1

EPA/540/S-97/502

April 1997

United States

Environmental Protection

Agency

Office of Solid Waste

and Emergency

Response

Office of

Research and

Development

Ground Water Issue

How Heat Can Enhance In-situ Soil and Aquifer Remediation:

Important Chemical Properties and Guidance on

Choosing the Appropriate Technique

Superfund Technology Support Center for Ground Water

National Risk Management Research Laboratory

Subsurface Protection and Remediation Division

Robert S. Kerr Environmental Research Center

Ada, Oklahoma

* National Risk Management Research Laboratory, U.S. EPA

Eva L. Davis *

Technology Innovation Office

Office of Solid Waste and Emergency

Response, US EPA, Washington, DC

Walter W. Kovalick, Jr., Ph.D.

Director

Background

The EPA Regional Ground Water Forum is a group of EPA

professionals representing Regional Superfund and Resource

Conservation and Recovery Act (RCRA) Offices, committed to

the identification and resolution of ground-water issues impacting the remediation of Superfund and RCRA sites. Innovative

technologies for subsurface remediation, including in-situ techniques based on heating the subsurface to enhance the recovery

of organic contaminants, are being evaluated more often for

specific sites as the limitations to the conventionally-used techniques are recognized. The purpose of this Issue Paper and the

three companion Issue Papers (Davis, 1997a, b, c) is to provide

to those involved in assessing remediation technologies some

basic information on the thermal remediation techniques. In

order to understand how heat can enhance a remediation

process, it is essential to understand the properties of organic

contaminants that affect their recovery. Thus, this Issue Paper

contains in-depth information on the properties of some common

organic contaminants which can affect their movement in and

recovery from the subsurface, as well as information on how

these properties are affected by temperature. Then, some basic

information on which of the heat-based remediation techniques

may be most appropriate for the subsurface conditions and the

contaminants is also provided, as well as a comparison of the

heat-based techniques to other in-situ remediation techniques.

The three companion Issue Papers have been written to provide

an explanation of how each of the three general types of

processes (steam or hot air injection, electrical heating, and hot

water injection) works, as well as preliminary information on the

design of a system and some estimates of the expected costs.

Thus, once the ground-water remediation specialist has determined which of the thermal methods may be appropriate for a

particular site, the Issue Paper on that method may be consulted

for more detailed information on how the process may be

applied.

Introduction

Thermal treatment is a common and proven technology for the

remediation of contaminated soils (Lighty et al., 1990), but in the

past thermal treatment has been applied mainly to soils that have

been excavated and are then incinerated to release and/or

destroy the contaminants. However, excavation of contaminated soils is not always practical and can be extremely costly

when the contamination occurs at great depths or covers a large

area. Excavation also increases the risk of exposure to and

further dispersion of the contaminants during material handling

steps (Dev et al., 1989; Superfund Report, August 10, 1994).

Heat-based in-situ remediation methods can be used in many

places where excavation is not possible, such as under and

around surface structures, and around empty underground

tanks and utilities (U.S. EPA, 1995d). In many instances, heatbased in-situ remediation techniques have been found to be cost

effective compared to the excavation and incineration option or

other remediation techniques (Dev et al., 1989; Basile and

Smith, 1994; Yow et al., 1995).
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The most commonly used remediation technique for the recovery of organic contaminants from ground water has been pumpand-treat, which recovers contaminants dissolved in the aqueous phase. Vacuum extraction (also called soil venting) is

quickly becoming popular for removal of volatile organic contaminants from the unsaturated zone in the gaseous phase (Ho

and Udell, 1992; Shah et al., 1995). Both of these techniques

can, in the initial remediation phase, rapidly recover contaminants at concentrations approximately equal to the solubility limit

(pump-and-treat), or the maximum gas phase concentration of

the contaminant (vacuum extraction). The maximum gas phase

concentration will depend on whether the contaminant is present

as a free phase or as a solute in the aqueous phase. During this

initial phase, large amounts of the contaminants may be removed. The second phase of the remediation, however, is

characterized by rapidly declining contaminant concentration in

the effluent as the rate of mass transfer into the flowing phase

controls the rate of removal. The third phase of the remediation

is characterized by a tailing in the effluent of low contaminant

concentrations. However, low effluent concentrations may not

be a reliable indication of low contaminant levels remaining in the

subsurface. Diffusion of contaminants from less-permeable

areas into the regions where flow is occurring or the slow

desorption of contaminants from the soil surface may control

contaminant removal during this phase, and termination of the

extraction process before these processes are complete may

lead to significant rebounding of the ground water and/or soil air

concentrations (Mackay and Cherry, 1989).

Thus, the rate-limiting properties of the systems are different in

each of the three phases of the remediation: in the first phase,

the solubility of the contaminant in the aqueous phase (pumpand-treat) or its maximum gas phase concentration (vacuum

extraction); in the second phase, it is the mass transfer step, i.e,

dissolution into the aqueous phase (pump-and-treat), or vaporization (vacuum extraction); and, during the third phase, it is

diffusion from low permeability areas or the desorption rate

(Shah et al., 1995).

Instances of soil and aquifer contamination by oily contaminants

such as automatic transmission fluid (Abdul et al., 1990), coal tar

(Gerencher et al., 1992) and creosote (Johnson, 1994) have

been documented. Contaminants such as these oils are practically insoluble in water and have essentially no vapor pressure.

Thus, they will be present in the subsurface as a nonaqueous

phase liquid and must be recovered as such. The recovery of

these types of products is often limited by slow movement to

recovery wells caused by their high viscosity and the significant

residual saturation left behind. Heat-based in-situ remediation

techniques which overcome or lessen the influence of each of

these limitations to the recovery of organic contaminants have

been developed and are being field tested.

Properties of the Contaminants

Organic contaminants in the subsurface can be present as a

separate nonaqueous phase liquid, dissolved in the aqueous

phase, in the vapor phase in the soil gas, partitioned into the soil

organic matter, or adsorbed onto the solid mineral phase. The

relative amount of the contaminant in each of these phases is

determined largely by the properties of the contaminant. Generally, the most important property of the soil in determining the

distribution of contaminants is the soil organic matter, which

normally controls the absorption of hydrophobic compounds.

Table 1 lists some of the relevant physical properties of volatile

and semivolatile organic chemicals that have been, or have the

potential to be, recovered using in-situ thermal techniques.

Many of these are commonly found at Superfund sites and other

sites where ground-water contamination has occurred (Roberts

et al., 1982; Esposito et al., 1989; Newell et al., 1995). All of the

properties listed are temperature dependent. Most of these

organics are essentially immiscible with water; acetone and

methanol are the exceptions. Many of these compounds have

low viscosities and, thus, have the potential to flow readily in the

liquid phase. Approximately half of these compounds are less

dense than water; the other half are more dense than water. The

density of an organic liquid relative to that of water is important

in determining the vertical mobility of the contaminant. Those

that are less dense than water will tend to float on the groundwater table, while those that are more dense than water may

move downward through the aquifer if the pressure in the organic

liquid is greater than the displacement pressure of the aquifer

materials. Low permeability clay layers in the aquifer may

restrict the vertical movement and allow the liquids to accumulate on top of the layer.

Table 1 is set up to list the chemicals in the order of lowest to

highest boiling points. Observation of the vapor pressures

shows that, in general, the lower the boiling point the higher the

vapor pressure at 25°C. The contaminants with the lowest

boiling points also generally have a lower heat of vaporization,

thus these contaminants are relatively easy to volatilize. Compounds with higher boiling points have lower vapor pressures at

ambient temperatures and a higher heat of vaporization, thus

more energy is required to convert them to the gaseous phase.

Laboratory experiments have shown that vaporization of even

highly volatile compounds can cause a measurable decrease in

the temperature of the system (Lingineni and Dhir, 1992).

Henry’s law constants indicate whether the compound prefers to

be in the gaseous or aqueous state. Henry’s law applies to dilute

solutions, and can be written as: PA = XAHc

, where PA is the partial

pressure of chemical A, XA is the mole fraction of chemical A in

solution, and Hc

 is a constant, commonly called the Henry’s law

constant. Thus, the Henry’s law constants are a function of the

aqueous solubility and vapor pressure of a compound, and the

greater the Henry’s law constant, the greater the extent to which

the compound partitions to the air phase (Atkins, 1986). The

constants given in the table are expressed in the dimensionless

form, which is a ratio of the concentrations, Cair/Cwater, where both

concentrations are in mol/m3. Due to difficulties involved in

determining solubilities and vapor pressures, reported values of

Henry’s law constants sometimes vary by two orders of magnitude or more depending on the source of the data. Mackay and

Shiu (1981) performed a critical review of the available data, and

determined recommended values for many chemicals of environmental interest, and a standard deviation of the reported
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Table 1. Properties of some organic chemicals that have been found at contaminated sites.

Organic Boiling Density Viscosity Water Vapor Vapor Henry’s Law Octanol- Diffusion Diffusion Heat ofContaminant Point gm/cm3 cP Solubility Pressure Pressure Constant Water Coefficient Coefficient Vaporiz-

°C 25°C mg/l mm Hg mm Hg dimensionless Partition in Water in Air ationb

T1, °C T2, °C 25°C Coefficient cm2/day cm2/day kJ/mol

25°C

Methylene 40 1.3182 0.413 20,000 260.9 >760 0.105 ± 0.008n 17.78q 28.82

Chloride 25°C 20°Ce 10°C 50°C

(Dichloromethane)

1,2-Dichloro- 49 1.2444 0.317 600 198.7 >760ethylene (trans) 20°Ce 10°C 50°CAcetone 56.3 0.7899 0.306 ∞ 121.7 622.4 0.000842k 1.74e 1.106b,p,h 9417.6s 30.99

10°C 50°C 25°C 0°C

1,1-Dichloro- 57.4 1.17 0.464 5500 125.8 608.6 0.234 ± 0.008n 30d 30.62

ethane 20°Ce 10°C 50°C 61.7i

1,2-Dichloro- 60 1.2649 0.445 800 104.8 580.0ethylene (cis) 25°C 20°Ce 10°C 50°CTrichloro- 61.2 1.49 0.537 8000 98.6 541.3 0.153 ± 0.012n 90d 7862s 31.28

methane 20°Ce 10°C 50°C 79.4i 0°C

(Chloroform) 93.3b

91.2q

1-Hexene 63.5 0.675 0.252 50 90.0 485.3 16.87 ± 0.40n 2455r,h 6212h 30.61

20°Ce 10°C 50°C 20°C

Methanol 64.6 0.791 0.544 ∞ 58.5 400 0.151 - 1.11o 15°C 14,688g 37.43

10°C 50°C 0.219 1.6s 25°C

1.43h 25°C

n-Hexane 68.7 0.659 0.300 9.5 80.8 407.5 68.6 ± 10.1n 10,000b 6143h 31.56

20°Ce 10°C 50°C 12,883r 20°C

1,1,1-Tri- 74.1 1.3303 0.793 4400 67.4 360.1 1.13 ± 0.016n 309b 32.50

chloroethane 25°C 20°C 10°C 50°C 300d

147.9i

Continued on next page.
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Organic Boiling Density Viscosity Water Vapor Vapor Henry’s Law Octanol- Diffusion Diffusion Heat ofContaminant Point gm/cm3 cP Solubility Pressure Pressure Constant Water Coefficient Coefficient Vaporiz-

°C 25°C mg/l mm Hg mm Hg dimensionless Partition in Water in Air ationb

T1, °C T2, °C 25°C Coefficient cm2/day cm2/day kJ/mol

25°C

Carbon 76.8 1.5833 0.908 800 58.3 332.8 0.807 ± 0.161n 676.1b,q 32.43

Tetrachloride 25°C 20°Ce 10°C 50°C 436.5e,i

2-Butanone 79.6 0.7994 0.405 26,800 52.6 314.3 0.0010j 1.820e 34.76

(Methyl Ethyl 25°C 10°C 50°C 0.00112c,k

Ketone)Benzene 80.1 0.88 0.604 1770 47.8 307.8 0.22 ± 0.01n 134.90b,e,h,i,l 0.881b,h 7460f 33.83

25°C 10°C 50°C 20°C 7819.2h

0.501h 2°C 20°C

6653s 0°C

Cyclohexane 80.7 0.7731 0.894 58 50.5 272.3 7.27 ± 0.81n 2754h 0.726b,h 7430.4s 33.01

25°C 25°C 10°C 50°C 20°C 45°C

0.397h 2°C 6212h

20°C

1,2-Dichloro- 84 1.257 0.779 8700 40.0 278.6 0.044 ± 0.004n 30.2b,i 35.61

ethane 20°C 10°C 50°C

Trichloro- 87.3 1.4578 0.545 1100 37.6 256.7 0.397f 195b 0.830f 7030f 34.54

ethylene 25°C 25°Ce 10°C 50°C 0.38i 200d

0.372a 339r

Toluene 110.6 0.8647 0.56 515 - 540 14.3 579.1 0.27 ± 0.014n 537b 0.734b,h 6570f 38.01

25°C 25°C 10°C 100°C 490d,e,h,i,l 20°C 7119.4h

447r 0.389h 2°C 20°C

6566s 0°C7603s

30°C7430o26°C7949o59°C

Continued on next page.

Table 1 -- Continued.
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Organic Boiling Density Viscosity Water Vapor Vapor Henry’s Law Octanol- Diffusion Diffusion Heat ofContaminant Point gm/cm3 cP Solubility Pressure Pressure Constant Water Coefficient Coefficient Vaporiz-

°C 25°C mg/l mm Hg mm Hg dimensionless Partition in Water in Air ationb

T1, °C T2, °C 25°C Coefficient cm2/day cm2/day kJ/mol

25°C

4-Methyl 116.6 0.802 0.545 19000 4.3 381.0 0.0063c 40.61

2-Pentanone 10°C 100°C

Tetrachloro- 121.3 1.613 0.844 150 9.0 400 0.928 ± 0.161n 400d,e 39.68

ethylene 25°C 25°C 10°C 100°C 407q

n-Octane 126 0.6986 0.508 0.7 6.5 368.7 121 ± 20n 104,700l 4910f 41.49

25°C 20°Ce 10°C 100°C 151,356h,r 5166.7h

20°C4363s 0°C

Chloro- 131.7 1.1007 0.753 490 6.9 323.7 0.14 ± 0.02n 691.8b,e,i 6394o 40.97

benzene 25°C 25°C 10°C 100°C 955r 26°C

7776o59°C6480s30°C

Ethylbenzene 136.2 0.8654 0.631 160 6.0 295.7 0.323 ± 0.028n 1412.5b,h,i 0.700b,h 6333h 42.24

25°C 25°C 10°C 100°C 1349r 20°C 20°C

0.380h 2°C

Xylenes 138.4 - 0.8577 0.608 - 160 - 4.5 - 5.6 238.9 - 0.20f 1412 - 5980f 42.40 -

 144.4 - 0.802 180 10°C 280.8 9400j 1585b 5771.5h 43.43

0.8764 25°C 100°C 0.202 - 0.286c 588.8 - 20°C

25°C 0.214a 1584.9e,h,l

1349 -1585r

n-Decane 174.2 0.730 0.838 0.052 3.0 77.7 282.5 ± 121n 51.38

25°C 100°C

Dichloro- 173 - 1.2988 1.044 - 80 - 150 2.2 67.1 0.048 - 0.073b 2399 - 36.18 -

Benzene 180 25°C 1.324 25°C 25°C 100°C 3981b 49.00

(3 isomers) (ortho 2399 -

 isomer) 2455e

Dodecane 216.5 0.75 1.383 0.0034 19.5 302.7 ± 100.9n 1537l 61.51

100°C 13 x 106 h

Continued on next page.

Table 1 -- Continued.
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Organic Boiling Density Viscosity Water Vapor Vapor Henry’s Law Octanol- Diffusion Diffusion Heat ofContaminant Point gm/cm3 cP Solubility Pressure Pressure Constant Water Coefficient Coefficient Vaporiz-

°C 25°C mg/l mm Hg mm Hg dimensionless Partition in Water in Air ationb

T1, °C T2, °C 25°C Coefficient cm2/day cm2/day kJ/mol

25°C

Naphthalene 218 0.97 32 22.7 0.02b,i 2239b,r 4432s 0°C

25°C 100°C 0.05m 2344.2i

0.017 ± 0.002n 1738l

1-Methyl 244.8 1.020 28.5 0.043 0.0182 ± 7413rnaphthalene kg/m3 0.0016nHexadecane 286.9 0.773 3.032 0.0063 < 1 < 1 81.38Phenanthrene 340 0.98 1.18 0.0016i,m 28,840e,i 72.50

0.0016 ± 37154r

0.00032n

Gasoline 0.73 0.45 100 - 300 6272.6h

0.7182h 0.4 - 0.6h 30 - 120h 20°C

20°C 20°C

Superscripts

a - Baehr, 1987 b - Lide, 1993 c - Newell et al., 1995 d - Hunt et al., 1988 e - Verschueren, 1983 f - Ong et al., 1992g - Thoma et al., 1992 h - Lyman et al., 1991 i - Ryan et al., 1988 j - Jury et al., 1990 k - Sanders, 1995 l - Johnson et al., 1990m - Jury et al., 1984 n - Mackay and Shiu, 1981 o - Treybal, 1980 p - Tyn and Calus, 1975 q - Valsaraj, 1988r - Miller et al., 1985 s - Perry and Chilton, 1973

Table 1 -- Continued.
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values that were thought to be reliable. For chemicals for which

their recommended values are available, they are listed in

Table 1. For the other chemicals, all reported values that were

located in the literature are listed to show the range in reported

values. The Henry’s law constants listed in the table show that

at ambient temperatures, the alkanes and similar compounds,

such as 1-hexene and cyclohexane, have a strong preference

for the air phase rather than the aqueous phase, while the

chlorinated solvents and compounds that contain a benzene

ring tend to concentrate more in the aqueous phase. As the

number of benzene rings in the compound increases, its preference for the aqueous phase increases. The ketones listed in

Table 1 also have a very strong preference for the water phase.

The partition coefficient is defined as the ratio of the equilibrium

concentration C of a dissolved substance in a system containing

two largely immiscible solvents. Thus, the octanol-water partition coefficient is defined as Kow = Coctanol/Cwater. The octanolwater partition coefficient has proven useful as a means to

predict soil absorption as well as biological uptake and

biomagnification and related phenomenon (Verschueren, 1983).

In general, the more hydrophobic a compound is, the greater its

octanol-water partition coefficient and the greater its absorption

onto soil organic matter (Karickhoff et al., 1979). However, this

generalization is limited to hydrophobic organic compounds and

soils which contain significant amounts of organic matter, on the

order of at least 0.1 percent (Schwarzenbach and Westall, 1981;

Weber et al., 1991). Most of the organic compounds listed in

Table 1 are at least moderately hydrophobic, thus the octanol/

water partition coefficient might be expected to indicate the

degree of absorption of these compounds in surface soils or

other soils with high organic carbon contents. It can be seen in

Table 1 that there are also some large differences in the reported

values for the octanol/water partition coefficients for a given

organic compound depending on the source of the data. Despite

these differences, the reported values show that most of the

compounds listed have a strong preference for organic matter

rather than the water phase, with the exception being the

ketones and methanol. The absorption of contaminants into the

soil organic matter will tend to limit the rate at which they can be

recovered in either the aqueous or gaseous phase.

To illustrate what the Henry’s law constants and octanol-water

partition coefficients indicate about the distribution of an organic

chemical in the subsurface, a few calculations were carried out

using the equations given by Feenstra et al. (1991). The results

of these calculations are shown in Table 2. A surface soil of loam

texture with a bulk density of 1.28 gm/cm3, a porosity of 0.30, a

water saturation of 50 percent, and an organic matter content of

2 percent was assumed. Karickhoff et al.’s (1979) relationship

between the octanol-water partition coefficient and the adsorption coefficient, Kd, was assumed to be valid: Kd

 0.6focKow, where

f

oc is the fraction of organic matter in the soil. In order to

demonstrate the effect of soil properties on the distribution of an

organic chemical, calculations were also done for the same

chemicals for a sandy soil of bulk density 1.86 gm/cm3, porosity

0.30, and organic matter 0.1 percent. Again, a 50 percent water

saturation was assumed.

Table 2 shows that in soils with high organic carbon content,

organic compounds may be highly associated with the solid

material. Calculations for 1,1,1-trichloroethane (TCA) were

carried out using the range of values that have been reported for

Kow for this chemical to illustrate the difference this can make in

the calculated distributions. For soils with high organic matter

content there is very little difference as most of the chemical is

still associated with the solids. For low organic matter conditions

the difference in the distribution is significant, and the lower Kow

value means that more of the chemical is recoverable in the

water or air phase. Trichloroethylene (TCE) and TCA have

similar Kow values at ambient temperatures, but TCA has a

Henry’s law constant that is greater than that of TCE at temperatures around 20°C and, thus, much more of the TCA will be in the

air phase. TCE is a volatile organic compound but, at 20°C,

significantly more of its mass will be in the aqueous phase rather

than in the gas phase; a very significant proportion of it will be

associated with the solids. Acetone is highly volatile with a

boiling point of 56°C, but its extremely low Kow and Hc concentrate

it in the aqueous phase. Because acetone is miscible with water

and is a good solvent for many organic chemicals, acetone may

significantly increase the transport of less soluble organic chemicals in ground water (Huling, 1989; Udell and Stewart, 1989).

The diffusion coefficient measures the rate at which molecules

spread down a concentration gradient, and is dependent on the

chemical nature of the system and the concentration, as well as

the temperature and pressure. As can be seen from the

coefficients listed in Table 1, diffusion in gases is much greater

than diffusion in liquids, which is due to the considerably higher

molecular concentration of liquids. When experimental data is

not available, the diffusion coefficient for gases can be estimated

fairly accurately by equations that have been developed based

on the kinetic theory of gases. For liquids, diffusion coefficients

cannot be estimated with the same degree of accuracy because

a sound theory of the structure of liquids has not been developed. However, empirical correlations have been developed

and can be used in the absence of laboratory data (Treybal,

1980). Observation of the values in the table shows that most of

the organic compounds for which values could be located fall in

the range of 5000 to 8000 cm2

/day for diffusion in air; methanol

and acetone are again the exceptions with larger coefficients. All

of the water diffusion coefficients, also, fall in a rather narrow

range from 0.7 to 1.6 cm2

/day.

The properties listed in Table 1 are for pure chemicals, and do

not consider the effects of a porous solid on the properties and

behavior of the chemical. The vapor pressure, the diffusion

coefficient in both air and water, and perhaps the viscosity, are

all significantly affected by the presence of the chemical in

porous media and the properties of that media. The partial

pressure of a liquid is dependent on the curvature of the interface

between the liquid and gaseous phases, and the values contained in Table 1 are for a flat interface. When the interface is

curved, the partial pressure of the liquid is reduced. Although

this effect is very small in sandy soils with a less than one percent

reduction in vapor pressure, it becomes important in clay soils

when the pore sizes are less than approximately 10-6 cm,
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Table 2. Distribution of chemicals in low and high organic matter content soils.

High Organic Matter Low Organic Matter

Air Water Solids Air Water Solids

1,1,1-Trichloroethane 5% 5% 90% 26% 23% 51%

 Poct = 300

1,1,1-Trichloroethane 6% 6% 88% 35% 31% 34%

 Poct = 147.9

Trichloroethylene 1.6% 4.6% 93.8% 14% 35% 52%

 20°C

Trichloroethylene 30% 7% 63% 73% 16% 11%

 90°C

Acetone 0.07% 85% 15% 0.08% 99% 1%

causing a decrease in vapor pressure of approximately 40 percent (Wilson et al., 1988). This effect can also be important when

the medium is dry and the remaining liquid has receded into the

smallest pores (Bear and Gilman, 1995). These capillarity

effects will determine the level of cleanup that can be achieved

by a venting process at a given temperature (Lingineni and Dhir,

1992).

Adsorption of the organic onto the solid phase or partitioning into

the soil organic matter can also have the effect of lowering the

partial pressure of an organic compound. Adsorption can occur

from the liquid onto the solid and, when the water content is very

low, from the vapor phase onto the soil surface (Lighty et al,

1990; Tognotti et al., 1991). Fares et al. (1995) have studied

desorption of TCE from soils, and their data shows that the

equilibrium vapor pressure of TCE as it desorbs from a soil is

approximately an order of magnitude lower than the partial vapor

pressure of TCE when no soil is present. Experiments performed by Arthurs et al. (1995) showed that the rate of volatilization of the liquid in the presence of a soil is about two orders of

magnitude slower than from the pure liquid, and that the vaporization rate increases as the vapor pressure of the compound

increases. Fares et al. (1995) and Keyes and Silcox (1994)

found that the rate of desorption was linearly correlated with the

inverse of the soil particle diameter. Hatzinger and Alexander

(1995) found that organic compounds may become more tightly

bound to the soil or organic matter with time, which reduces their

desorption. Thus, the equilibrium partial pressure of a volatile

contaminant in the pore space and mass transfer from the

aqueous liquid or adsorbed phase will depend on the properties

of the chemical and the soil environment in which it resides.

The diffusion coefficients listed in the table are for bulk air or

water. In the pore spaces of a soil, diffusion in both the air and

water phase is reduced because of the reduced cross-sectional

area and increased path length caused by the presence of solid

and liquid obstacles (Millington, 1959). Diffusion coefficients in

porous media are a function of both the porosity of the soil and

the water (or air) content of the pores. At low water contents,

diffusivity in the water is also limited by the continuity of the water

phase. For diffusion in water in porous media, Porter et al. (1960)

found a nearly linear increase in effective diffusion as the water

content increased, ranging from 4 to 30 percent of its diffusion in

bulk water. Jin and Jury (1996) recommended the use of a

second model developed by Millington (1959) for predicting gas

phase diffusion in disturbed porous media: Ds

/Da = a2/n2/3, where

Ds

 is the diffusion coefficient of a gas through soil, Da is the

diffusion coefficient in free air, a is the volumetric air content of

the pores, and n is the porosity. They did not find a unique

relationship for gaseous diffusion in undisturbed soils due to

their heterogeneous nature. Measured gas phase diffusion in

undisturbed soils is both higher and lower than predicted by this

theory, but is not generally greater than about 40 percent of the

diffusion in air, and the diffusion drops off rapidly as the air-filled

porosity decreases.

Adsorption of water onto solid surfaces, particularly onto the

reactive surfaces of clays, will increase the viscosity of the water

in the layers immediately adjacent to the clay surface. Theoretical and experimental results of Kemper (1961a, 1961b) indicated that the first layer of water on the surface of a clay may

have a viscosity on the order of 10 times that of the bulk water,

and the viscosity of each adjacent layer of water then decreases
9

rapidly to the viscosity of the bulk water. How much effect this

has on bulk flow in soils depends on the thickness of the water

films in proportion to the pore sizes. Neutral organic species will

generally not be adsorbed to the surface of the soil as strongly

as water. Therefore, neutral organics may have a smaller

effective viscosity relative to that of water in porous media,

allowing them to flow more readily. For clays that are highly

reactive and swell in the presence of water, research has shown

that the presence of organic chemicals may shrink the clay.

Cracks may form which allow a much greater flow of the organic

chemical than was possible with water (Anderson et al., 1985;

Brown and Thomas, 1987; Fernandez and Quigley, 1988).

Table 3 lists a few of the viscous oils that have been found

contaminating the subsurface. These oils are essentially nonvolatile and are not soluble to an appreciable degree in water.

Thus, they remain a separate liquid phase in the subsurface. All

of these oils are mixtures of many different hydrocarbons. Coal

tar and creosote also contain polycyclic aromatic hydrocarbons.

When oils such as these are spilled to the subsurface and are

exposed to air and water, "weathering" will occur, as the more

volatile hydrocarbons volatilize to the pore air, and the more

soluble hydrocarbons dissolve in the pore water. Water and air

moving past these oils while they are trapped in the subsurface

will enhance the weathering process. The loss of the “light”

hydrocarbons by the weathering process will increase the specific gravity and viscosity of the remaining oil, and lower the

surface and interfacial tensions. Measurements made in the

author’s laboratory, Robert S. Kerr Environmental Research

Center, showed that a significant proportion of the crude oil was

volatilized at room temperature, causing a 7-fold increase in

viscosity. Creosote was found to contain few volatiles at room

temperature, and weathering did not change its viscosity significantly. The specific gravity of both oils increased, and the

surface and interfacial tensions decreased by approximately 5 to

10 dynes/cm by the weathering. Mungan (1964, 1966) has

found that decreasing the interfacial tension in an immiscible

displacement will increase recovery of the oil. Although decreases in interfacial tension will favor recovery of these oils, that

may be offset by a decrease in mobility caused by the higher

viscosities.

Research in the author’s laboratory has shown that oils such as

the crude oil listed in Table 3 flows more readily through some

silica sands than would be predicted based on the intrinsic

permeability of the sand measured with water and the density

and viscosity of the crude oil. This is likely due to the nonpolar

nature of these hydrocarbons which limits their adsorption to soil

surfaces. The greater mobility of the crude oil would give it a

greater tendency to spread as it enters the subsurface, but also

should aid in its recovery by a displacement process such as hot

water injection.

Mechanisms for Enhanced Recovery

In general, when an organic chemical is heated, its density is

reduced, its vapor pressure is increased, its adsorption onto

solid phases or absorption into soil organic matter is decreased,

and its molecular diffusion in the aqueous and gaseous phase is

increased (Isherwood et al., 1992). The viscosity of a liquid will

decrease as the temperature is increased, but the viscosity of

gases increases with temperature. Which of these effects of

heat is important for the enhanced recovery of a particular

contaminant depends mostly on the properties of the contaminant and the mechanism limiting the removal rate of the contaminant in the particular circumstance.

Available data on the expansion of organic chemicals such as

those listed in Table 1 with temperature shows that these

chemicals expand approximately 0.1 percent per degree Celsius. Thus, increasing the temperature by 100°C will increase

the liquid volume by approximately 10 percent. Since the volume

of a gas is directly proportional to temperature given in Kelvin, a

100°C increase in temperature will cause approximately a 30 percent increase in the gas volume. These changes are small

compared to the volume change that occurs when a liquid is

converted to a gas; water at 100°C has approximately a 1600-fold

increase in volume when it is converted from a liquid to a vapor.

The expansion of liquids with temperature causes a reduction in

the interaction between molecules, and thus a reduction in its

viscosity. For the organic chemicals listed in Table 1, generally

there is about a one percent change in viscosity per degree

Celsius. Thus, the higher the viscosity of the liquid at ambient

temperatures, the greater the reduction in viscosity as the

temperature is increased. The viscosity of gases at ambient

temperatures is approximately one to two orders of magnitude

lower than the viscosity of liquids. However, the increase in the

velocity of gas molecules with temperature is such that it causes

greater interaction between molecules as the temperature increases, causing an increase in viscosity with temperature. This

increase is proportional to the temperature in degrees Kelvin, so

that a 100°C increase in temperature will increase the viscosity

of a gas by about 30 percent.

The effect of temperature on solubility is dependent on the

chemical. Increasing temperature will reduce the water-water,

water-solute, and solute-solute interactions, so the net effect of

temperature on solubility will depend on which interactions are

affected to the greatest extent (Yalkowsky and Banerjee, 1992).

Thus, some chemicals show increasing solubility with temperature while others show decreasing solubility with temperature.

Maximum or minimum solubilities with temperature have also

been found for some chemicals; many organic liquids exhibit

minima in solubility at about room temperature (Yalkowsky and

Banerjee, 1992). Measurements by Stephenson (1992) and the

data compiled by Horvath (1982) show that the solubility of the

organic chemicals listed in Table 1 often decreases in the

temperature range of 0°C to 90°C, but the change in solubility in

this temperature range is generally less than a factor of two.

Vapor pressures always increase with temperature. For the

organics listed that have a boiling point of less than 100°C, the

vapor pressure increases by a factor of 5 to 7 as the temperature

increases from 10°C to 50°C. For those compounds that have

a boiling point greater than 100°C, their vapor pressure generally

increases by a factor of 40 to 50 by raising the temperature from

10°C to 100°C. Limited data on the desorption of organics from
10

soils shows that the exponential increase in the vapor pressure

with temperature also holds when the organic chemical is in the

presence of soils (Fares et al., 1995).

The combination of only small changes in solubility with temperature but large increases in vapor pressure results in increases in Henry’s constant as a function of temperature.

However, very few measurements of Henry’s constants for

chemicals of environmental concern as a function of temperature have been made, and most of these measurements are over

a limited temperature range. Heron et al. (1996) calculated and

measured Hc values for TCE as a function of temperature and

found an order of magnitude increase when the temperature was

raised from 20°C to 90°C. For the more soluble compounds such

as dichloromethane or 2-butanone, and the water-miscible compounds such as acetone and methanol, Hc

 may not be influenced

significantly by temperature.

Few measurements of Kow and/or Kd

 have been made as a

function of temperature, and most of the measurements that

have been made are over very small temperature ranges.

Although it has been shown for some systems that adsorption

may increase with temperature over narrow temperature ranges

(Weber et al., 1983), adsorption is, in general, an exothermic

process and, thus, will decrease as the temperature increases.

The magnitude of the effect of temperature is dependent on the

particular chemical, the soil, and the water content, as these

factors will determine the mechanism causing the adsorption

(Cancela et al., 1992; Piatt et al., 1996). Heron et al. (1996)

showed theoretically, based on heats of sorption, that adsorption from the aqueous phase onto soils can be expected to

decrease by a factor of approximately 2.2 as the temperature is

increased from 20°C to 90°C. Adsorption from the vapor phase

onto dry soils generally has a larger heat of sorption, which leads

to a greater influence of temperature on the adsorption process.

For TCE, Heron et al. (1996) found approximately an order of

magnitude decrease in adsorption onto dry soil as the temperature was increased from 20°C to 90°C. For high molecular

weight organics such as PCBs, a large fraction of the organic

may remain adsorbed to the soil at ambient temperatures, and

significantly higher temperatures (300°C to 400°C) may be

required for desorption to occur (Uzgiris et al., 1995).

Measured data has shown that the diffusion coefficient in liquids

is proportional to temperature in degrees Kelvin. Increasing the

temperature from 10°C to 100°C will increase the diffusion of a

Table 3. Properties of some oily contaminants.

Boiling Specific Viscosity Water Vapor Surface Interfacial

Range Gravity cp Solubility Pressure Tension Tension

mg/l mm Hg dynes/cm dynes/cm

Automatic > 350°C 0.875 < 50 < 2 x 10-4 33.7

Transmission 20°C 20°C

Fluida

Coal Tarb 25°C 0.9744 25°C 41.4

60°C 0.9469 60°C 1.65

85°C 0.9263 85°C 1.16

Coal Tarc 50% can be 7°C 1.028 7°C 18.98 28.8 22

distilled at 15°C 1.017 50°C 5.04 22°C

270°C 38°C 0.991 60°C 3.89

60°C 0.985

Creosoted 45 to 65% 10°C 1.1060 10°C 35.7 10°C 32.4 9.65 10°C

can be 20°C 1.1027 20°C 19.8 20°C 33.5 7.83 20°C

distilled at 30°C 1.0957 30°C 12.4 30°C 29.0 6.16 30°C

315°Ce 40°C 1.0893 40°C 8.57 40°C 25.0 5.31 40°C

50°C 1.0816 50°C 6.17 50°C 26.8 5.90 50°C

Crude Oild 10°C 0.8953 10°C 160.2 10°C 26.2 22.0 10°C

20°C 0.8883 20°C 63.0 20°C 24.5 21.0 20°C

30°C 0.8820 30°C 34.8 30°C 23.2 20.5 30°C

40°C 0.8760 40°C 23.2 40°C 23.3 20.2 40°C

50°C 0.8680 50°C 16.4 50°C 22.8 21.4 50°C

a - Abdul et al., 1990 b - Johnson and Guffey, 1990 c - Villaume et al., 1983 d - unpublished data from the author’s lab

e - American Wood Preservers’ Association Standards
11

solute in the aqueous phase by approximately 30 percent

(Treybal, 1980). The diffusion coefficient for gases is also

dependent on temperature. Observation of the theoretical

equation for diffusivity in the gas phase developed for mixtures

of nonpolar gases or of a polar with a nonpolar gas shows that

the diffusivity varies almost as T3/2 (Treybal, 1980). Increasing

the temperature from 10°C to 100°C will increase diffusion in the

air phase by approximately 50 percent, while a temperature

increase from 10°C to 300°C will increase diffusion by approximately 200 percent.

Essentially all of these changes with temperature can aid in the

recovery of contaminants from the subsurface. The thermal

expansion of a liquid with its accompanying decrease in viscosity will allow the heated liquid to flow more readily. For gases, the

expansion with temperature will be largely offset by the increase

in viscosity. However, since the viscosity of gases is approximately two orders of magnitude lower than the viscosity of

liquids, conversion of a liquid to a gas will greatly increase its

mobility. The act of expansion itself will aid in moving the fluids

out of the pore space, with the greatest effects coming from the

vaporization of a liquid to a gas. The increased diffusion of

contaminants as the temperature increases in both the aqueous

and gaseous phases will help to move contaminants from areas

of low permeability to areas of high permeability and speed their

recovery.

To demonstrate the effects of temperature on the distribution of

organic contaminants between the phases in the subsurface,

calculations were carried out using the data of Heron et al. (1996)

for TCE at 90°C. The results are shown in Table 2. It can be seen

that raising the temperature to 90°C significantly increases the

concentration in the air phase under both the high and low soil

organic matter conditions, while significantly decreasing the

amount that is associated with the solids. Only small amounts

remain in the liquid phase. Thus, as the temperature is increased, significantly more of the TCE can be recovered in the

vapor phase. If the high organic matter content soil is considered

under water saturated conditions, the amount of TCE in the

water would approximately double as the temperature was

increased from 20°C to 90°C, but 82 percent of the TCE would

remain adsorbed to the solids. Under the low organic matter/water

saturated conditions, there would be approximately a 30 percent

increase in the amount of TCE in the water phase when the

temperature is increased from 20°C to 90°C, leaving approximately 25 percent adsorbed to the solids.

This small effect of temperature on the concentration in the

aqueous phase shows that raising the temperature would have

a limited effect on the recovery in a pump-and-treat system. For

the volatile and semivolatile organic compounds, such as those

listed in Table 1, the enhanced vapor pressure and rate of

vaporization are generally the most important mechanisms for

enhanced recovery using the in-situ heat based remediation

techniques. Some of the most volatile compounds, which

includes TCE, benzene, and toluene, can be removed fairly

efficiently from sandy soils by vacuum extraction alone (Ho and

Udell, 1992; Gauglitz et al., 1994; Shah et al., 1995), and

laboratory experiments on vacuum extraction have shown that

the addition of heat had little effect on the vaporization of the less

volatile compounds. For the higher boiling point compounds and

when clays are present in the subsurface, the addition of heat as

part of the remediation process will significantly increase volatilization and enhance the vacuum extraction process (Lingineni

and Dhir, 1992). Recovery of most of these chemicals from the

subsurface will be enhanced by either steam or hot air injection

or by electrical heating processes.

For the volatile and semivolatile contaminants, steam stripping

and steam distillation can also be important recovery mechanisms (Dev et al., 1989; Stewart and Udell, 1988). Steam

distillation occurs when an immiscible liquid is present along with

water, as the mixture of immiscible liquids will boil when the total

pressure reaches one atmosphere, rather than when the pressure of the individual component reaches one atmosphere.

Because both liquids are contributing to the vapor pressure, the

vapor pressure reaches one atmosphere at a lower temperature

than either of the individual components would (Atkins, 1986).

Steam stripping enhances volatilization of a volatile organic

compound by removing the vapor phase from contact with the

aqueous phase, thus preventing the liquid and vapor phases

from reaching equilibrium and allowing volatilization to continue.

Steam stripping becomes important when an immiscible phase

is not present (Dev et al., 1989).

For the types of contaminants listed in Table 3, the greatest

enhancement effect coming from the addition of heat is likely to

be a reduction in the viscosity of the oil phase. Highly viscous oils

will generally show a substantial decrease in viscosity with only

a moderate temperature increase above ambient temperatures

(Herbeck et al., 1976), and the rate of decrease with temperature

then drops off rapidly with continuing increases in temperature.

Edmondson (1965) found that the greater the dependence of

viscosity of the oil on temperature, the greater the increase in its

recovery by a hot water displacement as the temperature increased. Other mechanisms for the increased recovery of oils

by hot water include the thermal swelling of liquids (Willman et

al., 1961), shifts in the relative permeabilities to oil and water with

temperature, and decreases in the residual oil saturation

(Edmondson, 1965; Davidson, 1969; Poston et al., 1970; Davis

and Lien, 1993). Decreases in the interfacial tension with

temperature for contaminants such as creosote may also aid in

its recovery from the subsurface. Different researchers have

found shifts of varying magnitudes and directions in relative

permeability curves, but in all cases the increase in oil recovery

as the temperature increases always appears to be greater than

would be predicted based on the viscosity reduction alone.

Capillary pressure-saturation curves measured for two phase

water/oil systems have shown substantial decreases in the

residual oil phase as the temperature increased (Davis, 1994),

and thus, a greater portion of the oil may be recoverable as the

temperature is increased.

Heat-based In-situ Remediation Techniques

There are three general methods that can be used to inject or

apply heat to the subsurface to enhance remediation: injection

in the form of hot gases such as steam or air, electromagnetic
12

energy heating, and hot water injection. Another thermal

remediation technique that relies on thermal conduction of soil to

heat the subsurface is also under development (Iben et al.,

1996), but will not be discussed here. All of these methods were

first developed by the petroleum industry for enhanced oil

recovery, and have more recently been adapted to soil and

aquifer remediation applications. The two applications have

significantly different objectives. In oil recovery operations, the

reservoir initially has a large oil saturation, and the objective is

to recover as much as possible economically. In these operations, a large residual oil saturation of as much as 50 percent or

more may be acceptable. In contamination remediation applications, the initial saturation of the contaminant may be anywhere

from essentially fully saturated to less than residual saturation,

and the objective is to reduce the contaminant concentration to

very low levels. The techniques of steam injection, electrical

energy application, and hot water injection have been the

subject of extensive research and development, and it has been

established that these techniques are effective for the remediation

of organic contaminants when they are appropriately applied

(Fulton et al., 1991; Davis and Lien, 1993; U.S. EPA, 1995a;

Newmark and Aines, 1995).

Injection of hot air has been tried in the laboratory and found to

enhance the removal of contaminants from one-dimensional soil

columns (Lingineni and Dhir, 1992; Shah et al., 1995). However,

the use of hot air in the field is limited by the very low heat

capacity of air (approximately 1 kJ/kg °C) (Ramey, 1967).

Steam, with a heat capacity that is approximately four times that

of air (approximately 4 kJ/kg °C), and heat of evaporation of more

than 2000 kJ/kg, has been used successfully to heat soils,

aquifers and reservoirs to enhance the recovery of contaminants

and oils. However, the injection of steam will always leave

behind a residual water saturation (Stewart and Udell, 1988),

and contaminants that have a significant solubility in water may

remain at high concentrations in this residual water or may even

appear to increase in concentration (Udell and Stewart, 1989;

U.S. EPA, 1991). For these situations, recovery of the contaminants may require that the soil be dried, and hot air injection may

be applicable (Farrington, 1996).

Steam, hot air and hot water injection rely on contact between the

injected fluid and the contaminant for the transfer of heat to and

recovery of the contaminant. Steam injection will displace

mobile contaminants in front of the steam as well as vaporize

volatile residual contaminants, and therefore can recover volatile contaminants in both the liquid or vapor phase. Hot air

injection has been used to recover contaminants only in the

vapor phase. Hot water injection generally recovers contaminants only in the liquid phase. Thus, steam injection is applicable to volatile and semivolatile organic compounds that are

immiscible with water, hot air is applicable to volatile and

semivolatile organics that are water soluble, and hot water

injection is applicable for the oils that have low volatility and very

low solubility in water. The main mechanism for enhanced

recovery using hot water is generally a reduction in the viscosity.

Changes in relative permeability and reductions in residual

saturation are likely to also aid in the recovery of nonvolatile oils.

Hot water injection is most likely effective only when the nonaqueous phase is present in quantities greater than the residual

saturation, as the main recovery mechanism is the physical

displacement of the nonaqueous phase. Hot water injection may

be most effective for light oils that are floating on top of the water

table, as the lower-density hot water has a tendency to rise if

injected below the water table. For oils that are more dense than

water at ambient temperatures but less dense than water at the

displacement temperature, heating of the subsurface by hot

water injection may help to float these oils, which may aid in their

recovery (Johnson and Guffey, 1990). Steam injection has a

definite advantage over hot water injection when the contaminants have a low boiling point and are present as an immiscible

phase, and thus can be steam distilled at the temperatures

achieved by steam injection (Willman et al., 1961). Field trials

have shown that steam injection can be carried out above or

below the water table (Udell and Stewart, 1989; U.S. EPA, 1991;

Aines et al., 1992).

There are limitations on the pressures that can be used for steam

and hot water displacement processes, and this limits the

viscosity of the fluid that can be displaced from a media with a

certain permeability. However, volatilization processes using

steam or hot air may still be possible in low permeability media

(Farrington, 1996), and for highly viscous oil, it may be possible

to heat the oil and lower its viscosity sufficiently to recover at

least a portion of the oil by either a displacement process or

gravity drainage (Hall and Bowman, 1973; Vogel, 1992).

Electrical energy has been applied to the soil in the low frequency range used for electrical power (called electromagnetic

(EM), alternating current (AC), or resistivity heating) as well as

in the radio frequency (RF) range. When EM heating is used, the

water in the pore spaces of the soil absorbs essentially all the

applied energy, so the evaporation of water limits the transport

of energy in the soil and, therefore, limits the heating process.

Thus, for the low frequency methods, the boiling point of water

is the highest temperature that can be achieved. For semivolatile

organic contaminants, the vapor pressure at 100°C may not be

adequate to effectively recover the contaminants. For this

reason, researchers have also developed the use of RF energy

for soil heating. RF energy can be absorbed by the soil itself, and

thus is not limited by a lack of water in the pore space. Using RF

energy, the upper temperature limit of the technique is 300°C to

400°C (Dev, 1993; Sresty, 1994). For electrical heating, the

electrical properties of the soil and the presence of water are

important in determining the efficiency of the heating process

(Dev et al., 1989; Marley et al., 1993). The electrical heating

techniques are recommended for the removal of organic contaminants which exhibit a vapor pressure of at least 10 mm Hg

in the treatment temperature range (Sresty, 1994). There is

some evidence that high molecular weight organics, such as

organopesticides, can be broken down to simpler organics such

as acetone, benzene and toluene, at the temperatures and

conditions that can be achieved by RF heating (U.S. EPA,

1995b&c; Swanstrom and Besmer, 1995). Electrical heating

has been proven effective in sandy media (Dev, 1986), and also

has a greater potential than steam or hot water injection to be

effective in less permeable media such as clays. The higher

water content generally found in the clay will aid in directing the
13

electromagnetic energy to the clay and allow a faster heating

rate and higher temperatures to be achieved. RF heating,

however, is limited to the unsaturated zone. For contaminants

trapped below the water table, dewatering would have to be

done prior to electrical heating (U.S. EPA, 1995b).

Because steam injection (at least in its initial stages) and hot

water injection are displacement processes, they can also

recover nonvolatile contaminants dissolved in the aqueous

phase, such as salts (Vaughan et al., 1993), but heat does not

necessarily enhance the recovery of this type of inorganic

contaminant. Metals, with the exception of mercury, cannot be

recovered from soils by thermal means. Mercury has a significant vapor pressure at ambient temperatures, and it increases

as the temperature increases. Adsorption onto soils and other

materials can reduce its partial vapor pressure significantly, and

it was found that drying soil samples to 100°C did not recover

measurable amounts of mercury. At temperatures of 200°C and

greater, significant amounts of mercury can be recovered from

soils, reducing the residual remaining in the soil to as little as 1

part per million at 400°C (Dewing and Schluter, 1994).

Each of these thermal methods is generally applicable only to

certain types of contaminated sites, and it is important that the

appropriate heat-based remediation technique is chosen for a

given site. The choice of technique must be based on both the

characteristics of the subsurface and of the contaminants to be

recovered. Steam or hot air injection or the electrical heating

techniques are generally applicable for the types of chemicals

that are listed in Table 1, while hot water injection is generally

applicable for the nonvolatile oils listed in Table 3. The permeability of the media, the amount and type of heterogeneity, the

amount of adsorption, and the solubility of the contaminant must

all be considered when choosing between the technologies.

Electrical heating may be favored in low permeable media and

when there is significant heterogeneity. For highly soluble

contaminants, drying the soil may be necessary and, thus, hot air

or RF heating may be more applicable. Because desorption can

be a slow process, higher temperatures and/or longer remediation

times may be necessary when adsorption is significant.

Figure 1 can be used as a quick guide to determine which of the

techniques would likely be applicable for a given situation; in

some cases, more than one technique may be applicable. The

principle that has been applied in developing this figure is to

recommend the least severe technique, in terms of temperature

and pressure requirements, that is likely to be able to recover the

contaminants. For example, although hot water and steam

injection may both be able to recover a nonvolatile, viscous oil,

hot water injection is recommended because it will generally

recover the same amount of this type of oil (Willman et al., 1961)

at a lower temperature. Equipment and facilities for generating

and handling hot water are relatively simple and inexpensive

(Harmsen, 1971), but the generation and transport of steam

involves more complex and expensive systems. Higher operating temperatures also mean greater safety risks (Herbeck et al.,

1976). The Issue Paper specific to that technique can then be

consulted for further information.

Comparison to Other In-situ Techniques

The one significant advantage of heat-based remediation techniques over other in-situ remediation techniques is that these

methods do not require that chemicals of any sort be injected into

the subsurface as part of the remediation effort. This is a very

significant advantage over the surfactant and cosolvent techniques because surfactants and cosolvents may themselves

have toxic properties, and it may not be possible to recover all of

the injected chemicals. Also, when surfactants or cosolvents are

used, the technique may be limited by a lack of contact between

the injected chemical and the contaminants, which can be

caused by low permeability layers within the media or by reductions in relative permeability to one phase because of the

presence in the pores of another phase (Peters et al., 1991).

With the in-situ heating techniques discussed in these Issue

Papers, heating of the entire area to be treated has generally

been accomplished (Aines et al., 1992; Gauglitz et al., 1994).

These heating techniques can be used with a technique such as

vacuum extraction, with benefits that may be significantly greater

than for either process used separately (Udell and Stewart,

1989; Jarosch et al., 1994). Also, these processes are applicable in heavily contaminated soils, the “hot spots” of contaminated sites, which generally are very important to clean up in

order to stop the spread of the contamination, and where

biological treatment may not be effective (Johnson and Guffey,

1990; Aines et al., 1992).

These thermal techniques initially may be limited by subsurface

heterogeneities, which affects all other in-situ remediation techniques. When low permeability clay lenses are present in an

aquifer, the injected fluids often bypass these low permeability

areas and, therefore, do not contact the contaminants contained

within them. With time, however, the heat will be conducted into

the lower permeability areas. Also, the Dynamic Underground

Stripping Process, developed by the Lawrence Livermore National Laboratory, circumvented this problem for the case where

relatively thick clay layers are interbeded with sandy layers by

combining both steam injection and electromagnetic heating

with vacuum extraction (Newmark and Aines, 1995; Yow et al.,

1995).

Research has shown that complete desorption of chemicals

from clay soils, or soils containing a significant amount of natural

organic material, may require extreme temperature conditions

(Lighty et al., 1988; Tognotti et al., 1991), which may not be

achievable in-situ. Thus, in many cases, a secondary or polishing step may be required to achieve very low contaminant

concentrations (Yow et al., 1995). Hot water injection or shallow

steam injection applications where low temperature and pressures are used may leave the subsurface system amenable to

bioremediation. In fact, raising the temperature above ambient

temperatures may, in many instances, enhance naturally occurring biodegradation of contaminants (Isherwood et al., 1992).

However, if steam is injected into the deeper subsurface at high

temperatures and pressures, or if the soil is heated to high

temperatures using RF heating, the soil will likely require cooling
14

before reestablishment of the microbial population can take

place (U.S. EPA, 1991).

Notice

The U.S. Environmental Protection Agency through its Office of

Research and Development funded the research described

here. It has been subjected to the Agency’s peer and administrative review and has been approved for publication as an EPA

document. Mention of trade names or commercial products

does not constitute endorsement or recommendation for use.

References

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for the removal of petroleum products from shallow sandy

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Ramirez, W. Siegel, M. Buettner, W. Daily, P. Krauter, E.

Folsom, A. Boegel, D. Bishop, Dynamic underground stripping project, UCRL-JC-109902, Waste Management Symposium, Tucson, Arizona, March 1-5, 1992.

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of compacted clay soils to water and organic liquids, Waste

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Resour. Res., 23(10):1926-1938, 1987.

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zone caused by heating, Trans. Porous Media, 19(2):139-

156, 1995.

Figure 1. Flow Chart to indicate which of the thermal techniques may be applicable for a particular site.

Volatile

Contaminant?

Sandy

Media ?

Unsaturated

Conditions ?

Vacuum

Extraction

Steam or

Hot Air

Injection

Air Sparging

Nonvolatile

Contaminant ?

Sandy

Media ?

Steam or Hot

Air Injection

Hot Water

Injection

Electrical

Heating

Steam

Injection

No No

No

No

No

No No

No

Yes

Yes Yes

Yes

Semivolatile

Contaminant ?

Sandy

Media ?

Unsaturated

Conditions ?

Yes

Yes

or Yes Yes

Electrical

Heating
15

Brown, K. W., and J. C. Thomas, A mechanism by which organic

liquids increase the hydraulic conductivity of compacted clay

materials, Soil Sci. Soc. Am. J., 51:1451-1459, 1987.

Cancela, G. D., E. R. Taboada, and F. Sanchez-Rasero,

Carbendazim adsorption on montmorillonite, peat, and soils,

J. Soil Sci., 43:99-111, 1992.

Davidson, L. B., The effect of temperature on the permeability

ratio of different fluid pairs in two-phase systems, J. Petrol.

Tech., 1037-1046, August 1969.

Davis, E. L., Effect of temperature and pore size on the hydraulic

properties and flow of a hydrocarbon oil in the subsurface, J.

Contamin. Hydrol., 16:55-86, 1994.

Davis, E. L., Steam injection for soil and aquifer remediation,

Ground Water Issue Paper, in preparation, 1997a.

Davis, E. L., Electrical heating of the subsurface for soil and

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