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

Abstract

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 microg 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.

Hazard-ranking of agricultural pesticides for chronic health effects in Yuma County, Arizona

“…Measuring maneb concentrations in the home during high and low winds can be used to assess the effect of wind-driven resuspension of particles. On the other hand, metam sodium is more likely to contaminate homes by air infiltration from pesticide spray drift, due to its high vapor pressure and volatilization flux (Lee et al, 2002; Li et al, 2006). High levels of metam sodium’s toxic breakdown product, methyl isothiocyante (MITC), have been previously detected in air samples in an agricultural community (Merriman and Hebert, 2007).…”

Soil Fate of Agricultural Fumigants in Raised-Bed, Plasticulture Systems in the Southeastern United States

“…Previously, studies have been initiated to model fumigant soil flux and atmospheric emissions over large areas using methods and conditions representative of commercial production practices (Cryer and Wesenbeeck, 2009; Li et al, 2006). However, corresponding data documenting the fate of fumigants in soil when applications were made under GAPs are lacking, most notably when applications are made over large areas using methods and conditions representative of commercial production practices.…”

Methyl Isothiocyanate Residential Community Air Assessment for South Franklin County, Washington

“…The high vapor pressure of MITC (2.5-2.8 kPa at 20°C) can lead to substantial surface emission and subsequent off-target air mass movement if metam is not properly applied to the soil (Lee et al, 2002;Li et al, 2006;Sullivan et al, 2004). Thus, MITC may present a significant human inhalation exposure pathway in communities at the urbanagricultural interface (USEPA, 2005b).…”