Estimating the Transient Source Strength of Soil Fumigants for Use In Air Dispersion Calculations
Steven A. Cryer, R&D Operational Excellence, Dow AgroSciences, 9330 Zionsville Rd, Indianapolis, IN 46268 and Ian J. Van Wesenbeeck, R&D Reg Labs, Dow AgroSciences, 9330 Zionsville Rd, Indianapolis, IN 46268.
Soil fumigants are compounds with high vapor pressures and 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. Field studies help quantify the transient flux profile observed under typical agronomic conditions. Unfortunately, field trials only represent a small proportion of the near semi-infinite parameter combinations of environmental, agronomic, and meteorological conditions. Therefore, soil physics modeling is used to expand field knowledge and provide estimates of transient source strength for use in air dispersion modeling. The USDA model CHAIN_2D simulates chemical transport in variable saturated/unsaturated porous media. This model has largely been underutilized by the agricultural community due to lack of pre and post processing tools, slow CPU execution times, and lack of simulation of physical constraints often associated with agriculture. However, CHAIN_2D has been modified to make amenable to soil fumigation practices, with automated input/output and finite grid generation capabilities. Significant modifications include additions of a tarp boundary condition proposed by Wang et al (1998, 1999), along with the ability to simulate tarp tearing and tarp removal at user specified time intervals post application, and coupling to air dispersion modeling. A new air/soil boundary condition can be specified that ties meteorological conditions to the resistance to mass transfer at the air/soil interface, or the default and original stagnant (constant) boundary layer thickness of CHAIN_2D can be assumed. This tool, developed using Microsoft Excel with custom VBA programming, allows easy exploration of different best management practices (BMP's). A sensitivity analysis for CHAIN_2D, comparison to field observations for the fumigant 1-3-dichloropropene, and several BMPs examples illustrating the impact of source strength and resulting near-field air concentrations are provided.