Published online 3 April 2006
Published in J Environ Qual 35:707-713 (2006)
DOI: 10.2134/jeq2005.0216
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Modeling Methyl Isothiocyanate Soil Flux and Emission Ratio from a Field following a Chemigation of Metam-Sodium
Lin Ying Lia,*,
Terrell Barrya,
Kevin Mongarb and
Pamela Wofforda
a California Environmental Protection Agency, Department of Pesticide Regulation, Environmental Monitoring Branch, P.O. Box 4015, Sacramento, CA 95812-4015. L.Y. Li, current address: California Environmental Protection Agency, Air Resources Board, Planning and Technical Support Division, Sacramento, CA 95812-2815
b California Environmental Protection Agency, Air Resources Board, Monitoring and Laboratory Division, P.O. Box 2815, Sacramento, CA 95812-2815
* Corresponding author (lli{at}arb.ca.gov)
Received for publication May 31, 2005.
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ABSTRACT
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Metam-sodium had become the most heavily used soil fumigant in recent years as the deadline approached for methyl bromide to phase out in January 2005. After application, metam-sodium decomposes rapidly to methyl isothiocyanate (MITC), a highly toxic compound capable of killing a wide spectrum of soil-borne pests. Inhalation risk of MITC ranked high among airborne agricultural pesticides in California. Information about off-gassing intensity and percentage of emission is essential for exposure risk assessment and mitigation measures, but is limited, especially for new application methods such as drip chemigation. Air concentrations of MITC were monitored around a field treated with metam-sodium through surface drip irrigation system. The field was tarped with plastic films before the chemigation. The air concentrations at receptor locations were simulated for the period of air monitoring with the Industrial Source Complex (ISC3) Dispersion Model, and soil flux density of MITC at various periods after chemigation was estimated through a back-calculation procedure. The estimated soil flux density of MITC showed a diurnal pattern, with the daytime flux stronger than nighttime. However, the average air concentration at nighttime was higher than that at daytime. Soil flux density peaked at 4.30 µg m–2 s–1 in the first 12-h period after chemigation, then declined with time. The MITC emission percentage in the first 60-h was 2.65% of applied mass, of which 57% occurred in the first 24-h after chemigation. The study indicated that the tarped bed drip application method of metam-sodium had a relatively good control of MITC emission from soil.
Abbreviations: ARB, Air Resources Board • CFAC, California Food and Agriculture Code • DPR, Department of Pesticide Regulation • HDPE, high-density polyethylene • ISC3, Industrial Source Complex Dispersion Model • MITC, methyl isothiocyanate • QA/QC, quality assurance and quality control • TAC, toxic air contaminant • VIF, virtually impermeable films
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INTRODUCTION
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WITH the phase out of methyl bromide, metam-sodium (C2H4NaS2) has become the most heavily used soil fumigant in California, with about 6800 Mg use annually (CDPR, 2005). After application, metam-sodium rapidly decomposes to methyl isothiocyanate (CH3NCS) (MITC), a highly toxic compound capable of killing a wide spectrum of soil-borne pests (USEPA, 1997). Thus, metam-sodium is considered one of the best methyl bromide alternatives (Noling and Becker, 1994).
The volatility of MITC is much lower than methyl bromide; however, a certain amount of MITC escapes into the atmosphere after soil fumigation with metam-sodium. Human health effects are associated with excessive exposure to MITC (Pruett et al., 2001). Inhalation risk of MITC ranked high among airborne agricultural pesticides in California communities (Lee et al., 2002). The California Pesticide Illness Surveillance Program has documented many complaints of odor, eye and nose irritation, sore throats, nausea, dizziness, and headache in bystanders and residents living near fields treated with metam-sodium (CDPR, 2002).
Various approaches have been proposed to reduce fumigant emission, including deep shank injection, tarped with plastic films, soil compaction, and water sealing through regular and intermittent irrigation (Papiernik et al., 2004a). Effectiveness of metam-sodium as a soil fumigant largely depends on the uniformity of its distribution in treated fields. Drip chemigation is an emerging application method for fumigants with low vapor pressure and moderate water solubility, including metam-sodium and 1,3-dicloropropene. Drip chemigation is desirable for fumigants that can be applied in liquid form because it reduces evaporative loss of fumigants, results in uniformity of spatial distribution, and lowers the risk of worker exposure (Papiernik et al., 2004b; Sullivan et al., 2004a).
Saeed et al. (2000) studied the MITC volatilization losses from chemigation and direct injection of metam sodium application. The MITC off-gassing profile is critical to designing, evaluating, and implementing mitigation measures to reduce loss of MITC to the atmosphere following application. However, until recently, there was little information available on soil emission. Ajwa et al. (2002) discussed techniques to improve efficacy of soluble fumigants applied through drip irrigation systems. Papiernik et al. (2004a, 2004b) studied the effects of surface tarp and application methods on emission and distribution of drip-applied fumigants. The Metam-Sodium Task Force conducted a series of field experiments to study the movement of MITC in soil and the atmosphere around fumigated fields, and the effects of application and sealing methods on MITC loss (Sullivan et al., 2004ab).
Methyl isothiocyanate is designated as a toxic air contaminant (TAC) in California (P.E. Helliker, personal communication, 2002). Under the authority of California Food and Agriculture Code (CFAC) 14022, the director of California Department of Pesticide Regulation (DPR) requested that the Air Resources Board (ARB) conduct MITC air monitoring following a tarped bed drip application of metam-sodium (CARB, 2004). The objective of this monitoring study was to collect data for the evaluation of MITC exposure risk and to support regulatory actions, such as mitigation measures and reevaluation. In this study, the ARB MITC air monitoring results were used to estimate the soil flux density and emission ratio (a proportion of applied mass that is lost following application) from a field following a tarped drip application of metam-sodium.
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MATERIALS AND METHODS
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Field Description, Chemigation, and Air Sampling
The 3.6-ha experiment field was located in Ventura County, CA (Fig. 1). The latitude and longitude of the experiment site were 34.25° N, –119.00°W, respectively. The field consisted of two plots, one rectangular and one irregular, separated by 9.15 m (Fig. 1). Eighty-four rows of drip lines were evenly spaced through the field. Each row was 1.73 m wide, and the drip line length varied from 189.4 to 290.3 m. Each row consisted of tarped bed, slope, and furrow (Fig. 2). Chemigation was conducted through surface drip lines positioned midbed under the plastic tarp. Sectagon 42 (42.2% metam-sodium by weight) was applied to the entire field from 0720 to 1000 h on 9 May 2002 (CARB, 2004). The actual application rate of metam-sodium was 60.6 g m–2 over treated bed area, and the effective broadcast rate was 26.8 g m–2 over the field area. The molecular weights of metam-sodium and MITC are 129.2 and 73.1 g mol–1, respectively, and the conversion ratio from metam-sodium to MITC was assumed to be 1:1. Therefore, the equivalent MITC application rate over the entire field was 15.2 g m–2.
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Fig. 2. Diagram of rows in the chemigated field. Each row consisted of raised bed, slope, and furrow, and their dimensions were illustrated respectively on the diagram. Drip lines were located in the middle of bed, which was tarped with the plastic film.
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The background samples were collected from 1000 h on 7 May to 1015 h on 8 May 2002. Air monitoring around the treated field was conducted from 9 to 12 May 2002. Eight air samplers, one on each side and one at each corner, were positioned 11.6 to 20.7 m from the field edge (Fig. 1). Sampling interval duration was approximately 12 h, from sunset to sunrise or from sunrise to sunset, representing overnight and daytime concentrations, respectively. The first sampling interval covered application and post-application hours (2.67 h during the application, and 9.33 h immediately following the application). The average air concentration over each sampling interval was determined through laboratory analysis of air samples.
The weather conditions were clear to partially cloudy during the study period. A weather station was positioned to the south of the southwest corner of the field (Fig. 1). Weather elements, including air temperature and wind speed as well as wind direction, were averaged every 15 min, and stored into a data logger.
The recovery rate of the lab spikes, trip spikes, and field spikes using extraction solvent made before 14 May 2002 ranged from 55 to 59%, and the recovery rate of the lab spikes using freshly made extraction solvent was 80% (CARB, 2004). The air sampling, lab analysis protocol, and quality assurance and quality control (QA/QC) measures were discussed in detail in the ARB report (CARB, 2004).
ISCST3 Air Dispersion Modeling
The USEPA ISCST3 model is capable of simulating the spatiotemporal distribution of air concentration from various emission sources. The ISCST3 model requires as input the geometry of emission source, emission rate, meteorology conditions, and location of receptors. The process of preparing and running the ISCST3 model is described in detail in the user's guide of ISCST3 model (USEPA, 1995, 2002) and Johnson et al. (1999).
Field Geometry and Receptor Location
Based on the field information, source geometry and receptor location were quantified with a user-defined coordinate system (Fig. 1). The origin point is located in the southwest corner of the field. Under this coordinate system, the locations of receptors are shown in Table 1. The upper bed was a rectangle, an area easily defined by four corner points. The lower bed was an irregular shape, but could be approximated by a polygon (USEPA, 2002). The polygon was defined by a series of vertex of corner points.
Meteorology and Stability Classification
Weather data were processed to generate hourly average meteorological data in ISCST3 model format (Johnson et al., 1999). A computer program that uses latitude and Julian day as input variables was employed to determine sunrise time, sunset time, and solar elevation angle (Johnson et al., 1999). A program was developed to determine hourly stability class based on look-up tables and hourly meteorological data (Budney, 1977). The program uses wind speed, day/night, cloud type, sky cover, and solar elevation as input variables, and outputs Pasquill stability class. Cloud type was assumed to be thin high clouds, and sky coverage zero. The program also adjusts the stability class, so that stability classes of two adjacent hours do not differ by more than one class.
Running the ISCST3 Model
Input variables described above were input to the ISCST3 model through a control file. In this study, we are interested in modeling the average air concentration over each sampling period. Therefore, there was a control file and a meteorological file corresponding to each air sampling period. When running the ISCST3 model, a nominal soil flux density (E0) of 0.0001 g m–2 s–1 was used for all five periods.
Back Calculation of Soil Flux Density
The soil flux density was determined through a back-calculation procedure (Ross et al., 1996), in which measured air concentration was regressed to the simulated air concentration as follows:
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where Cm (µg m–3) and Cs (µg m–3) represent measured and simulated air concentrations, respectively, and a and b were intercept and slope of the regression line, respectively. The 95% confidence intervals for the slope and intercept were calculated (Agresti and Finlay, 1986) to examine if a regression was statistically significant.
In the Gaussian model simulated air concentrations are directly proportional to the source flux density. The back-calculation procedure uses the slope of the regression line to adjust the nominal soil flux density to produce simulated air concentrations of the same magnitude observed in the measured air concentrations as follows:
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where E0 (g m–2 s–1) is the nominal soil flux density used in ISCST3 modeling, b is the slope of regression line, and E (g m–2 s–1) is the estimated soil flux density. The MITC emission ratio (R) is calculated as as follows:
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where A is the effective broadcast application rate of MITC (15.2 g m–2), and E (g m–2 s–1) is the estimated soil emission flux density. Constants 12 and 3600 are time conversion coefficients (assuming 12 h for each period).
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RESULTS AND DISCUSSION
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Simulated Air Concentrations and Regression
The average air concentration at all sampling sites was simulated using the nominal soil flux density (E0) for each sampling period (Table 2). The simulated air concentrations were generally proportional to the measured values within each sampling period (Fig. 3). When MITC was not detected (below the method detection limit, or MDL), simulated air concentrations were either zero or a small value. However, the simulated air concentrations were generally higher by one or two orders of magnitude than the measured air concentrations (Table 2), implying the nominal soil flux density (E0) used for the simulation was higher than that actually occurred in the field. For each sampling period, measured air concentrations (Y) were regressed on simulated air concentrations (X) (Fig. 3). The R2 ranged from 0.62 to 0.94. All five periods showed regression intercepts not significantly different from zero, and slopes significantly different from zero (Table 3). Therefore, all regressions were significant (p = 0.05). The slope of each regression model was used to adjust the simulated air concentration to the magnitude of the measured air concentrations for that sampling interval (Table 2). After the adjustment on soil flux density, the simulated concentrations matched well to the measured air concentrations (Fig. 4).
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Fig. 3. Linear regressions between measured air concentration and simulated air concentration using the nominal soil flux density (E0) of 0.0001 g m–2 s–1. As the simulated air concentration is directly proportional to the soil flux density, the simulated air concentration using the nominal soil flux density might be far off from the measured air concentration.
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Fig. 4. Measured air concentration vs. simulated air concentration using back-calculated soil flux density. The back-calculated soil flux density (E) was obtained by adjusting the nominal soil flux density (E0) with the slope of the regression model. The simulated air concentrations using the back-calculated soil flux density (E) show a good match to the measured air concentrations.
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Soil Flux Density and Emission Ratio
The peak soil flux density was estimated to be 4.30 µg m–2 s–1, which occurred in the first sampling period (Table 4). Soil flux density showed a general decline over time, but diurnal patterns were evident. Emission during the daytime was higher than during the nighttime. Daytime soil flux density decreased by about 50% each day, and the soil flux density for the second night was also about the 56% of the first night. About 1.22% MITC was lost in the first 12 h following application, and 2.65% in the first 60 h. The emission ratio of the second day was about 53.5% of the first day. Compared with other soil fumigants, MITC has lower vapor pressure and higher water solubility, and the tarped drip chemigation method of MITC from this study showed lower emission ratio. For example, the emission ratio of 1,3-dicloropropene was reported in a range of 32 to 77% (Basile et al., 1986; Wang et al., 2001). Methyl bromide shows an emission ratio as high as 60 to 70% over an 8-d period after application under typical application methods (Yates et al., 1996).
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Table 4. Methyl isothiocyanate (MITC) soil flux density and emission ratio of various sampling periods following the metam-sodium chemigation.
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The diurnal pattern of estimated soil flux density indicates that the emission largely occurred during the daytime. Daytime emission was approximately three times higher than that of nighttime during the first 48 h (Fig. 5a). The higher temperature of daytime likely created a greater gradient of vapor pressure of MITC between soil and the near surface boundary layer, serving as a driving force in the off-gassing process. However, nighttime air concentration was higher than that of daytime (Fig. 5b). For example, the average daytime air concentration for the first 12 h from all sampling points was 12.6 µg m–3 (Period 1), whereas the nighttime average was 15.2 µg m–3 (Period 2). In the second day, the daytime average of 4.8 µg m–3 (Period 3) was also lower than the nighttime average of 5.7 µg m–3 (Period 4). Meteorological conditions of nighttime tend to result in a more stable condition near the surface, and reduce dispersion of the fumigant emitted from soil. Calm, clear nights commonly are accompanied by inversion conditions that produce high air concentrations. The maximum air concentration of each period might be of more interest for the exposure risk assessment for farmers and bystanders. The maximum air concentration declined continuously after the first night after application (Fig. 5c). Unlike the average air concentration, the maximum air concentration reflects wind direction and its persistence. Therefore, it will be measured at receptors located in the predominant downwind direction.
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Fig. 5. (a) Estimated average soil flux density, (b) measured average air concentration, and (c) measured maximum air concentration over various sampling periods. Estimated average soil flux density over daytime (Periods 1, 3, and 5) was significantly greater than that over nighttime (Periods 2 and 4). However, the average air concentration over nighttime was slightly higher than that over daytime. The measured maximum air concentration declined with time after the first night of chemigation.
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Papiernik et al. (2004b) estimated from a sand mescocosm experiment a cumulative MITC emission ratio for the first 40 h after tarped bed drip chemigation of 1.6 and 3.4% from a field experiment, respectively. Both experiments used a standard 1-mil high-density polyethylene (HDPE) tarp to cover the soil surface, and soil flux density was measured with passive metal chambers. The soil flux density and emission ratio also varied with tarp materials. For example, the cumulative emission ratio from a field tarped with Hytibar, a kind of virtually impermeable films (VIF), was only about 20% of that tarped with HDPE (Papiernik et al., 2004b). The Hytibar tarp significantly reduced soil flux density in the first 60 h, so that the fumigation might be more effective. Sullivan et al. (2004b) studied the effects of application and soil sealing methods on MITC cumulative emission ratio. The cumulative emission ratio in the first 72 h ranged from 1 to 18%, depending on the method of application and soil sealing methods (Sullivan et al., 2004b). Instead of using films to cover the fields, a water-sealing method followed application. Sprinkler chemigation and shank injection with standard water sealing method showed a reversed diurnal pattern of soil flux density, with the higher flux at night.
In summary, simulated and measured MITC air concentrations in this study were in reasonable agreement in all five monitoring periods. The soil flux density exhibited a diurnal pattern, with daytime higher than nighttime. The peak soil flux density was estimated to be 4.30 µg m–2 s–1, which occurred in the first sampling interval after the chemigation. Soil flux density decreased approximately 50% each day following the chemigation. The cumulative MITC emission ratio from soil in the first 60 h was approximately 2.65%, with 57% of the total loss occurring in the first 24 h. The study indicated that the tarped bed drip application method of metam-sodium had a relatively good control of MITC emission from soil.
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ACKNOWLEDGMENTS
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The authors thank the California Air Resources Board, Monitoring and Laboratory Division, for providing the air monitoring data and meteorological data for this analysis.
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REFERENCES
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- Agresti, A., and B. Finlay. 1986. Statistical methods for the social sciences. 2nd ed. Dellen Publ. Co., San Francisco, CA.
- Ajwa, H.A., T. Trout, J. Mueller, S. Wilhelm, S.D. Nelson, R. Soppe, and D. Shatley. 2002. Application of alternative fumigants through drip irrigation systems. Phytopathology 92:1349–1355.
- Basile, M., N. Senesi, and F. Lamberti. 1986. A study of some factors affecting volatilization losses of 1,3-dichloropropene (1,3-D) from soil. Agric. Ecosyst. Environ. 17:269–279.[CrossRef]
- Budney, L.J., 1977. Guidelines for air quality maintenance planning and analysis. Vol. 10. Procedures for evaluating air quality impact of new stationary sources. USEPA Rep. 450/4-77-001. USEPA, Office of Air Quality and Standards, Research Triangle Park, NC.
- CARB. 2004. Report for air monitoring around a tarped, drip irrigation application of metam sodium in Ventura County, Spring 2002. California Environ. Protection Agency, Air Resources Board, Sacramento, CA.
- CDPR. 2002. Evaluation of methyl isothiocyanate as a toxic air contaminant (TAC-2002-01). California Environ. Protection Agency, D. of Pesticide Regulation, Sacramento, CA.
- CDPR. 2005. Summary of pesticide use report data in 2003: Indexed by chemical. California Environ. Protection Agency, Dep. of Pesticide Regulation, Sacramento, CA.
- Johnson, B., T. Barry, and P. Wofford. 1999. Workbook for Gaussian modeling analysis of air concentration measurements. California Environ. Protection Agency, Dep. of Pesticide Regulation, Sacramento, CA.
- Lee, S., R. McLaughlin, M. Harnly, R. Gunier, and R. Kreutzer. 2002. Community exposures to airborne agricultural pesticides in California: Ranking of inhalation risks. Environ. Health Perspect. 110:1175–1184.[ISI][Medline]
- Noling, J.W., and J.O. Becker. 1994. The challenge of research and extension to define and implement alternatives to methyl bromide. J. Nematol. Suppl. 26(4s):573–586.
- Papiernik, S.K., R.S. Dungan, W. Zheng, et al. 2004a. Effect of application variables on emissions and distribution of fumigants applied via subsurface drip irrigation. Environ. Sci. Technol. 38:5489–5496.[Medline]
- Papiernik, S.K., et al. 2004b. Effect of surface tarp on emissions and distribution of drip-applied fumigants. Environ. Sci. Technol. 38:4254–4262.[Medline]
- Pruett, S.B., L.P. Myers, and D.E. Keil. 2001. Toxicology of metam sodium. J. Toxicol. Environ. Health 4:207–222.
- Ross, L.J., B. Johnson, K.D. Kim, and J. Hsu. 1996. Prediction of methyl bromide flux from area sources using the ISCST model. J Environ. Qual. 25:885–891.[Abstract/Free Full Text]
- Saeed, I.A.M., D.I. Rouse, and J.M. Harkin. 2000. Methyl isothiocyanate volatilization from fields treated with metam-sodium. Pestic. Manage. Sci. 56:813–817.[CrossRef]
- Sullivan, D.A., M.T. Holdsworth, and D.J. Hlinka. 2004a. Control of off-gassing rates of methyl isothiocyanate from the application of metam-sodium by chemigation and shank injection. Atmos. Environ. 38:2457–2470.[CrossRef]
- Sullivan, D.A., M.T. Holdsworth, and D.J. Hlinka. 2004b. Monte Carlo–based dispersion modeling of off-gassing releases from the fumigant metam-sodium for determining distances to exposure endpoints. Atmos. Environ. 38:2471–2481.[CrossRef]
- USEPA. 1995. User's guide for the Industrial Source Complex (ISC3) dispersion models. Vol. 1. User instructions and Vol. 2. Description of model algorithms. USEPA Rep. 4545/B-95-003a&b. USEPA, Office of Air Quality and Standards, Research Triangle Park, NC.
- USEPA. 1997. Metam sodium as an alternative to methyl bromide for fruit and vegetable production and orchard replanting. Methyl bromide alternatives case studies. Vol. 3. USEPA Rep. 430-R-97-030. USEPA, Washington, DC.
- USEPA. 2002. Addendum: User's guide for the Industrial Source Complex (ISC3) dispersion models. Vol. 1. User instructions. USEPA, Office of Air Quality and Standards, Research Triangle Park, NC.
- Wang, D., S.R. Yates, F.F. Ernst, J.A. Knuteson, and G.E. Brown, Jr. 2001. Volatilization of 1,3-dichloropropene under different application methods. Water Air Soil Pollut. 127:109–123.[CrossRef]
- Yates, S.R., F.F. Ernst, J.Y. Gan, F. Gao, and M.V. Yates. 1996. Methyl bromide emissions from a covered field: II. Volatilization. J. Environ. Qual. 25:192–202.[Abstract/Free Full Text]
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ABSTRACT
INTRODUCTION
