Removal of pooled dense, nonaqueous phase liquid from saturated porous media using upward gradient alcohol floods

Laboratory experiments employing 90% by volume alcohol solutions are used to compare the abilities of ethanol and 1-propanol to remove pooled tetrachloroethene (PCE) from saturated porous media using low upward hydraulic gradients. Equilibrium ternary phase diagrams measured for the systems water/PCE/ethanol and water/PCE/1-propanol indicate that for alcohol concentrations below the miscibility envelope, 1-propanol will partition predominantly into the dense, nonaqueous phase liquid (DNAPL) phase while ethanol remains in the aqueous phase. Interfacial tension and phase density measurements show that while both systems demonstrate a reduction in interfacial tension with increasing alcohol content, the density difference between the aqueous and DNAPL phases is only reduced for the 1-propanol system. Two-dimensional experiments in saturated porous media using alcohol floods ranging in size from 0.125 pore volumes (PV) to 1.0 PV recovered between 5.7% and 98.7% of the PCE mass. The removal mechanisms for the ethanol floods included enhanced dissolution followed by miscible displacement, while the 1-propanol floods removed PCE by DNAPL swelling and interfacial tension reduction leading to immiscible displacement followed by miscible displacement. Recovery results and effluent composition histories indicate that hydrodynamic instabilities and dispersion cause significant alcohol slug deterioration and confirm the necessity of using an appropriate size alcohol slug of sufficient concentration for efficient PCE mass recovery.