Fundamental Principles of In Situ Thermal Treatment

Thermal techniques for in situ remediation have been gaining popularity, both in the U. S. A. and internationally, due to their robustness and effectiveness. The success of such techniques is due to the thermodynamic fact that once the soil-water-NAPL system is heated to temperatures approaching the boiling point of water, the NAPL vaporizes. The vaporization processes occurs at a rate determined by the heat transfer rate, not the mass transfer rate. Thus, removal of NAPLs as vapors from water/NAPL-saturated low permeability zones is controlled by the bulk thermal diffusivity, not aqueous phase molecular diffusivities. Since thermal diffusivities are four to five orders of magnitude greater than molecular diffusivities, thermal remediation timescales are thousands of times faster than pump and treat. In addition, chemicals that tend to partition into the vapor phase can be easily removed from the aqueous phase through depressurization, electrical heating or radio frequency heating if the soils are at the water boiling point. Thus, thermodynamic pathways exist at elevated temperatures which lead to rapid and complete removal of the NAPL and reductions of groundwater concentrations to values representative of MCLs.

To illustrate these mechanisms, the results of a set of experiments, analytical theory, numerical simulations, and a field demonstrations are presented.