EPA

Superfund Technology Support Center for Ground Water

Robert S. Kerr Environmental

Research Laboratory

Ada, OK

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, RSKERLAda, 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

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 paramount to 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

Scott G. Huling

Facilitated Transport

Superfund

Ground Water Issue

Office of Solid Waste

and Emergency

Response

United States

Environmental Protection

Agency

Office of

Research and

Development

EPA/540/4-89/003

August 1989
determine how a particular solute will be distributed and

transported in the subsurface.

In a mixed solvent system consisting of water and one or

more water miscible 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 of the hydrophobic 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 water or as a separate phase could greatly underestimate

the mobility of chemicals.

Colloidal Processes

The transport of contaminants in ground water has typically

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

2

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

macromolecular phase (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 organic colloidal 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
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

greater the potential mobility of the contaminant by this facilitated

transport mechanism. Additional organic colloid features which

have been reported to affect the 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, the transport of the 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).

Several polar organic contaminants are characterized as

cationic (positively charged). The sorption of these contaminants

3

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 yet to 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 of the 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 of the

aqueous environment. Laboratory column experiments indicate

that clay particles passing through porous media are increasingly

retained as the salinity of the 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.

Little information is currently available concerning the

association between inorganic colloids and organic contaminants.
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 construction 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 from the 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

characteristics of formation materials in the vicinity of the 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 groundwater 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.

4
5

 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.G., 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

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1985.

 6. Fu, J.K., and Luthy, R.G., Aromatic Compound Solubility in

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 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

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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

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11. Kan, A.T., Tomson, M.B., Factors Affecting the Movement

of Organic Compounds in Soil-Facilitated Transport by

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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

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502, 1989.

14. McDowell-Boyer, L.M., Hunt, J.R., Sitar, N., Particle

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16. Nkedi-Kizza, P., Rao, P.S.C., and Hornsby, A.G., Influence

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17. Piwoni, M., Facilitated Transport, An Overview, USEPARSKERL Internal Report.

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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. WileyInterscience, New York, NY, 1984.

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