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Behavior of methylisothiocyanate under varying soil water regime and soil properties Frick, Arlan V. H.


Soil fumigants have been identified as a potential risk to groundwater quality in unconfined aquifers in the Lower Fraser Valley of British Columbia. Metam-sodium, which converts in soil primarily to the volatile biocidal compound methylisothiocyanate (MITC), has seen increased usage in the area in recent years. The objective of this study was to determine the effects of soil water regime and soil properties on the behavior of MITC in soil. The partitioning of MITC to leaching loss, volatilization loss, soil extractable MITC, and MITC degradation were determined in repacked soil columns under three differing leaching regimes and in four soils with differing clay and organic carbon contents. Degradation of MITC was generally rapid. In the four soil types examined, > 90 % of applied MITC was degraded in less than 83 days. Volatilization loss of MITC increased with increasing soil air filled porosity and was inhibited by water application. In another experiment, with no water application in a coarse textured soil, approximately 30 % of applied MITC was volatilized over a 35 day period. MITC was very mobile in soil; negligible soil extractable MITC concentrations were measured after completion of the experiments and MITC movement was only slightly retarded compared to bromide, a conservative tracer. The extent of MITC retention compared to bromide increased with increasing soil organic carbon content, suggesting retention by sorption to soil organic matter. The proportion of MITC lost by leaching was dependent on the amount of MITC remaining in the soil after losses by degradation and volatilization processes, and was not related to soil clay or organic carbon content. These results suggest MITC would not normally present a risk to groundwater quality, but because of its high mobility in soil some risk is possible in situations where degradation and volatilization processes are slow in removing MITC from the soil, and water infiltration is high.

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