The impact of physical heterogeneity and connectivity on LNAPL entrapment and dissolution

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0408895SillimanThis work examines the transport behavior of LNAPLs in the region bounding thewater 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 theregion below the water table in which multiple phases (air, water, and/or LNAPL) arepresent. 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 thesubstantial literature on LNAPL behavior (e.g., Nambi and Powers, 2000; Illangasekareet 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 ofheterogeneity and the distribution / dissolution of LNAPLs under conditions of afluctuating water table. The work is based on a combination of two-dimensionallaboratory and numerical experiments on the distribution of pure- and dissolved-phaseLNAPL in a two-zone porous medium (coarse sand and fine sand). Within thelaboratory, visual and TDR techniques will be used to monitor LNAPL saturation. Massof dissolved-phase LNAPL constituents will be monitored at the outflow from the porousmedia. Numerical experiments will be performed on simulated random and scaledporous media and T2VOC, an extension of TOUGH2 [Transport Of UnsaturatedGroundwater and Heat, version II], developed at Lawrence Berkeley NationalLaboratory. These experiments will be based on generating a series of realizations ofrandom distributions of the coarse and fine sands (based on both stationary randompermeability distributions and structured permeability distributions), followed bysimulation of a fluctuating water table with significant LNAPL phase above the originalwater table. The laboratory experiments will be utilized to verify LNAPL behavior aspredicted by the numerical model, as well as to provide visualization of LNAPLbehavior. The numerical model will be utilized to address the central hypotheses:The degree of connectivity (defined within the proposal) of coarse sand lenseswithin a fine sand matrix will influence the amount of LNAPL entrapped withinthe PSF. Specifically, as the connectivity of the coarse regions increases (with thesame relative volume of coarse and fine sands), the volume of entrapped LNAPLwill decrease. An increase in the degree of connectivity of coarse sand lenses within a fine sandmatrix will result in a reduced overall rate of dissolution of LNAPL entrappedwithin the PSF.In terms of intellectual merit, this work advances the understanding of controls ofLNAPL distribution and dissolution in the PSF. From a theoretical standpoint, thisprovides greater insight into the impact of physical structure on transport characteristicsnear the water table. From an applied standpoint, extension of this work holds promisefor innovative means of remediation of LNAPL contaminated systems.In terms of broader impact, this work will provide training for one Ph.D. and oneMasters student, as well as contributing to undergraduate and graduate courses ingroundwater hydrology and remediation. In addition, this work will aid in furtherdevelopment of research collaborations with colleagues in western Africa (Benin).