Transport of Organic Chemicals by Gas Advection in Structured or Heterogeneous Porous Media: Development of a Model and Application to Column Experiments

Abstract

The use of soil venting or vacuum extraction for remediation of contaminated soil has lead to an increased interest in modeling gas‐phase processes. The majority of existing transport models have been developed assuming homogeneous porous media properties. It is well known, however, that the assumption of homogeneity will almost always be invalid for field systems. In addition, sorption has been described using the local equilibrium assumption. This assumption has also been shown to be invalid under certain conditions. A one‐dimensional model is presented that accounts for a structured or heterogeneous porous medium and for rate‐limited sorption. The model is designed for cases where transport occurs by advection and dispersion in the gas phase and where the liquid phase is immobile. A sensitivity analysis is presented, and parameter determination is discussed. The performance of the model was evaluated by comparing predicted simulations to data obtained from the literature. The model performed well, especially considering that the values for all input parameters were obtained independent of curve fitting.

Contaminant mass transfer during gas‐phase transport in unsaturated porous media

“…To quantify and distinguish between the mass transfer processes, the multiprocess nonequilibrium model developed by Brusseau [1991], as adapted to our system (equations (7) and (8) trichloroethene was retained in the bulk water and by sorption. The values of both Damkohler numbers suggest that transport of trichloroethene through the unsaturated heterogeneous porous media is subject to nonideality due to gas-phase mass transfer between advective and nonadvective domains (to' < 1) and due to diffusion within immobile water combined with rate-limited sorption/desorption (to < 1).…”

“…Brusseau [1991] developed a model for gas transport through structured porous media that accounted for multiple rate-limited mass transfer processes. The author used the model to describe data collected from the literature and found that both rate-limited sorption/desorption and diffusion within immobile water significantly influenced transport.…”

Volatilization of Non-Aqueous Phase Liquid in Porous Media: Vapor Phase Mass Transfer Characteristics

“…Hence, it appears that diffusion predominates over mechanical dispersion for gas phase transport at low velocity. Gas phase diffusion can occur not only by molecular diffusion but also by Knudsen diffusion or Klinkenberg effect (Brusseau, 1991;Bear, 1972) . Equations based on Fick's law are then not adequate to represent diffusion in the systems where Knudsen diffusion are significant.…”

Solutions to equations incorporating the effect of rate‐limited contaminant mass transfer on vadose zone remediation by soil vapor extraction

“…For further details of the model and the model assumptions, see Brusseau [1991] and Goltz and Oxley [1994]. Using symbols defined in the notation list, nondimensional equations describing the system are …”

“…Because of the impact of rate-limited mass transfer on SVE efficacy, models incorporating the process are increasingly being developed and used [Brusseau, 1991;Cho et incorporates the following: (1) radially convergent advectivedispersive transport of contaminant in the gas phase, with advective velocity inversely proportional to the radial distance from the extraction well and dispersion described using a constant dispersion coefficient, (2) equilibrium partitioning of contaminant between the gas phase and water residing in macropores adjacent to flowing gas (the so-called advective domain), (3) sorption of contaminant from water to the aquifer solids, where sorption is assumed to be instantaneous for a fraction of the sorbent and rate-limited for the remainder, and (4) ratelimited mass transfer between the gas and water residing in micropores that is not directly adjacent to flowing gas (the so-called nonadvective domain). For further details of the model and the model assumptions, see Brusseau [1991] and Goltz and Oxley [1994].…”