&EPA
United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada, OK 74820
Research and Development
EPA/600/S-93/004 May 1993
ENVIRONMENTAL
RESEARCH BRIEF
COMPLEX MIXTURES AND GROUNDWATER QUALITY
M.L Brusseau*
INTRODUCTION
The occurrence of organic chemicals in soil and groundwater
has become an issue of great interest and import. Concomitantly,
research on the transport and fate of organic contaminants in
subsurface environments has expanded greatly in recentyears.
Much of this research has been focused on dissolved
constituents in aqueous systems. However, the behavior of
"complex mixtures" is beginning to receive increased attention.
By complex mixture we mean any system other than the simple
system of water containing a single solute. Examples of
pertinent problems involving complex mixtures include the
transport of oxygenated gasoline in the subsurface, the
dissolution of diesel fuel and coal-tar, and the use of chemical
agents such as surfactants or solvents to enhance the removal
of contaminants by pump-andrtreat remediation. A discussion
of these few selected examples will serve to highlight some of
the issues associated with complex mixtures, with a focus on
potential groundwater contamination and remediation.
COMPLEX MIXTURES AND SUBSURFACE
CONTAMINATION
Miscible Organic Liquids and Alternative Fuels
Concern about air pollution and the dependency on foreign
sources of oil has led to major programs promoting the use of
alternative fuels in the U.S.A. Currently, oxygenates, either
neat or as additives, appear to be the principal alternative fuel
Department of Soil and Water Science
University of Arizona, Tucson, Arizona
candidates (Haggin, 1989). Of the oxygenates, methanol and
ethanol are the primary miscible compounds in use (Hanson,
1991). The advent of alternative fuels has fomented increased
interest in the transport and fate of miscible organic liquids in
the subsurface. It has also increased interest in the effects of
these liquids on the transport and fate of other contaminants.
1. Transport and Fate of Miscible Organic Liquids in
the Subsurface
The sorption of miscible organic liquids by soil is generally
extremely low. Little sorption is expected for compounds such
as methanol and ethanol because of their polarity and large
(infinite) aqueous solubility. The minimal sorption of alcohols
has been widely demonstrated in the chromatography literature.
Limited data for soil systems has also shown negligible sorption
of alcohols (cf., Garrettetal., 1986; Wood etal., 1990). Hence,
these compounds will be minimally retarded and will travel
through the subsurface at essentially the velocity of water. This
large mobility can be a useful characteristic. For example,
alcohols may be useful as an "early warning" sign of the
impending arrival of a contaminant plume emanating from a fuel
spill. In regard to the use of alcohols for in:situ soil washing, the
greater mobility means that an injected pulse of alcohol may be
able to overtake a plume of a retarded solute.
Alcohols such as methanol have been reported to be
biodegradable under both aerobic and anaerobic conditions
(cf., Colby et al., 1979; Lettinga et al., 1981; Novak et al., 1885).
However, the concentrations of alcohol at which biodegradation
occurred were less than 1 %. Large concentrations (> 10%) of
alcohol are generally considered to be toxic to most
microorganisms and therefore not biodegradable.
dS> Printed on Recycled Paper
-------
2. Effect of Miscible Organic Liquids on the
Subsurface Environment
The addition of a miscible organic liquid, such as methanol, to
water results in a reduction of surface tension. For example,
surface tension is reduced by approximately one-half in systems
containing 5% acetone (Paluch and Rybska, 1991) or 50%
methanol (Wells, 1981). Very large reductions in interfacial
(liquid-liquid) tension are required to mobilize immiscible liquids
trapped in porous media (Puig et al., 1982). Using the surface
tension data as a guide, cosolvents will probably not produce
such large reductions in interfacial tension. Thus, the presence
of a cosolvent is not expected to produce emulsions or to
mobilize residuals of immiscible liquids.
The presence of organic liquids has been shown to cause
shrinking of clay materials and of soils consisting of large
portions of clay. For example, clay materials have been
demonstrated to shrink (in relation to status in aqueous system)
with the addition of acetone or ethanol (Green et al., 1983;
Brown and Thomas, 1987; Chen et al., 1987). This shrinkage
can result in an increase in hydraulic conductivity (Brown and
Thomas, 1987). Thus, it is possible that the presence of large
concentrations of cosolvent could cause shrinking and cracking
of subsurface domains containing large fractions of clay. This
perturbation may alter the hydraulic conductivity and, thereby,
affect fluid flow and solute transport.
The presence of organic liquids can also affect the properties of
naturally occurring organic components of the soil. It is well
known in polymer science that organic liquids can cause organic
polymers to swell. The degree of swelling is dependent upon
the properties of the solvent (polarity) and of the polymer (type,
structure). The addition of an organic liquid has been shown to
cause natural organic materials to swell (Freeman and Cheung,
1981; Lyon and Rhodes, 1991). One potential effect of the
swelling of organic matter associated with the subsurface solid
phase is a reduction in permeability due to blockage of pores.
Given the relatively small content of organic matter associated
with most subsurface materials, this effect will probably not lead
to a measurable reduction in permeability in most cases.
Another potential effect is the dissolution of components (e.g.,
humic or fulvic acids) from the solid-phase organic matter.. A
great deal of research has been reported describing the effect
of dissolved organic matter on the solubility, sorption, and
transport of organic and inorganic compounds. There is a
possibility that large concentrations of cosolvents could extract
organic material from the soil, and that this dissolved organic
matter could affect the transport of contaminants. This effect
will probably be of importance for limited conditions, i.e., for
systems with high organic-carbon content soils and highly
hydrophobia compounds.
Another potential effect of the swelling of organic matter is the
enhanced releaseof organic compounds (contaminants) residing
in the matrix of organic matter. It is generally accepted that the
organic fraction of soil is the predominant sorbent for low-
polarity organic compounds. It is likely that sorbed organic
compounds reside in internal as well as external domains of the
organic matter. It is quite possible that high concentrations of
cosolvents could enhance the release of organic contaminants
retained within the organic phase. The swelling of the organic
matrix with the addition of a cosolvent allows greater diffusive
mass transfer and, thus, enhances the release of sorbed
compounds (Freeman and Cheung, 1982; Brusseau et al.,
1991 a). This concept is used in analytical chemistry in terms of
solvent extraction of contaminated soils. This is discussed in
more detail in the following section.
As previously mentioned, large concentrations (> 10%) of
alcohol are generally considered to be toxic to most
microorganisms. Hence, it is possible that a release of a fuel
containing large concentrations of alcohol could deleteriously
affect the subsurface biota. The potential effect of large
concentrations of alcohols on microbial communities in the
subsurface appears to have received minimal attention.
3. Effect of Miscible Organic Liquids on the Transport
and Fate of Organic Contaminants in the
Subsurface
The influence of an organic liquid (cosolvent) on the solution-
phase activity of organic compounds is dependent upon the
nature of the solute and of the solvent-cosolvent system. For
many of the systems of environmental interest, water is the
solvent, the cosolvent is less polar than water, and the solutes
are of relatively low polarity. For this case, the addition of a
cosolvent tends to increase the amount of solute that can reside
in solution under equilibrium conditions. A simple relationship
describing the influence of cosolvent on the solubility of a solute
in the mixed-solvent system is the log-linear cosolvency model
(Yalkowsky et al., 1972)
log Sm = log Sw + o fr
(D
where S is the solubility in water (w) and mixed-solvent (m), 0
represents the cosolvency power of the cosolvent expressed as
the slope of the solubilization profile (i.e., log solubility versus
fc), and fc is the volume fraction of organic cosolvent.
Given that the sorption of low-polarity organic compounds by
soils, sediments, and aquifer materials ("soil") is considered to
be driven primarily by an entropic, solute-solvent interaction
process, it is expected that the presence of a cosolvent should
significantly affect sorption. A log-linear cosolvency model,
relating the equilibrium sorption constant (K ) to the volume
fraction of cosolvent, for sorption of organic solutes from binary
mixed solvents has been presented in the chromatography and
soil science literature (Dolanetal., 1979; Rao etal., 1985). This
equation is:
log K = log K w - oof,
p,w
(2)
where Kp m and Kp w are the equilibrium sorption constants
(ml g- 1)' for the'mixed-solvent and aqueous systems,
respectively, and a is an empirical constant that represents
any deviation of the sorption-fc functionality from that observed
for solubilization. The latter term is generally considered to
represent solvent-sorbent interactions.
The decrease in Kp caused by addition of a cosolvent results in
a reduction in retardation (i.e., retardation factor, R, = 1 +
(p/6)Kp where p and 6 are soil bulk density and volumetric
water content, respectively). The cosolvency effect has
been demonstrated by experiment to cause a decrease in the
sorption and retardation of many organic solutes (cf., Nkedi-
Kizza et al., 1985; 1987; 1989;'Fu and Luthy, 1986; Wood et al.,
-------
1990; Brusseau et al., 1991 a). An example of this effect is
shown in Figure 1, where log Kp values obtained for sorption of
anthracene by a sandy soil are plotted versus volume fraction
of methanol (Figure 1 A). The effect of methanol on the transport
of anthracene in a column packed with the sandy soil is shown
in Figure 1B.
The discussion of the cosolvency effect presented above was
focused on low-polarity organic compounds. A number of
environmentally important compounds, however, are ionizable
acids or bases (e.g., phenols, amines). The impact of organic
cosolvents on the sorption of ionizable organic solutes has
received very little attention to date. The decreases in sorption
of ionizable solutes, present in the neutral form, obtained with
increasing fraction of cosolvent were similar to those observed
for nonionizable solutes (Fu and Luthy, 1986; Leeetal., 1991),
as might be expected. In these cases, however, the system pH
was fixed. The impact of cosolvents on sorption of ionizable
solutes in systems where pH is not controlled is of great
interest, considering the effect organic cosolvents can have on
the pH of the system and on the pKa of the solute. The pKa of
an ionizable solute changes with the composition of the solvent
because of the so-called medium effect, which results from
differences in solvent-solvent and solute-solvent interactions
(cf., Bates, 1969). The pKa value of an organic acid will increase
with increasing fraction of cosolvent (cf., Parsons and Rochester,
1975; Rubino and Berryhill, 1986), while that of an organic base
will decrease (cf., Gowland and Schmid, 1969). Observe that
for both cases, the shift in pKa promotes formation of the neutral
species. This shift in speciation could significantly affect the
nature and magnitude of sorption.
To illustrate the impact of cosolvent on transport of ionizable
solutes, breakthrough curves obtained for pentafluorobenzoate
in water and methanol systems are compared in Figure 2. Note
that no sorption is observed for the aqueous system and that the
retardation factor is, therefore, 1. No sorption of
pentafluorobenzoate is expected since it is in the anionic form
under the experimental conditions. The fact that sorption is
essentially nonexistent for many organic acids under conditions
typical to the subsurface (pKa« pH; net negative surfaces) has
fomented the use of these organic acids as groundwater tracers.
In contrast, R is greater than 1 for the methanol system. This
change in R would negate the use of pentafluorobenzoate as a
tracer to delineate the velocity of fluid flow. The increase in
retardation with addition of an organic cosolvent has also been
observed for other acids such as dicamba, 2,4-
dichlorophenoxyacetic acid, and chlorophenols (Hassett et al.,
1981; Brusseau, 1990; Lee et al., 1993). This phenomenon
may be important at waste-disposal sites, where ionogenic
chemicals may co-exist with organic solvents.
In comparison to the amount of research devoted to the effect
of cosolvents on solubility and equilibrium sorption of organic
contaminants, there has been little work reported on the impact
of cosolvents on nonequilibrium sorption of organic solutes. A
decrease in the asymmetry of breakthrough curves with
increasing volume fraction of cosolvent was reported by Nkedi-
Kizza et al., (1987), who were investigating the transport of two
herbicides (diuron and atrazine) in columns packed with a
sandy soil. Breakthrough-curve asymmetry, which was
attributed by the authors to nonequilibrium sorption, decreased
with increasing cosolvent content suggesting that the rate of
sorption is greater in the presence of a cosolvent. The sorption
of dioxins by soils from water/methanol mixtures was observed
to be more rapid at higher methanol contents (Walters and
Guiseppi-Elie, 1988). The desorption rate constant (k2) has
been observed to increase with increasing fraction of cosolvent
(Nkedi-Kizzaetal., 1989; Shorten and Elzerman, 1990; Brusseau
etal., 1991 a; Leeetal., 1991).
1.5 -
0.5
-0.5
-1.5
log Kp=-4.07 fc +1.635
r2 = 0.988
0.3 0.5
Volume Fraction Methanol, f c
0.7
g
1
c
o
O
CD
1
(D
cc
1
.
.
0.8
.
0.6
0.4
.
0.2
,
n
H&03 « * * *
F° .* B
P
3A ^ * fc = 0.3
? »0 . 0 fc = 0-5
A A fc = 0.7
A
k. 0 *
A _.
^ 0
A
^ L
!»»%. i^Ba-. ' i i i i ' i ' * «» ' '
20 40 60 80 100 120 140 160
Pore Volume
Figure 1. The influence of methanol on the sorption and transport of anthracene in a Eustis sand; A) the log-linear relationship between
the equilibrium sorption constant (K ) and volume fraction of cosolvent (fc). B) The influence of cosolvent on the retardation
and transport of anthracene. Data from Brusseau et al., 1991 a.
-------
1
I
<3
o
QT
1
0.8
t\ o
O.O
0.4
0.2
Q
f~' * *
A -.
t
* 100% water, R = 1
A 100% methanol, R = 1.7
.
A -
Solute - pentafluorobenzoic acid
A * Soil - Eustis sand
9
A-. 1 1 I
0 2 4 , 6 8
Pore Volumes
Figure 2. The effect of methanol on the transport of an organic
acid (pentafluorobenzoate) in a sandy soil; data from
Brusseau 1990.
A quantitative investigation of the impact of organic cosolvents
on nonequilibrium sorption of organic solutes was presented by
Brusseau et al. (1991 a). They presented a model that predicts
a log-linear relationship between k2 and fc:
log ka = log k2 w + <|> fc (3)
where k2 and k,w are the reverse sorption-rate constants for
the mixed-solventand aqueous systems, respectively; 4> = acco;
and a is the slope of the linear relationship between log k~ w and
log Kpw. The validity of this model was substantiatedusing
experimental data. Examples of their results are presented in
Figures. The mechanism responsible for the cosolvency effect
on sorption kinetics was postulated to involve changes in
conformation of the organic carbon associated with the sorbent.
These conformational changes were induced by the changes in
solvent polarity that resulted from the addition of a cosolvent.
The concentrations of cosolvent required to produce substantial
enhancementin solubility and reduction in sorption are relatively
large (% level) for many solutes of interest. Thus, it has been
difficult to envision scenarios wherein cosolvency could be
important. The use of oxygenated and alternative fuels, however,
has presented cases where cosolvency could be very important.
For example, the presence of the cosolvent in alternative fuels
(e.g., 50% methanol, 50% gasoline) could enhance the transport
of the gasoline constituents contained in the fuel, thus increasing
the potential for groundwater contamination resulting from a
spill. In any case, the effect would probably be limited to the
region near the spill (i.e., the near-field domain).
Immiscible Liquids: Multi-Component Systems,
Dissolution Kinetics, and Transport of Co-Solutes
The disposition of immiscible organic liquids in the subsurface
is of interest to environmental scientists, hydrologists,
environmental/civil engineers, and petroleum engineers. The
vast majority of research performed by these groups has
focused on the movement, entrapment, and displacement of
the liquid (cf., Marie, 1981; Schwille, 1988). This reflects
concerns associated with petroleum-reservoir engineering as
well as remediation of solvent- and petroleum- contaminated
sites. Other aspects that have begun to receive attention are
the dissolution of residual immiscible phases, including the
partitioning behavior of multi-component liquids and the rate of
mass transfer to the aqueous phase, and the effect of immiscible
liquids on the transport of co-solutes.
1. Transport, Entrapment, and Dissolution of
Immiscible Organic Liquids in the Subsurface
The movement, entrapment, and mobilization of immiscible
organic liquids in porous media has been the focus of a
tremendous research effort. Entire volumes have been published
on this subject and there is no need to reproduce this material.
Instead, the dissolution of immiscible organic liquids, a topic
that has received less attention, will be briefly discusjsed.
Mass transfer of a constituent between two liquids can be
represented by (Cussler, 1984):
= kr (KrCm - Cr)
(4)
where Cr and Cm are the concentrations of the solute in the
residual and aqueous phases, respectively; Kr is the liquid-
liquid partition coefficient; kristhe mass-transfer constant (1/T);
and t is time (T). The appropriate driving force for mass transfer
is the difference between the actual solute concentration in the
residual phase and that attained at equilibrium (KrCm) (Cussler,
1984). Equation 4 is based on a macroscopic approach and the
mass transfer term is a global parameter. Microscopic
approaches where mass transfer across individual interfaces is
explicitly simulated have also been developed. This latter
approach, however, is constrained by the difficulty of specifying
the nature and magnitude of the interfaces present in the
system.
Consideration of the kinetics of dissolution of residual phases
of immiscible organic liquids is a departure from the majority of
models developed for multi-phase systems, which are based on
instantaneous attainment of equilibrium between residual and
water phases. The results of several laboratory experiments
have suggested that mass transfer between immiscible liquid
and water is relatively rapid (cf., van der Waarden et al., 1971;
Fried et al., 1979; Schwille, 1988; Miller et al., 1990). Other
investigations, however, have shown that liquid-liquid transfer
can be significantly rate-limited, especially under conditions
that may be found in the field (cf., Hunt et al., 1988; Powers et
al., 1991; Brusseau, 1992a). Thus, the use of the equilibrium
assumption for mass transfer in the development of mathematical
models is still open to question. Much additional research is
needed in this area to identify the conditions under which
dissolution will be rate limited and the local equilibrium
assumption is not valid. Liquid-liquid mass transfer in
heterogeneous porous media is of special concern.
2. Partitioning of Multi-Component Liquids
While some of the most widely studied immiscible liquids are
composed of a single component (e.g., trichloroethene), many
others (e.g., gasoline, diesel fuel, coal tar) are multi-component
liquids. Knowledge of the partitioning behavior of multi-
component liquids is essential to the prediction of their impact
on groundwater quality. The partitioning of components into
-------
D)
O
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
Naphthalene
logK2 = 1.87fc-0.87
r2 = 0.99
0 0.1 0.2 0.3
Volume Fraction Methanol, fc
2.5
2
1.5
£ 1
O)
2 0.5
0
-0.5
-1
Anthracene
log K2 = 3.55 f c - 0.38
r2 = 0.98
B
0.2 0.4 0.6
Volume Fraction Methanol, fc
0.4
0.2
£
§> 0
-0.2
-0.4
Phenanthrene
log K2 = 2.10 fc- 1.20
r2 = 0.97
0.4 0.5 0.6 0.7
Volume Fraction Methanol, f c
IogK2 = 1.24fc-1.20
r2 = 0.88
0.1 0.2 0.3 0.4
Volume Fraction Methanol, f c
Figure 3. The effect of methanol on the reverse sorption rate coefficient (k2); figure adapted from Brusseau et al., 1991a.
water is controlled by the aqueous solubility of the component
and the composition of the liquid. A simple approach to
estimating partitioning involves an assumption of ideal behavior
in both aqueous and organic phases and the application of
Raoult's law:
C^XOjS*, (5)
where CW| is aqueous concentration (mol/l) of component i, SW|
is aqueous solubility (mol/l) of component i, and X°j is mole
fraction of component i in the organic liquid. The liquid-liquid
partition coefficient, K|, is given by:
Kr = C°JX°, Sw,
(6)
where C°| is concentration (mol/l) of i in the organic liquid and
where C0//0, is equivalent to the inverse of the molar volume of
the organic liquid. The Raoult's law-based approach has been
used successfully to predict aqueous-phase concentrations of
compounds (or partition coefficients) for gasoline (Cline et al.,
1991), diesel fuel (Lee et al., 1992a), and coal-tar (Lee et al.,
1992b) systems (see Figure 4). One result of this and other
work (Banerjee, 1984; Piceletal., 1988; Vadasetal., 1991) is
that it appears that many multi-component liquids can be
approximated as ideal mixtures.
3. Effect of Immiscible Liquids on Solute Transport.
The impact of immiscible liquids present as a separate phase
on the sorption and transport of organic solutes was evaluated
by Brusseau (1990). An analysis of experimental data obtained
from systems where an immiscible liquid (e.g., toluene) was the
mobile phase showed that the retardation of organic solutes
(e.g., benzene) was near unity and much lower than that which
would be obtained with water as the solvent. This enhanced
transport by mobile immiscible liquids is to be expected based
upon the relative solubilities of low-polarity organic solutes in
organic liquids and water.
The opposite effect is observed, however, when the immiscible
liquid is present as a fixed residual phase. The residual phase
serves as a sink for organic solutes, resulting in enhanced
retention and retardation. For example, the presence of a
residual phase of aviation gas was observed to increase retention
of petroleum constituents (e.g., toluene) in columns packed
with an aquifer material (Bouchard etal., 1989). The presence
of residual petroleum or PCS oils was shown to increase the
sorption of pentachlorophenol, toluene, and 2-chlorobiphenyl
(Boyd and Sun, 1990). A large increase in retardation of
naphthalene was observed when a residual phase of
tetrachloroethene was emplaced in a column packed with
-------
* Measured Values
95% Confidence Interval
inzene
1,2,3-trimethyIbenzene
S.^-SDtvttaluene .
£thyfoenzene
xylenes ^~':--^_ toluene
MW0-102g/mole
(Data from Cllne et al., 1991)
MWd|-226g/mo!e
(Data from Lee et al., 1992a)
1-methylnaphthalene
(Data from Lee et al.. 19925}
-4.5
log [S, moles/L]
Figure 4. Comparison of data obtained from multi-component
partitioning experiments to ideal behavior predicted
by use of Raoult's Law. A) Gasoline system, data from
Cllne etal., 1991; B) Diesel Fuel system, data from Lee
et al., 1992a; C) Coal Tar system, data from Lee et al.,
1992b. S is aqueous solubility of the compound, Ktw
Kg and Kgw are the equilibrium partition coefficients
of trie compounds for distribution between the organic
and aqueous phases.
aquifer material (Brusseau, 1990). A mathematical model
describing the effect of immobile immiscible organic phases on
the transport of solutes was presented by Brusseau (1992a).
The model was used to predict the transport of toluene in a
column packed with an aquifer material contaminated with a
residual of aviation gas (data reported by Bouchard et al.,
1989). The simulated prediction produced with the model
provided a good description of the data (see Figure 5). Based
on these investigations, it appears possible that residual phases
of immiscible organic liquids can serve as long-term sinks and
sources for organic solutes.
When multiple contaminants are present in solution, a primary
question to be addressed is the occurrence of antagonistic or
synergistic interactions among the solutes, and between the
solutes and the solid and aqueous phases. The presence of a
cosolute at high concentrations can affect the behavior of
organic compounds in several ways, resulting in the following
three phenomena: (1) competitive sorption; (2) cooperative
sorption; and (3) cosolvency. The first and third phenomena
reduce sorption and thus enhance the transport of solutes,
whereas cooperative sorption has the opposite effect. A potential
source of these multi-contaminant solutions is the dissolution of
immiscible liquids into water residing in or entering the
subsurface, the relatively slow movement of water in the
subsurface creates the possibility of relatively high solute
concentrations (e.g., near X0, Sw, limit) in the vicinity of the
immiscible liquid phases.
Competitive sorption, where sorption of a solute is reduced by
the presence of a co-solute, has been investigated by several
researchers and their results have generally shown no
competition for nonionic, low-polarity organic solutes such as
naphthalene and chlorinated benzenes (of., Kariekhbff et al.,
1979; Chiou etal., 1983). Indeed, non-competition is considered
a defining characteristic of the sorption of nonionic, low-polarity
organic solutes by a "partitioning" mechanism (Chiou et al.,
1983). However, some researchers have reported relatively
small decreases in sorption resulting from competition (Maclntyre
and deFur, 1985; Abdul and Gibson, 1986; McGinley et al.,
1989). The vast majority of studies on sorption in multi-solute
systems have used sorbents with relatively high organic-carbon
contents (i.e., greater than 0.1 %). Conversely" few studies
have been reported for systems comprised of sorbents containing
small organic-carbon contents, which are representative of
many sand aquifers. The sorption of trichloroethene and p-
xylene from single and binary solute solutions by two ;organic-
carbon-poor aquifer materials was examined by Lee et al.
(1988). They observed no difference in sorption between the
single and binary systems. The sorption of trichloroethene by
a sandy aquifer material in single and ternary solute systems
was observed by Brusseau and Rao (1991) to be essentially
identical.
Cooperative sorption, where sorption of nonionic, low-polarity
organic solutes is enhanced by the presence of other nonionic,
low-polarity organic solutes, has been studied by few
researchers. Brusseau (1991) investigated the effect of a
nonionic, low-polarity cosolute (tetrachloroethene) on the
sorption of three nonionic, low-polarity organic chemicals
(naphthalene, p-xylene, 1,4-dichlorobenzene) by two aquifer
materials with small organic-carbon contents (< 0.03 %). In all
cases, the sorption of the primary solute was enhanced by the
presence of high concentrations of tetrachloroethene.
Equilibrium sorption constants measured in binary-solute
systems were 1.5 to 3 times larger than those measured forthe
single-solute systems. Hence, tetrachloroethene had a
synergistic (i.e., cooperative), rather than an antagonistic (i.e.,
competitive), effect on the sorption of the primary solutes. The
enhanced sorption was postulated to result from sorbed
tetrachloroethene increasing the effective organic carbon content
of the sorbent. Enhanced sorption was observed by Onken and
Traina (1991) in recently reported experiments that used
synthetic organo-clay complexes. They examined the sorption
of pyrene by CaCO, treated with humic acid to obtain an organic
carbon content of 0.003%. The sorption of pyrene in a binary
-------
log K2 = 3.55 f c - 0.38
r2 = 0.98
logK2 = 1.87fc-0.87
r2 = 0.99
0 0.1 0.2 0.3
Volume Fraction Methanol, f c
0 0.2 0.4 0.6
Volume Fraction Methanol, f c
0.4
0.2
g> 0
-0.2
-0.4
Phenanthrene
log K2 = 2.10 fc- 1.20
r2 = 0.97
0.4 0.5 0.6 0.7
Volume Fraction Methanol, f c
logK2 = 1.24fc-1.20
0.88
0.1 0.2 0.3 0.4
Volume Fraction Methanol, f c
Figure 3. The effect of methanol on the reverse sorption rate coefficient (k2); figure adapted from Brusseau et al., 1991a.
water is controlled by the aqueous solubility of the component
and the composition of the liquid. A simple approach to
estimating partitioning involves an assumption of ideal behavior
in both aqueous and organic phases and the application of
Raoult's law:
Cw _
-
(5)
where CW| is aqueous concentration (mol/l) of component i, SW|
is aqueous solubility (mol/l) of component i, and X°| is mole
fraction of component i in the organic liquid. The liquid-liquid
partition coefficient, Kit is given by:
where C°j is concentration (mol/l) of i in the organic liquid and
where C°j/X°, is equivalent to the inverse of the molar volume of
the organic liquid. The Raoult's law-based approach has been
used successfully to predict aqueous-phase concentrations of
compounds (or partition coefficients) for gasoline (Cline et al.,
1991), diesel fuel (Lee et al., 1992a), and coal-tar (Lee et al.,
1992b) systems (see Figure 4). One result of this and other
work (Banerjee, 1984; Picel et al., 1988; Vadas et al., 1991) is
that it appears that many multi-component liquids can be
approximated as ideal mixtures.
3. Effect of Immiscible Liquids on Solute Transport.
The impact of immiscible liquids present as a separate phase
on the sorption and transport of organic solutes was evaluated
by Brusseau (1990). An analysis of experimental data obtained
from systems where an immiscible liquid (e.g., toluene) was the
mobile phase showed that the retardation of organic solutes
(e.g., benzene) was near unity and much lower than that which
would be obtained with water as the solvent. This enhanced
transport: by mobile immiscible liquids is to be expected based
upon the relative solubilities of low-polarity organic solutes in
organic liquids and water.
The opposite effect is observed, however, when the immiscible
liquid is present as a fixed residual phase. The residual phase
serves a.s a sink for organic solutes, resulting in enhanced
retention and retardation. For example, the presence of a
residual phase of aviation gas was observed to increase retention
of petroleum constituents (e.g., toluene) in columns packed
with an aquifer material (Bouchard et al., 1989). The presence
of residual petroleum or PCB oils was shown to increase the
sorption of pentachlorophenol, toluene, and 2-chlorobiphenyl
(Boyd and Sun, 1990). A large increase in retardation of
naphthalene was observed when a residual phase of
tetrachloroethene was emplaced in a column packed with
-------
G>
J2 2
1
0
n-propylbenzene
rt,2,3-trimethylbenzene
3.4-6BlyKatuen0
9 Measured Values
95% Confidence Interval
MTBE
(Data from Cline et al., 1991)
-4.5 -4 -3.5
log [S, moles/L]
Figure 4. Comparison of data obtained from multi-component
partitioning experiments to ideal behavior predicted
by use of Raoult's Law. A) Gasoline system, data from
Cline et al., 1991; B) Diesel Fuel system, data from Lee
et al., 1992a; C) Coal Tar system, data from Lee et al.,
1992b. S is aqueous solubility of the compound, Ktw
K^ and Kgw are the equilibrium partition coefficients'
ofthecompoundsfordistribution between the organic
and aqueous phases.
aquifer material (Brusseau, 1990). A mathematical model
describing the effect of immobile immiscible organic phases on
the transport of solutes was presented by Brusseau (1992a).
The model was used to predict the transport of toluene in a
column packed with an aquifer material contaminated with a
residual of aviation gas (data reported by Bouchard et al.,
1989). The simulated prediction produced with the model
provided a good description of the data (see Figure 5). Based
on these investigations, it appears possible that residual phases
of immiscible organic liquids can serve as long-term sinks and
sources for organic solutes.
When multiple contaminants are present in solution, a primary
question to be addressed is the occurrence of antagonistic or
synergistic interactions among the solutes, and between the
solutes and the solid and aqueous phases. The presence of a
cosolute at high concentrations can affect the behavior of
organic compounds in several ways, resulting in the following
three phenomena: (1) competitive sorption; (2) cooperative
sorption; and (3) cosolvency. The first and third phenomena
reduce sorption and thus enhance the transport of solutes,
whereas cooperative sorption has the opposite effect. A potential
source of these multi-contaminant solutions is the dissolution of
immiscible liquids into water residing in or entering the
subsurface, the relatively slow movement of water in the
subsurface creates the possibility of relatively high solute
concentrations (e.g., near X^ SWj limit) in the vicinity of the
immiscible liquid phases.
Competitive sorption, where sorption of a solute is reduced by
the presence of a co-solute, has been investigated by several
researchers and their results have generally shown no
competition for nonionic, low-polarity organic solutes such as
naphthalene and chlorinated benzenes (cf., Karickhoff et al.,
1979; Chiouetal., 1983). Indeed, non-competition is considered
a defining characteristic of the sorption of nonionic, low-polarity
organic solutes by a "partitioning" mechanism (Chiou et al.,
1983). However, some researchers have reported relatively
small decreases in sorption resulting from competition (Maclntyre
and deFur, 1985; Abdul and Gibson, 1986; McGinley et al.,
1989). The vast majority of studies on sorption in multi-solute
systems have used sorbents with relatively high organic-carbon
contents (i.e., greater than 0.1 %). Conversely, few studies
have been reported for systemseomprised of sorbents containing
small organic-carbon contents, which are representative of
many sand aquifers. The sorption of trichloroethene and p-
xylene from single and binary solute solutions by two brganic-
carbon-poor aquifer materials was examined by Lee et al.
(1988). They observed no difference in sorption between the
single and binary systems. The sorption of trichloroethene by
a sandy aquifer material in single and ternary solute systems
was observed by Brusseau and Rao (1991) to be essentially
identical.
Cooperative sorption, where sorption of nonionic, lowrpolarity
organic solutes is enhanced by the presence of other nonionic,
low-polarity organic solutes, has been studied by few
researchers. Brusseau (1991) investigated the effect of a
nonionic, low-polarity cosolute (tetrachloroethene) on the
sorption of three nonionic, low-polarity organic chemicals
(naphthalene, p-xylene, 1,4-dichlorobenzene) by two aquifer
materials with small organic-carbon contents (< 0.03 %). In all
cases, the sorption of the primary solute was enhanced by the
presence of high concentrations of tetrachloroethene.
Equilibrium sorption constants measured in binary-solute
systems were 1.5 to 3 times larger than those measured for the
single-solute systems. Hence, tetrachloroethene had a
synergistic (i.e., cooperative), rather than an antagonistic (i.e.,
competitive), effect on the sorption of the primary solutes. The
enhanced sorption was postulated to result from sorbed
tetrachloroethene increasing the effective organic carbon content
of the sorbent. Enhanced sorption was observed by Onken and
Traina (1991) in recently reported experiments that used
synthetic organo-clay complexes. They examined the sorption
of pyrene by CaCO3 treated with humic acid to obtain an organic
carbon content of 0.003%. The sorption of pyrene in a binary
-------
0.8
0.6
o
O
I
-------
disequilibrium between the advective and non-advective
domains and result in delayed removal (i.e., "tailing"). The
effects of variable velocity fields caused by hydraulic
conductivity heterogeneity can also be caused by sorption
capacity variability.
2. Sorption/desorption kinetics: Recent research has revealed
that adsorption/desorption of organic solutes by aquifer
materials can be significantly rate limited (Lee et al., 1988;
Ball and Roberts, 1991; Brusseau and Reid, 1991; Brusseau
et al., 1991b). The rate-limiting mechanism apparently
involves constrained diffusion within the sorbent matrix
(Ball and Roberts, 1991; Brusseau et al., 1991c). The
validity of the local equilibrium assumption is dependent, in
part, upon the hydrodynamic residence time of the
contaminant in the system, which is a function of, among
other factors, pore-water velocity. Increasing the velocity,
as is done in pump-and-treat, can cause or enhance
nonequilibrium conditions as a result of reduced residence
time. Nonequilibrium will produce aqueous-phase
concentration values lower than those obtained under
ideal, equilibrium conditions. Thus, tailing will occur and
removal by flushing will take longer.
3. Immiscible liquid dissolution kinetics: In many cases,
residual phases of immiscible organic liquids may exist in
portions of the contaminated subsurface. It has been
shown that very large pore-water velocities (i.e., hydraulic
gradients) are required to displace residual saturation
(Wilson and Conrad, 1984; Willhite, 1986; Hunt et al.,
1988). Hence, the primary means of removal will be
dissolution into water and volatilization into the soil
atmosphere. The immiscible liquid, therefore, serves as a
long-term source of contaminant. As discussed above, the
dissolution of immiscible liquid into water may be rate
limited and, in such cases, would be dependent upon pore-
water velocity. Increased velocity would enhance
nonequilibrium conditions and, thus, result in tailing and
delayed removal.
4. Contaminant Aging: Recent research has shown that
contaminants that have been in contact with porous media for
long times are much more resistant to desorption, extraction,
and degradation. For example, contaminated soil samples
taken from field sites exhibit solid:aqueous distribution ratios
that are much larger than those measured or estimated based
on spiking the porous media with the same contaminant (e.g.,
adding contaminantto uncontaminated sample) (Steinberg et
al., 1987; Pignatello etal., 1990; Smith etal., 1990; Scribner
et al. 1992). In addition, the desorption rate coefficients
determined for previously contaminated media collected from
the field have been shown to be much smaller than the values
obtained for spiked samples (Steinberg et al., 1987). These
field-based observations are supported by laboratory
experimentsthatshowdesorptionratecoefficientstodecrease
with increasing time of contact prior to desorption (Karickhoff,
1980; McCall and Agin, 1985; Coates and Elzerman, 1986;
Brusseau et al., 1991c). These phenomena are significant
not only because of the delayed removal they can cause, but
also because the aged contaminants appear to be highly
resistantto degradative processes (cf., Steinberg etal., 1987;
Scribner etal., 1992). Thus, these aged contaminant residues
may be resistant to remediation, except perhaps by use of an
enhancement technique.
5. Other Factors: Otherfactors, such as nonuniform flowpaths
and stagnation zones, can contribute to observed nonideal
phenomena such as tailing during a pump-and-treat
remediation. The effects of these factors are, however,
much more a function of well-field dynamics than
contaminant-media interactions and, as such, would not be
affected by chemical enhancements.
It is apparent from the above discussion that several factors
influencing contaminant transport can have deleterious effects
on the efficacy of pump-and-treat remediation. These effects
can create conditions where the expected, desirable result of
large decreases in remediation time is not obtained when
pumping is initiated or increased. These factors must be
considered when designing pump-and-treat remediation
systems.
Unfortunately, there has been very little quantitative analysis of
the impact of nonideal transport on aquifer flushing. An example
taken from one of the few such analyses is presented in Figure
6 (adapted from Brusseau, 1993). The data presented in the
figure were obtained from a pilot-scale aquifer flushing system
wherein a two-well injection-withdrawal couplet was used to
evaluate the effect of injecting clean water into a contaminated
aquifer (Whiffen and Bahr, 1984). These, as well as other, data
were used by Brusseau (1992b) to evaluate the ability of a multi-
factor nonideality model to predict field-scale solute transport.
The data were subsequently used to quantitatively evaluate the
effect of porous-media heterogeneity and nonequilibrium
sorption on the effectiveness of pump-and-treat (Brusseau,
1993). The predicted removal curve for the case of uniform
aquifer properties and instantaneous sorption/desorption is
shown in Figure 6. It is evident that the prediction greatly
underestimates the volume of water required to remove the
contaminant. The predicted simulation obtained for the case of
variable hydraulic conductivity and rate-limited sorption/
desorption matches the field data extremely well (see Figure 6).
A comparison of this prediction to the one obtained for ideal
conditions clearly illustrates the effect that nonideal transport
factors can have on aquifer flushing.
The predicted removal of contaminant for the case of spatially
variable hydraulic conductivity and instantaneous sorption/
desorption is also shown in Figure 6. While this prediction does
not match the early field observations, at large pore volumes
the simulated curve approaches the curve obtained by including
the combined effects of variable conductivity and rate-limited
sorption/desorption. This suggests that, while both;factors
contribute to nonideal transport, spatially variable conductivity
may be the more important factor constraining the efficacy of
aquifer flushing in this system. The knowledge of which factor
or factors is the major cause of nonideal transport is essential
in the design of an effective method for enhancing the efficiency
of a pump-and-treat operation.
Chemical Enhancement of Pump-and-Treat
Remediation
Several chemical-based techniques for enhancing contaminant
removal in thesubsurface are under investigation (e.g., addition
of surfactants, cosolvents, complexing agents), and each has
advantages and disadvantages. A detailed discussion of
chemical enhancement techniques was presented by Palmer
and Fish (1992). However, several aspects relating to the
-------
o
I
0.8
0.6
O
o
0.4
111
cc
diethylether field data
Predicted simulation
(variable hydraulic conductivity)
(rate-limited desorption)
"~ Predicted simulation
(variable hydraulic conductivity)
(instantaneous desorption)
Predicted simulation
(uniform hydraulic conductivity)
(instantaneous desorption)
10 15 20
PORE VOLUMES
25
Figure 6.
The effect of nonideal transport on removal of organic
contaminants from aquifers by flushing. Field data
from Whiffen and Bahr, 1984; model used for
simulations from Brusseauetal. 1989. Figure adapted
from Brusseau 1993.
impact of nonideal transport phenomena on the efficacy of
chemical enhancement were not discussed.
Surfactants are currently the focus of the research effort on
chemical enhancements and, based on preliminary laboratory
data, appear to have promise for enhancing pump-and-treat
remediation in some situations. The use of dissolved organic
matter (DOM) and of cosolvents is also being investigated,
albeit at a smaller scale. Miscible cosolvents, such as methanol,
reduce the net polarity of the mixed solvent when added to
water and thereby increase the quantity of a nonionic organic
compound that can dissolve in the mixed solvent. This increase,
in turn, results in a smaller equilibrium sorption constant and
less attendant retardation. Thus, the addition of a cosolvent
can reduce the volume of water required to flush a contaminant
from porous media by altering the equilibrium phase distribution.
A similar result is obtained with surfactants and DOM, although
by different mechanisms. Hence, surfactants, DOM, and
cosolvents act to increase the aqueous-phase concentration of
organic compounds, the so-called "solubilization" effect. This
effect is of special interest for the removal of residual phases of
immiscible liquids. Theother major method of removing trapped
residual phases, mobilization, will not be considered in the
present discussion.
A comparison of the relative degree to which aqueous-phase
concentration of contaminant is enhanced by the various
additives favors the surfactants. However, a comparison of this
type can be very misleading without considering such factors as
potential interactions between the additive and the porous
media. It is well known, for example, that surfactant molecules
(of., Ducreux et al., 1990; Kan and Tomson, 1990; Jafvert and
Heath, 1991) and DOM (of., Dunnivantetai., 1992; Moore etal.,
1992) can sorb to surfaces of solids, thereby reducing the
concentration of additiveavailablefordissolvingthecontaminant.
In addition, surfactants and DOM may precipitate under certain
conditions. In contrast, most subsurface solids have a low
affinity for miscible solvents such as methanol. Thus, it is
possible that, whereas the "active" mass of a surfactant or DOM
may be significantly less than the total mass injected into the
subsurface, that of a solvent may be essentially the same.
A comparison of the impact of several potential chemical
additives on the apparent solubility of selected organic
compounds was developed by collecting and synthesizing data
reported in the literature (see Table 1). The effect of sorption
and precipitation of the additives was taken into account. The
results of the analyses are presented in Figure 7a-c. For all
three solutes, the nonionic surfactant (Triton), with low assumed
sorption, produced the greatest enhancement. The cosolvent
(ethanol) produced the lowest degree of enhancement for all
three solutes. The solubilization effect of ethanol increases
dramatically at cosolvent concentrations above those used in
these analyses. It is readily apparent that the relative
enhancement effect will vary by solute, and by other factors
such as the nature of the sorbent. The comparison of the
effectiveness of various additives under a range of conditions
is a topic requiring more research.
The primary criterion upon which chemical enhancement
additives are judged is their solubilization potential. The impact
of interactions between the additive and the solid phase on this
enhancement is an important factor to consider, as discussed
above. However, there are several other factors that should
also be considered when selecting an enhancement agent. In
this regard, cosolvents have several benefits that surfactants
and DOM do not.
First, the addition of a cosolvent increases the magnitude of the
desorption rate coefficient (not to be confused with an increase
in the rate of desorption), thereby reducing the time required to
attain equilibrium. This reduction in thedegreeofnonequilibrium
would result in ^reduced tailing during pumping. This, in turn,
would decrease the volume of water and the time required to
remove the contaminant by flushing. As previously discussed,
rate-limited desorption may impose a significant constraint on
the efficacy of pump-and-treat remediation. If so, the ability of
a cosolvent to reduce the degree of nonequilibrium would be a
major attribute. There is no reason to expect surfactants or
DOM to increase desorption rate coefficients.
Second, cosolvents may be able to "extract1' the highly retained,
aged contaminants that have been observed in field studies
(see discussion above). There is no reason to suppose that
surfactants or DOM could act in an "extractive" manner.
Conversely, there is good reason to suppose that cosolvents
could enhance the release of aged contaminants, based on the
results of solvent extraction techniques used in the analysis of
contaminated soils (cf., Sawhney et al., 1988) and on the
results of experiments that evaluated the effect of cosolvents on
the desorption of organic compounds (Freeman and Cheung,
1981; Nkedi-Kizza etal., 1989; Brusseau et al., 1991 a).
Third, cosolvents may be able to access contaminant that is
residing in low hydraulic-conductivity domains such as clay
lenses. During a pump-and-treat remediation, as discussed
above, contaminant in these domains is probably removed
primarily through diffusion. The clay particles provide a large
surface area with which a surfactant or DOM may interact and
thereby reduce its availability for enhancing contaminant
-------
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33
£
c
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p
1
1
14
12
10
B
6
4
2
1
Trichloroethene
Ettianol
SOS
Humicacid
O "O Triton. R.2
D-" D Triton. R.10
^'
,<*"'
. ,£>'' _-
ii " i '~~ """ i
A
...-''
..-''
..--''
_..-''
-'"
_""" E
, ...Q
i i
20 40 60 80
Additive Concentration (g/l)
100
I
120
100
60
40
20
Naphthalene
B
EUunol
SDS
Humicaod
O-O Triton, fl-2
p,,,,,Q Triton.R.10
.G'
20 40 60 80
Additive Concentration (g/l)
5000
c 4000
I
g 3000
2000
1000
Pyrene
Ethanol
SDS
Humicacid
©O Triton. R.2
rj,,,, ,,Q Triton. R-io
.
-,
1
*
20 40 60 80
Additive Concentration (g/l)
100
Figure 7.
The effect of several additives on the aqueous-phase
concentration of (A) trichloroethene, (B) naphthalene,
and (C) pyrene.
removal. In addition, thesorption of the surfactant or DOM can
enhance the retention of the organic solutes by providing an,
increase in stationary organic carbon. Surfactant micelles and
larger DOM particles may possibly be excluded from the smaller
pore-size domains, which would limit accessibility. Cosplvents
such as methanol do not sorb significantly to solid surfaces and,
because of their small size, would not be excluded from any
pore domains in which contaminants would be found. In
addition, as discussed above, cosolvents have been found to
cause cracking of clayey materials. This cracking results in
larger permeabilities, which could enhance the rate of
contaminant removal from the lenses. Thus, in comparison to
surfactants and DOM, cosolvents may have a much greater
potential for enhancing the release of contaminants trapped in
fine-grained media.
Fourth, cosolvents have the potential for being used in an
integrated, chemical-biological remediation technique. For
example, methanol is the initial intermediate in the oxidation of
methane by methanotrophic bacteria. The addition of methanol
to the groundwater environment at low concentrations may
stimulate useful cometabolic transformations, causing the
destruction of otherwise refractory contaminants such as
trichloroethene. Under (locally) anaerobic conditions, cosolvent
addition may also drive reductive dehalogenation, particularly
of compounds such as tetrachloroethene (cf., DiStefano et al.,
1991; Gibson and Sewell, 1992). It is possible to envision
situations where addition of cosolvents such as methanol or
ethanol may initiate transformations that result directly or
indirectly in degradation to non-toxic products. The negative
effects of high concentrations of cosolvent on the subsurface
microbial community may initially preclude the development of
biodegradative activity. However, such activity could occur
following dilution of the cosolvent during transport.
Considering the preceding discussion, cosolvents may have
specific properties that make them useful for enhanced pump-
and-treat. However, given these same properties, it is likely
that the use of cosolvents will be limited to smaller scale
problems. The clean-up of near-field contamination problems
is probably where cosolvents can be put to best use.
CONCLUSION
Experience has shown that many soil and groundwater
contamination problems involve complex mixtures of chemicals.
As discussed in this monograph, these mixtures may affect
contaminant behaviorthrough a variety of mechanisms. Because
many of these mechanisms are not well understood, approaches
for dealing with complex mixtures in the subsurface often
involve direct application or untested extrapolation of knowledge
derived from relatively simple aqueous systems. Not surprisingly,
the results are frequently less than satisfactory.
The primary purpose of this paper is to identify and discuss, in
a generic sense, some of the important processes which must
be considered when dealing with complex mixtures in the
subsurface, and to illustrate how these may impact groundwater
quality. From the discussion, it is apparent that complex
mixtures may play a role in groundwater reclamation as well as
degradation of groundwater quality. Equally apparent, however,
is the need for improved scientific understanding of the processes
associated with the transport of complex mixtures and of the
10
-------
Table 1. Enhanced Solubilization Data Collected From the Literature
Additive
Compound
Sorption of Additive*
SDS TCE (Shiau et al., 1992)
SDS Naphthalene (Gannon et al., 1989)
SDS Pyrene (Jafvert, 1991)
Triton TCE (West, 1992)
Triton Naphthalene (Edwards et al., 1991)
Triton Pyrene (Edwards et al., 1991)
Ethanol TCE (Morris et al., 1988)
Ethanol Naphthalene (Morris et al., 1988)
Ethanol Pyrene (Morris et al., 1988)
Humic Acid TCE (Garbarini and Lion, 1986)
Humic Acid Naphthalene (McCarthy and Jiminez, 1985)
Humic Acid Pyrene (Gauthier et a!., 1987)
R = variable (Jafvert and Heath, 1991)
R = variable "
R = variable "
R = 2,10 (Kan and Tomson, 1990)
R = 2,10
R = 2,10
R = 1 (Woodetal., 1990)
R = 1
R = 1
R = 2 (Dunnivant et al., 1992)
R = 2
R = 2
SDS = sodium dodecyl sulfate; TCE = trichloroethene; Triton = triton X-100; Humic Acid = Aldrich humic acid; #Retardation
factor, R, of an additive in a hypothetical soil was estimated from data reported in the references cited in this column.
influence that chemical mixtures have on the behavior of
specific contaminants.
Disclaimer
The information in this document has been funded in partby the
U.S. Environmental Protection Agency under Cooperative
Agreement No. CR-818757. This document has been subjected
to the Agency's peer review and has been approved for
publication as an EPA document.
Quality Assurance Statement
This project did not involve physical measurements and, as
such, did not require a QA plan.
11
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PCB congeners from river sediments. J.Contam. HydroL, 1,191,
1986.
Colby, J., Dalton, H., and Whittenburg, R., Biological and
biochemical aspects of microbial growth on C, compounds, Ann.
Rev. Microbiol., 33,481,1979.
Cussler, E.L., Diffusion: Mass Transfer in Fluid Systems, Cambridge
Univ. Press, Cambridge, 1984.
DiStefano, T.D., Gosset, J.M., and Zinder, S.H., Reductive
dechlorination of high concentrations of tetrachloroethene to ethene
by an anaerobic enrichment culture in the absence of
methanogenesis. App. Environ. Micro., 57,2287,1991.
Dolan, J.W., Gant, J.R., and Snyder, L.R., Gradient elution in high-
performance liquid chromatography, J. Chromat., 165,31,1979.
Ducreux, J., Bocard, C., Muntzer, P., Razakarisoa, O., and Zilliox,
L., Mobility of soluble and non-soluble hydrocarbons in
contaminated aquifer, Water Sci. Tech., 22,27,1990.
Dunnivant, F.M., Jardine, P.M., Taylor, D.L., and McCarthy J.F.,
Transport of naturally occurring dissolved organic carbon in
laboratory columns containing aquifer material, Soil Sci. Soc.
Amer. J., 56, 437,1992.
Edwards, D.A., Luthy, R.G., and Liu, Z., Solubilization of Polycyclic
aromatic hydrocarbons in micellar nonionic surfactant solutions,
Environ. Sci. Technol., 25,127,1991.
Freeman, D.H. and Cheung, L.W., A gel-partition model for
organic desorption from a pond sediment, Science, 214, 790,
1981.
12
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Table 1. Enhanced Solubilization Data Collected From the Literature
Additive
Compound
Sorption of Additive*
SDS TCE (Shiau et al., 1992)
SDS Naphthalene (Gannon et al., 1989)
SDS Pyrene (Jafvert, 1991)
Triton TCE (West, 1992)
Triton Naphthalene (Edwards et al., 1991)
Triton Pyrene (Edwards et al., 1991)
Ethanol TCE (Morris et al., 1988)
Ethanol Naphthalene (Morris et al., 1988)
Ethanol Pyrene (Morris et al., 1988)
Humic Acid TCE (Garbarini and Lion, 1986)
Humic Acid Naphthalene (McCarthy and Jiminez, 1985)
Humic Acid Pyrene (Gauthier et al., 1987)
R = variable (Jafvert and Heath, 1991)
R = variable "
R = variable "
R = 2,10 (Kan and Tomson, 1990)
R = 2,10
R = 2,10
R = 1 (Wood etal., 1990)
R = 1
R = 1
R = 2 (Dunnivant et al., 1992)
R = 2
R = 2
SDS = sodium dodecyl sulfate; TCE = trichloroethene; Triton = triton X-100; Humic Acid = Aldrich humic acid; ^Retardation
factor, R, of an additive in a hypothetical soil was estimated from data reported in the references cited in this column.
influence that chemical mixtures have on the behavior of
specific contaminants.
Disclaimer
The information in this document has been funded in part by the
U.S. Environmental Protection Agency under Cooperative
Agreement No. CR-818757. This document has been subjected
to the Agency's peer review and has been approved for
publication as an EPA document.
Quality Assurance Statement
This project did not involve physical measurements and, as
such, did not require a QA plan.
11
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15
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