Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2006
Publication Date: 10/10/2006
Citation: Zheng, W., Yates, S.R., Papiernik, S.K. 2006. Conversion of metam sodium and emission of fumigant from soil columns. Atmospheric Environment. 40:7046-7056. Interpretive Summary: Agricultural fumigants have been used extensively to control plant pathogens in economically important crops. Atmospheric emission of volatile organic contaminants (VOCs) is a major problem associated with fumigant use and many management practices have been developed to reduce emissions. Emissions increase the risk of bystander exposure to toxic chemicals and may also contribute to the formation of near surface tropospheric ozone, which has become a serious concern with U.S. EPA’s implementation of a new federal 8-hour ozone standard with a 2007 deadline. Near-surface ozone, a serious problem in the interior valleys of California and is formed from the photochemical oxidation of NOx and VOCs (i.e., fumigants). This is leading to increased regulation of agricultural VOC sources and may seriously affect food production in these regions. This paper describes a laboratory experiment where soil column system was used to determine the emission and distribution of MITC after subsurface and surface application of metam sodium. Volatilization flux and cumulative emission loss of MITC were substantially reduced with surface water sealing compared to uncovered soil columns after subsurface metam sodium application. This research project will provide valuable information that can be used to determine if fumigant chemicals and their transformation products significantly contribute to near-surface ozone production and bystander risk.
Technical Abstract: Metam sodium is the most widely used soil fumigant in the United States. Methyl isothiocyanate (MITC) is the primary breakdown product of metam sodium with a high toxicity and a great potential for volatilization. Reducing atmospheric emissions of MITC is therefore critical to maintain air quality. The objective of this study was to examine the rate and efficiency of conversion of metam sodium to MITC in soil and to investigate the potential of using surface water sealing to reduce MITC emissions. The conversion of metam sodium to MITC was a rapid abiotic decomposition process. At concentrations typical of current metam sodium application rates, the conversion efficiency depended on the application rate, but was independent of soil moisture, soil type, and soil atmospheric conditions. A soil column system was used to determine the emission and distribution of MITC after subsurface and surface application of metam sodium. Volatilization flux and cumulative emission loss of MITC was substantially reduced with surface water sealing compared to uncovered soil columns after subsurface metam sodium application. When metam sodium was surface applied in simulated chemigation, surface water sealing was ineffective, suggesting the need for additional emissions reduction practices when metam sodium is broadcast. Concentrations of MITC were higher in soil columns subject to surface water sealing than those with no water seal after subsurface application of metam sodium, especially at times >24 h. The results of these experiments indicate that surface water sealing with subsurface metam sodium application may be an effective and economical strategy to reduce MITC emissions while maintaining pest control efficacy.