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Title: Phase partitioning, retention kinetics, and leaching of fumigant methyl iodide in agricultural soils

item ASHWORTH, DANIEL - University Of California
item Yates, Scott
item LUO, LIFANG - University Of California
item XUAN, RICHENG - University Of California

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2012
Publication Date: 6/20/2012
Citation: Ashworth, D.J., Yates, S.R., Luo, L., Xuan, R. 2012. Phase partitioning, retention kinetics, and leaching of fumigant methyl iodide in agricultural soils. Science of the Total Environment. 432:122-127.

Interpretive Summary: Methyl iodide is regarded as an important soil fumigant for the killing of plant pests prior to the planting of crops. However, its potential use has caused significant controversy, especially in California, due to its high toxicity. There is concern that its gaseous loss from soil to air may impact human health. As a result, its sale within the USA has been halted but it may continue to be sold in the future and is still used in other countries. Therefore, it is important to understand how this chemical behaves in the soil environment. Although other work has assessed the loss of methyl iodide from soil to air, very little work has determined if methyl iodide residues remain in soil after fumigation, and if such residues can be leached down the soil profile by rain/irrigation water. Our results showed that methyl iodide persisted in soil, especially when the soil was rich in organic matter. In addition, methyl iodide was found to be present within soil water. In a laboratory leaching experiment, methyl iodide and its degradation product, iodide, were readily transported down the soil profile. This represents a potentially important pathway in terms of groundwater and surface water contamination.

Technical Abstract: Although it is not currently being sold in the USA, the recent US registration of the fumigant methyl iodide has led to an increased interest in its environmental fate and transport. Although some work has now considered its volatile emissions from soil, there remains a lack of experimental data regarding its ability to be retained in soil and ultimately become transported with irrigation/rain waters. Using laboratory batch and soil column experiments, we aimed to better understand the phase partitioning of MeI, the ability of soils to retain MeI on the solid phase, and the potential for leaching of MeI and its primary degradation product, iodide, down a soil profile. Results indicated that MeI was retained by the solid phase of soil, being protected from volatilization and degradation, particularly in the presence of elevated organic matter. Retention was greater at lower moisture content, and maximum retention occurred after 56 days of incubation. At higher moisture content, the liquid phase also became important in retaining MeI within soil. Together with low observed KD values (0.10 to 0.57 mL/g), these data suggest that MeI may be prone to leaching. Indeed, in a steady-state soil column study, initially retained MeI was transported with interstitial water. The MeI degradation product, iodide, was also readily transported in this manner. The data highlight a potentially significant process by which MeI fate and transport within the environment may be impacted.