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TECHNICAL REPORTS

Modeling Methyl Isothiocyanate Soil Flux and Emission Ratio from a Field following a Chemigation of Metam-Sodium

^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. 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