Journal of Soil Contamination, 3(4): (1994)
Transport and Fate of Chlorinated
Hydrocarbons in the Vadose
Zone — A Literature Review with
Discussions on Regulatory Implications
Samuel C. T. Yu, D. Env.
California Environmental Protection Agency, Regional Water Quality Control
Board, Los Angeles Region
ABSTRACT: Chlorinated hydrocarbons (CHCs) are one of the pollutant groups most com-
monly found in hazardous waste sites. Understanding the transport and fate of these compounds
in the vadose zone is crucial to identifying pollution sources, assessing the threat to groundwater,
and evaluating the need and adequacy of cleanup. CHCs exist in various physical phases in the
subsurface, linked by interphasal processes. These, combined with unsaturated water flow in the
vadose zone, which is complicated by the multifluid (air and water) situation, geologic hetero-
geneity, and a highly site-specific preferential flow pattern, create a possibly four-fluid flow
condition, with interactions among different phases of CHCs and between chemicals and
geologic materials. Superimposed on these are various biotic and abiotic transformations.
Integration of these processes is being attempted by mathematical modeling; however, natural
heterogeneity and site-specific factors render these efforts partially successful at best. Regulators
are faced with considerable uncertainties and cannot expect simple solutions with the current
level of understanding of subsurface processes.
KEY WORDS: chlorinated solvents, solute transport, subsurface transport, unsaturated zone,
contaminant migration, non-queous phase liquid (NAPL), multiphase transport, subsurface
environment.
I. INTRODUCTION
The vadose zone usually consists of a heterogeneous geologic medium that pro-
vides crucial pollution protection to the groundwater through various physical,
chemical, and biological processes. Vadose zone investigation provides valuable
information regarding the source, extent, and strength of subsurface contamina-
tion, its (potential) impact on groundwater, and implications for remediation, such
as evaluating the need and adequacy of certain remedial actions. Findings from
vadose zone investigations also have important regulatory ramifications for iden-
tifying sources of groundwater contamination.
Copyright© 1996, CRC Press, Inc. — Files may be downloaded for personal use only. Reproduction of this
material without the consent of the publisher is prohibited.
1
, Due to the importance of groundwater as a natural resource, a large number of
studies on subsurface contamination were performed in the last decade. It became
clear that there are significant uncertainties in the study of subsurface contamina-
tions, especially those by toxic organic chemicals such as chlorinated solvents.
These studies also highlighted the multidisciplinary nature of this particular envi-
ronmental problem.
There are a number of recent reviews on this topic, each with different emphases
(e.g., Jury and Flühler, 1992; Nielsen et al.., 1986; Schwille, 1984; USEPA, 1989,
1990, 1991). The present review contains a progressive discussion of important
components and processes in the vadose zone environment for common one- and
two-carbon chlorinated hydrocarbons (CHCs), such as tetrachloroethene (PCE),
trichloroethene (TCE), 1,1,1-trichloroethane (TCA), carbon tetrachloride (CT),
and their precursors and/or degradation derivatives. These compounds are widely
used as industrial solvents and are found in many contaminated aquifers. Because
of the potential health effects of these toxic chemicals, their subsurface transport
and fate processes elicit considerable regulatory interest.
II. MULTIPHASE DISTRIBUTION
Once introduced into the vadose zone, organic contaminants, including CHCs,
gradually partition into different phases based on their physicochemical properties.
Figure 1 illustrates a four-phase system in the vadose zone, consisting of soil
solids, soil water, interstitial air, and nonaqueous phase liquid.
A. Nonaqueous-Phase Liquid
Nonaqueous-phase liquid (NAPL) refers to organic contaminants immiscible with
water and that therefore exist in a purely organic phase in the subsurface environ-
ment. NAPL is usually further divided into “light” (lNAPL) and “dense” (dNAPL),
based on the specific gravity. All of the CHCs selected for this study fall into the
category of “dense” NAPL.
In most of the leaking underground tanks, pipes, and surface spills, NAPL is the
primary phase of contaminant being introduced to the subsurface. The presence of
NAPL in the subsurface has several important consequences:
1. NAPL may migrate under the influence of gravity. The transport is affected
by the multiphase interactions among NAPL, soil water and air, and the
porous medium. Given time, NAPL may penetrate the vadose zone (or even
saturated zone in the case of dNAPL) and reach the groundwater, where it
serves as a continuous source of contamination, or the migration may cease
before the NAPL front reaches the water table.
Copyright© 1996, CRC Press, Inc. — Files may be downloaded for personal use only. Reproduction of this
material without the consent of the publisher is prohibited.
2
,FIGURE 1. Distribution of chlorinated hydrocarbons in four phases in the Vadose Zone.
2. In the trail of NAPL migration, “blobs” or “ganglions” of NAPL are left in
the vadose zone; these are called “residual saturation”. Schwille (1988)
demonstrated the formation of NAPL blobs in an idealized situation using
spherical glass beads. Hunt et al.. (1988), on the other hand, discussed the
immobilization of NAPL in the vadose zone through the counteraction of
gravity and capillary forces, and presented a method for estimating NAPL
ganglion dimensions. In a recent work, Conrad et al.. (1992) focused on the
visualization of residual organic liquid trapped in aquifers by fixing and
isolating the chemically transformed NAPL blobs for physical observations.
These residual saturations of NAPL act as a continuous source for vapor and
dissolved-phase contaminant migration (Schwille, 1984, 1988).
3. The residual saturation of NAPL may occupy a relatively small volume in
the vadose zone, but contains a disproportional mass of contaminants com-
pared to other phases. These concentrated blobs and ganglions, which are
heterogeneously distributed in the subsurface, represent a special problem
to the detection, characterization, and remediation of soil contamination. A
recent study by Poulsen and Kueper (1992) indicated that the heterogeneity
of a pure PCE distribution in a sandy soil is dependent on source strength
and the intrinsic properties of the porous medium; and the sample size
required to obtain a true measurement of the distribution of residual satura-
Copyright© 1996, CRC Press, Inc. — Files may be downloaded for personal use only. Reproduction of this
material without the consent of the publisher is prohibited.
3
, tion is on the order of the scale of permeability variation, which, in their
study, is millimeters.
B. Dissolved Phase
Dissolved CHC may either be directly discharged to the ground through various
industrial activities that generate CHC containing waste water or be a result of the
dissolution of dNAPL in the vadose zone. The extent of dissolution depends on the
solubility and availability of CHC. Dissolved contaminants migrate with soil
water, although these dissolved chemicals, or solutes, usually do not move as fast
as water because of the many interactions to be discussed later.
C. Sorbed Phase
“Sorption” is a generic term describing a range of physicochemical interactions
between organic contaminants and soil particle surfaces. Chiou (1989) used the
term for uptake of a solute or vapor by soil regardless of mechanism, while
“adsorption” means condensation of vapor or solute on the surface or interior pores
of soil particles by physical or chemical forces, and “partition” describes a model
in which the sorbed material essentially dissolves into the network of an organic
phase (in the case of CHC sorption in the subsurface, the organic phase is usually
organic matter on soil particles). Weber et al.. (1991) presented detailed discus-
sions of the basic principles at work in various forms of sorption phenomena.
Sorbed contaminant molecules are in effect retained by the soil particles. There is
a dynamic equilibrium between contaminants in the sorbed phase and other phases.
It is commonly believed that the majority of sorbed contaminants, based on Chiou’s
(1989) definition, are partitioned into organic matter on soil particle surfaces, while
a minor portion is adsorbed to inorganic or mineral surface features on soil particles
(e.g., Bouchard et al.., 1989; Lee et al.., 1989; Piwoni and Banerjee, 1989).
On the whole, sorbed-phase contaminant may comprise a significant part of the
total contaminant mass, depending on the organic carbon content of the soil and the
physicochemical property of the CHC. Interactions between contaminants sorbed
on the immobile soil matrix and those in the mobile phases significantly affect the
transport rate of the contaminant in the subsurface.
D. Vapor Phase
Due to the high vapor pressure of the CHCs in question, a considerable portion of
the contaminants may exist as vapor in the subsurface. The relative concentration
of the vapor phase depends on the physicochemical properties of the compound.
Copyright© 1996, CRC Press, Inc. — Files may be downloaded for personal use only. Reproduction of this
material without the consent of the publisher is prohibited.
4
Transport and Fate of Chlorinated
Hydrocarbons in the Vadose
Zone — A Literature Review with
Discussions on Regulatory Implications
Samuel C. T. Yu, D. Env.
California Environmental Protection Agency, Regional Water Quality Control
Board, Los Angeles Region
ABSTRACT: Chlorinated hydrocarbons (CHCs) are one of the pollutant groups most com-
monly found in hazardous waste sites. Understanding the transport and fate of these compounds
in the vadose zone is crucial to identifying pollution sources, assessing the threat to groundwater,
and evaluating the need and adequacy of cleanup. CHCs exist in various physical phases in the
subsurface, linked by interphasal processes. These, combined with unsaturated water flow in the
vadose zone, which is complicated by the multifluid (air and water) situation, geologic hetero-
geneity, and a highly site-specific preferential flow pattern, create a possibly four-fluid flow
condition, with interactions among different phases of CHCs and between chemicals and
geologic materials. Superimposed on these are various biotic and abiotic transformations.
Integration of these processes is being attempted by mathematical modeling; however, natural
heterogeneity and site-specific factors render these efforts partially successful at best. Regulators
are faced with considerable uncertainties and cannot expect simple solutions with the current
level of understanding of subsurface processes.
KEY WORDS: chlorinated solvents, solute transport, subsurface transport, unsaturated zone,
contaminant migration, non-queous phase liquid (NAPL), multiphase transport, subsurface
environment.
I. INTRODUCTION
The vadose zone usually consists of a heterogeneous geologic medium that pro-
vides crucial pollution protection to the groundwater through various physical,
chemical, and biological processes. Vadose zone investigation provides valuable
information regarding the source, extent, and strength of subsurface contamina-
tion, its (potential) impact on groundwater, and implications for remediation, such
as evaluating the need and adequacy of certain remedial actions. Findings from
vadose zone investigations also have important regulatory ramifications for iden-
tifying sources of groundwater contamination.
Copyright© 1996, CRC Press, Inc. — Files may be downloaded for personal use only. Reproduction of this
material without the consent of the publisher is prohibited.
1
, Due to the importance of groundwater as a natural resource, a large number of
studies on subsurface contamination were performed in the last decade. It became
clear that there are significant uncertainties in the study of subsurface contamina-
tions, especially those by toxic organic chemicals such as chlorinated solvents.
These studies also highlighted the multidisciplinary nature of this particular envi-
ronmental problem.
There are a number of recent reviews on this topic, each with different emphases
(e.g., Jury and Flühler, 1992; Nielsen et al.., 1986; Schwille, 1984; USEPA, 1989,
1990, 1991). The present review contains a progressive discussion of important
components and processes in the vadose zone environment for common one- and
two-carbon chlorinated hydrocarbons (CHCs), such as tetrachloroethene (PCE),
trichloroethene (TCE), 1,1,1-trichloroethane (TCA), carbon tetrachloride (CT),
and their precursors and/or degradation derivatives. These compounds are widely
used as industrial solvents and are found in many contaminated aquifers. Because
of the potential health effects of these toxic chemicals, their subsurface transport
and fate processes elicit considerable regulatory interest.
II. MULTIPHASE DISTRIBUTION
Once introduced into the vadose zone, organic contaminants, including CHCs,
gradually partition into different phases based on their physicochemical properties.
Figure 1 illustrates a four-phase system in the vadose zone, consisting of soil
solids, soil water, interstitial air, and nonaqueous phase liquid.
A. Nonaqueous-Phase Liquid
Nonaqueous-phase liquid (NAPL) refers to organic contaminants immiscible with
water and that therefore exist in a purely organic phase in the subsurface environ-
ment. NAPL is usually further divided into “light” (lNAPL) and “dense” (dNAPL),
based on the specific gravity. All of the CHCs selected for this study fall into the
category of “dense” NAPL.
In most of the leaking underground tanks, pipes, and surface spills, NAPL is the
primary phase of contaminant being introduced to the subsurface. The presence of
NAPL in the subsurface has several important consequences:
1. NAPL may migrate under the influence of gravity. The transport is affected
by the multiphase interactions among NAPL, soil water and air, and the
porous medium. Given time, NAPL may penetrate the vadose zone (or even
saturated zone in the case of dNAPL) and reach the groundwater, where it
serves as a continuous source of contamination, or the migration may cease
before the NAPL front reaches the water table.
Copyright© 1996, CRC Press, Inc. — Files may be downloaded for personal use only. Reproduction of this
material without the consent of the publisher is prohibited.
2
,FIGURE 1. Distribution of chlorinated hydrocarbons in four phases in the Vadose Zone.
2. In the trail of NAPL migration, “blobs” or “ganglions” of NAPL are left in
the vadose zone; these are called “residual saturation”. Schwille (1988)
demonstrated the formation of NAPL blobs in an idealized situation using
spherical glass beads. Hunt et al.. (1988), on the other hand, discussed the
immobilization of NAPL in the vadose zone through the counteraction of
gravity and capillary forces, and presented a method for estimating NAPL
ganglion dimensions. In a recent work, Conrad et al.. (1992) focused on the
visualization of residual organic liquid trapped in aquifers by fixing and
isolating the chemically transformed NAPL blobs for physical observations.
These residual saturations of NAPL act as a continuous source for vapor and
dissolved-phase contaminant migration (Schwille, 1984, 1988).
3. The residual saturation of NAPL may occupy a relatively small volume in
the vadose zone, but contains a disproportional mass of contaminants com-
pared to other phases. These concentrated blobs and ganglions, which are
heterogeneously distributed in the subsurface, represent a special problem
to the detection, characterization, and remediation of soil contamination. A
recent study by Poulsen and Kueper (1992) indicated that the heterogeneity
of a pure PCE distribution in a sandy soil is dependent on source strength
and the intrinsic properties of the porous medium; and the sample size
required to obtain a true measurement of the distribution of residual satura-
Copyright© 1996, CRC Press, Inc. — Files may be downloaded for personal use only. Reproduction of this
material without the consent of the publisher is prohibited.
3
, tion is on the order of the scale of permeability variation, which, in their
study, is millimeters.
B. Dissolved Phase
Dissolved CHC may either be directly discharged to the ground through various
industrial activities that generate CHC containing waste water or be a result of the
dissolution of dNAPL in the vadose zone. The extent of dissolution depends on the
solubility and availability of CHC. Dissolved contaminants migrate with soil
water, although these dissolved chemicals, or solutes, usually do not move as fast
as water because of the many interactions to be discussed later.
C. Sorbed Phase
“Sorption” is a generic term describing a range of physicochemical interactions
between organic contaminants and soil particle surfaces. Chiou (1989) used the
term for uptake of a solute or vapor by soil regardless of mechanism, while
“adsorption” means condensation of vapor or solute on the surface or interior pores
of soil particles by physical or chemical forces, and “partition” describes a model
in which the sorbed material essentially dissolves into the network of an organic
phase (in the case of CHC sorption in the subsurface, the organic phase is usually
organic matter on soil particles). Weber et al.. (1991) presented detailed discus-
sions of the basic principles at work in various forms of sorption phenomena.
Sorbed contaminant molecules are in effect retained by the soil particles. There is
a dynamic equilibrium between contaminants in the sorbed phase and other phases.
It is commonly believed that the majority of sorbed contaminants, based on Chiou’s
(1989) definition, are partitioned into organic matter on soil particle surfaces, while
a minor portion is adsorbed to inorganic or mineral surface features on soil particles
(e.g., Bouchard et al.., 1989; Lee et al.., 1989; Piwoni and Banerjee, 1989).
On the whole, sorbed-phase contaminant may comprise a significant part of the
total contaminant mass, depending on the organic carbon content of the soil and the
physicochemical property of the CHC. Interactions between contaminants sorbed
on the immobile soil matrix and those in the mobile phases significantly affect the
transport rate of the contaminant in the subsurface.
D. Vapor Phase
Due to the high vapor pressure of the CHCs in question, a considerable portion of
the contaminants may exist as vapor in the subsurface. The relative concentration
of the vapor phase depends on the physicochemical properties of the compound.
Copyright© 1996, CRC Press, Inc. — Files may be downloaded for personal use only. Reproduction of this
material without the consent of the publisher is prohibited.
4
