Soil fumigants, used by the agricultural community to control nematodes and plant disease, often have sufficiently high vapor pressures to become airborne and poise inhalation risks to bystanders. Fumigant lost from soil following treatment has historically been estimated from large field trials or approximated using soil physics models that solve mass and energy conservation equations. This work offers an empirical-hybrid alternative for fumigant loss based upon multiple field study observations using the soil fumigant 1,3-dichloropropene. Simple empirical, 1st order kinetic material balance expressions can capture representative field observation trends. Material balances are characterized by two rate constants that can be approximated using legacy soil physics models or fit to field observations. These rate constants are further parameterized by soil/air temperatures to collapse empirical flux loss predictions to field measurements. This hybrid approach for soil flux estimates can be used in the absence of data or when field observations are limited. Such estimates of flux loss can be used as initial conditions for procedures to “back calculate” the transient volatility mass loss based upon external air concentration measurements and air dispersion modeling, or directly in air dispersion models as the transient source. Examples determining the transient flux rate (back calculation method) using an optimization algorithm, concentration measurements near a fumigant treated field, and several air dispersion models are provided. Resulting air concentrations, bystander, and ecological risk can subsequently be addressed once a representative transient fumigant flux rates from soil are known.