SEPA
United States
Environmental Protection
Agency
Office of
Research and
Development
Office of Solid Waste
and Emergency
Response
EPA/540/4-89/003
August 1989
Superfund
Ground Water Issue
Facilitated Transport
Scott G. Huling
The Regional Superfund Ground Water Forum is a group
of ground-water scientists representing EPA's Regional
Superfund Offices, organized to exchange up to date information
related to ground-water remediation at Superfund sites.
Facilitated transport is an issue identified by the forum as a
concern of Superfund decision-makers.
Any process that has the potential to speed the transport
of a pollutant beyond what is expected based solely on
considerations of idealized Darcian flow and equilibrium sorptive
interactions with an immobile solid phase has been broadly
termed, "facilitated transport". Hydrodynamic dispersion, a
transport mechanism which fits this description of facilitated
transport, is not discussed herein.
Research and literature indicates that hydrophobic organic
contaminants (HOC's)(i.e., PCB's, DDT, dioxins, polynuclear
aromatic hydrocarbons (PAH's)) and heavy metals have a high
affinity for mobile subsurface particles and that such an attraction
may alter the mobility of the contaminant. Facilitated transport
is a relatively new area of study in the field of contaminant
transport. Considerable research and interest is currently
focused in this area. Although incompletely understood at this
point, the effects of facilitated transport at Superfund sites may
range from paramountto negligible. There may be an abundance
of field data currently available that identifies both the occurrence
and the importance of these transport mechanisms. However,
relatively little information is available in the scientific literature
which attempts to correlate the occurrence of these transport
mechanisms with field data.
Most Superfund Sites are characterized as having the
following conditions: a complex mixture of organic and inorganic
wastes; highly variable chemical and physical characteristics; a
broad range of chemical concentrations; and a broad spectrum
of soil and hydrogeological characteristics. Therefore, several
facilitated transport mechanisms may be occurring
simultaneously at any site.
Idealized laboratory experiments reported in the literature
have been designed to simulate specific physical and chemical
conditions. These laboratory conditions have allowed
researchers to control the variables which affect the behavior of
contaminants in the subsurface and to identify the mechanisms
which are likely to occur in the field. An understanding of the
various mechanisms of facilitated transport will provide a more
thorough understanding of the fate and transport of contaminants
in the ground water and ultimately will provide the framework for
further development of ground-water remediation technology.
The following is a brief technical overview of facilitated transport
prepared in support of the Regional Superfund Ground Water
Forum.
For further information, contact Scott G. Huling, EPA,
RSKERL-Ada, FTS 743-2313; Candida West, EPA, RSKERL-
Ada, FTS 743-2257; Robert Puls, EPA, RSKERL-Ada, FTS 743-
2262; Dermont Bouchard, EPA, RSKERL-Ada, FTS 743-2321.
Cosolvent Effects
Many releases from land disposal or waste storage
systems consist of a mixture of water and organic compounds.
High concentrations of organic compounds or solvents in water
have significant potential for facilitated transport of usually
immobile HOC's. An HOC which partitions into a solvent may
exhibit increased mobility, above which is typically predicted
from idealized Darcian flow and adsorption/desorption kinetics,
due to its intimate association with the mobile solvent. In a mixed
solvent (cosolvent) system, organic solutes are subjected to a
wide range of chemical and physical processes which ultimately
Superfund Technology Support Center for Ground Water
Robert S. Kerr Environmental
Research Laboratory
Ada, OK
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determine how a particular solute will be distributed and
transported in the subsurface.
In a mixed solvent system consisting of water and one or
morewatermiscible organic compounds(i.e., methanol, acetone,
methyl ethyl ketone, etc...), sorption of HOC's onto the solid
phase does not follow the same sorption behavior as seen for
water without the solvent mixture. Instead, as the fraction of
cosolvent in the mixture increases, the solubility of the HOC
increases exponentially (6). Correspondingly, the sorption
coefficient decreases logarithmically and the retardation factor
decreases drastically (6,7,15,16,18,22). As the sorption
coefficient decreases, less HOC will be sorbed onto the solid
phase and subsequently becomes more mobile. The decrease
in the sorption coefficient has been shown to be a function of the
increased solubility ofthehydrophobic compound in the cosolvent
(18). In one set of laboratory column studies, the breakthrough
curves for some pesticides in a cosolvent system were equal to
that of a conservative tracer, suggesting no measurable
retardation of the pesticide had occurred (16). This mechanism
of facilitated transport is significant at cosolvent concentrations
above a few percent. Therefore, the effects of this transport
mechanism are expected to be greater near the source, prior to
dilution. A model has been developed to quantitatively describe
the sorption and transport of hydrophobic organic chemicals in
an aqueous and mixed solvent system (18).
Research has shown that an organic cosolvent can also
accelerate the movement of metals through a soil matrix. Based
on the results of laboratory soil column experiments, ethylene
glycol was shown to increase the rate of cadmium migration
through three soils compared to water, while 2-propanol increased
the rate of cadmium migration through two of the three soils (19).
This research indicates that metal contaminants may be found
deeper in soils than originally expected because of cosolvent
effects.
Immiscible flow of solvents and petroleum fluids in the
subsurface has been observed at numerous waste disposal
sites. The presence of a mobile immiscible phase can facilitate
the transport of HOC's in both the saturated and unsaturated
zones. The impact of this facilitated transport mechanism has
been described utilizing an analytical chemical transport model
(5). Using this method of analysis, it was observed that ignoring
the presence of organic compounds moving either with the
ground wateror as a separate phase could greatly underestimate
the mobility of chemicals.
Colloidal Processes
The transport of contaminants in ground water hastypically
been characterized as a mass balance of the contaminant
governed by the partitioning of the contaminant between the
mobile aqueous phase and the immobile solid phase. However,
under certain conditions, small solid phase particles and
macromolecules, which exist in some subsurface environments,
are transported in the aqueous phase and have been
characterized as mobile sorbent.
Colloids, defined as particles with diameters less than 10
micrometers (20), are widely recognized as mobile particles in
both the unsaturated and saturated zones of the subsurface.
Many hydrophobic organic contaminants, generally considered
to be highly retarded due to strong interactions with immobile
aquifer material, have a high affinity for the mobile colloidal
material. Consequently, the association between the contaminant
and colloid ultimately affects the transport of the contaminant
(12). Research has found that batch adsorption experiments
which are often relied upon to predict the adsorption potential of
a compound may give misleading results if consideration is not
given to the presence of the nonsettleable particulate phase or
macromolecularphase(9). Furthermore, current solute transport
models that assume partitioning between a mobile aqueous
phase and a stationary organic carbon or mineral phase may
significantly underestimate contaminant mobility (2).
The significance of dissolved colloidal material to the fate
of contaminants depends on the following factors: (a) the
identity and concentration of dissolved colloidal matter; (b) the
nature of the interaction between the contaminants and the
dissolved colloidal matter; and (c) the mobility of the colloidal
matter in an aquifer (11).
Ground water typically contains a few mg/l dissolved
organic matter, but the dissolved organic matter may reach a few
hundred mg/l in surface water and ground water near a dump site
(11). Consequently, the potential effects of facilitated transport
is likely to be greater in waste disposal areas. The nature of the
interactions between contaminants and colloidal material are
diverse. Colloids typically are divided into two groups: organic
and inorganic and the contaminants which colloids interact with
are typically divided into three groups: organic compounds,
metals, and radionuclides. Therefore, it is apparent that there
are many interactions which may occur in a complex mixture of
colloids and contaminants. The various interactions between
colloidal material and contaminants are further discussed below.
The mobility of a diverse range of colloidal matter has been
reported by one reviewer to occur in the ground water under a
variety of conditions (13). However, the ability to accurately
assess the mobility of colloidal material in the subsurface is
difficult and at present, is incompletely understood.
Organic Colloids
Natural and anthropogenic organic colloids occurring in
the subsurface can assume the role of either the sorbent in the
adsorption-desorption and cation exchange mechanisms or the
solvent in a solvent-cosolvent scenario. Organic colloids range
in size over several orders of magnitude. There are at least three
general classes of organic colloids: (a) biocolloids, such as
bacteria, spores, and viruses, (b) macromolecules, such as high
molecular weight polymers, humic substances, pulp fibers,
proteins, and (c) nonaqueous-phase liquids, such as oil droplets
or detergent micelles (12,21).
Organic Colloids: Interactions with Metal Contaminants
The association of metals with organic matter is a
relationship that has been documented repeatedly both in the
field and the laboratory. Due to the large surface area per unit
mass and anionic surface functional groups associated with
some organiccolloidal material, metals have a significant potential
to be adsorbed. Due to the association with mobile colloidal
matter, the metal may become more mobile.
The complexation of metal ions with organic colloids is
reported in the literature to vary considerably with a number of
experimental variables. Complexation increases at higher pH's
and higher humic substance concentrations and decreases at
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higher ionic strengths. Generally, complexation also varies with
the nature of metal ion (12).
The chemical and/or physical reaction which influences
the metal complexation with organic colloids is a reversible
process. Parameters which influence reversibility include: pH,
ionic strength, and metal and organic compound concentrations.
Complexation reversibility may have important repercussions
when ground water from various flow regimes mix together in
common hydrogeological units. When complexation reactions
are reversed, the fate and transport mechanisms associated
with the complexation may change accordingly.
Although the association between organic matter and
metals has been investigated intensively, little information is
available on this association with respect to transport in a porous
media system. This is an area of considerable research effort at
the present time.
Organic Colloids: Interactions with Organic Contaminants
Organic colloids are reported to associate with HOC's by:
(a) sorbing organic contaminants; (b) behaving as a solvent to
the organic contaminant; and (c) participating in ionic exchange
reactions with cationic organic compounds. The association of
the mobile organic colloidal matter results in the increased
mobility of the contaminant through the porous media. A
summary report on the role of colloids in contaminant transport
processes indicates that the sorption of organic contaminants
onto colloids appears to be a simple partitioning process between
the water and the organic colloidal phase. This sorption process
is also found to be mathematically predictable (12).
Hydrophobic organic contaminants have a high affinity
for association with organic macromolecules, i.e., humic
substances. Enhancement of the solubility of HOC's by the
organic macromolecules can be accounted for by a partition-like
interaction of the HOC's with the macromolecule (3,11). Enhanced
solubility, also referred to as apparent solubility, as used in
reference to colloids describes the increased contaminant
concentration in an aqueous sample due to the presence of
colloids. These two terms do not describe a condition where the
water solubility, i.e., the physical constant, of a contaminant
increases. Instead, the suspended colloids provide a mechanism
whereby the chemical stays associated with the solid phase
while suspended in the liquid phase.
The apparent solubility of the HOC's increases with an
increase in the colloid concentration (12) and with a decrease in
HOC solubility. Therefore, the greater the concentration of
organic colloids, and the more hydrophobic the compound, the
greaterthe potential mobility of the contaminant by this facilitated
transport mechanism. Additional organic colloid features which
have been reported to affectthe apparent solubility of the HOC's
include: molecular size, polarity, and molecular configuration
(3). In a laboratory column experiment where an aqueous
mixture of organic macromolecules and hexachlorobenzene
were introduced together, thetransportofthe hexachlorobenzene
occurred more rapidly than in a mixture without macromolecules
(1). The presence of hydrophilic macromolecules may change
the relative mobility of HOCs' by an order of magnitude in low
organic carbon soils (2).
onto the solid phase is governed by both electrostatic and
hydrophobic forces. Therefore, the retention of these
contaminants not only depends on its physical and chemical
characteristics and the colloid organic carbon content, but also
on the cation exchange capacity of the colloid.
Surfactants
Aggregates of surfactant molecules or micelles may be
classified as organic colloids or organic microdroplets that may
interact with both metal and organic contaminants and increase
the mobility of these contaminants. At critical concentration
levels, surfactants form discrete structures called micelles.
Micelles are distinctly different from the bulk aqueous phase, and
in most instances, serve as efficient media for the partitioning of
hydrophobic pollutants (17). The micelle then can strongly
influence the transportation of the contaminants (14). The
existence of such micelles in leachates has yetto be demonstrated
(17).
Presently, there is little information in the literature which
correlates the facilitated transport of organic contaminants by
organic colloids with field data. However, the literature does
contain several publications of laboratory studies which
demonstrate facilitated transport processes involving colloids
(1,3,11,23).
In conjunction with the numerous interactions which may
occur between colloids and contaminants is the high degree of
variability and uncertainty ofthe chemical, physical, and biological
subsurface environment. Consequently, estimating the effects
of facilitated transport in the field is often difficult. However, the
concept of facilitated transport of trace organic compounds helps
rationalize the occurrence of hydrophobic contaminants 30 meters
below waste disposal sites in Ohio (1), a distance much greater
than predicted by conventional sorption theory.
Inorganic Colloids
Inorganic colloids include clay, metal oxides, and inorganic
precipitates in the sub-micrometer size range (12). These
colloids occur both naturally and from anthropogenic sources.
Anthropogenic formation of ferrous phosphate colloids was
reported to occur when sewage-derived phosphate combined
with the ferrous iron that was released from aquifer solids (8).
These colloids were detected in the ground water downgradient
from the disposal site indicating that the colloids were mobile in
the aquifer system.
Although most Superfund sites do not contain radioactive
wastes, radionuclide research of inorganic colloids is useful to
identify facilitated transport mechanisms. The transport of clay
particles has been reported to vary with the ionic strength ofthe
aqueous environment. Laboratory column experiments indicate
that clay particles passing through porous media are increasingly
retained as the salinity ofthe solution is increased. Additionally,
saline aqueous solutions are less likely to adsorb cesium from
solution and, once adsorbed, are less likely to desorb into saline
water (4). These studies show that kaolinite colloids pass readily
through various soil media and readily adsorb cationic
radionuclides, indicating that facilitated transport may potentially
occur at high level radioactive waste repositories.
Several polar organic contaminants are characterized as
cationic (positively charged). The sorption of these contaminants
Little information is currently available concerning the
association between inorganic colloids and organic contaminants.
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Due to both the existing data base concerning organic
contaminant sorption to typical subsurface mineral surfaces (12)
and to inorganic colloidal transport, the scientific framework
suggests facilitated transport is a viable transport mechanism.
Further research is necessary to elucidate specific mechanisms
and the importance of this particular colloid-contaminant
association.
Practical Considerations
The potential role of facilitated transport should be
considered while assessing the areal and vertical extent of
contaminants in the ground water, particularly if the following
pertain: the contaminant is known to associate strongly with
organic or inorganic surfaces (e.g., hydrophobic organic
compounds, metals); the ground water contains a relatively high
concentration of dissolved organic carbon, total dissolved solids,
or total suspended solids; the aquifer is relatively porous or
fractured, and flow rates are relatively high; or aqueous chemistry
undergoes natural or contaminant associated alterations that
could mobilize colloidal particles (12). Facilitated transport has
the potential to disperse contaminants which are usually relatively
immobile thereby increasing problems associated with
contaminant migration control. On the other hand, low sorption
of contaminants in the saturated zone material is desirable from
the standpoint of ground-water remediation in that solute removal
by pumping to the surface is facilitated (2).
There are several areas related to field work which must
be given special consideration to determine whether facilitated
transport may be playing an important role in contaminant
transport. These areas are as follows (12):
Drilling Methods
Drilling operations, by nature, disrupt the subsurface
environment. Drilling may redistribute material and create fine
particles as a result of the associated abrasive activities. In
addition, many drilling techniques involve the injection of foreign
materials into the borehole such as drilling muds, water, and
compressed air. The particles introduced into the system may
become associated with the contaminants entering the well
screen area. Depending on numerous factors surrounding the
particle-contaminant association and sampling technique, the
contaminant may be undetected in the ground water. Augering
is the least disruptive technique available for shallow holes in
unconsolidated material. In deeper holes or harder materials,
casing drive techniques may be required. However, there will be
occasions when the more traditional drilling methods are
necessary. Careful evaluation of the impacts of the drilling
technique and materials is essential to evaluate contaminant
transport in the well area.
Well Construction
Materials used in the const ruction of recovery or monitoring
wells may have an impact on the subsurface chemical
environment. Sampling artifacts may arise through contact of
various well materials with the water that is drawn into the well
and sampled. In particular, the sand pack may act as a source
of fine particles. As previously discussed, these particles may
result in the contaminant being undetected. If the sand (filter)
pack is constructed with very fine material, it may function as a
filter medium effectively removing larger colloids that may have
contaminants adsorbed to their surface.
Well Development
One purpose of well development is to remove drilling
muds and fine particles introduced or created during the course
of well construction. This process involves dislodging and
transporting the particles fromthe system. Although the objective
is to remove the artificially placed particles, this activity may
introduce naturally occurring particulate material into the well
area. However, every type of drilling operation alters the
character! sties of formation materials in the vicinity ofthe borehole.
Therefore, well development is generally recommended to
eliminate the particulate matter potentially available to interfere
with contaminant transport to the well.
Well Purging
The well is a conduit for the surface atmosphere to
artificially contact the ground water. Ground water in the area of
the well is in contact not only with the atmosphere, but with the
construction material of the well. Additionally, the ground water
in the well becomes stagnant and unrepresentative of actual
ground-water quality. For these reasons, it is standard practice
in ground-water sampling to purge a predetermined volume of
water from the well before taking the sample. The purpose is to
draw in fresh and presumably representative formation water to
be sampled. Excessive rapid pumping, however, may create a
dramatically different ground-water gradient and flow pattern
from the natural state and affect the distribution of suspended
particles in the sampled water. Recently, field methods were
implemented during a ground-water investigation to distinguish
whether colloidal particles were introduced during sampling
operations or if the colloids were truly suspended and moving
with the ground water in-situ (10). This was performed by using
very low pumping rates to purge the well in conjunction with a
dissolved oxygen sample handling technique to prevent the
atmospheric exposure of ground-water samples. These steps
yielded ground-water colloid suspensions which the researchers
believed were representative of ground-water quality.
Sample Handling
Perhaps the single most important aspect of ground-
water sampling that has paramount effects on the detection of
contaminants resulting from facilitated transport is filtering the
ground-water sample. Essentially, filtering the sample (usually
with a 0.45 micron filter) removes some of the colloid and
macromolecule material which may be responsible for
contaminant transport. Therefore, when the filtered sample is
analyzed, there is reduced probability that the contaminant will
be detected in the ground water. Further discussion on the
repercussion of filtering ground-water samples can be found in
Superfund Issue Paper No. 1, entitled, "Ground Water Sampling
for Metal Analyses."
References
1. Bengtsson, G., Enfield, C.G., and Lindqvist, R.,
Macromolecules Facilitate Transport of Trace Organics, Sci. of
the Total Environment. Vol. 67, pp. 159-169.
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2. Bouchard, D.C., Enfield, C.G., and Piwoni, M.D., Transport
Processes Involving Organic Chemicals, Chapter 16 for
Reactions and Movement of Organic Chemicals in Soils. Soil
Sci. Society of Am. Special Publications Press, 1989.
3. Chiou, C.T., Malcom, R.L., Brinton, T.I., and Kile, D.E.,
Water Solubility Enhancement of Some Organic Pollutants and
Particles by Dissolved Humic and Fulvic Acids. Environ. Sci.
Technol.. Vol. 20, No. 5, 1986.
4. Eichholz, G.G., Wahlig, B.C., Powell, G.F., and Craft, T.F.,
Subsurface Migration of Radioactive Waste Materials by
Particulate Transport, Nuclear Technology. Vol. 58, pp. 511-
519, Sep. 1982.
5. Enfield, C.G., Chemical Transport Facilitated by Multiphase
Flow Systems, Water Sci. and Tech.. Vol. 17, No. 9, pp. 1-12,
1985.
6. Fu, J.K., and Luthy, R.G., Aromatic Compound Solubility in
Solvent/Water Mixtures, Journal of Environmental Engineering.
Vol. 112, No. 2, pp. 328-345, April 1986.
7. Fu, J.K., and Luthy, R.G., Effect of Organic Solvent on
Sorption of Aromatic Solutes onto Soils, Journal of Environmental
Engineering. Vol. 112, No. 2, pp. 346-366, April 1986.
8. Gschwend, P.M., and Reynolds, M.D., Monodisperse
Ferrous Phosphate Colloids in an Anoxic Groundwater Plume,
Journal of Contaminant Hydrology, pp. 309-327, 1987.
9. Gschwend, P.M., and Wu, S.C., On the Constancy of
Sediment-Water Partition Coefficients of Hydrophobic Organic
Pollutants, Environ. Sci. Technol.. Vol. 19, No. 1, 1985.
10. Gschwend, P.M., Backus, D.A., and MacFarlane, J.K.,
Mobilization of Colloids in Groundwater Due to Infiltration of
Water Near an Electric Generating Station. Submitted to Water
Resources Research. 1989.
11. Kan, AT., Tomson, M.B., Factors Affecting the Movement
of Organic Compounds in Soil-Facilitated Transport by
Macromolecules and Micelles. Final Report. National Centerfor
Ground Water Research, October 1988.
12. McCarthy, J.F., Summary Report of Transport of
Contaminants in the Subsurface: The Role of Organic and
Inorganic Colloidal Particles. International Series of Interactive
Seminars, October 6-9, 1986.
13. McCarthy, J.F., and Zachara, J.M., Subsurface Transport of
Contaminants. Environ. Sci. Technol.. Vol. 23., No. 5, pp. 496-
502, 1989.
14. McDowell-Boyer, L.M., Hunt, J.R., Sitar, N., Particle
Transport through Porous Media. Water Resources Research.
Vol. 22, No. 13, pp. 1901-1921, December 1986.
15. Nkedi-Kizza, P., Rao, P.S.C., and Hornsby, A.G., Influence
of Organic Cosolvents on Sorption of Hydrophobic Organic
Chemicals by Soils, Environ. Sci. Technol.. Vol. 19, No. 10, pp.
975-979, 1985.
16. Nkedi-Kizza, P., Rao, P.S.C., and Hornsby, A.G., Influence
of Organic Cosolvents on Leaching of Hydrophobic Organic
Chemicals through Soils, Environ. Sci. Technol.. Vol. 21, No. 11,
pp. 1107-1111, 1987.
17. Piwoni, M., Facilitated Transport. An Overview. USEPA-
RSKERL Internal Report.
18. Rao, P.S.C., Hornsby, A.G., Kilcrease, D.P., and Nkedi-
Kizza, P., Sorption and Transport of Hydrophobic Organic
Chemicals in Aqueous and Mixed Solvent Systems: Model
Development and Preliminary Evaluation,Journal Environ. Qual..
Vol. 14, No. 3, pp. 376-383, 1985.
19. Sheets, P.J., and Fuller, W.H., Transport of Cadmium by
Organic Solvents through Soil, Soil Sci. Am. J.. Vol. 50, pp. 24-
28, 1986.
20. Stumm, W., and Morgan, J.J., Aquatic Chemistry. Wiley-
Interscience, New York, NY, 1984.
21. Weber. W.J..Physicochemical Processes for Water Quality
Control. Wiley-lnterscience, New York, NY, 1972.
22. Woodburn, K.B., Rao, P.S.C., Fukui, M., and Kizza, P.N.,
Solvophobic Approach for Predicting Sorption of Hydrophobic
Organic Chemicals on Synthetic Sorbents and Soils, Journal of
Contaminant Hydrology. Vol. 1, pp. 227-241, 1986.
23. West, C.C. Dissolved Organic Carbon Facilitated Transport
of Neutral Organic Compounds in Subsurface Systems. Ph.D.
Thesis. Rice University, Houston, TX. 1984.
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