Chapter 10
Strategies for Reducing Fumigant Loss to the Atmosphere 1
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William A. Jury , Yan Jin , Jianying Gan , and Thomas Gimmi
Downloaded by UNIV OF GUELPH LIBRARY on September 6, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1997-0652.ch010
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Department of Soil and Environmental Sciences, University of California, 2208 Geology, Riverside, CA 92521 Department of Plant and Soil Science, University of Delaware, 149 Townsend Hall, Newark, DE 19716 2
A model is developed to describe transport and loss of methyl bromide (MeBr) in soil following application as a soil fumigant. The model is used to investigate the effect of soil and management factors on MeBr volatilization. Factors studied include depth of injection, soil water content, presence or absence of tarp, depth to downward barrier, and irrigation after injection. Of these factors, the most important was irrigation after injection followed by covering with the tarp, which increased the diffusive resistance of the soil and prevented early loss of MeBr. The model offers an explanation for the apparently contradictory observations of earlier field studies of MeBr volatilization from soils under different conditions. The model was also used to calculate the concentration-time index for various management alternatives, showing that the irrigation application did not make the surface soil more difficult to fumigate, except at very early times. There fore, irrigation shows promise for reducing fumigant loss while at the same time permitting control of target organisms during fumigation. Introduction Current preplant field fumigation guidelines mandate that a > 1 mil polyethy lene film or tarp be put on the soil surface after a shallow (15^30 cm) injection of fumigant. Until recently, there was no information available about how effective the tarp was at restricting volatilization of methyl bromide (MeBr) to the atmo sphere following application. In the past few years, several experimental studies of emission of MeBr from fumigated fields have been conducted to determine the amount of chemical entering the atmosphere after application. Theirfindingsare somewhat contradictory and difficult to interpret. The first published field scale study (1) showed that 87% of applied MeBr was lost to the atmosphere within 7 days after a commercial fumigation on a tarped field, and much of that occurred in the first few hours after application. When the same group repeated their experiment on the same soil, however, they observed a loss of only 34% of applied MeBr (2). Majewski et al. (3) conducted experiments 0097-6156/96/0652-0104$15.00/0 © 1996 American Chemical Society
In Fumigants; Seiber, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV OF GUELPH LIBRARY on September 6, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1997-0652.ch010
10. JURY ET AL.
Strategies for Reducing Fumigant Loss to the Atmosphere
on adjacent fields and found 89% losses of MeBr to the atmosphere from the one that had no tarp over the surface, but smaller losses from the tarped field, i.e. 32%. Finally, Yates et al. (4) measured 64% losses from a tarped field in which MeBr was injected at the 25-30 cm depth. A laboratory study conducted by Jin and Jury (5) demonstrated that MeBr could move readily through a 1 mil polyethylene tarp covering the soil surface, apparently by dissolving into the tarp and diffusing through it. In their system, essentially 100% of the MeBr introduced at the bottom of a 22 cm soil column volatilized out the top before degrading, even if the tarp was present. The authors were able to reduce this loss substantially by adding water before covering the surface with the tarp, to as low as 4% cumulative volatilization when 1.6 cm of water was applied. These results imply that many factors can influence fumigant volatilization loss from soil. From the limited experimental information available, however, it is not clear what these factors are, and what relative importance they have. The purpose of this study is to employ a simplified mathematical model of the fu migation process to identify the factors influencing fumigant loss and to explore the effects of different fumigant and soil management strategies on reducing this loss. Since it will not be beneficial to reduce atmospheric loading if the fumi gant is rendered ineffective at pest control in the process, we also examine the concentration-time index CT (the product of the average fumigant concentration and the time of exposure) for each scenario studied. This index has been shown in previous studies to be correlated with the efficacy of fumigant dosage (6). Theory Transport Equations, Initial and Boundary Conditions The scenario that will be used for the calculation of volatilization losses is as follows: 1. The soil consists of two uniform layers of different water content #i, #2- The upper layer (0 < ζ < Η) is wetter than the lower, to represent the aftermath of a recent irrigation. 2. Fumigant moves by vapor diffusion only. This is not true for the first hour or so when density and pressure gradients contribute significantly to vapor advection, but approximates the stage that initiates when the fumigant partial pressure lowers to the point where massflowbecomes less important than diffusion. 3. Loss to the atmosphere occurs either through bare soil or through a polyethylene tarpaulin represented by a diffusive transfer coefficient h. 4. Fumigant is initially present in a narrow band of concentration at a depth L in the soil. This is an idealization of the dispersed initial distribution resulting from the pressure-driven and density-driven early stages.
In Fumigants; Seiber, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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FUMIGANTS: ENVIRONMENTAL FATE, EXPOSURE, AND ANALYSIS
5. A barrier to gaseous diffusion is present at ζ — P. This depth may be finite or infinite. 6. The fumigant partitions linearly and instantaneously into the dissolved and sorbed phases. 7. The fumigant undergoes first-order degradation in the soil, described by a degradation rate coefficient μ that is constant and independent of location.
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Under these assumptions, we can write the transport equations in layers 1 and 2 as follows: + R -^ 2
= D -^-; 1
+ /ii? C = D -^; 2
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2
0< ζ
