Overview:

LNAPLs are nonaqueous phase liquids (NAPLs) that are lighter than water such as gasoline and other fuels. DNAPLs are nonaqueous phase liquids that are denser than water. Chlorinated solvents are particularly problematic DNAPL contaminants found in groundwater. They are in widespread use for metal cleaning and degreasing, dry cleaning, paint removal and as adhesives and aerosols, and invade saturated groundwater zones in complex, erratic migration pathways that are dependent on fluid and soil properties. Because of the complexity of these flow patterns, and the significant influence of small-scale heterogeneities, it is important that pore scale models of DNAPL invasion be easy to use, visual, and interactive.

SAM (Trantham and Durnford, 1999) is a modified DLA model that transitions from a diffusion limited aggregation (DLA) to anti-DLA or compact, stable flow (Lenormand et al.,1988). As an example, figure 1 shows single model realizations for the DNAPL 1,2 dibromoethane (EDB). EDB is denser and slightly more viscous than water with a viscosity of 1.49 cp and a density of 2.17 g/cm³. The photographs show systematic changes in stability as the flow rate and angle from horizontal of the plane changes. The change is stability is manifested by smooth transitions from thin fingers, to fingers with thicker branches, to compact fronts. Values of the sticking probabilities range from 0.6(1a) to 0.02 (1d) for this DNAPL under the conditions shown.

(a)	(b)
(c)	(d)

Figure 1. The displacement of water by 1,2 Dibromoethane at different flow rates and different angles of inclination from the horizontal.

References:

Lenormand, R., Touboul, E. and Zarcone, C., 1988. Numerical models and experiments on immiscible displacements in porous media. J. Fluid Mech, 189:165-187.

Trantham, H. H. and Durnford, D. S., 1999. Stochastic aggregation model for DNAPL-water displacement in porous media. Journal of Contaminant Hydrology (in press).