Estimating mass discharge from dense nonaqueous phase liquid source zones using upscaled mass transfer coefficients: An evaluation using multiphase numerical simulations

[1] Difficulties associated with identifying the dense nonaqueous phase liquid (DNAPL) source zone architecture at the field scale, combined with the computational costs of field-scale DNAPL dissolution simulations, have motivated the development of a number of simplified models that rely upon upscaled (i.e., domain-averaged) mass transfer coefficients to approximate field-scale dissolution processes. While conceptually attractive, these upscaled models have yet to be fully evaluated for prediction of mass recovery from a range of nonuniform, three-dimensional DNAPL source zones. This study compares upscaled model predictions of flux-weighted downstream concentrations and source longevity to predictions derived from three-dimensional multiphase numerical simulation of tetrachloroethene (PCE)-NAPL dissolution for realizations of a statistically homogeneous, nonuniform aquifer. Although the functional forms of the upscaled models are generally shown to be mathematically equivalent, upscaled model flux-weighted concentration predictions varied by over one order of magnitude, with variations attributed to the dependence of the upscaled model parameters on the specific source zone scenario used for model calibration. Replacement of upscaled model calibration parameters with source zone parameters that can be obtained from site characterization information (specifically, the initial flux-weighted concentration and source zone ganglia-to-pool (GTP) mass ratio) reduced the root-mean-square error between upscaled and numerical model predictions by approximately 80%. Application of this modified model to a range of source zone scenarios (0.4 < GTP < 1) demonstrates the efficacy of the model for use as a screening tool to relate DNAPL mass removal and flux-weighted concentrations when mass removal is less than 80%.