Dissolution of residual tetrachloroethylene in fractional wettability porous media: Incorporation of interfacial area estimates

Available data from a recent study of entrapped tetrachloroethylene (PCE) dissolution [Bradford et al., 1999] reveal that soil wettability and grain size distribution characteristics can dramatically influence dissolution behavior. This paper explores the modeling of the long-term dissolution of residual nonaqueous phase liquids (NAPLs) entrapped in fractional wettability porous media. Here dissolution is modeled with a linear driving force expression, and the lumped mass transfer coefficient is represented using independent estimates of the film mass transfer coefficient and the NAPL-water interfacial area. In fractional wettability porous media the NAPL saturation is assumed to occur as both NAPL films and ganglia, and NAPL-water interfacial area is thus modeled as the sum of contributions from films and ganglia. The interfacial area model is calibrated by fitting several parameters to the available experimental data. Trends in fitted parameters suggest that (1) residual NAPL ganglia are less accessible to a flowing aqueous phase in finer textured soils; (2) the dissolution rate depends on the interfacial area in a nonlinear manner; and (3) NAPL ganglia entrapment depends on system wettability and grain size distribution. Mass transfer coefficient correlations are established from these fitted parameters. The calibrated model predicts that for a given NAPL saturation, an increasing PCE-wet sand fraction results in longer periods of high effluent concentrations, followed by increased rates of concentration reduction and more persistent subsequent low-concentration tailing. The dependence of dissolution behavior on mean grain size is also captured by this model. Simulations demonstrate a significant improvement in dissolution predictions using the proposed model in comparison with those obtained using a previously developed mass transfer correlation based upon a power law function of saturation.