1 Introduction

Subsurface fluids are a mixture of multiple constituents. Vadose zone gas often contains nitrogen, oxygen, carbon dioxide, argon, water vapor, and, perhaps, vapors from volatile liquids (e.g., gasoline, solvents). Other GW-Project books are concerned with fluid motion as described by Darcy’s law, which calculates the motion of a mixture without regard to composition. But individual species in fluids of non-uniform composition experience motion that is in addition to that imparted by viscous flow and is subject to different driving and resisting forces. This additional increment of motion is called diffusion and is the subject of this book.

Diffusion in the gas phase is an important phenomenon in a variety of problems important to earth scientists and engineers. For example, the supply and elimination of gases to and from the root zones of plants is affected by diffusion in soil gas and is studied by soil and plant scientists. Hydrogeologists, engineers, and environmental scientists are interested in the migration of vapors from volatile sources of contamination lodged in the vadose zone. Among the concerns is the creation of a migrating vapor plume in the vadose zone that might act as a source of groundwater contamination via partitioning of chemicals from the gas phase to contiguous groundwater. Diffusion in the gas phase is partially responsible for the so-called vapor intrusion problem, where contaminated air in basements and crawl spaces of buildings can be linked to subsurface sources as indicated in the cover figure of this book. Diffusion of contaminants from low permeability strata may influence the performance and efficacy of soil-vapor extraction systems. Natural degradation of liquid petroleum present in the vadose zone often generates diffusive fluxes of gases such as oxygen, carbon dioxide, and methane. Analysis and measurement of such diffusive fluxes sometimes assists in the estimation of the rates of natural degradation and the projection of source longevity. Emanation of gases from landfills is yet another circumstance in which diffusion in the gas phase plays an important role.


Flux Equations for Gas Diffusion in Porous Media Copyright © 2021 by David B. McWhorter. All Rights Reserved.