1

Ground Water Issue

Light Nonaqueous Phase Liquids

Robert S. Kerr Environmental

Research Laboratory

Ada, Oklahoma

Superfund Technology Support Center for

Ground Water Technology Innovation Office

Office of Solid Waste and Emergency

Response, US EPA, Washington, DC

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

Director

United States

Environmental Protection

Agency

Office of Solid Waste

and Emergency

Response

Office of

Research and

Development

Charles J. Newell,* Steven D. Acree,** Randall R. Ross,**

and Scott G. Huling**

varying degrees of water solubility. Some additives (e.g.,

methyl tertiary-butyl ether and alcohols) are highly soluble.

Other components (e.g., benzene, toluene, ethylbenzene, and

xylenes) are slightly soluble. Many components (e.g., ndodecane and n-heptane) have relatively low water solubility

under ideal conditions. Physical and chemical properties

which affect transport and fate of selected LNAPL compounds

and refined petroleum products are presented in Table 1. In

general, LNAPLs represent potential long-term sources for

continued ground-water contamination at many sites.

LNAPL TRANSPORT THROUGH POROUS MEDIA

General Conceptual Model

Movement of LNAPLs in the subsurface is controlled by

several processes described in the following simplified

scenario (Figure 1). Upon release to the environment, NAPL

(i.e., LNAPL or DNAPL) will migrate downward under the

force of gravity. If a small volume of NAPL is released to the

subsurface, it will move through the unsaturated zone where a

fraction of the hydrocarbon will be retained by capillary forces

as residual globules in the soil pores, thereby depleting the

contiguous NAPL mass until movement ceases. If sufficient

LNAPL is released, it will migrate until it encounters a physical

barrier (e.g., low permeability strata) or is affected by

buoyancy forces near the water table. Once the capillary

fringe is reached, the LNAPL may move laterally as a

continuous, free-phase layer along the upper boundary of the

water-saturated zone due to gravity and capillary forces.

The Regional Superfund Ground-Water Forum is a group of

scientists representing EPA’s Regional Superfund Offices,

committed to the identification and resolution of ground-water

issues affecting the remediation of Superfund sites. Light

nonaqueous phase liquids (LNAPLs) have been identified by

the Forum as an issue of concern to decision makers. This

issue paper focuses on transport, fate, characterization, and

remediation of LNAPLs in the environment.

For further information contact Steve Acree (405) 436-8609,

Randall Ross (405) 436-8611, or Scott Huling (405) 436-8610

at RSKERL-Ada.

INTRODUCTION

Nonaqueous phase liquids (NAPLs) are hydrocarbons that

exist as a separate, immiscible phase when in contact with

water and/or air. Differences in the physical and chemical

properties of water and NAPL result in the formation of a

physical interface between the liquids which prevents the two

fluids from mixing. Nonaqueous phase liquids are typically

classified as either light nonaqueous phase liquids (LNAPLs)

which have densities less than that of water, or dense

nonaqueous phase liquids (DNAPLs) which have densities

greater than that of water. A previous issue paper developed

by the Robert S. Kerr Environmental Research Laboratory

reviews processes and management issues pertaining to

DNAPLs (Huling and Weaver, 1991).

Light nonaqueous phase liquids affect ground-water quality at

many sites across the country. The most common LNAPLrelated ground-water contamination problems result from the

release of petroleum products. These products are typically

multicomponent organic mixtures composed of chemicals with

* Groundwater Services, Inc.

** Robert S. Kerr Environmental Research Laboratory.

EPA/540/S-95/500
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Hydrocarbon

Vapors

Ground-Water

Flow

After Mercer and Cohen (1990)

Capillary

Fringe

Dissolved

Contaminants

LNAPL Contamination

Water Table

Chemical Density† Dynamic† Water† Vapor† Henry's Law†

Viscosity Solubility Pressure Constant

(g/cm3) (cp) (mg/l) (mm Hg) (atm-m3 /mol)

Methyl Ethyl Ketone 0.805 0.40 2.68 E+05 71.2 2.74 E-05 (2)

4-Methyl-2-Pentanone 0.8017 0.5848 1.9 E+04 16 1.55 E-04 (2)

Tetrahydrofuran 0.8892 0.55 3 E+05(1) 45.6 (2) 1.1 E-04 (2)

Benzene 0.8765 0.6468 1.78 E+03 76 5.43 E-03 (1)

Ethyl Benzene 0.867 0.678 1.52 E+02 7 7.9 E-03 (1)

Styrene 0.9060 0.751 3 E+02 5 2.28 E-03

Toluene 0.8669 0.58 5.15 E+02 22 6.61 E-03 (1)

m-Xylene 0.8642 (1) 0.608 2 E+02 9 6.91 E-03 (1)

o-Xylene 0.880 (1) 0.802 1.7 E+02 7 4.94 E-03 (1)

p-Xylene 0.8610 (1) 0.635 1.98 E+02 (1) 9 7.01 E-03 (1)

Water 0.998 (6) 1.14 (6) ---- ---- ----

Common Petroleum Products

Automotive gasoline 0.72-0.76 (3) 0.36-0.49 (3) ---- ---- ----

#2 Fuel Oil 0.87-0.95 1.15-1.97 (5) ---- ---- ----

#6 Fuel Oil 0.87-0.95 14.5-493.5 (4) ---- ---- ----

Jet Fuel (JP-4) ~0.75 ~0.83 (5) ---- ---- ----

Mineral Base

 Crankcase Oil 0.84-0.96 (6) ~275(4) ---- ---- ----

† Values are given at 20°C unless noted.

(1) Value is at 25°C.

(2) Value is at unknown temperature but is assumed to be 20°- 30°C.

(3) Value is at 15.6°C.

(4) Value is at 38°C.

(5) Value is at 21°C.

(6) Value is at 15°C.

Table 1. Representative properties of selected LNAPL chemicals commonly found at Superfund sites (U.S.EPA, 1990), water, and

selected petroleum products (Lyman and Noonan, 1990)

Although principal migration may be in the direction of the

maximum decrease in water-table elevation, some migration

may occur initially in other directions. A large continuousphase LNAPL mass may hydrostatically depress the capillary

fringe and water table. Once the source is removed,

mounded LNAPL migrates laterally, LNAPL hydrostatic

pressure is removed, and the water table eventually rebounds.

Infiltrating precipitation and passing ground water in contact

with residual or mobile LNAPL will dissolve soluble

components and form an aqueous-phase contaminant plume.

In addition, volatilization may result in further spreading of

contamination.

Contaminant Phase Distribution

LNAPL constituents may exist in any of four phases within the

subsurface. The NAPL, aqueous, and gaseous phases were

mentioned above. Contaminants may also partition to the

solid-phase material (i.e.,soil or aquifer materials). In the

unsaturated zone contaminants may exist in all four phases

Figure 1. Simplified conceptual model for LNAPL release and

migration.
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Soil Gas

Solid

Water

LNAPL

After Huling & Weaver (1991)

Figure 3. Partitioning of LNAPL among the four phases

potentially found in the unsaturated zone.

Water Soil

LNAPL

Soil

Gas

After DiGiulio and Cho (1990)

Figure 2. Contamination in the unsaturated zone may be present

in four physical states: gas, sorbed to soil materials,

dissolved in water, or immiscible liquid.

generally decreases as temperature increases.

Consequently, the density of fluids considered to be DNAPLs

under normal subsurface conditions may decrease during

remedial actions which impart heat to the subsurface (e.g.,

Johnson and Leuschner, 1992). A decrease in density of

DNAPLs which have densities near that of water (e.g., some

coal tar residues) may result in sufficient reduction to

temporarily convert the DNAPL to an LNAPL. Density not only

affects the buoyancy of a liquid but also the subsurface

mobility. The hydraulic conductivity of a porous medium is a

function of the density and viscosity of the liquid. As the

density increases, the hydraulic conductivity with respect to

the liquid also increases.

Viscosity

Viscosity is the resistance of a fluid to flow. Dynamic, or

absolute, viscosity is expressed in units of mass per unit

length per unit time. This resistance is also temperature

dependent. The viscosity of most fluids will decrease as the

temperature increases. The lower the viscosity, the less

energy required for a fluid to flow in a porous medium. The

hydraulic conductivity increases as the fluid viscosity

decreases.

Interfacial Tension

When two liquids, which are immiscible, are in contact, an

interfacial energy exists between the fluids, resulting in a

physical interface. Interfacial tension is the surface energy at

the interface that results from differences in the forces of

molecular attraction within the fluids and at the interface

(Bear, 1972). It is expressed in units of energy per unit area.

In general, the greater the interfacial tension, the greater the

stability of the interface between the liquids. Interfacial

tension is affected by temperature (Davis and Lien, 1993),

changes in pH and the presence of surfactants and dissolved

gases. It is an important factor affecting wettability (Mercer

and Cohen, 1990).

Wettability

Wettability is generally defined as the overall tendency of one

fluid to spread on or adhere to a solid surface (i.e.,

(Figure 2). In the saturated zone NAPL-related contaminants

may be present in the aqueous, solid, and NAPL phases.

NAPL constituents may partition, or move from one phase to

another, depending on environmental conditions (Figure 3).

For example, soluble components may dissolve from the

NAPL into passing ground water. The same molecule may

adsorb onto a solid surface, and subsequently desorb into

passing ground water. The tendency for a contaminant to

partition from one phase to another may be described by

partition coefficients such as Henry's Law constant for

partitioning between water and soil gas. These empirical

coefficients are dependent on the properties of the subsurface

materials and the NAPL. A clear understanding of the phase

distribution of contaminants is critical to evaluating remedial

decisions (Huling and Weaver, 1991). It is important to note

that this distribution is not static and may vary over time due to

remedial actions and natural processes.

LNAPL Transport Parameters

Characteristics of the LNAPL and subsurface materials govern

transport at both the pore scale and field scale. At the pore

scale, the following transport and fate parameters control

LNAPL migration and distribution. At the field scale, LNAPL

migration is much more difficult to predict due to such factors

as complex release history and, most importantly, subsurface

heterogeneity. However, the following discussion of porescale principles is necessary for development of conceptual

models incorporating observations made at the field scale. A

more detailed explanation of these concepts ( Mercer and

Cohen, 1990) and methods for measuring these properties

(Cohen and Mercer, 1993) are available in the literature.

Density

Density is defined as the mass of a substance per unit

volume. One way to express density of a fluid is the specific

gravity (S.G.) which is the ratio of the mass of a given volume

of substance at a specified temperature to the mass of the

same volume of water at the same temperature. If a NAPL

has an S.G. less than water, generally less than 1.0, it is less

dense than water (i.e., LNAPL) and will float on water. If it has

an S.G. greater than water, generally greater than 1.0, it is

denser than water (DNAPL). The density of most fluids
4

preferentially coat) in the presence of another fluid with which

it is immiscible. This concept has been used to describe fluid

distribution at the pore scale. In a multiphase system, the

wetting fluid will preferentially coat (wet) the solid surfaces and

tend to occupy smaller pore spaces. The non-wetting fluid will

generally be restricted to the largest interconnected pore

spaces. In the vadose zone, where air, water, and LNAPL are

present, liquids, usually water, preferentially wet solid

surfaces. However, under conditions where only LNAPL and

air are present, LNAPL will preferentially coat the mineral

surfaces and displace air from pore spaces. In the saturated

zone, with only water and LNAPL present, water will generally

be the wetting fluid and will displace LNAPL from pore spaces.

Wettability is affected by such factors as NAPL and aqueousphase composition, presence of organic matter, surfactants,

mineralogy, and saturation history of the porous medium

(Mercer and Cohen, 1990). Some researchers have

concluded that wetting of subsurface media by NAPL may be

heterogeneous due to subsurface variabilility and the many

factors that influence wettability (Anderson, 1986). In

summary, wettability is a qualitative indicator useful to

understand the general behavior of NAPLs in multiphase

systems and has been used extensively in the petroleum

industry (Anderson, 1986). Actual wettability measurements

of NAPLs on solid surfaces are usually reported for flat,

homogeneous material which is unrepresentative of complex

aquifer and soil material. Refined petroleum products typically

found at Superfund sites can generally be considered the nonwetting fluid in water:LNAPL systems, and the wetting fluid in

LNAPL:air systems.

Capillary Pressure

Capillary pressure is the pressure difference across the

interface between the wetting and non-wetting phases and is

often expressed as the height of an equivalent water column.

It determines the size of the pores in which an interface can

exist. It is a measure of the relative attraction of the molecules

of a liquid (cohesion) for each other and for a solid surface

(adhesion). Capillary pressure is represented by the tendency

of the porous medium to attract the wetting fluid and repel the

non-wetting fluid (Bear, 1972). The capillary pressure of the

largest pore spaces must be exceeded before the non-wetting

fluid (generally NAPL) can enter the porous medium. The

minimum pressure required for the NAPL to enter the medium

is termed the entry pressure.

In general, capillary pressure increases with decreasing pore

size, decreasing initial moisture content, and increasing

interfacial tension. Capillary conditions affect the

configuration and magnitude of trapped residual NAPL. Field

observations of the effects of capillary pressure include

preferential LNAPL migration through coarse-grained

materials (e.g., sands and gravels), rather than fine-grained

materials (e.g., silts and clays). Analytical expressions

describing relationships between capillary pressure and NAPL

movement under hydrostatic and hydrodynamic conditions are

compiled by Mercer and Cohen (1990). Although these

approximate expressions do not account for complicated pore

geometries and distributions present in most systems,

evaluation of site conditions using such expressions may often

be useful in refining the conceptual model for NAPL transport.

Although the capillary forces that hold residual NAPL in pores

are relatively strong, they can be overcome to some degree

by viscous forces associated with ground-water flow.

However, complete mobilization of residual hydrocarbons is

very difficult or impossible to achieve in most aquifers by

manipulating hydraulic gradient alone (Wilson and Conrad,

1984). The required hydraulic gradients are so high for many

aquifers (greater than 1 ft/ft) that no reasonable configuration

of pumping and injection wells could sweep all of the residual

NAPL trapped in the pores of the aquifer.

Saturation and Residual Saturation

Saturation is the relative fraction of total pore space containing

a particular fluid (e.g., NAPL) in a representative volume of a

porous medium. The mobility of an LNAPL is related to its

saturation in the medium as described by the relative

permeability function discussed below. The saturation level

where a continuous NAPL becomes discontinuous and is

immobilized by capillary forces is known as the residual

saturation (Sr

). Residual saturation of LNAPL represents a

potential source for continued ground-water contamination

that is tightly held in the pore spaces and not readily removed

using currently available remediation technologies. The

magnitude of residual saturation is affected by several factors

including pore-size distribution, wetting properties of the fluids

and soil solids, interfacial tension, hydraulic gradients, ratios

of fluid viscosities and densities, gravity, buoyancy forces, and

flow rates (Mercer and Cohen, 1990; Demond and Roberts,

1991). Due to the known heterogeneity of subsurface

systems with regard to these factors, it follows that residual

saturation in the subsurface is also highly variable.

Data compiled by Mercer and Cohen (1990) indicate the

residual saturation of most NAPLs in these studies ranged

from about 10% to 20% in the unsaturated zone and about

15% to 50% of the total pore volume in the saturated zone.

The potential for higher retention of NAPLs in the saturated

zone than in the unsaturated zone is due to several factors

including: 1) potential existence of the NAPL as the wetting

fluid relative to air in the unsaturated zone resulting in NAPL

spreading to adjacent pores with residual held in small pore

spaces, 2) existence of the NAPL as the non-wetting fluid in

the saturated zone resulting in NAPL present as blobs in

larger pore spaces, and 3) the relatively high fluid density ratio

of NAPL to air in the vadose zone resulting in drainage

(Anderson, 1988).

Relative Permeability

Relative permeability is the ratio of the effective permeability

of the medium to a fluid at a specified saturation and the

permeability of the medium to the fluid at 100% saturation.

Values for relative permeability range between 0 and 1. A

simplified relative permeability diagram for a hypothetical

LNAPL/water system (Figure 4) illustrates how two fluids

interfere with each other to reduce mobility. Similar, yet more

complex relationships exist in the unsaturated zone where

three fluids (air, water, and NAPL) may be present (van Dam,

1967; Ferrand et al., 1989). At most points on the curves, the

relative permeabilities of NAPL and water do not sum to one

because interference reduces the overall mobility of both

fluids in the porous medium. The curves also illustrate that a

minimum saturation must be attained before the permeability

to a fluid is non-zero (Schwille, 1988). The minimum

saturation for the wetting fluid has been termed irreducible

saturation (Sir) and for the non-wetting fluid, generally NAPL,
5

has been termed residual saturation. It should be noted that

the example described above is highly simplified for purposes

of this discussion. In reality, an infinite set of curves, bounded

by main curves for drainage and imbibition, describe the

relative permeability function.

Relative permeability curves (Figure 4) can be used to

describe different types of multiphase flow regimes, all of

which may exist at any particular site (Williams and Wilder,

1971):

Zone I: LNAPL occurs as a potentially mobile, continuous

phase and saturation is high. Water is restricted to

small pores. The relative permeability of water is

low. Such conditions may be observed within large

mobile product accumulations.

Zone II: Both LNAPL and water occur as continuous phases,

but, generally, do not share the same pore spaces.

However, the relative permeability of each fluid is

greatly reduced by the saturation of the other fluid.

Such conditions may be representative of zones of

smaller mobile product accumulations at the water

table.

Zone III: LNAPL is discontinuous and trapped as residual in

isolated pores. Flow is almost exclusively the

movement of water, not LNAPL. Examples of such

conditions may be found within zones of residual

LNAPL retained below the water table.

LNAPL Migration at the Field Scale

Darcy’s Law

Various forms of Darcy’s Law may be used to describe fluid

migration in porous media under many conditions. For

example, a form of Darcy’s Law, including relative permeability

as a function of saturation, may be used to describe LNAPL

flow when LNAPL saturation is less than 100%. However,

movement of LNAPL can be examined using a simple

conceptual model to understand the effects of LNAPL physical

properties on mobility. The one-dimensional migration of

LNAPL in an LNAPL-saturated system can be represented

using the following form:

v = -(k ρ g / μ ) (dh / dl) (1)

where

v = Darcy velocity (L/T)

k = intrinsic permeability (L2

)

ρ = density of NAPL (M/L3

)

g = force of gravity (L/T2)

μ = dynamic (absolute) viscosity (M/L∗T)

dh/dl = hydraulic gradient of NAPL mass (L/L)

Hydraulic conductivity of the medium is proportional to the

density and inversely proportional to the viscosity of the

LNAPL. The potential mobility of various fluids may be

compared using the ratio of density to viscosity. High ratios

correspond to greater potential mobility. In the subsurface,

LNAPLS are subjected to biotic and abiotic (volatilization and

solubilization) weathering processes which change the

composition (mass fraction) of product (Johnson et al.,

1990b). The weathering process may alter the overall LNAPL

properties, such as increasing the viscosity. A significant

change in these properties will affect the potential mobility of

the LNAPL.

Field Scale Versus Pore Scale

While LNAPL migration at the pore scale can be described

using some of the physical relationships presented above,

migration and distribution of LNAPL at the field scale is

controlled by a complex combination of release factors, soil/

aquifer properties, and LNAPL characteristics (Mercer and

Cohen, 1990) including:

• volume of LNAPL released;

• release rate (e.g., one-time “slug” event vs. long-term

continual discharge);

• LNAPL infiltration area at the release site;

• properties of the LNAPL (e.g., density, viscosity);

• properties of the soil/aquifer media (e.g.,

permeability, pore size distribution);

• fluid/porous media relationships (e.g., wettability);

• lithology and stratigraphy; and

• macro-scale features (e.g., fractures, root holes).

To illustrate how these factors can combine to control LNAPL

migration, a series of conceptual models is presented below

which describe a general LNAPL release scenario. These

conceptual models are intended to convey several Figure 4. Hypothetical relative permeability curves for water and

an LNAPL in a porous medium.

II

Mixed Flow

III

Water Flow

Water Saturation

Kr = relative permeability

Kr

After Williams and Wilder (1971) 

1.00

0.01

LNAPL Saturation

1.00

Sir

0 100%

Sr

100% 0

 LNAPL

Kr Water

I

LNAPL

Flow

0.10 0.10

0.01
6

Laboratory column experiments (Abdul, 1988) indicate LNAPL

may migrate freely through the upper region of the vadose

zone where water is present at irreducible saturation.

Movement through regions of increasing water saturation at

the capillary fringe depends on displacement of water from

these zones. In general, increasing LNAPL head is required

to displace water with increasing depth within these zones.

Rate and volume of LNAPL release and capillary properties of

subsurface materials are factors that affect the depth of

LNAPL penetration within the water-saturated regions (Abdul,

1988). In general, increases in release rates and volumes

lead to increased LNAPL head and increased depth of

penetration. Increases in capillary forces associated with fine

grained materials result in increased LNAPL head required to

penetrate and displace pore water.

The conceptual model presented above is a relatively

simplified representation of potential subsurface conditions.

As discussed by Farr et al. (1990) and Lenhard and Parker

(1990), sharp interfaces between zones saturated with

LNAPL, water, and air generally do not exist in the subsurface

at most sites. Porous media above the water-saturated zone

are generally filled with varying saturations of LNAPL, water,

and/or air (Figure 7). Thus, the conceptual model often

reported in the literature indicating a discrete LNAPL-water

interface should not be assumed. Instead, it is reasonable to

assume that the relative saturation ranges reported for the

LNAPL and water phases in the mixed flow region (i.e., region

II) of Figure 4 are more representative of actual LNAPL-water

interface areas. From a remediation perspective, it is

apparent that LNAPL mobility is compromised due to the

water saturation in this interface area.

LNAPL Smearing Due to Fluctuating Water Table

Accumulations of LNAPL at or near the water table are

susceptible to “smearing” from changes in water-table

elevation such as those that occur due to seasonal changes in

recharge/discharge or tidal influence in coastal environments.

Mobile LNAPL floating above the water-saturated zone will

move vertically as the ground-water elevation fluctuates

(Figure 8). As the water table rises or falls, LNAPL will be

 LNAPL

Release

After Waterloo Centre for Groundwater Research (1989)

Figure 6. LNAPL affected by low permeability units prior to

reaching water table.

Vadose

Zone

Residual Saturation of

LNAPL in Soil

Ground-Water

Flow

After Waterloo Centre for Groundwater Research (1989)

Leachate

Dissolved

Contaminants

Hydrocarbon

Vapors

fundamental principles. However, actual release scenarios

are strongly affected by numerous site-specific parameters

that may not necessarily be represented in this discussion.

LNAPL Migration Through Vadose Zone

After release on the surface, LNAPL moves vertically

downward under the force of gravity. For small volume

LNAPL releases, all of the LNAPL may eventually be retained

in pores and fractures in the unsaturated zone (Figure 5).

Infiltration of water through the residual LNAPL due to

recharge or increased water-table elevations slowly dissolves

soluble constituents, resulting in an aqueous-phase

contaminant plume. Migration of vapors may also spread

contamination (Mendoza and McAlary, 1989). LNAPL

migration is influenced by heterogeneity in subsurface media

and may be complex. For example, it may preferentially

migrate laterally through more permeable pathways or

accumulate and migrate along low permeability layers above

the water table (Figure 6).

Accumulation at the Water Table

If a sufficient volume of LNAPL is released, it may migrate

through the unsaturated zone toward the water table and zone

of water held by capillary forces above the water table (i.e.,

capillary fringe). As LNAPL approaches the water table,

entering regions of increasing water saturation, it may migrate

laterally. Lateral migration is controlled by the LNAPL head

distribution. In general, migration may be expected to be

greatest in the direction of ground-water flow (i.e., maximum

decrease in water-table elevation). However, migration may

occur initially in other directions in response to the hydraulic

gradients induced by an LNAPL mound (Figure 1). A

relatively large LNAPL accumulation may result in

compression or collapse of the capillary fringe and, potentially,

depression of the water table.

Figure 5. LNAPL is retained at residual saturation in unsaturated

zone. Leaching by infiltrating water and migration of

hydrocarbon vapors result in ground-water

contamination.
7

- LNAPL trapped by capillary forces

- Mobile LNAPL

Prior to Rise in Water Table Following Rise in Water Table

After API (1989)

Figure 8. Effect of rising water table on LNAPL distribution in porous medium. A similar effect may be seen with a falling water table.

Water and Air

LNAPL, Water and Air

LNAPL and Water

Water

Porous Medium

Water

LNAPL

Air

Well

After Farr et al. (1990)

Figure 7. Conceptualization of a multiphase fluid distribution in

porous medium and monitoring well screened within

the medium.

retained in the soil pores, leaving behind a residual LNAPL

“smear zone”. If smearing occurs during a decline in groundwater elevations, residual LNAPL may be trapped below the

water table when ground-water elevations rise.

A similar situation may develop during product recovery

efforts. LNAPL will flow towards a recovery well or trench in

response to the gradient induced by water-table depression.

LNAPL residual will be retained below the water table as the

water-table elevation returns to pre-pumping conditions.

LNAPL Migration in Fractured Media

LNAPL introduced into an unsaturated, fractured medium

follows a complex pathway based on the characteristics of the

rock, such as fracture density, orientation, and aperture

distribution (Figure 9). The degree of fracture interconnectivity

may not be definable due to the extreme heterogeneity of

most fractured systems and the lack of economical aquifer

characterization technologies (U.S.EPA, 1992a). Many clay

units, once considered to be impermeable, often act as

fractured media with preferential pathways for vertical and

horizontal NAPL migration. Although this concern is

particularly true for DNAPL, it is applicable to LNAPL in

unsaturated systems.
8

Figure 9. Potential LNAPL migration in fractured medium.

phases when the system is at equilibrium. Most volatilization

calculations are dependent on the assumption that the LNAPL

and air are in chemical equilibrium (Johnson et al., 1990a).

However, it is reasonable to assume that equilibrium

conditions do not occur in many volatilization-based

subsurface remediation systems. The actual transfer process

in a dynamic and complex subsurface system must take into

account numerous parameters that are not included in these

general partitioning equations. A practical approach to

evaluating soil ventilating systems involves an in-depth

analysis of numerous site-specific parameters and processes

(Johnson et al., 1990b).

Dissolution

A NAPL in physical contact with ground water will dissolve

(solubilize, partition) into the aqueous phase. The solubility of

an organic compound is the equilibrium concentration of the

compound in water at a specified temperature and pressure.

For all practical purposes, the solubility represents the

maximum concentration of that compound in water. The

solubilities of the compounds most commonly found at

Superfund sites range over several orders of magnitude.

Several parameters affecting solubility include temperature,

pH, cosolvents, dissolved organic matter, and dissolved

inorganic compounds (salinity).

For a multicomponent NAPL in contact with water, the

equilibrium dissolved-phase concentrations may be estimated

using the solubility of the pure liquid in water and its mole

fraction in the NAPL mixture (Feenstra et al., 1991). The

maximum concentration that can be achieved in this scenario

is referred to as the effective solubility, as indicated in

Equation 2.

Se

i

 = Xi Si (2)

where

Se

i

 = effective aqueous solubility of compound i in

NAPL mixture

Xi

 = mole fraction of compound i in NAPL mixture

Si

 = aqueous solubility of the pure-phase compound

Two examples are presented which illustrate the effect of

dissolution on multicomponent LNAPLs. First, consider a twocomponent NAPL with equal mole fraction (0.5), where the

solubilities of the pure-phase compounds are 1000 mg/l and

10 mg/l, respectively, and the effective solubilities are 500

mg/l and 5 mg/l, respectively. The more soluble compound

will potentially partition into ground water 100-fold more

readily than the less soluble compound. According, less

soluble compounds will primarily be associated with the NAPL

phase and dissolution and transport in the aqueous phase will

be limited relative to more soluble components. Second,

consider a “gasoline” containing numerous compounds

(Johnson et al., 1990b) where the mole fractions of benzene

were 0.0076 and 0.0021 for fresh and weathered gasoline,

respectively. The solubility for benzene is 1780 mg/l

(U.S.EPA, 1990). Yet, the predicted effective solubility would

be only 13.5 mg/l and 3.78 mg/l for fresh and weathered

gasoline, respectively.

LNAPL Migration Through Man-Made Pathways

LNAPL will move through man-made preferential pathways,

such as improperly grouted monitoring wells or trenches

containing distribution piping or utilities. Trenches backfilled

with pea gravel or other coarse-grained material can provide a

horizontal pathway for LNAPL migration in the near-surface

environment.

FATE OF LNAPLs IN THE SUBSURFACE

As illustrated in Figure 3, organic compounds which compose

NAPLs may partition among four separate phases in the

subsurface: gaseous, solid, aqueous, and NAPL. The

subsurface fate of chemicals in these phases is largely

determined by volatilization, dissolution, sorption, and

degradation processes.

Volatilization

Volatilization of organic compounds occurs by two primary

pathways: volatilization from water and NAPL. Henry’s Law

describes the partitioning of an organic compound between

the aqueous and gaseous phases. For a dilute solution,

concentrations less than approximately 10-3moles/l, the ideal

gas vapor pressure of a volatile solute is proportional to its

mole fraction in solution. More simply stated, the escaping

tendency of the solute molecules from the water phase to the

air phase is proportional to the concentration in the water.

This relationship assumes local equilibrium between water

and air and is useful for estimating the potential for organic

chemical transport from water to air, and from hydrocarbon

vapors to water (Mendoza and McAlary, 1989). For more

concentrated solutions or pure phase compounds,

volatilization is best described by Raoult’s Law, which states

that the vapor pressure over a solution is equal to the mole

fraction of the solute times the vapor pressure of the pure

phase liquid. Again, this is a measure of the escaping

tendency of molecules from the NAPL to the gaseous phase.

Henry’s Law and Raoult’s Law are useful to describe the

partitioning of organic compounds between fluid and gaseous

Sand

Vadose

Zone

Residual

LNAPL

 Fractured

Rock or

Clay

After Waterloo Centre for Groundwater Research (1989)
9

The effective solubility represents the concentration that may

occur at equilibrium under ideal conditions. Laboratory

studies (Banerjee, 1984) indicate that effective solubilities

calculated using Equation 2 are reasonable approximations

for mixtures of organic liquids that are hydrophobic,

structurally similar, and have low solubilities. Effective

solubilities of components in more complex mixtures, such as

petroleum products, appear to be in error by no more than a

factor of two (Leinonen and Mackay, 1973). However, the

degree to which these compounds partition to the water phase

is a function of many variables including cosolvency (Rao et

al., 1991). Cosolvency effects may occur in cases where

dissolution of highly soluble components (e.g., alcohols)

significantly increase the solubility of other components.

In general, higher dissolution rates may be associated with

higher ground-water velocities, higher LNAPL saturation in the

subsurface, increased contact area between LNAPL and

water, and LNAPLs with a high fraction of soluble components

(Mercer and Cohen, 1990; Miller et al., 1990). However,

recent studies (e.g., Powers et al., 1992) indicate that nonequilibrium effects may limit contaminant mass transfer by

dissolution under certain conditions such as high groundwater velocity. Laboratory and modeling studies conducted by

many researchers (e.g., Borden and Kao, 1992; Geller and

Hunt, 1993; Powers et al., 1991) indicate that complete

dissolution of an LNAPL may require hundreds or thousands

of pore volumes of water under ideal field conditions. These

studies observed initially high aqueous contaminant

concentrations which were followed by a period of rapid

decline and an asymptotic period during which concentrations

declined slowly.

Sorption

Sorption is defined as the interaction of a contaminant with a

solid (Piwoni and Keeley, 1990). In soil or aquifer material

contaminated with LNAPL, contaminants from the LNAPL will

partition onto solid phase material. The primary pathway in

which this process occurs is through the water phase, as

indicated in Figure 3. For example, when LNAPL is released

into the subsurface, components will dissolve into the

aqueous phase, then partition onto aquifer material.

Numerous parameters affect sorption at hazardous waste

sites including solubility, polarity, ionic charge, pH, redox

potential, and the octanol/water partition coefficient (Piwoni

and Keeley, 1990).

In general, solid-phase (adsorbed) contaminants may

represent a small fraction of the total contaminant mass in soil

and aquifer material where continuous phase or residual

NAPL exists. The majority of the contaminant mass in these

systems is typically present in the nonaqueous liquid phase.

Desorption of the contaminant (i.e. mass transfer of

contaminant from the solid phase to the water phase) is often

a rate-limited step and is partially responsible for the tailing

effect commonly observed in ground-water pump-and-treat

systems.

Analytical results from an LNAPL-contaminated aquifer or soil

sample extracted and analyzed in the laboratory represent the

total contaminant mass associated with the sample. Careful

evaluation of the chemical and physical properties of the

contaminant(s) and soil/aquifer sample is necessary to

evaluate the phase(s) in which the contaminant exists. A

practical approach has been developed to evaluate

contaminant phase distribution by applying equilibrium

partitioning theory (Feenstra et al., 1991).

Biodegradation

Many of the LNAPL-related compounds are amenable to

biological degradation in the aqueous phase by naturally

occurring microorganisms in the subsurface. However, there

is an important distinction between aqueous-phase and NAPL

biodegradation. The distinction is the inability to create and

maintain conditions that are conducive to microbial activity

within a NAPL. In brief, biodegradation of pure phase

hydrocarbon does not appear to be practical and has not been

demonstrated. Considerable research has focused on

evaluating aerobic and anaerobic biodegradation and

transformation processes. These processes play an important

role in the ultimate fate of LNAPLs in the subsurface, both in

the form of naturally occurring and actively engineered

remediation processes (Norris et al., 1994).

LNAPL SITE CHARACTERIZATION

Success or failure of an LNAPL remediation program depends

in large measure on the remediation objectives and adequacy

of the site characterization. This section will focus on issues

pertaining to LNAPL investigations. Discussion of strategies

and techniques for characterization of aqueous-phase and

sorbed contamination will be limited. More information

regarding site characterization concepts, techniques, and

strategies is provided by U.S.EPA (1991) and Cohen and

Mercer (1993).

The focus of site characterization is often to provide

information necessary to define and evaluate potential

remedial options. Specific objectives may include

determination/delineation of subsurface contamination in the

aqueous, gaseous, solid, and LNAPL phases; mobile and

residual LNAPL; migration rates/directions of the mobile

phases; geologic controls on LNAPL movement; LNAPL

properties; and other pertinent fluid/media properties. The

level of detail and the type of data required will be sitespecific, partially dictated by the remedial technologies under

consideration and practical economic constraints, and often

restricted by available characterization technologies. Such

limitations include lack of practicable methods for detailed

delineation of many parameters of interest including LNAPL

distribution and saturation and hydraulic conductivity

distributions.

Conceptual Model

The conceptual model is the interpretation and assimilation of

all site-related information into assumptions and hypotheses

regarding contaminant sources, subsurface contaminant

distribution, and dominant transport/fate processes. The basis

for this conceptual model may include one or more of the

conceptual models discussed in the previous section.

Characterization is best conducted in a phased approach,

beginning with a review of site history, contaminant properties,

and regional/local studies. This review should include the

following (API, 1989):
10

1. Information on storage, transportation, use,

monitoring, and disposal of LNAPLs at the site;

2. Locations, volumes, and timing of any known LNAPL

releases;

3. Locations of underground piping, structures, or

utilities which might influence LNAPL flow;

4. Regional/local geologic and hydrogeologic studies,

soil surveys, climatic data, and pertinent maps/

historic photographs of the site; and

5. Preliminary information, available from the literature,

concerning pertinent contaminant transport and fate

parameters for the site-specific contaminants.

An initial conceptual model for contaminant distribution,

transport, and fate at the site is then formulated and used to

plan further characterization. Each characterization phase is

designed to test and refine this model. The iterative process

continues throughout remedial design and during remedial

operations.

Soil/Aquifer Material

Lithologic, stratigraphic, and structural features may often be

dominant influences on LNAPL movement. As expected,

LNAPLs migrate faster through relatively permeable features

(e.g., root holes, fractures, sandy layers, utility trenches, etc.)

in the unsaturated zone than through less permeable

materials. Characterization of subsurface heterogeneity to the

extent practicable may provide valuable information

concerning potential LNAPL migration and distribution.

Analyses of geologic materials exposed in trenches and

obtained from borings may be used at some sites to improve

the conceptual model and identify features that potentially

serve as preferential pathways for LNAPL. Non-invasive

geophysical methods may also provide information regarding

the distribution of geologic materials and stratigraphic or

structural features that may control contaminant migration.

However, detailed characterization of heterogeneity and

LNAPL distribution often will not be possible due to

subsurface complexity and limitations in characterization tools

and techniques (U.S.EPA, 1992a).

Fractured rock and karstic settings may represent extreme

examples of heterogeneous environments. These settings

pose exceptionally difficult problems in site characterization

due to the complexity of transport pathways. Efficient

techniques for characterizing contaminant transport (i.e.,

LNAPL, aqueous phase, and gaseous phase) at such sites

are not available currently.

Hazards of Invasive Characterization Methods

Certain hazards may exist during invasive characterization at

LNAPL contamination sites. The potential for increasing the

vertical extent of contamination should be considered when

evaluating drilling and well installation programs. Drilling

through mobile LNAPL (e.g., liquids perched on low

permeability units above the water table) may result in

contaminating deeper intervals. Such risks may be reduced

using appropriate techniques such as detailed observation

and screening of geologic materials. The risk of explosion or

fire (API, 1989) may exist at sites contaminated with

flammable materials (e.g., most liquid petroleum products).

During drilling operations, LNAPL, contaminated soil and

aquifer material, and vapors are brought to the surface where

conditions for ignition may exist. LNAPL is not only a concern

as a fire and explosion hazard, but also with regard to

chemical exposure to the driller and sampling crews.

Monitoring and mitigation of explosion and exposure risks

should be considered when planning field operations.

Information from Borings and Excavations

Multiple borings with continuous sampling of subsurface

media or excavation of pits/trenches will generally be required

to define stratigraphy and estimate properties of the

subsurface media and fluids. Media properties useful in

contaminant fate and transport studies at NAPL sites include

texture, porosity, permeability, organic carbon content,

isotropy, and heterogeneity. Estimation of fluid/media

properties, such as fluid saturation and capillary pressuresaturation relationships, may also be useful. Such data may

provide the basis for evaluating potential LNAPL distribution

and mobility at the macro scale. Various laboratory methods

for measuring such properties are discussed in Cohen and

Mercer (1993). Applicability of these methods will be site

specific. Hydrogeologic information, including the depth to

ground water, delineation of perched ground water or LNAPL,

hydraulic gradient, and hydraulic conductivity of saturated

materials, is also essential. This information may be used to

evaluate ground-water flow directions which will also be the

potential flow directions for the LNAPL. Characterization of

temporal as well as spatial variations in many of these

parameters generally will be required. For example,

information on seasonal variations in water-table elevations

may aid in evaluating mobile LNAPL flow directions and

LNAPL distribution above and below the average water-table

elevation. Installation of wells will be required to obtain much

of this information. These data will provide the basis for

evaluating LNAPL mobility on the site scale.

Methods for direct detection of NAPL in soil samples are

relatively limited. Visual observation often has been relied

upon to make this determination in the field. However, it may

be difficult or impossible to observe NAPLs which are

colorless or clear, heterogeneously distributed in the sample,

or present at low saturations (Huling and Weaver, 1991).

Recently, qualitative techniques to enhance rapid, visual

observations of NAPL in drill cuttings or cores have been

evaluated (Cohen et al. 1992). Centrifugation, examination

under ultraviolet light, soil-water separation tests, and addition

of hydrophobic dye to preferentially stain NAPL (i.e., Sudan

IV) were studied. Of these methods, the use of hydrophobic

dye and ultraviolet light examination for fluorescent NAPLs

were found to be the simplest and most effective methods.

These methods provide only limited, qualitative information

concerning the presence of LNAPLs. However, this

information may be useful to delineate the presence of LNAPL

along a vertical profile when limited resources prevent the use

of more expensive, quantitative techniques.

Screening of soil headspace vapors using a flame ionization

detector was also found useful in evaluating the presence of

volatile NAPL (Cohen et al., 1992). However, a poor

correlation between NAPL saturation in the samples and
11

headspace vapor concentrations was obtained. This result

was principally due to physical constraints on contaminant

concentrations in the vapor phase.

Chemical analysis of soil samples will generally be required to

identify individual compounds and their concentrations and aid

in evaluating NAPL present at low residual saturations (Huling

and Weaver, 1991). Chemical analyses do not directly

distinguish between gaseous, aqueous, sorbed, and NAPL

contaminants. However, relatively high constituent

concentrations indicate the constituent may be present as a

NAPL or sorbed to subsurface materials (Huling and Weaver,

1991). An example of sampling and analysis techniques

applied at the site of an aviation gasoline spill to determine the

LNAPL distribution in the capillary fringe is described by

Ostendorf et al. (1992).

Feenstra et al. (1991) proposed a method to indirectly assess

the presence of NAPL in soil samples by applying equilibrium

partitioning theory. Application of this method to DNAPL

assessments is explained in U.S.EPA (1992b). In the

absence of NAPL, there is a theoretical maximum contaminant

mass which can be present in a sample. The maximum

constituent concentration in the sample is dependent on the

constituent solubility in water, the sorptive capacity of the soil,

and the saturated soil gas concentration. If NAPL is present,

the constituent concentration detected in the sample will

exceed the calculated maximum concentration and the

calculated constituent concentration in pore water would

exceed the effective constituent solubility. This method

requires determination of soil moisture content, organic

carbon content, porosity, potential LNAPL composition,

sorption parameters, effective solubilities, and total constituent

concentrations in soil samples. The methods are most useful

when relatively large quantities of NAPL are present, pore

water/soil partitioning coefficients are determined rather than

estimated from available literature, and effective solubilities

are measured. However, many of these data may be

unavailable at many sites and the methods are less reliable

when parameters are estimated rather than measured (Cohen

and Mercer, 1993; Feenstra et al., 1991).

Cone Penetrometer

The cone penetrometer (ASTM Standard D3441) is a

geotechnical tool that originated in the construction industry.

This tool is capable of rapidly providing valuable stratigraphic

information useful in assessing potential LNAPL distribution.

Data collected during penetrometer testing have also been

used to evaluate soil saturation, potentiometric surfaces, and

horizontal soil permeability. In its simplest form, truckmounted hydraulic rams force a cone-shaped instrument

containing strain gages through the soil, sending back

continuous readings of tip resistance, sliding friction, and

inclination. Correlations developed between subsurface

materials cored in adjacent borings and corresponding

penetrometer measurements may allow rapid interpretations

of stratigraphy in each test hole (Chiang et al., 1992).

Penetration depths depend on the subsurface material

properties but are often limited to less than approximately

thirty meters.

Modified penetrometers have been equipped to collect

subsurface samples of fluids and aquifer materials useful in

delineating LNAPL (Chiang et al., 1992). Other researchers

are incorporating additional in situ sensing technology into

cone penetrometers (Seitz, 1990) for real-time detection of

organic contaminants which may eventually provide a rapid

means for defining LNAPL distribution. It should be noted that

such methods are still in the developmental stage and should

be applied with caution.

Geophysical Methods for Hydrogeologic Characterization

Surface geophysical methods, where applicable, allow noninvasive investigation of subsurface physical and chemical

properties. Certain techniques, including seismic, electrical,

magnetic, and ground-penetrating radar, have been applied

successfully to site characterization (e.g., Benson et al., 1982;

Walther et al., 1986). These methods may provide valuable

hydrogeologic information concerning lithologic and

stratigraphic boundaries, fracture orientation, locations of

underground utilities, and depths to ground water and

bedrock. Variations of these and other methods are useful

borehole techniques for hydrogeologic site characterization

(Keys and MacCary, 1971; Keys, 1989). Use of such

techniques should be considered as part of an integrated

approach to delineation of hydrogeologic controls on groundwater flow and potential LNAPL migration.

LNAPL

Light nonaqueous phase liquids may exist as continuous, freephase liquids and/or as residual liquids trapped by capillary

forces above and below the water table. Evaluation of LNAPL

remedial options requires delineation of LNAPL distribution,

composition, and properties. Installation of wells and

excavations coupled with previously discussed soil sampling

and analysis generally will be required to estimate distribution

and potential mobility. LNAPL fate, transport, and distribution

has been presented from a theoretical and conceptual model

perspective. However, site characterization may be

technically challenging due to the heterogeneity of subsurface

media and variability of subsurface conditions. It is not

uncommon to observe a “patchy” distribution of LNAPL over a

relatively small area at a site, or the transient presence of

LNAPL in a well.

Mobile LNAPL

Monitoring wells, borings, and test pits installed in areas of

potential LNAPL releases are one of the key sources of

information concerning the distribution of mobile LNAPL.

Wells used for this purpose must be screened across the air/

LNAPL interface and the water table to provide uninhibited

access for floating LNAPL (API, 1989). The screen generally

should encompass the water table and LNAPL during

seasonal fluctuations in ground-water elevations. Additional

design considerations include use of appropriate construction

materials. In general, conventional construction practices use

a filter pack coarser than the surrounding formation to allow

entry of the LNAPL. However, research regarding

construction materials specifically designed for LNAPL

monitoring is relatively limited. Recent studies (Hampton and

Heuvelhorst, 1990; Hampton et al., 1991) of filter packs for

LNAPL recovery wells indicated traditional design practices

may not produce an optimum recovery well. These studies

indicated LNAPL recovery rates were increased using packs

with a grain size approximately half of conventional

recommendations for recovery wells. Based on such results,
12

Well

Air

Well

Screen

Apparent

LNAPL

Thickness

Water

Water

Air and Water

LNAPL Trapped by

Capillary Forces

Mobile LNAPL

Water

Table

Capillary

Fringe

Figure 10. Simplistic conceptualization of LNAPL thickness

measured in well and LNAPL distributed information.

it appears that additional research regarding optimum filter

pack design for LNAPL monitoring wells may be warranted. In

addition, compatibility of materials, including pumps, casing,

screens, and bentonite, with the particular LNAPL must be

considered (Mercer and Cohen, 1990). The use of additional

tools such as well points and grab samplers driven to desired

depths (e.g., Smolley and Kappmeyer, 1991) may provide

useful screening information for monitoring network design.

Non aqueous phase liquids held in pore spaces under tension

as a result of capillary forces are not free to migrate to wells

and boreholes. Such contamination may be more extensive

than the mobile LNAPL and may be a major source for

continuing contamination of ground water. Monitoring wells

are not useful for defining this contamination (Abdul et al.,

1989). Thus, lack of detection of mobile LNAPL is not an

indication of the absence of such liquids.

Apparent LNAPL Thickness

The LNAPL thickness measured in a monitoring well has been

reported to typically exceed the LNAPL-saturated formation

thickness by a factor estimated to range between

approximately 2 and 10 (Mercer and Cohen, 1990). Due to

this difference, the LNAPL thickness measured in a monitoring

well has been referred to as an apparent thickness (Figure

10). This difference generally will not be uniform throughout a

site as a result of heterogeneity (Abdul et al., 1989).

In a well screened across the water table and capillary fringe

(Figure 10), the difference has been related to several factors.

These factors include capillary forces in the formation,

hydrocarbon density, and volume/rate of LNAPL release (e.g.,

Blake and Hall, 1984; Hall et al., 1984). In general, the

difference increases with decreasing grain size of the

formation materials due to increased capillary forces and

capillary fringe height (Mercer and Cohen, 1990). The

ground-water elevation in the well will be lower than the

elevation of the capillary fringe on which the mobile LNAPL

may accumulate as the capillary fringe does not exist within

the well. The LNAPL may then migrate to the well, a low point

on the capillary fringe. However, thick LNAPL accumulations

in the formation may depress the capillary fringe. In this case,

the difference between the apparent thickness and the true

thickness may be reduced (Testa and Paczkowski, 1989).

This difference also increases with increasing density of the

LNAPL due to increased depression of the LNAPL/water

interface in the well (Hall et al., 1984). The weight of the

LNAPL column in the well depresses the interface. The

greater the LNAPL density, the greater the weight of the

LNAPL column and the greater the depression of the LNAPL/

water interface.

The relative difference between actual and apparent LNAPL

thicknesses may also be affected by the existence of LNAPL

perched on lower permeability layers above the water table

and capillary fringe. The apparent LNAPL thickness

measured in the well may be related to the height of the

mobile, perched LNAPL above the water table (Testa and

Paczkowski, 1989). Wells installed through such perched

zones provide potential pathways for LNAPL flow.

Fluctuations in ground-water elevation may affect the LNAPL

thickness measured in wells (Blake and Hall, 1984;

Kemblowski and Chiang, 1990; Yaniga, 1984). A gradual

decline in the water table may result in increased volume of

mobile LNAPL and increased apparent thickness due to

drainage from the unsaturated zone (API, 1989). Preferential

fluid flow through the well, particularly in low permeability

formations, may also result in increased apparent LNAPL

thickness during a water-table decline (Kemblowski and

Chiang, 1990). In periods of rising water-table, a reduction in

apparent thickness due to compression of the capillary fringe

may occur (Blake and Hall, 1984; Yaniga, 1984). A rising or

falling water table may promote entry of mobile LNAPL into

areas not previously contaminated with these liquids or

regions of lower LNAPL saturation. This results in trapping of

additional LNAPL in soil pores with reduction in the volume of

mobile LNAPL (Figure 8). Changes in apparent LNAPL

thickness correlated with tidal induced fluctuations in groundwater elevation have also been noted in several studies (e.g.,

Hunt et al., 1989).

Measurement Techniques for Apparent LNAPL Thickness

Common methods for measuring the apparent LNAPL

thickness in wells include the use of measuring tapes coated

with water-sensitive and hydrocarbon-sensitive pastes,

interface probes, and transparent bailers (API, 1989). A steel

measuring tape coated with pastes sensitive to the presence

of water and hydrocarbons may be used to determine the

depths to the air/LNAPL and LNAPL/water interfaces by

monitoring color changes in the pastes. Alternatively, an

electronic probe designed to detect the electrical

conductivities of different fluids (interface probe) may be used

to determine the depths to the interfaces. A third common

method is the direct measurement of LNAPL thickness in a

transparent, bottom-filling bailer. The bailer is slowly lowered

until it spans the LNAPL layer, withdrawn, and LNAPL
13

and apparent LNAPL thickness. In a controlled study,

Wickramanayake et al. (1991) compared the methods

proposed by De Pastrovich et al. (1979), Hall et al. (1984),

and Lenhard and Parker (1990) to estimate LNAPL volume

from a known release. A known quantity of JP-4 fuel was

released into a model aquifer equipped with observation wells.

In this study, the method proposed by Lenhard and Parker

(1990) provided the best estimate of LNAPL release after the

system had reached equilibrium. However, all estimates were

within an order of magnitude of the actual release volume.

Durnford et al. (1991) identified several potential limitations in

applying many of these relationships. These problems

include:

1. Water table fluctuations can result in differences in

apparent LNAPL thickness without significant in situ

changes. This is a major source of uncertainty in

estimating in situ distribution.

2. Several of these relationships are based on soil and

fluid properties that require measurements of

capillary pressure-saturation curves which are

difficult to obtain and may not be representative of

actual field conditions.

3. Spatial variability in subsurface properties (i.e.,

heterogeneity) and the effects on apparent and in

situ LNAPL thickness are not easily evaluated using

these relationships.

4. LNAPL held by capillary forces above and below the

water table, which is an important potential source for

ground-water contamination, is not addressed using

many of these methods.

In addition, many of the methods are based on an assumed

equilibrium distribution of the fluids. Such assumptions will not

be applicable at many sites. This implies that simple

relationships and proportionalities may not be sufficient to

estimate mobile LNAPL thickness or volume from apparent

thickness information. In addition, methods requiring

estimates of subsurface media properties may be subject to

much uncertainty due to heterogeneity and uncertainty in

parameter estimates. However, the techniques described

above may yield order-of-magnitude estimates of mobile

LNAPL distribution at some sites. In summary, proven field

methods for accurate and reliable estimation of mobile LNAPL

volume using well thickness information are not currently

available. Further research and development of methods for

directly assessing subsurface LNAPL distribution are

warranted.

Geophysical Methods for Contaminant Detection

Application of geophysical methods for direct detection of

organic contamination, including LNAPL, is currently an area

of research and development (U.S.EPA, 1992a). Surface

geophysical methods which have been applied with limited

success at a small number of sites include ground penetrating

radar, complex resistivity, electromagnetic induction, and

direct current resistivity (e.g., Olhoeft, 1986; King and Olhoeft,

1989; Holzer, 1976). Borehole techniques which have been

studied include dielectric logging (Keech, 1988) and a driven

probe equipped with galvanic and conductivity circuits for

sensing soil moisture and a hydrocarbon indicator for sensing

LNAPL (Hampton et al., 1990). Such techniques are currently

in the developmental stage and should generally be

thickness is measured. LNAPL thickness in the bailer may be

slightly greater than the apparent thickness present in the well

due to fluid displacement by the bailer walls (API, 1989;

Mercer and Cohen, 1990). Detection of clear or colorless

LNAPLs may be enhanced by addition of a hydrophobic dye

that preferentially stains the organic liquid (Cohen et al.,

1992).

Recharge and Bail-Down Tests

Methods used to estimate mobile LNAPL thickness in the

formation have included studies referred to as bail-down,

recovery, or recharge tests (e.g., Hughes et al., 1988;

Gruszczenski, 1987). These tests involve monitoring LNAPL

recharge to a well following its removal by pumping or bailing.

The mobile LNAPL thickness in the formation at each well is

then estimated by various interpretations of depth-to-product,

depth-to-water, and product thickness as a function of time.

Several potential sources for error exist in the performance

and interpretation of these tests. In general, these

procedures have not been adequately proven in a variety of

field situations (Testa and Paczkowski, 1989) and do not

provide information concerning LNAPL trapped by capillary

forces (Durnford et al., 1991). However, the performance of

such tests may provide qualitative information concerning the

potential for LNAPL recovery using conventional pumping

technology (API, 1989).

Apparent LNAPL Thickness Relationships

Studies regarding the relationship between apparent LNAPL

thickness and in situ LNAPL distribution have been reported

by many researchers (e.g., Ballestero et al., 1994; Blake and

Hall, 1984; Farr et al., 1990; Gruszczenski, 1987; Hall et al.,

1984; Hampton and Miller, 1988; Kemblowski and Chiang,

1990; Lenhard and Parker, 1990; Pantazidou and Sitar, 1993;

Testa and Paczkowski, 1989; Yaniga, 1984; Yaniga and

Warburton, 1984). Methods proposed for estimating in situ

mobile LNAPL thickness and, ultimately, LNAPL volume range

from simple relationships based on hydrocarbon density (e.g.,

De Pastrovich et al., 1979) to more complex relationships

incorporating properties of the porous media (e.g., Farr et al.,

1990). Commercial software packages implementing some of

these methods are currently available.

Based on laboratory studies and a review of many of these

methods, Hampton and Miller (1988) concluded that the

relationships investigated in their study were not sufficient to

reliably predict hydrocarbon thickness in the formation. Of

these relationships, the De Pastrovich et al. (1979) equation

was found to yield crude, order-of-magnitude approximations

of mobile LNAPL thickness. Wagner et al. (1989) compared

estimates using various techniques including simple and

complex relationships, bail-down tests, and chemical analysis

of soil samples. The study indicated that estimates from baildown tests, analysis of soil samples from a test pit, a

developmental hydrocarbon-sensing probe, and the

relationship proposed by De Pastrovich et al. (1979) yielded

comparable results at one field site. However, none of the

aforementioned methods have been adequately evaluated

under a variety of controlled and field conditions.

Farr et al. (1990) and Lenhard and Parker (1990) developed

methods for evaluating LNAPL volume in porous media under

equilibrium conditions based on fluid and media properties
14

considered research applications. The utility of geophysics at

most sites will not be in the direct detection of LNAPL, but in

hydrogeologic site characterization (U.S.EPA, 1992a).

LNAPL Samples

Determination of physical and chemical properties of LNAPL

obtained from wells or separated from soil samples will often

be required to evaluate many aspects of LNAPL site

characterization and remedial design. For example,

information concerning physical properties such as density

and viscosity may be used to assess LNAPL mobility and

distribution (Reidy et al., 1990; Cohen and Mercer, 1993) and

potential extraction designs. Analyses of LNAPL will be

necessary to determine the chemical composition which may

be used to compute the effective solubility of LNAPL

components, identify potential LNAPL sources, and evaluate

applicability of certain remedial technologies such as soil

vapor extraction.

Depending on the study objectives and potential LNAPL

composition, many analytical methodologies may be

applicable for chemical analysis of LNAPL samples. Common

techniques include infrared spectrometry, gas and liquid

chromatography, mass spectrometry, and nuclear magnetic

resonance spectrometry. Such techniques are used to

qualitatively and quantitatively determine hydrocarbon

composition. Cohen and Mercer (1993) describe many of

these techniques in the context of characterizing DNAPL

samples. This information is also applicable to LNAPL sample

characterization. Multiple techniques may be required to

characterize complex, multicomponent LNAPL mixtures. In

addition, analyses for indicator parameters such as total

petroleum hydrocarbons and total organic carbon may be

useful in screening level investigations at some sites.

Current guidance dictates LNAPL samples should be obtained

prior to purging of the well. Purging of the LNAPL from the

well prior to sampling may be conceptually desirable to obtain

a more representative sample. However, in many cases

purging may result in an inability to sample due to slow or no

LNAPL recharge and emulsification of the sample. Specific

sampling techniques should be evaluated with regard to study

objectives and site conditions. A bailer will generally be

adequate for LNAPL sample collection from wells (API, 1989).

Additional equipment such as a bladder pump, peristaltic

pump, or specialized equipment used for LNAPL recovery

may be useful at many sites.

Soil Gas

Depending on site conditions, soil gas analysis for volatile

organic compounds may be useful in locating contaminants

present in the subsurface. Several techniques have been

developed for conducting such surveys (e.g., Marrin and

Kerfoot, 1988; Thompson and Marrin, 1987). However,

limitations in the use and interpretation of data from soil gas

surveys exist (Kerfoot, 1988; Marrin, 1988). Soil gas surveys

may provide qualitative information useful as a screening tool

in delineating areas of LNAPL contamination as well as

aqueous-phase contamination (e.g., Evans and Thompson,

1986; Devitt et al., 1987). Anomalously high soil gas

concentrations may be an indication of NAPL in the vadose

zone near the sample location.

Ground Water

Ground-water elevations in wells containing LNAPL require

correction for the depression of the LNAPL/water interface in

the well to obtain total hydraulic head. The depression is

caused by the weight of the hydrocarbon. The correction is

accomplished by multiplying the apparent LNAPL thickness in

the well by the specific gravity of the LNAPL. The result is

then added to the elevation of the LNAPL/water interface to

obtain the total hydraulic head (API, 1989; Hudak et al., 1993).

The computed hydraulic head facilitates a more accurate

assessment of hydraulic gradient which leads to better

conclusions regarding potential contaminant sources, extent

of free-product accumulation, and optimal areas for focusing

remediation efforts (Hudak et al., 1993). It should be noted

that this approach may be inappropriate for conditions in

which LNAPL enters the well from a low-permeability unit

perched above the water table.

The influence of this correction on data interpretation is

dependent on several factors, including apparent LNAPL

thickness and hydraulic gradient. The greater the thickness of

LNAPL in the well and the lower the hydraulic gradient, the

greater the potential influence of this correction. The

necessity of performing this calculation should be evaluated

on a site-specific basis. Accumulation of LNAPL in a well

introduces an additional degree of uncertainty into the

calculation of hydraulic head and gradient. Density of the

LNAPL obtained from the well must be measured and, often,

will not be constant across a site due to such factors as

LNAPL release history and differences in the degree of

“weathering” following release. Measurement of the apparent

thickness using any of the methods previously described is

subject to a higher degree of uncertainty than measurement of

ground-water elevation in a well which does not contain

LNAPL. The additional uncertainty introduced by LNAPL

accumulation in wells is site specific and should be evaluated

during any investigation.

Although contaminant concentrations in ground-water

samples may not approach the effective solubility of each

compound, analyses for LNAPL components may aid in

evaluating the potential presence of LNAPL and delineating

potential source areas. Several factors may account for

lower-than-expected concentrations of constituents in ground

water from LNAPL zones. First, the effective solubility of a

single chemical from a multicomponent LNAPL is less than the

solubility of the pure chemical in water. The difference is often

an order of magnitude or more. Other explanations for the

relatively low concentrations of dissolved constituents in

LNAPL zones may include variability in subsurface LNAPL

distribution, mixing of ground water from different intervals in a

well during sampling, and effects resulting from non-uniform

ground-water flow (Cohen et al., 1992). Non-equilibrium

dissolution may also be a factor at some sites (Powers et al.,

1991).

REMEDIATION

Applicability of potential remedial technologies depends on

site-specific hydrogeologic characteristics, nature/distribution

of contaminants, and remedial objectives. Technologies for

removal of mobile LNAPL exist and may be applicable at

some sites. However, technologies for removal of residual

LNAPL, particularly those liquids trapped in the saturated

zone, are not well developed. Subsurface restoration to pre-
15

Wells

or

Sumps

Drain

Permeable

Fill

Ground-Water

Flow

Ground-Water

Flow

Mobile

LNAPL

Mobile LNAPL

Recovery Well with

Open Interval

LNAPL or

LNAPL/Water

Plan View Cross Section

After API (1989)

Figure 11. Plan and cross sectional views of a drain designed for mobile LNAPL recovery.

contamination conditions may require removal of virtually all

LNAPL and much of the contamination sorbed to aquifer

material. Technological limitations to complete LNAPL

removal may exist at many sites.

Excavation

The remedial alternative(s) selected for an LNAPL site may

depend on the volume of LNAPL released. If the volume is

small enough to be retained within the upper portion of the

vadose zone, excavation and above-ground treatment/

disposal may be selected in lieu of in situ remediation. Some

of the advantages of excavation are:

• Costs of shallow excavation are generally low

compared to other remediation technologies.

• Remediation time is relatively short.

• Experienced contractors are more readily available.

• A wide variety of contaminants often can be

remediated with a high degree of reliability.

The potential disadvantages include:

 • Contamination may not be completely removed due

to inability to delineate LNAPL distribution in detail.

 • Structures, roads, or other features can restrict the

area available for excavation.

 • Air emissions of volatile constituents may be

significant.

 • Costs of deep excavation may be high.

Mobile LNAPL Recovery - Trenches/Drains/Wells

Recovery potential for mobile LNAPL is controlled by such

factors as LNAPL viscosity and density and relative

permeability (Testa and Paczkowski, 1989). Liquids trapped

by capillary forces are not recoverable using conventional

trench, drain, or well systems. High LNAPL viscosity, high

residual water saturation, and low permeability reduce LNAPL

recovery rates. Under optimum circumstances, these systems

may remove less than 50% of the total LNAPL volume in the

subsurface (Abdul, 1992). It is estimated that only 20% to

30% of the total release volume is typically recovered (Testa

and Paczkowski, 1989). The remaining LNAPL will generally

be sufficient to result in continued ground-water

contamination. However, there may be important benefits to

mobile LNAPL recovery other than simple contaminant mass

removal.

Some of the other potential reasons for recovery of these

liquids include:

• Mobility reduction. Residual LNAPL held by capillary

forces is relatively immobile under normal

hydrogeologic conditions. Recovery of mobile

LNAPL will limit the migration of these liquids.

• Increase LNAPL transformation. Reduction in

LNAPL saturation may increase LNAPL surface area

and may result in more rapid dissolution,

degradation, and volatilization of the LNAPL. This

may be an advantage to the remediation program at

some sites.
16

Trench/Drain Systems

Trenches/drains may be used to recover mobile LNAPL at

shallow depths. Generally, they are used at sites where the

LNAPL is located within 15 to 20 feet of the surface with

access for construction equipment (U.S.EPA, 1988).

Depending on available excavation equipment, costs, and soil

stability, deeper drains may be cost effective (API, 1989).

Trench/drain systems provide the most hydraulically efficient

means for removing fluids from the aquifer. Therefore, these

systems often are well suited for low-permeability units and

heterogeneous sites which would require a large number of

wells to control LNAPL flow (API, 1989). Trenches are usually

excavated perpendicular to the general direction of groundwater flow, downgradient and/or within mobile LNAPL (Figure

11). The trench generally should be as long as the width of

the mobile LNAPL to provide containment. It should be

excavated to a depth several feet below the lowest expected

seasonal water-table elevation to prevent the migration of

LNAPL below the collection system. However, trenches for

recovery of LNAPL perched on low permeability layers should

not penetrate the supporting unit allowing further downward

migration to the water table. In general, trench width is not a

factor in LNAPL recovery (U.S.EPA, 1988) and may be as

narrow as possible to reduce costs (API, 1989). A low

permeability barrier may be installed on the downgradient side

of the trench to limit hydrocarbon migration while allowing

water to flow underneath. However, such a barrier may not be

an essential component of the design. In case of a trench/

drain constructed in fine-grained native materials using a

coarse grained fill, a capillary barrier effect may exist. Even

without installation of an artificial barrier, the non-wetting

LNAPL may tend to be trapped in the trench due to the higher

elevation of the capillary fringe in the formation on each side

of the trench. As long as product is removed from the trench,

LNAPL migration beyond the trench may be limited.

Open trenches may be converted to drains by backfilling with

appropriate graded filter materials (e.g., gravel). Sumps or

wells may be installed along the drain to collect LNAPL (API,

1989). Perforated pipe installed in the trench and connected

to the sumps may be used to improve system efficiency.

Recovery of LNAPL from trenches/drains or wells may be

accomplished using several options including pumps for

collecting total fluids (i.e., LNAPL and water) and skimming

systems and pumps for collecting only LNAPL (API, 1989;

U.S.EPA, 1988).

Trenches/drains may be operated to recover only LNAPL

migrating under the influence of the natural hydraulic gradient.

A system operated in this manner would generally result in a

relatively low recovery rate. Recovery time frames may also

be relatively long unless multiple trenches/drains are used.

Such systems should generally use continuous LNAPL

recovery to prevent accumulation and migration around the

ends of the trench (API, 1989). Ground-water extraction to

increase hydraulic gradients toward the trench/drain may be

used to increase LNAPL recovery rate and establish

hydrodynamic control. However, depressing the water table

may also result in LNAPL migration into deeper portions of the

saturated zone previously uncontaminated by these liquids. In

this situation, residual LNAPL will remain trapped below the

water table following mobile LNAPL recovery. The potential

consequences of such system operation should be carefully

considered before implementation.

Recovery Wells

Wells used to recover mobile LNAPL offer more flexibility in

system design and operation than trench/drain systems.

Wells may be most useful at sites with moderate to high

hydraulic conductivities, but may be used in less conductive

materials. Conventional recovery well designs have relied on

modified water-well design procedures to maximize well

efficiency and increase LNAPL recovery rates (Blake and

Lewis, 1983). However, well construction materials and

recovery equipment compatible with the LNAPL should be

used (Mercer and Cohen, 1990). Potential fire and explosion

hazards should also be considered.

Wells may be designed to remove only LNAPL, LNAPL and

water separately, or a combined fluid mixture (API, 1989;

Blake and Lewis, 1983; U.S.EPA, 1988). Well construction

depends on system design. Screens generally are set across

the air/LNAPL/water interfaces in the well. Such screens

should be long enough to encompass the anticipated changes

in position of these interfaces due to pumping and other

influences. Maximization of open screen area through use of

continuous wire-wrapped designs has been recommended

(Blake and Lewis, 1983) to maximize well efficiency and

extend intervals between maintenance. Well diameter

depends, in part, on the proposed pumping equipment. Many

conventional designs have used wells of 6-inch diameter or

larger. However, recent equipment innovations have allowed

use of smaller diameter wells.

Standard water-well design procedures traditionally have been

used for gravel pack design. However, studies by Sullivan et

al. (1988) indicate that filter packs which are too coarse may

reduce hydrocarbon recovery efficiency. Laboratory studies

reported by Hampton and Heuvelhorst (1990) indicate that

LNAPL recovery may be enhanced using hydrophobic filter

packs and a grain size approximately half as large as

traditional designs. Further laboratory studies (Hampton et al,.

1991) again indicated that hydrophobic materials were

superior to other materials for filter pack construction. Of the

remaining materials which were tested, the mixture containing

the least quartz, most angular grain shape, and least uniform

size distribution produced the most efficient filter pack.

Further research, including field studies, regarding well

construction for LNAPL recovery appear to be warranted.

Fluid recovery rate for wells designed to recover only LNAPL

generally will be relatively low. Thus, the reduction in

hydraulic head due to removal may be minimal. This results in

a relatively small capture zone around each well and,

generally, hydrodynamic control is not maintained. Conditions

in which such systems may be most applicable include

situations where water-table depression is not necessary or

desired and water treatment/disposal capacities are limited

(API, 1989).

Equipment designed for recovery of only LNAPL operates

using a variety of mechanisms (API, 1989) including:
17

diaphragm pumps or down-hole pneumatic pumps (API,

1989). Aqueous-phase constituent concentrations in

recovered water are also relatively high due to LNAPL/water

mixing. Although relatively high yields are possible (Hayes et

al., 1989), total fluids removal may be effectively used in

situations where the hydraulic conductivity is too low to permit

the efficient use of higher capacity pumps (API, 1989; Blake

and Lewis, 1983).

Operation of recovery well systems using total fluids recovery

may sometimes be enhanced by sealing the wellhead and

creating a vacuum within the well (API, 1989; Hayes et al.,

1989). Vacuum generation enhances recovery by reducing

pressure within the casing, increasing the effective head

difference between the formation and the well. Wells

constructed using this design may be installed with the top of

the screen below the mobile LNAPL (Hayes et al., 1989). The

water table is depressed sufficiently during pumping to allow

LNAPL to enter the well. This results in less vacuum loss

through minimal screening of the unsaturated zone.

Dual pump systems (Figure 12) use one pump located near

the bottom of the well to extract only water, creating a cone of

depression to initiate LNAPL movement towards the well (API,

1989; Blake and Lewis, 1983; U.S.EPA, 1988). A separate

Water

LNAPL

Mobile

LNAPL

LNAPL

Pump

Water

Pump Well

Screen

Filter

Pack

LNAPL

Sensors

After API (1989)

1. top mounted intakes allowing fluid collection from the

LNAPL/water interface,

2. density-sensitive float valves,

3. conductivity sensors for pump activation, and

4. hydrophobic filters that preferentially allow

hydrocarbons to pass.

Such equipment can recover LNAPL to a thickness of a

fraction of an inch for systems equipped with conductivity

sensors and to a sheen for systems using a hydrophobic

membrane (U.S.EPA, 1988).

Wells designed to recover total fluids (i.e., LNAPL and water)

may use surface mounted suction-lift or submersible pumps

(API, 1989). Recent applications of this technology are similar

to wellpoint dewatering systems used in the construction

industry (Hayes et al., 1989). In this application, several

closely spaced driven or drilled wells may be manifolded to a

single high volume vacuum pump. However, practical

recovery depths using suction-lift pumps are limited to

approximately 20 feet. Total fluids recovery systems generally

require LNAPL/water separation which may be difficult due to

emulsification. Emulsification problems can be minimized by

proper pump selection. Centrifugal pumps generally will

emulsify the LNAPL/water mixture more than surface

Figure 12. Conceptual system design for separate recovery of LNAPL and water in a single well.
18

pump located in the upper portion of the screened interval is

used to remove LNAPL. Both pumps rely on sensors to

ensure that only water and only LNAPL are extracted in their

respective pumps (API, 1989). Dual pump systems may also

use submersible or surface mounted pumps. Positioning of

the water pump generally should be well below the LNAPL/

water interface to minimize constituent concentrations and

treatment requirements for recovered ground water. Single

units, combining separate LNAPL and water pumps, equipped

with sensors to prevent pumping mixed fluids are also

available (U.S.EPA, 1988).

Dual well LNAPL recovery systems extract water and LNAPL

from separate wells. As with single well/dual pump systems,

one well produces only water and induces a hydraulic gradient

to promote LNAPL movement. The adjacent well is used to

recover LNAPL that accumulates in the cone of depression.

Conditions under which dual pump and dual well systems are

normally used include (API, 1989):

1. Water-table depression to increase LNAPL recovery

rates is desirable and not detrimental to the

remediation objectives.

2. Hydraulic conductivity and saturated thickness are

large enough to sustain flows of separate fluid

streams.

3. LNAPL/water separation and treatment capacities

are limited.

Installation of horizontal wells is an emerging technology in

the environmental field (Morgan, 1992). Such wells could

greatly increase contaminant removal efficiency, including

LNAPL removal, at some sites. Reported use of this

technology at contamination sites (e.g., Looney et al., 1992;

Oakley et al., 1992) has been limited. Additional information

regarding construction and use of horizontal wells is available

in U.S.EPA (1994).

System Design and Operation

At sites where a systematic design approach has been used,

traditional LNAPL extraction system designs generally have

relied on estimation of the recovery well capture zone. Well

locations are then selected to capture and remove

hydrocarbons (e.g., API, 1989; Blake and Lewis, 1983).

Injection of recovered ground water also has been used to

increase LNAPL recovery rates by increasing hydraulic

gradients and provide a hydraulic barrier to LNAPL migration

(e.g., Zinner et al., 1991). Careful evaluation of the effects of

reinjecting treated/untreated ground water is recommended to

minimize the potential for increasing subsurface

contamination.

Much of the optimization of LNAPL recovery has been

conducted in the field through manipulation of LNAPL and

water pumping rates. Several factors should be considered

during system design and operation. One important design

consideration is the effect of water-table depression used to

increase LNAPL recovery rates (API, 1989; Blake and Lewis,

1983; Chiang et al., 1990). Although high water extraction

rates initially may yield a higher hydrocarbon recovery rate,

the ultimate recovery may be greatly reduced. Much of the

LNAPL that was originally mobile may be smeared into

uncontaminated portions of the aquifer as the water table is

depressed, trapping it as immobile, residual LNAPL in soil

pores. This residual LNAPL will remain trapped below the

water table when the recovery system is turned off providing a

continuing source for ground-water contamination. At many

sites, multiple wells pumping water and LNAPL at lower rates

may ultimately recover more LNAPL than fewer wells pumping

water at higher rates to create large water-table depressions

(API, 1989). In addition, if the recovery rate exceeds the

LNAPL migration rate to the well, water saturation around the

well may increase, resulting in lower relative permeability with

respect to LNAPL and reduced recovery (Abdul, 1988; Chiang

et al., 1990).

Similar considerations exist for systems using water injection

to increase LNAPL recovery rates. Water-table mounding

may result in pushing LNAPL upward into previously

uncontaminated intervals (Testa et al., 1992). Injection may

also cause undesired lateral LNAPL migration. These

potential effects should be considered during system design.

Additional variables which may affect optimal LNAPL recovery

rates include thickness of the mobile LNAPL and formation

permeability to the LNAPL (Charbeneau et al., 1989; Chiang

et al., 1990). As the mobile LNAPL thickness increases, the

transmissivity of the formation with respect to LNAPL also

increases resulting in increased recovery. In low permeability

situations, the overall system recovery rate may become a

linear function of the number of wells. This is often due, in

part, to a lack of hydraulic interference between wells.

In recent studies, several researchers have recommended

strategies for maximizing the overall recovery of LNAPL from

systems using LNAPL and water extraction. One

recommended strategy is to pump the LNAPL layer at

relatively low rates in order to maintain the LNAPL as a

flowing continuous mass (Abdul, 1988; Charbeneau et. al.,

1989; Chiang et al., 1990). Water pumping, if used, is

carefully controlled to minimize smearing of the LNAPL layer

and to prevent upconing of the water table. As expected,

these studies indicate that this approach results in a relatively

small capture zone for each pumping well. Although this

implies that the total LNAPL recovery rate may be a linear

function of the number of wells which are used, a greater

volume of LNAPL may be recoverable than is possible using

other approaches such as maximizing water-table depression.

Based on mathematical simulations, Chiang et al. (1990)

produced a series of nomographs to aid in estimating the

optimal LNAPL recovery rate for a well under various

conditions of hydraulic conductivity, LNAPL thickness, density,

and viscosity. However, assumptions such as homogeneity of

subsurface materials and LNAPL thickness were used in

development of the nomographs, limiting their applicability at

many sites. Comparison with site-specific data would be

required to verify the utility of these nomographs for a specific

site. Charbeneau et al. (1992) also proposed a general

method for selecting the optimum hydrocarbon pumping rate

in dual water/LNAPL pumping systems as a function of water

pumping rate. However, this method only aids in estimating

an LNAPL pumping rate and does not indicate an optimal

pumping rate for water.

The time frame required for mobile LNAPL recovery using

extraction wells, trenches, or drains depends on many factors
19

including LNAPL and porous media properties, LNAPL volume

and distribution, and recovery system design. Time frames

generally will be difficult to estimate due to heterogeneity and

uncertainty in the values of pertinent parameters. At many

sites the time frame required for recovery will be a direct

function of the number of wells, trenches, and drains installed.

LNAPL Modeling

Several multiphase flow models have been developed which

are capable of simulating LNAPL transport (e.g., API, 1988;

Charbeneau et al., 1989; Faust et al., 1989; Huyakorn et al.,

1992; Kaluarachchi and Parker, 1989; Kaluarachchi et al.,

1990; Katyal et al., 1991; van der Heijde and Elnawawy, 1993;

Weaver et al., 1994). Such models have been used in site

characterization to simulate potential contaminant distribution

and LNAPL recovery system design. However, multiphase

flow is a complex problem, particularly in a heterogeneous

environment. Models incorporate simplifying assumptions to

facilitate utility. Recognition of the underlying assumptions

and evaluation of the site-specific applicability of the model is

required. Data requirements may also be extensive. Certain

parameters may not be readily measured in the field due to

site characterization technology limitations. Many of the

models are sensitive to parameters such as permeability,

porosity, and LNAPL spill history that are often unknown or

poorly defined. Thus, significant uncertainty in the accuracy of

the results may exist, even at relatively well characterized

sites.

The applicability of these models at many sites may be limited.

Objectives of modeling, required quality/quantity of

characterization data, and required confidence in the model

results should be evaluated prior to initiation of the exercise

(Huling and Weaver, 1991). Use of models as screening level

and site characterization tools may be beneficial at some

sites. For example, Weaver et al. (1994) developed a simple

collection of integrated models called the Hydrocarbon Spill

Screening Model to help predict the environmental impact on

ground water from LNAPL spills. Included are models for

simulating the downward migration of LNAPL through the

unsaturated zone, radial transport of the oil lens on the water

table, and advection and dispersion of the dissolution products

in ground water. The modeling package is intended to be

used as a screening tool for estimating the effects of

parameters such as LNAPL loadings, adsorption, groundwater velocity, and other major factors. Results of simulations

using other appropriate models may also aid in designing

LNAPL recovery systems. However, as in any simulation of

subsurface processes, significant uncertainty in the accuracy

of the results generally exists and should be considered in the

modeling and remedial design processes.

Soil Vapor Extraction

Soil vapor extraction (SVE) is a rapidly developing technology

with applications for removal of volatile contaminants from the

vadose zone. In a simple system, air is drawn through the

affected area by applying a vacuum to vapor extraction wells

(Figure 13), stripping volatile organic compounds into the

moving air stream. The air containing the organic vapors is

then treated, if necessary, and discharged to the atmosphere.

More complex systems may incorporate trenches, air injection

wells, and surface seals to direct the air flow through the

AS

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Figure 13. Conceptual design of soil vapor extraction system

with vapor treatment prior to discharge.

desired remediation zone (e.g., Johnson et al., 1990b; Lyman

and Noonan, 1990; Pedersen and Curtis, 1991).

Success of the SVE system at LNAPL removal is based on

three factors: 1) chemical composition of the LNAPL, 2) vapor

flow rates through the unsaturated zone, and 3) flow path of

the carrier vapors relative to the zone of contamination

(Johnson et al., 1990a). In general, SVE may be best suited

for the removal of compounds with vapor pressures greater

than approximately 14 mm Hg at 20o

 C (Sims, 1990). With

multiple component LNAPLs, preferential removal is observed

for chemical compounds with higher vapor pressures and

higher mole fractions in the LNAPL mixture.

Subsurface media must be permeable enough to allow

adequate vapor movement. Vapor flow rates are dependent

on the vacuum drawn in the SVE wells and the properties of

the unsaturated zone (e.g., soil moisture content, soil texture,

macropores, and LNAPL distribution). The ability to direct flow

through the contaminated zone is another primary concern.

Vapor flow paths to wells are affected by the degree of shortcircuiting from the surface that occurs, locations of subsurface

heterogeneities, and the presence of preferential flow paths.

A design process based on vapor pressure data for the

contaminants and soil/air permeability data was developed by

Johnson et al. (1990a, 1990b) to determine if a site is

amenable to SVE and to estimate the size of the required

system. Additional information concerning design and

application of SVE systems is available from DiGiulio (1992),

Pedersen and Curtis (1991), and U.S.EPA (1992c).

Ground-water extraction may be used with SVE systems to

counteract upconing of the water table near the SVE well

caused by pressure reduction (Johnson et al., 1990a).

Increased ground-water extraction may dewater deeper

intervals exposing contaminated materials for recovery by
20

Air Air

Vapors

Air

 Channels

Air

 Flow

 Clay Lens

Figure 14. Conceptual design of air sparging system.

SVE at some sites. Controlled studies conducted in a model

aquifer by Johnson et al. (1992) indicate this application of

SVE may be very effective in removing much of the residual

NAPL previously trapped below the water table.

Enhancements which have been applied to SVE systems

include in situ soil heating and induced fracturing.

Contaminant volatility, which can be one of the limitations to

the effectiveness of SVE, increases with increasing

temperature. Steam or hot air injection has been investigated

for use in increasing removal of volatile and semivolatile

constituents (DePaoli and Hutzler, 1992; Sittler et al., 1992;

U.S.EPA, 1992a). Studies using radio-frequency heating

have also been performed (U.S.EPA, 1992a). Field scale

trials have been relatively limited to date. Hydraulic and

pneumatic methods for inducing fractures in fine grained

vadose zone materials have also resulted in increased vapor

yields and contaminant removal during field trials (U.S.EPA,

1993a; U.S.EPA, 1993b).

Another recent enhancement to the SVE concept is the use of

induced air flow to increase biodegradation of contaminants in

the vadose zone (e.g., Kampbell, 1992; van Eyk, 1992). The

term “bioventing” has been applied to such systems. The

system is operated to deliver oxygen to the indigenous

microbes, thereby promoting degradation of contaminants.

Research efforts are currently focusing on design issues such

as the need for nutrient addition and the kinetics of the

biological reactions.

There are practical limitations to the effectiveness and

efficiency of SVE at many sites. For example, site conditions

resulting in vapor flow around but not through contaminated

zones will result in contaminant mass transfer limitations

(Johnson et al., 1990a; Travis and Macinnis, 1992).

Situations in which such conditions may occur include;

1) contaminated low permeability materials surrounded or

layered with higher permeability materials, and 2) LNAPL

removal from a relatively thick layer with a high liquid

saturation and a correspondingly low vapor permeability.

Contaminant mass transfer in these cases is limited by

diffusion from the contaminated zone. The effectiveness of

SVE in removing contaminants from the water-saturated zone

is similarly limited by diffusive transfer. In general, soil vapor

extraction may be applicable for removal of much of the

LNAPL located above the water table, including liquids

retained by capillary forces, at many sites. Selection of SVE

or conventional pumping technology for removal of mobile

LNAPL will depend on site conditions and remedial objectives.

Air Sparging

Air sparging is a relatively new technology that is being

implemented to remove volatile contaminants below the water

table in unconfined aquifers (e.g., Loden, 1992; Marley et al.,

1992a; U.S.EPA, 1992a). Air is injected from a well screened

in the saturated zone (Figure 14). Two potential objectives

cited for such systems are: 1) strip volatile hydrocarbons from

the aqueous phase and from any NAPLs present along the
21

Enhanced Oil Recovery Technologies

Methods for enhancing oil recovery which were pioneered by

the petroleum industry are being developed for environmental

applications. Technologies under investigation include

injection of hot water or steam, cosolvents (e.g., ethanol),

surfactants, alkaline agents, and polymers. Currently many

researchers are evaluating the applicability of enhanced oil

recovery (EOR) technology to the remediation of LNAPL sites.

Most of these technologies are still experimental. Very little

information regarding field applications is available. Current

reviews of the development of EOR technologies for DNAPL

removal (U.S.EPA, 1992a) and other hazardous waste site

remediations (Sims, 1990) are available. Information from

these sources is also applicable to LNAPL site remediation.

Primary LNAPL recovery systems (e.g., drains, pumping

wells) will generally result in the removal of significantly less

than 50% of the total LNAPL volume. Enhanced oil recovery

may remove more of the LNAPL. However, there are practical

limitations to the effectiveness of these techniques.

Conditions such as subsurface heterogeneity, low

permeability units, and relative permeability reductions caused

by the presence of NAPL can prevent remediation fluids from

making contact with the NAPL. Significant quantities of

LNAPL may remain in place following EOR applications

(Mercer and Cohen, 1990).

Recovery of LNAPL may be enhanced by injecting fluids to

increase hydraulic gradients, reduce the NAPL/water

interfacial tension, reduce NAPL viscosity, increase wettingphase viscosity, and/or increase NAPL solubility (U.S.EPA,

1992a). The application of heat to viscous LNAPL will

enhance mobility by decreasing viscosity and may result in

increased solubility. Volatile NAPL components may also

volatilize and condense in advance of a steam or hot water

flood increasing NAPL saturation and relative permeability.

Steam/hot water/hot air flooding, electrical heating, radio

frequency heating, and conduction heating are possible

techniques which have been investigated (e.g., Davis and

Lien, 1993; Fulton et al., 1991; Hunt et al., 1988a; Hunt et al.,

1988b; Johnson and Leuschner, 1992; Sims, 1990; Udell,

1992; U.S.EPA, 1992a). Thermal techniques have potential

application for increased removal of LNAPL in both the

saturated and unsaturated zones. These techniques have

been evaluated in a relatively limited number of field scale

studies (U.S.EPA, 1992a).

Chemically enhanced recovery techniques appear to be

promising technologies for increasing LNAPL solubility and

mobility (U.S.EPA, 1992a). Injection of surfactant solutions

has been investigated for use in increasing the solubility of

NAPL constituents and increasing NAPL mobility through

reduction of interfacial tension (Fountain, 1992). Surfactants

are potentially capable of increasing NAPL solubility by

several orders of magnitude. Injection of alkaline agents has

been used in the petroleum industry to increase NAPL mobility

through in situ surfactant production resulting from reaction

with organic acids in the NAPL. Polymer flooding has been

used as a component of water flooding technology by the

petroleum industry. Polymers are used to displace some

portion of the residual NAPL by increasing the viscosity of the

water flood. Application of these technologies in the

environmental field has been very limited (U.S.EPA, 1992a).

In addition, use of cosolvents has been proposed for

increasing solubility of NAPL compounds (e.g., Augustijin et

path of air flow, and 2) add oxygen to the water to encourage

biodegradation of amenable constituents. The compounds

must be volatile to be stripped and transported to the surface.

Additionally, the compounds must be biodegradable for the

biodegradation component of this technology to be effective.

In the case of multicomponent LNAPLs, a certain fraction of

the LNAPL may be volatile and biodegradable. However, a

certain fraction of the LNAPL may be relatively unaffected by

air sparging. After the air make its way to the unsaturated

zone, a soil vapor extraction system is used to remove the

vapors for treatment prior to release to the atmosphere.

Air sparging generally is proposed in conjunction with vacuum

extraction, therefore, design limitations, and concerns

associated with vacuum extraction are also applicable in air

sparging systems. Specific design parameters for air sparging

are numerous and detailed discussions are beyond the scope

of this review. Generally, these parameters fall under three

categories: hydrogeological constraints, contaminant

properties and distribution, and system operations (Marley et

al., 1992a; Brown, 1994). However, the primary design

consideration is contaminant volatility, (i.e., volatilization from

the water phase (Henry’s Law) or from the NAPL (Raoult’s

Law)).

One limitation of air sparging is the vulnerability to preferential

pathways and heterogeneities. Short circuiting along

manmade and natural pathways (sheet piles, wells, sparge

points, root holes, high hydraulic conductivity layers,

stratigraphic windows, etc.) reduces the overall effectiveness

of air sparging. Subsurface heterogeneity results in

channeling air flow through preferential pathways (Ji et al.,

1993; Johnson et al., 1993). Sparging below an impervious

stratigraphic unit may direct air laterally, spreading

contamination (Brown, 1994). As with other technologies,

overall effectiveness may be reduced due to rate limited

mechanisms such as contaminant desorption and slow

contaminant diffusion from low permeability materials.

Consequently, the radius of influence of a sparge well is

difficult to evaluate, and may be relatively small.

Additional potential concerns with use of this technology

include increased contaminant migration and in situ

precipitation of dissolved minerals (Marley et al., 1992b).

Mechanisms such as formation of gas pockets may result in

lateral displacement of contaminated ground water or LNAPL

into previously clean areas. Uncontrolled vapor migration in

the vadose zone may also spread contamination. Clearly,

control of both the vapor emissions in the unsaturated zone

and the ground-water plume generally must be monitored and

maintained. Mineral precipitation due to changes in

geochemical conditions may result in undesired permeability

reduction at some sites. Adequate site characterization,

system design, and monitoring would be required to mitigate

such concerns.

Controlled studies of the effectiveness of this technology are

relatively limited. In a study conducted in a model aquifer

(Johnson et al., 1992) contaminant mass removal using air

sparging was observed. However, significant contaminant

mass appeared to remain following sparging. Preferential

pathways and a limited radius of influence were also

observed. The overall performance of this technology has not

been adequately assessed under a variety of field conditions.

Carefully monitored field demonstrations are required to better

determine applicability and effectiveness.
22

containment operations. Depending on site conditions, time

frames of many decades or centuries may be required to

remove LNAPLs trapped in the saturated zone using

dissolution alone.

Despite the limitations, ground-water extraction and injection

technology will be an applicable component of the overall

remediation strategy at most sites. Pump-and-treat systems

may be most useful for establishing hydrodynamic control to

prevent contaminant migration and for remediation of

aqueous-phase contamination in some situations in which

contaminant sources, including NAPLs, have been removed

or isolated. However, many factors, including subsurface

heterogeneities, may limit the effectiveness of contaminant

removal.

Physical Barriers

Low permeability barriers (e.g., grout curtains, slurry walls,

sheet piling) for control of ground-water and LNAPL flow

(Mitchell and van Court, 1992) may be applicable as

components of remedial operations at many sites. Potential

uses include containment of ground-water and/or mobile

LNAPL during remediation. However, several of the concerns

cited regarding difficulties in DNAPL containment (Huling and

Weaver, 1991) will exist for LNAPL containment. Concerns

regarding difficulty in assessing barrier integrity and materials

compatibility issues should always be considered.

Permeable treatment walls (e.g., Brown et al., 1992; Gillham

and Burris, 1992) are an emerging technology for passive

control of aqueous-phase contaminant migration. The

technology is in its infancy with very few field-scale trials

reported to date. Conceptually, reactive materials or

substances creating a reactive zone are placed to form a

vertical wall (Figure 15). The reactive materials remove or

transform dissolved contaminants in ground water that passes

through the wall. Low permeability barriers may be

incorporated to channel ground water through the reactive

al., 1992; Boyd and Farley, 1992; Rao et al., 1991; U.S.EPA,

1992a). In general, chemically enhanced recovery of

hydrocarbons for environmental applications is in the

developmental stage (U.S.EPA, 1992a).

Bioremediation

Practical biological degradation of LNAPL pools has not been

demonstrated. Bioremediation of immiscible hydrocarbon is

limited due to the following: (1) NAPLs present a highly hostile

environment to the survival of most soil microbes, (2) the basic

requirements for microbial proliferation (nutrients, terminal

electron acceptor, pH, moisture, osmotic potential, etc.) are

difficult if not impossible to deliver or maintain in the NAPL

(Huling and Weaver, 1991). Correspondingly, bioremediation

may be limited to the periphery of the NAPL zone in both

saturated and unsaturated media. It has been postulated that

biologically-produced surfactants resulting from microbial

activity near a NAPL have increased the rate of NAPL

solubilization (Wilson and Brown, 1989). However, this has

not been proven. Through degradation of the solubilized

constituents, microbes may also increase contaminant mass

transfer rates from the NAPL by creating steeper

concentration gradients than solubilization alone.

Many aqueous-phase compounds dissolved from LNAPL

sources are amenable to degradation by naturally occurring

microorganisms in the subsurface (Norris et al., 1994; Wilson

et al., 1986). While in situ biodegradation occurs naturally at

most sites, the overall rate of the reaction may be limited by a

lack of nutrients, electron acceptors, or both (Thomas and

Ward, 1989). Therefore, in situ biodegradation projects

attempt to reduce limitations by injecting necessary nutrients

and electron acceptors into the contaminated zone and

stimulating naturally-occurring microorganisms. Effectiveness

of this approach may be limited by the inability to deliver

nutrients and electron acceptor into heterogeneously

contaminated and low permeability materials. Although

limitations exist, biodegradation of aqueous-phase

contaminants derived from many LNAPL sources, such as

petroleum products, is a process that is potentially applicable

as one component of site management at many LNAPL sites.

The greatest utility of enhanced biodegradation may be as a

polishing step following removal, to the extent practicable, of

mobile and residual NAPL. More comprehensive discussions

of the application of biodegradation to contaminant removal

from soils and ground water are available (Norris et al., 1994;

Sims et al., 1989; Sims et al., 1992).

Ground-Water Pump-and-Treat

Traditional pump-and-treat systems extract contaminated

ground water for above-ground treatment. Such systems

have primarily been designed to recover aqueous-phase

contaminants. Flushing of hundreds or thousands of pore

volumes of ground water may be required to significantly

diminish contaminant levels at some sites (e.g., Borden and

Kao, 1992; Geller and Hunt, 1993; Hunt et al., 1988a; Newell

et. al., 1990). Complete mobilization of LNAPL trapped below

the water table using increased hydraulic gradients alone is

not practical under conditions encountered in the field (Hunt et

al., 1988a; Wilson and Conrad, 1984). Many of the more

soluble LNAPL components may continue to dissolve in

ground water resulting in ground-water contamination at

unacceptable concentrations, potentially necessitating

Figure 15. Map view of reactive treatment wall and low

permeability barriers used to channel ground-water

flow.

Low Permeability

Barrier

Low Permeability

Barrier

Ground-Water Flow

High Concentration

Aqueous-Phase Plume Low

Concentration

Plume

Treatment Wall
23

wall. A biological treatment wall, for example, may slowly

allow dissolution of oxygen and nutrients into ground water,

encouraging in situ biodegradation to proceed at an

accelerated pace.

Treatment Train

Remediation may require the use of more than one

technology. It is likely that several remediation techniques,

used in series and/or parallel applications, will be required for

maximum contaminant removal. This collaborative effort may

be referred to as a treatment train approach (U.S.EPA,

1992a). A conceptual example of a treatment train which

might be effective at an LNAPL site includes use of

conventional pumping technology for mobile LNAPL removal.

This phase might be followed by vapor extraction for removal

of residual LNAPL and possibly coupled with ground-water

extraction to lower the water table for increased contaminant

removal. Additional technologies such as bioremediation

might be used to further reduce contaminant concentrations.

Optimum sequencing of remedial actions in a treatment train

will be site specific and will depend on such factors as LNAPL

migration rates and distribution, remedial objectives, and

applicable remedial technologies. Containment of migrating

LNAPL may be an appropriate objective of initial actions at

many sites. Removal of LNAPL to the extent practicable will

also be an objective during early stages of remediation at

many sites.

The treatment train concept acknowledges the strengths and

weaknesses of various remediation strategies and couples

promising technologies to overcome limitations. A successful

treatment train will require a thorough understanding of the

hydrogeologic and geochemical characteristics of the site.

Detailed site characterization efforts and an in-depth

understanding of the processes which affect the transport and

fate of LNAPLs in the subsurface will permit the optimization

of all possible remedial actions, maximize predictability of

remediation effectiveness, minimize remediation costs, and

make cost estimates more reliable (API, 1989; U.S.EPA,

1992a; Wilson et al., 1986).

REFERENCES

Abdul, A.S., 1988. Migration of petroleum products through

sandy hydrogeologic systems, Ground Water Monit. Rev.,

8(4): 73-81.

Abdul, A.S., 1992. A new pumping strategy for petroleum

product recovery from contaminated hydrogeologic systems:

Laboratory and field evaluations, Ground Water Monit. Rev.,

12(1): 105-114.

Abdul, A.S., S.F. Kia, and T.L. Gibson, 1989. Limitations of

monitoring wells for the detection and quantification of

petroleum products in soils and aquifers, Ground Water Monit.

Rev., 9(2): 90-99.

Anderson, M.R.,1988. The dissolution and transport of dense

nonaqueous phase liquids in saturated porous media. Ph.D.

Dissertation, Oregon Graduate Center, Beaverton, OR,

260 pp.

Anderson, W.G., 1986. Wettability literature survey -- part 1:

Rock/oil/brine interactions and the effects of core handling on

wettability. J. Petroleum Technology, October, 1125-1144.

API (American Petroleum Institute), 1988. Phase separated

hydrocarbon contaminant modeling for corrective action, Publ.

4474, API, Washington, DC, 125 pp.

API (American Petroleum Institute), 1989. A guide to the

assessment and remediation of underground petroleum

releases, Publ. 1628, API, Washington, DC, 81 pp.

Augustijin, D.C.M., L.S. Lee, R.E. Jessup, and P.S.C. Rao,

1992. Remediation of soils contaminated with oily wastes:

Experimental and theoretical basis for the use of cosolvents,

in Proc. Subsurface Restoration Conf., Dallas, TX, Rice Univ.,

Dept. of Environ. Sci. & Eng., Houston, TX, 49-50.

Ballestero, T.P., F.R. Fiedler, and N.E. Kinner, 1994. An

investigation of the relationship between actual and apparent

gasoline thickness in a uniform sand aquifer, Ground Water,

32(5): 708-718.

Banerjee, S., 1984. Solubility of organic mixtures in water,

Environ. Sci. Technol., 18(8): 587-591.

Bear, J., 1972. Dynamics of Fluids in Porous Media, American

Elsevier Publishing Co., New York, 763 pp.

Benson, R.C., R.A. Glaccum, and M.R. Noel, 1982.

Geophysical Techniques for Sensing Buried Wastes and

Waste Migration, Natl. Ground Water Assoc., Dublin, OH,

236 pp.

Blake, S.B., and R.A. Hall, 1984. Monitoring petroleum spills

with wells: Some problems and solutions, in Proc. Fourth Natl.

Symp. on Aquifer Restoration and Ground Water Monitoring,

Natl. Ground Water Assoc., Dublin, OH, 305-310.

Blake, S.B., and R.W. Lewis, 1983. Underground oil recovery,

Ground Water Monit. Rev., 3(2): 40-46.

Borden, R.C., and C.M. Kao, 1992. Evaluation of groundwater

extraction for remediation of petroleum-contaminated aquifers,

Water Environ. Res., 64(1): 28-36.

Boyd, G.R., and K.J. Farley, 1992. NAPL removal from

groundwater by alcohol flooding: Laboratory studies and

applications, in Hydrocarbon Contaminated Soils and

Groundwater, Volume 2, edited by E.J. Calabrese and P.T.

Kostecki, Lewis Publishers, Boca Raton, FL, 437-460.

Brown, M.J., D.R. Burris, J.A. Cherry, and D.M. Mackay, 1992.

Enhancement of organic contaminant retardation by the

modification of aquifer material with cationic surfactants, in

Proc. Subsurface Restoration Conf., Dallas, TX, Rice Univ.,

Dept. of Environ. Sci. & Eng., Houston, TX, 194-196.

Brown, R., 1994. Treatment of petroleum hydrocarbons in

ground water by air sparging, in Handbook of Bioremediation,

Lewis Publishers, Boca Raton, FL, 257 pp.
24

Evans, O.D., and G.M. Thompson, 1986. Field and

interpretation techniques for delineating subsurface petroleum

hydrocarbon spills using soil gas analysis, in Proc. NWWA/API

Conf. on Petroleum Hydrocarbons and Organic Chemicals in

Ground Water: Prevention, Detection, and Restoration, Natl.

Ground Water Assoc., Dublin, OH, 444-455.

Farr, A.M., R.J. Houghtalen, and D.B. McWhorter, 1990.

Volume estimation of light nonaqueous phase liquids in

porous media, Ground Water, 28(1): 48-56.

Faust, C.R., J.H. Guswa, and J.W. Mercer, 1989. Simulation

of three-dimensional flow of immiscible fluids within and below

the unsaturated zone, Water Resour. Res., 25(12): 2449-

2464.

Feenstra, S., D.M. Mackay, and J.A. Cherry, 1991. A method

for assessing residual NAPL based on organic chemical

concentrations in soil samples, Ground Water Monit. Rev.,

11(2): 128-136.

Ferrand, L.A., P.C.D. Milly, and G.F. Pinder, 1989.

Experimental determination of three-fluid saturation profiles in

porous media, J. Contam. Hydrol., 4: 373-395.

Fountain, J.C., 1992. Removal of non-aqueous phase liquids

using surfactants, in Proc. Subsurface Restoration Conf.,

Dallas, TX, Rice Univ., Dept. of Environ. Sci. & Eng., Houston,

TX, 36-37.

Fulton, D.E., G.J. Reuter, and T.E. Buscheck, 1991. Hot water

enhanced recovery of phase separated lubricating oil, in Proc.

Conf. on Petroleum Hydrocarbons and Organic Chemicals in

Ground Water: Prevention, Detection, and Restoration, Natl.

Ground Water Assoc., Dublin, OH, 143-156.

Geller, J.T., and J.R. Hunt, 1993. Mass transfer from

nonaqueous phase organic liquids in water-saturated porous

media, Water Resour. Res., 29(4): 833-845.

Gillham, R.W., and D.R. Burris, 1992. In situ treatment walls -

Chemical dehalogenation, denitrification, and bioaugmentation, in Proc. Subsurface Restoration Conf., Dallas, TX, Rice

Univ., Dept. of Environ. Sci. & Eng., Houston, TX, 66-68.

Gruszczenski, T.S., 1987. Determination of a realistic estimate

of the actual formation product thickness using monitor wells:

A field bailout test, in Proc. Conf. on Petroleum Hydrocarbons

and Organic Chemicals in Ground Water : Prevention,

Detection, and Restoration, Natl. Ground Water Assoc.,

Dublin, OH, 235-253.

Hall, R.A., S.B. Blake, and S.C. Champlin, Jr., 1984.

Determination of hydrocarbon thicknesses in sediments using

borehole data, in Proc. Fourth Natl. Symp. on Aquifer

Restoration and Ground Water Monitoring, Natl. Ground

Water Assoc., Dublin, OH, 300-304.

Hampton, D.R., and H.G. Heuvelhorst, 1990. Designing gravel

packs to improve separate-phase hydrocarbon recovery:

Laboratory experiments, in Proc. Conf. on Petroleum

Hydrocarbons and Organic Chemicals in Ground Water :

Prevention, Detection, and Restoration, Natl. Ground Water

Assoc, Dublin, OH, 195-209.

Charbeneau, R.J., N. Wanakule, C.Y. Chiang, J.P. Nevin, and

C.L. Klein, 1989. A two-layer model to simulate floating free

product recovery: Formulation and applications, in Proc. Conf.

on Petroleum Hydrocarbons and Organic Chemicals in

Ground Water: Prevention, Detection, and Restoration, Natl.

Ground Water Assoc., Dublin, OH, 333-345.

Charbeneau, R.J., P.B. Bedient, and R.C. Loehr, eds., 1992.

Groundwater Remediation, Water Qual. Mgmt. Library, Vol. 8,

Technomic Publishing Co., Lancaster, PA, 185 pp.

Chiang, C.Y., J.P. Nevin, and R.J. Charbeneau, 1990. Optimal

free hydrocarbon recovery from a single pumping well, in

Proc. Conf. on Petroleum Hydrocarbons and Organic

Chemicals in Ground Water: Prevention, Detection, and

Restoration, Natl. Ground Water Assoc., Dublin, OH, 161-178.

Chiang, C.Y., K.R. Loos, and R.A. Klopp, 1992. Field

determination of geological/chemical properties of an aquifer

by cone penetrometry and headspace analysis, Ground

Water, 30(3): 428-436.

Cohen, R.M., and J.W. Mercer, 1993. DNAPL Site Evaluation,

CRC Press, Boca Raton, FL, 314 pp.

Cohen, R.M., A.P. Bryda, S.T. Shaw, and C.P. Spalding,

1992. Evaluation of visual methods to detect NAPL in soil and

water, Ground Water Monit. Rev., 12(4): 132-141.

Davis, E.L., and B.K. Lien, 1993. Laboratory study on the use

of hot water to recover light oily wastes from sands, EPA/600/

R-93/021, U.S.EPA, R.S. Kerr Environ. Res. Lab., Ada, OK,

59 pp.

Demond, A.H., and P.V. Roberts, 1991. Effect of interfacial

forces on two-phase capillary pressure-saturation

relationships, Water Resour. Res., 27(3): 423-437.

DePaoli, D.W., and N.J. Hutzler, 1992. Field test of

enhancement of soil venting by heating, in Proc. Symp. on

Soil Venting, EPA/600/R-92/174, U.S.EPA, R.S. Kerr Environ.

Res. Lab., Ada, OK, 173-192.

De Pastrovich, T.L., Y. Baradat, R. Barthel, A. Chiarelli, and

D.R. Fussell, 1979. Protection of ground water from oil

pollution, CONCAWE, The Hague, 61 pp.

Devitt, D.A., R.B. Evans, W.A. Jury, and T.H. Starks, 1987.

Soil Gas Sensing For Detection and Mapping of Volatile

Organics, Natl. Ground Water Assoc., Dublin, OH, 270 pp.

DiGiulio, D.C., 1992. Evaluation of soil venting application,

Ground Water Issue, EPA/540/S-92/004, U.S.EPA, R.S. Kerr

Environ. Res. Lab, Ada, OK, 7 pp.

DiGiulio, D.C., and J.S. Cho, 1990. Conducting field tests for

evaluation of soil vacuum extraction application, in Proc.

Fourth Natl. Outdoor Action Conf. on Aquifer Restoration,

Ground Water Monitoring, and Geophysical Methods, Natl.

Ground Water Assoc., Dublin, OH, 587-601.

Durnford, D., J. Brookman, J. Billica, and J. Milligan, 1991.

LNAPL distribution in a cohesionless soil: A field investigation

and cryogenic sampler, Ground Water Monit. Rev., 11(3): 115-

122.
25

Hampton, D.R., and P.D.G. Miller, 1988. Laboratory

investigation of the relationship between actual and apparent

product thickness in sands, in Proc. Conf. on Petroleum

Hydrocarbons and Organic Chemicals in Ground Water:

Prevention, Detection, and Restoration, Natl. Ground Water

Assoc., Dublin, OH, 157-181.

Hampton, D.R., R.B. Wagner, and H.G. Huevelhorst, 1990. A

new tool to measure petroleum thickness in shallow aquifers,

in Proc. Fourth Natl. Outdoor Action Conf. on Aquifer

Restoration, Ground Water Monitoring and Geophysical

Methods, Natl. Ground Water Assoc., Dublin, OH.

Hampton, D.R., M.M. Smith, and S.J. Shank, 1991, Further

laboratory studies of gravel pack design for hydrocarbon

recovery wells, in Proc. Conf. on Petroleum Hydrocarbons and

Organic Chemicals in Ground Water : Prevention, Detection,

and Restoration, Natl. Ground Water Assoc, Dublin, OH, 615-

629.

Hayes, D., E.C. Henry, and S.M. Testa, 1989. A practical

approach to shallow petroleum hydrocarbon recovery, Ground

Water Monit. Rev., 9(1): 180-185.

Holzer, T.L., 1976. Application of ground-water flow theory to

a subsurface oil spill, Ground Water, 14(3): 138-145.

Hudak, P.F., K.M. Clements, and H.A. Loaiciga, 1993. Watertable correction factors applied to gasoline contamination, J.

Environ. Eng., 119(3): 578-584.

Hughes, J.P., C.R. Sullivan, and R.E. Zinner, 1988. Two

techniques for determining the true hydrocarbon thickness in

an unconfined sandy aquifer, in Proc. Conf. on Petroleum

Hydrocarbons and Organic Chemicals in Ground Water:

Prevention, Detection, and Restoration, Natl. Ground Water

Assoc., Dublin, OH, 291-314.

Huling, S.G., and J.W. Weaver, 1991. Dense nonaqueous

phase liquids, Ground Water Issue, EPA/540/4-91-002,

U.S.EPA, R.S. Kerr Environ. Res. Lab., Ada, OK, 21 pp.

Hunt, J.R., N. Sitar, and K.S. Udell, 1988a. Nonaqueous

phase liquid transport and cleanup, 1: Analysis of

mechanisms, Water Resour. Res., 24(8): 1247-1258.

Hunt. J.R., N. Sitar, and K.S. Udell, 1988b. Nonaqueous

phase liquid transport and cleanup, 2: Experimental studies,

Water Resour. Res., 24(8): 1259-1269.

Hunt, W.T., J.W. Wiegand, and J.D. Trompeter, 1989. Free

gasoline thickness in monitoring wells related to ground water

elevation change, in Proc. Conf. on New Field Techniques for

Quantifying the Physical and Chemical Properties of Heterogeneous Aquifers, Natl. Ground Water Assoc., Dublin, OH,

671-692.

Huyakorn, P.S., Y.S. Wu, and S.Panday, 1992. A

comprehensive three-dimensional numerical model for

predicting the fate of petroleum hydrocarbons in the

subsurface, in Proc. Conf. on Petroleum Hydrocarbons and

Organic Chemicals in Ground Water: Prevention, Detection,

and Restoration, Natl Ground Water Assoc., Dublin, OH, 239-

253.

Ji, W., A. Dahmani, D.P. Ahlfeld, J.D. Lin, and E. Hill III, 1993.

Laboratory study of air sparging: Air flow visualization,

Ground Water Monit. and Rem., 13(4): 115-126.

Johnson, Jr., L.A., and A.P. Leuschner, 1992. The CROW

process and bioremediation for in situ treatment of hazardous

waste sites, in Hydrocarbon Contaminated Soils and

Groundwater, Volume 2, edited by E.J. Calabrese and P.T.

Kostecki, Lewis Publishers, Boca Raton, FL, 343-357.

Johnson, P.C., M.W. Kemblowski, and J.D. Colthart, 1990a.

Quantitative analysis for the cleanup of hydrocarboncontaminated soils by in-situ soil venting, Ground Water,

28(3): 413-429.

Johnson, P.C., C.C. Stanley, M.W. Kemblowski, D.L. Byers,

and J.D. Colthart, 1990b. A practical approach to the design,

operation, and monitoring of in situ soil-venting systems,

Ground Water Monit. Rev., 10(2): 159-178.

Johnson, R.L., W. Bagby, M. Perrott, and C. Chen, 1992.

Experimental examination of integrated soil vapor extraction

techniques, in Proc. Conf. on Petroleum Hydrocarbons and

Organic Chemicals in Ground Water: Prevention, Detection,

and Restoration, Natl. Ground Water Assoc., Dublin, OH, 441-

452.

Johnson, R.L., P.C. Johnson, D.B. McWhorter, R.E. Hinchee,

and I. Goodman, 1993. An overview of in situ air sparging,

Ground Water Monit. and Rem., 13(4): 127-135.

Kaluarachchi, J.J., and J.C. Parker, 1989. An efficient finite

element model for modeling multiphase flow in porous media,

Water Resour. Res., 25(1): 43-54.

Kaluarachchi, J.J., J.C. Parker, and R.J. Lenhard, 1990. A

numerical model for areal migration of water and light

hydrocarbon in unconfined aquifers, Adv. in Water Resour.,

13: 29-40.

Kampbell, D.H., 1992. Subsurface remediation at a gasoline

spill site using a biovent approach, in Proc. Symp. on Soil

Venting, EPA/600/R-92/174, U.S.EPA, R.S. Kerr Environ.

Res. Lab., Ada, OK, 309-315.

Katyal, A.K., J.J. Kaluarachchi, and J.C. Parker, 1991.

MOFAT: A two-dimensional finite element program for

multiphase flow and multicomponent transport, program

documentation and user’s guide, EPA-600/2-91-020, NTIS

PB91-191692, U.S.EPA, R.S. Kerr Environ. Res. Lab., Ada,

OK, 109 pp.

Keech, D.A., 1988. Hydrocarbon thickness on groundwater by

dielectric well logging, in Proc. Conf. on Petroleum

Hydrocarbons and Organic Chemicals in Ground Water:

Prevention, Detection, and Restoration, Natl. Ground Water

Assoc., Dublin, OH, 275-290.

Kemblowski, M.W., and C.Y. Chiang, 1990. Hydrocarbon

thickness fluctuations in monitoring wells, Ground Water,

28(2): 244-252.
26

Mercer, J.W., and R.M. Cohen, 1990. A review of immiscible

fluids in the subsurface: Properties, models, characterization,

and remediation, J. Contam. Hydrol., 6: 107-163.

Miller, C.T., M.M. Poirier-McNeill, and A.S. Mayer, 1990.

Dissolution of trapped nonaqueous phase liquids: Mass

transfer characteristics, Water Resour. Res., 26(11): 2783-

2796.

Mitchell, J.K., and W.A. van Court, 1992. Barriers - walls and

covers, in Proc. Subsurface Restoration Conf., Dallas, TX,

Rice Univ., Dept. of Environ. Sci. & Eng., Houston, TX, 34-35.

Morgan, J.H., 1992. Horizontal drilling applications of

petroleum technologies for environmental purposes, Ground

Water Monit. Rev., 12(3): 98-102.

Newell, C.J., J.A. Conner, and D.K. Wilson, 1990. Pilot test for

evaluating the effectiveness of enhanced in-situ

biodegradation for soil remediation, in Proc. Conf. on

Petroleum Hydrocarbons and Organic Chemicals in Ground

Water: Prevention, Detection, and Restoration, Natl. Ground

Water Assoc., Dublin, OH, 369-383.

Norris, R.D., R.E. Hinchee, R. Brown, P.L. McCarty, L.

Semprini, J.T. Wilson, D.H. Kampbell, M. Reinhard, E.J.

Bouwer, R.C. Borden, T.M. Vogel, J.M. Thomas, and C.H.

Ward, 1994. Handbook of Bioremediation, Lewis Publishers,

Boca Raton, FL, 257 pp.

Oakley, D., M. Thacker, K. Singer, J. Koelsch, and B. Mabson,

1992. The use of horizontal wells in remediating and

containing a jet fuel plume-preliminary findings, in Proc. Conf.

on Petroleum Hydrocarbons and Organic Chemicals in

Ground Water: Prevention, Detection, and Restoration, Natl.

Ground Water Assoc., Dublin, OH, 1-13.

Olhoeft, G.R., 1986. Direct detection of hydrocarbon and

organic chemicals with ground penetrating radar and complex

resistivity, in Proc. NWWA/API Conf. on Petroleum

Hydrocarbons and Organic Chemicals in Ground Water:

Prevention, Detection, and Restoration, Natl. Ground Water

Assoc., Dublin, OH, 284-305.

Ostendorf, D.W., E.E. Moyer, R.J. Richards, E.S. Hinlein,

Y. Xie, and R.V. Rajan, 1992. LNAPL distribution and

hydrocarbon vapor transport in the capillary fringe, EPA/600/

R-92/247, U.S.EPA, R.S. Kerr Environ. Res. Lab., Ada, OK,

122 pp.

Pantazidou, M., and N. Sitar, 1993. Emplacement of

nonaqueous liquids in the vadose zone, Water Resour. Res.,

29(3): 705-722.

Pedersen, T.A., and J.T. Curtis, 1991. Soil vapor extraction

technology, Reference Handbook, EPA/540/2-91/003,

U.S.EPA, Risk Red. Eng. Lab., Cincinnati, OH, 316 pp.

Piwoni, M.D., and J.W. Keeley, 1990. Basic concepts of

contaminant sorption at hazardous waste sites, Ground Water

Issue, EPA/540/4-90/053, U.S.EPA, R.S. Kerr Environ. Res.

Lab., Ada, OK, 7 pp.

Kerfoot, H.B., 1988. Is soil-gas analysis an effective means of

tracking contaminant plumes in ground water? What are the

limitations of the technology currently employed?, Ground

Water Monit. Rev., 8(2): 54-57.

Keys, W.S., 1989. Borehole Geophysics Applied to Ground

Water Investigations, Natl. Ground Water Assoc., Dublin, OH,

313 pp.

Keys, W.S. and L.M. MacCary, 1971. Application of borehole

geophysics to water-resource investigations, in Techniques of

Water-Resource Investigations of the United States

Geological Survey, Book 2, U.S. Govt. Printing Off.,

Washington, D.C., 126 pp.

King, T.V.V., and G.R. Olhoeft, 1989. Mapping organic

contamination by detection of clay-organic processes, in Proc.

Conf. on Petroleum Hydrocarbons and Organic Chemicals in

Ground Water: Prevention, Detection, and Restoration, Natl.

Ground Water Assoc., Dublin, OH, 627-640.

Leinonen, P.J., and D. Mackay, 1973. The multicomponent

solubility of hydrocarbons in water, Can. J. Chem. Eng., 51:

230-233.

Lenhard, R.J., and J.C. Parker, 1990. Estimation of free

hydrocarbon volume from fluid levels in monitoring wells,

Ground Water, 28(1): 57-67.

Loden, M.E., 1992. Project summary: A technology

assessment of soil vapor extraction and air sparging, EPA/

600/SR-92/173, U.S.EPA, Risk Red. Eng. Lab., Cincinnati,

OH, 3 pp.

Looney, B.B., D.S. Kaback, and J.C. Corey, 1992.

Environmental restoration using horizontal wells: A field

demonstration, in Proc. Subsurface Restoration Conf., Dallas,

TX, Rice Univ., Dept. of Environ. Sci. & Eng., Houston, TX,

41-43.

Lyman, W.J., and D.C. Noonan, 1990. Assessing UST

corrective action technologies: Site assessment and selection

of unsaturated zone treatment technologies, EPA/600/2-90/

011, U.S.EPA, Risk Red. Eng. Lab., Cincinnati, OH, 107 pp.

Marley, M.C., D.J. Hazebrouck, and M.T. Walsh, 1992a. The

application of in situ air sparging as an innovative soils and

ground water remediation technology, Ground Water Monit.

Rev., 12(2): 137-145.

Marley, M., D. Hazebrouck, and M. Walsh, 1992b. Tiny

bubbles pop to deep clean, Soils, October: 8-12.

Marrin, D.L., 1988. Soil-gas sampling and misinterpretation,

Ground Water Monit. Rev., 8(2): 51-54.

Marrin, D.L., and H.B. Kerfoot, 1988. Soil-gas surveying

techniques, Environ. Sci. Technol., 22(7): 740-745.

Mendoza, C.A., and T.A. McAlary, 1989. Modeling of

groundwater contamination caused by organic solvent vapors,

Ground Water, 28(2): 199-206.
27

Powers, S.E., C.O. Loureiro, L.M. Abriola, and W.J. Weber,

Jr., 1991. Theoretical study of the significance of

nonequilibrium dissolution of nonaqueous phase liquids in

subsurface systems, Water Resour. Res., 27(4): 463-477.

Powers, S.E., L.M. Abriola, and W.J. Weber, Jr., 1992. An

experimental investigation of nonaqueous phase liquid

dissolution in saturated subsurface systems: Steady state

mass transfer rates, Water Resour. Res., 28(10): 2691-2705.

Rao, P.S.C., L.S. Lee, and A.L. Wood, 1991. Solubility,

sorption, and transport of hydrophobic organic chemicals in

complex mixtures, Environmental Research Brief, EPA/600/M91/009, U.S.EPA, R.S. Kerr Environ. Res. Lab., Ada, OK,

14 pp.

Reidy, P.J., W.J. Lyman, and D.C. Noonan, 1990. Assessing

UST corrective action technologies: Early screening of

cleanup technologies for the saturated zone, EPA/600/2-90/

027, U.S.EPA, Risk Red. Eng. Lab., Cincinnati, OH, 124 pp.

Schwille, F., 1988. Dense Chlorinated Solvents in Porous and

Fractured Media, Lewis Publishers, Chelsea, MI, 146 pp.

Seitz, W.R., 1990. In situ detection of contaminant plumes in

ground water, Special Report 90-27, U.S. Army Corps of

Engineers, Cold Regions Research & Engineering Laboratory,

12 pp.

Sims, J.L., R.C. Sims, and J.E. Matthews, 1989.

Bioremediation of contaminated surface soils, EPA 600/9-89/

073, U.S.EPA, R.S. Kerr Environ. Res. Lab., Ada, OK, 23 pp.

Sims, J.L., J.M. Suflita, and H.H. Russell, 1992. In-situ

bioremediation of contaminated ground water, Ground Water

Issue, EPA/540/S-92/003, U.S.EPA, R.S. Kerr Environ. Res.

Lab., Ada, OK, 11 pp.

Sims, R.C., 1990. Soil remediation techniques at uncontrolled

hazardous waste sites, J. Air Waste Manage. Assoc., 40(5):

704-730.

Sittler, S.P., G.L. Swinford, and D.G. Gardner, 1992. Use of

thermal-enhanced soil vapor extraction to accelerate

remediation of diesel-affected soils, in Proc. Conf. on

Petroleum Hydrocarbons and Organic Chemicals in Ground

Water: Prevention, Detection, and Restoration, Natl. Ground

Water Assoc., Dublin, OH, 413-426.

Smolley, M., and J.C. Kappmeyer, 1991. Cone penetrometer

tests and HydroPunch sampling: A screening technique for

plume definition, Ground Water Monit. Rev., 11(2): 101-106.

Sullivan, C.R., R.E. Zinner, and J.P. Hughes, 1988. The

occurrence of hydrocarbon on an unconfined aquifer and

implications for liquid recovery, in Proc. Conf. on Petroleum

Hydrocarbons and Organic Chemicals in Ground Water:

Prevention, Detection, and Restoration, Natl. Ground Water

Assoc., Dublin, OH, 135-156.

Testa, S.M., and M.T. Paczkowski, 1989. Volume

determination and recoverability of free hydrocarbon, Ground

Water Monit. Rev., 9(1): 120-128.

Testa, S.M., D.L. Winegardner, and C.B. Burris, 1992.

Reinjection of coproduced groundwater in relation to LNAPL

occurrence, in Proc. Conf. on Petroleum Hydrocarbons and

Organic Chemicals in Ground Water: Prevention, Detection,

and Restoration, Natl. Ground Water Assoc., Dublin, OH, 127-

145.

Thomas, T.M., and C.H. Ward, 1989. In situ biorestoration of

organic contaminants in the subsurface, Environ. Sci.

Technol., 23: 760-765.

Thompson, G.M., and D.L. Marrin, 1987. Soil gas contaminant

investigations: A dynamic approach, Ground Water Monit.

Rev., 7(3): 88-93.

Travis, C.C., and J.M. Macinnis, 1992. Vapor extraction of

organics from subsurface soils-Is it effective?, Environ. Sci.

Technol., 26(10): 1885-1887.

Udell, K.S., 1992. Thermally enhanced removal of the oily

phase, in Proc. Subsurface Restoration Conf., Dallas, TX,

Rice Univ., Dept. of Environ. Sci. & Eng., Houston, TX, 51-53.

U.S.EPA, 1988. Cleanup of releases from petroleum USTs:

Selected technologies, EPA/530/UST-88/001, U.S.EPA,

Washington, DC, 110 pp.

U.S.EPA, 1990. Subsurface contamination reference guide,

EPA/540/2-90/011, U.S.EPA, Washington, DC, 13 pp.

U.S.EPA, 1991. Seminar publication: Site characterization for

subsurface remediation, EPA/625/4-91/026, U.S.EPA, R.S.

Kerr Environ. Res. Lab., Ada, OK, 259 pp.

U.S.EPA, 1992a. Dense nonaqueous phase liquids -- A

workshop summary, Dallas, Texas, April 16-18, 1991, EPA/

600/R-92/030, U.S.EPA, R.S. Kerr Environ. Res. Lab., Ada,

OK, 81 pp.

U.S.EPA, 1992b. Estimating potential for occurrence of

DNAPL at Superfund sites, Publ. 9355.4-07FS, U.S.EPA, R.S.

Kerr Environ. Res. Lab., Ada, OK, 9 pp.

U.S.EPA, 1992c. Proceedings of the symposium on soil

venting, April 29 - May 1, 1991, Houston, Texas, EPA/600/R92/174, U.S.EPA, R.S. Kerr Environ. Res. Lab., Ada, OK, 334

pp.

U.S.EPA, 1993a. Accutech pneumatic fracturing extraction

and hot gas injection, Phase 1, EPA/540/AR-93/509,

U.S.EPA, Risk Red. Eng. Lab., Cincinnati, OH, 44 pp.

U.S.EPA, 1993b. Hydraulic fracturing technology, Applications

analysis and technology evaluation report, EPA/540/R-93/505,

U.S.EPA, Risk Red. Eng. Lab., Cincinnati, OH, 119 pp.

U.S.EPA, 1994. Manual: Alternative methods for fluid delivery

and recovery, EPA/625/R-94/003, U.S.EPA, Risk Red. Eng.

Lab., Cincinnati, OH, 87 pp.

van Dam, J., 1967. The migration of hydrocarbons in a water

bearing stratum, in The Joint Problems of the Oil and Water

Industries, P. Hepple, ed., Elsevier, Amsterdam, 55-96.
28

Yaniga, P.M., and J.G. Warburton, 1984. Discrimination

between real and apparent accumulation of immiscible

hydrocarbons on the water table: A theoretical and empirical

analysis, in Proc. Fourth Natl. Symp. and Expo. on Aquifer

Restoration and Ground Water Monitoring, Natl. Ground

Water Assoc, Dublin, OH, 311-315.

Zinner, R.E., E.A. Hodder, W.E. Carroll, and C.A. Peck, 1991.

Utilizing groundwater reinjection in the design of a liquid

hydrocarbon recovery system, in Proc. Conf. on Petroleum

Hydrocarbons and Organic Chemicals in Ground Water:

Prevention, Detection, and Restoration, Natl. Ground Water

Assoc., Dublin, OH, 469-483.

van der Heijde, P.K.M., and O.A. Elnawawy, 1993.

Compilation of ground-water models, EPA/600/2-93/118,

U.S.EPA, R.S. Kerr Environ. Res. Lab., Ada, OK, 279 pp.

van Eyk, J., 1992. Bioventing: An in situ remedial technology,

scope and limitations, in Proc. Symp. on Soil Venting, EPA/

600/R-92/174, U.S. EPA, R.S.Kerr Environ. Res. Lab., Ada,

OK, 317-334.

Wagner, R.B., D.R. Hampton, and J.A. Howell, 1989. A new

tool to determine the actual thickness of free product in a

shallow aquifer, in Proc. Conf. on Petroleum Hydrocarbons

and Organic Chemicals in Ground Water: Prevention,

Detection, and Restoration, Natl. Ground Water Assoc.,

Dublin, OH, 45-59.

Walther, E.G., A.M. Pitchford, and G.R. Olhoeft, 1986. A

strategy for detecting subsurface organic contaminants, in

Proc. NWWA/API Conf. on Petroleum Hydrocarbons and

Organic Chemicals in Ground Water: Prevention, Detection,

and Restoration, Natl. Ground Water Assoc., Dublin, OH, 357-

381.

Waterloo Centre For Ground Water Research, 1989.

University of Waterloo Short Course, “Dense Immiscible

Phase Liquid Contaminants in Porous and Fractured Media”,

Kitchner, Ontario, Canada, Nov. 6-9, 1989.

Weaver, J.W., R.J. Charbeneau, J.D. Tauxe, B.K. Lien, and

J.B. Provost, 1994, The Hydrocarbon Spill Screening Model

(HSSM) Volume 1: User’s Guide, EPA/600/R-94/039a, U.S.

EPA, R.S. Kerr Environ. Res. Lab., Ada, OK, 212 pp.

Wickramanayake, G.B., N. Gupta, R.E. Hinchee, and B.J.

Nielsen, 1991. Free petroleum hydrocarbon volume estimates

from monitoring well data, J. Environ. Eng., 117(5): 686-691.

Williams, D.E., and D.G. Wilder, 1971. Gasoline pollution of a

ground-water reservoir - A case history, Ground Water, 9(6):

50-54.

Wilson, J.L., and S.H. Conrad, 1984. Is physical displacement

of residual hydrocarbons a realistic possibility in aquifer

restoration?, in Proc. NWWA/API Conf. on Petroleum

Hydrocarbons and Organic Chemical in Ground Water:

Prevention, Detection, and Restoration, Natl. Ground Water

Assoc., Dublin, OH, 274-298.

Wilson, J.T., L.E. Leach, M. Henson and J.N. Jones, 1986. In

situ biorestoration as a ground water remediation technique,

Ground Water Monit. Rev., 6(4): 56-64.

Wilson, S.B., and R. A. Brown, 1989. In situ bioreclamation: A

cost effective technology to remediate subsurface organic

contamination, Ground Water Monit. Rev., 9(1): 175-179.

Yaniga, P.M., 1984. Hydrocarbon retrieval and apparent

hydrocarbon thickness: Interrelationships to recharging/

discharging aquifer conditions, in Proc. NWWA/API Conf. on

Petroleum Hydrocarbons and Organic Chemicals in Ground

Water: Prevention, Detection and Restoration, Natl. Ground

Water Assoc., Dublin, OH, 299-329.