Modeling the movement of volatile organic chemicals in columns of unsaturated soil

Abstract

Mechanisms affecting the fate of nondegradable volatile organic chemicals in soils include (1) advection in air and water, (2) dispersion in air and water, (3) air-water mass transfer and equilibrium, (4) diffusion in immobile water, (5) mass transfer between mobile and immobile water, and (6) sorption. A deterministic model was developed to account for these processes in laboratory columns of unsaturated soil. The general form of the model was solved numerically. The numerical solution was verified with analytic solutions for simplified conditions. Column experiments were conducted to validate the model and to determine the relative importance of each mechanism in two soil types. The movement of trichloroethene was measured in a column packed with a uniform sand and one packed with uniformly sized aggregates that were made from clay. Parameter values for the model predictions were independently determined from direct measurements and literature correlations. Bromide tracer studies were performed to determine parameter values that could not be measured directly or were not estimated accurately by literature correlations. For the sand column the amount of immobile water, the rate of liquid diffusion, and the liquid dispersion coefficient were measured in a tracer study. A batch rate study was used to measure the rate of intraaggregate diffusion in the clay aggregates. The liquid dispersion coefficient for the column containing aggregates was measured in a tracer study. These parameter values were used in the model to predict the breakthrough and elution of trichloroethene in the two columns. To describe the column data, however, Henry's constant was increased from a literature value of 0.4 to 0.7, and the predicted gas dispersion coefficient was reduced by a factor of 10. INTRODUCTION Subsurface contaminant transport and attenuation is governed by a number of spreading, retardation, and transformation mechanisms such as advection, dispersion, diffusion, and interfacial mass transfer; adsorption and vo!atilization; and biological and chemical reactions. These mechanisms and their impacts on chemical fate are discussed in detail by MacKay et al. [1985] and Nielsen et al. [1986]. As a chemical travels through soil with fluid flow, the shape of its concentration profile is affected by dispersing or spreading mechanisms, the profile's position is slowed by retardation mechanisms, and the concentration may also decrease due to biological and chemical transformations. Previous work on modeling unsaturated transport has focused in three areas: (1) vapor transport in the upper soil layer for predicting pesticide movement [Rolston et al., 1969; Mayer et al., 1974] and for assessing the behavior of organic chemicals [Jury et al., 1980, 1983], (2) tracer and nonvolatile chemical transport for simulating the onedimensional movement of salts and heavy metals [van Genuchten and Wierenga, 1976; Jury, 1982], and (3) threedimensional subsurface movement of liquids and vapors for estimating the travel time of organic solvents and pe...

Transport Models for Radioactive Carbon Dioxide at RWMC

“…The migration of a reactive gas moving through a variably saturated media has been investigated (Weeks, Earp and Thompson 1982;Kreamer, Weeks, and Thompson 1988;Gierke, Hutzler, and Crittenden 1990). Partitioning between the gas phase and the aqueous phase and between the aqueous phase and solid phases retards the diffusion of the reactive gas.…”

Hydrodynamic dispersion and mass transfer in unsaturated flow

“…Hydrodynamic dispersion coefficients for the unsaturated zone have not bccn sufficiently investigated, although practical values arc nccdcd for exploring NAPL (non-aqueous phase liquid) transport in an aquifer [Crittenden et al, 1986 [Wilson, 1995;Anderson, 1994Anderson, , 1995 and soil vapor extraction [Gierke et al, 1990[Gierke et al, , 1992; Armstrong et al, 1994; Conant et al, 1996;Fischer et al, 1996]. In particular, there is little information available for cases in the range of high water saturation.…”

Buoyant Advection of Gases in Unsaturated Soil

“…A number of these models have simulated forced advection of dilute gases in soils, including Mohsen et al (1980) and Metcalfe and Farquhar (1987), who address methane release from landfills; Loureiro et al (1990), who model radon transport into the basement of buildings, and a number of models related to the transport of volatile organic solvents in unsaturated soil (Abriola and Pinder 1985;Baehr and Corapcioglu 1987;Gierke et al 1990). With the recognition that volatile organic liquids become trapped in the subsurface following their release, gas transport models have included the effects of volatilization and buoyancy-induced advection (Sleep and Sykes 1989;Falta et al 1989;Mendoza and Frind 1990;Mendoza and McAlary 1990).…”