THE USE OF UPWARD GRADIENTS TO ARREST DOWNWARD DENSE, NONAQUEOUS PHASE LIQUID (DNAPL) MIGRATION IN THE PRESENCE OF SOLUBILIZING SURFACTANTS

This paper presents a mathematical analysis and the results of a series of laboratory experiments investigating in situ dissolution of dense, nonaqueous phase liquid (DNAPL) pools from porous media using solubilizing surfactants under upward-flowing conditions. Surfactant flood experiments were conducted in a one-dimensional glass column packed with natural sand. Perchloroethylene (PCE) was emplaced in the column as a pool perched above a coarse-fine sand interface to reproduce conditions occurring when DNAPL pools on a low permeability layer in the subsurface. Aqueous surfactant solution was directed vertically upward to counteract downward pool mobilization resulting from reduction of the PCE-water interfacial tension in the presence of surfactant. Experiments were conducted at overall hydraulic gradients across the sand pack of 0.0, 0.1, 0.2, 0.3, and 0.5. The surfactant flood solution consisted of a 1:1 blend of two commercial surfactants, Witconol NP-100 and Rexophos 25/97, in a 2% aqueous solution. Surfactant flooding was conducted until all visible traces of PCE were removed from the column. Subsequent water flooding was continued until effluent PCE concentrations exiting the column dropped below 5 ppm. In the zero gradient experiment, the PCE pool invaded the fine sand layer immediately upon contact with the surfactant because of a lowering of interfacial tension. PCE entry into the fine sand did not occur in the 0.3 and 0.5 gradient runs. Minor amounts of PCE mobilization occurred in the late stages of the 0.1 and 0.2 gradient runs and is attributed to a lowering of interfacial tension with time upon exposure to surfactant.