Nonaqueous-phase-liquid dissolution in variable-aperture fractures: Development of a depth-averaged computational model with comparison to a physical experiment

Dissolution of nonaqueous-phase liquids (NAPLs) from variable-aperture fractures couples fluid flow, transport of the dissolved NAPL, interphase mass transfer, and the corresponding NAPL-water-interface movement. Each of these fundamental processes is controlled by fracture-aperture variability and entrapped-NAPL geometry. We develop a depth-averaged computational model of dissolution that incorporates the fundamental processes that control dissolution at spatial resolutions that include all scales of variability within the flow field. Thus this model does not require empirical descriptions of local mass transfer rates. Furthermore, the depth-averaged approach allows us to simulate dissolution at scales that are larger than the scale of the largest entrapped NAPL blobs. We compare simulation results with an experiment in which we dissolved residual entrapped trichloroethylene (TCE) from a 15.4 30.3 cm, analog, variable-aperture fracture. We measured both fracture aperture and the TCE distribution within the fracture at high spatial resolution using light transmission techniques. Digital images acquired over the duration of the experiment recorded the evolution of the TCE distribution within the fracture and are directly compared with the results of a computational simulation. The evolution with time of the distribution of the entrapped TCE and the total TCE saturation are both predicted well by the dissolution model. These results suggest that detailed parametric studies, employing the depth-averaged dissolution model, can be used to develop a comprehensive understanding of NAPL dissolution in terms of parameters characterizing aperture variability, phase structure, and hydrodynamic conditions.