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

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 Mel, the ability of soils to retain Mel on the solid phase, and the potential for leaching of Mel and its primary degradation product, iodide, down a soil profile. Results indicated that Mel 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 Mel within soil. Together with low observed KD values (0.10 to 0.57 mL g―1), these data suggest that Mel may be prone to leaching. Indeed, in a steady-state soil column study, initially retained Mel was transported with interstitial water. The Mel degradation product, iodide, was also readily transported in this manner. The data highlight a potentially significant process by which Mel fate and transport within the environment may be impacted.