Monte Carlo-based dispersion modeling of off-gassing releases from the fumigant metam-sodium for determining distances to exposure endpoints

Abstract

Fumigants, such as metam-sodium, can help control pests in the soil and improve the quality and yields for a wide range of crops. Enhanced control of field loss, or off-gassing, to the atmosphere is an important means of promoting fumigant efficacy and sound environmental management of fumigants. Metam-sodium has been used for over 40 years and is one of the most widely used fumigants in the United States. This paper describes a modeling method, the fumigant emissions modeling system (FEMS), which was developed to promote more realistic assessment of downwind concentrations in the vicinity of an applied field. Two models of the US Environmental Protection Agency, i.e., the industrial source complex short-term (ISCST3) model and the toxic modeling system short-term (TOXST) model are used, in conjunction with a pre-processing program, PCRAMMET, and the FEMS programming code to provide a Monte Carlo treatment for all key model inputs. Start times for applications are triggered on a Monte Carlo basis consistent with the annual application frequency. Typically, 1000–10,000 years of applications are simulated, outputting data to display average exceedances per year of selected concentration endpoints as a function of distance from the edge of the treatment zone. A sensitivity analysis is displayed for key meteorological variables, showing uncertainty in emission rates to be a significant input parameter. Some comparisons also are made between ambient concentrations and modeled personal and indoor exposures, showing perspective on the benefits of the indoor environment to buffer peak concentrations.