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Environ. Sci. Technol. 2010, 44, 5793–5798

1,3-Dichloropropene and
Chloropicrin Emissions Following
Simulated Drip Irrigation to Raised
Beds under Plastic Films
D. J. ASHWORTH,* L. LUO, R. XUAN,
AND S. R. YATES
USDA-ARS, United States Salinity Laboratory, 450 W. Big
Springs Rd, Riverside, California 92507

Received February 26, 2010. Revised manuscript received
June 8, 2010. Accepted June 10, 2010.

Using laboratory soil chambers a nonscaled representation
of an agricultural raised bed was constructed. For a sandy loam
soil, 1,3-dichloropropene (1,3-D) and chloropicrin (CP) were
applied at 5 cm depth with an excess of water (simulated drip
irrigation). Application was made under both high density
polyethylene (HDPE) and virtually impermeable film (VIF) covering
the soil bed (the furrow was left uncovered). Soil gas distribution
of the fumigants, together with emissions into the headspace
above the bed, sidewall and furrow were determined over time.
Total emissions from the HDPE treatment were cis 1,3-D
28%, trans 1,3-D 24%, and CP 8%. Due to its lower permeability,
the values for VIF were 13%, 7%, and 1.5%, respectively.
With HDPE, the majority (86-93%) of the emissions occurred
from the bed, while for VIF the majority (92-99%) of the emissions
was from the furrow. Compared to a range of literature
values for shank injection, the use of drip application appears
to offer a benefit in reducing 1,3-D and CP emissions.
However, the most meaningful comparison is with our previous
data for simulated shank injection where the same soil was
covered (completely) with the same plastic films (1). In
this comparison, only 1,3-D emissions under HDPE were lower
with drip application; 1,3-D emissions under VIF and CP
emissions under both films were greater with the drip application.

Introduction
Reducing the soil-to-air emission of agricultural fumigants
is increasingly important to protect both human and
environmental health. The high vapor pressure of preplant
fumigants such as 1,3-dichloropropene (1,3-D) and chloropicrin (CP) (Table 1) renders them liable to transfer from
the soil and thereby adversely affect air quality. The presence
of the fumigants in air, together with their role in the
formation of near-surface ozone (smog) are two areas of
particular concern. Therefore, research is focusing on approaches that reduce emissions either by application strategy
(e.g., reducing amounts of applied chemicals, differing
application type) or by postapplication strategy (e.g., plastic
film covering, water sealing, surface amendment).
One approach to potentially reduce emissions of fumigants is the use a subsurface drip application under plastic
film. This reduction is based on the degree of solubility and
relatively low Henry’s constants of fumigants such as 1,3-D
and CP (Table 1). For example, compared to fumigants such
* Corresponding author.
10.1021/es100641q

 2010 American Chemical Society

Published on Web 07/02/2010

as methyl bromide and methyl iodide, 1,3-D and CP have a
lesser propensity to partition into the gaseous soil phase. In
contrast to shank injection therefore, where the predominant
mechanism of fumigant movement is vapor diffusion, drip
applied 1,3-D and CP are likely to be initially transported
with the irrigation water downward and laterally into the
soil. Under such conditions, the presence of the fumigants
in the water phase, the increased path length to the soil
surface, and the increase in soil moisture content all serve
to potentially lower emissions. It has been suggested (2) that
drip application of soluble formulations may ensure a more
uniform distribution of fumigants in the soil, and is likely to
reduce emissions, worker exposure, and the amount of
chemicals applied relative to conventional shank application.
For example, comparison of a 46 cm shank injection to a 20
cm drip application for both 1,3-D and CP (3) showed that
under HDPE, 1,3-D emissions were reduced from 43% with
shank injection to 12% with drip application, and CP
emissions were reduced from 17% (shank) to 2% (drip). A
high level of uniformity in the distribution of drip-applied
fumigants within the soil is also likely to improve pest control.
This may be particularly important for fumigants with
relatively low vapor pressures (e.g., compared to methyl
bromide and, to a lesser extent methyl iodide) where extensive
gas phase diffusion away from the point of application may
not be apparent. Indeed, gas phase distribution was markedly
improved by drip application when compared to shank
injection (2).
Nevertheless, shank injection of 1,3-D and chloropicrin
typically takes place at 30 or 46 cm depth, whereas drip
application usually takes place at, or just below, the soil
surface. Therefore, any potential benefit in emissions reduction offered by the irrigation water may be offset by the
closeness of the application to the soil surface. For example,
if the irrigation water is inefficient in transporting the
fumigant to greater soil depth, high emissions may occur
due to a short soil-to-atmosphere path length. This can be
somewhat mitigated by the presence of plastic film over the
bed surface since this acts as a barrier at the soil-atmosphere
interface. The rate of gas diffusion across this barrier is then
controlled by the permeability of the film to a specific
fumigant. High density polyethylene (HDPE) is the standard
film used in such operations but has been frequently noted
to offer poor impermeability to fumigants such as 1,3-D and
CP (4-6). However, plastics with lower permeability, for
example, virtually impermeable film (VIF), although more
expensive, can offer dramatic reductions in the emissions of
such fumigants. By maintaining fumigants in the soil, such
films increase pesticidal efficacy by increasing fumigantpest contact time (i.e., concentration × time index) and can
ultimately lead to the chemical/biological degradation of the
fumigant within the soil.
Often, 1,3-D and CP are applied together to provide a
synergistic effect in relation to pest kill. Although 1,3-D is
very effective as a nematicide and herbicide, its relatively
poor fungicidal efficacy is compensated for by the CP (7). As
the commercial product “Inline” (Dow Agrosciences) the two
chemicals (approximately 65:35 1,3-D:CP) are commonly
used in combination for drip application situations (e.g.,
raised bed agriculture). Using laboratory soil columns, the
aim of the work described in this paper was to approximate
a drip application (i.e., fumigant application with an excess
of water to transport the fumigant throughout the soil bed)
of 1,3-D and CP to raised beds under both HDPE and VIF,
and determine volatile fumigant emissions from the soil.
VOL. 44, NO. 15, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Selected Properties of cis 1,3-D, trans 1,3-D, and
Chloropicrin
solubility at vapor pressure at Henry’s constant
25 °C (mg L-1)
25 °C (mm Hg)
(dimensionless)
cis 1,3-D
trans 1,3-D
CP

2.18a
2.32a
2.00b

34.3a
23.0a
23.8b

0.056c
0.037c
0.103c

a
From Thomas et al. (19). b From California Department
of Pesticide Regulations (20). c From Wang et al. (18).

Materials and Methods
The experiment was performed using purpose-built aluminum soil chambers that allowed for an actual size (nonscaled)
construction of half-a-bed and half-a-furrow. The design of
these chambers has been described in detail previously (8),
therefore only a brief description will be given here.
Rectangular soil chambers (120 cm high ×80 cm wide ×10
cm deep) were packed with soil in such a way as to form half
a bed (50 cm wide) and half a furrow (30 cm deep, 30 cm
wide at its top and 25 cm wide at its base) across the width
of the chamber. A field moist sandy loam soil from Buttonwillow, CA (thermic Typic Haplargids; Milham series) was
packed into the chamber to a bulk density of 1.5 g cm-3. The
surface layer of the soil had a pH of 7.9 and an organic matter
content of around 2%. All treatments were performed in
duplicate.
Either a 1 mil HDPE film (Dow Chemical Company,
Midland, MI) or a 1.5 mil Hytibar VIF (Klerk’s Plastics,
Hoogstraten, Belgium) was sealed over the surface of the
bed and sidewall (furrow base was left uncovered) and a
volatilization chamber placed atop. In these chambers, it
was possible to seal and isolate the headspace above the
bed, sidewall, and furrow compartments and thus determine
the contribution of each compartment to the overall emissions loss. The 1,3-D (as Telone II, 50:50 cis:trans isomers)
and CP (both from Dow Agrosciences, Indianapolis, IN) were
applied at a depth of 5 cm below the bed surface at twothirds distance from the bed shoulder. Application equated
to field application rates of approximately 80 kg ha-1 cis 1,3D, 80 kg ha-1 trans 1,3-D and 84 kg ha-1 CP (based on bed
surface area). These rates closely matched the ratio of 1,3-D
to CP in the commercial drip application product Inline,
(although in contrast to the commercial product, no surfactant was added to our system). Immediately following
injection of the fumigants via a sealable port in the face of
the chamber, 1 L water was applied through the same port
at a rate of 8 mL min-1. Thus, the application took place over
a period of around 2 h. The relatively high solubility of both
fumigants (Table 1) ensured the downward and lateral
movement with the flow of irrigation water from the point
of application. This procedure was used as a surrogate for
a typical 2 h field drip-application of 1,3-D and CP using
standard 0.67 gallon min-1 100 ft-1 drip tape. Application
was commenced at 11:00 h (Time 0).
The temperature of the surface soil was manipulated to
approximate a diurnal regime typical of temperatures
recorded at the Buttonwillow site during September 2007
(ranging from 23 to 32 °C at 5 cm depth). This was achieved
by controlling the air temperature of the experimental room
in which the chambers were housed. Below 30 cm depth, the
outside of the chambers were insulated with foam to lessen
temperature variation at depth. Sampling of fumigant
emissions from the soil was carried out by pulling the
headspace air through XAD-4 (2 section 400/200 mg) sorbent
tubes (SKC Inc., Eighty Four, PA). Due to the large size of the
volatilization chambers, a mass flow rate of 1 L min-1 was
used to sweep the headspace air, of which a subsample of
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 15, 2010

50 mL min-1 was directed through the sorbent tubes. Tubes
were initially sampled for 2 h during the day (7 a.m. to 7 p.m.)
and 12 h at night. The daytime sampling period was increased
later in the experiment when emissions were expected to be
lower. On a daily basis, fumigant distribution within the soil
pore space was determined by removing a 250 µL of soil gas
sample from a series of ports installed into the face of the
chambers.
XAD-4 tubes were stored at -19 °C until extraction and
analysis. The tubes were extracted first by separating their
two sections and placing each into 20 mL glass vial. After the
addition of 4 mL of hexane, the vials were immediately capped
with a Teflon-faced butyl rubber septum and aluminum
crimp seal, shaken for 30 min, and around 1.5 mL of
supernatant solution transferred to a glass vial for analysis.
The two sections of the XAD-4 tubes were extracted and
analyzed separately.
Analysis of XAD-4 extracts was carried out using an Agilent
Technologies 7890C gas chromatograph (GC), equipped with
a microelectron capture detector. The column was a DBVRX 122-1534 with dimensions of 30 m × 250 µM × 1.4 µm
(Agilent Technologies) running at a flow rate of 1.6 mL min-1
and with He as the carrier gas. The inlet temperature was 240
°C and the detector temperature 290 °C. The GC oven
temperature was maintained at 45 °C for 1 min after sample
injection, increasing to 75 °C at a rate of 2.5 °C min-1, and
then to 140 °C at a rate of 35 °C min-1 before being held at
this temperature for 3 min. Under these conditions, retention
times of cis 1,3-D, trans 1,3-D and CP were 10.4, 11.7, and
13.1 min, respectively. A set of 10 standards encompassing
the range of cis and trans 1,3-D and CP concentrations of the
samples were prepared in hexane.
Soil gas samples were analyzed using a Hewlett-Packard
HP6890 GC coupled with a G1888 Network Headspace Sampler
(Agilent Technologies). Similar GC conditions to those described
above were used. The operating conditions for the headspace
sampler were as follows: oven temperature 80 °C, loop temperature 90 °C, transfer line temperature 100 °C, vial equilibration time 5 min, and sample loop volume 0.2 mL. For all
analyses, five standards, encompassing the range of cis and
trans 1,3-D and CP concentrations observed in the samples
were prepared in a small volume (1-5 µL) of hexane.

Results
Flux rates (arithmetic means) of the fumigants from the HDPE
and VIF treatments over the course of the experiment are
shown in Figure 1a and b, respectively. These are the total
fluxes, that is, from the bed, sidewall, and furrow combined.
In general, the cis and trans isomers showed similar emission
pattern, although, in most cases the rates for the cis isomer
were higher. In each case, the 1,3-D isomers showed greater
flux rates than the CP. However, this difference was least
marked in the HDPE treatment where CP fluxes were
comparable to those of trans 1,3-D. Fluxes of the 1,3-D
isomers occurred over longer time periods than for CP. In
general, CP emissions ceased after around 75 h under HDPE
and around 150 h under VIF.
Comparing HDPE and VIF, it is noticeable that the HDPE
led to a more rapid release of the fumigants causing an initial
peak followed by extensive tailing over time. In the VIF
treatment the, albeit lower, fluxes were spread over a longer
period and had no obvious single emissions peak. Under
HDPE, emission fluxes rapidly peaked before tailing off over
time. The highest emission value was reached most rapidly
for the cis 1,3-D isomer (14 h), followed by the trans and CP
(both at 21 h). At these times, the mean flux rates were around
14.5 µg m-2 s-1 for the cis isomer, around 10.5 µg m-2 s-1 for
the trans isomer, and around 10 µg m-2 s-1 for CP. Under
VIF, emission fluxes increased rapidly at the start of the

The distribution of cis 1,3-D at 24 h after injection under
HDPE and VIF are shown in Figure 2a and b, respectively.
The distribution of trans 1,3-D and CP were similar to the
cis isomer and are not presented here. In the VIF chamber,
the data show concentrations decreasing radially from the
point of injection. In the HDPE chamber it is noticeable that
the point of highest concentration was not at the point of
injection and it is unclear why this should be the case.
Nevertheless, a similar radial distribution from this point is
observed. Gas phase concentration was generally higher
under VIF, and the peak concentration was almost double
that of the HDPE treatment. In both the VIF and HDPE
treatments, a widespread distribution of the fumigants
throughout the bed region was observed.

Discussion

FIGURE 1. Arithmetic mean (n ) 2) flux rates of each fumigant
over the course of the experiment from the (a) HDPE and (b) VIF
treatments. Application was carried out at 11:00 h (Time 0).
experiment but, overall, the emission curves were much
broader than for HDPE. For the 1,3-D isomers, emissions
were highest from around 25 to 100 h. For CP, emissions
were highest from around 30 to 60 h. Highest emission rate
for the cis isomer was 5.1 µg m-2 s-1 (at 53 h), for the trans
isomer was 2.9 µg m-2 s-1 (at 99 h) and for CP was 2.1 µg m-2
s-1 (at 49 and 53 h).
When expressed as a percentage of the total amount of
fumigant added to the system (Table 2) it is clear that in all
cases CP emissions were lower than for the 1,3-D isomers.
Moreover, Table 2 highlights differences between the plastic
types. VIF dramatically reduced total emissions compared
to HDPE. For example, for cis 1,3-D this reduction was 54%,
for trans 1,3-D was 71%, and for CP was 81%. The distribution
of emissions between the three compartments of the raised
bed system is also shown in Table 2. Under HDPE the
predominant emission loss for each fumigant was via the
bed surface, that is, close to the point of application. However,
under VIF, emissions occurred primarily via the furrow.

In relation to risk assessment associated with fumigant use,
the flux rate (a precursor of time course air concentrations)
is an important consideration. High flux rates are of concern
in terms of the health of agricultural workers and local
populations, particularly during the initial period following
fumigation when rates are usually highest. Clearly, the rapid,
relatively high flux rates observed in the HDPE treatment
demonstrate that this risk would be substantially greater when
HDPE, rather than VIF, is used as a surface containment.
The permeability of these films was tested previously (1) using
a previously reported technique (5). Mass transfer coefficients
at 20 °C (cm h-1) were determined for HDPE as cis 2.0; trans
3.7, CP 0.6, and for VIF as cis 0.001; trans 0.002; CP 0.0002.
Clearly, the higher emissions from the HDPE treatment were
a result of the much greater permeability of this film to each
fumigant. The lower CP mass transfer coefficients for both
films are consistent with the lower emissions observed for
this fumigant compared to 1,3-D. However, these lower
emissions are also likely due to the difference in degradation
rate, half-life, which was previously measured as 2.9 h for CP
and 90 h for 1,3-D in this soil (1). Therefore, this more rapid
degradation loss pathway for CP will have significantly
reduced emission losses from the soil surface. In contrast,
the relatively long half-life and higher mass transfer coefficients for cis and trans 1,3-D resulted in higher emissions.
The higher vapor pressure for cis 1,3-D (Table 1) explains its
greater emissions when compared to trans 1,3-D. In field
experiments comparing the emissions of 1,3-D and CP from
surface drip-application under both HDPE and VIF (9) a
similar extent of emission reduction due to VIF use was
observed.
Tarp type also had a strong effect upon soil gas concentrations of the fumigants. This can be seen in contrasting soil
gas distribution under the two films, where greater concentrations and wider distribution under the VIF were observed.

TABLE 2. Arithmetic Mean (n = 2) and Range of Emissions (% of Total Applied) from Each Chamber Compartment within Each
Experimental Treatment
cis 1,3-D

trans 1,3-D

CP

HDPE

bed
sidewall
furrow
drip total
shank totala

24 ((4)
3.5 ((0.3)
0.07 ((0.02)
28
42

22 ((4)
2.0 ((1.1)
0.06 ((0.03)
24
37

7.4 ((0.5)
0.19 ((0.07)
0.01 ((0)
8
1.1

VIF

bed
sidewall
furrow
drip total
shank totala

0.58 ((0.29)
0.40 ((0.22)
12 ((1)
13
2.3

0.24 ((0.17)
0.29 ((0.25)
6.9 ((0.9)
7
2.5

0.02 ((0.01)
0.03 ((0.02)
1.4 ((0.8)
1.5
0.0008

a
Comparative data for simulated shank injection (30 cm depth) to same soils (nonbedded) under same films, where
whole soil surface was covered with film (1).

VOL. 44, NO. 15, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Soil gas cis 1,3-D concentrations under (a) HDPE and (b) VIF after 24 h.

FIGURE 3. Repeating 24 h diurnal pattern in soil temperature at
5 cm depth. Arrow shows timing of fumigant application (11:00
h; Time 0 in Figure 1a and b).
The maintenance of high soil gas fumigant concentrations
is critical in ensuring successful pest disinfestation. In
addition, the presence of fumigants throughout the bed
region of the chambers at 24 h suggests that the application
procedure was effective in facilitating fumigant distribution
within the crop rooting zone. Overall, comparing the two
films, the higher total emissions from HDPE covered soil
would be likely to have a greater negative impact in relation
to air quality, for example, via contribution to near-surface
ozone formation. In addition, this relatively large loss from
the soil led to low soil gas concentrations and potentially
less efficacious pest control.
For comparison to the emission fluxes, the 24 h diurnal
pattern in soil temperature measured at 5 cm depth within
the chambers is presented in Figure 3. The influence of soil
temperature is evidenced by the higher fluxes generally
corresponding to afternoon and early evening times. Due to
its influence over vapor pressure, the role of soil temperature
in influencing the rate of 1,3-D (10, 11) and CP (8) volatilization from bare soil, has been previously reported. In covered
soils, the effect of temperature on the permeability of the
plastic film must also be considered since increasing perme5796

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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 15, 2010

ability of plastic film at higher temperatures has been reported
(4, 6), and differing films may exhibit differing temperaturedependent permeability. This is again consistent with the
observed increases in emission fluxes during the warmer
parts of the temperature cycle. Although both films appeared
to exhibit this effect, the VIF seemed to be most strongly
affected; exhibiting many fluctuations in emission flux over
time. It has been reported that low density polyethylene
(LDPE) may be more strongly affected by temperature than
VIF (12).
The type of plastic film clearly had a marked impact on
the point of fumigant release from the bed furrow system.
With the higher permeability HDPE, the emission release of
both 1,3-D and CP from the three headspace compartments
followed the order bed > sidewall > furrow. This would be
the expected pattern based on the positioning of the fumigant
application and would also be the expected pattern for a
nontarped raised bed system. It has been reported (8, 9) that
the vast majority of fumigant emissions occurred from the
bed surface with bare soil or relatively permeable plastic
covering (e.g., HDPE, LDPE, and semi-impermeable film).
In contrast, our current data show that emissions from the
VIF treatment were always greatest from the furrow. It seems
therefore, that the very low permeability of this film,
evidenced by the very low level of emissions directly above
the application point (i.e., from the bed compartment),
resulted in the diffusive transport of the fumigant both
vertically and horizontally. Since both the bed and sidewall
were covered with the VIF, the furrow base was the only
region from which emissions could readily occur. Consequently, a delay in emissions from the furrow was observed
due to the time taken for gas diffusion to this region. Indeed,
the broad nature of the flux rate curve from the VIF chamber
(Figure 1b) is mostly due to this emissions delay.
In sand mesocosms and field studies (13), similar results
using the same VIF (Hytibar) were reported; noting that the
majority of cis/trans 1,3-D, MITC and propargyl bromide
emissions occurred from the furrow following drip application. In practical terms, the fact that more than 90% of the
total fumigant emission from the VIF treatment occurred

from the furrow (Table 2), indicates that a supplementary
emission reduction strategy applied to the furrow may be
beneficial. A possible approach may be the furrow application
of a chemical amendment (e.g., thiosulfate solution) to
enhance fumigant degradation in this region. Potassium
thiosulfate used in the furrows of VIF covered beds reduced
CP emissions from 17% (VIF only) to 11% (12). However, in
LDPE covered beds, the thiosulfate did not yield further
emission reductions over plastic film alone.
Comparison of the present data with literature data
suggest that when 1,3-D and CP are applied under plastic
films but in the absence of excess irrigation water and to
nonbedded systems (e.g., shank/broadcast applied), total
emissions are greater than observed in the present study.
For a field study (3), 1,3-D (cis + trans) emissions for shank
injection (46 cm) under HDPE of 43% and for VIF 19% were
reported. For CP, the values were 17% and 8%, respectively.
These values are higher than those observed in Table 2 for
the same treatments. In the same study, these workers also
determined 1,3-D emissions of 12%, and CP emissions of 2%
from a drip application at 20 cm depth under HDPE. This
greater depth may explain the lower emissions than were
found in the present study. Also in field studies (14), CP
emissions of 9.5 and 18% for two soils following shank
injection at 20-25 cm under HDPE were found. For simulated
shank injected CP in column studies, emissions of 20% under
HDPE and 4% under VIF (15), and 30% under HDPE (16)
have been reported. For 1,3-D, emissions of 45% under HDPE,
and 10% under VIF, for a 30 cm simulated shank injection
to soil columns were observed (17). Overall, these comparisons suggest an emissions benefit in the application of
fumigants with drip irrigation to raised bed systems.
However, in a recently published paper (1), we reported
data for 1,3-D and CP emissions from laboratory soil columns
where a simulated shank injection was made at 30 cm depth
to nonbedded soil completely covered with plastic film. The
soil, plastic films and application rate were the same as those
used in the present study and so provide a useful basis for
comparing the two simulated application scenarios. Only in
the case of 1,3-D (both isomers) under HDPE did the dripapplied, raised-bed application lead to a reduction in
fumigant flux rates when compared to the shank injection.
Peak emission of the cis isomer was reduced from around
29 µg m-2 s-1 to around 14.5 µg m-2 s-1, and that of the trans
isomer from around 23 µg m-2 s-1 to 10.5 µg m-2 s-1. However,
within the same treatment, CP emissions doubled with the
drip-applied, raised-bed application (peak emission increasing from 5 µg m-2 s-1 to around 10 µg m-2 s-1). Furthermore,
under VIF, emissions of both 1,3-D isomers and CP were
increased with the drip-applied, raised-bed application. 1,3-D
isomer flux values were typically around an order of
magnitude greater, and CP around 2 orders of magnitude
greater, compared to shank injection. When emissions are
expressed as a percentage of the total amount of fumigant
added to the system, clear differences between the application
methods were again observed. Total emissions from the shank
injected soils were, under HDPE: cis 42%; trans 37% and CP
1.1%. And under VIF: cis 2.3%; trans 2.5% and CP 0.0008%.
Comparing these values (Table 2), it is evident that total
emissions of cis and trans 1,3-D were both lower with the
drip/raised-bed application under HDPE, but that in all other
cases this application resulted in higher total emissions than
the shank injection.
We believe that the reason the drip/raised-bed application
did not produce lower emissions of 1,3-D and CP under VIF
was due to the furrow region of the chambers not being
covered with film. As is evident in Table 2, the majority of
emissions from the VIF chambers occurred from this region,
that is, the fumigants diffused to this region through the soil
because they could not pass through the highly impermeable

film over the bed/sidewall surface. In contrast, with the shank
injection under VIF, the entire soil surface was covered with
the film and resulted in very low emissions. The same is not
true for HDPE because of its greater permeability, that is, the
vast majority of emission occurred from the bed surface
(Table 2). Under HDPE, the 1,3-D and CP clearly behaved
very differently in relation to application method and we
believe this may have been due to differences in their
physical/chemical properties. Although they exhibit similar
solubility, the dimensionless Henry’s constant of CP is
significantly higher than that of both cis and trans 1,3-D.
Indeed, Wang et al. (18) indicated a CP value almost twice
that of cis 1,3-D and almost three times that of trans 1,3-D
(Table 1). Therefore, under conditions of drip irrigation, trans
1,3-D would be expected to most readily remain in the water
phase and CP to more readily convert to the gas phase (with
cis 1,3-D intermediate). With the drip application taking place
much closer to the soil surface than the shank application,
a conversion of CP to the gas phase soon after drip application
would likely lead to higher emissions, that is, due to a short
path length to the soil surface. For the 1,3-D isomers, their
greater affinity for the water phase likely led to solute transport
to greater depths with the irrigation water, and an increased
path length to the soil surface. In combination with fumigant
partitioning into the liquid phase and the clogging of soil
pores by the irrigation water, this will have led to reduced
emissions compared to the shank injection.
Overall, the results suggest that, compared to HDPE, VIF
can offer significant reductions in emissions of 1,3-D and CP
from raised bed systems. 1,3-D under HDPE was the only
combination that followed the expected behavior of reduced
emissions from drip-applied, tarped, raised-beds when
compared to shank injection under plastic film. On this basis,
we conclude that further direct comparisons (i.e., using the
same soils, plastic films, and experimental conditions)
between shank and drip application systems are required to
fully determine emission reduction benefits of either
approach.

Acknowledgments
We thank Q. Zhang and Y. Wang for their technical assistance
in conducting these experiments. This work was funded by
California Air Resources Board. The use of trade, firm, or
corporation names in this paper is for the information and
convenience of the reader. Such use does not constitute an
official endorsement or approval by the United States
Department of Agriculture or the Agricultural Research
Service of any product or service to the exclusion of others
that may be suitable.

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