Principal Investigator: S.R. Yates, U.S. Salinity Laboratory, Riverside, CA.

Cooperators: J. Gan, F.F. Ernst, D. Wang, W.A. Jury, M.V. Yates, F. Gao, A. Mutziger, U.S. Salinity Laboratory and University of California, Riverside.

Methyl iodide (MeI) has been recently proposed as an ideal replacement for methyl bromide in soil fumigation. Current research indicates that MeI is as effective as methyl bromide (MeBr) in controlling pests and is readily degraded in the lower atmosphere; hence should not significantly deplete stratospheric ozone. Since MeI is not a registered pesticide, however, there is virtually no information regarding its environmental behavior in agricultural soils. With the impending cancellation of MeBr and previous cancellation of other soil fumigants, it has become imperative to find and adopt methods for reducing emissions so that the remaining soil fumigants do not suffer a similar fate. Also, any decision to use alternative fumigants should be weighed in terms of the probable environmental consequences and the associated economic implications, and should be compared with methyl bromide.

There is a great need for more information on the soil­chemical parameters which characterize the fate and transport of MeI. Since MeI has a similar chemical structure to MeBr, one would expect similar environmental behavior. Some information which is important in determining environmental fate and transport includes: atmospheric degradation, field­scale soil degradation, liquid­gas phase and liquid­solid phase partitioning, effective diffusion in soil and the various resistances to mass transfer between the soil and the atmosphere (e.g. volatilization). Other useful information on MeI which is generally available from chemical handbooks includes: water solubility (14,190 mg/L),
log K_ow (octanol­water coefficient, 1.69), molecular weight (141.9 g/mole), vapor pressure (400 mmHg @ 25C), vapor density (4.89 mg/mL) and boiling point (42.5 C).

We have recently started to obtain basic information on MeI transport. For example, although MeI and MeBr (1420 mmHg @ 20C) have very different vapor pressures, their Henry's Law constants are similar (K_h,0.25 for MeBr and 0.22 for MeI at 25C). Henry's Law describes the equilibrium partitioning between the gas and liquid concentration and is an important measure of how readily a fumigant will diffuse through soils; since vapor diffusion is much greater than liquid diffusion. The free­air diffusion coefficients can be obtained using the Fuller­Schettler­Giddings correlation (Broadkey and Hershey, 1988) and is 6.96 cm²/min for MeBr and is 6.28 cm²/min for MeI. Coupling these values to the gas­phase tortuosity gives the gas­diffusion coefficient for the soil.

Soil degradation is the transport parameter which is least known. Recent experiments indicate that the degradation half­life for MeBr is approximately 10 days in a Greenfield sandy loam. Preliminary studies suggest that the MeI half­life is approximately 50 days in this soil. For a Linne clay loam soil with higher organic matter content, the half­lives were 5 and 10 days, respectively, for MeBr and MeI. A longer half­life for MeI would promote an increased residence time in soils. This may cause delays in planting, if the soil needs venting to reduce phyto­toxicity, and would produce a greater total emission into the atmosphere compared to MeBr. Also, if irrigation follows fumigation, MeI's high water solubility and long residence time coupled to the reduced air­filled pore space after irrigation, will increase the probability of ground water contamination.

To demonstrate these ideas, a simulation of gas­phase diffusion in a sandy loam soil was conducted. The selected soil conditions are typical for fumigation and the model includes the affects of liquid­gas and liquid­solid partitioning, soil diffusion, degradation and volatilization at the surface through either a polyethylene or high­barrier plastic cover. The following soil properties were used for both MeBr and MeI: porosity 0.4 cm³/cm³, water content 0.15 cm³/cm³, K_d 0.2 cm³/g, bulk density 1.6 g/cm³ and mass applied 250 kg/ha at a depth of 25 cm. The Henry's law constants were 0.25 (MeBr) and 0.22 (MeI); soil degradation half­lives were 10 days (MeBr) and 50 days (MeI); and the effective soil diffusion coefficients (e.g., D_e = D_soil/R_g) were 0.376 (MeBr) and 0.301 (MeI). The resistance offered by the plastic material was obtained using Table 2 of Yates et al. (1996) which reports the flux density of various fumigants through polyethylene and Hytibar (high­barrier) plastics. For MeBr and MeI, respectively, the resistance offered by the high­barrier plastic is 75 and 148 times greater than 1.4 mil polyethylene.

When 1.4 mil polyethylene plastic covers the soil, the model predicts that 43.5% of the applied MeBr and 67% of the applied MeI will volatilize. The peak flux densities were 0.10 mg/cm²/min for both MeBr and MeI. When a high­barrier plastic is used, the model predicts that only 1.4% and 1.9% of the applied MeBr and MeI escape from the soil. This suggests that the use of high­barrier plastics offer a important control mechanism for reducing emissions. Note that these estimates assume that the plastic remains on the field until soil gas­phase concentrations are zero, which is considerably longer than the time period used in typical fumigations. Clearly, field experiments and simulations using more comprehensive models are necessary to refine these estimates.

The model predicts that MeI will move deeper into soils than MeBr and that using a high­barrier plastic will enhance downward movement. When the surface is covered with polyethylene, the model predicts that 2.0% and 0.2%, respectively, of the applied MeBr will move below 3m and 5m deep in soil. For MeI, 6.1% and 2.0% is predicted. When high­barrier plastic is used, the fraction of MeBr moving below 3m and 5m is slightly higher at 3.5% and 0.4%. For MeI, using a high­barrier plastic causes significant increases in deep movement: 17.9% of applied passes 3m and 5.5% at 5m. This is due, principally, to slower degradation and improved containment.

Although this gives a first glimpse into the behavior of MeI in soils, the effect of numerous model simplifications need to be investigated before making any conclusions using the figures given above. Also, there is a great need for more information on the relevant environmental parameters for a wider range of soil and environmental conditions.

Last Updated: October 15, 1996