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Published in Vadose Zone Journal 3:656-667 (2004)
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

ORIGINAL RESEARCH

Effect of Soil Moisture and Irrigation on Propargyl Bromide Volatilization and Movement in Soil

S. E. Allaire*,a, S. R. Yatesb and F. F. Ernstb

a Département des sols et de génie agroalimentaire, Pavillon de l'Envirotron, FSAA, Université Laval, QC, Canada G1K 7P4
b George E. Brown Jr. Salinity Laboratory, USDA-ARS, 450 W. Big Springs Rd, Riverside, CA 92507
* Corresponding author (suzanne.allaire{at}sga.ulaval.ca).

Received 24 July 2003.



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Propargyl bromide (3-bromo-propyne, 3BP) is a potential replacement for the soil fumigant methyl bromide. Since little is known about its movement in soil, a study was conducted to compare the volatilization and movement of 3BP in the soil profile for different irrigation treatments. A rectangular soil column was used to simulate a bed–furrow system. The surface of the bed was covered with high-density polyethylene (HDPE) plastic (i.e., a tarp). The furrow was left uncovered. Multiple volatilization chambers were used to measure emissions from the furrows, the slopes of the bed, and the bed. The soil was fumigated by injecting 1.0 mL of 3BP to the center of the column. Three treatments were studied, no irrigation, a single 5-h surface irrigation 24 h after fumigation, and a 2-h daily surface irrigation. Volatilization was about three times greater from nonirrigated soil. Irrigation and higher initial soil moisture content were more effective in controlling 3BP volatilization than the use of a HDPE tarp. Volatilization and degradation were similar for both irrigation treatments, but the 2-h irrigation had the advantage of requiring one-third less water. Volatilization rates from the slopes of the bed were lower than from the bed surface. To obtain accurate total mass, volatilization chambers should cover the whole bed–furrow system. Short advective gas and liquid fluxes created by the irrigation had pronounced and prolonged effect on 3BP distribution and degradation. Henry's Law could not be used to predict the 3BP distribution pattern in the liquid phase even long after the irrigation ceased.

Abbreviations: GC, gas chromatography • HDPE, high-density polyethylene • 3BP, 3-bromo-propyne


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOIL FUMIGANTS are used to disinfest soil and control soilborne pathogens (Noling and Becker, 1994). Methyl bromide has been one of the primary fumigants used for decades to control nematodes, weeds, and fungi, but will no longer be available due to environmental concerns. This will affect an array of agricultural production systems. Potential chemical alternatives include 1,3-dichloropropene (Chen et al., 1995), methyl isothiocyanate (Saeed et al., 2000), methyl iodide (Ohr et al., 1996), and propargyl bromide (3-bromo-propyne, 3BP) (Yates and Gan, 1998). These chemicals are similar to methyl bromide but have lower ozone depletion potentials due to their rapid breakdown in the troposphere. 3-Bromo-propyne was developed by Dow Chemical Co. (Midland, MI) but was not further developed because it is relatively unstable. Little information is available for 3BP because it was never registered as a pesticide. The phasing out of methyl bromide has generated a renewed interest in 3BP as a replacement fumigant.

Before 3BP can be used as soil fumigant, a better understanding of its behavior in the environment is needed, as well as information on the effects of potential management practices aimed at decreasing volatilization while maintaining effective pest control. The dominant factors influencing pesticide volatilization are physicochemical properties of the pesticides (e.g., vapor pressure and water solubility), their persistence in soil, and various environmental conditions such as temperature, soil organic matter, and soil water content (Taylor and Spencer, 1990; van den Berg et al., 1999).

Propargyl bromide has a relatively high vapor density (388 mg L–1 at 25°C) that is about twice that of 1,3-dichloropropene and about 10% that of methyl bromide. While limited information is available on the effect of temperature and organic matter on the degradation of propargyl bromide in soil, its persistence in soil seems to be relatively short-lived compared with methyl bromide. Yates and Gan (1998) estimated that the propargyl bromide half-life is about 5 d in a sandy loam soil. Papiernik et al. (2002) found that the apparent first-order decay rate decreased with increasing initial concentration in four tested soils. The predominant degradation processes are hydrolysis and alkylation, both of which release Br– ions (Papiernik et al., 2000).

Different management techniques have been used to control fumigant volatilization (Yates et al., 1996). Improving methods to hold a fumigant at the treatment site may allow sufficient time for degradation and decrease cumulative volatilization. The most common containment method is the use of plastic films, also known as tarps. Plastic films are installed on the soil surface and removed at a later time. The permeability of plastic film depends on the type of fumigant and the plastic material. Multiple-layer and co-extruded films tend to be less permeable to fumigants than single-layer or single composition plastics (Papiernik et al., 2001). Although film permeability increases with temperature (Wang et al., 1999), fumigant loss from volatilization is generally less for soils covered by plastic films (Yates et al., 1996).

Another management technique consists of modifying soil water content. For many pesticides, an increase in soil water content results in an increase in volatilization (Lembrich et al., 1999; Taylor and Spencer, 1990). For some pesticides, however, volatilization may be independent of water content (Whang et al., 1993). In the case of fumigants, an increase in soil moisture tends to decrease volatilization. Gan et al. (1996) found that methyl bromide volatilization from a sandy loam soil decreased by nearly 15% as the water content increased from 0.058 to 0.124 m3 m–3. Also, an application of water following a 0.2-m-deep injection of 1,3-dichloropropene substantially reduced volatilization (Gan et al., 1998).

Several other management practices affect fumigant volatilization. Deep injection of fumigants increases their residence time and, consequently, their degradation (Yates et al., 1996). Increasing soil bulk density by compacting the soil surface may help to control volatilization (Gan et al., 1996) because dense soil has lower air-filled porosity and decreased intrinsic diffusion. The addition of soil amendments to the soil surface may also accelerate the degradation process. For example, Wang et al. (2000) showed that the addition of ammonium thiosulfate to the soil leads to rapid dehalogenation of halogenated fumigants such as 3BP.

Information on the fate and transport of 3BP in soil is very limited. Experiments were conducted to investigate the behavior of 3BP in soil and to obtain information on potential fumigant management practices. The objectives of this paper were to (i) study the movement of 3BP in the soil profile and its partitioning between the liquid and gas phases under transient conditions, (ii) study the spatial and temporal variability of 3BP volatilization in a bed–furrow system, (iii) compare different irrigation treatments on 3BP volatilization and its movement in soil, and (iv) characterize the distribution of Br– ion, a 3BP degradation product, in the soil profile under different treatments.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A 0.60 by 0.60 by 0.025 m soil column was packed by hand with Arlington sandy loam (coarse loamy, mixed, thermic Haplic Durixeralf) to a density of 1650 kg m–3. The soil surface was configured to reproduce a bed–furrow production system that is commonly used for strawberry (Fragaria x ananassa Duchesne ex Rozier) in Southern California. The experimental system was scaled to half the size of a typical production system. The surface of the bed (bed), the slopes (slope), and the furrows (furrow) measured 0.075, 0.075, and 0.3 m wide, respectively, and the bed was 0.15 m above the furrow (Fig. 1) . The bed was placed in the middle of the column to preserve symmetry. A HDPE plastic (0.0254-mm thickness, from Tri-Cal, Hollister, CA) was placed over the bed (top and slope sections) while the furrow remained uncovered (Fig. 1).



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  		Fig. 1. Soil column and sampling system.

 
The treatments were as follows. In Treatment M-0 the soil was initially moist (0.17 m3 m–3) and no irrigation was applied. For Treatment M-5 the soil was initially moist and a 5-h furrow irrigation (0.27 pore volume in each furrow) containing 0.005 M CaCl2 was applied 24 h after fumigation. In Treatment M-M the soil was initially moist and a 2-h irrigation was applied 2 h after the injection and repeated every 24 h thereafter for four consecutive days, for a total of 0.18 pore volume in each furrow. During irrigation, a 0.03-m pressure head was maintained in the furrow with a Mariotte bottle system. As a preliminary experiment, an initially dry soil (Treatment D-5) was also subjected to the same irrigation treatment as M-5. Only partial data on volatilization was obtained. The data related to concentrations in the soil profile are complete. All treatments were fumigated with 1.0 mL of liquid 3BP (97% pure from Fluka, Buchs, Switzerland) in the center of the column.

The instrumentation was similar to Allaire et al. (2002). The rectangular soil column was made of acrylic. A five-section volatilization chamber made of acrylic was attached to the top of the column. Each section of the volatilization chamber corresponded to a furrow, a slope of the bed, or to the top of the bed (Fig. 1). Computer-controlled solenoid valves directed chamber exhaust to a gas chromatograph so that a targeted section of the volatilization chamber was automatically sampled and analyzed at fixed time intervals. Gas chromatography (GC), following the method of Gan et al. (1995), was used in the analysis of 3BP. Volatilization was measured every 25 min in each section of the chamber.

The 3BP distribution in the gas phase of the soil profile was determined using gas-tight syringes by sampling 50 µL of soil air at 62 sites in a radial pattern around the injection port (Fig. 1). Samples were collected several times during the first 2 d, and then once a day for 9 d.

Pressure distribution in the gas phase was measured every 5 min with a pressure transducer (0–30 KPa, Sensym, Milpitas, CA). Sampling frequency was increased at the beginning and at the end of irrigation. The pressure transducer was connected with a manifold valve and tubing to 12 sampling ports located in half of the column. Vertical symmetry was assumed for estimating the pressure distribution in the other half of the column. Data collection was controlled by a data logger (21X, Campbell Scientific, Logan, UT).

Each treatment was maintained for 9 d. Then one side of the column was removed and covered with plastic. Seventy-three soil samples (5 g) were taken on a regular grid for soil water content analysis. Another 73 soil samples (5 g) were collected on the same grid for 3BP concentration in the liquid phase. These samples were placed into headspace vials containing 5 g of sodium sulfate and 10 mL of ethyl acetate. The vials were capped immediately with aluminum seals and Teflon-faced butyl rubber septa and were shaken for 1 h. The extracts were analyzed by GC.

All known mechanisms (hydrolysis, methylation of organic matter, and microbial degradation) of 3BP degradation result in the release of Br– (Papiernik et al., 2000). An additional 73 soil samples (5 g) were taken to determine Br– distribution in the profile. Bromide ions were extracted by adding 5 mL of ultra-pure water to the samples. The soil solutions were shaken for 1 h. Bromide concentration in the extract was measured with an ion chromatograph.

The volatilization flux density (J, µg m–2 s–1) was calculated as follows:

[1]
where F is the mean air flow rate (m3 s–1) as measured by flow meters, and Ci and Co are 3BP concentration (µg m–3) in the air at the inlet and the outlet of the chamber, respectively. Ci was 0 µg m–3. A (m2) is the area of the chamber section. The experiments were conducted at a constant 22°C. Assuming that sorption to the plastic film reached equilibrium instantaneously, the mass transfer coefficient of the plastic film (h, m s–1) was calculated as follows:

[2]
where Cgb is the 3BP concentration (µg m–3) in the soil gas phase in the bed directly underneath the plastic. The value of Co was assumed to be the same as in Eq. [1] because the air inlets of the volatilization chambers were only a few millimeters above the soil surface, resulting in well-mixed air above the tarp. Thus, a negligible stagnant boundary layer above the tarp was assumed.

The mass balance was calculated as follows (Jury et al., 1991):

[3]
where

[4a]

[4b]

MT is the total mass in the soil (µg). CTz (µg m–3) is the total 3BP concentration at a sampling location z in the profile (sampling port), and CBrz (µg m–3) is the concentration of Br– in equivalent 3BP at a sampling location z at the end of the experiment. V is the volume of soil corresponding to one sampling port (m3). The indices i, j, and z refer to the sampling time, the section of the volatilization chamber, and the sampling location in the profile, respectively. The parameter t is the time (s), {rho}b is the soil bulk density (kg m–3), {theta} is the volumetric soil water content (m3 m–3), a is the air-filled porosity (m3 m–3). Ca (µg kg–1), Cl (µg m–3), and Cg (µg m–3) are the concentrations, respectively, in the sorbed, liquid, and gas phases. KH is the Henry's constant and Kd is the sorption coefficient (m3 kg–1). A mass balance was calculated by integrating the volatilization rate in time, integrating the total 3BP concentration (sorbed, liquid, and gas phases) in the soil, and integrating the Br– concentration in the soil at the end of the experiment. All data were corrected for recovery due to losses through analysis, handling, and storing and to losses through sorption onto instrumentation (Allaire et al., 2003).

The retardation factor (R, dimensionless) for a volatile compound represents the relative time needed for the pesticide to move a specific distance, {Delta}x (m), as compared with a nonsorbing tracer. The retardation factor can therefore be used as an indicator of mobility and is computed for the liquid (Rl, dimensionless) and gas (Rg, dimensionless) phases as follows (Jury et al., 1991):

[5]

[6]

Assuming that Darcy's Law is applicable and that linear laminar flow occurs, the average linear gas velocity, v, in one direction (m s–1) due to a pressure gradient can be calculated as follows (Fischer et al., 1996):

[7]
where ka is the air permeability (m2), kra is the relative air permeability (dimensionless), µg is the dynamic gas viscosity (µg m–1 s–1), {Delta}P/{Delta}x is the pressure difference between two points (µg m–1s–2), and ka can be calculated from Moldrup et al. (1998):

[8]
where rg is the equivalent radius of the pore space (m). The values of some of the parameters described above are given in Table 1.


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  		Table 1. Soil and propargyl bromide (3BP) characteristics.

 

   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Mass Balance
Table 2 shows the mass balance for the different treatments. Treatments M-0 and M-M had a mass balance of 91 and 99%, respectively, and the concentration ratios (i.e., Cg/Cl) for the same treatments were 0.07 and 0.02. Since the Henry's Law constant has been reported to be 0.046 at 25°C (Yates and Gan, 1998), this result indicates good experimental procedures. The slightly lower mass balance for M-5 may be due to losses and to inherent difficulties in capturing spatial variability. It may also be due to errors associated with the extraction from the liquid phase at the end of the experiment. Indeed, the Cl/Br– ratio was close to 1:1 for Treatments M-0 and M-M, but was nearly 1:2 for Treatment M-5 while all other mass components between M-5 and M-M were in the same range.


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  		Table 2. Mass balance of 3BP for different treatments.

 
Volatilization
The peak volatilization occurred at approximately the same time in all irrigated treatments (24 h after fumigant injection) and corresponded to the start of the irrigation (Fig. 2) . The total volatilization was of the same order of magnitude in Treatments M-5 and M-M. The reported difference is probably not significant given the variability of the system and the much greater difference between these two treatments and Treatment M-0. The advantage of the multiple short irrigations (M-M) is that one-third less water is used compared with the long irrigation in Treatment M-5.



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  		Fig. 2. Volatilization rate and cumulative volatilization rate through time in Treatments M-5 and M-M.

 
Irrigation Effects
Irrigation had a significant effect on cumulative volatilization. The volatilization decreased from 21% of the applied mass in Treatment M-O to 6.3% in M-M and 3.0% in M-5 (Table 2). The effect of irrigation on volatilization can be better understood by studying the water distribution through time in the different sections of the bed–furrow system. The effect of irrigation on the volatilization from the bed, the slope, and the furrow in Treatment M-5 is shown in Fig. 2. The volatilization rate was the highest in the furrows, followed by the bed and the slopes before irrigation started. This is due to the fact that the furrows were closer to the injection point and were not covered with plastic film. The bed slopes, which represent nearly 25% of the surface area, had lower emission because they were covered by plastic material and were more humid than the bed. Therefore, chambers installed only on the bed would overestimate volatilization through the plastic covered area. This indicates that the instruments should cover the entire cross section of the bed–furrow system. In addition, the result demonstrates that a furrow–bed system with only the bed tarped, which is often used by producers, is not an effective method for controlling 3BP volatilization. This is in agreement with Wang and Yates (1998) for methyl bromide.

As soon as irrigation started, furrow volatilization rates in Treatment M-5 sharply decreased to rates lower than those in the bed, while the rates in the bed increased significantly during the same period (Fig. 2). Irrigation had a lesser effect on volatilization in Treatment M-M, since the irrigation period was shorter (2 vs. 5 h). Even so, the effect was noticeable during each irrigation event as volatilization in the bed increased to a level higher than in the furrow (Fig. 2). This indicates that a layer of water above the soil is more effective in controlling volatilization than HDPE. Gan et al. (1997)(1998) also found that polyethylene plastic was less effective than irrigation in controlling the volatilization of 1,3-dicholopropene.

The higher volatilization from the bed, during and after irrigation, was not due to punctures in the plastic or improper installation since the phenomenon occurred in all irrigated treatments. In this experiment, the effective permeability of the HDPE film to 3BP was between 0.5 and 2 µm s–1 (Table 3), which is close to the range (2.7–4 µm s–1) estimated in other studies for the same plastic material (Papiernik et al., 2001; Wang et al., 1999). The effective permeability of the plastic film in this experiment was lower than more direct permeability measurements due to a number of factors. For example, a thin film of water may have formed on the underside of the film due to increased soil moisture. This water film could have enhanced the effectiveness of the plastic film in blocking emissions to the atmosphere, thus decreasing the estimated permeability at 216 h as compared with 48 h.


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  		Table 3. Estimated physical properties of the system.

 
The results related to volatilization indicate the following:
  1. Volatilization of 3BP tends to behave as methyl bromide.
  2. Repeated irrigation lowers 3BP volatilization as well as a single prolonged irrigation and has the advantage of using less water.
  3. There is considerable spatial variability in 3BP volatilization in a bed–furrow system. Sampling emissions above the flat area of the bed is not sufficient and would overestimate the volatilization of the plastic covered surface in these systems.
  4. The HDPE film is not as effective as a layer of water at the soil surface in controlling volatilization but a thin film of water vapor on one side of the plastic appears to increase its effectiveness.
  5. Farmers tend to partially cover their fields with tarps to save money, but our results indicate that partial coverage with plastic film (only the bed) is not sufficient for controlling 3BP volatilization. Either furrow irrigation should be applied or the soil should be completely covered with a less permeable plastic film.

Movement in the Profile
Initial Soil Moisture
The 3BP gas-phase concentration in D-5 was 100 times lower than measured in the other treatments (Table 2). This difference is due to the fact that more of the chemical volatilized during the first 48 h in this treatment. For many pesticides, however, an increase in soil water content leads to an increase in volatilization (Taylor and Spencer, 1990). Quick estimate of Rl in the system indicates that the fumigant is less retarded and thus moves faster in moist soil than in dry soil (Table 3), which is consistent with the behavior of most pesticides. However, the observations in this study indicate otherwise. This can be explained by the fact that Rl refers to the retardation in the liquid phase. Although 3BP is soluble in water and distributed preferably in the liquid phase (KH = 0.05), it is also highly volatile. The system had a large amount of air-filled pores that contributes to its movement in the gas phase.

The total volatilization from Treatment D-5 can be estimated from the mass balance by assuming 100% recovery. For this case, the volatilization from D-5 would be nearly 68%, or about three times higher than for Treatment M-0 (Table 2). The high rate is due to diffusion in the air phase, which is about 10000 times greater than in pure water. Gas-phase movement of pesticides with high vapor pressure, such as 3BP, can be rapid. Calculating Rg indicates that 3BP would be less retarded (Table 3) and would move faster in dry soil. Indeed, Rg was more affected by soil water content than Rl and indicates faster movement in dry soil. Like methyl bromide, the initial soil moisture content had a large effect on 3BP movement even though the KH of 3BP is much lower.

The effect of irrigation 24 h after fumigation was not as important in Treatment D-5 as in other treatments since there was little chemical left in the profile for transport through convection. For the same reason, Br– mass in Treatment D-5 was less than one-half that in the other treatments at the end of the experiment (Table 2).

Irrigation Effects
3-Bromo-propyne in the gas phase moved relatively unimpeded from the point of injection by diffusion before irrigation (Fig. 3 for M-0 and Fig. 4 at 5 h). 3-Bromo-propyne distribution in M-5 was almost radial around the injection point when it was measured after 24 h, a few minutes before irrigation took place. As soon as irrigation began, 3BP distribution in the gas phase was no longer radial.



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  		Fig. 3. Bromide distribution 216 h after fumigation for different treatments.

 


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  		Fig. 4. 3-Bromo-propyne concentration (µg cm–3) distribution in the soil gas phase at different times after its injection in Treatment M-5.

 
Water infiltrating into the profile created a significant pressure gradient in both water and gas phases (Fig. 5) . The gas phase in Treatment M-5 and M-M was compressed up to 1.8 x 106 Pa directly underneath the furrow for a second and then remained at 1 x 103 Pa for 2 h after the start of irrigation before returning to equilibrium. Air compression occurred because the air outlet at the bottom of the column was closed and the soil surface was ponded in the furrows and covered with plastic film elsewhere. This created a significant convective gas flux from the soil surrounding the furrow toward the center of the column and toward the center of the bed near the surface (Fig. 5). A maximum convective velocity of 33 m s–1 occurred between the furrow and the middle of the bed immediately following irrigation (Table 3), pushing the gas toward the bed. The maximum velocity in Treatment M-M from the center of the column to the center of the bed during the first seconds of irrigation was 0.003 m s–1 (Table 3). Maximum velocities in any direction (Table 3) were lower in the initially dry soil (D-5) than in moist soil (M-M). At these velocities, the gas-phase convective flux had a significant impact on the 3BP distribution in the soil profile, even if it only occurred for a very short period of time.



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  		Fig. 5. Pressure distribution in the gas phase and gas phase concentration associated with the direction of their flux in Treatment M-5 at the beginning of the irrigation.

 
When irrigation ceased, the draining water created a negative pressure in the gas phase near the furrow for a few seconds, down to –1 x 104 Pa in Treatment D-5, a little less for Treatments M-M and M-5, and a positive pressure near the wetting front. The pressure gradient created during drainage lasted for about 2 h (29.5 to 32.5 h) before pressure equilibrium in the gas phase was reestablished. The maximum convective flux, after irrigation was stopped, occurred from the middle of the column to the furrow (i.e., in the same direction as diffusion and in the opposite direction to water flow).

Figure 6 shows the final 3BP distribution in the gas phase for different treatments. Although 3BP distribution occurs more readily in water, it is obvious that 3BP distribution did not correspond to water content distribution (Fig. 7) . This was due, in part, to the pressure gradient created by the infiltrating water. The effect of infiltrating water on gas movement was similar in all irrigated treatments.



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  		Fig. 6. 3-Bromo-propyne distribution in the gas phase 216 h after fumigation for different treatments.

 


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  		Fig. 7. Water content distribution in the profile 216 h after fumigation for different treatments.

 
In this experiment, the pressure gradient would probably not have built up in the soil profile if the outlets at the bottom of the column had been opened. There would also have been a noticeable change in 3BP movement and volatilization. However, pressure gradients may occur when the water table is near the soil surface, the water infiltration rate is high (e.g., heavy rain from a tropical storm) or a dense soil layer is present close to the surface (e.g., dense plow layer, shallow soil with rock underneath).

Distribution Between the Phases
The Br–, water, Cg, and Cl had different distribution patterns in the profile (Fig. 3, 6, 7, and 8) , except in Treatment M-0 where only diffusion occurred resulting in a radial distribution pattern for all phases. This indicates the effect of convective gas and liquid fluxes. The irrigation effect was less dramatic in Treatment D-5 than in Treatments M-5 and M-M since there was only half as much chemical in the D-5 profile when irrigation started. In M-5 and M-M, the distribution of both treatments was similar. The soil moisture was higher near the furrow and decreased with depth. The wetter soil region did not correspond to the higher 3BP concentration in either liquid or gas phases. This is partly due to the following:

  1. As a result of the diffusion process, the highest concentration remained at the injection point.
  2. Volatilization near the soil surface decreased 3BP concentration in the furrow.
  3. Convective flux occurred in the opposite direction to diffusion. This resulted in a quasihorizontal distribution of Cl (Fig. 8) and a slightly more vertical distribution of Cg (Fig. 6).



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  		Fig. 8. 3-Bromo-propyne distribution in the liquid phase 216 h after fumigation for different treatments.

 
The liquid and gas phases had slightly different distributions because they occupy complementary porosity. During water infiltration, the gas phase was rapidly pushed toward the bed where it volatilized, and toward the bottom of the column where it remained trapped. During the same period, the liquid phase moved mostly downward with water at a rate much slower than the gas phase. The results show that although there is continuous and rapid exchange between the liquid and gas phases, equilibrium was not achieved. The use of Henry's Law is questionable for precisely estimating the distribution in one phase from the distribution of the other phase as it is routinely done, unless equilibrium is achieved. The convective flux affected them differently, resulting in distinct distribution profiles. This difference needs to be taken into consideration when estimating pest control areas around the injection point depending upon the specific pest response to the gas and liquid phases.

However, if the spatial average concentration in one phase is known, it can be roughly used to estimate the concentration in the other phase since the ratio of Cg to Cl (Table 2) is comparable to the Henry's constant (Table 1). However, a precise estimate of the distributions cannot be obtained if convective fluxes occurred, since the processes affect the gas and liquid phases differently.

After 9 d, Treatments M-5 and M-M had the lowest Br– concentration. Their mass was about half of that applied (Table 2). This is in agreement with Yates and Gan (1998) and Papiernik et al. (2001) (Table 1) who estimated that the 3BP half-life in this soil was between 4 and 12 d.

The Br– concentration distribution (Fig. 3) 216 h after fumigation differed from water (Fig. 7) and Cl distributions (Fig. 8). The Br– concentration distribution had higher values near the center and near the bottom of the column (Fig. 3). Since Br– is soluble and nonsorbing, the Br– distribution would be affected by the pattern of water infiltration. The Br– concentration was low near the furrows because of volatilization, which reduced the time available for degradation, and because infiltrating water carried Br– downward. Bromide concentration underneath the bed was low due to volatilization and because 3BP reached this area much later (2 d), leaving less time for degradation. The concentration was high at the lower end of the column because the gas-phase 3BP was pushed downward by a pressure gradient and there was more air space available at the lower end of the column. Thus, 3BP was trapped at the lower end of the column where it had sufficient time to degrade. 3-Bromo-propyne also diffused upward through the wetted soil (recall that 3BP has a higher affinity for water than for air). Since the water was moving in the opposite direction to the gas, the water had a longer contact time with 3BP in the center and lower end of the column than with other regions of the soil profile. Therefore, the final distribution of Cl, Cg, and water could not be used as a predictor of Br– distribution.

The movement of 3BP in the soil profile indicates the following:

  1. In an initially dry soil, little 3BP remains in the profile after 24 h. This reduces the importance of irrigation and results in lower concentration of degradation products. Irrigation should be applied within a few hours following 3BP fumigation.
  2. Without irrigation, the movement of 3BP is radial and diffusive.
  3. Irrigation causes significant convective fluxes in both liquid and gas phases, at some times in the opposite direction to diffusive fluxes. Prediction models usually assume only diffusion process in the gas phase. The results indicate that it is not sufficient to predict the distribution in the profile if water infiltrates into the soil.
  4. Irrigation reduces 3BP volatilization and consequently increases residence time and emissions to the atmosphere thereafter.
  5. Irrigation affects the overall 3BP distribution in the profile, modifying the pattern for pest control around the injection point.
  6. The distribution in one phase can only be used to obtain a rough estimate of the mass in the other phase using the Henry's constant since both phases are affected differently by convective and diffusive fluxes. In this study, it could not be used to estimate the distribution patterns.
  7. The distribution pattern of the degradation product does not follow the distribution pattern of water, Cl, or Cg and can be only predicted with the knowledge of convective and diffusive fluxes of both phases.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
3-Bromo-propyne volatilization decreased with irrigation. Irrigation was more effective in controlling 3BP volatilization than the HDPE tarp. Repeated short-duration irrigation used less water and controlled 3BP volatilization as effectively as a single long-lasting irrigation. Irrigation gave rise to variability in volatilization from a bed–furrow system, even in a homogeneous soil. Volatilization chambers should cover the entire cross section of a bed–furrow system when the soil is partially covered by plastic. Irrigation also created convective fluxes in the gas and liquid phases affecting Cl, Cg, and Br– distributions in the profile differently. Henry's law could not be used to predict Cl distribution in the profile from Cg because of the nonequilibrium of the system for treatments with irrigation, even a long time after irrigation ceases. Overall, higher initial soil moisture and irrigation proved to be as effective as a plastic tarp for controlling volatilization and could potentially be used to manage 3BP.


   ACKNOWLEDGMENTS
 
The authors thank Ms. Zhang and C. Taylor for their assistance with the chemical analyses and R. Austin for his support with electronic equipment.


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





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