In-Line and low density polyethylene tarp employed in this study resulted in a volatilization of 26.5% of the applied 1,3-D and 11.2% of the applied chloropicrin, as determined using the AD method. Estimates of volatilization obtained from dynamic FCs were 23.6% for 1,3-D and 18.0% for chloropicrin. The FC method predicted similar mass loss patterns to the aerodynamic method for both compounds. The AD and FC methods each offer advantages and disadvantages, depending on the research goals. While the AD method integrates mass flux over a large area, FCs can be used to make smaller scale ‘point’ measurements of flux and thus enable the use of much smaller treated plots. This can result in significant cost savings. As a result of their small scale of measurement, FCs are useful for comparing mass loss from the various physical components found in bedded agriculture, such as flux from a raised bed area, a furrow, or the sides of raised beds, as well as allowing the replication of treatments. For this reason, FCs are a valuable tool for evaluating best management practices for minimizing soil fumigant emissions. Drawbacks of the FCs such as their inherent impact on surface boundary conditions can be mitigated by moving the chambers to different locations after each sampling interval. Additionally, more FC replicates may be required to account for the smaller scale spatial variability for FC measurements (spatial scale on the order of <1 m) when comparing to field-scale estimates, which are made on a spatial scale on the order of >100 m. Flux of 1,3-D and chloropicrin are significantly affected by the use of tarp, with VIF film providing the greatest reduction in flux density. Although tarp appears to have a significant effect on reducing gaseous emissions, the effect of different tarps (e.g., semi-impermeable film [SIF]) in various management and cropping systems requires more research.


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





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