Abstract
Soil fumigants are often applied to agricultural land to control nematode populations and crop diseases. Lowering fumigant losses to the atmosphere improves air quality and is now mandated by the California Department of Pesticide Regulations (CDPR) for the state of California. New impermeable and semi-impermeable membranes (tarps) are being developed to minimize volatility losses of fumigants from soil. A low-cost soil column and data analysis experimental system was summarized for estimating membrane permeability. This apparatus was designed using engineering principles (computational fluid dynamics) and readily available materials (commercial 30-gallon steel drums). Soil columns, in the absence of soil, can act as dynamic permeability chambers for measuring membrane parameters. Historical, theoretical derivations are extended to include important physical processes such as evaporation rates from the liquid to vapor phase, kinetic and equilibrium sorption of the fumigant to film, and degradation of fumigant in the vapor phase. Estimates of the mass transfer coefficient (mtc) for the soil fumigants 1,3-dichloropropene isomers and chloropicrin across 1-mil HDPE film using this apparatus were almost twice as high as published results by the USDA using static permeability chambers. Higher mtc predictions were corroborated by recent and independent fieldwork for membrane permeability under field conditions. Increased net mass transfer coefficients were consistent with expectations for a solute continually swept from the membrane surface as the concentration gradient across the membrane is maximized due to the convective transport of solute away from the film surface. Thus, a dynamic system provides a beneficial alternative to smaller static permeability chambers, especially if the impact of airflow across the membrane is important and accurate bridging parameters are required for numerical predictions of field-scale soil fumigant flux loss and exposure once transported to air.