Fate of Non-Aqueous Phase Liquids: Modeling of Surfactant Effects

Abstract

When a sufficient volume of an organic liquid is released into the environment, it may persist in the environment in the form of a residual saturation in either the unsaturated or the saturated zones of the subsurface. In the case of organic liquids less dense than water (referred to as LNAPLs, Light Non-Aqueous Phase Liquids), if the spill is of sufficient volume to reach the water table (Figure 1), some fraction of the liquid will remain as a residual saturation in the pores of the soil in the unsaturated zone. The liquid that reaches the water table will form a liquid lens, with some depression of the water table. As the volume of the LNAPL lens decreases, the water table may return to its original height, resulting in the formation of a residual saturation of the LNAPL in the aquifer medium (Weber, et al., 1991; Johnson, et al., 1989). If the liquid is a Dense Non-Aqueous Phase Liquid (DNAPL)—a chlorinated hydrocarbon solvent such as trichloroethylene is an example—its fate in the soil above the water table is essentially the same as that of the LNAPL. Upon contact with the ground water, however, the chlorocarbon, being up to 60% more dense than water, can continue to move down through the aquifer, displacing the ground water (Figure 2). Within the aquifer, the chlorocarbon may also become trapped in pores of the aquifer medium by capillary forces. If the size of the spill is sufficient, pools of the chlorocarbon may collect in the aquifer on an impermeable layer. Though the chlorocarbon is immiscible with the ground water, it has sufficient solubility to contaminate the ground water to an unacceptable level. Chlorocarbon dissolved in the ground water will not only move away from the site of the residual saturation, but will also partition between the ground water and the solid phase of the aquifer as it moves away from the spill (Huling and Weaver, 1991). A residual saturation is particularly persistent in the environment because only a limited interfacial area may be exposed to the ground water; this has the effect of reducing the overall mass transfer rate away, from the site of the residual saturation which in turn increases the persistence of the contamination. In case of a residual saturation in a ground water environment, traditional pump-and-treat methods may require that many hundreds of pore volumes of water be circulated through the contaminated region before the residual saturation has been removed. This situation has led to an interest in the use of surfactants for enhancing the rate of removal of DNAPLs from a subsurface environment (Palmer and Fish, 1992).