United States
Environmental Protection
Agency
Office of
Research and
Development
Office of Solid Waste
and Emergency
Response
EPA/540/4-91-002
March 1991
&EPA Ground Water Issue
DENSE NONAQUEOUS PHASE LIQUIDS
Scott G. Huling* and James W. Weaver**
Background
The Regional Superfund Ground Water Forum is a group of
EPA professionals representing EPA's Regional Superfund
Offices, committed to the identification and the resolution of
ground water issues impacting the remediation of Superfund
sites. The Forum is supported by and advises the Superfund
Technical Support Project. Dense nonaqueous phase liquids
is an issue identified by the Forum as a concern of
Superfund decision-makers. For further information contact
Scott G. Huling (FTS:743-2313), Jim Weaver (FTS:743-
2420), or Randall R. Ross (FTS: 743-2355).
Introduction
Dense nonaqueous phase liquids (DNAPLs) are present at
numerous hazardous waste sites and are suspected to exist
at many more. Due to the numerous variables influencing
DNAPL transport and fate in the subsurface, and
consequently, the ensuing complexity, DNAPLs are largely
undetected and yet are likely to be a significant limiting factor
in site remediation. This issue paper is a literature evaluation
focusing on DNAPLs and provides an overview from a
conceptual fate and transport point of view of DNAPL phase
distribution, monitoring, site characterization, remediation,
and modeling.
A nonaqueous phase liquid (NAPL) is a term used to
describe the physical and chemical differences between a
hydrocarbon liquid and water which result in a physical
interface between a mixture of the two liquids. The interface
is a physical dividing surface between the bulk phases of the
two liquids, but compounds found in the NAPL are not
prevented from solubilizing into the ground water.
Immiscibility is typically determined based on the visual
observation of a physical interface in a water- hydrocarbon
mixture. There are numerous methods, however, which are
used to quantify the physical and chemical properties of
hydrocarbon liquids (31).
Nonaqueous phase liquids have typically been divided into
two general categories, dense and light. These terms
describe the specific gravity, or the weight of the nonaqueous
phase liquid relative to water. Correspondingly, the dense
nonaqueous phase liquids have a specific gravity greater
than water, and the light nonaqueous phase liquids (LNAPL)
have a specific gravity less than water.
Several of the most common compounds associated with
DNAPLs found at Superfund sites are included in Table 1.
These compounds are a partial list of a larger list identified
by a national screening of the most prevalent compounds
found at Superfund sites (65). The general chemical
categories are halogenated/non-halogenated semi-volatiles
and halogenated volatiles. These compounds are typically
found in the following wastes and waste-producing
processes: solvents, wood preserving wastes (creosote,
pentachlorophenol), coal tars, and pesticides. The most
frequently cited group of these contaminants to date are the
chlorinated solvents.
DNAPL Transport and Fate - Conceptual Approach
Fate and transport of DNAPLs in the subsurface will be
presented from a conceptual point of view. Figures have
been selected for various spill scenarios which illustrate the
general behavior of DNAPL in the subsurface. Following the
conceptual approach, detailed information will be presented
explaining the specific mechanisms, processes, and
variables which influence DNAPL fate and transport. This
includes DNAPL characteristics, subsurface media
characteristics, and saturation dependent parameters.
Unsaturated Zone
Figure 1 indicates the general scenario of a release of
DNAPL into the soil which subsequently migrates vertically
under both the forces of gravity and soil capillarity. Soil
capillarity is also responsible for the lateral migration of
DNAPL. A point is reached at which the DNAPL no longer
holds together as a continuous phase, but rather is present
as isolated residual globules. The fraction of the hydrocarbon
that is retained by capillary forces in the porous media is
Environmental Engineer," Research Hydro-legist, U.S.
Environmental Protection Agency, Robert S. Kerr Environmental
Research Laboratory, Ada, Oklahoma.
0-noA/
v
Superfund Technology Support Center for
Ground Water
Robert S. Kerr Environmental
Research Laboratory
Ada, Oklahoma
Technology Innovation Office
Office of Solid Waste and Emergency
Response, US EPA, Washington, D.C.
Walter W. Kovalick, Jr., Ph.D.
Director
-------
Table 1. Most prevalent chemical compounds at U.S. Superfund Sites (65) with a specific gravity
greater than one.
Vapor[6]
Compound
Density
[1]
Dynamic[2] Kinematic
Viscosity Viscosity[3] Solub.
Water[4]
Constant[5]
Henry's Law
Pressure
Halogenated Semi-volatiles
1 ,4-Dichlorobenzene
1 ,2-Dichlorobenzene
Aroclor 1242
Aroclor 1260
Aroclor 1254
Chlordane
Dieldrin
2,3,4,6-Tetrachlorophenol
Pentachlorophenol
Halogenated Volatiles
Chlorobenzene
1 ,2-Dichloropropane
1,1-Dichloroethane
1,1-Dichloroethylene
1,2-Dichloroethane
Trans-1 ,2-Dichloroethylene
Cis-1 ,2-Dichloroethylene
1,1,1-Trichloroethane
Methylene Chloride
1,1,2-Trichloroethane
Trichloroethylene
Chloroform
Carbon Tetrachloride
1 ,1 ,2,2-Tetrachloroethane
Tetrachloroethylene
Ethylene Dibromide
1.2475
1.3060
1.3850
1.4400
1.5380
1.6
1.7500
1.8390
1.9780
1.1060
1.1580
1.1750
1.2140
1.2530
1.2570
1.2480
1.3250
1.3250
1.4436
1.4620
1.4850
1.5947
1.6
1.6250
2.1720
1.2580
1.3020
1.1040
0.7560
0.8400
0.3770
0.3300
0.8400
0.4040
0.4670
0.8580
0.4300
0.1190
0.5700
0.5630
0.9650
1.7700
0.8900
1.6760
1.008
0.997
0.69
0.683
0.72
0.321
0.27
0.67
0.321
0.364
0.647
0.324
0.824
0.390
0.379
0.605
1.10
0.54
0.79
8. 0 E+01
1. 0 E+02
4.5 E-01
2.7 E-03
1.2 E-02
5.6 E-02
1.86 E-01
1. 0 E+03
1.4 E+01
4.9 E+02
2.7 E+03
5.5 E+03
4. 0 E+02
8.69 E+03
6.3 E+03
3.5 E+03
9.5 E+02
1.32E+04
4.5 E+03
1. 0 E+03
8.22 E+03
8.0 E+02
2.9 E+03
1.5 E+02
3.4 E+03
1.58 E-03
1.88 E-03
3.4 E-04
3.4 E-04
2.8 E-04
2.2 E-04
9.7 E-06
2.8 E-06
3.46 E-03
3.6 E-03
5.45 E-04
1.49 E-03
1.1 E-03
5.32 E-03
7.5 E-03
4.08 E-03
2.57 E-03
1.17 E-03
8.92 E-03
3.75 E-03
2. 0 E-02
5. 0 E-04
2.27 E-02
3. 18 E-04
6 E-01
9.6 E-01
4.06 E-04
4.05 E-05
7.71 E-05
1 E-05
1.78 E-07
1.1 E-04
8.8 E+00
3.95 E+01
1.82 E+02
5 E+02
6.37 E+01
2.65 E+02
2 E+02
1 E+02
3.5 E+02
1.88 E+01
5.87 E+01
1.6 E+02
9.13 E+01
4.9 E+00
1.4 E+01
1.1 E+01
Non-halogenated Semi-volatiles
2-Methyl Napthalene
o-Cresol
p-Cresol
2,4-Dimethylphenol
m-Cresol
Phenol
Naphthalene
Benzo(a)Anthracene
Flourene
Acenaphthene
Anthracene
Dibenz(a,h)Anthracene
Fluoranthene
Pyrene
Chrysene
2,4-Dinitrophenol
Miscellaneous
Coal Tar
Creosote
1.0058
1.0273
1.0347
1.0360
1.0380
1.0576
1.1620
1.1740
1.2030
1.2250
1.2500
1.2520
1.2520
1.2710
1.2740
1.6800
1.028<7>
1.05
21. 0
18.98<7>
1.08<8>
20
3.87
2.54 E+01
3.1 E+04
2.4 E+04
6.2 E+03
2.35 E+04
8.4 E+04
3.1 E+01
1.4 E-02
1.9 E+00
3.88 E+00
7.5 E-02
2.5 E-03
2.65 E-01
1.48 E-01
6. 0 E-03
6. 0 E+03
5.06 E-02
4.7 E-05
3.5 E-04
2.5 E-06
3.8 E-05
7.8 E-07
1.27 E-03
4.5 E-06
7.65 E-05
1.2 E-03
3.38 E-05
7.33 E-08
6.5 E-06
1.2 E-05
1.05 E-06
6.45 E-10
6.80 E-02
2.45 E-01
1.08 E-01
9.8 E-02
1.53 E-01
5.293E-01
2.336E-01
1.16 E-09
6.67 E-04
2.31 E-02
1.08 E-05
1 E-10
E-02 E-06
6.67 E-06
6.3 E-09
1.49 E-05
[1] g/cc
[2] centipoise (cp), water has a dynamic viscosity of 1 cp at
20°C.
[3] centistokes (cs)
[4] mg/l
[5] atm-rrWmol
[6] mm Hg
[7] 45° F (70)
[8] 15.5°C, varies with creosote mix (62)
-------
Residual Saturation of
DNAPL in Soil From Spill
Residual
Saturation of
DNAPL Gaseous DNAPL in
Vapors /" VadoseZone
Infiltration, Leaching an
Mobile DNAPL Vapor:
Groundwater
Plume From DNAPL Flow
Residual Saturation
Plume From DNAPL
Soil Vapor
After, Waterloo Centre for Groundwater Research
Figure 1. The entire volume of DNAPL is exhausted by residual
saturation in the vadose zone prior to DNAPL reaching
the water table. Soluble phase compounds may be
leached from the DNAPL residual saturation and
contaminate the ground water.
referred to as residual saturation. In this spill scenario, the
residual saturation in the unsaturated zone exhausted the
volume of DNAPL, preventing it from reaching the water
table. This figure also shows the subsequent leaching
(solubilization) of the DNAPL residual saturation by water
percolating through the unsaturated zone (vadose zone). The
leachate reaching the saturated zone results in ground-water
contamination by the soluble phase components of the
hydrocarbon. Additionally, the residual saturation at or near
the water table is also subjected to leaching from the rise and
fall of the water table (seasonal, sea level, etc.).
Increasing information is drawing attention to the importance
of the possibility that gaseous-phase vapors from NAPL in
the unsaturated zone are responsible for contaminating the
ground water and soil (18,47). It is reported that the greater
"relative vapor density" of gaseous vapors to air will be
affected by gravity and will tend to sink. In subsurface
systems where lateral spreading is not restricted, spreading
of the vapors may occur as indicated in Figure 2. The result
is that a greater amount of soils and ground water will be
exposed to the DNAPL vapors and may result in further
contamination. The extent of contamination will depend
largely on the partitioning of the DNAPL vapor phase
between the aqueous and solid phases.
DNAPL Phase Distribution - Four Phase System
It is apparent from Figures 1 and 2 that the DNAPL may be
present in the subsurface in various physical states or what is
referred to as phases. As illustrated in Figure 3, there are
four possible phases: gaseous, solid, water, and immiscible
hydrocarbon (DNAPL) in the unsaturated zone.
Contaminants associated with the release of DNAPL can,
therefore, occur in four phases described as follows:
1. Air phase - contaminants may be present as vapors;
2. Solid phase - contaminants may adsorb or partition onto
the soil or aquifer material;
3. Water phase - contaminants may dissolve into the water
according to their solubility; and
Figure 2. Migration of DNAPL vapors from the spill area and
subsequent contamination of the soils and ground
water.
4. Immiscible phase - contaminants may be present as
dense nonaqueous phase liquids.
The four phase system is the most complex scenario
because there are four phases and the contaminant can
partition between any one or all four of these phases, as
illustrated in Figure 4. For example, TCE introduced into the
subsurface as a DNAPL may partition onto the soil phase,
volatilize into the soil gas, and solubilize into the water phase
resulting in contamination in all four phases. TCE can also
partition between the water and soil, water and air, and
between the soil and air. There are six pathways of phase
distribution in the unsaturated zone. The distribution of a
contaminant between these phases can be represented by
empirical relationships referred to as partition coefficients.
The partition coefficients, or the distribution of the DNAPL
between the four phases, is highly site-specific and highly
dependent on the characteristics of both the soil/aquifer
matrix and the DNAPL. Therefore, the distribution between
phases may change with time and/or location at the same
site and during different stages of site remediation.
Solid
Water
DNAPL
Figure 3. A DNAPL contaminated unsaturated zone has four
physical states or phases (air, solid, water,
immiscible). The contaminant may be present in any
one, or all four phases.
-------
Wat§
Four Phase System
Partition Coefficients
K = Soil-water partition coefficient
KH = Henry's Constant
K' = DNAPL-water partition coefficient
K" = DNAPL-air partition coefficient
after DiGiulio, 1990 (!
Figure 4. Distribution of DNAPL between the four phases found
in the vadose zone.
Figure 5. DNAPL spilled into fractured rock systems may
follow a complex distribution of the preferential pathways.
The concept of phase distribution is critical in decision-
making. Understanding the phase distribution of a DNAPL
introduced into the subsurface provides significant insight in
determining which tools are viable options with respect to
site characterization and remediation.
DNAPL represented by residual saturation in the four phase
diagram is largely immobile under the usual subsurface
pressure conditions and can migrate further only: 1) in water
according to its solubility; or 2) in the gas phase of the
unsaturated zone (47). DNAPL components adsorbed onto
the soil are also considered immobile. The mobile phases
are, therefore, the soluble and volatile components of the
DNAPL in the water and air, respectively.
The pore space in the unsaturated zone may be filled with
one or all three fluid phases (gaseous, aqueous, immiscible).
The presence of DNAPL as a continuous immiscible phase
has the potential to be mobile. The mobility of DNAPL in the
subsurface must be evaluated on a case by case basis. The
maximum number of potentially mobile fluid phases is three.
Simultaneous flow of the three phases (air, water, and
immiscible) is considerably more complicated than two-
phase flow (46). The mobility of three phase flow in a four-
phase system is complex, poorly understood, and is beyond
the scope of this DNAPL overview. The relative mobility of
the two phases, water and DNAPL, in a three-phase system
is presented below in the section entitled "Relative
Permeability."
Generally, rock aquifers contain a myriad of cracks
(fractures) of various lengths, widths, and apertures (32).
Fractured rock systems have been described as rock blocks
bounded by discrete discontinuities comprised of fractures,
joints, and shear zones which may be open, mineral-filled,
deformed, or any combination thereof (61). The unsaturated
zone overlying these fractured rock systems also contain the
myriad of preferential pathways. DNAPL introduced into such
formations (Figure 5) follow complex pathways due to the
heterogeneous distribution of the cracks, conduits, and
fractures', i.e., preferential pathways. Transport of DNAPL
may follow non-Darcian flow in the open fractures and/or
Darcian flow in the porous media filled fractures. Relatively
small volumes of NAPL may move deep, quickly into the rock
because the retention capacity offered by the dead-end
fractures and the immobile fragments and globules in the
larger fractures is so small (32). Currently, the capability to
collect the detailed information fora complete description of
a contaminated fractured rock system is regarded as neither
technically possible nor economically feasible (61).
Low permeability stratigraphic units such as high clay content
formations may also contain a heterogeneous distribution of
preferential pathways. As illustrated in Figure 6, DNAPL
transport in these preferential pathways is correspondingly
complex. Typically, it is assumed that high clay content
formations are impervious to DNAPL. However, as DNAPL
spreads out on low permeable formations it tends to seek out
zones of higher permeability. As a result, preferential
pathways allow the DNAPL to migrate further into the low
permeable formation, or through it to underlying stratigraphic
units. It is apparent from Figures 5 and 6 that the complexity
of DNAPL transport may be significant prior to reaching the
water table.
Saturated Zone
The second general scenario is one in which the volume of
DNAPL is sufficient to overcome the fraction depleted by the
residual saturation in the vadose zone, as illustrated in
Figure 7. Consequently, the DNAPL reaches the water table
and contaminates the ground water directly. The specific
gravity of DNAPL is greater than water, therefore, the DNAPL
migrates into the saturated zone. In this scenario, DNAPL
continues the vertical migration through the saturated zone
until the volume is eventually exhausted by the residual
saturation process or until it is intercepted by a low
permeable formation where it begins to migrate laterally.
DNAPL Phase Distribution - Three Phase System
Due to the lack of the gaseous phase, the saturated zone
containing DNAPL is considered a three-phase system
consisting of the solid, water, and immiscible hydrocarbon
(Figure 8). Contaminant distribution in the three-phase
-------
Water
DNAPL
Solid
Figure 6. DNAPL spilled into a low permeable formation may
follow a complex distribution of preferential pathways.
The volume of DNAPL is exhausted in the vadose
zone prior to reaching the water table.
Figure 8. A DNAPL contaminated saturated zone has three
phases (solid, water, immiscible). The contaminant
may be present in any one, or all three phases.
system is less complex than the four-phase system. Again,
this is highly dependent on the characteristics of both the
aquifer matrix and the DNAPL. Figure 9 indicates the three
phases and the transfer of the mass of contaminant between
the phases. In this scenario, there are only three pathways of
phase distribution in the saturated zone.
Note that when the DNAPL is represented by residual
saturation in the three-phase system, the mobile phase of the
contaminant is the water soluble components of the DNAPL
and the immobile phases are the residual saturation and the
adsorbed components of the DNAPL associated with the
aquifer material. The main mobilization mechanism of the
residual saturation is removal of soluble phase components
into the ground water. When the DNAPL is present as a
continuous immiscible phase, it too is considered one of the
mobile phases of the contaminant. While the continuous
phase DNAPL has the potential to be mobile, immobile
continuous phase DNAPL may also exist in the subsurface.
Although the saturated zone is considered a three-phase
system, gaseous vapors from DNAPL in the unsaturated
zone does have the potential to affect ground-water quality,
as was indicated earlier in Figure 2.
Assuming the residual saturation in the saturated zone does
not deplete the entire volume of the DNAPL, the DNAPL will
continue migrating vertically until it encounters a zone or
stratigraphic unit of lower permeability. Upon reaching the
zone of lower permeability, the DNAPL will begin to migrate
laterally. The hydraulic conductivity in the vertical direction is
typically less than in the horizontal direction. It is not
uncommon to find vertical conductivity that is one-fifth or one-
tenth the horizontal value (4). It is expected that DNAPL
spilled into the subsurface will have a significant potential to
migrate laterally. If the lower permeable boundary is "bowl
shaped", the DNAPL will pond as a reservoir (refer to Figure
10). As illustrated in Figure 11, it is not uncommon to observe
a perched DNAPL reservoir where a discontinuous
impermeable layer; i.e., silt or clay lens, intercepts the
vertical migration of DNAPL. When a sufficient volume of
Residual
Saturation of
DNAPL in Soil
From Spill
Groundwater
Flow
Residual
Saturation in Saturated Zone
After, Waterloo Centre for Groundwater Research
Figure 7. The volume of DNAPL is sufficient to overcome the
residual saturation in the vadose zone and
consequently penetrates the water table.
Three Phase System
Water
K' = DNAPL-water partition coefficient
K = Soil-water partition coefficient
Figure 9. Distribution of DNAPL between the three phases
found in the saturated zone.
-------
Low Permeable
Stratigraphic Unit*
Groundwater
Flow
CLAY
After, Waterloo Centre for Groundwater Research!
Figure 10. Migration of DNAPL through the vadose zone to an
impermeable boundary.
Figure 12. Perched and deep DNAPL reservoirs.
DNAPL has been released and multiple discontinuous
impermeable layers exist, the DNAPL may be present in
several perched reservoirs as well as a deep reservoir (refer
to Figure 12). Lateral migration continues until either the
residual saturation depletes the DNAPL or an impermeable
depression immobilizes the DNAPL in a reservoir type
scenario. Soluble-phase components of the DNAPL will
partition into the ground water from both the residual
saturation or DNAPL pools. The migration of DNAPL
vertically through the aquifer results in the release of soluble-
phase components of the DNAPL across the entire thickness
of the aquifer. Note, that ground water becomes
contaminated as it flows through, and around, the DNAPL
contaminated zone.
As indicated earlier, DNAPL will migrate laterally upon
reaching a stratigraphic unit of lower permeability. Transport
of DNAPL will therefore be largely dependent on the gradient
of the stratigraphy. Occasionally, the directional gradient of
an impermeable stratigraphic unit may be different than the
direction of ground-water flow as illustrated in Figure 13a.
This may result in the migration of the continuous phase
DNAPL in a direction different from the ground-water flow.
Nonhorizontal stratigraphic units with varying hydraulic
conductivity may also convey DNAPL in a different direction
Low Permeable
Stratigraphic Unit
than ground-water flow, and at different rates (refer to Figure
13b). Determination of the direction of impermeable
stratigraphic units will therefore provide useful information
concerning the direction of DNAPL transport.
Similar to the unsaturated zone, the saturated zone also
contains a complex distribution of preferential pathways from
cracks, fractures, joints, etc. DNAPL introduced into such
formations correspondingly follow the complex network of
pathways through an otherwise relatively impermeable rock
material. Other pathways which may behave as vertical
conduits for DNAPL include root holes, stratigraphic
windows, disposal wells, unsealed geotechnical boreholes,
improperly sealed hydrogeological investigation sampling
holes and monitoring wells, and old uncased/unsealed water
supply wells (72). Transport of the DNAPL may migrate very
rapidly in these open conduits or follow Darcian flow in the
surrounding porous media or porous media filled fractures. A
relatively small volume of DNAPL can move deep into a
fractured system due to the low retentive capacity of the
fractured system. Consequently, fractured clay or rock
stratigraphic units, which are often considered lower DNAPL
Figure 11. Perched DNAPL reservoir.
Figure 13a. Stratigraphic gradient different from ground water
gradient results in a different direction of flow of
the ground water and continuous phase DNAPL.
-------
Where Kx2> toi> Kxs
Kx= Horizontal Hydraulic Conductivity
Figure 13b. Non-horizontal stratigraphic units with variable
hydraulic conductivity may convey DNAPL in a
different direction than the ground water flow
direction.
boundary conditions, may have preferential pathways leading
to lower formations, as depicted in Figure 14. Careful
inspection of soil cores at one Superfund site indicated that
DNAPL flow mainly occurred through preferential pathways
and was not uniformly distributed throughout the soil mass
(8). Due to the complex distribution of preferential pathways,
characterization of the volume distribution of the DNAPL is
difficult.
Important DNAPL Transport and Fate Parameters
There are several characteristics associated with both the
subsurface media and the DNAPL which largely determine
the fate and transport of the DNAPL. A brief discussion of
these parameters is included to help identify the specific
details of DNAPL transport mechanisms. Several of the
distinctive DNAPL phenomena observed on the field-scale
relates back to phenomena at the pore-scale. Therefore, it is
Kesidual ~
DNAPL Groundwater Sand
Flow
>ermeable Boundary
After, Waterloo Centre for Ground Water Researc
important to understand the principles from the pore-scale
level to develop an understanding of field-scale observations,
which is the scale at which much of the Superfund work
occurs. A more complete and comprehensive review of
these parameters is available (2,36,71).
DNAPL Characteristics
Fluid density is defined as the mass of fluid per unit volume,
i.e. g/cm3. Density of an immiscible hydrocarbon fluid is the
parameter which delineates LNAPL's from DNAPL's. The
property varies not only with molecular weight but also
molecular interaction and structure. In general, the density
varies with temperature and pressure (2). Equivalent
methods of expressing density are specific weight and
specific gravity. The specific weight is defined as the weight
of fluid per unit volume, i.e. Ib/ft3. The specific gravity (S.G.)
or the relative density of a fluid is defined as the ratio of the
weight of a given volume of substance at a specified
temperature to the weight of the same volume of water at a
given temperature (31). The S.G. is a relative indicator which
ultimately determines whether the fluid will float (S.G.< 1.0)
on, or penetrate into (S.G.>1.0) the water table. Table 1
contains a list of compounds with a density greater than one
that are considered DNAPL's. Note, however, that while the
specific gravity of pentachlorophenol and the non-
halogenated semi-volatiles is greater than 1.00, these
compounds are a solid at room temperature and would not
be expected to be found as an immiscible phase liquid at
wood preserving sites but are commonly found as contami-
nants. Pentachlorophenol is commonly used as a wood
preservant and is typically dissolved (4-7%) in No. 2 or 3 fuel
oil.
Viscosity
The viscosity of a fluid is a measure of its resistance to flow.
Molecular cohesion is the main cause of viscosity. As the
temperature increases in a liquid, the cohesive forces
decrease and the absolute viscosity decreases. The lower
the viscosity, the more readily a fluid will penetrate a porous
media. The hydraulic conductivity of porous media is a
function of both the density and viscosity of the fluid as
indicated in equation [1]. It is apparent from this equation
that fluids with either a viscosity less than water or fluids with
a density greater than water have the potential to be more
mobile in the subsurface, than water.
K= kpg where,
Figure 14. DNAPL transport in fracture and porous media
stratigraphic units.
K = hydraulic conductivity [1]
k = intrinsic permeability
p = fluid mass density
g = gravity
|i = dynamic (absolute) viscosity
Results from laboratory experiments indicated that several
chlorinated hydrocarbons which have low viscosity
(methylene chloride, perchloroethylene, 1,1,1-TCA, TCE) will
infiltrate into soil notably faster than will water (47). The
relative value of NAPL viscosity and density, to water,
indicates how fast it will flow in porous media (100%
saturated) with respect to water. For example, several low
-------
viscosity chlorinated hydrocarbons (TCE,
tetrachloroethylene, 1,1,1-TCA, Methylene Chloride,
Chloroform, Carbon Tetrachloride, refer to Table 1) will flow
1.5-3.0 times as fast as water and higher viscosity
compounds including light heating oil, diesel fuel, jet fuel, and
crude oil (i.e. LNAPL's) will flow 2-10 times slower than water
(45). Both coal tar and creosote typically have a specific
gravity greater than one and a viscosity greater than water. It
is interesting to note that the viscosity of NAPL may change
with time (36). As fresh crude oils lose the lighter volatile
components from evaporation, the oils become more viscous
as the heavier components compose a larger fraction of the
oily mixture resulting in an increase in viscosity.
Solubility
When an organic chemical is in physical contact with water,
the organic chemical will partition into the aqueous phase.
The equilibrium concentration of the organic chemical in the
aqueous phase is referred to as its solubility. Table 1
presents the solubility of several of the most commonly found
DNAPL's at EPA Superfund Sites. The solubility of organic
compounds varies considerably from the infinitely miscible
compounds, including alcohols (ethanol, methanol) to
extremely low
solubility compounds such as polynuclear aromatic
compounds.
Numerous variables influence the solubility of organic
compounds. The pH may affect the solubility of some organic
compounds. Organic acids may be expected to increase in
solubility with increasing pH, while organic bases may act in
the opposite way (31). For example, pentachlorophenol is an
acid which is ionized at higher pH's. In the ionized form,
pentachlorophenol would be more soluble in water (59).
Solubility in water is a function of the temperature, but the
strength and direction of this function varies. The presence of
dissolved salts or minerals in water leads to moderate
decreases in solubility (31). In a mixed solvent system,
consisting of water and one or more water-miscible
compounds, as the fraction of the cosolvent in the mixture
increases, the solubility of the organic chemical increases
exponentially (12). In general, the greater the molecular
weight and structural complexity of the organic compound,
the lower the solubility.
Organic compounds are only rarely found in ground water at
concentrations approaching their solubility limits, even when
organic liquid phases are known or suspected to be present.
The observed concentrations are usually more than a factor
of 10 lower than the solubility presumably due to diffusional
limitations of dissolution and the dilution of the dissolved
organic contaminants by dispersion (74). This has also been
attributed to: reduced solubility due to the presence of other
soluble compounds, the heterogeneous distribution of
DNAPL in the subsurface, and dilution from monitoring wells
with long intake lengths (10). Detection of DNAPL
components in the subsurface below the solubility should
clearly not be interpreted as a negative indicator for the
presence of DNAPL.
In a DNAPL spill scenario where the DNAPL or its vapors are
in contact with the ground water, the concentration of the
soluble phase components may range from non-detectable
up to the solubility of the compound. The rate of dissolution
has been expressed as a function of the properties of the
DNAPL components (solubility), ground water flow
conditions, differential between the actual and solubility
concentration, and the contact area between the DNAPL and
the ground water (10). The contact area is expected to be
heterogeneous and difficult to quantify. Additionally, as the
time of contact increases between the DNAPL and the water,
the concentration in the aqueous phase increases.
Vapor Pressure
The vapor pressure is that characteristic of the organic
chemical which determines how readily vapors volatilize or
evaporate from the pure phase liquid. Specifically, the partial
pressure exerted at the surface by these free molecules is
known as the vapor pressure (30). Molecular activity in a
liquid tends to free some surface molecules and this
tendency towards vaporization is mainly dependent on
temperature. The vapor pressure of DNAPL's can actually be
greater than the vapor pressure of volatile organic
compounds. For example, at 20 C, the ratio of the vapor
pressures of TCE and benzene is 1.4 (1).
Volatility
The volatility of a compound is a measure of the transfer of
the compound from the aqueous phase to the gaseous
phase. The transfer process from the water to the
atmosphere is dependent on the chemical and physical
properties of the compound, the presence of other
compounds, and the physical properties (velocity, turbulence,
depth) of the water body and atmosphere above it. The
factors that control volatilization are the solubility, molecular
weight, vapor pressure, and the nature of the air-water
interface through which it must pass (31). The Henry's
constant is a valuable parameter which can be used to help
evaluate the propensity of an organic compound to volatilize
from the water. The Henry's law constant is defined as the
vapor pressure divided by the aqueous solubility. Therefore,
the greater the Henry's law constant, the greater the
tendency to volatilize from the aqueous phase, refer to Table
1.
Interfacial Tension
The unique behavior of DNAPLs in porous media is largely
attributed to the interfacial tension which exists between
DNAPL and water, and between DNAPL and air. These
interfacial tensions, result in distinct interfaces between these
fluids at the pore-scale. When two immiscible liquids are in
contact, there is an interfacial energy which exists between
the fluids resulting in a physical interface. The interfacial
energy arises from the difference between the inward
attraction of the molecules in the interior of each phase and
those at the surface of contact (2). The greater the interfacial
tension between two immiscible liquids; the less likely
emulsions will form; emulsions will be more stable if formed,
and the better the phase separation after mixing. The
magnitude of the interfacial tension is less than the larger of
the surface tension values for the pure liquids, because the
mutual attraction of unlike molecules at the interface reduces
the large imbalance offerees (31). Interfacial tension
-------
decreases with increasing temperature, and may be affected
by pH, surfactants, and gases in solution (36). When this
force is encountered between a liquid and a gaseous phase,
the same force is called the surface tension (66).
The displacement of water by DNAPL and the displacement
of DNAPL by water in porous media often involves a
phenomena referred to as immiscible fingering. The lower the
interfacial tension between immiscible fluids, the greater the
instability of the waterDNAPL interface and thus the greater
the immiscible fingering (27). The distribution of the fingering
effects in porous media has been reported to be a function of
the density, viscosity, surface tension (27) and the
displacement velocity (13) of the fluids involved as well as
the porous media heterogeneity (28).
Wettabilitv
Wettability refers to the relative affinity of the soil for the
various fluids -water, air, and the organic phase. On a solid
surface, exposed to two different fluids, the wettability can be
inferred from the contact angle (66), also referred to as the
wetting angle, refer to Figure 15. In general, if the wetting
angle is less than 90 degrees, the fluid is said to be the
wetting fluid. In this scenario, water will preferentially occupy
the smaller pores and will be found on solid surfaces (14).
When the wetting angle is near 90 degrees, neither fluid is
preferentially attracted to the solid surfaces. If the wetting
angle is greater than 90 degrees, the DNAPL is said to be
the wetting fluid. The wetting angle is an indicator used to
determine whether the porous material will be preferentially
wetted by either the hydrocarbon or the aqueous phase (71).
Wettability, therefore, describes the preferential spreading of
one fluid over solid surfaces in a two-fluid system. The
wetting angle, which is a measure of wettability, is a solid-
liquid interaction and can actually be defined in terms of
interfacial tensions (71). Several methods have been
developed to measure the wetting angle (36,71). In most
natural systems, water is the wetting fluid, and the immiscible
fluid is the non-wetting fluid. Coal tar may be the exception
(i.e. contact angle greater than 90 degrees), which is mainly
attributed to the presence of surfactants (70). The wetting
fluid will tend to coat the surface of grains and occupy
smaller spaces (i.e. pore throats) in porous media, the non-
wetting fluid will tend to be restricted to the largest openings
(47).
Water
0 <90C
Wetting Fluid: Water
Water
Fluid Relationships:
System Wetting Fluid
airwater water
air: DNAPL DNAPL
water: DNAPL water
air:DNAPL:water water>organic>air(1)
(1) Wetting fluid order
Non-Wetting Fluid
air
air
DNAPL
After, Waterloo Centre for
Groundwater Research, 1989.
Figure 15. Wetting angle and typical wetting fluid
relationships.
The wetting angle depends on the character of the solid
surface on which the test is conducted. The test is conducted
on flat plates composed of minerals which are believed
representative of the media, or on glass. Contact angle
measurements for crude oil indicates that the wetting angles
vary widely depending on the mineral surface (53). Soil and
aquifer material are not composed of homogeneous mineral
composition nor flat surfaces. The measured wetting angle
can only be viewed as a qualitative indicator of wetting
behavior.
The reader is recommended to refer to reference No. 31 for
review of the basic principles and for various techniques to
measure the following DNAPL parameters: density, viscosity,
interfacial tension, solubility, vapor pressure, and volatility.
Subsurface Media Characteristics
Capillary Force/Pressure
Capillary pressure is important in DNAPL transport because
it largely determines the magnitude of the residual saturation
that is left behind after a spill incident. The greater the
capillary pressure, the greater the potential for residual
saturation. In general, the capillary force increases in the
following order; sand, silt, clay. Correspondingly, the residual
saturation increases in the same order. Capillary pressure is
a measure of the tendency of a porous medium to suck in the
wetting fluid phase or to repel the nonwetting phase (2).
Capillary forces are closely related to the wettability of the
porous media. The preferential attraction of the wetting fluid
to the solid surfaces cause that fluid to be drawn into the
porous media. Capillary forces are due to both adhesion
forces (the attractive force of liquid for the solids on the walls
of the channels through which it moves) and cohesion forces
(the attraction forces between the molecules of the liquid)
(32). The capillary pressure depends on the geometry of the
void space, the nature of solids and liquids, the degree of
saturation (2) and in general, in-creases with a decrease in
the wetting angle and in pore size, and with an increase in
the interfacial tension (71). All pores have some value of
capillary pressure. Before a nonwetting fluid can enter
porous media, the capillary pressure of the largest pores
(smallest capillary pressure) must be exceeded. This
minimum capillary pressure is called the entry pressure.
In the unsaturated zone, pore space may be occupied by
water, air (vapors), or immiscible hydrocarbon. In this
scenario, capillary pressure retains the water (wetting phase)
mainly in the smaller pores where the capillary pressure is
greatest. This restricts the migration of the DNAPL (non-
wetting phase) through the larger pores unoccupied by water.
Typically, DNAPL does not displace the pore water from the
smaller pores. It is interesting to note that the migration of
DNAPL through fine material (high capillary pressure) will be
impeded upon reaching coarser material (low capillary
pressure).
The capillary fringe will obstruct the entry of the DNAPL into
the saturated zone. When a sufficient volume of DNAPL has
been released and the "DNAPL pressure head" exceeds the
water capillary pressure at the capillary fringe (entry
pressure), the DNAPL will penetrate the water table. This is
why DNAPL is sometimes observed to temporarily flatten out
30
-------
on top of the water table. Similarly, laboratory experiments
have been conducted in which DNAPL (tetrachloroethylene)
infiltrating through porous media was found to flow laterally
and cascade off lenses too fine to penetrate (28), (refer to
Figure 11). This was attributed to the inability of the DNAPL
to overcome the high capillary pressure associated with the
lenses. Logically, when "DNAPL pressure head" exceeds the
capillary pressure, the DNAPL will penetrate into the smaller
pores. These laboratory experiments are important because
they illustrate that small differences in the capillary
characteristics of porous media can induce significant lateral
flow of non-wetting fluids.
A comprehensive investigation of capillary trapping and
multiphase flow of organic liquids in unconsolidated porous
media revealed many intricacies of this process in the
vadose and saturated zone (66). An important note is that
while capillary pressure is rarely measured at hazardous
waste sites, the soil texture (sand, silt, clay) is usually
recorded during drilling operations and soil surveys. This
information, along with soil core analyses will help to
delineate the stratigraphy and the volume distribution of
NAPL.
Pore Size Distribution/Initial Moisture Content
In natural porous media, the geometry of the pore space is
extremely irregular and complex (2). The heterogeneity of the
subsurface environment i.e. the variability of the pore size
distribution, directly affects the distribution of the capillary
pressures along the interfaces between the aqueous and
immiscible phases (50). In saturated column experiments, it
was observed that NAPL preferentially traveled through
strings of macropores, almost completely by-passing the
water filled micropores (66). In the same study, a
heterogeneous distribution of coarse and fine porous material
was simulated. Most of the incoming organic liquid
preferentially traveled through the coarse lens material.
In short term column drainage experiments, results indicated
that the particle grain size is of primary importance in
controlling the residual saturation of a gasoline hydrocarbon
(19). Fine and coarse sands (dry) were found to have 55%
and 14% residual saturation, respectively. The finer the sand,
the greater the residual saturation. During these experiments,
the residual saturation was reduced 20-30% in a medium
sand and 60% in a fine sand when the sands were initially
wet. Soil pore water held tightly by capillary forces in the
small pores will limit the NAPL to the larger pores, and thus,
result in lower residual saturation. In a similar laboratory
(unsaturated) column study, the smaller the grain size used
in the experiment, the greater the residual saturation of the
NAPL (74). The residual saturation in the saturated column
experiments was found to be greater than the unsaturated
columns and was independent of the particle size distri-
bution.
These observations follow traditional capillary force theory.
Residual saturation resulting from a DNAPL spill in the
unsaturated zone is highly dependent on the antecedent
moisture content in the porous media. When the moisture
content is low, the strong capillary forces in the smaller pores
will tenaciously draw in and hold the DNAPL. When the
moisture content is high, the capillary forces in the smaller
pores will retain the soil pore water, and DNAPL residual
saturation will mainly occur in the larger pores. Therefore,
greater residual saturation can be expected in dryer soils.
Correspondingly, NAPL will migrate further in a wetter soil,
and displacement of NAPL from small pores is expected to
be more difficult than from large pores.
Stratigraphic Gradient
DNAPL migrating vertically will likely encounter a zone or
stratigraphic unit of lower vertical permeability. A reduction in
the vertical permeability of the porous media will induce
lateral flow of the DNAPL. The gradient of the lower
permeable stratigraphic unit will largely determine the
direction in which the DNAPL will flow. This is applicable to
both the saturated and unsaturated zones. As depicted in
Figures 13a and 13b, the lateral direction of DNAPL flow may
be in a different direction than ground-water flow.
Ground Water Flow Velocity
The ground water flow velocity is a dynamic stress parameter
which tends to mobilize the hydrocarbon (39). As the ground
water velocity increases, the dynamic pressure and viscous
forces increase. Mobilization of DNAPL occurs when the
viscous forces of the ground water acting on the DNAPL,
exceeds the porous media capillary forces retaining the
DNAPL.
Saturation Dependent Functions
Residual Saturation
Residual saturation is defined as the volume of hydrocarbon
trapped in the pores relative to the total volume of pores (38)
and therefore is measured as such (74). Residual saturation
has also been described as the saturation at which NAPL
becomes discontinuous and is immobilized by capillary
forces (36). The values of residual saturation vary from as
low as 0.75 -1.25% for light oil in highly permeable media to
as much as 20% for heavy oil (50). Residual saturation
values have also been reported to range from 10% to 50% of
the total pore space (39,74). Other researchers reported that
residual saturation values appear to be relatively insensitive
to fluid properties and very sensitive to soil properties (and
heterogeneities) (66). Laboratory studies conducted to
predict the residual saturation in soils with similar texture and
grain size distribution yielded significantly different values. It
was concluded that minor amounts of clay or silt in a soil may
play a significant role in the observed values.
In the unsaturated zone during low moisture conditions, the
DNAPL residual saturation will wet the grains in a pendular
state (a ring of liquid wrapped around the contact point of a
pair of adjacent grains). During high moisture conditions, the
wetting fluid, which is typically water, will preferentially
occupy the pendular area of adjacent grains and the
hydrocarbon will occupy other available pore space, possibly
as isolated droplets. In the saturated zone, the DNAPL
residual saturation will be present as isolated drops in the
open pores (47). Furthermore, results of laboratory
experimentation indicated that residual saturation increased
with decreasing hydraulic conductivity in both the saturated
10
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and unsaturated zones and that the residual saturation is
greatest in the saturated zone. Laboratory experiments
indicated that vadose zone residual saturation was roughly
one third of the residual saturation in the saturated zone (66).
The increase in residual saturation in the saturated zone is
due to the following: [1] the fluid density ratio (DNAPLair
versus DNAPLwater above and below the water table,
respectively) favors greater drainage in the vadose zone; [2]
as the non-wetting fluid in most saturated media, NAPL is
trapped in the larger pores; and, [3] as the wetting fluid in the
vadose zone, NAPL tends to spread into adjacent pores and
leave a lower residual content behind, a process that is
1.00
Kr = relative permeability
1.00
Increasing DNAPL Saturation
Increasing Water Saturation
c
After Williams and Wilder, 19111
Figure 16. Relative permeability graph.
inhibited in the saturated zone (36). Thus, the capacity for
retention of DNAPLs in the unsaturated zone is less than the
saturated zone.
Relative Permeability
Relative permeability is defined as the ratio of the
permeability of a fluid at a given saturation to its permeability
at 100% saturation. Thus it can have a value between 0 and
1 (71). Figure 16 illustrates a relative permeability graph for a
two fluid phase system showing the relationship between the
observed permeability of each fluid for various saturations to
that of the observed permeability if the sample were 100%
saturated with that fluid (73). The three regions of this graph
are explained as follows (71): Region I has a high saturation
of DNAPL and is considered a continuous phase while the
water is a discontinuous phase, therefore, water permeability
is low. Assuming the DNAPL is the non-wetting fluid, water
would fill the smaller capillaries and flow through small
irregular pores. In Region II, both water and DNAPL are
continuous phases although not necessarily in the same
pores. Both water and NAPL flow simultaneously. However,
as saturation of either phase increases, the relative
permeability of the other phase correspondingly decreases.
Region III exhibits a high saturation of water while the
DNAPL phase is mainly discontinuous. Water flow dominates
100%
100
After Schwille, 1988^)
Figure 17. The relative permeability curves for water and a
DNAPL in a porous medium as a function of the
pore space saturation.
Site Characterization for DNAPL
Characterization of the subsurface environment at hazardous
waste sites containing DNAPL is complex and will likely be
expensive. Specific details associated with the volume and
timing of the DNAPL release are usually poor or are not
available and subsurface heterogeneity is responsible for the
complicated and unpredictable migration pathway of
subsurface DNAPL transport. As discussed previously, slight
changes in vertical permeability may induce a significant
horizontal component to DNAPL migration.
Site characterization typically involves a significant
investment in ground-water analyses. Although analysis of
ground water provides useful information on the distribution
of the soluble components of the DNAPL, the presence of
other phases of the DNAPL may go unrecognized. The
investigation must, therefore, be more detailed to obtain
information concerning the phase distribution of the DNAPL
at a site. Site characterization may require analyses on all
four phases (aqueous, gaseous, solid, immiscible) to yield
the appropriate information (refer to Table 2). In brief, data
collected on the various phases must be compiled, evaluated
and used to help identify: where the contaminant is presently
located; where it has been; what phases it occurs in; and
what direction the mobile phases may be going. A
comprehensive review of site characterization for subsurface
investigations is available (68). Development of monitoring
and remediation strategies can be focused more effectively
and efficiently after a clear definition of the phase distribution
has been completed.
Ground Water
Ground water analyses for organic compounds, in
conjunction with ground water flow direction data, has
repeatedly been used to: delineate the extent of ground
11
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Table 2 - Phase Distribution of DNAPL in the Subsurface
MATRIX
PHASE
1. ground water aqueous - soluble components of
DNAPL
2. soil/aquifer solid - adsorbed components of DNAPL
material on solid phase material
3. DNAPL immiscible - continuous phase (mobile),
residual saturation (immobile)
4. soil gas
gaseous -volatile components
water contamination from DNAPL; determine the direction of
plume migration; and to identify probable DNAPL source
area(s). While this approach has been used successfully to
characterize the distribution of contaminants in the
subsurface, there are limitations. For example, since DNAPL
and ground water may flow in different directions, as
indicated in Figures 13a and 13b, ground water analyses
may not necessarily identify the direction of DNAPL
migration.
Ground water analyses may be useful to identify probable
DNAPL source areas, but, estimating the volume of DNAPL
in the subsurface is limited using this approach. Soluble
phase components of DNAPL are rarely found in excess of
10% of the solubility even when organic liquids are known or
suspected to be present. The concentration of soluble
DNAPL components in the ground water is not only a
function of the amount of DNAPL present, but also the
chemical and physical characteristics of the DNAPL, the
contact area and time between the ground water and
DNAPL, and numerous transport and fate parameters
(retardation, biodegradation, dispersion, etc.). One
technique has been developed using chemical ratios in the
ground water as a means of source identification and
contaminant fate prediction (18).
Soil/Aquifer Material
Exploratory Borings
Physical and chemical analyses of soil and aquifer material
(drill cuttings, cores) from exploratory borings will provide
useful information in the delineation of the horizontal and
vertical mass distribution of DNAPL. While simple visual
examination for physical presence or absence of
contamination might seem like a worthwhile technique, it can
be deceiving and does nothing to sort out the various liquid
phases and their relationship to each other (71). A
quantitative approach is necessary to determine DNAPL
distribution.
Drill cuttings or core material brought to the surface from
exploratory borings can be screened initially to help delineate
the depth at which volatile components from the various
phases of the hydrocarbon exists. The organic vapor
analyzer and the HNU are small portable instruments that
can detect certain volatile compounds in the air. These
methods are used to initially screen subsurface materials for
volatile components of DNAPL. Identification of individual
compounds and their concentrations may be confirmed by
other, more precise, analyses.
Analysis of the soil or aquifer material by more accurate
means, such as gas chromatography or high pressure liquid
chromatography, will take longer but will provide more
specific information on a larger group of organic compounds,
i.e., volatile/non-volatile, and on specific compounds. This
information is necessary to help fix the horizontal and vertical
mass distribution of the contaminant and to help delineate
the phase distribution. These analyses do not distinguish
between soluble, sorbed or free-phase hydrocarbon,
however; a low relative concentration indicates that the
contaminant may mainly be present in the gaseous or
aqueous phases; and a high relative concentration indicates
the presence of sorbed contaminant or free phase liquid
either as continuous-phase or residual saturation. A more
rigorous set of analyses is required to distinguish between
the various phases.
Additional tests to identify the presence of NAPL in soil or
aquifer core sample are currently undeveloped and research
in this area is warranted. Squeezing and immiscible
displacement techniques have been used to obtain the pore
water from cores (40). Other methods of phase separation
involving vacuum or centrifugation may also be developed for
this use. A paint filter test was proposed in one Superfund
DNAPL field investigation where aquifer cores were placed in
a filter/funnel apparatus, water was added, and the filtrate
was examined for separate phases. These core analysis
techniques have potential to provide valuable field data to
characterize NAPL distribution.
Cone Penetrometer
The cone penetrometer (ASTM D3441-86)(69) has been
used for some time to supply data on the engineering
properties of soils. Recently, the application of this
technology has made the leap to the hazardous waste arena.
The resistance of the formation is measured by the cone
penetrometer as it is driven vertically into the subsurface.
The resistance is interpreted as a measure of pore pressure,
and thus provides information on the relative stratigraphic
nature of the subsurface. Petroleum and chlorinated
hydrocarbon plumes can be detected most effectively when
the cone penetrometer is used in conjunction with in-situ
sensing technologies (48). Features of the cone
penetrometer include: a continuous reading of the
stratigraphy/permeability; in-situ measurement; immediate
results are available; time requirements are minimal; vertical
accuracy of stratigraphic composition is high; ground-water
samples can be collected in-situ; and the cost is relatively
low.
Data from the cone penetrometer can be used to delineate
probable pathways of DNAPL transport. This is accomplished
by identifying permeability profiles in the subsurface. A zone
of low permeability underlying a more permeable
stratigraphic unit will likely impede vertical transport of the
DNAPL. Where such a scenario is found, a collection of
DNAPL is probable and further steps can be implemented to
more accurately and economically investigate and confirm
such an occurrence. This general approach has successfully
been implemented at one Superfund site (8).
12
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DNAPL
Well Level Measurements
In an effort to delineate the horizontal and vertical extent of
the DNAPL at a spill site, it is important to determine the
elevation of DNAPL in the subsurface. Monitoring DNAPL
elevation overtime will indicate the mobility of the DNAPL.
There are several methods that can be used to determine the
presence of DNAPL in a monitoring well. One method relies
on the difference in electrical conductivity between the
DNAPL and water. A conductivity or resistivity sensor is
lowered into the well and a profile is measured. The interface
of the DNAPL is accurately determined when the difference
in conductivity is detected between the two fluids. This
instrument may also be used to delineate LNAPL. A
transparent, bottom-loading bailer can also be used to
measure the thickness (and to sample) of DNAPL in a well
(36). The transparent bailer is raised to the surface and the
thickness of the DNAPL is made by visual measurement.
Several laboratory and field studies have been performed
which investigate the anomaly between the actual and
measured LNAPL levels in ground-water wells (15,16,24,25).
The anomaly between actual and measured NAPL thickness
in the subsurface is also applicable to DNAPL, but for
different reasons. The location of the screening interval is the
key to understanding both scenarios. First, if the well screen
interval is situated entirely in the DNAPL layer, and the
hydrostatic head (water) in the well is reduced by pumping or
bailing, then to maintain hydrostatic equilibrium, the DNAPL
will rise in the well (36,44,71) (refer to Figure 18). Secondly,
if the well screen extends into the barrier layer, the DNAPL
measured thickness will exceed that in the formation by the
length of the well below the barrier surface (36) (refer to
Figure 19). Both of these scenarios will result in a greater
DNAPL thickness in the well and thus a false indication
(overestimate) of the actual DNAPL thickness will result. One
of the main purposes of the monitoring well in a DNAPL
Measured > Actual
DNAPL PC
Figure 18. A well screened only in the DNAPL in conjunction
with lower hydrostatic head (i.e. water) in the well
may result in an overestimation of DNAPL
thickness.
Measured > Actual
DNAPL F
Figure 19. A well screened into an impermeable boundary
may result in an over-estimation of the DNAPL
thickness.
investigation is to provide information on the thickness of the
DNAPL in the aquifer. Therefore, construction of the well
screen should intercept the ground waterDNAPL interface
and the lower end of the screen should be placed as close as
possible to the impermeable stratigraphic unit.
DNAPL Sampling
Sampling of DNAPL from a well is necessary to perform
chemical and physical analyses on the sample. Two of the
most common methods used to retrieve a DNAPL sample
from a monitoring well are the peristaltic pump and the bailer.
A peristaltic pump can be used to collect a sample if the
DNAPL is not beyond the effective reach of the pump, which
is typically less than 25 feet. The best method to sample
DNAPL is to use a double check valve bailer. The key to
sample collection is controlled, slow lowering (and raising) of
the bailer to the bottom of the well (57). The dense phase
should be collected prior to purging activities.
Soil-Gas Surveys
A soil-gas survey refers to the analysis of the soil air phase
as a means to delineate underground contamination from
volatile organic chemicals and several techniques have been
developed (34,52). This investigative tool is mainly used as a
preliminary screening procedure to delineate the areal extent
of volatile organic compounds in the soil and ground water.
This method is quick, less expensive than drilling wells and
can provide greater plume resolution (33).
Data from a soil-gas survey is a valuable aid in the
development of a more detailed subsurface investigation
where ground water monitoring wells and exploratory borings
are strategically located for further site characterization.
There are limitations to soil-gas surveys (26,52) and data
interpretation must be performed carefully (35,49). Soil-gas
investigations have mainly been conducted to identify the
location of the organic contaminants in ground water. At the
13
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time of this publication, the scientific literature did not contain
information specifically applicable to the delineation of
DNAPL from soil-gas survey data. However, it is surmisable
that soil- gas surveys can be used to help delineate DNAPL
residual saturation in the unsaturated zone or the location of
perched DNAPL reservoirs.
Miscellaneous
The vertical migration of DNAPL in the saturated zone will
eventually be challenged by a low permeability stratigraphic
unit. According to the principles of capillary pressure, the
lower permeability unit will exhibit a greater capillary
pressure. Displacement of water by DNAPL requires that the
hydrostatic force from the mounding DNAPL exceed the
capillary force of the low permeability unit. The Hobson
formula is used to compute the critical height calculation to
overcome the capillary pressure under different pore size
conditions (70).
In an effort to minimize further DNAPL contamination as a
result of drilling investigations, precautionary steps should be
taken. Penetration of DNAPL reservoirs in the subsurface
during drilling activities offers a conduit for the DNAPL to
migrate vertically into previously uncontaminated areas. It is
very easy to unknowingly drill through a DNAPL pool and the
bed it sits on, causing the pool to drain down the hole into a
deeper part of the aquifer or into a different aquifer (32).
Special attention to grouting and sealing details during and
after drilling operations will help prevent cross-contamination.
Precautionary efforts should also be considered when a
DNAPL reservoir is encountered during drilling operations.
The recommended approach is to cease drilling operations
and install a well screen over the DNAPL zone and cease
further drilling activities in the well. If it is necessary to drill
deeper, construction of an adjacent well is recommended.
Alternatively, if it is not necessary to screen off that interval, it
is recommended to carefully seal off the DNAPL zone prior to
drilling deeper.
Well construction material compatibility with DNAPL should
be investigated to minimize downhole material failure. A
construction material compatibility review and possible
testing will prevent the costly failure of well construction
material. The manufacturers of well construction material are
likely to have the most extensive compatibility data and
information available.
Remediation
Remediation of DNAPL mainly involves physical removal by
either pumping ortrench-drainline systems. Removal of
DNAPL early in the remediation process will eliminate the
main source of contaminants. This step will substantially
improve the overall recovery efficiency of the various DNAPL
phases including the long term pump and treat remediation
efforts for soluble components. Remediation technologies
such as vacuum extraction, biodegradation, ground water
pumping, and soil flushing is mainly directed at the immobile
DNAPL and the various phases in which its components
occur. Physical barriers can be used in an effort to minimize
further migration of the DNAPL.
Clean-up of DNAPL can involve sizable expenditures: they
are difficult to extract and the technology for their removal is
just evolving (43). Historically, field recovery efforts usually
proceed with a poor understanding of the volume distribution
of the DNAPL. This reflects the difficulties involved in
adequate site characterization, poor documentation of the
release, and the complexity associated with the DNAPL
transport in the subsurface.
Pumping Systems
Pumping represents an important measure to stop the mobile
DNAPL from migrating as a separate phase by creating a
hydraulic containment and by removal of DNAPL (44). Very
simply, DNAPL recovery is highly dependent on whether the
DNAPL can be located in the subsurface. The best recovery
scenario is one in which the DNAPL is continuous and has
collected as a reservoir in a shallow, impermeable
subsurface depression. Once the DNAPL has been located
and recovery wells are properly installed, pumping of pure
phase DNAPL is a possible option but depends largely on
site specific conditions which include, but are not limited to:
DNAPL thickness, viscosity, and permeability.
Many DNAPL reservoirs in the subsurface are of limited
volume and areal extent. Therefore, it can be expected that
both the level of DNAPL (saturated thickness) in the well will
decline from the prepumping position and the percentage of
DNAPL in the DNAPLwater mixture will decrease rather
rapidly. Correspondingly, DNAPL recovery efficiency
decreases. Field results indicate that recovery wells
screened only in the DNAPL layer will maintain maximum
DNAPL:water ratios (44). Well diameter was not found to
influence long term DNAPL recovery; however, large
diameter wells allow high volume pumping for short
durations; and small diameter wells result in lower
DNAPL:water mixtures and greater drawdown.
An enhanced DNAPL recovery scheme may be used to
improve recovery efficiency. An additional well is constructed
with a screen interval in the ground water zone located
vertically upward from the DNAPL screen intake. Ground
High Level
High Level
Storage
Treatment
i
"-SI
WE
J
Hydraulically Induced
DNAPL Level
Figure 20. A DNAPL recovery system where deliberate
upwelling of the static coal-tar surface is used to
increase the flow of product into the recovery wells.
14
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water is withdrawn from the upper screen which results in an
upwelling of the DNAPL (70), refer to Figure 20. The
upwelling of the DNAPL, coal tar in this case, improved the
rate (twofold) at which the coal tar was recovered resulting in
a more efficient operation. The ground water withdrawal rate
must be carefully determined; too much will result in the coal
tar from rising excessively and being either mixed
(emulsions) with or suppressed by the higher water velocity
above; too low will not caused upwelling. An estimate of this
upwelling can be calculated using the simplified Ghyben-
Herzberg Principle under ideal conditions (4). Laboratory
studies indicated that dimethyl phthalate (1.19 g/cc) recovery
rate was doubled or tripled over the conventional, non-
upconing, recovery scheme (75). A similar application of this
technique was used to increase the level of DNAPL
(solvents) in a sandstone bedrock formation (11). Other
enhanced DNAPL recovery techniques were implemented
utilizing both water flooding and wellbore vacuum.
Essentially, this minimized drawdown, allowing a maximum
pumping rate of the DNAPLwater mixture. Both techniques
offered significant advantages in terms of the rate and
potential degree of DNAPL removal (8).
The highly corrosive nature of some DNAPL's may increase
maintenance problems associated with the recovery system.
A design consideration during any DNAPL recovery program
should include a material compatibility review to minimize
downhole failures. This is applicable to the well construction
material and the various appurtenances of the recovery
system. Manufacturers of the construction material would
most likely have the best compatibility information available.
While most scientists agree that the residual saturation of
immiscible hydrocarbon droplets in porous media are
immobile, researchers have investigated the mobility of
residual saturation in porous media for enhanced oil recovery
and for NAPL remediation at spill sites. Specifically, this
includes a complex interplay between four forces (viscous,
gravity, capillary, buoyancy). These forces are dependent on
both the chemical and physical characteristics of the DNAPL
and porous media. The mobilization of residual saturation
mainly hinges on either increasing the ground water velocity
which increases the viscous forces between the residual
saturation and the ground water, or decreasing the interfacial
tension between the residual saturation and the ground water
which decreases the capillary forces.
The capillary number is an empirical relationship which
measures the ratio between the controlling dynamic stresses
(absolute viscosity and ground water velocity) and static
stresses (interfacial tension) of the residual saturation (39).
The former are the viscous stresses and the dynamic
pressure in the water which tend to move the oil. The latter
are the capillary stresses in the curved water/oil interfaces
which tend to hold the oil in place. As the capillary number is
increased, the mobility of the residual saturation increases. In
a laboratory column study, the capillary number had to be
increased two orders of magnitude from when motion was
initiated to complete displacement of the hydrocarbon in a
sandstone core (74). In a glass bead packed column, only
one order of magnitude increase was required. However, a
higher capillary number was required to initiate mobility. The
difference in mobility between the two columns was attributed
to the pore geometry, i.e. size, shape.
There are limitations to residual saturation mobilization. The
ground water gradient (dh/dl) necessary to obtain the critical
capillary number to initiate blob mobilization would be 0.24.
To obtain complete NAPL removal would require a gradient
of 18 (3). Ground water gradients of this magnitude are
unrealistic. Another estimate of the gradient necessary to
mobilize carbon tetrachloride in a fine gravel and medium
sand was 0.09 and 9.0 respectively (74). The former gradient
is steep but not unreasonable and the latter gradient is very
steep and impractical to achieve in the field. The same
researchers concluded from more recent, comprehensive
studies, that the earlier predictions were optimistic, and that
the gradient necessary to mobilize residual organic liquid is
clearly impractical (66). Another limitation is that along with
residual saturation mobilization, the NAPL blobs disperse into
smaller blobs and that the blob distribution was dependent on
the resulting capillary number (6). Recovery of the NAPL
residual saturation by pumping ground water may be more
feasible where the porous media is coarse and capillary
forces are low, i.e. coarse sands and gravel. However, even
in this scenario, it is expected that the radius of residual
saturation mobilization would be narrow.
It is held in petroleum engineering theory that the only
practical means of raising the capillary number dramatically
is by lowering the interfacial tension (39) and that this can be
achieved by using surfactants (66). Surfactants reduce the
interfacial tension between two liquids, and therefore, are
injected into the subsurface for enhanced recovery of
immiscible hydrocarbons. In laboratory experiments,
surfactant flushing solutions produced dramatic gains in
flushing even after substantial water flushing had taken place
(54). Unfortunately, surfactants can be quite expensive and
cost prohibitive in NAPL recovery operations. Surfactants are
usually polymeric in nature and a surfactant residue may be
left behind in the porous media which may not be
environmentally acceptable. Additionally, surfactants may be
alkaline and thus affect the pH of the subsurface
environment. It has been suggested that such a surfactant
may inhibit bacterial metabolism and thus preclude
subsequent use of biological technologies at the site.
Significant research in this area is currently underway which
may uncover information improving the economics and
feasibility of this promising technology.
In summary, practical considerations and recommendations
concerning the mobilization and recovery of residual
saturation include the following: greater effectiveness in very
coarse porous media i.e. coarse sands and gravel; recovery
wells should be installed close to the source to minimize flow
path distance; a large volume of water will require treatment/
disposal at the surface; compounds with high interfacial
tension or viscosity will be difficult to mobilize; and implemen-
tation of linear one-dimensional sweeps through the zones of
residual saturation (74) and surfactants will optimize
recovery.
Pumping the soluble components (aqueous phase) of
DNAPL from the immiscible (continuous and residual
saturation), solid (sorbed), and gaseous phases has been
perhaps one of the most effective means to date to both
recover DNAPL from the subsurface and to prevent plume
migration. Recovery of soluble components quite often has
been the only remediation means available. This is largely
15
-------
? Ground Water Surface
DNAPL Surface
Water Drainline
Oil Distribution
DNAPL denser than ground v
has accumulated at the base
alluvium.
r Ground Water Surface
Ground Surface
Ground Water Surface
DNAPL Surface o _,
Ground Surface
DNAPL Mounding
Drawdown of the overlying w;
table by pumping the water d
results in mounding of the Dl*
DNAPL Recovery
Pumping from both the water
DNAPL drainline induces incr
DNAPL flow to the DNAPL dr
Separate production of DNAF
ground water reduces above
separation requirements.
A flow path of maximum form
permeability to DNAPL is est;
at the base of the alluvium.
After, Saleetal., 191
Figure 21. Trench recovery system of DNAPL utilizing the dual
drainline concept.
attributed to the inability to locate DNAPL pools and due to
low, DNAPL yielding formations. The basic principles and
theory of pump and treat technology and the successes and
failures have been summarized in other publications (64,67)
and is beyond the scope of this publication.
Pumping solubilized DNAPL components from fractured rock
aquifers historically has been plagued with a poor recovery
efficiency. Although the rock matrix has a relatively small
intergranular porosity, it is commonly large enough to allow
dissolved contaminants from the fractures to enter the matrix
by diffusion and be stored there by adsorption (32). The
release of these components is expected to be a slow
diffusion dominated process. This is because little or no
water flushes through dead-end fracture segments or through
the porous, impervious rock matrix. Therefore, clean-up
potential is estimated to be less than that expected for sand
and gravel aquifers.
Trench Systems
Trench systems have also been used successfully to recover
DNAPL and are used when the reservoir is located near the
ground surface. Trench systems are also effective when the
DNAPL is of limited thickness. Recovery lines are placed
horizontally on top of the impermeable stratigraphic unit.
DNAPL flows into the collection trenches and seep into the
recovery lines. The lines usually drain to a collection sump
where the DNAPL is pumped to the surface. Similar to the
pumping system, an enhanced DNAPL recovery scheme
may be implemented using drain lines to improve recovery
efficiency. This "dual drain line system" (41) utilizes a drain
line located in the ground water vertically upward from the
DNAPL line. Ground water is withdrawn from the upper
screen which results in an upwelling of the DNAPL which is
collected in the lower line, refer to Figure 21. This increases
the hydrostatic head of the DNAPL. Excessive pumping of
either single or dual drain line systems may result in the
ground water "pinching off" the flow of DNAPL to the drain
line. An advantage of the dual drain system is that the
oil:water separation requirements at the surface are reduced.
Vacuum Extraction
Soil vacuum extraction (SVE) is a remediation technology
which involves applying a vacuum to unsaturated subsurface
strata to induce air flow. Figure 22 illustrates that the volatile
contaminants present in the contaminated strata will
evaporate and the vapors are recovered at the surface and
treated. Common methods of treatment include granular
activated carbon, catalytic oxidation, and direct combustion.
SVE can effectively remove DNAPL present as residual
saturation or its soluble phase components in the
unsaturated zone. In general, vacuum extraction is expected
to be more applicable for the chlorinated solvents (PCE,
TCE, DCE) than the polycyclic aromatic compounds (wood
preserving wastes, coal tars, etc.). When DNAPL is present
in perched pools (Figure 12) it is more effective to remove
the continuous phase DNAPL prior to the implementation of
SVE. The same strategy is applicable in the saturated zone
where DNAPL removal by SVE is attempted concomitantly
with lowering the water table. Upon lowering the water table,
SVE can be used to remove the remnant volatile wastes not
previously recovered. Often, the precise location of the
DNAPL is unknown; therefore, SVE can be used to
remediate the general areas where the presence of DNAPL
is suspected. Removal of DNAPL by SVE is not expected to
be as rapid as direct removal of the pure phase compound.
One advantage of SVE however, is that the precise location
of the DNAPL need not be known.
Important parameters influencing the efficacy of SVE concern
both the DNAPL and porous media. Porous media specific
parameters include: soil permeability, porosity, organic
carbon, moisture, structure, and particle size distribution.
DNAPL specific parameters include: vapor pressure, Henry's
constant, solubility, adsorption equilibrium, density, and
viscosity (20). These parameters and their relationships must
be evaluated on a site specific basis when considering the
feasibility of vacuum extraction and a practical approach to
the design, construction, and operation of venting systems
(22). Additionally, soil gas surveys which delineate vapor
concentration as a function of depth is critical in locating the
contaminant source and designing an SVE system.
Historically, SVE has been used to remove volatile
compounds from the soil. Recently it has been observed that
SVE enhances the biodegradation of volatile and semivolatile
organic compounds in the subsurface. While SVE removes
volatile components from the subsurface, it also aids in
supplying oxygen to biological degradation processes in the
unsaturated zone. Prior to soil venting, it was believed that
biodegradation in the unsaturated zone was limited due to
inadequate concentrations of oxygen (17). In a field study
where soil venting was used to recover jet fuel, it was
16
-------
Figure 22. Vacuum extraction of DNAPL volatile components
in the unsaturated zone. As shown here, vapors
are treated by thermal combustion or carbon
adsorption and the air is discharged to the
atmosphere.
observed that approximately 15% of the contaminant removal
was from the result of microbial degradation. Enhanced
aerobic biodegradation during SVE increases the cost
effectiveness of the technology due to the reduction in the
required above ground treatment.
Vacuum extraction is one form of pump and treat which
occurs in the saturated zone where the fluid is a gas mixture.
Therefore, many of the same limitations to ground water
pump and treat are also applicable to vacuum extraction.
While the application of vacuum extraction is conceptually
simple, its success depends on understanding complex
subsurface chemical, physical, and biological processes
which provide insight into factors limiting its performance (9).
Biodegradation
The potential for biodegradation of immiscible hydrocarbon is
highly limited for several reasons. First, pure phase
hydrocarbon liquid is a highly hostile environment to the
survival of most microorganisms. Secondly, the basic
requirements for microbiological proliferation (nutrients,
electron acceptor, pH, moisture, osmotic potential, etc.) is
difficult if not impossible to deliver or maintain in the DNAPL.
A major limitation to aerobic bioremediation of high
concentrations of hydrocarbon is the inability to deliver
sufficient oxygen. A feasible remediation approach at sites
where immiscible hydrocarbon is present is a phased
technology approach. Initial efforts should focus on pure
phase hydrocarbon recovery to minimize further migration
and to decrease the volume of NAPL requiring remediation.
Following NAPL recovery, other technologies could be
phased into the remediation effort. Bioremediation may be
one such technology that could be utilized to further reduce
the mass of contaminants at the site. NAPL recovery
preceding bioremediation will improve bioremediation
feasibility by reducing the toxicity, time, resources, and labor.
Similar to other remediation technologies, a comprehensive
feasibility study evaluating the potential effectiveness of
bioremediation is critical and must be evaluated on a site
specific basis. A comprehensive review of biodegradation of
surface soils, ground water, and subsoils of wood preserving
wastes, i.e. PAH's (29,37,51,62,63) are available. A
comprehensive review of microbial decomposition of
chlorinated aromatic compounds is also available (58).
Soil Flushing
Soil flushing utilizing surfactants is a technology that was
developed years ago as a method to enhance oil recovery in
the petroleum industry. This technology is new to the
hazardous waste arena and available information has mainly
been generated from laboratory studies. Surfactant soil
flushing can proceed on two distinctly different mechanistic
levels: enhanced dissolution of adsorbed and dissolved
phase contaminants, and displacement of free-phase
nonaqueous contaminants. These two mechanisms may
occur simultaneously during soil flushing (42).
Surfactants, alkalis, and polymers are chemicals used to
modify the pore-level physical forces responsible for
immobilizing DNAPL. In brief, surfactants and alkalis reduce
the surface tension between the DNAPL and water which
increases the mobility. Polymers are added to increase the
viscosity of the flushing fluid to minimize the fingering effects
and to maintain hydraulic control and improve flushing
efficiency. Based on successful laboratory optimization
studies where an alkali-polymer-surfactant mixture was used,
field studies were conducted on DNAPL (creosote) which
resulted in recovery of 94% of the original DNAPL (42).
Laboratory research has also been conducted which
indicated that aqueous surfactants resulted in orders of
magnitude greater removal efficiency of adsorbed and
dissolved phase contaminants than water flushing (55).
Depth to contamination, DNAPL distribution, permeability,
heterogeneities, soil/water incompatibility, permeability
reduction, and chemical retention are important factors when
considering soil flushing (42). Prior to this technology being
cost effective in the field, surfactant recycling will be
necessary to optimize surfactant use (55). Soil flushing is
complex from a physical and chemical point of view; is
relatively untested in the field; and will likely be challenged
regulatorily. Considerable research currently being
conducted in this area may result in the increased use of this
technology to improve DNAPL recovery in the future.
Thermal methods of soil flushing involve injecting hot water
or steam in an effort to mobilize the NAPL. The elevated
temperature increases volatilization and solubilization and
decreases viscosity and density. A cold-water cap is used to
prevent volatilization. The mobile phases of the DNAPL are
then recovered using a secondary approach, i.e. pumping,
vacuum extraction etc. This approach (Contained Recovery
of Oily Wastes) to enhance recovery of DNAPL is currently
under EPA's Superfund Innovative Technology Evaluation
Program and a pilot-scale demonstration is forthcoming (21).
A limitation in the use of thermal methods is that the DNAPL
17
-------
may be converted to LNAPL due to density changes (36).
The adverse effects from this are that the DNAPL, existing as
a thin layer, becomes buoyant and mobilizes vertically
resulting in a wider dispersal of the contaminant. Other
limitations involve the high energy costs associated with the
elevated water temperature and the heat loss in the
formation (36).
Physical Barriers
Physical barriers may be used to prevent the migration of
DNAPL's in the subsurface and are typically used in
conjunction with other recovery means. One feature of
physical barriers is the hydraulic control it offers providing the
opportunity to focus remediation strategies in treatment cells.
Unfortunately, physical barriers, while satisfactory in terms of
ground water control and containment of dissolved-phase
plumes, may contain small gaps or discontinuities which
could permit escape of DNAPL (7). Chemical compatibility
between physical barriers and construction material must
agree to insure the physical integrity of the barrier. The
history of the performance of these containment technologies
is poorly documented and is mainly offered here for
completeness of review. A more complete review of these
physical barriers is available (5,56).
Sheet piling involves driving lengths of steel that connect
together into the ground to form an impermeable barrier to
lateral migration of DNAPL. Ideally, the bottom of the sheet
pile should be partially driven into an impermeable layer to
complete the seal. Slurry walls involve construction of a
trench which is backfilled with an impermeable slurry
(bentonite) mixture. Grouting is a process where an
impermeable mixture is either injected into the ground or is
pumped into a series of interconnected boreholes which
together form an impermeable boundary. Again, the main
feature of these techniques is to physically isolate the
DNAPL.
In summary, site characterization and remediation options for
sites containing DNAPL are limited. Field data from site
characterization and remediation efforts are also limited.
This is largely due to the complexity of DNAPL transport and
fate in the subsurface, poorly developed techniques currently
available to observe and predict DNAPL in the subsurface,
and to the fact that this issue has not been widely recognized
until recently. Clearly, there is a growing realization within
the scientific and regulatory community that DNAPL is a
significant factor in limiting site remediation.
Correspondingly, current research efforts within the private,
industrial, and public sectors are focusing on both the
fundamentals and applications aspects of DNAPL behavior in
subsurface systems. Additionally, the number of field
investigations reflecting an increased awareness of DNAPLs,
is growing.
DNAPL Modeling
A modeling overview report identified nineteen (numeric and
analytic) multiphase flow models which are currently available
(60). Most of these models were developed for saltwater
intrusion, LNAPL transport, and heat flow. Four models are
qualitatively described as immiscible flow models but do not
specifically indicate DNAPL. A more recent model has been
developed which simulates density driven, three phase flow,
that is capable of modeling DNAPL transport (23). Presently,
very little information is available on DNAPL modeling in the
scientific literature.
Multiphase flow modeling involves modeling systems where
more than one continuous fluid phase (NAPL, water,
gaseous) is present. Modeling any subsurface system
requires a conceptual understanding of the chemical,
physical, and biological processes occurring at the site.
Modeling of simultaneous flow of more than one fluid phase
requires a conceptual understanding of the fluids and the
relationship between the fluid phases. The significance of
multiphase flow over single phase flow is the increased
complexity of fluid flow and the additional data requirements
necessary for modeling. As presented earlier, numerous
variables strongly influence DNAPL transport and fate, and
consequently, the mathematical relationship of these
variables is complex. Therefore, it follows that DNAPL
modeling presents paramount technical challenges.
Presently, it is exceedingly difficult to obtain accurate field
data which quantitatively describes DNAPL transport and fate
variables within reasonable economic constraints. DNAPL
transport is highly sensitive to subsurface heterogeneities
(8,27,28) which compounds the complexity of modeling.
Heterogeneities are, by nature, difficult to identify and
quantify and models are not well equipped to accommodate
the influence of heterogeneities. Additionally, relative
permeability and capillary pressure functions must be
quantified to identify the relationship between fluids and
between the fluids and the porous media. Unfortunately,
these parameters are very difficult to measure, particularly in
three phase systems. Prior to an investment of time and
money to model a given site, a careful evaluation of the
specific objectives and the confidence of the input and
anticipated output data should be performed. This will help
illuminate the costs, benefits, and therefore, the relative
value of modeling in the Superfund decision making process.
In summary, DNAPL modeling at Superfund sites is presently
of limited use. This is mainly due to: the fact that very little
information is available in the scientific literature to evaluate
previous work; accurate and quantitative input data is
expected to be costly; the sensitivity of DNAPL transport to
subsurface heterogeneities; and, the difficulty in defining the
heterogeneities in the field and reflecting those in a model.
However, multiphase flow models are valuable as learning
tools.
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