Phenomenological models for transient NAPL-water mass-transfer processes

Abstract

A phenomenological model for transient NAPL dissolution, referred to as the “sphere” model, is developed. In this model, the mass-transfer coefficient and specific surface areas are quantified independently. The unique aspect of this work is the assumption that the complex distribution of NAPL blob shapes and sizes can be represented by a set of spheres with a range of diameters. The sphere diameter distributions are determined through the entrapment, polymerization and characterization of styrene blobs in sandy media. Mass-transfer coefficients are estimated from experimental measurements of the dissolution of naphthalene emplaced as solid spheres within sandy media. The phenomenological model describing NAPL dissolution rates is incorporated into a solute transport equation and solved with a numerical simulator. Simulations show that the sphere model can be calibrated to experimental NAPL dissolution data from column studies presented previously. The inclusion of a range of sphere sizes resulted in a model which represents experimental data better than a model based on mono-size spheres. The sphere model also adequately predicts dissolution for NAPL's with a range of solubilities provided the NAPL blob size distributions can be adequately estimated. The primary limitation of the sphere model is the qiantity of input data required to describe the distribution of effective sphere sizes. While these data requirements are extensive, the ability to define the NAPL blob distribution within the model makes the sphere model much more versatile for extrapolation to heterogeneous systems than other available dissolution models.