Many groundwater contamination incidents begin with the release of an essentially immiscible fluid into the subsurface environment. Once in the subsurface, an immiscible fluid participates in a complex pattern of transport processes. For immiscible fluids that are commonly found in contaminated groundwater environments the interphase mass transfer between the nonaqueous phase liquids (NAPLs) phase and the aqueous phase is an important process. An experimental apparatus and procedure were used to isolate and measure mass transfer between toluene and water in glass bead media systems. The rate of interphase mass transfer was investigated in two-fluid systems as a function of aqueous phase velocity, aqueous-and nonaqueous-phase fluid saturations, and porous media characteristics. The rate of interphase mass transfer is found to be directly related to aqueous phase velocity and nonaqueous phase fluid saturation level, but no significant relation to mean particle size is found. Correlation expressions for the rate of interphase mass transfer are developed using relevant dimensionless parameters and are compared to literature values. Equilibrium between the two fluid phases investigated is shown to be achieved rapidly, over wide ranges of nonaqueous phase fluid saturations and aqueous phase velocities. The derived correlations provide a means for estimating the appropriateness of the local equilibrium assumption for a nonaqueous phase liquid-aqueous phase couple in multiphase groundwater systems. A. S. Mayer and C. T. Miller

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A two‐dimensional multiphase flow and species transport model was developed and applied to the case of nonaqueous phase liquid (NAPL) emplacement and dissolution in both homogeneous and heterogeneous porous media systems. Simulations were performed to observe dissolution rate variations and the degree of NAPL‐aqueous phase nonequilibrium as a function of two aqueous phase velocities and five forms of the NAPL‐aqueous phase mass transfer formulation. An integrated form of the Damkohler number was introduced to analyze the degree of NAPL‐aqueous phase nonequilibrium. Mass removal rates for homogeneous media were insensitive to the form of the NAPL‐aqueous phase mass transfer formulation, yielding results similar to a local equilibrium approach for all but one mass transfer formulation. This latter formulation was most sensitive to NAPL saturation and yielded significant nonequilibrium behavior, which was manifested as a decrease in NAPL dissolution rates as the NAPL volume fraction decreased. Variations in mass elution rates between homogeneous and heterogeneous media were observed, with more significant variations found for variances in porous media properties than for horizontal correlation lengths. In heterogeneous media, decreases in dissolution rates were attributed to the existence of relatively immobile regions of NAPL with saturations greater than the residual saturation of the media, so‐called NAPL pools. These results illustrate the importance of the statistical characteristics of heterogeneous porous media on NAPL distribution and dissolution processes.

Abstract. A new optimization formulation for dynamic groundwater remediation management is developed by simultaneously using well locations and the corresponding pumping rates as the decision variables. The genetic algorithm is applied to search for optimal pumping rates and the discrete space of well locations. The optimization model is applied to hypothetical, three-dimensional, contaminated aquifer systems with homogeneous and heterogeneous porous media properties. Optimal well locations and pumping rates obtained with the moving-well model are less expensive than solutions obtained with a comparable fixed-well model. Optimization with a linear objective function formulation identifies some of the optimal well locations obtained with a nonlinear formulation but results in higher pumping rates than the nonlinear formulation and ignores the higher drawdowns produced in low-permeability areas. Optimal well locations are found along the mass centerline of the contaminant plumes and in high-permeability areas in the heterogeneous system. Dynamic pumping rates and well locations produce more cost-effective solutions relative to a static model. The well location search path and convergence behavior indicate that the genetic algorithm is an effective alternative solution scheme and that well location optimization is more important than pumping rate optimization.