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

Area of Science:

• Environmental Engineering
• Geotechnical Engineering
• Remediation Technologies

Background:

• Thermal conduction heating (TCH) is a subsurface remediation technology for removing volatile and semi-volatile non-aqueous phase liquids (NAPLs).
• Gas generation during TCH aids NAPL removal but can also cause NAPL redistribution within and beyond source zones.

Purpose of the Study:

• To investigate the redistribution of trichloroethene (TCE) and creosote in coarse and medium sands during bench-scale TCH experiments.
• To understand the mechanisms of NAPL transport and removal under different soil conditions.

Main Methods:

• Conducted bench-scale experiments using TCH on coarse and medium sand matrices.
• Introduced trichloroethene (TCE) and creosote as representative NAPLs.
• Monitored NAPL transport and phase changes (e.g., co-boiling) during heating.

Main Results:

• In coarse sand, discontinuous gas flow facilitated upward NAPL transport via double displacement and spreading.
• In medium sand, a continuous gas channel led to limited TCE redistribution but significant upward and lateral creosote transport.
• TCE removal occurred via co-boiling, while only higher volatility creosote components were depleted.

Conclusions:

• TCH poses a risk of NAPL transport away from the heated zone, necessitating thorough site investigation and potentially perimeter heaters.
• Complete heating of the entire treatment zone is essential to ensure removal of displaced NAPLs.
• Understanding soil properties is critical for predicting and mitigating NAPL redistribution during TCH remediation.