HR: 1340h
AN: H23A-1416    [Abstracts]
TI: Dissolution of DNAPLs from variable-aperture fractures: Parametric simulations and a proposed fracture-scale model of mass-transfer rates
AU: Rajaram, H
EM: detwiler@llnl.gov
AF: University of Colorado, 428 UCB, Boulder, CO 80903 United States
AU: * Detwiler, R L
EM: detwiler@llnl.gov
AF: Lawrence Livermore National Laboratrory, 7000 East Ave., Livermore, CA 94550 United States
AU: Glass, R J
EM: rjglass@sandia.gov
AF: Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185 United States
AB: DNAPLs entrapped in variable aperture fractures exhibit a complex geometry that is sensitive to capillary, viscous and gravitational forces during the migration of the DNAPL into the fracture. Development of effective models of mass transfer rates from DNAPL sources located in fractured media must consider the full range of potential phase structures as well as the hydrodynamic conditions during dissolution. We have carried out an extensive series of computational simulations to systematically explore the influence of entrapped DNAPL structure and flow conditions on mass transfer rates from variable aperture fractures. The simulations for this study used a modified invasion percolation (MIP) algorithm to displace DNAPL from initially DNAPL-saturated fractures by fracture with water. We varied the relative magnitude of in-plane and out-of-plane interfacial curvature, the magnitude of a linear stabilizing gradient, and boundary conditions during the invasion process and the hydrodynamic boundary conditions during dissolution. The result was DNAPL geometries ranging from large complex DNAPL ganglia to small uniformly distributed DNAPL blobs representative of a broad range of potential subsurface conditions. To simulate dissolution, we coupled a depth-averaged model of flow, transport and interphase mass transfer with the modified invasion percolation algorithm to predict mass-transfer-induced interfacial movement. The model explicitly calculates mass transfer from individual DNAPL ganglia and thus effectively incorporates the small-scale processes that control fracture-scale mass-transfer rates. Previous comparison to high-resolution physical experiments have demonstrated that the MIP model reproduces entrapped phase structures reasonably well and that the dissolution model accurately calculates local mass transfer rates. Results suggest that despite the wide range of entrapped DNAPL geometries, DNAPL saturation (S_NAPL) strongly influences both fracture-scale interphase mass fluxes (mass transfer rate per unit area, J) and evolving relative permeability (k_r). Furthermore, the interfacial area scales approximately linearly with S_NAPL suggesting the potential for developing fracture-scale mass transfer relationships that are effective over a broad range of conditions. We propose models of relative permeability and interphase mass flux as functions of SNAPL and demonstrate the ability of these models to estimate mass removal rates through direct comparison to our parametric simulations. This work was performed under the auspices of the DOE by the University of California, Lawrence Livermore National Laboratory under contract No. 7405-Eng-48.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1875 Vadose zone
SC: Hydrology [H]
MN: Fall Meeting 2005