Impact of Source Zone Architecture on DNAPL Dissolution in Systems Containing Organic- Wet Porous Media

Subsurface wettability is an important property that impacts our ability to accurately characterize DNAPL source zones. Its variability creates uncertainty in predictions of the fate of DNAPLs in the subsurface. The purpose of this study was to explore and quantify the impact of spatial wettability variations on DNAPL migration, entrapment, and dissolution. Tetrachloroethylene (PCE), a representative DNAPL, was injected into a water saturated two-dimensional sandbox packed predominantly with organic-wet coarse sand and containing lenses of organic- and water-wet sands of various textures. Experimental observations indicated that the PCE migrated downwards through the coarse textured organic-wet sand, overcoming capillary forces. However, PCE was retained to a significant extent in the medium textured organic-wet sand lens. During the dissolution portion of the study PCE concentrations were monitored at a number of sampling locations in the sandbox as well as in the effluent. The infiltration portion of the experiment was modeled using a version of the MVALOR multiphase flow simulator, modified to include non water-wet conditions. Simulation results indicate that the model yields good predictions of the migration and retention of PCE in non water-wet porous media. The dissolution portion of the sandbox experiment was also simulated to assess the predictive capability of a wettability modified version of the numerical simulator MISER. The influence of source zone architecture on measured and simulated downstream PCE concentrations and associated mass flux for this mixed-wettability system is discussed. In general, model simulation comparisons reveal that heterogeneities in wettability can considerably control source zone mass distribution and the persistence of DNAPLs in the subsurface.