Risk reduction during chemical flooding: Preconditioning DNAPL density in situ prior to recovery by miscible displacement

Dense, nonaqueous phase liquids (DNAPLs) are separate phase compounds that commonly contaminate groundwater supplies. Miscible displacement methods using surfactants and alcohols to recover the DNAPLs have been proposed, but concerns have been raised about mobilizing the DNAPLs deeper into previously uncontaminated media. In this paper, the concerns are addressed by reducing DNAPL density prior to elimination of interfacial tension. Laboratory-measured equilibrium phase behavior demonstrates the ability of 2-butanol to reverse the density contrast between tetrachloroethene (PCE) and water prior to miscibility, resulting in a DNAPL phase less dense than the aqueous phase. Laboratory experiments using an upward gradient flow cell demonstrate that 2-butanol, introduced as an aqueous solution to a PCE pool suspended within a water-saturated sandpack, partitions strongly into the PCE. Pools of PCE exposed to 1 pore volume of water saturated with 2-butanol and subsequently recovered by miscible displacements with 1-propanol showed that between 56.6% and 97.6% of the DNAPL mass was recovered for total alcohol complements of between 0.24 and 1.0 pore volume. In all two-phase samples produced, the NAPL phase was less dense than the aqueous phase. Analysis of a static DNAPL pool shows that the use of a 2-butanol preflood may prevent the downward mobilization predicted to occur for more conventional alcohol floods.