US EPA

Chemicals that readily vaporize at relatively low temperatures can migrate from contaminated soils into the atmosphere via a process called volatilization. Volatilization represents a potentially significant exposure pathway because humans can come in contact with volatilized compounds through inhalation. The EMSOFT manual describes a computer screening model that may be used 1) to determine concentrations of contaminants remaining in the soil over a given time (when the initial soil concentration is known); 2) to quantify the mass flux (rate of transfer) of contaminants into the atmosphere over time; and 3) to subsequently calculate contaminant air concentrations by inputting mass flux values into atmospheric dispersion models. Ingestion of contaminated soil and dermal contact are also potential exposure pathways requiring knowledge of the contaminant soil concentration to estimate potential risks. EMSOFT can also be used by risk assessors and exposure modelers to calculate average chemical concentrations at a given depth over time.

Through a series of menus the user is prompted for several input choices to select a calculation method, chemical data, soil properties, and layer properties. The various calculation methods include Time-Averaged Flux, Flux vs. Time, Time- and Depth-Averaged Soil Concentration, Depth-Averaged Soil Concentration vs. Time and Soil Concentration vs. Depth. The user also has the option to determine cover layer thickness; the number of contaminant layers and their corresponding thickness; and the time period for averaging flux and soil concentration.

Input variables and their effect on model output are discussed within the manual. The model assumes the following conditions 1) a steady porewater flux, 2) homogeneous soil properties, and 3) chemicals are present in dissolved form and at low concentrations. The user friendly, menu driven EMSOFT model utilizes a DOS operating system and is written in Fortran programming language.

The EMSOFT model is based on the work of Jury et al. (1990) and is used to estimate the emission flux of volatile soil contaminants to ambient air. The model includes a closed-form solution for both partitioning of a total volume soil concentration into multiple phases (i.e., sorbed-phase, aqueous-phase, and vapor-phase) as well as a solution for vapor transport of the vapor-phase contaminants to the soil surface and subsequent flux to ambient air. At the present time, the model also accounts for solute transport within the soil column including both downward flux (i.e., leaching) and upward flux (i.e., capillary rise and evaporation at the soil surface). The model does not account for subsurface transformation processes such as biodegradation, hydrolysis, redox reactions, etc. These processes can have significant effects on the availability of vapors for diffusion upward to the soil surface. In particular, biodegradation may be a significant vector for contaminant loss, especially for petroleum-based contaminants. Modeling these transformation processes and accounting for their effects on vapor-phase diffusion is difficult and perhaps beyond the present scientific understanding. These processes can be accounted for, however, by reworking the Jury flux solution to provide soil vapor concentrations as a function of depth. Comparison of these predicted depth-specific soil vapor concentrations with measured values at the same depths can then be made and the model solution altered so that the measured and modeled values coincide. In this way, the model can be calibrated for site-specific conditions.

EMSOFT input parameters include various chemical and soil properties. EMSOFT produces calculated values for 1) time averaged flux, 2) flux as a function of time, 3) time and depth averaged soil concentrations, 4) depth averaged soil concentrations as a function of time and 5) soil concentrations as a function of depth.

U.S. EPA. Emsoft User's Guide and Modeling Software (2002 Update). U.S. Environmental Protection Agency, Office of Research and Development, National Center for Environmental Assessment, Washington Office, Washington, DC, NCEA/W-0073, NCEA/W-0073, 2002.