United States
Environmental Protection
Agency
Office of
Research and
Development
Office of Solid Waste
and Emergency
Response
EPA/540/S-95/500
July 1995
&EPA Ground Water Issue
Light Nonaqueous Phase Liquids
Charles J. Newell,* Steven D. Acree,** Randall R. Ross/
and Scott G. Huling**
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 LNAPL-
related ground-water contamination problems result from the
release of petroleum products. These products are typically
multicomponent organic mixtures composed of chemicals with
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., n-
dodecane 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.
Groundwater Services, Inc.
Roberts. Kerr Environmental Research Laboratory.
^ I0'v •
Superfund Technology Support Center for
Ground Water
Robert S. Kerr Environmental
Research Laboratory
Ada, Oklahoma
Oflte* of soRrf Waste and i wr
tft, SO
Wafter W, Kovaficfc* Jr,
Printed on Recycled Paper
-------
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)
to****
Methyl Ethyl Ketone
4-Methyl-2-Pentanone
Tetrahydrofuran
ftmsfyt
0.805
0.8017
0.8892
ttyftsunicf
Vi$tX>$lty
0.40
0.5848
0.55
Waterf
2.68 E+05
1 .9 E+04
3 E+05<1>
V^porf
fmmH0}
71.2
16
45.6 (2)
Hemy^ tawf
O»>$fc&tf
2.74 E-05 <2>
1 .55 E-04 (2)
1.1 E-04 <2>
Benzene 0.8765 0.6468
Ethyl Benzene 0.867 0.678
Styrene 0.9060 0.751
Toluene 0.8669 0.58
m-Xylene 0.8642 <1> 0.608
o-Xylene 0.880 <1> 0.802
p-Xylene 0.8610 <1> 0.635
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 <">
Jet Fuel (JP-4) -0.75 -0.83 <5>
Mineral Base
Crankcase Oil 0.84-0.96 <6> ~275<4>
1.78
1.52
3
5.15
2
1 .7
1.98
E+03
E+02
E+02
E+02
E+02
E+02
E+02<1>
76
7
5
22
9
7
9
Values are given at 20°C unless noted.
Value is at 25°C.
Value is at unknown temperature but is assumed to be 20°- 30°C.
Value is at 15.6°C.
Value is at 38°C.
Value is at 21 °C.
Value is at 15°C.
5.43
7.9
2.28
6.61
6.91
4.94
7.01
E-03 <1>
E-03 <1>
E-03
E-03<1>
E-03 <1>
E-03 <1>
E-03 <1>
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 continuous-
phase 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
Dissolved
Contaminants
Water Table
LNAPL Contamination
Ground-Water
"* Flow
After Mercer and Cohen (1990)
Figure 1. Simplified conceptual model for LNAPL release and
migration.
-------
Solid
Soil Gas
Water
LNAPL
After Huling & Weaver (1991)
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.
Water
After DiGiulio and Cho (1990)
Figure 3. Partitioning of LNAPL among the four phases
potentially found in the unsaturated zone.
(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 pore-
scale 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
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.,
-------
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 aqueous-
phase 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 variability 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 non-
wetting 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
interracial 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 (Sjr) and for the non-wetting fluid, generally NAPL,
-------
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
bw. 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.
1.00
Kr = relative permeability
o.
<
0.10-
0.01
LNAPL
Flow
Mixed Flow
0.10
£
k_
^
0.01
or-
Water Saturation
H 100%
LNAPL Saturation
-H o
After Williams and Wilder (1971)
Figure 4. Hypothetical relative permeability curves for water and
an LNAPL in a porous medium.
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 p g/jL) (dh/dl)
(1)
where
v = Darcy velocity (L/T)
k = intrinsic permeability (L2)
p = density of NAPL (M/L3)
g = force of gravity (L/T2)
p. = dynamic (absolute) viscosity (M/L*T)
dh/dl = hydraulic gradient of NAPL mass (LA)
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
-------
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 (Mendozaand 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.
Residual Saturation of
LNAPL in Soil
Ground-Water
•* Flow
After Waterloo Centre for Groundwater Research (1989)
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.
LNAPL
Release
Low
Permeability
Units
After Waterloo Centre for Groundwater Research (1989)
Figure 6. LNAPL affected by low permeability units prior to
reaching water table.
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
-------
Water and Air
UNAPU water an* Air
k V»s
Porous Medium
LNAPL
IVater
Well
After Farretal. (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 ground-
water 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.
Prior to Rise in Water Table
Following Rise in Water Table
LNAPL trapped by capillary forces
Mobile LNAPL
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.
-------
After Waterloo Centre for Groundwater Research (1989)
Figure 9. Potential LNAPL migration in fractured medium.
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 103moles/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
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.
KS,
(2)
where
S" = effective aqueous solubility of compound i in
NAPL mixture
X{ = 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 two-
component 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.
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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 hydrophobia,
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 LINAPL 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 non-
equilibrium effects may limit contaminant mass transfer by
dissolution under certain conditions such as high ground-
water 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-lreat
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 evalgate
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 site-
specific, 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):
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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
Lrthologic, 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 pressure-
saturation 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
10
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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, truck-
mounted 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 non-
invasive 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 ground-
water flow and potential LNAPL migration.
LNAPL
Light nonaqueous phase liquids may exist as continuous, free-
phase 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,
11
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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 bentonrte, 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 all, 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 ILNAPL/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
Well
Capillary
Fringe
Well
Screen
Apparent
LNAPL
Thickness
Water
LNAPL Trapped by
Capillary Forces
Mobile LNAPL
Figure 10. Simplistic conceptualization of LNAPL thickness
measured in well and LNAPL distributed information.
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 ground-
water elevation have also been noted in several studies (e.g.,
Huntetal., 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 towered
until it spans the LNAPL layer, withdrawn, and LNAPL
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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 Ball-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 bail-
down 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
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
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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 em unification 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.
So/7 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 etal., 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-
14
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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.
Recovery Well with
Open Interval
Plan View
Cross Section
After API (1989)
Figure 11. Plan and cross sectional views of a drain designed for mobile LNAPL recovery.
15
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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 ground-
water 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:
16
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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. hydrophobia 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
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
Well
Screen
Water
Pump
After API (1989)
Figure 12. Conceptual system design for separate recovery of LNAPL and water in a single well.
17
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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
etal., 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
18
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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, ground-
water 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.
So/7 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
After Huling and Weaver (1991)
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 20° 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 short-
circuiting 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
19
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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
Air
Air
Channels
Vapors
4
Air
Figure 14. Conceptual design of air sparging system.
20
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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., 19926).
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.
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 wetting-
phase 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
21
-------
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
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
Low Permeability
Barrier ^^
Low
Concentration
Plume
High Concentration
Aqueous-Phase Plume
r*\ *
>w Permeability %
Ground-Water Flow
Low Permeability
Barrier
Figure 15. Map view of reactive treatment wall and low
permeability barriers used to channel ground-water
flow.
22
-------
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 etal., 1986).
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