DISPLACEMENT OF NON-AQUEOUS PHASE LIQUID BY GAS FLOW DURING THERMAL REMEDIATION OF TRICHLOROETHENE

Gas flow occurs during the application of many in situ remediation technologies used for the treatment of contaminated soil and groundwater. In situ thermal treatment (ISTT) technologies, such as electrical resistance heating (ERH) and thermal conductive heating (TCH), rely on the formation of gases at elevated temperature followed by their transport, capture, and treatment in an aboveground treatment system. If these gases are not captured and extracted, vaporized volatile organic compounds (VOCs) can condense after transport into colder areas or after thermal treatment has stopped, potentially preventing the achievement of remediation objectives. However, when applied to source zones containing non-aqueous phase liquids (NAPLs) gases compete with NAPL for the larger pore spaces and can displace NAPL to previously uncontaminated regions. This study investigated the heating of trichloroethene (TCE) pools in a 40 cm tall × 20 cm wide × 1 cm thick sand-packed glass-walled chamber. Heating was conducted in a manner similar to an ERH application, where electric current was applied between two horizontal graphite electrodes spaced 16 cm apart. Temperatures were measured using thermocouples installed through the side walls of the chamber and digital images were captured throughout the heating process using light transmission techniques. These images were used to quantify dynamic gas saturations at fine spatial and temporal resolutions and allowed observations of NAPL removal and displacement. Results showed that the co-boiling of water and TCE produced discontinuous gas (bubble) flow capable of transporting NAPL out of the original pool zone by both displacement from the larger pore spaces where gas nucleation and growth was occurring and at the leading edge of steam fronts moving through colder regions. Displacement was particularly significant when the pool was initially present in a coarse layer surrounded by finer material. These results demonstrate the potential for NAPL to be displaced during the early development and flow of gas, and highlight the need to carefully consider gases in the design of heating and extraction systems in field-scale ISTT applications.