Background/Objectives. Thermal technologies are widely used to target high mass contaminated sites where Non-Aqueous Phase Liquids (NAPLs) are known to be present. While the geology and hydrogeology of a site is typically a driving factor for technology selection, the mass distribution, chemical composition and thermal behavior of the site contaminants are often the key drivers in determining which phase the mass is mobilized, and thus the extraction strategy. For VOCs, mobilization in the vapor phase typically fully governs the mass removal. However, for more complex mixtures of high boiling point hydrocarbons, that may not be the case. This presentation focuses on mass removal and design considerations at thermal sites, where contaminant masses are counted in the hundred-thousand to million pounds range.

Approach/Activities. As more complex sites are targeted using thermal technologies, and more focus is drawn towards eliminating the environmental risk of source zones rather than ensuring full mass removal, removal of contaminants in the NAPL phase appears to be increasingly dominant. For creosote, jet fuel, coal tar, PCBs and general oil-based source zones, the reduction in liquid viscosity with temperature can be several orders of magnitudes and therefore plays a major role in contaminant behavior during the thermal remedy. Thus, it is crucial for a successful thermal design to include provisions for proper NAPL removal and management – to both capture the chemical mass in the subsurface and to properly handle the mobilized mass in the treatment system when brought to the surface. This changes the design focus of not only the subsurface components, but also the above grade treatment system features and infrastructure.

Results/Lessons Learned.  In this presentation, results from five TCH and SEE sites where NAPL removal in the liquid phase was shown to be the dominant contaminant mobilization mechanism, will be presented. This includes several high mass sites where more than 80-90% of the total mass was recovered as a NAPL, including former manufactured gas plant sites, oil sites and sites where mixed contaminants were present as NAPL. This presentation focuses on design considerations and lessons learned for these high mass source zones.