An assessment of the rate limited liquid-liquid interphase mass transfer in DNAPL source-zones: Modeling techniques and applications in site characterization

Remediation of dense nonaqueous phase liquids (DNAPLs) remains an important challenge for the environmental engineering community. Many characterization and remediation technologies rely on the mass transfer of solutes between the aqueous and DNAPL phases. Therefore, accurate description of the solute mass transfer rate is essential to the design and implementation of these applications. This research focused on modeling studies to quantify rate-limited interphase mass transfer between liquid-liquid phases within DNAPL source zones in laboratory and field-scale applications. Numerical modeling of experimental data from laboratory scale column experiments indicated that intra-NAPL diffusion offered negligible resistance to interphase mass transfer. Thus, a linear driving force approximation of aqueous phase diffusion adequately described interphase partitioning within homogeneous source zones. The effect of source zone heterogeneities on the effective mass transfer rate coefficient was explored through a series of 2D simulations. A predictive model for the upscaled mass transfer coefficient was developed and verified. The immobile phase spreading in the vertical direction, Reynolds number, pool fraction, and the effective organic phase saturation were identified as the parameters controlling partitioning rates. 3D field scale simulation of push-pull tracer tests was used to investigate the hypothesis that solute partitioning in heterogeneous source zones could be effectively described using the rate-limited model developed here, in contrast to the current practice based on a local equilibrium assumption. For the examples considered herein, the upscaled model was capable of estimating the overall NAPL saturation with less than three percent error, but not the pool fraction or the vertical spreading of NAPL.;Results of this research may be used to estimate the mass exchange rate between the separate liquid phases in heterogeneous porous media and improve our understanding of the transport of partitioning solutes in DNAPL source zones. The models developed in this research, based on rate-limited partitioning as opposed to the current approach of using local equilibrium models, provide more precise methods to analyze field scale partitioning tracer tests. Partitioning tracer tests in conjunction with other site characterization methods provide techniques to quantity the presence and distribution of NAPL in the subsurface. Future research in this area should focus on diversifying and increasing number of 3D simulations over a broader range of parameter values, i.e. saturation and mass distribution metrics, to further explore the predictive potential of the developed model for estimating spatial distribution of the organic phase.