Nonaqueous Phase Liquid Removal by Postconventional Techniques

Nonaqueous phase liquids (NAPL) tend to be trapped within aquifers. The review first covers conventional technologies that rely on pumping water, hot water, and/or air sparging with vapor extraction. The review then addresses polymer and foam delivery, which are intended to directly add solutions to low-permeability zones, where NAPL resides. Based on data from the literature, the removal of hydrocarbons by any of the flushing techniques, including polymer and foams apply well for porous media whose hydraulic conductivity is greater than 10-4 m/s, excluding silt and clay materials. For those lower permeability soils, electrokinetics (EK) appears appropriate. EK relies on imposing a voltage of DC current across the soil, which would engender three types of flows: electromigration, which causes ions to move to the electrode of opposite sign; electrophoresis, which causes charged particles, such as negatively charged clay particles or bacteria (mostly negatively charged but also some positively charged) to the electrode of the opposite sign; and electroosmosis, which occurs only when a zeta potential exists in the soil (typical of clay and silt) that would cause the movement of water and potentially NAPL. EK could be used to deliver anionic surfactants through electromigration or nonionic surfactants through electroosmosis (in clay or silt). An emerging hydraulic technique is chaotic advection, and it maximizes the contact between the delivered solution and the soil region of interest. The main challenge of applying EK in field studies is the familiarity of operators and scalability, as the electrodes cannot be more than 15-20 m apart.