Contrast Between Theoretical and Measured Emission Rates of Chemicals Volatilizing from Soil. R. J. Machado, A. Johnston, and A. S. Kao, ENVIRON Corporation, 4350 North Fairfax Drive, Suite 300, Arlington, VA 22203
A risk assessment was recently conducted at a Superfund site in which high levels of contamination existed in several distinct areas of the site. Emission rates of chemicals volatilizing from the soils were estimated based on soil sampling data in order to assess potential inhalation risks associated with contaminated soils at the site. The estimates of time-averaged mass flux rates from these area sources were determined based on a volatilization model described in USEPA’s Risk Assessment Guidance for Superfund document. The available emission models are based on an idealized representation of contaminant distribution that frequently does not reflect actual conditions at sites with historical contamination. To provide a more representative characterization of emissions from the site, flux chamber measurements were made from several areas of potentially high emissions at the site. The modeled emission rates were found to exceed those predicted using the flux-chamber measurements by several orders of magnitude. Conservative assumptions incorporated into the volatilization model, the limitations of these models, and their impact on the results will be discussed.