Dissolution of non-aqueous-phase liquids and aqueous-phase contaminant transport in discretely-fractured porous media

Abstract

A transient, two-dimensional groundwater flow and contaminant transport model is developed here to study DNAPL dissolution in discrete fracture networks. The model accounts for matrix diffusion, equilibrium sorption and non-linear relative transmissivity effects that influence groundwater flow within fractures as the DNAPL disappears. A mixed analytical-numerical approach is incorporated to accurately calculate the disappearance of the DNAPL from the source fractures by aqueous-phase diffusion into the matrix. Locally, equilibrium partitioning of the contaminant between the non-aqueous and aqueous phases is assumed. Sensitivity analyses indicate that the dissolution time of the DNAPL is a function of the flow and transport properties of the fracture network and the rock matrix, with the degree of fracture connection playing an important role. In relatively porous rock types, the DNAPL is removed relatively rapidly from the hydraulically-active fractures by matrix diffusion and by aqueous-phase transport along these fractures. In poorly connected or dead-end fractures, the rate of dissolution of the DNAPL is controlled by matrix diffusion if the rock is porous but of low permeability, or by groundwater movement into the matrix if the matrix is sufficiently permeable. In all cases, dissulution is encouraged by the maintenance of high concentration gradients adjacent to the DNAPL source, either naturally or by hydraulic manipulation.