EPA/540/4-90/053

October 1990

Robert S. Kerr Environmental

Research Laboratory

Ada, OK

Superfund Technology Support Center for Ground Water

Basic Concepts of Contaminant

Sorption at Hazardous Waste Sites

Marvin D. Piwoni* and Jack W. Keeley**

United States

Environmental Protection

Agency

Office of Solid Waste

and Emergency

Response

Office of

Research and

Development

Introduction

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 of Superfund sites. One of the major

issues of concern to the Forum is the transport and fate of

contaminants in soil and ground water as related to subsurface

remediation. Processes which influence the behavior of

contaminants in the subsurface must be considered both in

evaluating the potential for movement as well as in designing

remediation activities at hazardous waste sites. Such factors not

only tend to regulate the mobility of contaminants, but also their

form and stability. Sorption is often the paramount process

controlling the behavior of contaminants in the subsurface. This

paper summarizes the basic concepts of sorption in soil and

ground water with emphasis on nonpolar organic contaminants.

For further information contact: Joe Williams, FTS 743-2246;

Bert Bledsoe, 743-2324; or Dom DiGiulio, 743-2271 at RSKERLAda.

The Concept of Sorption

Sorption can be defined as the interaction of a contaminant with

a solid. More specifically, the term can be further divided into

adsorption and absorption. The former refers to an excess

contaminant concentration at the surface of a solid while the latter

implies a more or less uniform penetration of the solid by a

contaminant. In most environmental settings this distinction

serves little purpose as there is seldom information concerning

the specific nature of the interaction. The term sorption is used

in a generic way to encompass both phenomena.

There are a number of factors which control the interaction of a

contaminant and the surface of soil or aquifer materials. These

include chemical and physical characteristics of the contaminant,

composition of the surface of the solid, and the fluid media

encompassing both. By gaining an understanding of these

factors, logical conclusions can often be drawn about the impact

of sorption on the movement and distribution of contaminants in

the subsurface. The failure to take sorption into account can

result in a significant underestimation of the amount of a

contaminant at a site as well as the time required for it to move

from one point to another.

In introducing sorption theory it is necessary to define the terms

sorbate and sorbent. The sorbate is the contaminant that

adheres to the sorbent, or sorbing material. In this discussion the

sorbate will usually be an organic molecule and the sorbent will

be the soil or aquifer matrix.

This Issue Paper is condensed from a presentation given at the

EPA/EPRI Workshop on Leachate Testing Methods in Houston,

Texas, in January 1989.

Factors Influencing Sorption

The properties of a contaminant have a profound impact on its

sorption behavior. Some of these include:

● Water Solubility

● Polar/Ionic Character

● Octanol/Water Partition Coefficient

● Acid/Base Chemistry

● Oxidation/Reduction Chemistry

Ground Water Issue

* Laboratory Services Manager, Hazardous Waste Research and

Information Center, Illinois Department of Energy and Natural

Resources.

** Environmental Engineer, Dynamac Corporation, Robert S. Kerr

Environmental Research Laboratory.
-

Contaminant Characteristics

In discussing sorption it is useful to divide chemicals into three

groups. Although there are many ways to divide chemicals into

subgroups, for this purpose three categories are presented

which transcend normal boundaries between inorganic and

organic species. These are: (1) ionic or charged species; (2)

uncharged polar species; and, (3) uncharged nonpolar species.

Most inorganic chemicals in aqueous solution will occur as ionic

or charged species. This applies to metals and metalloids, and

to other molecules such as cyanide and ammonia. However, in

contaminated water, metals and other inorganic constituents can

exist as polar or nonpolar neutral species. In any event, the

chemical form of a contaminant will have a profound effect on its

sorption and, therefore, its environmental mobility.

Organic contaminants have representatives in all three of the

sorption categories. Many of the more common organic groundwater contaminants are of the nonpolar species, including

trichloroethene (TCE), tetrachloroethene (PCE), the chlorinated

benzenes, and the more soluble components of hydrocarbon

fuels such as benzene, toluene and xylene. Other important

organic contaminants including many of the pesticides, phenols

and dyes exist in solution as either charged or polar molecules.

Still other, larger organics, such as surfactants, can have both

polar and nonpolar ends within the same molecule. The

environmental mobility of contaminants with these distinctive

properties has been less thoroughly studied than nonpolar

organics; therefore, site-specific investigations may provide the

most reliable information for their transport characteristics.

Soil Characteristics

If one avoids the difference between positive and negative

charges, a simple rule of sorption might be: for charged species,

"opposites attract" and for uncharged species, “likes interact with

likes.” Likes refers to the three categories of contaminants and

to the properties of the soil matrix. Some of the most important

characteristics of soil affecting the sorptive behavior of subsurface

materials include:

● Mineralogy

● Permeability/Porosity

● Texture

● Homogeneity

● Organic Carbon Content

● Surface Charge

● Surface Area

Soil, in its natural state, is primarily composed of sand, silt, clay,

water, and a highly variable amount of natural organic carbon.

The latter profoundly complicates a soil’s sorptive properties.

The combination of these characteristics describes the surfaces

offered as sorptive sites to contaminants in water passing through

the subsurface matrix. For example, silts and clays have much

higher surface areas than sand, usually carry a negative charge,

and almost invariably associate with natural organic matter.

It can be deduced that sandy materials offer little in the way of

sorptive surfaces to passing contaminants while silts and clays,

particularly those having substantial amounts of organic matter,

provide a rich sorptive environment for all three categories of

contaminants. Even the most porous and highly productive

aquifers, composed of sands and gravels, usually have some

fine grained material, and a few percent of silts and clays can

result in a substantial increase in the sorptive behavior of the

aquifer material.

Fluid Media Characteristics

Under most contamination situations the primary transporting

fluid is water. One of the most important properties of this solvent

phase is pH for it dictates the chemical form and, therefore, the

mobility, of all contaminants susceptible to the gain or loss of a

proton. As an example, pentachlorophenol will primarily be an

uncharged polar molecule in an aqueous solution whose pH is

below about 4.7 and an anion when the pH is above that value,

increasing its solubility from 14 to 90 mg/l.

Other characteristics of water that can influence the behavior of

contaminants include the salt content and the dissolved organic

carbon content. Chlorides, for example, which are not usually of

much concern when dealing with organic contaminants, can

have an important effect on the mobility of various metals.

Dissolved organic matter, at relatively high concentrations found

in many leachates, has a significant effect on the mobility of most

nonpolar organics.

Implications of These Characteristics

Although somewhat simplified, it can be assumed for purposes

of this discussion, that charged and polar species tend to interact

with charged and polar surfaces, and nonpolar compounds

interact with nonpolar components of soil, usually the natural

organic carbon. In order to make a first estimate of the significance

of sorption at a site, it is necessary to determine the polar and

nonpolar nature of the material with which the contaminant will

come into contact. This is usually done by measuring the cation

exchange capacity and the natural organic carbon content,

respectively.

The cation exchange capacity (CEC) provides an estimate of the

total negatively-charged sites on the surface of the soil. It is

determined by measuring the mass of a standard cation, usually

ammonia, that displaces another cation held by the soil. Under

normal field conditions these sites will be occupied by cations

common to the flowing or percolating water, such as Na+, K+,

Ca2+, and Mg2+. Larger organic cations and highly-charged metal

ions like Hg2+ or Cr3+ will be preferentially retained at these sites

by “exchanging” with their normal occupants. Thus large organic

cations and heavy metals would not normally be expected to

move far through soils with a measurable cation exchange

capacity.

At contaminated sites, however, conditions may not be “normal”

and Hg2+ may be codisposed with high levels of chloride salts. In

the complexation chemistry shown in Figure 1, Hg2+ may be

replaced by the neutral complex HgCl2 or the negative ion HgCl3

,

both of which move through the soil more quickly than the cationic

form.

Sorption of Nonpolar Organics

As mentioned above the chemicals at many contaminated sites

are nonpolar organics. It was representatives of these types of

compounds (DDT and other chlorinated hydrocarbon pesticides)

2
Octanol-Water Partition Coefficient:

Concentration Octanol

Kow =

Concentration Water

Almost always presented as Log10 because the

numbers are so large for hydrophobic compounds.

Sorption Coefficient:

Concentration Solid Phase

Kp =

Concentration Solution

 mg/kg

Units are , which is L/kg.

 mg/L

Carbon Normalized Sorption Coefficient:

 Sorption Coefficient, Kp

Koc =

Fraction Organic Carbon

Figure 2. Relationships Pertinent to Nonpolar Organic

Contaminant Transport

that first focused attention on the potential hazards of chemicals

in the environment because of their widespread use, potential

human toxicity, and recalcitrance.

Transport and fate characteristics of these compounds have

been well studied, first by the agricultural community and later by

environmental scientists. As a result, an understanding of the

sorptive behavior of these compounds has evolved which can be

used to assess the environmental consequences posed at a

waste disposal site.

Many organics of environmental concern have a limited solubility

in water because of their nonpolarity and molecular size: that is,

the solubility of an organic contaminant decreases with decreasing

polarity and increasing molecular size. But even with limited

solubilities, many hazardous chemicals at equilibrium are at

measurable, and sometimes toxic concentrations in water. Polar

molecules, such as ethanol, are compatible with water. Their

combination results in a homogeneous solution regardless of the

proportions that are mixed.

Nonpolar organic compounds interact with soil organic matter

through a process known as “hydrophobic sorption” which can

be explained as the affinity of organic compounds for phases

other than water. For example, water being a polar molecule is

not compatible with other nonpolar molecules, such as DDT,

which is immiscible with water.

Octanol-Water Partitioning

Organic molecules of increasing size, decreasing polarity and

therefore water solubility, are said to exhibit increasing

“hydrophobicity” which can be quantified by their octanol-water

partition coefficient. It is a measure of the distribution of the

chemical between a water and an organic (octanol) phase with

which it is in contact. The more hydrophobic the contaminant, the

more likely it is to partition into the octanol phase. The partition

coefficient provides a fairly accurate understanding of the sorptive

process occurring between water and the soil, more specifically,

the soil organic matter.

The octanol-water partition coefficient, expressed as Kow in

Figure 2, is determined by measuring the concentration of a

particular compound in the water and the octanol phases after a

period of mixing. It is important to note that the more hydrophobic

the compound the less accurate the test, and the results should

The complexation reactions are driven right and down by

increasing chloride concentrations, often characteristic of

waste waters. Increased complexation produces increased

environmental mobility of the mercury.

Figure 1. Mercury Ion Complexation in Chloride-Rich Water

Hg + Cl 

HgCl + Cl 

HgCl + Cl 

HgCl + Cl

2+

+

0

2

-

3

-

-

-

-

HgCl 

HgCl 

HgCl 

HgCl

+

0

2

-

3

2-

4

be viewed accordingly. It is often sufficient to know that an

extremely high coefficient means that the compound is very

hydrophobic. Since measured Kow values can be in the millions

for important environmental contaminants (PCB’s, chlorinated

pesticides, dioxins and furans), they are often expressed as the

base10 logarithm, Log Kow.

The Kow has two attributes that make it especially useful in

environmental assessments. First, it varies in a predictable way

within classes of organic compounds. For example, as shown in

Figures 3 and 4, if Kow is known for one member of a class of

compounds it can be used reasonably well to estimate a value for

other members of the same family. In the examples shown, the

Kow can be correlated to the number of chlorine atoms or the

number of rings in the molecular structure of classes of

contaminants.

The second attribute results from the work of a number of

agricultural and environmental researchers who correlated

sorption on the organic matter of soils with the Kow of the

compounds involved. By using these attributes of the Kow, it is

possible to estimate the potential sorption of organic contaminants

based on the structure of the compounds and the organic carbon

content of the soil or aquifer material.

Sorption to Soils

Thus far it has been suggested that nonpolar organic compounds

are sorbed by soils as a function of their hydrophobicity (Kow) and

the organic carbon content of the soil. There has been

considerable research which suggests that the slow kinetics of

the sorption process may be significant in swiftly moving ground

water. Sorption studies using flow-through columns produce

results sensitive to the flow rate, and batch tests indicate that

3
0 1234 56

3

4

5

6

Chlorinated 

Benzenes

# of Chlorine Atoms

Figure 3. Relationship of Molecular Structure to

Hydrophobic Character

Figure 4. Relationship of Molecular Structure to Octanol Water

Partition Coefficient

1

2

3

4

5

Rings

1 2 345

6

Anthracene

Naphthalene

Benzene

Benzanthracene

Benzo(a)pyrene

Increasingly Hydrophobic

AAAA AAAA AAAA AAAA AAAAAAAA

AAAA

AAAA

AAAA

AAAA

Chlorinated 

Benzenes

Anthracene

5

4

3

2

01234 56

Log Kow

Figure 5. Partitioning on Soil Organic Carbon as Function of

 Octanol-Water Partition Coefficient

contaminants associated with the solid phase to that in solution,

and is, therefore, conceptually similar to Kow. The usefulness of

Kow in estimating sorption stems from the fact that the soil organic

matter serves the same function as octanol in the octanol-water

test. As a result, there have been many empirical relationships

developed for estimating sorption from the Kow and the soil

organic carbon content. One expression, developed in the

laboratory by Piwoni and Banerjee, 1989, for the sorption of

common environmental contaminants with a low aquifer organic

carbon, is:

 Log Koc = 0.69 Log Kow + 0.22

When applying such a relationship, it is important to select a

study in which the compounds used are similar to those of

interest at the site under investigation. However, as shown in

Figure 5, even when applying the empirical relationship to a

structurally dissimilar compound such as anthracene, if it is a

nonpolar organic, the error of estimate should be less than a

factor of five.

These estimates of sorption are based, in large measure, on a

good evaluation of the soil organic carbon content at a site which

is obtained from the degradation of naturally occurring organic

matter. In this regard it is important to realize that soils and aquifer

materials are very heterogeneous and the organic carbon content

can vary considerably both in the vertical and horizontal dimension.

Fortunately, this variability tends to be the greatest in the vertical

soil profile while most site investigations are concerned with

contaminant movement in the ground water away from the

source. While the soil organic carbon content in the horizontal

plane usually differs by a factor of ten or less, it can vary by a

factor of 10 to 100 in the vertical dimension.

4

increased sorption occurs with longer exposure times. The

practical implication of these findings may be that sorption is

overestimated in aquifer systems with relatively high flow rates.

Sorption is expressed in terms of a partition coefficient Kp

, which

is defined in Figure 2 as the ratio of the concentration of
In order to determine the soil organic carbon content at a site,

samples are usually obtained using split spoon sampler or other

standard soil sampling devices. Representatives portions of the

soil are then burned in an O2

 atmosphere and the produced CO2

is measured by IR spectrophotometry. Before burning soil

samples must be acidized to remove inorganic carbon. The

accuracy of measuring organic carbon content can also be

questionable, particularly at low levels and in carbonate soils.

Existing analytical methods for measuring soil organic carbon

were developed for the higher concentrations found near the

surface. Therefore, at the low levels found in deeper soils and

ground water, the same quality assurance procedures used in

determining contaminant levels in water should be followed in

determining the subsurface organic carbon content.

The processes driving hydrophobic sorption are nonspecific and

depend upon small amounts of energy gained by moving

contaminants out of the aqueous phase. The extent to which the

process proceeds is dependent upon how receptive the soil

matrix is to the organic molecule, which is a function of the

organic content. But even when the organic carbon content is

very low, some sorption of the most hydrophobic molecules

continues because of the soil's mineral surfaces.

Sorption Estimation

In order to use the information provided above in estimating the

amount of a contaminant associated with the aqueous and solid

phases of an aquifer, it is necessary to develop a contamination

scenario. To that end it is assumed that the contaminant at an

industrial landfill is 1,4-dichlorobenzene, and there is sufficient

data to indicate that: (1) most of the contamination is below the

water table; (2) the contaminant concentration in ground water

averages 1 mg/l; (3) the measured soil organic carbon is 0.2

percent; and (4) the pore water occupies 50 percent of the aquifer

volume. Steps leading to an estimate of the contaminant’s

distribution between the aqueous and solid phases are:

Field Measurements:

Average contaminant concentration

in monitoring wells = 1.0 mg/l

Soil organic carbon = 0.2 percent,

therefore foc = 0.002

Pore water occupies 50 percent of

the aquifer's volume.

From The Literature:

Log Kow (1,4-dichlorobenzene) = 3.6

Piwoni and Banerjee Regression,

Log Koc = 0.69 Kow + 0.22

Calculated:

Log Koc = 0.69(3.6) + 0.22 = 2.70

therefore: Koc = 506

Kp

 = Koc(foc) = 506 (0.002) 3 1.0 3 Sorbed C

 Solution C

5

Conclusion;

The contaminant, equally distributed between each phase,

is expressed as mg/kg (soil) and mg/l (water). Since soil is

about 2.5 times more dense than water, 2 liters of aquifer

would contain 1 liter of water and 2.5 kg of soil. Therefore,

1.0 mg/l of the contaminant would be associated with the

water and 2.5 mg (70 percent) would be sorbed to the

aquifer’s solid phase.

As can be seen from this example, sorption tends to complicate

remediation techniques that require pumping water to the surface

for treatment. The desorption process has kinetic constraints

that can render a pump-and-treat system ineffective. Slow

desorption kinetics result in progressively lower contaminant

concentrations at the surface, and less cost-effective contaminant

removal. It is not uncommon to pump a system until the

contaminant concentration in the pumped water meets a

mandated restoration level, while the aquifer’s solid phase still

contains a substantial mass of contaminant. If the pumps are

turned off, concentrations in the ground water will soon return to

their equilibrium level.

Measuring Sorption

It is preferable to obtain the best information possible on which to

base an estimate of sorption. Therefore, tests should be made

with the contaminants of concern, as well as soils and aquifer

material from a specific site. The goal is to obtain a partition

coefficient, Kp

, for use in the prediction of contaminant movement.

There are essentially two methods for measuring the partition

coefficient, those being batch and dynamic techniques. Batch

techniques are quicker and easier to perform and, therefore,

more amenable to replication and quality control. Dynamic or

flow through techniques offer the advantage of more closely

representing processes occurring in the field.

The standard approach to determine the partition coefficient is to

generate a sorption isotherm, a graphical representation of the

amount of material sorbed at a variety of solute concentrations.

The Freundlich isotherm, S = Kp

C1/n, is the representation most

often used for the sorption of nonpolar organics to soils and

aquifer materials. In this equation, S is the mass sorbed per mass

of sorbent (mg/kg), C is the solute concentration at equilibrium

(mg/l), Kp

 is the Freundlich partition coefficient, and 1/n is a fitting

factor. The equation can be expressed in a linear form for

convenience:

 Log S = Log Kp

 + 1/n Log C

As shown in Figure 6, Log Kp can be estimated by determining

the intercept of the regression of a Log-Log plot of S and C.

Summary

This has been a discussion of the concepts involved in estimating

contaminant sorption, particularly nonpolar organics, at hazardous

waste sites. After determining the types of contaminants present

at a site, it is possible to estimate Kp

 using Kow values from the

literature, an appropriate sorption coefficient/Kow regression

equation, and some organic carbon values.

If sorption determinations are within the scope of the project, site

representative soil samples and contaminants should be selected
Brown, D.S., and E.W. Flagg. 1981. Empirical Prediction of

Organic Pollutant Sorption in Natural Sediments. J. Environ.

Qual. 10:382-386.

Carlson, D.J., L.M. Mayer, M.L. Brann, and T.H. Mague. 1985.

Binding of Monomeric Organic Compounds to Macromolecular

Dissolved Organic Matter In Seawater. Mar. Chem. 16:141-163.

Carlson, R.M., R.E. Carlson, and H.L. Kopperman. 1975.

Determination of Partition Coefficients by Liquid Chromatography.

J. Chromatogr. 107:219-223.

Caron, G., H. Suffet, and T. Belton. 1985. Effect of Dissolved

Organic Carbon on the Environmental Distribution of Nonpolar

Organic Compounds. Chemosphere 14:993-1000.

Carter, C.W., and I.H. Suffet. 1982. Binding of DDT to Dissolved

Humic Materials. Environ. Sci. Technol. 16:735-740.

Chin, Y., W.J. Weber, and T.C. Voice. 1986. Determination of

Partition Coefficients and Aqueous Solubilities by Reverse Phase

Chromatography-II. Water Res. 20:1443-1450.

Chiou, C.T., R.L. Malcolm, T.I. Brinton, and D.E. Kile. 1986.

Water Solubility Enhancement of Some Organic Pollutants and

Pesticides by Dissolved Humic and Fulvic Acids. Environ. Sci.

Technol. 20:502-508.

Dragun, James. 1988. The Soil Chemistry of Hazardous

Materials. Hazardous Materials Control Research Institute.

Silver Spring, MD.

Enfield, C.G. 1985. Chemical Transport Facilitated by Multiphase

Flow Systems. Water Sci. Technol. 17:1-12.

Enfield, C.G., D.M. Walters, R.F. Carsell, and S.Z. Cohen. 1982.

Approximating Transport of Organic Pollutants to Groundwater.

Ground Water 20:711-722.

Garbarini, D.R., and L.W. Lion. 1986. Influence of the Nature of

Soil Organics on the Sorption of Toluene and Trichloroethylene.

Environ. Sci. Technol. 20:1263-1269.

Gamerdinger, A.P. R.J. Wagonet, and M. th. van Genuchten.

1990. Application of Two-Site/Two-Region Models for Studying

Simultaneous Transport and Degradation of Pesticides. Soil Sci.

Soc. Am. J. 54:957-963.

Gauthier, T.D., W.R. Seitz, and C.L. Grant. 1987. Effects of

Structural and Compositional Variations of Dissolved Humic

Materials on Pyrene Koc Values. Environ. Sci. Technol. 21:243-

248.

Griffin, R.A. and W.R. Roy. 1985. Interaction of Organic Solvents

with Saturated Soil-Water Systems. Open File Report prepared

for the Environmental Institute for Waste Management Studies,

University of Alabama.

Gschwend, P.M., and S. Wu. 1985. On the Constancy of SedimentWater Partition Coefficients of Hydrophobic Organic Pollutants.

Environ. Sci. Technol. 19:90-96.

Hassett, J.P., and M.A. Anderson. 1982. Effect of Dissolved

Organic Matter on Adsorption of Hydrophobic Organic

Compounds by River-and Sewage-Borne Particulate Matter.

Water Res. 16:681-686.

Figure 6. Sorption of 1,4-Dichlorobenzene

1.00

0.50

0.10

0.05

0.01

Kp=0.74=Intercept

0.01 0.05 0.10 0.15 1.00

C=Solution Concentration mg/L

from the tests. The measured sorption information is best used

to evaluate the validity of preliminary estimates. If the measured

partition coefficients differ from the estimates by more than a

factor of 2 or 3, it may be useful to select other contaminants from

the site and determine Kp

 values the same soil samples. A plot

of Kp

 values versus Kow values will provide a useful guide for

predicting the sorption characteristics of other contaminants at

the site.

Selected References

American Society of Agronomy Soil Sciences of America. 1982.

Methods of Soil Analyses. Part 2-Chemical and Microbiological

Properties-Second Addition. Agronomy No. 9, Part 2.

Ballard, T.M. 1971. Role of Humic Carrier Substances in DDT

Movement Through Forrest Soil. Soil Sci. Soc. Am. Proc. 35:145-

147.

Banerjee, P., M.D. Piwoni, and K. Ebeid. 1985. Sorption of

Organic Contaminants to Low Carbon Substrate Core.

Chemosphere 14:1057-1067.

Bouchard, D.C., R.M. Powell, and D.A. Clark. 1988. Organic

Cation Effects on the Sorption of Metals and Neutral Organic

Compounds on Aquifer Material. J. Environ. Sci. Health Part A

23:585-601.

Bouchard, D.C., and A.L. Wood. 1988. Pesticide Sorption on

Geologic Material of Varying Organic Carbon Content. Toxic.

Industr. Health 4:341-349.

Briggs, G.G. 1981. Theoretical and Experimental Relationships

Between Soil Adsorption, Octanol-Water Partition Coefficients,

Water Solubilities, Bioconcentration Factors, and the Parachor.

J. Agric. Food Chem. 29:1050-1059.

6
7

Karickhoff, S.W. 1981. Semi-Empirical Estimation of Sorption of

Hydrophobic Pollutants on Natural Sediments and Soils.

Chemosphere 10:833-846.

Karickhoff, S.W., D.S. Brown, and T.A. Scott. 1979. Sorption of

Hydrophobic Pollutants on Natural Sediments. Water Res. 13:241-

248.

Landrum, P.F., S.R. Nihart, B.J. Eadie, and W.S. Gardner. 1984.

Reverse-Phase Separation Method for Determining Pollutant

Binding to Aldrich Humic Acid and Dissolved Organic Carbon of

Natural Waters. Environ. Sci. Technol. 18:187-192.

McCarty, P.L., M. Reinhard, and B.E. Rittman. 1981. Trace

Organics in Groundwater. Environ. Sci. Technol. 15:40-51.

Morrow, N.R., and I. Chatzis. 1982. Measurement and Correlation

of Conditions for Entrapment and Mobilization of Residual Oil.

DOE/BC/10310-20.

Mortland, M.M., S. Shaobai, and S.A. Boyd. 1986. Clay-Organic

Adsorbents for Phenol and Chlorophenol. Clays and Clay Minerals.

34:581-585.

Piwoni, M.D., and P. Banerjee. 1989. Sorption of Volatile

Organic Solvents from Aquifer Solution onto Subsurface Solids.

Journal of Contaminant Hydrology. 4:163-179.

Poirrier, M.A., B.R. Bordelon, and J.L. Laseter. 1972. Adsorption

and Concentration of Dissolved Carbon-14 DDT by Coloring

Colloids in Surface Waters. Environ. Sci. Technol. 6:1033-1035.

Rao, P.S.C., and R.E. Jessup. 1983. Sorption and Movement of

Pesticides and other Toxic Substances in Soils. p. 183-201. In

D.W. Nelson et al. (ed.) Chemical Mobility and Reactivity in Soil

Systems. SSA Spec. Publ. II, Madison, WI.

Schwarzenbach, R.P. and J. Westall. 1981. Transport of Nonpolar

Organic Compounds from Surface Water to Groundwater.

Laboratory Sorption Studies. Environ. Sci. Technol. 15:1360-

1367.

Thurman, E.M. 1985. Humic Substances in Groundwater. p. 87-

103. In G.R. Aiken et al. (ed.) Humic Substances in Soil, Sediment,

and Water. Wiley-Interscience, New York.

Voice, T.C., C.P. Rice, and W.J. Weber, Jr. 1983. Effect of Solids

Concentration on the Sorptive Partitioning of Hydrophobic

Pollutants in Aquatic Systems. Environ. Sci. Technol. 17:513-

518.

Weber, W.J., Y. Chin, and C.P. Rice. 1986. Determination of

Partition Coefficients and Aqueous Solubilities by Reverse Phase

Chromatography-I. Water Res. 20:1433-1442.

Wershaw, R.L., P.J. Burcar, and M.C. Goldberg. 1969. Interaction

of Pesticides with Natural Organic Material. Environ. Sci. Technol.

3:271-273.

Whitehouse, B. 1985. The Effects of Dissolved Organic Matter on

the Aqueous Partitioning of Polynuclear Aromatic Hydrocarbons.

Estuarine Coastal Shelf Sci. 20:393-402.

Wilson, J.L., and S.H. Conrad. 1984. Is Physical Displacement of

Residual Hydrocarbons a Realistic Possibility in Aquifer

Restoration? p. 274-298. In Proc. Petroleum Hydrocarbons

Organic Chemicals Groundwater, Houston, TX.

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