Article pubs.acs.org/JAFC
Effect of Co-formulation of 1,3-Dichloropropene and Chloropicrin on Evaporative Emissions from Soil Daniel J. Ashworth,*,†,§ Scott R. Yates,§ Ian J. Van Wesenbeeck,# and Mike Stanghellini⊥ †
Department of Environmental Sciences, University of California, Riverside, California 92521, United States U.S. Salinity Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 450 West Big Springs Road, Riverside, California 92507, United States # Dow AgroSciences, 9330 Zionsville Road, Indianapolis, Indiana 46268, United States ⊥ TriCal Inc., P.O. Box 1327, Hollister, California 95024, United States §
ABSTRACT: Co-formulations of 1,3-dichloropropene (1,3-D) and chloropicrin (CP) are commonly used for preplant fumigation in the production of high-value crops. Various ratios of 1,3-D to CP are available in these co-formulations. Collation of previous field data suggested that when the two fumigants were co-applied, the emissions of CP were significantly lower than when CP was applied singly. However, none of these previous studies had a control treatment with CP applied alone, alongside a treatment where CP was co-applied with 1,3-D under the same climatic and edaphic conditions. This work aimed to address this issue by measuring emission fluxes from soil columns maintained under controlled conditions in which 1,3-D and CP were applied alone and as four commercial co-formulations with various 1,3-D:CP ratios. A strong positive relationship between CP emissions and CP percentage in the formulation was observed. Furthermore, strong positive relationships between CP degradation half-life and CP percentage in the formulation and between CP degradation half-life and total column emissions suggested that the lower emissions were due to faster CP degradation when the CP percentage (and hence initial application mass) in the formulation was low. The presence of 1,3-D did not significantly affect the degradation rate of CP, and, therefore, it is hypothesized that co-application was, in itself, not a significant factor in emission losses from the columns. The findings have implications for the accurate modeling of CP because the effect of initial mass applied on CP degradation rate is not usually considered. KEYWORDS: fumigants, degradation, half-life, soil columns, Telone II, Tri-Clor, Telone C-17, Telone C-35, Pic-Clor 60, Pic-Clor 80
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1,3-D is given as 1.7−53 days.3 For CP, half-life is reported as a few hours to several days6 but is commonly considered to be approximately 4.5 days in sandy loam soil.7 In California, fumigant use is increasing. Pesticide Use Reports published annually by California Department of Pesticide Regulations (CDPR)8 indicate that CP use increased from 2.49 × 106 kg in 2007 to 4.09 × 106 kg in 2012, and that 1,3-D use increased from 4.35 × 106 kg in 2007 to 5.57 × 106 kg in 2012. Indeed, 1,3-D and CP were ranked the first and third most used fumigants (based on mass applied) in California in 2012, respectively. In the same year, they were ranked the third and fifth most used pesticides (based on mass applied), respectively. Strawberry production is an important, and increasing, user of fumigants. According to CDPR9 the fumigated land area for this commodity increased by 16% from 2011 to 2012. The majority of the 2011−2012 increase in fumigant use in strawberry production was due to the fumigants metam sodium (72% increase), 1,3-D (24% increase), and CP (11% increase). Given their increasing use and importance, it is necessary to consider the emissions behavior of 1,3-D and CP. These fumigants can be applied separately or as co-formulations with
INTRODUCTION The use of chemical fumigants is critical in agricultural areas of states such as California and Florida, where high-value crops, requiring efficacious pest control, are commonly grown.1 Although they provide excellent plant pest control, fumigants are highly volatile, making them prone to evaporation into the atmosphere. In addition to the toxic nature of these volatile chemicals, they also have the potential to contribute to the formation of near-surface ozone if sufficient NOx is available for reaction. Therefore, degradation of air quality and associated inhalation health risks in agricultural workers and local populations are potential hazards associated with fumigant application2 that warrant management via mitigation strategies such as emission reduction (e.g., tarps and irrigation) and buffer zones. The ban on the use of methyl bromide, together with the recent withdrawal of methyl iodide in the United States, has limited the options for growers requiring the high degree of pest control offered by soil fumigants. 1,3-Dichloropropene (1,3-D) and chloropicrin (CP) are broad-spectrum fumigants with fungicidal, herbicidal, insecticidal, and nematicidal properties; however, weed control with these products is limited.3,4 Often, 1,3-D and CP are applied together to enhance pest control. For example, commercial products of 1,3-D are often combined with CP to enhance the control of fungi.5 These fumigants can be applied using shank injection or drip application, with or without tarps. The aerobic soil half-life of © 2014 American Chemical Society
Received: August 11, 2014 Accepted: December 22, 2014 Published: December 22, 2014 415
DOI: 10.1021/jf506010b J. Agric. Food Chem. 2015, 63, 415−421
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Journal of Agricultural and Food Chemistry Table 1. Total Emissions and Maximum Fluxes of Chloropicrin (CP) in Collated Field Study Data Where either Coformulations (C-17, C-35, or Inline) of CP and 1,3-Dichloropropene (1,3-D), or CP Alone, Have Been Applied chloropicrin +1,3-D
chloropicrin only
CP emissions
CP emissions
total (%)
max fluxa (μg m2 s−1)
application
tarp?
ref
total (%)
max fluxa (μg m2 s−1)
application
tarp?
ref
11 17 8 3 1 9 30 9 7 8 3 8 11 8 6 5 2 10 9 9 5 3
14 8 3 3 2 10 23 6 9 7 5 19 11 6 5 4 2 19 18 13 7 1
S S S S S S S S S S S D D D D D D D D D D D
Y Y Y Y Y Y N N N N N Y Y Y Y Y Y Y Y Y Y N
22 18 18 17 17 17 18 18 17 17 17 23 10 24 24 24 18 25 25 25 25
46 49 63 69 37 34 51 50 65 61 22 17 15 12 22 9 14 3
21 23 64 75 17 21 37 23 106 133 47 28 24 15 39 13 28 6
S S S S S S S S S S D D D D D D D D
Y Y Y Y Y Y N N N N Y Y Y Y Y Y Y Y
26 26 27b 27b 27b 27b 26 26 27b 27b 24 24 24 24 24 24 24 24
av (all) SD (all) n (all)
8 6 22
9 6 22
36 22 18
40 34 18
av (shank) SD (shank) n (shank)
10 8 11
8 6 11
53 12 10
52 41 10
av (drip) SD (drip) n (drip)
7 3 11
10 7 11
14 6 8
25 14 8
av (tarped) SD (tarped) n (tarped)
8 4 16
9 6 16
29 21 14
30 20 14
av (nontarped) SD (nontarped) n (nontarped)
10 10 6
9 8 6
57 7 4
75 53 4
18
Maximum CP flux normalized to an application rate of 112 kg ha−1 (100 lb acre−1). Application types were either shank (S) or drip (D). bData from Chloropicrin Manufacturers Task Force (CMTF) study, reported by Cryer and Van Wesenbeeck.27
a
co-formulation with 1,3-D, percentage emissions of CP are significantly lower than when it is applied singly. In fact, in some field studies, zero, or an extremely low amount of, detectable CP was emitted from the soil.10 A possible explanation for these findings is that the presence of 1,3-D enhances the degradation of CP. There is evidence in the literature11 showing that the degradation rate of CP increased by approximately 15% in soil containing 1,3-D compared to that in soil with no 1,3-D. A major limitation of this collated emissions data, as it is used for this comparison, is that these previous studies were not designed specifically to test the effect of co-application of 1,3-D
various 1,3-D:CP ratios. However, there appears to be an effect of using co-formulations on the total emission losses of CP measured. We collated data (Table 1) from several field experiments in which CP was applied either alone or as the 1,3D:CP co-formulations Telone C-17 (∼17% CP, 83% 1,3-D), Telone C-35 (∼35% CP, 65% 1,3-D), and Inline (∼33% CP, 61% 1,3-D). Meta-analysis of these data showed that, on average, total CP emissions (% of applied) were ∼4.5 times lower when CP was applied as a co-formulation with 1,3-D compared to being applied singly. Such observations have not been made for 1,3-D. These data are discussed in more detail later in this paper, but ostensibly suggest that when added as a 416
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tubes were cut into their A and B sections and each was placed into separate glass vials. After the addition of 4 mL of n-hexane, the vials were immediately capped and shaken for 1 h, and then 1 mL of supernatant solvent was removed for gas chromatography (GC) analysis. For the charcoal backup tubes, the procedure was the same except that the entire charcoal medium was removed into a glass vial, and 4 mL of acetone was used for the extraction. Each co-formulation treatment was performed in triplicate, although for the PC-80 treatment, one of the three replicates yielded data that were deemed unreliable since they were very different from the other two replicates and also outside the trend observed across the experimental data set as a whole. This replicate was therefore excluded. When appropriate, one-way analysis of variance (ANOVA) was used to statistically compare emission losses between treatments at the 95% confidence level. This analysis was performed in Microsoft Office Excel 2007. Degradation Study. Degradation kinetics of 1,3-D and CP in each formulation were also determined for the Arlington soil. Moist (5% gravimetric moisture content) soil (10 g) was placed into a 20 mL glass vial and 200 μg of fumigant added (total amount of 1,3-D plus CP) for each treatment/replicate. The vials were immediately capped (Teflon-faced butyl caps) and placed at 25 °C. At times 0 (immediately after adding fumigant), 2, 4, 8, 24, 48, 72, 144, and 240 h triplicate vials were removed and placed at −19 °C. At the end of the experiment, 10 g of anhydrous sodium sulfate and 10 mL of ethyl acetate were quickly added to each frozen vial before immediate recapping, shaking for 1 h, and subsequent removal of 1 mL of supernatant to an amber GC vial for GC analysis. Extraction efficiencies were determined as ∼80% for both chemicals. Degradation loss of 1,3-D and CP from the soil was fitted with a first-order kinetic model to calculate the degradation rate constant and half-life for each chemical in each treatment. Analysis. Analysis of sorbent tube and soil solvent extracts was carried out using an Agilent Technologies 7890C GC, equipped with a microelectron capture detector. The column was a DB-VRX 122-1534 with dimensions of 30 m × 0.25 mm × 1.4 μm (Agilent Technologies) running at a flow rate of 1.4 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 80 °C at a rate of 2.5 °C min−1 and then to 120 °C at a rate of 30 °C min−1, before being held at this temperature for 2 min. Under these conditions, retention times of cis1,3-D, trans-1,3-D, and CP were 11.0, 12.3, and 13.8 min, respectively. Sets of eight standards encompassing the range of concentrations of the samples were prepared from the Telone II (1,3-D only) and TriClor (CP only) formulations. Standards were prepared in the same solvent as the samples. 1,3-D data are reported as total 1,3-D (i.e., the sum of the cis and trans isomers).
and CP on CP emissions. Therefore, none of these studies had a control treatment with CP applied alone alongside a treatment in which CP was co-applied with 1,3-D under the exact same climatic and edaphic conditions. The goal of this work was to address this issue by measuring emission fluxes from soil columns maintained under controlled conditions in which 1,3-D and CP were applied alone and at various 1,3D:CP ratios. Our hypothesis was that co-application of 1,3-D and CP would result in lower total emissions of CP than when CP was applied alone due to increased (or changes in) fumigant degradation kinetics when CP concentrations in soil are reduced.
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MATERIALS AND METHODS
Chemicals, Sampling Tubes, and Soil. The commercially available 1,3-D and CP formulations used in this study were Telone II (1,3-D only), Tri-Clor (CP only), Telone C-17 (∼17% CP, 83% 1,3-D), Telone C-35 (∼35% CP, 65% 1,3-D), Pic-Clor 60 (∼60% CP, 40% 1,3-D), and Pic-Clor 80 (∼80% CP, 20% 1,3-D). Telone II, C-17, and C-35 were donated by Dow Agrosciences (Indianapolis, IN, USA) and Tri-Clor, PC-60, and PC-80 by TriCal Inc. (Hollister, CA, USA). Sodium sulfate (anhydrous), n-hexane, and ethyl acetate were obtained from Fisher Scientific (Fair Lawn, NJ, USA). Acetone was obtained from Burdick and Jackson (Muskegon, MI, USA). XAD-4 (2 section 400/200 mg) and Anasorb CSC charcoal (2 section 400/200 mg) sorbent tubes were purchased from SKC Inc. (Eighty Four, PA, USA). Deionized water was used in the preparation of solutions. Sandy loam soil (75% sand, 18% silt, 7% clay; 0.92% organic matter; Arlington soil series) was collected from the plowed, upper 30 cm of field 2B of the University of California Riverside Agricultural Station. This field has been used in numerous field and laboratory fumigant emission studies performed by the U.S. Salinity Laboratory (see, e.g., refs 12−14); however, the soil for this study was collected from an area of the field that had not been previously fumigated. The soil was sieved to 5 mm and stored in sealed plastic bags in a cool environment. The gravimetric moisture content of the soil used in the experiments was between 4 and 5%. Soil Column Study. Stainless steel soil columns (12 cm diameter × 150 cm length) were used to study the surface emissions of coformulations of 1,3-D and CP, following a simulated shank injection at 46 cm soil depth. The general design and setup of the columns has been described previously.15,16 Soils were packed into the columns in 5 cm intervals to a dry bulk density of 1.5 g cm−3. The top of each column was sealed with a stainless steel flux chamber. The columns were housed in a controlled temperature room where the ambient temperature was maintained at 25 °C. Approximately 177 mg of a fumigant formulation was injected into the center of a soil column at 46 cm depth (equivalent field application rate of approximately 156 kg ha−1). Specifically, the amounts of 1,3-D and CP applied for each formulation were as follows: Telone II, 177.1 mg of 1,3-D, 0 mg of CP; Telone C-17, 143.1 mg of 1,3-D, 29.0 mg of CP; Telone C-35, 115.2 mg of 1,3-D, 62.8 mg of CP; Pic-Clor 60, 71.4 mg of 1,3-D, 106.6 mg of CP; Pic-Clor 80, 35.1 mg of 1,3-D, 142.9 mg of CP; and Tri-Clor, 0 mg of 1,3-D, 176.4 mg of CP. Immediately after application, a vacuum (100 mL min−1) was applied to the outlet of the flux chamber to sweep clean air across the soil surface. In this way, headspace gas vapors were collected on in-line sorbent tubes (XAD-4 filter media) that were sampled every 4 h throughout the experiment. Backup sorbent tubes were used to check for fumigant breakthrough. All sampled tubes were capped on both ends and stored in a freezer at −19 °C. Background air within the controlled column environment was sampled daily by pulling the air through an XAD-4 sorbent tube for 24 h periods. In addition, at the start of the experiment, blank XAD-4 and charcoal tubes were placed in the sample storage freezer (i.e., storage blanks). Analysis of all storage blank and background air sampling tubes yielded nondetectable levels of each fumigant, and so no correction was required for these potential sources of contamination. At the end of the experiment (10 days), XAD-4
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RESULTS AND DISCUSSION
Figure 1 shows total emission losses of 1,3-D and CP (as a percentage of the amounts added) for each formulation. Total emissions of 1,3-D (∼30−35% across all treatments) were broadly consistent with previously published values for both field17−19 and laboratory15,16 studies. One-way ANOVA testing showed no statistically significant differences (p > 0.05) in 1,3D emissions across the treatments. The relative consistency of 1,3-D total emissions across the treatments indicates that coformulation of this fumigant with CP did not affect 1,3-D emission behavior from the columns, consistent with previously published work (Table 1). Conversely, total CP emissions varied widely across the treatments, increasing monotonically from 0.018 to 14% as the amount of CP in the formulation increased (Figure 1). In general, emission flux curves followed the expected trend of a peak in emissions shortly after application, followed by a gradual decline and tailing of the curve over time (data not 417
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suggested a hypothesis that chloropicrin emissions are reduced in the presence of 1,3-D. For example, on average, for this data set as a whole, co-application across 22 studies yielded approximately 4.5 times lower CP emissions (8% total emissions) than single CP application across 18 independent studies (36% total emissions). If only shank applications are considered (the same as emulated in the present study), the difference in CP emissions between co-formulation (10% total CP emissions) and single application (53% total CP emissions) is even greater. For drip applications, the difference in total CP emissions is lower but still marked (7% for co-application and 14% for single CP application). Clearly, emission reduction strategies, such as tarping, may be an important factor affecting emission levels across the collated studies, some of which were tarped whereas some were not. However, large differences in CP emissions between co-application and single CP application are similarly observed if tarping is removed as a potential factor. For example, for tarped shank applications, CP emissions averaged 8% with co-application and 50% with single application. With nontarped shank applications the values were 11 and 57%, respectively. In addition to an apparent effect on total CP emissions, the maximum emission flux of CP also seemed to be affected by co-formulation of CP with 1,3-D across these studies (Table 1). The maximum CP emission flux (normalized to an effective application rate of 112 kg ha−1 (100 lb acre−1)) for shank applications was reduced by >5 times from 52 μg m2 s−1 with a single application to 8 μg m2 s−1 with co-application. On the basis of anecdotal evidence, these results are also consistent with the experience of growers and applicators in the field, who have observed that chloropicrin is rarely detected (by olfactory sense or lachrimation) in Telone C-17 applications in the Pacific Northwest. Overall, these collated field data broadly concur with our soil column studies in suggesting that when CP is co-applied with 1,3-D, total CP emissions and emission fluxes are reduced compared with CP applied alone. However, our column studies allow for a more detailed analysis of the relationship between the percentage of CP in a co-formulation and CP emissions. Indeed, for the column data, one-way ANOVA testing revealed that CP percentage in the formulation was statistically a highly significant (p = 0.008) factor affecting total CP emissions. We further observed a strong positive relationship (R2 = 0.99) between CP percentage in a formulation and CP emissions (Figure 2), indicating that as CP levels decreased (and, consequently, 1,3-D levels increased) in the co-formulation, a process occurred to limit CP emissions from the soil. However, this relationship was not linear, but was rather best fitted with a
Figure 1. Total 1,3-D and CP emission losses (as percentage of amount added) for each treatment. Average CP emissions for the C-17 treatment were 0.018% (standard deviation = 0.018%).
shown). For formulations containing both 1,3-D and CP, this tailing occurred over a longer period for 1,3-D than for CP, probably due to the longer degradation half-life of the former.15 Of significance in the assessment of the effects on air quality and risk assessment of fumigant operations is the maximum flux, in terms of both its magnitude and its timing. For each treatment, these variables are shown in Table 2. Peak emission flux generally occurred very rapidly after fumigant application, typically within 12 h. Compared to each fumigant applied singly, peak fluxes across the treatments generally declined in response to decreasing percentage (or mass) of each fumigant in the treatment. However, this relationship was not proportional as is particularly noticeable for the treatments with low CP levels. For example, the peak CP flux in the C-17 treatment was only 0.07% of the peak flux in the CP-only treatment despite containing 17% of the total CP level of this treatment. Similarly, the value for the C-35 treatment was just 6.3%, despite containing 35% of the CP amount in the CP-only treatment. Such dramatically disproportional differences were not observed across the 1,3-D treatments. Overall, CP again showed very different emission flux behavior from 1,3-D. These experiments were conducted in an attempt to elucidate observations made during a meta-analysis of a number of published field studies in which co-application of 1,3-D and CP as the commercial product C-17, C-35, or Inline (a commercial drip application product with 33% CP and 61% 1,3-D), or as CP applied singly, was performed (Table 1). This meta-analysis, where the data from a number of independent studies were combined in a weight-of-evidence approach,
Table 2. Average Peak Emission Flux and Its Timing, Degradation Rate Constant, and Half-Life for 1,3-Dichloropropene (1,3D) and Chloropicrin (CP) across All Treatments 1,3-D
CP
emissions Telone II (1,3-D)a C-17 C-35 PC-60 PC-80 TriClor (CP)
degradation
peak flux (μg m2 s−1)
peak flux time (h)
k (h−1)
t1/2 (h)
80.4 29.8 22.3 13.1
4−8 8−12 8−12 12−16 8−12
0.0038 0.0048 0.0033 0.0030 0.0031
182.4 144.4 210.0 231.0 223.5
emissions
degradation
peak flux (μg m2 s−1)
peak flux time (h)
k (h−1)
t1/2 (h)
0.016
8−12 8−12 12−16
0.280 0.047 0.020 0.012 0.010
2.5 14.7 35.4 59.7 66.6
9.5 16.6 23.5
8−12
Emission flux of 1,3-D for the Telone II treatment showed a relatively high degree of variation due to one of the three replicates exhibiting a relatively rapid and high peak emission flux. a
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of CP was related to the co-presence of 1,3-D, half-lives for various initial CP masses were determined in a separate degradation study in which approximately equivalent initial amounts of TriClor (CP only) were added to soils (i.e., in the absence of 1,3-D). The strong overlap between the data sets for CP degradation both in the presence and in the absence of 1,3D (Figure 3) indicates that CP degradation rate was not affected by the presence of 1,3-D. Because the total amount of fumigant (1,3-D + CP) added to each treatment of the degradation study was the same, the amounts of applied CP varied across the treatments. As shown in Figure 3, the initial CP mass controlled its degradation rate within the soil, with smaller masses degrading more rapidly. The dependency of fumigant degradation upon initial mass or concentration has been previously observed,20,21 and rapid degradation (half-life of 2.9 h) of CP in sandy loam soil has also been reported when the initial concentration was low (10 μg−1).15 With a lower initial concentration, the number of biological and chemical sites available for degradation is greater per unit mass of fumigant. At higher initial concentrations, competition for such sites is likely greater, leading to a lower degree of degradation. Although this process may explain the degradation behavior of CP, it does not explain why 1,3-D was not affected in a similar way. However, Zheng et al.11 noted that for soil collected from the same field as the current study, autoclave sterilization led to a >6 times increase in CP half-life from 9 days to 57 days when compared to control soil, demonstrating the significant influence microbial populations have on CP degradation rates. For 1,3-D, however, the increase due to sterilization was much less marked, from 139 to 193 days (averages of cis and trans isomers). This apparently strong dependence upon biological processes suggests that the suppression or death of soil microbes at high CP concentrations may induce slower CP degradation. The observed differences in CP degradation rate across the formulations provide a highly probable explanation for the observed trend in column CP total emissions and emission fluxes. A positive linear relationship (R2 = 0.97) was observed between CP halflife and total emissions across the formulations. This strongly suggests that dramatically faster degradation in formulations with lower levels of CP resulted in relatively low amounts of CP available for emission to air. The degradation studies further suggested that this effect occurred irrespective of 1,3-D presence. Therefore, although there were significant differences in CP emissions across the formulations, this cannot be attributed to a 1,3-D/CP co-formulation effect. This suggests that we would expect the same CP emissions as those observed in the current column experiment if it was carried out using equivalent initial application masses of CP alone. From an air quality perspective, the low CP emissions from certain co-formulations are clearly beneficial. The results have implications for volatility and environmental fate modeling of CP because its degradation rate is not often considered in relation to its initial application mass. The calculation of buffer zone sizes around fumigated fields is based on a linear scaling of empirically derived emission losses from studies utilizing the maximum labeled application rates. Therefore, the relationship between emission flux and application rate is assumed to be linear and have a slope of 1; that is, doubling the application rate would double the emission flux. Translating the current work into this broader context can be achieved by regressing maximum CP flux (commonly used as the primary indicator of fumigant impact on local air quality) against CP application rate
Figure 2. Relationship between percentage CP in the formulation and total CP emissions (percentage of amount added). Fitted line is a third-order polynomial.
third-order polynomial equation, which suggested a disproportional relationship between total CP emissions and CP percentage in the formulation (similar to the CP emission flux data). We are not aware of any previously published data that report this relationship. In the column system, two processes are considered to dominate fumigant fate and transport, emissions and degradation. Because determining in situ degradation rates within the column is not possible, a separate degradation study was performed to quantify this parameter for each formulation product. In this degradation study, the total fumigant mass (1,3D + CP) addition at the start of the experiment was the same for each formulation treatment. Subsequent loss of fumigant was attributed to soil degradation. Fitting a first-order kinetic model to the loss of fumigant over time yielded the degradation rate constants and half-lives shown in Table 2. At the lowest level of CP addition (C-17), CP half-life was just 2.5 h and at the highest level (CP only), was 66.6 h. As shown in Figure 3, a
Figure 3. Relationship between initial CP mass added and CP degradation half-life in the presence (co-formulation data points) and absence (CP-only data points) of 1,3-D. The co-formulation data points are, from left to right, C-17, C-35, PC-60, and PC-80. Fitted line is third-order polynomial.
strong positive relationship between initial CP mass added in the formulation and CP half-life was observed across all treatments. For 1,3-D, across the six formulation products, halflives ranged from 144 to 231 h, and no consistent relationship between half-life and percentage of 1,3-D in the formulation was observed. Therefore, in contrast to CP, it was concluded that the initial mass of 1,3-D was not significant in dictating the half-life of this fumigant. To determine whether the degradation 419
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(Figure 4). In this case, although the relationship is linear, the slope is
