This work examines the transport behavior of LNAPLs in the region bounding the water table. Defined as the Partially Saturated Fringe, or PSF (Berkowitz et al., 2003), the region bounding the water table includes the classic Capillary Fringe (CF) and the region below the water table in which multiple phases (air, water, and/or LNAPL) are present. This work builds on prior work within our laboratory on Air Entry Barriers (e.g., Silliman et al., 2002; Dunn and Silliman, 2003; Dunn et al., 2003) as well as the substantial literature on LNAPL behavior (e.g., Nambi and Powers, 2000; Illangasekare et al., 1995a,b; Schroth et al., 1998; Walser et al., 1999; Van Dijke and Van Der Zee, 1997). Of particular interest for the present study is the interaction of the structure of heterogeneity and the distribution / dissolution of LNAPLs under conditions of a fluctuating water table. The work is based on a combination of two-dimensional laboratory and numerical experiments on the distribution of pure- and dissolved-phase LNAPL in a two-zone porous medium (coarse sand and fine sand). Within the laboratory, visual and TDR techniques will be used to monitor LNAPL saturation. Mass of dissolved-phase LNAPL constituents will be monitored at the outflow from the porous media. Numerical experiments will be performed on simulated random and scaled porous media and T2VOC, an extension of TOUGH2 [Transport Of Unsaturated Groundwater and Heat, version II], developed at Lawrence Berkeley National Laboratory. These experiments will be based on generating a series of realizations of random distributions of the coarse and fine sands (based on both stationary random permeability distributions and structured permeability distributions), followed by simulation of a fluctuating water table with significant LNAPL phase above the original water table. The laboratory experiments will be utilized to verify LNAPL behavior as predicted by the numerical model, as well as to provide visualization of LNAPL behavior. The numerical model will be utilized to address the central hypotheses: The degree of connectivity (defined within the proposal) of coarse sand lenses within a fine sand matrix will influence the amount of LNAPL entrapped within the PSF. Specifically, as the connectivity of the coarse regions increases (with the same relative volume of coarse and fine sands), the volume of entrapped LNAPL will decrease. An increase in the degree of connectivity of coarse sand lenses within a fine sand matrix will result in a reduced overall rate of dissolution of LNAPL entrapped within the PSF. In terms of intellectual merit, this work advances the understanding of controls of LNAPL distribution and dissolution in the PSF. From a theoretical standpoint, this provides greater insight into the impact of physical structure on transport characteristics near the water table. From an applied standpoint, extension of this work holds promise for innovative means of remediation of LNAPL contaminated systems. In terms of broader impact, this work will provide training for one Ph.D. and one Masters student, as well as contributing to undergraduate and graduate courses in groundwater hydrology and remediation. In addition, this work will aid in further development of research collaborations with colleagues in western Africa (Benin).
