[1] Mass transfer from entrapped dense nonaqueous phase liquids (DNAPLs) in heterogeneous aquifers takes place in natural, three-dimensional groundwater flow fields. However, mass transfer processes are characterized in the laboratory in columns or flow cells under conditions of one-dimensional or two-dimensional water flow. Dissolution data generated in these test systems are used to determine empirical parameters of correlations that relate a dimensionless form of a mass transfer coefficient (Sherwood number) to other dimensionless groups that capture the basic processes contributing to mass transfer (e.g., Reynolds and Schmidt numbers). These phenomologically based empirical correlations, which are referred to as Gilland-Sherwood models, are not directly applicable in predicting the dissolution of DNAPLs in the field as they do not capture the effects of aquifer heterogeneity and DNAPL entrapment morphology (referred to as DNAPL entrapment architecture). Numerical simulation of DNAPL dissolution requires the discretization of the problem domain into computational grid blocks and assignment of an effective mass transfer coefficient to each of these blocks containing DNAPL. A methodology for upscaling is needed to determine the grid-scale effective mass transfer coefficient from the laboratory-determined empirical correlations. It is our hypothesis that the upscaled effective mass transfer coefficient needs to contain information on the field-scale heterogeneity and DNAPL entrapment architecture. This hypothesis was tested through a Monte-Carlo-based numerical experiment using a laboratory-validated dissolution model with the goal of developing and testing an upscaling method. The developed upscaling method involves the use of geostatistical parameters that capture the aquifer heterogeneity and the DNAPL entrapment architecture. These parameters were determined to be the variance of log hydraulic conductivity, correlation lengths, and the normalized second moments of DNAPL mass distribution in the entrapment zone. Monte Carlo numerical simulation experiments combined with inverse modeling were used in this theoretical development and to determine parameters of the proposed upscaled Gilland-Sherwood mass transfer correlation. Through these numerical modeling studies, the upscaled mass transfer correlation was successfully verified. Sensitivity analyses indicate that the normalized second moment, which describes the spreading of DNAPL mass in the vertical directions, was the most sensitive parameter in simulating the mass transfer at large scales.Citation: Saenton, S., and T. H. Illangasekare (2007), Upscaling of mass transfer rate coefficient for the numerical simulation of dense nonaqueous phase liquid dissolution in heterogeneous aquifers, Water Resour.

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Field-flow fractionation (FFF) techniques were used to separate various strains of bacteria and differentiate live from dead bacteria. The sedimentation, flow, and electrical FFF separations were accomplished in less than 15 minutes using the rapid hyperlayer mode. The bacteria used in these studies include Pseudomonas putida, Escherichia coli, and Staphylococcus epidermidis. Sedimentation FFF gives the highest resolution separations of a mixture of spherical and rod-shaped bacteria, and a mixture of two rod-shaped bacteria.

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The benefits of partial source zone treatment of non-aqueous phase liquids (NAPL)-contaminated sites (not fully removing the entrapped free-phase NAPL sources) with respect to achieving cleanup goals and reducing concentrations of dissolved constituents in downstream plumes are being debated. Uncertainty associated with the removal of NAPLs from source zones could be attributed to a number of factors including lack of information on the extent or timing of spills, complex entrapment configurations created by unstable behavior (fingering), geologic heterogeneity, and unavailability of accurate techniques for characterizing these heterogeneities, and uncertainty in locating source zone and estimating NAPL mass. Data for the resolution of issues related to benefits of partial source zone treatment are not expected to come from field sites. Laboratory studies in intermediate-scale test tanks can provide accurate data sets to investigate this issue, as it is possible to conduct controlled experiments under known conditions of aquifer heterogeneity. At this scale, source depletion and downstream concentrations in dissolved plumes can be monitored during remediation. The data generated in controlled experiments are used to validate numerical models to conduct theoretical analysis. This paper discusses this approach and presents results from such a study where the benefits of partial source zone treatment using surfactants were evaluated using intermediate-scale testing and numerical modeling. Results from both experiment and numerical simulations agreed conceptually where they suggested that a very large fraction of NAPL has to be removed from the entrapment zone to significantly reduce downstream plume concentrations. Keywords Dissolution Á DNAPL Á Heterogeneity Á Numerical modeling Á Surfactants Abbreviations b Sorption parameter in the Langmuir isotherm (L 3 M-1) c Aqueous concentration (M L-3) c* Apparent aqueous solubility of PCE in the presence of Tween-80 (M L-3) c s Aqueous solubility of PCE under normal condition (i.e., no surfactant) (M L-3) c tw Aqueous concentration of Tween-80 (M L-3) d 50 Average grain size (L) D m Molecular diffusion coefficient (L 2 T-1) J NAPL-water mass transfer rate per unit volume of porous medium (M L-3 T-1) k La Overall mass transfer coefficient (T-1) k r,w Relative permeability of water in the sand (-) K Effective hydraulic conductivity (L T-1) K s Saturated hydraulic conductivity (L T-1) ln K s Average of (natural) log of K s L* Characteristic or dissolution length (L) m 0