Simulating the dissolution of a complex dense nonaqueous phase liquid source zone: 2. Experimental validation of an interfacial area–based mass transfer model

Abstract

A multiphase flow–aqueous phase transport numerical model (DNAPL3D‐MT) is developed to simulate the dissolution of complex source zones containing both pooled and residual dense nonaqueous phase liquids (DNAPLs). The multiphase flow model (DNAPL3D) is coupled to the aqueous species transport code (MT3D) via a flexible mass transfer function, which can employ the local equilibrium assumption or the single–boundary layer expression for rate‐limited dissolution either incorporating a lumped (correlation function) coefficient or explicitly accounting for the interfacial area (IFA) between the fluids. For the latter, this work employs the thermodynamically based Explicit IFA Submodel (Grant and Gerhard, 2007), which provides IFA as a function of saturation and saturation history. A bench‐scale experiment is presented involving the complete, natural dissolution of a DNAPL source zone emplaced by a point source release into heterogeneous porous media. DNAPL3D‐MT simulations of the experiment, involving no calibration to results, are compared with the observed evolution of both (1) measured downgradient dissolved phase concentrations and (2) DNAPL source zone configuration. The model, employing a mass transfer expression equipped with the Explicit IFA Submodel, simulates the experiment more accurately than when equipped with either a local equilibrium assumption or a published empirical correlation expression. Sensitivity simulations indicate that this model validation is sensitive to a number of the key assumptions in the Submodel derivation except one: the relationship between interfacial area and residual DNAPL saturations. The employed assumption of a single mass transfer coefficient value is supported by an analysis of the evolution of Peclet numbers throughout the DNAPL source zone, which reveals that the low hydraulic gradient employed resulted in diffusion‐dominated mass transfer conditions throughout the experiment. This study suggests that simulations of global mass flux from complex DNAPL source zones are sensitive to the interrelationship of rate‐limited mass transfer and groundwater velocity (and thus aqueous phase relative permeability and DNAPL saturation) at the local scale.