Comparison of Simple Fate and Transport Models Used in Various Risk Assessment and Risk-Based Cleanup Goals Guidance Documents. C. Susiene, K. Zhang, M. Garry, C. M. Velicer, and M. G. Norman, Foster Wheeler Environmental, 10900 NE 8th St., Suite 1300, Bellevue, WA 98004-4405
For indirect pathways, such as chemical volatilization from soil or groundwater to ambient air, fate and transport modeling is often required to estimate exposure point concentrations and calculate corresponding cleanup goals. Fate and transport models vary in degrees of complexity. Simple analytical models may be implemented with a single set of equations and parameters. Other models can be solved analytically, but must be evaluated over the exposure period at discrete time intervals. Finally, some models cannot be solved analytically. Solutions to these models must be determined numerically from multiple iterations using software programs. This paper deals with the simplest type of analytical models.
Each year, more states and regulatory agencies are adopting tiered approaches to risk assessment and risk-based cleanup goal development, such as those presented in ASTM’s Risk-Based Corrective Action for Petroleum Release Sites and EPA’s Soil Screening Guidance. In these tiered approaches, the lower tiers start with simple analytical default models, and then progress to more complicated models if the need arises. On the surface, the default models presented in various guidance documents may appear to be quite different. However, many of these models are quite similar and differ only in the default parameters. This paper explains the fundamental theory behind several of these simple models; compares the default models and parameters presented in various guidance documents; describes the applicability of the various default models and parameters; describes different fate and transport components that are often added to the base models; and describes for which chemicals and scenarios the addition of these alternate components would be most beneficial.