Gas-facilitated NAPL transport during bench-scale thermal conduction heating experiments

Thermal conduction heating (TCH) is a subsurface remediation technology used for the removal of volatile and semi-volatile non-aqueous phase liquids (NAPLs). Gas produced during TCH facilitates NAPL removal and above ground treatment but can also redistribute NAPL within and outside of source zones. Bench-scale TCH experiments were conducted to investigate the redistribution of trichloroethene (TCE) and creosote in both coarse and medium sand. In the coarse sand, both NAPLs were transported upwards through the gas zone via double displacement and spreading mechanisms, facilitated by discontinuous gas flow. With sustained heating, TCE was removed via co-boiling, but only the higher volatility components in the transported creosote were depleted. In the medium sand, a continuous gas channel expanded away from the heater, resulting in limited TCE redistribution due to co-boiling, but substantial upwards and lateral transport of creosote away from the gas zone. This work highlights the potential risk for NAPL transport away from the heated region during TCH, which may be mitigated through accurate site investigation and the installation of perimeter heaters. This work also emphasizes the need to heat the entire treatment zone to the target temperature to promote removal of any displaced NAPL.