Experimental Study of NAPL Dissolution Fingering in Two-Dimensional Heterogeneous Porous Media

Laboratory studies were conducted to evaluate the influence of heterogeneity on NAPL dissolution fingering. During a spill event heterogeneous porous media favor the formation of nonuniform NAPL saturation fields, including NAPL residual and NAPL pools, that may affect the fingering mechanism. A light transmission technique was used to measure trichloroethylene saturation fields at a 0.05-cm resolution that resulted from spills in two heterogeneous packings of a laboratory test cell. The correlation length of the permeability field transverse to be the mean water flow direction was selected to be similar to (1.0 cm) or significantly greater than (6.0 cm) the expected wavelength of dissolution fingers. As the entrapped NAPL dissolved into water, preferential NAPL dissolution patterns occurred in both experiments, with patterns strongly affected by the heterogeneities. Experimental results were used to validate the utility of a numerical simulator for capturing the growth of centimeter-scale preferential NAPL dissolution patterns. Using data from these experiments, four different methods for upscaling the mass transfer rate coefficient for NAPL dissolution were examined [Imhoff et al., 2003; Saenton and Illangasekare, 2007; Christ et al., 2006; and Basu et al., 2008]. These models were developed to account for the influence of dissolution fingering or NAPL architecture on the long-term flux of contaminants from NAPL source zones. In the packing where the correlation scale of permeability perpendicular to the mean water flow direction was 6.0 cm, greater than the scale of the dissolution fingers, all upscaling approaches predicted effluent concentrations reasonably well. When the correlation scale of the heterogeneities was smaller (1.0 cm), the models performed much poorer. These results and their implications on the applicability of the upscaling models will be discussed.