£EPA
           United States
           Environmental Protection
           Agency
            Office of
            Development
            Washington, DC 20460
EPA/600/R-92/030
February 1992
Dense Nonaqueous
Phase Liquids --
A Workshop Summary
           Dallas, Texas
           April 16-18, 1991

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                                            EPA/600/R-92/030
Dense Nonaqueous Phase Liquids
       A Workshop Summary
             Dallas, Texas
          April 16-18,1991
                                             Agency
                                       -i2J)
                                       :cyard, 12th Floor
Robert S. Kerr Environmental Research Laboratory
      Office of Research and Development
     U.S. Environmental Protection Agency
              Ada, OK 74820
                                    VyC Printed on Recycled Paper

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Notice


The information in this document has been funded wholly or in part by the United States Environmental Protection Agency.
It has been subjected to the Agency's peer review and administrative review, and it has been approved for publication as an
EPA document.

All research projects making conclusions or recommendations based on environmentally related measurements and funded
by the Environmental Protection Agency are required to participate in the Agency Quality Assurance Program.  This project
did not involve environmentally related measurements and did not involve a Quality Assurance Project Plan. Mention of
trade names or commercial products does not constitute endorsement or recommendation for use.

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Contents


1.   Introduction	1

2.   Summary and Conclusions	1

3.   Background  	2
    3.1 Extent of problem	2
    3.2 Types of DNAPLs	3
    3.3 Movement of DNAPL in the subsurface	3
    3.4 Identifying a DNAPL site	4
       3.4.1  Soil	4
       3.4.2 Ground Water	5

4.   DNAPL Site Characterization	5
    4.1 Conceptual Approach	5
    4.2 Noninvasive Techniques	6
    4.3 Invasive Techniques	7

5.   DNAPL Remediation	8
    5.1 Containment versus Restoration	10
    5.2 DNAPL Recovery Methods	10
    5.3 Enhanced DNAPL Recovery Methods	10
       5.3.1  Induced Gradient/Water Flooding	10
       5.3.2 Chemically Enhanced Recovery	11
       5.3.3  Thermally Enhanced Recovery	11
    5.4 Pump and Treat	11
    5.5 Vacuum Extraction	12
    5.6 Bioremediation	12
    5.7 Treatment Train	12

Table 1  	14

Appendices

    Appendix A: Dense Nonaqueous Phase Liquids, a
       USEPA Ground-Water Issue Paper	23
    Appendix B: DNAPL Bibliography	47
    Appendix C: DNAPL Workshop Attendee List	71
    Appendix D: Glossary of Terms	77

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1.     Introduction
site characterization, and, therefore, DNAPL
remediation, can be expected.
Dense nonaqueous phase liquids (DNAPLs) in the
subsurface are long-term sources of ground-water
contamination, and may persist for centuries before
dissolving completely in adjacent ground water. In response
to increasing recognition among scientists and engineers
that DNAPL contamination is widespread, a two-day
workshop concerning DNAPL site characterization and
remediation was held in Dallas, Texas on April 16-18,1991.
The workshop was sponsored and organized by EPA's
Ground Water Forum, Robert S. Kerr Environmental
Research Laboratory, and Office of Solid Waste and
Emergency Response, and the University of Waterloo,
Ontario, Canada. Professionals from government,
academia, and private industry with experience in DNAPL
contamination presented papers and participated in
discussions concerning the options for characterization and
remediation of DNAPL  sites.

This document was prepared to summarize the main
observations and conclusions of the meeting, and draws
freely from transcripts, papers, and comments prepared by
the participants.  A second meeting of the participants was
held in Dallas on September 5-6, 1991 to review the draft
summary. The summary is intended to be a nontechnical
document of general interest to environmental scientists and
engineers from Federal,  State, and local agencies,
universities, and private industry. Readers interested in
more specific technical information should consult the
extensive bibliography compiled in Appendix B.  The
material presented here represents "state-of-the-art"
information, and, as such, can be expected to change as the
science progresses.

To provide background material and an overview of the
DNAPL issue, an EPA Ground Water Issue Paper entitled
"Dense Nonaqueous Phase Liquids" (Huling and Weaver,
1991) is included as Appendix A. Appendix B is an
extensive bibliography concerned with DNAPLs, and
Appendix C is a list of workshop participants. Appendix D
is a list of terms and definitions.
2.     Summary and
        Conclusions
      The potential for extensive contamination of ground
      water by DNAPLs is high, but reliable, quantitative
      information concerning the extent of contamination is
      not currently available. Historically, many site
      investigations were not designed or equipped to detect
      or delineate the presence of DNAPL. As DNAPL
      awareness increases, an improvement in the quality of
Failure to directly observe DNAPL at a site does not
mean it does not exist. Often, only very low aqueous
concentrations of DNAPL constituents are detected in
monitoring wells at known DNAPL sites.  If dense,
free-phase chemicals were widely used, handled, and
disposed of according to standard industry practices
common more than several years ago, chances are high
that DNAPL is present. Indirect detection methods,
based on soil and ground-water chemical compositions,
can often be used to infer the presence of DNAPLs.

DNAPLs can be broadly classified on the basis of
physical properties such as density, viscosity, and
solubility.  Of the various types of DNAPLs found in
the subsurface, chlorinated solvents and creosote/coal
tar are apparently the most common. These two types
of DNAPLs, however, present ground-water and
remediation problems of a very different nature due to
differences in their physical properties. Some of the
conclusions applicable to one are not generally appli-
cable to the other. Physical characteristics can guide
the choice of characterization and remediation options.

The relative importance of the forces that control the
rate, flow direction and ultimate fate of DNAPL is
different from the relative importance of those that
control the distribution of dissolved-phase plumes.
DNAPL behavior is only loosely coupled, if at all, to
the behavior of water. Movement of DNAPLs is
remarkably sensitive  to the capillary properties of the
subsurface, and the distribution of those properties
controls the distribution of the DNAPL. Thus,
knowledge of geologic conditions is relatively more
important than knowledge of hydrologic conditions for
adequate conceptualization and characterization of
DNAPL sites.

Obtaining a detailed delineation of subsurface DNAPL
distribution is difficult and may be impractical using
conventional site characterization techniques.  DNAPL
migrates preferentially through relatively permeable
pathways (soil and rock fractures, root holes, sandy
layers, etc.) and is influenced by small-scale
heterogeneities (such as bedding dip and slight textural
changes) due to density, capillary forces, and viscous
forces.  As a result, the movement and distribution of
DNAPL is difficult to determine even at sites with
relatively homogeneous soil and a known, uniform
DNAPL source. This difficulty is compounded by
fractured bedrock, heterogeneous strata, mixtures of
DNAPLs, etc. The relative importance of small-scale
heterogeneities may depend on the volume of the
release.

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The risk of causing DNAPL remobilization must be
assessed during site investigation. Conventional
drilling technologies have a high potential for
promoting vertical DNAPL movement. The
appropriate investigation strategy is dependent on site-
specific conditions, including the geology and the type
of DNAPL. Nonintrusive, low-risk investigation
methods such as surface geophysical techniques and
reviews of site history and existing data should first be
used to develop and improve the conceptual model of
DNAPL presence and lessen the risks associated with
subsequent drilling.

Currently, surface geophysical methods capable  of
delineating subsurface DNAPL and the availability of
geophysicists trained in investigating DNAPL
problems are extremely limited.  However, even
routine, noninvasive geophysical techniques can be
used to evaluate site geology, which has a great
influence on DNAPL migration.

Site characterization should be a continuous, iterative
process, whereby each phase of investigation and
remediation is used to refine the conceptual model of
the  site. The time required to define site characteristics
cannot easily be reduced because of the heterogeneous,
site-specific nature of subsurface environments and the
evolutionary process required. However, the
information required to implement early containment
of dissolved-phase contamination is less extensive than
that required to design final remedial alternatives.

DNAPL in fractured rock poses exceptionally difficult
problems for site investigation and remediation because
fracture networks are complex, DNAPL retention
capacity (mass of DNAPL per unit volume of rock) is
generally small, and the depth to which DNAPL may
penetrate can be very large.

DNAPL, as a long-term source of dissolved phase
ground-water contamination, is a pollution prevention
issue. Unless controlled and/or remediated, DNAPLs
in the subsurface may persist for decades or centuries
dissolving into and contaminating vast quantities of
ground water.

Factors controlling DNAPL dissolution into ground
water include the effective aqueous solubility of
DNAPL chemicals, ground-water velocity, DNAPL-
water contact area, and the diffusion of DNAPL
chemicals in water.

Ground-water remediation is a relatively new science.
Rigorous scientific performance evaluations of existing
remediation systems are essentially nonexistent.  Only
within the last half-decade has ground-water
      remediation been attempted on a significant scale.
      Technologies for DNAPL recovery are generally
      unproven under field-scale conditions. There are no
      methods or series of methods which have been
      demonstrated to completely remove DNAPL from the
      subsurface; however, several experimental DNAPL
      recovery and treatment technologies have undergone
      limited laboratory and pilot-scale field testing.

      There are no proven technologies to restore DNAPL
      zones in aquifers to drinking water standards. There
      are, however, technologies available to remove some
      portion of the DNAPL. Significant benefit, including
      reduced DNAPL mobility, can often be realized by use
      of such technologies. Technologies such as pump-and-
      treat can often be used to control dissolved-phase
      contamination.

      As with remediation, physical and/or hydraulic
      containment of DNAPLs has not been adequately
      demonstrated under a variety of field conditions,
      particularly with respect to vertical containment.
      Performance evaluations will be necessary to compare
      containment with other remediation options, and
      effective containment design may require as much site
      characterization information as effective remediation
      design.

      The complexity of the DNAPL problem dictates that
      site investigators have a sophisticated knowledge of
      DNAPL contaminant hydrogeology. Most decisions
      regarding site investigations and cleanup depend on
      site-specific conditions regarding the types and
      volumes of DNAPL and local hydrogeologic
      conditions. Formal environmental education is critical
      for future success in remediation. Progress in
      education and research in subsurface restoration has
      been slow, due to a paucity of funding mechanisms.
3.     Background

3.1     Extent of problem

The potential for extensive contamination of ground water by
dense nonaqueous phase chemicals is high because of their
widespread production, transport, utilization, and disposal.
There are many DNAPL contamination sites in North
America, and dissolved constituents derived from DNAPLs
are frequently detected in ground-water supplies. The most
common DNAPLs are chlorinated solvents (e.g.,
trichloroethylene, tetrachloroethylene, and trichloroethane),
creosote, and coal tar. DNAPL contamination is associated
with a wide range of industries and operations, for
example:

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   •   wood treatment operations (creosote);
   •   manufactured gas plants (coal tar);
   •   transformer oil production, reprocessing,
      and disposal facilities (PCBs and chlorinated solvents);
   •   chemical industry facilities (chlorinated solvents,
      pesticides, herbicides, and other dense organic
      compounds);
      steel industry coking operations (coal tar);
   •   dry cleaners (chlorinated solvents);
   •   electronic instrument manufacturers (chlorinated
      solvents);
   •   machine shops (chlorinated solvents);
   •   print shops (chlorinated solvents);
   •   metal works (chlorinated solvents);
   •   waste disposal facilities (all types)

3.2     Types of DNAPLs

Although DNAPL compounds exhibit a continuum of
physical properties such as density, viscosity, solubility, and
sorptive properties, it is imperative to examine the
characteristics of common DNAPLs.  Many chlorinated
solvents are characterized by relatively high density and low
viscosity. However, creosote and coal tar compounds have
relatively lower densities and higher viscosities than
chlorinated solvents. The important physical differences
between compounds in each group lead to distinctive
problems with regard to characterization and remediation.

Many chlorinated solvents have relatively low viscosities and
specific gravities significantly greater than that of water.
Thus, the nonaqueous phase of these compounds may be
relatively mobile in the subsurface and the movement may be
strongly influenced by gravity. The aqueous (dissolved) phase
may also be relatively mobile due to the sorption properties
and significant solubilities of some constituents.  Therefore,
dissolved contamination may migrate relatively large
distances at some sites.

Creosote and coal tars typically have specific gravities only
slightly greater than water, and viscosities ten or twenty times
greater than water. The nonaqueous phase is, therefore, less
mobile in the subsurface, and its movement may  be less
gravity-dominated than nonaqueous chlorinated solvents.
Aqueous-phase creosote and coal tar constituents
(predominantly polyaromatic hydrocarbons (PAHs)) are
generally much less mobile than aqueous-phase chlorinated
solvents due to lower solubilities and concomitant stronger
sorption onto aquifer materials.

The differentiation of DNAPLs into groups is a useful, though
artificial, division of a continuum of density, viscosity, and
solubility differences. The advantage to Jhis classification is
that it clearly delineates types of DNAPLs on the basis of
physical and chemical characteristics. Differences in physical
characteristics control mobility of both the nonaqueous and
aqueous phases. In turn, mobility considerations will
determine site characterization methods and remedial options.
For example, optimal methods to characterize a chlorinated
solvent site with a mobile, nonaqueous phase may differ
significantly from methods used to characterize a site
contaminated with less mobile, nonaqueous creosote or coal
tar. However, it is important to note that many sites are
polluted with complex mixtures of DNAPLs.

3.3     Movement of DNAPL in the subsurface

The infiltration of a nonaqueous liquid into the subsurface is
controlled predominantly by gravity, viscous forces, and
capillary forces, which are in turn controlled by the
interactions of water, soil/aquifer material, and the
nonaqueous liquid.  Properties of the nonaqueous liquid which
influence its mobility include density, viscosity, and interfacial
tension with water.  Properties of the soil which influence
mobility of the nonaqueous liquid include soil heterogeneity,
intrinsic permeability, mineralogy, pore size, pore geometry
and macropores. DNAPL migration is also influenced greatly
by structural and stratigraphic features.

When released into the subsurface, gravity causes a DNAPL
to migrate downward through the vadose zone as a distinct
liquid.  Vertical migration is accompanied by some lateral
spreading due to capillary forces and spatial heterogeneities.
Residual liquid remains trapped by surface tension in pore
space as the DNAPL drains through the soil. DNAPLs also
dissolve into residual soil water and vaporize into soil gas.

When sufficient DNAPL reaches the vicinity of the water
table to overcome the capillary (entry) pressure, DNAPL
penetrates the saturated zone, and continues to migrate
downward driven by gravity. Preferential flow occurs where
DNAPL encounters relatively permeable layers and fractures.
The ultimate location of DNAPL in the subsurface may be far
from the DNAPL entry location.  In addition to accumulating
in DNAPL pools, residual DNAPL is trapped in pore spaces
within the saturated zone. The dissolution of DNAPL
constituents into passing ground water may result in the
contamination of large volumes of ground water.

Gravity forces generally dominate DNAPL movement until
the immiscible liquid reaches a permeability contrast, which
may be very subtle.  Capillary forces may inhibit penetration
of the DNAPL into the less permeable zone. Often, DNAPL
will move along bedding planes in the direction of the
geologic dip, even in a direction opposite to ground-water
flow. The actual direction of DNAPL movement will depend
on the dip of the underlying strata, the capillary and viscous
forces, and the density contrast.

The magnitude of capillary pressure resisting entry of DNAPL
into a water-saturated, porous medium  is inversely
proportional to the size of pore openings. DNAPL, therefore,

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migrates preferentially through relatively permeable pathways
such as fractures, root holes, and sandy layers, and may be
influenced by small-scale textural changes.  Clayey aquitards
which are relatively efficient at limiting vertical water
movement may not prevent downward leakage of DNAPLs
through minute fractures and root hairs. Chlorinated solvents
can migrate through openings smaller than a human hair (i.e.,
< 20 microns).  Thus, the normal concept of a confining layer
commonly used in hydrogeology may be inappropriate at
DNAPL sites.

The extent of contamination caused by a DNAPL is related to
the volume of DNAPL released to the environment, the
physical site characteristics, the length of time since release,
and the physical properties of the DNAPL itself. Most sites,
where large portions of aquifers contain clearly identifiable
free-phase DNAPL, are creosote or coal tar sites. However,
low viscosity, high density,  chlorinated solvents with relatively
high solubilities can cause widespread nonaqueous- and
aqueous-phase contamination. The nonaqueous phase of these
solvents is also more difficult  to locate in the subsurface than
creosote and coal tar.

3.4    Identifying a DNAPL site

An important problem which  should be addressed early in the
evaluation of potential DNAPL sites is the determination of
whether or not DNAPL exists at the site. The importance of
this identification cannot be overemphasized. The presence of
DNAPL at a site should dictate the methods used for site
characterization, investigation of the extent of contamination,
and remediation.  The current consensus is that a site has a
high probability of being a DNAPL site if chemical usage was
appreciable, and usage and  disposal were consistent with
industry practices common  more than several years ago.

Some DNAPLs, such as creosote and coal tar, can be readily
identified by visual inspection of soil and rock samples taken
during field investigation. Likewise, immiscible-phase
chlorinated solvents can sometimes be identified in ground-
water samples. Visual identification may be difficult,
however, where the DNAPL is colorless, present in low
residual content, or heterogeneously distributed. The inability
to directly verify the presence of a DNAPL should not be
taken as evidence of its absence.  In the case of chlorinated
solvents and other dense, low viscosity DNAPLs, it is
particularly important to assess the presence or absence of the
nonaqueous phase without drilling directly through a
nonaqueous pool or lens. It is possible to drill through a
DNAPL zone without detecting the DNAPL, and the risk of
remobilizing and spreading contamination during drilling is
significant.

There are methods of indirectly assessing chemical and
physical characteristics of both soil and ground water to
evaluate the presence of DNAPL in the subsurface. They are
based on equilibrium partitioning theory, and require
determination of total chemical concentrations, soil
moisture content, porosity, organic carbon content,
approximate composition of the possible DNAPL, sorption
parameters, and solubilities.

3.4.1    Soil

If no visible DNAPL is present, there are no direct methods
available to confirm the presence of residual DNAPL in soil
samples. Qualitatively, soil samples which exhibit
concentrations in the percent range are indicative of DNAPL
contamination. However, DNAPLs may also be present at
low residual saturations in soil samples. Analytical results
from these samples may exhibit much lower constituent
concentrations.

It is sometimes possible to qualitatively assess the presence of
DNAPL in such samples using equilibrium partitioning
theory. If no DNAPL is present, there is a theoretical
maximum mass of chemical which can be contained in a soil
sample. The theoretical maximum total soil concentration is
determined by the solubility of the chemical in water, the
saturated soil gas concentration, and the sorptive capacity of
the soil. If the nonaqueous liquid exists, the theoretical
maximum total soil concentration would be exceeded, and the
calculated pore water concentration would exceed the
chemical solubility. The conclusion would be that DNAPL is
present in the sample.

DNAPL can be a single chemical component, or a mixture of
several or many components. For mixtures, the dissolved-
phase concentrations in equilibrium with the mixture can be
estimated such that the effective solubility of a compound is
equal to the mole fraction of the compound multiplied by its
pure-phase solubility. This relationship is strictly valid only
for ideal liquid mixtures, but errors in the estimation of
effective solubility for complex hydrocarbon mixtures should
be less than a factor of two. A common problem at sites
suspected of having mixed chlorinated solvent DNAPL in the
aquifer is the lack of encounter of free-phase DNAPL in cores
or monitoring wells. In such cases, the theoretical solubilities
of the components cannot be estimated reliably due to the lack
of DNAPL composition data.

Solubility data reported in the literature for coal tar and
creosote constituents and PCBs is usually solid-phase
solubility. However, when estimating effective solubility of
these compounds in a DNAPL mixture, liquid-phase
solubility, which is often many times higher than solid-phase
solubility, should be used.

Methods for evaluating DNAPL  presence in soil are most
reliable when significant residual is present, pore water-soil
partition coefficients can be determined, and effective
solubilities can be calculated. Conclusions on the presence of

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 DNAPL are less reliable when sorption coefficients and effec-
 tive solubilities must be estimated. However, such calcula-
 tions are currently the only means of assessing the possible
 presence of DNAPL in soil and, as such, should be routinely
 performed when DNAPL contamination is suspected.

 3.4.2    Ground Water

 Laboratory experiments demonstrate that aqueous-phase
 chemical concentrations in contact with nonaqueous-phase
 liquids approach effective solubilities (saturation) rapidly.  For
 low and moderate ground-water flow rates, it would be logical
 to expect saturated dissolved concentrations in ground water
 leaving the DNAPL zone. However, saturated aqueous
 concentrations are rarely observed in ground water. Relatively
 low aqueous concentrations in the field probably result from
 (1) the variable distribution of DNAPL as residuals, lenses,
 and pools; (2) aquifer dispersion; (3) nonuniform ground-
 water flow; (4) dilution in monitoring wells; and (5) failure to
 account for effective rather than pure-phase solubilities.
 Importantly, field observations of low aqueous concentrations
 do not support the absence of a DNAPL phase. This has been
 demonstrated at known DNAPL sites.

 Unless a monitoring well is installed very close to the DNAPL
 zone and the well intake is short, saturated or near saturated
 levels of dissolved constituents will not be observed.
 Depending on the distribution of DNAPL, ground-water
 concentrations of 1% or less of effective solubility can be
 found even in the immediate proximity of the DNAPL. At
 some sites where permeability and ground-water velocity are
 exceptionally large, the contaminant concentrations found in
 monitoring wells are far below the 1% level in areas where
 DNAPL is known or expected to occur. Therefore, detection
 of such concentrations  should be viewed as indirect evidence
 of DNAPL presence, and detection of very low aqueous
 concentrations should not be interpreted as absence of
 DNAPL.
4.     DNAPL Site

        Characterization

4.1.    Conceptual Approach

Many characterizations at potential DNAPL sites were not
designed to assess the presence or extent of subsurface
DNAPL contamination. Due to low solubilities, DNAPLs are
expected to persist for decades or centuries before completely
dissolving in ground water. As a result, DNAPL acts as a
long-term source of dissolved-phase ground-water
contamination, resulting in large plumes with chemical
concentrations that may greatly exceed drinking water
standards. For example, one gallon of trichloroethylene can
 contaminate approximately fifteen million gallons of ground
 water to a level of 100 micrograms per liter. At most DNAPL
 sites, the mass of contaminant in the immiscible phase greatly
 exceeds the contaminant mass dissolved in ground water.
 Based on the adverse environmenlal and health impacts of
 subsurface DNAPLs, characterization of the nature, extent,
 and severity of DNAPL contamination is necessary to
 determine site-specific remediation approaches.

 Site characterization should be conducted in an evolutionary,
 phased approach. Characterization begins with a review of
 pertinent site history and available information from regional
 and local studies. This information is used to formulate an
 initial conceptual model of contaminant transport and fate at
 the site. Each characterization phase should be designed to
 test the conceptual model. The information acquired from
 each phase is used to refine the model. This iterative process
 should be continued throughout the remedial design and
 remedial action phases to provide better site characterization.

 Mathematical modeling of contaminant transport and fate
 processes for dissolved contaminants is a useful site
 characterization tool.  However, a limited number of
 multiphase flow models are currently available for modeling
 DNAPL transport. Fluid flow in a two-phase (water, NAPL)
 or three-phase (water, NAPL, gas) system is highly complex.
 Many variables affect DNAPL transport and fate and it is
 generally impractical to obtain the necessary field data to
 accurately model DNAPL movement in the subsurface.
 Accordingly, the utility of modeling DNAPL transport and
 fate at a site is presently very limited compared to other
 ground-water modeling applications.

 Subsurface heterogeneity affects the migration pathway of
 DNAPLs. DNAPLs migrate faster through relatively
 permeable features and rates of flow are influenced by subtle,
 small-scale heterogeneities. Migration and distribution of
 DNAPL are sensitive to the capillary properties of subsurface
 materials and the volume of DNAPL released. As a result, the
 movement and distribution of DNAPL may be complex and
 difficult to determine in detail, especially for low viscosity
 contaminants. This difficulty is increased where there is
 fractured rock, heterogeneous strata, mixtures of DNAPLs,
 and multiple release locations.

 Fractured rock settings pose exceptionally difficult problems
 in site characterization due to the complexity of fracture
 networks. DNAPL flow in fractured rock is especially
 difficult to trace due to the low retention capacity and the large
 depth to which DNAPL may penetrate even where fracture
 openings are small. Contaminant investigation in karstic
 settings is also extremely difficult. Efficient techniques for
characterizing such sites are not currently available.

Many sites are underlain by clayey soil aquitards which
conlain secondary permeability pathways such as fractures

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and root holes. These openings are particularly common at
shallow depths and in stiff clays, but may extend to
considerable depth and facilitate DNAPL migration to lower
aquifers. It is prudent to assume the presence of
interconnecting features capable of transmitting DNAPL
through a hydrogeologic aquitard. Therefore, characterization
of units beneath hydrogeologic aquitards may be warranted at
many sites.

For these reasons, information regarding geologic conditions
is important for adequate conceptualization and
characterization at a DNAPL site. This implies that much
work should be focused on characterizing the geological
setting including lithologic and stratigraphic features.
Manmade features (e.g., utility corridors) may play a
significant role as preferential migration pathways for DNAPL
transport and should also be characterized.  Adequate site
characterization is essential for design and evaluation of both
remediation and containment systems. The level of detail and
type of data required will be site-specific and dictated by the
potential remedial technologies.  In addition, less site
characterization information will generally be required to
implement early containment of dissolved-phase
contamination than to design final remedial alternatives.

An important consideration in conducting investigations at
DNAPL sites is the risk of expanding the zone of
contamination by creating pathways for DNAPL migration as
a result of drilling. Remobilization of DNAPL during site
investigation can have one or more of the following negative
impacts: (1) increase in ultimate risk to receptors; (2) increase
in difficulty and cost of site remediation; and (3)
misinterpretation of monitoring data, sometimes on the side of
conclusions indicating higher site risk or remediation
difficulty than is actually the case. It is important to prevent
downward migration of mobile, perched DNAPL. Soil
contaminated with residual DNAPL can also be carried to
greater depths during  drilling and is a source for cross-
contamination of hydrogeologic units. Given the difficulty in
predicting the presence and distribution of subsurface
DNAPL, the risks involved with drilling generally increase
with proximity to the disposal site or release location.

The degree of risk is related to  the physical properties of the
particular DNAPL present. DNAPLs with relatively high
densities and low viscosities (e.g., chlorinated solvents) are
more mobile and more easily mobilized to greater depths than
those with lower densities and higher viscosities (e.g., creosote
and coal tar). Unlike creosote or coal tar contamination,
which may be identifiable in subsurface material, many
DNAPL chemicals (e.g., chlorinated solvents) may not be
obvious during drilling. It is possible to drill through zones of
DNAPL contamination without realizing it. Even when the
locations of DNAPLs and the geology at the site are relatively
well known, the installation of wells in DNAPL areas using
the best available technology has its risks. For example,
bentonite pellets used to form annular seals during well
installation do not swell in the presence of certain DNAPLs.
Consequently, an effective annular seal may not be achieved.

The risks involved in the use of invasive methods mandate
evaluation of site history and existing data and using
noninvasive techniques (e.g., surface geophysical methods),
where applicable, prior to invasive studies (e.g., drilling of
boreholes). The information derived from these sources can
be used to formulate and refine the conceptual model for
transport of DNAPL at a site and to guide the use of invasive
techniques.

Methods used for determining DNAPL presence have
generally been limited to analysis of soil samples,
observations during drilling and other field activities, and
DNAPL detection in wells. Assessment of DNAPL mobility
has generally been limited to observing free-phase liquid in
wells or seeps. There is a need to develop, demonstrate, and
document (1) drilling methods which can be used to prevent
or reduce the risk of downward DNAPL migration, including
techniques allowing DNAPL detection while drilling; (2)
improved field methods for rapid and inexpensive detection of
DNAPL that cannot be readily identified by visual inspection
of drilling samples; and (3) methods to evaluate DNAPL
mobility.

4.2     Noninvasive Techniques

Noninvasive techniques which may be applicable to initial site
characterization at DNAPL sites include soil gas analysis and
surface geophysical methods. At certain sites, these
techniques may  provide useful site characterization
information. Geophysical techniques have been applied with
some success to characterization of site hydrogeology.
However, geophysical  techniques for detecting organic
contamination have not been adequately tested in a variety of
field situations.  The methods discussed below, which have
recently been applied to the detection of organic
contamination, are relatively untested and  should be
considered as emerging technologies. These techniques are
currently research tools and are not readily available or
applicable for use at most sites.  In addition, the use of
noninvasive techniques generally would not eliminate the
need for invasive characterization at a site. Instead, the results
from noninvasive studies may provide information to guide
the use of invasive methods.

Depending on site conditions, soil gas analysis may be useful
in situations where volatile constituents are present  High
vapor concentrations may be present near volatile, immiscible-
phase contaminants in the unsaturated zone.  Soil gas samples
from areas where volatile constituents are not present in the
immiscible phase or from areas where the  immiscible phase is
only present in the saturated zone may exhibit relatively lower
or nondetectable constituent concentrations.  The extent of

-------
 DNAPL contamination in the unsaturated zone may be
 relatively limited. Therefore, soil gas data should be
 examined for both trends and isolated high concentrations.
 These data may provide an indirect indication of immiscible-
 phase contaminants in the unsaturated zone.

 Surface geophysical methods, where applicable, allow
 noninvasive investigation of subsurface physical and chemical
 properties.  Variations of many of these methods are also
 useful borehole techniques.  Seismic, electrical, magnetic, and
 ground penetrating radar techniques have been successfully
 applied in site characterization. These methods may provide
 valuable information such as depth to ground water and
 bedrock, lithologic and stratigraphic boundaries, fracture
 orientation, locations of buried pipes, trenches, and drums, and
 inorganic contaminant distribution, without risk of spreading
 contamination.

 Ground penetrating radar, complex resistivity, and
 electromagnetic induction methods have been applied
 successfully to the detection of aqueous and nonaqueous-
 phase hydrocarbons and other organic chemicals at a very
 limited number of sites. Applications of these methods to
 organic contamination investigations are relatively untested
 and are state-of-the-art research techniques.  Consequently,
 these methods are not currently available for use at most
 sites.

 Ground penetrating radar is used to detect changes in
 dielectric properties of subsurface materials. These changes
 may be related to changes in physical properties such as water
 content or bulk density. Although principally used to aid in
 the characterization of stratigraphy and lithologic
 heterogeneity, this method has been used to map LNAPL.

 Complex resistivity measures resistivity in magnitude and
 phase as a function of frequency. The method is used to detect
 the presence or absence of chemical reactions. Higher phase
 shifts and nonlinearity generally indicate greater chemical
 activity. The activity  includes many clay-organic chemical
 reactions. The specific reactions which are occurring have not
 been characterized at most sites. However, the geophysical
 signatures from these reactions have been used to determine
 the presence of organic contamination. Recent laboratory
 investigations indicate most common organic contaminants
 react with montmorillonite and, therefore, may be detectable
 using this technique.

 Electromagnetic induction methods, which are used to map
changes in electrical conductivity in the subsurface, have been
 applied to the detection of organic chemicals with limited
success. The conductivity of subsurface materials is
predominantly a function of porosity, water saturation,
electrical conductivity of the pore fluid, and the clay content
of the soil.  Dissolved inorganic contaminants generally
increase the conductivity of the water and may mask any
 conductivity decreases due to the presence of organic
 chemicals. Electromagnetic induction methods are generally
 most applicable to organic contamination delineation when
 inorganic contaminants are not present or when immiscible-
 phase organic compounds preferentially wet soil, displace
 water, and decrease conductivity.

 The value of geophysical methods may be increased when
 applied repeatedly at successive points in time. Changes in
 the geophysical signatures of subsurface materials may be
 indicative of contaminant migration. Geophysical methods
 are best suited to detect DNAPL while the immiscible liquids
 are moving. Repeated geophysical surveys may provide
 information concerning subsurface changes caused by the
 moving DNAPLs as well as identify the pathways along
 which they are moving.  During remediation, repeated surveys
 may also be useful in monitoring changes caused by the
 remediation process.

 The utility of geophysics at most sites will not be in the direct
 detection of DNAPLs, but in aiding the characterization of
 where DNAPLs may be going based on the geologic context.
 The use of geophysical methods to provide information
 concerning the hydrogeology at a site is within the current
 state of practice.  The applicability of these techniques
 depends on site-specific conditions.  All techniques are not
 appropriate for every site and applications must be evaluated
 on a site-specific basis.

 Detection of organic compounds using geophysical methods is
 currently state-of-the-science and not readily available for use
 at most  sites. The techniques have only been applied
 successfully at a limited number of sites and should be
 considered as emerging technologies. The principal
 difficulties related to the use of geophysical methods in
 characterizing organic contamination are partly due to
 deficiencies in the knowledge of contaminant properties and
 processes, but predominantly due to a critical lack of
 environmental geophysicists trained in these techniques.

 43     Invasive Techniques

 Regardless of the use of noninvasive characterization
 techniques, invasive methods will continue to be
 indispensable for characterizing most DNAPL sites. Invasive
 techniques include subsurface soil sampling, piezometer and
 monitoring well installation, and the use of such tools as
 standard or modified cone penetrometers. Soil, rock, and fluid
 samples brought to the surface should be carefully inspected
 and analyzed as drilling progresses to identify DNAPL pools,
residual zones, and potential barrier layers. In addition, the
 type and degree of site characterization needed for application
of one DNAPL remedial technology are often not what is best
 suited for another. Therefore, site characterization strategies
appropriate for DNAPL sites are generally more complex than
those that are adequate for non-DNAPL sites.

-------
Two basic approaches to invasive characterization exist and
have been referred to as the outside-in and the inside-out
approaches. The outside-in strategy of conducting initial
invasive characterization outside suspected DNAPL areas and
working toward the source has the advantage of allowing
acquisition of significant geologic and hydrogeologic data at
relatively low risk.  These data can be used to refine the
conceptual model and guide additional investigations.
However, it should be noted that uncertainty exists in
determining the extent of the suspected DNAPL area at any
site. One disadvantage to the outside-in approach is that much
additional time and expense may be incurred during the study
if characterization is started too far from the suspected
DNAPL zone.  At some sites, it may be appropriate to avoid
drilling directly within areas of known or suspected DNAPL
contamination and focus on characterizing dissolved
contaminant plumes migrating from the source areas.
However, drilling in suspected DNAPL areas may be required
to provide the necessary information for site characterization
and remedial design (e.g., assessing the presence and locations
of DNAPLs and for application of in situ restoration
technologies).

The inside-out approach of initially drilling within the areas
suspected to be the most contaminated and subsequently
drilling in more remote areas to define the extent of
contamination has been the traditional approach for many site
investigations.  One potential advantage of this method is that
fewer boreholes may be required to determine the extent of
contamination when investigation is started within an area of
known contamination and progresses outward. The obvious
disadvantage to this strategy is the increased potential for
providing pathways for the rapid downward migration of
DNAPLs. The increased risks of remobilizing certain high
mobility DNAPLs (e.g., chlorinated solvents) may render the
inside-out approach the least desirable strategy at certain sites.
This strategy appears to be most applicable at DNAPL sites
where the immiscible liquids are relatively immobile (e.g.,
many creosote and coal tar sites). The relative time and cost
advantages of the inside-out approach may be offset by
additional investigation and remediation costs incurred if
DNAPLs are mobilized to greater depths.

The choice of characterization strategy depends on the
conceptual model of the site and the physical properties of the
contaminants.  Immiscible liquids with relatively high
densities and low viscosities (e.g., chlorinated solvents) will
be  relatively more mobile than less dense, more viscous
liquids (e.g., creosote or coal tar). More mobile liquids
represent greater risk for spreading contamination to deeper
zones during site characterization. In addition, these
contaminants may have migrated considerably farther from
the DNAPL entry location than less dense, more viscous
liquids. The use of site history and noninvasive techniques
may provide useful information for guiding invasive study.
However, it will generally not be possible to determine the
extent of immiscible liquids prior to invasive study. Locations
for invasive study must be chosen using the best available site
specific information and knowledge of DNAPL contaminant
transport principles. Regardless of whether the outside-in or
inside-out approach is chosen, characterization should proceed
from shallow depths to greater depths. In this manner, more
information is acquired concerning shallow geologic features
and contamination and the risk of mobilizing DNAPLs to
greater depths during drilling is reduced.


5.     DNAPL Remediation

Experimental technologies must undergo considerable
development in order to become proven technologies (Figure
1). During the experimental phase, problems and concepts are
identified and numerous technologies are screened using
bench study results. Technologies which pass the initial
screening may be evaluated during controlled field studies
with rigorous mass balance constraints. The most promising
emerging technologies can then be further assessed during
uncontrolled pilot investigations.  Those technologies which
produce favorable results may be implemented on a large
scale. The time necessary for an experimental ground-water
remedial technology to be considered proven varies, but may
be on the order of ten years. The  ultimate goal of a proven
technology is to restore a contaminated aquifer in a cost-
effective manner.

No in situ technologies for restoring DNAPL zones in aquifers
to drinking water standards have been adequately
demonstrated or subjected to rigorous scientific evaluation.
Some technologies appear to hold promise for use in DNAPL
remediation. The most promising technologies are considered
emerging technologies.  It is not possible at this time to fully
evaluate the efficacy of the emerging or experimental
technologies on the horizon due to the critical lack of data
from controlled field studies. Table 1 provides a summary of
remedial options potentially applicable at DNAPL sites.

Conceptually, a DNAPL site may be considered to consist of
two contamination zones (Figure 2), the DNAPL zone which
contains free-phase and/or residual DNAPL and the zone of
dissolved (and sorbed) contamination only. The zone of
dissolved contamination is formed by dissolution of DNAPL
constituents into passing ground water and encompasses the
DNAPL zone. The nature of contamination in each zone is
different and, accordingly, potential remediation technologies
for each zone also differ.

Technologies for containment and, potentially, restoration of
contaminated ground water within areas contaminated solely
with dissolved and sorbed constitutents are better defined than
technologies for containment and/or restoration of DNAPL
zones. However, effective implementation of technologies
designed for restoration of the zone of dissolved

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                        Proven
                      Technology
     Experimental
     Technology
                                       Uncontrolled Pilot
                                          Site Trials
                                       Controlled Field
                                     Experiments/Prototype*
              Emerging
             Technology
               Cost-Effective Technology
                                                         Large Trials
                        Prototype
                         Assessment
   Bench Studies
Concept Identification
Problem Identification
                             Field Studies with Rigorous
                              Mass Balance Constraints
                                                                       Screening of Numerous
                                                                         Technology Concepts
                                      CONCEPTS
                                                                              After Cherry (1992)
Figure 1. Conceptual Development of Technologies.
                            DNAPLZONE
                         (contains free-phase
                       and/or residual DNAPL)
                          DISSOLVED
                   CONTAMINATION ZONE
   DNAPL ENTRY LOCATION
  (such as a former waste pond)
                                                         Groundwater Flow Direction
Figure 2. Defined Areas at a DNAPL Site.

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contamination will require containment of ground water and
DNAPL within the DNAPL zone. Technologies for DNAPL
containment are promising but are not yet proven in a variety
of field situations.

5.1     Containment versus Restoration

There is considerable scientific and political controversy
concerning ultimate attainable cleanup levels at a DNAPL
site. Except for excavation, there are no proven technologies
to reduce the total mass of subsurface DNAPL to levels low
enough to effect full restoration of a contaminated aquifer.
However, there are techniques which can mitigate
contamination at many DNAPL sites. Therefore, much
attention in political, scientific, and industrial communities has
focussed on the concept of "containment" versus
"restoration."  In this context, restoration refers to full and
complete rehabilitation of the contaminated subsurface.
Containment refers to physical control of the aqueous and
nonaqueous phases in the subsurface and is commonly
coupled with partial cleanup of dissolved contamination
outside of the DNAPL zone. The hydraulic containment of
dissolved-phase contamination is fairly well understood and
technically feasible.

Although it is popular to envision containment of a
nonaqueous phase by physical or hydraulic barriers such as
cut-off walls, drains, wells, and artificial hydraulic gradients,
these mechanisms have not been critically evaluated under a
variety of field conditions, particularly for vertical
containment. In addition, containment technologies require
maintenance and monitoring following installation to ensure
DNAPL isolation.

Horizontal and vertical containment of DNAPLs present
different problems. Some scientific opinion holds that vertical
migration of DNAPLs through low permeability strata can be
limited or eliminated by creating an upward hydraulic
gradient. However, these results are from modeling studies,
and have not been demonstrated in the field. Natural conduits
through aquitards and chemical-induced desiccation of the
clays may allow downward vertical migration against a
hydraulic gradient.

The physical and chemical properties of a DNAPL may
influence the feasibility of containment and restoration
options. For example, the containment of low viscosity, high
density chlorinated solvents may not be possible under certain
conditions due to their high potential for mobility. In contrast,
highly viscous wastes (e.g., typical creosote and coal tar) may
be more readily contained.

Physical containment cannot be effected unless the location of
the potentially mobile DNAPL is known. To date, there are
no adequate means of locating chlorinated solvent DNAPLs in
the subsurface. Without adequate DNAPL zone delineation,
containment may not be possible to attain or adequately
demonstrate. Development of methods for site character-
ization is as critical for DNAPL containment options as it is
for other remedial options.

5.2     DNAPL Recovery Methods

DNAPL recovery systems consisting of extraction wells or
drain systems may remove large volumes of mobile DNAPL
by extracting total fluids (DNAPL and water) or DNAPL only.
The wells or drains are usually installed in stratigraphic traps
to optimize recovery where DNAPL pools are present. The
efficiency of a DNAPL recovery system generally decreases
rapidly as the DNAPL saturated thickness decreases.
Overpumping a product recovery well may result in truncation
of the DNAPL layer and significantly reduce the formation
transmissivity to DNAPL flow.

A significant quantity of DNAPL will remain trapped within
pore spaces subsequent to the removal of mobile DNAPL.
Generally, it will not be possible to reduce the DNAPL
residual saturation by increasing the hydraulic gradients alone.
This may be favorable in the sense that DNAPL is no longer
potentially mobile under normal hydraulic gradients, and the
high surface areas associated with residual ganglia and blobs
allow  greater dissolution per volume of DNAPL than with
pools or lenses.

53     Enhanced DNAPL Recovery Methods

DNAPL recovery may be enhanced by injecting fluids or
agents into the DNAPL zone to  increase hydraulic gradients,
reduce water/DNAPL interfacial tension, reduce DNAPL
viscosity, and/or increase DNAPL solubility. However, there
are practical technical problems which limit the effectiveness
of these methods in the field.  The complex subsurface
distribution of DNAPL is a function of geologic
heterogeneities.  These heterogeneities, in conjunction with
permeability reductions caused by the presence of DNAPL,
can prevent injected fluids or agents from making thorough
contact with subsurface DNAPL. The use of enhanced
DNAPL recovery techniques in the area of hazardous waste
remediation is in its infancy. Most of the techniques were
developed for enhanced oil recovery (EOR) applications.
Consequently, very little information is available concerning
field applications of enhanced DNAPL recovery techniques.

53.1    Induced Gradient/Water Flooding

Where DNAPL is pooled on lower permeability layers,
inducing DNAPL to flow to wells or drains may be an
effective means of achieving significant mass removal.  The
feasibility of removing DNAPL through induced flow is
influenced by site-specific characteristics such as pool
thickness, DNAPL viscosity, hydraulic conductivity, relative
                                                       10

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permeabilities, and capillary pressure-fluid content
relationships.

Long-term DNAPL recovery is increased by maintaining a
maximum thickness of DNAPL adjacent to the well or
drain. Capillary pressure and DNAPL saturation increase
with depth in a DNAPL pool, resulting in an increase in
relative permeability of the media to DNAPL with depth.
For this reason, DNAPL at the base of a pool is more mobile
than DNAPL at the top of a pool. As the thickness of a
DNAPL pool is reduced by pumping, the average relative
permeability to DNAPL declines, resulting in greater
resistance to DNAPL flow. This has important implications
for the efficacy of induced gradient DNAPL removal
systems.

Pumping water from wells or drains completed above a
DNAPL pool causes DNAPL upwelling which results in an
increase in formation transmissivity to DNAPL.  Pumping
wells or drains completed in the pool further induce DNAPL
flow.  To maximize the driving force on the DNAPL,
drawdown on both the DNAPL pool and the water table
should be maximized.  This may be achieved with dual
drain systems, dual well systems, an open trench to the
ground surface, fully screened wells, or water injection
wells. The physical and chemical properties (e.g., viscosity,
interfacial tension, etc.) of a DNAPL will control its
mobility and the extent to which the mobile product may be
removed. Highly viscous wastes may be more difficult to
mobilize, even under enhanced hydraulic gradients.

5.3.2    Chemically Enhanced Recovery

Surfactants, cosolvents, and alkaline agents are primarily
used to increase the solubility of DNAPL constituents in
ground water. The enhanced solubility increases the
efficiency of contaminant removal by significantly
increasing contaminant concentrations in ground water
recovered by extraction systems.  Surfactants are capable of
increasing the solubility of many DNAPLs by several orders
of magnitude.

However, in addition to increasing DNAPL solubility,
surfactants enhance DNAPL mobility by lowering the
interfacial tension between the nonaqueous phase and water.
The reduction of interfacial tension can produce
uncontrolled vertical and lateral migration of previously
immobile DNAPL trapped as isolated ganglia, lenses or
pools accumulated on lower permeability strata in the
aquifer.  The mobilization of such DNAPLs could greatly
exacerbate ground-water problems at DNAPL sites.

Surfactants and polymers have been used by the petroleum
industry to mobilize and recover a portion of the residual
hydrocarbons from  reservoirs.  Polymers have been used to
increase the viscosity of injected fluids and aid in the
displacement of residual hydrocarbons.  Recent laboratory
studies have demonstrated that under the proper conditions,
surfactants can enhance the removal of significant quantities
of DNAPL. Although it is known that surfactants can
enhance the rate of DNAPL mass removal, it is also known
that complexities related to geologic heterogeneity cause
limitations in mass removal such that restoration to drinking
water conditions is currently problematic.  Currently, several
pilot studies are under way to evaluate the effect of
surfactants on the solubility and mobility of DNAPL under
field conditions.

5.3.3    Thermally Enhanced Recovery

The application of heat to viscous DNAPLs will enhance their
mobility by decreasing viscosity and density. Additionally,
heating DNAPL increases the solubility of some constituents.
Steam and hot water flooding may be applicable in the
unsaturated and saturated zones. The injected steam yields
heat to the formadon and condenses into a zone that acts as a
hot water flood. Volatile DNAPL may condense to form a
DNAPL front, resulting in increased DNAPL saturation and
mobility. The continuous injection of steam drives the hot
water and nonaqueous phase toward extraction wells. Heating
certain DNAPLs may reduce their density, convert them to
lighter-than-water NAPLs, and, thereby, promote mobility due
to buoyancy forces and viscosity reduction.

Various steam and hot water displacement methods for
DNAPL recovery have been evaluated in laboratory and pilot
field studies. These methods have been successful at
removing significant quantities of DNAPL from the vadose
zone. However, the amount of residual DNAPL left in place
will maintain contaminant levels above regulatory levels for a
significant time.

5.4     Pump-and-Treat

Traditional pump-and-treat systems, which recover and treat
aqueous contamination, may be useful components in many
remediation programs. These systems may  be used to
prevent plume migration and reduce contaminant
concentrations in areas contaminated solely with aqueous and
sorbed constituents. Pump-and-treat may also be used to
remove residual DNAPL by solubilization.  However, these
systems are not efficient at removing DNAPL from the
subsurface.  Many pore volumes of water are needed to
completely remove even residual DNAPL under the best of
circumstances.  In general, subsurface heterogeneities limit
the ability to move water through many contaminated zones,
and thereby limit the ability of a pump-and-treat system to
cleanse a site.  However, when used in conjunction with other
remediation techniques designed to remove DNAPL, pump-
and-treat systems may prevent the exacerbation of ground-
water problems by capturing and removing aqueous-phase
contaminants.
                                                       11

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 5.5.    Vacuum Extraction

 Under certain conditions, soil vacuum extraction (SVE)
 technology has been demonstrated to remove large quantities
 of volatile DNAPLs from the vadose zone. This technique
 requires the application of vacuum to the unsaturated zone,
 thereby inducing air flow and enhancing the removal of many
 volatile organic contaminants (VOCs). The VOCs are
 removed from the subsurface by gas transport and treated
 above ground.

 SVE appears to have very limited applications for DNAPLs
 and aqueous-phase contaminants in the saturated zone.
 Molecular diffusion dominates vertical dispersion of VOCs
 from the capillary fringe and saturated zone. As a
 consequence, low mass transfer rates limit the mass of VOC
 which can be removed by SVE from ground water.  Steep
 vertical concentration gradients develop, but the amount of
 mass transferred from the saturated zone to the unsaturated
 zone is minimal.

 5.6    Bioremediation

 The applicability of bioremediation first depends on the
 inherent biodegradability of the specific contaminant. The
 inherent biodegradability of the individual constituents of
 DNAPLs varies widely.  Certain nonchlorinated, low-
 molecular weight contaminants are readily biodegraded under
 aerobic conditions. The biodegradability of high-molecular
 weight contaminants such as PCBs and the highly condensed
 polyaromatic hydrocarbons present in creosote is limited by
 their solubility and non-bioavailability. Effective
 biodegradation of chlorinated solvents and PCBs could require
 both an anaerobic dechlorination step and an aerobic step in
 which a supplemental carbon source is provided.

 The applicability of biodegradation for DNAPLs will also be
 limited by other factors.  Bioremediation in the saturated zone
 requires delivery of reagents required for biodegradation (e.g.,
 oxygen or other electron acceptor, inorganic nutrients) to the
 contaminated zone.  High contaminant concentrations in
 DNAPL zones will result in high stoichiometric reagent
 demands.  Limitations on reagent delivery associated with
 advective transport of the dissolved reagents into the
 contaminated zone will challenge the practicality of
 bioremediation in terms of the required delivery rate of
 reagent-enriched ground water and/or the duration of
 bioremediation. The high concentrations of toxic compounds
 in some DNAPLs may also be inhibitory or toxic to microbial
 activity.

 Due to the toxicity of most DNAPLs, and the lack of essential
 nutrients, electron acceptors, and other requirements for life in
 the NAPL pool itself, the potential for biologically-mediated
degradation of DNAPLs is limited. Bioremediation may be
most appropriate as a "polishing step" following methods such
 as free-phase liquid removal or in situ flushing that remove the
 majority of the DNAPL. Bioremediation may also be more
 applicable for remediation of plumes of contaminated ground
 water emanating from DNAPL zones. In these cases,
 bioremediation may be more effective than a pump-and-treat
 remediation approach due to more aggressive removal of
 contaminants sorbed to the aquifer matrix or present in small
 amounts of DNAPL. However, bioremediation will often be
 subject to some of the same factors that limit the effectiveness
 of pump-and-treat or other in situ approaches (e.g., preferred
 flow paths resulting from subsurface heterogeneities). There
 are a limited number of field-scale tests that have been
 conducted recently, or are under way, that are defining the
 applicability of the technology at DNAPL sites.  Numerous
 bench-scale research studies are also being conducted.

 5.7     Treatment Train

 If the intent of a remedial program at a DNAPL site is to
 restore an aquifer to its pre-contamination condition within a
 reasonable time span, more than one remedial technique may
 be required. An example of a treatment train presented at the
 workshop was a wood preserving site. It should be noted that
 despite the use of multiple techniques which resulted in
 significant DNAPL mass removal at this site, restoration has
 not been  achieved in the pilot test area.

 The facility treated railroad ties and other wood products for
 almost 100 years. Wood-preserving agents used in the
 operations included a creosote oil and asphalt-based petroleum
 oil mixture and pentachlorophenol. Approximately 100 acres
 of the site are contaminated with DNAPL in the underlying
 alluvial sands and gravels. Both nonaqueous and aqueous-
 phase contaminants were discharging through the alluvium
 into a nearby river.

 A bentonite slurry wall was constructed around the site and
 drain lines were installed inside and outside the wall to
 maintain  an inward and upward hydraulic gradient inside the
 containment area. Subsequently, a pilot program was initiated
 to evaluate a series of in situ treatment methods at the site.
 Selected remedial techniques included induced gradient
 product recovery  using a  dual drainline, chemically-enhanced
 in situ soil flushing, and in situ bioremediation.

 The induced gradient recovery method was designed to
 significantly reduce the total mass of contaminant to levels
conducive to cleanup by enhanced oil recovery techniques.
Following product recovery efforts, a site-specific,
chemically-enhanced in situ soil flushing method was applied.
 Surfactant, polymer, and alkaline agents were delivered to the
subsurface to mobilize the remaining oily wastes, further
reducing the subsurface DNAPL mass. The final step of the
pilot program was in situ  bioremediation. The naturally-
occurring microbial populations were stimulated with supplied
oxygen. Despite the apparent success of these techniques in
                                                         12

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removing a large DNAPL mass, significant amounts of
residual DNAPL remained in the test area.

At this site, numerous remedial technologies were employed
both in parallel and in series. Currently, it appears this type of
treatment train may be required to potentially restore DNAPL-
contaminated sites. However, there are many technological
and conceptual problems with present-day remedial systems
that require extensive research, development, and
demonstration. Implementation and successful use on large
scales may not be achievable.  Even at sites where several
emerging technologies have been applied, they have not been
fully successful at removing DNAPL. Nevertheless, ground-
water remediation is a relatively new science and as emerging
technologies evolve to proven technologies, significant
advances in DNAPL remediation can be anticipated.
This document was edited by Steven D. Acree, Randall R.
Ross, and Marion R. Scalf ofU.S. EPA Robert S. Ken-
Environmental Research Laboratory, Ada, OK, Ellen R.
Graber of U.S. EPA Region VI, on temporary detail at
RSKERL, and James W. Mercer and Robert M. Cohen of
GeoTrans, Inc.
                                                       13

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                                                      Table  1.   Summary of Remedial Operations Potentially Applicable at DNAPL
                                                                  Contamination Sites.
            APPLICATION
                                                                                            EFFECTIVENESS / ADVANTAGES / LIMITATIONS
                                                                                                                                                       EXPERIENCE
                                                                                                                                                                                        REFERENCES
H\ drauhc
Contain-
  ment
H \draulic
containment is
used to pre\ ent
the undesired
migration of
chemicals
through the
saturated zone
Hydraulic containment of dissolved chemicals may be
achieved by pumping groundwaler from wells and/or
drains. Full horizontal and vertical hydraulic containment
of DNAPL has not been demonstrated in the field. Fluid
flow control can be augmented by injecting w ater through
w ells and/or drains, and by the installation of physical
barriers icut-off walls and landfill covers). Monitor wells
are utilized to determine w hether or not the specified
h> drauhc gradients hav e been obtained and chemical
migration has been arrested.
Long-term hydraulic containment will be needed at many
DNAPL sites because residual and trapped pools of
DNAPL are long-term sources of groundwater contamina-
tion.  The effectiveness of a hydraulic containment system
depends largely on the adequacy of the design and operation
of the system.  Containment is eased where there is a
continuous barrier layer that prevents downward DNAPL
migration. It is theorized that downward migration may be
arrested at some sites by creating an upward hydraulic
gradient into the DNAPL zone that exceeds the density
difference between DNAPL and water.  Vertical hydraulic
containment of DNAPL, however, has yet to be demon-
strated in the field. The main drawbacks to hydraulic
containment systems are cost and the need for long-term
operation.
The components of a hydraulic
containment system (wells,
drains, cut-off walls, and covers)
have been widely used for
contamination site remediation
and other applications. Although
hydraulic containment is
generally a proven migration
control technology, its success
depends on adequate design and
implementation.
Cherry et al. (1990), Cohen et al.
(1987), Mackay and Cherry
(1989), Mercer etal. (1990)
 Contain-
   ment
   L SlT.i
 P-NilCii
 Ba'-.ers
Capillar. and
low permeabil-
it\ barriers
ifine-grained
walls') can be
constructed to
limit NAPL
migration in the
saturated zone
Low permeability, fine-grained barrier walls (i.e., slurry
walls, concrete walls, sheet piling with grouted joints, etc.)
can be constructed to impede the lateral migration of non-
wetung  DNAPL below the water table.  Where possible,
bamer w alls should be keyed into a low permeability,
capillar> bamer la\ er beneath the DNAPL contamination
zone.
Barrier walls can provide cost-effective control over NAPL
migration in favorable settings.  Bamer walls have not been
tested, however, to determine their capacity for long-term
impedance of NAPL migration. Small fractures or openings
will facilitate  DNAPL breakthrough.  The long-term
integrity of engineered subsurface barriers is not well-
known.  Consideration must be given to the compatibility of
bamer wall materials with subsurface chemicals, the
potential for inducing migration during wall construction,
and changes to the hydro-geologic system effected by wall
emplacement.
Cutoff walls have been installed
as part of containment systems at
many sites.
Cherry etal. (1990), Sale etal.
(1988)

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                                                    Table 1.   Summary of Remedial Operations Potentially Applicable at DNAPL
                                                                Contamination Sites (continued).
           APPLICATION
                                                                                         EFFECTIVENESS / ADVANTAGES / LIMITATIONS
                                                                                                                                                  EXPERIENCE
                                                                                                                                                                                  REFERENCES
 Product
Recovery
   by
Pumping
Mobile NAPL
can be pumped
from wells or
drams.
Mobile XAPL can be pumped from wells and drains
utilizing single pumps to extract total fluids or NAPL only,
or using individual pumps to withdraw water and NAPL
separately.
Wells should be placed in stratigraphic and structural traps
to optimize recovery where NAPL pools are present. Long-
term recovery is increased by maintaining a maximum
thickness and saturation of NAPL adjacent to the well.  This
can be achieved by pumping NAPL and water separately.
Pumping water above a DNAPL pool causes DNAPL
upwelling which works to increase the formation transmis-
sivity to DNAPL flow. A dual pumping system can be
operated using wells or drains. Overpumping the NAPL
may result in truncation of the NAPL layer at the well edge
and significantly reduce the formation transmissivity to
NAPL flow.  Pumping may cause NAPL to enter previously
uncontammated sections, thereby enlarging the contami-
nated zone.  In shallow, unconfmed formations, it will
generally not be possible to significantly diminish the
NAPL residual saturation by increasing hydraulic gradients
alone. Pumping can be used to remove mobile NAPL and
reduce the potential for continued NAPL migration,
however. Vacuum-enhanced pumping may be a way to
increase gradients for improved mobilization and hydraulic
control.
A great deal of experience has
been acquired by the oil industry
pumping LNAPL from crude oil
reservoirs and by the environ-
mental industry pumping
petroleum products from
contaminated shallow forma-
tions. Little documentation is
available, however, regarding
DNAPL product recovery at
contamination sites.
Blake et al. (1990), Ferry et al.
(1986),  McWhorter(1991),
Newell and Connor (1991), Sale
et al. (1988 and 1989), Sale and
Piontek (1988), Schmidtke et al.
(1987), Villaume (1991),
Wisniewsky et al. (1985)
EOR using
  Water
 Hooding
Waterflooding
can be used to
increase the
recovery of
NAPL from the
saturated zone.
Referred to as secondary recovery by the oil industry,
waterflooding involves the injection of water in wells or
drains to hydraulically sweep NAPL toward production
wells. Recovery can be enhanced because injection/
extraction systems (i.e., line-drive and five spot systems)
allow for the development and sustenance of increased
hydraulic gradients and flow rates, elimination of dead
zones, and overall improved flow control management.
                                                                                   Refer to the soil flushing comments.
                                                       Although routinely utilized for
                                                       secondary recovery by the oil
                                                       industry, waterflooding has been
                                                       utilized to recover NAPL at only
                                                       a few environmental contamina-
                                                       tion sites.
                                Anderson (1987c), Donaldson et
                                al. (1989), Newell and Connor
                                (1991), Rathmell et al. (1973),
                                Sale and Piontek (1988), Sale
                                etal. (1989), Sale etal. (1988)

-------
                                                       Table 1.    Summary of Remedial Operations Potentially Applicable at DNAPL
                                                                    Contamination Sites (continued).
METHOD
            APPLICATION
                                                                                              EFFECTIVENESS / ADVANTAGES / LIMITATIONS
                                                                                                                                                         EXPERIENCE
                                                                                                                                                                                           REFERENCES
    Soil
  Flushing
    by
  Flooding
    with
  Water,
  Steam,
  Surfac-
   tants,
  Alkaline
  Agents,
 Polymers,
    and
 Cosolvents
    [see
  specific
 flood EOR
  technolo-
    gies
  below]
 O\
In situ soil
flushing can be
used to enhance
recovery of
NAPLs,
adsorbed
chemicals, and
dissolved
chemicals from
the saturated or
unsaturated
Injection wells or drains, or surface application delivery
systems are used to flood the contaminated zone with
flushing solutions and sweep the contaminants to recovery
wells or drains (i.e., surfactants, cosolvents, alkaline agents,
polymers, steam, etc.). Drains are typically used to effect
line-drive sweeps; wells are typically used to effect line-
dnve or five-spot sweeps. The flood enhances recovery of
NAPL by reducing interfacial tension, reducing NAPL
viscosity, lowering the mobility ratio, increasing solubility,
and/or increasing hydraulic gradients (i.e. raising the
capillary number).  Recovery of adsorbed and/or dissolved
chemicals is also enhanced by these processes.  Displaced
NAPL and chemicals are recovered by pumping wells and/
or drains. At the conclusion of the flood, the flushing
solution can be displaced to the recovery system by
injecting water via the delivery system.  Soil flushing may
be used as an intermediate process in a train of remedial
Soil flushing is most effective in permeable, uniform
media. It can be used to speed the removal of contaminants
from the subsurface. Heterogeneous and low permeability
soils will generally result in reduced sweep efficiency,
longer project duration, and less successful recovery. Soil
flushing can, however, be used to reduce the NAPL
residual saturation to levels below the immobile saturation
at ambient site conditions.  The movement of contaminants
mobilized by the flood must be carefully controlled to
prevent detrimental migration.  Consideration must also be
given to: the toxicity and fate of the flushing solution and
potential adverse reactions (permeability reduction,
impairment of biodegradation rates, etc.) caused by the
solution. Site-specific bench and pilot-scale field tests are
generally recommended prior to implementation of a field-
scale remedial project.  The site-specific application of
EOR methods may prove to be effective within a train of
treatment measures to remediate NAPL-contammated sues.
Although the oil industry has
made extensive use of flooding
technology to enhance oil
recovery, few in-situ soil
flushing operations have been
conducted at contamination sites.
In general, the application of soil
flushing technologies to
remediate contamination sites is
at the pilot-test stage and the
effectiveness of these technolo-
gies in environmental applica-
tions is unknown.
Sims (1990), USEPA (1990a and
1990b) [see specific flooding
technologies below]
 EOR Using
 Surfactant
   Water
  Flooding
 Surfactant
 flooding can be
 used to increase
 NAPL recovery
 during a flood
 operation in the
 saturated zone.
Surfactant solution is injected as a slug in a flooding
sequence to decrease the interfacial tension between NAPL
and water by several orders of magnitude (i.e., from 20-50
dynes/cm to less than 0.01 dynes/cm).  The development of
ultra-low interfacial tension effects a commensurate 3-5
orders-of-magnitude increase in the capillary number (Nci)
which is the ratio of viscous to capillary forces, sometimes
expressed as NCI=HV/O"<|>,  where u, and v are the viscosity
and Darcy velocity of the displacing fluid, o" is the
interfacial tension, and ((> is the porosity. Ultra-low
interfacial tension and higher capillary numbers improve
the NAPL displacement efficiency of a flood, promote the
coalescence of NAPL ganglia and development of a NAPL
bank in front of the surfactant slug, and result in increased
NAPL recovery and reduced NAPL residual saturation.
Surfactant flooding can also enhance NAPL recovery by
causing increased NAPL  wetting, solubilization, and
emulsification. Some surfactants used in EOR operations
by the oil industry include petroleum sulfonates, synthetic
sulfonates, ethoxylated sulfonates, and ethoxylated
alcohols.
The high cost of surfactant chemicals has limited the
commercial application of surfactant flooding by the oil
industry. Using surfactant solutions to enhance NAPL
recovery at contamination sites, however, may be more
attractive given the higher costs associated with waste site
remediation.  At many sites, reducing interfacial tension
will be the only practical way to mobilize residual NAPL.
Refer to the soil flushing comments.
There were approximately 30
active EOR field-scale projects
using surfactant injection in the
United Stales in 1980. Applica-
tion of surfactant flooding by the
oil industry for EOR is limited by
its high cost relative to other
EOR methods. The application
of surfactant flooding to enhance
NAPL recovery at contamination
sites is in its infancy. Surfactants
were included in a soil flushing
solution of alkaline, polymer, and
surfactant agents (A-P-S) to
enhance DNAPL recovery at the
I^aramie Tie Plant.
 Akstmat(1981), Beikirch (1991),
 Donaldson et al. (1989), Ellis et
 al (1985), Flumerfeltetal
 (1981), Fountain (1991), Gogarty
 (1983), Hesselmk and Faber
 (1981), Manji and Stasiuk
 (1988), Nash (1988), Nelson
 (1989), Nelson etal. (1984),
 Neustadter (1984), Novosad
 (1981), Pitts etal. (1989),
 Salagarctal. (1979), Shah
 (1981), Sharma and Shah (1989)
 Tuck etal. (1988)

-------
                                                       Table 1.   Summary of Remedial Operations Potentially Applicable at DNAPL
                                                                   Contamination Sites (continued).
            APPLICATION
                                                                                             EFFECTIVENESS / ADVANTAGES / LIMITATIONS
                                                                                                                                                        KXPKKIKNt.K
                                                                                                                                                                                          RKKKRENCES
  EOR
 Using
Alkaline
 Water
Flooding
Alkaline
flooding can be
used to increase
NAPL recovery
during a flood
operation.
Alkaline waterflooding is an EOR process where
inexpensive caustics such as sodium carbonate, sodium
silicate, sodium hydroxide, and potassium hydroxide are
mixed with the injection water. The alkaline agents raise
the pH of the flood and react with organic acids that are
present in oil. This reaction generates surfactants at the oil-
water interface and leads to improved oil recovery due to
(1) greatly reduced interfacial tension, (2) emulsification
effects, and (3) wettability reversals, which can mobilize
entrapped oil ganglia. Alkaline waterflooding may be used
in conjunction with other EOR methods; for example, it is
reported to be an effective preflush for surfactant-polymer
floods.
NAPLs must have acidic components to react with the
alkali agents to form surfactants. Alkaline flooding is
relatively cheap compared to some other EOR methods, but
alkali consumption due to reaction with porous media may
be a limiting factor.  Refer to the soil flushing comments.
Numerous alkaline waterflood
EOR projects have been
conducted by the oil industry
(approximately 40 were reported
begun in the U.S  between 1979
and 1981)  The application of
alkaline flooding to enhance
NAPL recovery at contamination
sites is in its infancy. Alkaline
agents were included in a soil
flushing solution of alkaline,
polymer, and surfactant agents
(A-P-S) to enhance DNAPL
recovery at the Laramic Tie
Plant.
Brcitetal. (1981), Campbell
(1981), Castor el al. (1981 b),
Donaldson et al. (1989), Janssen-
VanRosmalen and I Icssehnk
(1981), Kumar etal. (1989),
Mayer el al. (1983), Nelson et al
(1984),  Pitlsctal. (1989), Sale
eial. (1989), Surkalo (1990)
 Polymer
  Water
 Flooding
 Polymer
 flooding can be
 used to increase
 NAPL recovery
 during a flood
 operation in the
 saturated  zone.
 Polymers are large molecules that can be dispersed in a
 waterflood to increase the viscosity of the flood, thereby
 reducing the mobility ratio and improving the volumetric
 sweep efficiency (NAPL recovery). The mobility ratio is
 defined as the mobility of the displacing fluid (effective
 permeability/viscosity for water) divided by the mobility of
 the displaced fluid (effective permeability/viscosity for
 NAPL). Lower mobility ratios favor NAPL displacement
 and recovery.  An effective polymer will impart a high
 viscosity at low concentration. Only two types of polymers
 are commonly used by the oil industry (polyacrylarmdes
 and polysacchandes).  In EOR operations, polymer
 flooding is often used as part of a phased injection
 sequence consisting of:  (1) a preflush to adjust the pH and
 salinity of the reservoir, (2) surfactants and/or alkaline
 agents to reduce interfacial tension; (3) polymer solution to
 increase viscosity and improve displacement efficiency,
 and (4) waterflood to displace the mobilized oil and EOR
 solutions.
The advantage of polymer flooding is that it improves the
volumetric sweep efficiency of a water flood process.  By
itself, polymer flooding will not, however, mobilize trapped
residual NAPL in a water-wet situation.  Potenual
limitations include: the risk of reduced mjectivity caused
by wellbore plugging, increased project durations and
slowed recovery due to the lower absolute flood mobility,
polymer degradation, excessive cost. Refer to the soil
flushing comments.
Polymer flooding had been
initiated at about 180 field-scale
EOR projects in the U S. by the
mid 1980s.  There is debate in the
oil industry, however, regarding
whether or not polymer flooding
by itself provides more than a
small incremental recovery. The
application of polymer flooding
to enhance NAPL recovery at
contamination sites  is in the early
development stage.  Polymers
were used in a soil flushing
solution of alkaline, polymer,
and surfactant agents (A-P-S) to
enhance DNAPL recovery at the
Laramie Tie Plant.
Caenn et al. (1989), Castor el al.
(198la), Chauveteau and Zauoun
(1981), Donaldson et al. (1989),
Hessehnk and Faber (1981),
Labastie and Vio (1981), Lin et
al. (1987), Littman (1988), Pitts
etal. (1989), Sale etal. (1989),
Shah (1981), Surkalo etal.
(1986), Yen etal. (1989)

-------
                                                      Table 1.   Summary of Remedial Operations Potentially Applicable at DNAPL
                                                                  Contamination Sites (continued).
METHOD
            APPLICATION
                                                   PROCESS
                                                                                          EFFECTIVENESS / ADVANTAGES / LIMITATIONS
                                                                                                                                                    EXPERIENCE
                                                                                                                                                                                     REFERENCES
EOR using
  Chemi-
   cally-
 Enhanced
Dissolution
(Cosolvents)
Cosolvents
can be used
to increase
the
dissolution
and recovery
of NAPL and
adsorbed
chemicals
from the
subsurface.
Cosolvents injected into a contamination zone via wells or
drains increase the dissolution of NAPLs and adsorbed
chemicals. Continued flooding of the contamination zone
with Cosolvents or another flood (water, polymers, etc.)
drives the elutriate to production wells or drains. The
elutriate may be treated and recycled through the system.
Refer to the soil flushing comments.
 Miscible flooding with carbon
 dioxide and/or hydrocarbon
 solvents has been tested at
 numerous sites for EOR by the
 oil industry. Several bench- and
 pilot-scale studies have been
 conducted on in-situ cosolvent
 flushing technology at contami-
 nation sues with variable success.
 Blackwell (1981), Fountain
 (1991), Groves (1988),
 McDermott et al. (1988),
 Mehdizadeh et al. (1989), Nash
 (1987), Nash and Traver (1986),
 Rao etal. (1991), Roy and
 Griffon (1988), Sayegh and
 McCaffery (1981), Taber (1981),
 USEPA (1990a and 1990b)
EOR Using
  Thermal
  Methods
 (Steam or
 Hot Water
 Flooding)
 Thermal
 methods (steam
 or hot water
 flooding) can
 be used to
 increase NAPL
 recovery.
 High-temperature steam is injected via wells into the
 contamination zone. The steam yields heat to the
 formation and condenses into a zone that acts as a hot
 water flood. Coupled with the continuous injection of
 steam behind it, this hot water drives NAPL to the recovery
 wells. NAPL recovery is enhanced because: (1) the NAPL
 becomes less viscous and more mobile upon heating; (2)
 NAPL solubility may be increased by the higher tempera-
 tures; (3) volatile NAPL vaporizes, moves ahead of the hot
 water and then condenses to form a NAPL bank; and, (4)
 the increased NAPL saturation in the NAPL condensate
 bank provides increased NAPL transmissivity and, under
 favorable conditions, results in a snowball effect.
 The application of thermal methods to enhance DNAPL
 recovery at contamination sites may become more popular
 based on the successful use of steam injection by the oil
 industry, the encouraging results of limited lab and pilot-
 scale testing of steam injection for DNAPL recovery, and
 the fact that additional chemicals need not be injected to
 recover the contaminants. Heating may convert DNAPL to
 LNAPL, thereby promoting mobility due to buoyancy
 forces.  Costs may be high due to heat loss and the need to
 heat large volumes of subsurface materials.  Refer to the
 soil flushing comments.
 There were approximately 200
 active thermal EOR projects in
 the United States in 1986. Steam
 and hot water displacement  of
 NAPLs at contamination sites has
 been evaluated in a few
 laboratory and pilot field studies.
 Blevin et al. (1984), Boberg
 (1988), Donaldson et al. (1989),
 Doscher and Ghassemi (1981),
 Goyal and Kumar (1989), Hunt
 etal. (1988aand 1988b),
 Leuschner and Johnson (1990),
 Mandl and Volek (1969),
 Menegus and Udell (1985),
 Miller (1975), Offennga et al.
 (1981),  Prats (1989), USEPA
 (1990b), Volek and Pryor
 (1972),  Willman et al. (1961),
 Yortsos and Gavalas (1981)
Pump-and-
   Treat
Dissolved
chemicals can
be removed
from the
saturated zone
by pumping
groundwater.
Contaminated groundwater is pumped from wells or drains
using conventional technology. Recovery rates can be
optimized by fine-tuning pumping rates, well locations, etc.
Pump-and-trcat is typically utilized as part of a hydraulic
containment or aquifer restoration program.  DNAPL,
where present, will be a long-term source of groundwater
contamination. Due to the relatively low solubility of most
DNAPLs, pumping and treatment is generally not efficient
in removing DNAPL from the subsurface.
Recovery of dissolved chemicals
by ground-water pumping is a
widely-used, proven technology,
but not for removal of NAPL.
Anderson et al. (1987,1991a and
1991b), Anderson (1988),
Feenstra and Cherry (1988), Hunt
et al. (1988a), Johnson (1991),
Johnson and Pankow (1991),
Keely (1989), Mackay and
Cherry (1985), Mercer et al.
(1990), Mercer and Cohen
(1990), Schwille (1988)

-------
                                                      Table 1.   Summary of Remedial Operations Potentially Applicable at DNAPL
                                                                  Contamination Sites (continued).
 METHOD
             APPLICATION
                                                                                            EFFECTIVENESS / ADVANTAGES / LIMITATIONS
                                                                                                                                                       EXPERIENCE
                                                                                                                                                                                        REFERENCES
 Vacuum
Extraction
   (VE)
VE may be
used to remove
volatile
chemicals from
the unsaturated
zone and to
prevent
uncontrolled
migration of
volatile
chemicals in
soil gas. If the
water table is
lowered, VE
may  be used to
remove residual
NAPLfrom
below the
original water
table elevation.
Using unsaturated zone wells equipped with blowers or
vacuum pumps, air is forced through soils contaminated
with volatile chemicals.  The air flow generates advective
vapor fluxes that change the vapor-liquid equilibrium,
inducing volatilization of contaminants. The resulting
vapors are collected and treated. Positive differential
pressure systems induce vapor flow away from the control
points and negative differential pressure systems induce
vapor flow toward control points. Experience had
demonstrated that generation of negative differential gas
pressures typically provides the most favorable field
results.
Vacuum extraction is most effective removing low
molecular weight, volatile chemicals (dimensionless
Henry's Law Constant > 0.01) from homogeneous,
permeable media. Intermittent vacuum extraction operation
is generally more efficient than constant operation.
Vacuum extraction systems can be installed using off-the-
shelf components and conventional drilling methods.
Vacuum extraction is less effective at removing volatile
chemicals from heterogeneous and low permeability soils
and is ineffective removing volatile chemicals from the
saturated zone.  Because it induces water table upwelling,
vacuum extraction can result in groundwater contamination
where chemicals are located just above the water table.
Groundwater recovery wells located adjacent to vacuum
extraction wells may be necessary. Alternatively, lowering
the water table to allow volatile chemical recovery by
vacuum extraction may promote DNAPL remobilization
and sinking.
In-situ vacuum extraction
processes have been employed at
more than 100 contamination
sites in the United States.
Agrelot et al. (1985), Ardito and
Billings (1990), Baehr et al.
(1989), Batchelderet al. (1986),
Blake et al. (1990), Blake and
Gates (1986), Crow et al. (1985
and 1987), DiGiulio and Cho
(1990), Dunlap (1984), Gierke et
al. (1990), Hutzler et al. (1989),
Johnson et al. (1988), Johnson et
al. (1990a and 1990b), Jury et al
(1990), Knshnayya et al. (1988),
Mackay et al. (1990), Marley and
Hoag (1984), Massmann (1989),
McClellen and Gdlham (1990),
O'Connor et al. (1984),
Rarhfelder (1989), Rathfelder et
al. (1991), Regalbuto et al.
(1988), Sims (1990), Stephanatos
(1988), Texas Research Inst
(1984), Thornton and Wootan
(1982), Wilson et al (1987),
USHPA (1989a, 1990aand
1990b)
Steam and
 Hot Air
 Injection
    to
 Enhance
 Vacuum
Extraction
Vacuum
extraction in the
unsaturated zone
can be enhanced
by steam and hot
air injection.
Steam or hot air injected into or below the zone of soil
contamination can improve the effectiveness of vacuum
extraction systems. Heating and increased soil gas
movement caused by steam and hot air injection raise the
vaporization rate of volatile and some semi-volatile
compounds. Additionally, contaminated soil water and
low viscosity NAPLs can be physically displaced by the
condensate that forms in front of the steam zone.
In general, steam or hot air injection will increase the
effectiveness of a vacuum extraction system.
HeaUng will reduce the viscosity and interfacial surface
tension of residual or trapped DNAPL in the unsaturated
zone, which may result in uncontrolled migration.
Similarly, accumulation of DNAPL at the steam condensate
front may result in uncontrolled downward or lateral
migration of the DNAPL.
Successful laboratory and pilot-
scale field studies have been
conducted using steam and hot
air stnppmg to enhance vacuum
extraction recovery of solvents
and petroleum contaminants from
soil. Two different systems have
been used:  (1) a mobile unit
consisting of a hollow stem auger
ng outfitted for steam/air
injection and vacuum extraction
of vapors, and (2) a fixed system
of injection and extraction wells.
Baum (1988), Hunt el al. (1988b),
DePauli (1990), de Herein (1990),
Houthoofd et al. (1991), llinchee
and Smith (1990), Johnson and
Guffey (1990), Lord et al (1988,
1989, 1990, and 1991), Nunno et
al. (1989), Udell and Stewart
(1989 and 1990), USliPA (1990a
and 199()b)

-------
                                                       Table 1.   Summary of Remedial Operations Potentially Applicable at DNAPL
                                                                   Contamination Sites (continued).
METHOD
            APPLICATION
                                                                                             EFFECTIVENESS / ADVANTAGES / LIMITATIONS
                                                                                                                                                         EXPKKII.Nr.K
                                                                                                                                                                                          KKFKKhNCKS
  Radio
Frequency
 Heating
 (RFH) to
 Enhance
 Vacuum
Extraction
Vacuum
extraction of
chemicals that
volatilize in the
temperature
range of 80° to
300° C can be
enhanced by
using radio
frequency
heating of
contaminated
soil.
In situ radio frequency heating (RFH) involves heating soil
with electromagnetic energy in the radio frequency band
(typically 6.7 MHz to 2.5 GHz).  Using a modified radio
transmitter as a power source, energy is transmitted to the
zone targeted for decontamination via electrodes placed in
an array of boreholes. This energy heats the soil to
temperatures between 150° and 300° C. Volatilized
chemicals are recovered with soil gas for treatment by
applying a vacuum to selected hollow electrodes. A rubber
sheet barrier may be spread over the soil surface to provide
thermal insulation and prevent fugitive emissions
hi general, RFH will increase the effectiveness of a vacuum
extraction system. The technology is only applied to the
unsaturated zone and us use is precluded where buned
metal objects are present.  As with vacuum extraction, this
method is best suited to sites where volatile chemicals are
present at shallow depth in homogeneous, coarse-grained
soils. The uniformity of heating provided by RFH (which
occurs due to dielectric heating mechanisms rather than the
thermal conductivity of the soil), however, may result in
more uniform decontamination than achieved using steam
or hot air injection methods and may make this method
more applicable to heterogeneous soils. Heating will
reduce the viscosity and possibly the interfacial surface
tension of residual or trapped DNAPL in the  unsaturated
zone, which may result in  uncontrolled migration
Several bench- and pilot-scale
tests and limited field-scale
testing have been conducted
using RFH to remove 70% to
99% of various  solvents, jet fuel,
and PCBs from  shallow soils.
Although RI'H continues to be in
the pilot- and held  scale
demonstration stage, at least one
company has announced the
availability of RI'H on a
c(jmmcrcial basis
Dev(1986), Dev et al  (1988),
Dev and Downey (1989),
Houthoofd el al (1991), Sims
(1990), SrestyetaJ.( 1986),
USHPA (1990)
Bioreme-
 diation
Bioremediation
involves
enhancement of
natural
processes to
degrade
hazardous
chemicals in the
subsurface.
Naturally occurring microbes can be used to degrade and/
or detoxify hazardous chemicals in the subsurface.
Bioremediation approaches include: (1) stimulation of
biochemical mechanisms for degrading chemicals; (2)
enhancement by delivery of exogeneous acclimated or
specialized microorganisms; (3) delivery of cell-free
enzymes; and (4) vegetative uptake. Typically, oxygen and
nutrients are delivered to the contamination zone via wells
and/or drains to increase the rate of aerobic biodegradation.
Adequate characterization of the hydraulic conductivity
distribution is necessary to achieve efficient delivery of
oxygen, nutrients, and/or microbes within the contaminated
zone. Many NAPLs are toxic to microbes and/or resistant
to biodegradation. As a result, degradation may be limited
to the periphery of NAPL contamination zones or to the
aqueous phase.  Degradation may  generate other undesir-
able chemicals.  Bench and pilot studies are recommended
Bioremediation  is typically used as a "polishing" step with
limited mass removal following the application of other
chemical recovery and treatment processes
F.fforts to stimulate biodegrada-
tion have been employed at full
field-scale at numerous
contamination sites with varying
degrees of success and d
-------
                                                        Table 1.   Summary of Remedial Operations Potentially Applicable at DNAPL
                                                                    Contamination Sites (concluded).
             APPLICATION
                                                                                              EFFECTIVENESS / ADVANTAGES / LIMITATIONS
                                                                                                                                                          EXPERIENCE
                                                                                                                                                                                            REFERENCES
  Contain-
  ment b>
 Solidifica-
   uofi,
 Stabiliza-
tion, and/or
  In-Situ
Vitrification
Solidification,
stabilization,
and in-situ
vitrification are
used to
immobilize
subsurface
contaminants.
Waste solidification involves mixing cementing agents with
soil to mechanically bind subsurface contaminants and
thereby reduce their rate of release.  Cementing agents (such
as pozzolan-portland cement, kme-flyash pozzolan, and
asphalt systems) can be combined with contaminated soils by
injection, in-situ mechanical mixing, or above ground
mechanical mixing.  During waste stabilization, reagents are
used to convert contaminants to their least toxic, soluble, or
mobile form

In-situ vitrification utilizes an electrical network (125 or
138kV) to melt contaminated soils and sludges at tempera-
tures of 1600 to 2000° C.  Electricity is transmitted from a
power source into contaminated soil via large electrodes.
Organic contaminants are pyrohzed and inorganic contami-
nants are incorporated within the vitnfied mass.  Vapors can
be captured at the surface for treatment.
Obtaining complete and uniform mixing of the solidifying
and/or stabilizing agents with the contaminated soil is a
critical factor determining the success of solidification/
stabilization systems.
Successful application of these methods becomes more
difficult with increasing depth and with increasing water
content. Costs are high, but may be competitive with other
remedies.  Bench studies and pilot field tests are generally
necessary. Mixed, complex wastes present special
challenges. Volatilization, mobilization, and migration of
contaminants may be caused by these processes. The long-
term stability and leaching characteristics of contaminated
materials that have been solidified, stabilized or vitnfied is
unknown.
Several different solidification-
stabilization processes have
undergone pilot tests and full-
scale field demonstrations. Six
full-scale demonstrations of in-
situ vitrification have been
conducted at the DOE Hanford
site, and more than 90 in-situ
vitrification tests of various
scales have been conducted on
PCB wastes, and other solid
combustibles and Liquid
chemicals.
Cuffinane et al. (1986),
Fitzpatrick et al. (1986), Sims
(1990), USEPA(1990a and
1990b)
Excavation
Excavation is
used to remove
contaminated
materials from
the subsurface.
Conventional excavating methods are used to remove
contaminated materials from the subsurface for subsequent
incineration, treatment and/or disposal.
                                                                                       Excavation can be a very effective site remedy where
                                                                                       contaminant penetration is limited to shallow soils and
                                                                                       where shallow contamination hot spots are identified. The
                                                                                       cost and difficulty of excavation increases with the depth of
                                                                                       contaminant migration and generally becomes prohibitive in
                                                                                       bedrock. Additional concerns include potential fugitive
                                                                                       dust, liquid, and gas emissions caused by excavating
                                                                                       contaminated materials, and the possibility that DNAPL
                                                                                       may have migrated beneath the excavation limit, thereby
                                                                                       reducing the effectiveness of excavation as a remedy.
                                                                                       Saturated zone difficulties include dewatering.
                                                          Excavation of contaminated
                                                          materials is a widely-used remedy
                                                          of proven value in appropriate

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Appendix A

Dense Nonaqueous Phase Liquids,
a USEPA Ground-Water Issue Paper
                         23

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                          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 (FTS743-2420), 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  referred to as residual
      Environmental Engineer," Research Hydrologist, U.S.
      Environmental Protection Agency, Robert S. Kerr Environmental
      Research Laboratory, Ada, Oklahoma.
                        Superfund Technology Support Center for
                        Ground Water

                        Robert S. Kerr Environmental
                        Research Laboratory
                        Ada, Oklahoma
                          Office of ScW Waste and Emergency
                          Response, US EPA, Washington, D.C,

                          Waiter W, Kqvalk*, Jr., Pn.D,
                          Dir&itar                     :
                                                   25

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Table 1.  Most prevalent chemical compounds at U.S. Superfund Sites (65) with a specific gravity
         greater than one.

Compound
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
Density
[1]

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
Dynamic[2]
Viscosity

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
Kinematic
Viscosity[3]

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
Water[4]
Solub.

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 .32 E+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
Henry's Law
Constant[5]

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
1.1 7 E-03
8.92 E-03
3.75 E-03
2. 0 E-02
5. 0 E-04
2.27 E-02
3.1 8 E-04
Vapor[6]
Pressure

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] g/cc
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(?>
1.05





21.0












18.98<7>
1 .08

[2] centipoise (cp), water has a dynamic viscosity of
[3] centistokes (cs)
[4] mg/l






20
3.87














1 cp at 20°C.

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



[5] atm-m3/mol
[6] mm Hg
[7] 45° F (70)
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






[8] 15 5°C, varies with creo
                                              26

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                             Residual Saturation of
                            DNAPL in Soil From Spill
             Plume of Dissolved
               • Contaminants
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.


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
              DNAPL Gaseous
                 Vapors
                      \
                               Residual
                             Saturation of
                              DNAPL in
                             Vadose Zone
                                                                        Vadose
                                                                         Zone
                                      Infiltration, Leaching and
                                       Mobile DNAPL Vapors
                                                                        Plume From DNAPL
                                                                           Soil Vapor
                                             Groundwater
                              Plume From DNAPL    Flow
                              Residual Saturation
                                                                                        After, Waterloo Centre for Groundwater Research. 1969
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 ONAPL 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.
                                                          27

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                 Four Phase System
                             Partition Coefficients
                             K  = Soil-water partition coefficient
                             KH « Henry's Constant
                             K'  = DNAPL-water partition coefficient
                             K"  = DNAPL-air partition coefficient
 Water
                                           alter DiGiuto, 1990(9)
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 filied 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 for a 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 Distributbn - 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 system
is less complex than the four-phase system. Again, this is
highly dependent on the characteristics of both the aquifer
                                                         28

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                                                                           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.


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
                                         Residual
                                       Saturation of
                                      DNAPL in Soil
                                         From Spill
            Plume of Dissolved
            •   Contaminants •
         ^    Groundwater
                 Flow
        Residual
Saturation in Saturated Zone
                          After, Waterloo Centre for Groundwater Research, 1989
Figure 7. The volume of DNAPL is sufficient to overcome the
        residual saturation in the vadose zone and
        consequently penetrates 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.


                             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 DNAPL has been released and multiple
                             discontinuous impermeable layers exist, the DNAPL may be
                             present  in several perched reservoirs as well as a deep
                                              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.
                                                         29

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Figure 10. Migration of ONAPL through the vadose zone to an
         impermeable boundary.
Figure 12. Perched and deep DNAPL reservoirs.
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 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.
                            Low Permeable
                           Stratigraphic Unit
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 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
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.
                                                         30

-------
    Where
    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.
 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 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

 Density

 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 [1j. 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.
                          After, Waterloo Centre (or Ground Water Research. 1989
Figure 14.  DNAPL transport in fracture and porous media
          stratigraphic units.
   K= kpg   where,  K  = hydraulic conductivity     [1]
         u,            k  = intrinsic permeability
                      p  = fluid mass density
                      g  = gravity
                      u,  = 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 Tetrachbride, 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
                                                          31

-------
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
of forces (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 water:DNAPL 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
                                                         32

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(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).

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.
             4     e>
         Wettng Fluid DNAPL
  Water
            <90°
                                      Wetting Ruid Water
                              Water
  Fluid Relationships:
    System           Wetting Fluid

    airwater          water
    air: DNAPL        DNAPL
    waterDNAPL      water
    air:DNAPL:water   water>organioair(
    (1) Wetting fluid order
 Non-Wetting Fluid

 air
 air
 DNAPL

 Alter, Waterloo Centre tor~\
Groundwater Research, 1989 J
Figure 15. Wetting angle and typical wetting fluid relationships.
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 unsaturaled 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 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,
                                                          33

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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 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 (DNAPL:air versus DNAPL:water 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 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).
                                                         34

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 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 H 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
                   : relative permeability
          o to
          008
          006

          004


          002


          001
100

0.80

060

040


020   i_
      0>
      «
0 10  ^
008  ^
006

004


002


001
                      • Increasing DNAPL Saturation
             Increasing Water Saturation
                                     After Wllarm tnd Wider, (971
Figure 16. Relative permeability graph.
NAPLflow 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 this region and there is
little or no flow of DNAPL.

Both fluids flow through only a part of the pore space  and thus
only a part of the cross section under consideration is available
for flow of each fluid. Therefore, the discharge of each fluid
must be lower corresponding to its proportion  of the cross
sectional area (46).

Figure 17 is another relative permeability graph which
demonstrates several points. Small increases in DNAPL
saturation results in a significant reduction in the relative
permeability of water. However, a small increase in water
saturation does not result in a significant reduction in DNAPL
relative permeability. This figure identifies two points, SO1 and
SO2, where the saturation of the DNAPL and the water are
greater than 0 before there is a relative permeability for this
fluid. The two fluids hinder the movement of the other to
different degrees and both must reach a minimum  saturation
before they achieve any mobility at all (47).  These minimum
saturations, for the water and DNAPL, are identified as
irreducible and residual saturation, respectively.
                      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 water contamination
from DNAPL; determine the direction of plume migration; and
                                                          35

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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
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 filler 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).

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
                                                         36

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 of DNAPL in the subsurface. Monitoring DNAPL elevation over
 time 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 investigation is to provide
 information on the thickness of the DNAPL in the aquifer.
 Therefore, construction of the well screen should intercept the
 ground water:DNAPL interface and the lower end of the screen
 should be placed as close as possible to the impermeable
 stratigraphic unit.
   Measured > Actual
                                            DNAPL Pool
                                    Impermeable Boundary'
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
                                               Jf
                                            DNAPL Pool
 Figure 19.   A well screened into an impermeable boundary
            may result in an over-estimation of the DNAPL
            thickness.
 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
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.
                                                         37

-------
Miscellaneous
Pumping Systems
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 or trench-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 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 DNAPL:water 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:wate;
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 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
         High Level


Storage
Treatment
1
*-^:

^ w
^

UUa
Controller

OK


                                       High Level
    Static DNAPL Level
    Hydraultcally Induced
       DNAPL Level
                                            Sand
                                      After J F Vittaume, el at, 1963
 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.
                                                         38

-------
 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 DNAPL:water 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 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
                                                         39

-------
      Ground Walet Surface
     DNAPL Surface
               ,Waler Dranine
     Oil Distribution


• DNAPL denser than ground water,
 has accumulated at Ihe base of the
 alluvium
                      Qiound Surface.
     Ground Waler Surface
                                       DNAPL Mounding
                                   • Drawdown of Ihe overlying water
                                    table by pumping Ihe water dranlne
                                    results n moundng of Ihe DNAPL
                                       DNAPL Recovery


                                    Pumping Ircm bolh Ihe watar and
                                    DNAPL drainlne induces increasing
                                    DNAPL low lo the DNAPL dramkne


                                    Separate production of DNAPL and
                                    ground water reduces above ground
                                    separation requirements


                                    A How path of maxmum formation
                                    permaabkty to DNAPL is established
                                    at lie base of the eluvium


                                              Atler. Sate « al. 19M
Figure 21.  Trench recovery system of DNAPL utilizing the dual
           drainline concept.
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 oiliwater 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, rt 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, rt was 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
                                                          40

-------
  Figure 22.  Vacuum extraction of DNAPL volatile components
            in the unsaturated zone.  As shown here, vapors are
            treated by thermal combustion or carbon adsorp-
            tion and the air is discharged to the atmosphere.
 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 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
                                                         41

-------
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 (bentonhe)
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 salt water
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.
References
 1.   Baehr, A.L, Selective Transport of Hydrocarbons in the
     Unsaturated Zone Due to Aqueous and Vapor Phase
     Partitioning, Water Resources Research. Vol. 23, No. 10,
     pp. 1926-1938, 1987.

2.    Bear, J., 1972, Dynamics of Fluids in Porous Media.
     American Elsevier Publishing Co., New York, 763 p.

3.    Bouchard, D., Contaminant Transport in the Subsurface:
     Sorption Equilibrium and the Role of Nonaqueous Phase
     Liquids, in, Intermedia Pollutant Transport: and Field
     Measurement. (David T. Allen, Yoram Cohen and Isaac
     R. Kaplan, Eds.), New York, Plenum Pub. Corp., pp. 189-
     211.

4.    Bower, H., Groundwater Hydrology. McGraw-Hill Book
     Co., 1978, 480pp.

5.    Canter, L. W. and R. C. Knox, Ground Water Pollution
     Control. Lewis Publishers Inc., Chelsea, Mich., 1986,526
     PP-

6.    Chatzis, I., M.S. Kuntamukkula, and N.R. Morrow, Blob-
     size Distribution as a Function of Capillary Number in
     Sandstones, Paper 13213, Presented at SPE Annual
     Tech. Conference and Exhibition. Houston, TX, 1984.
                                                         42

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7.    Cherry, J.A., S. Feenstra, B.H. Kueper and D.W.
     McWhorter, "Status of In Situ Technologies for Cleanup
     of Aquifers Contaminated by DNAPL's Below the Water
     Table," in, International Specialty Conference on How
     Clean is Clean? Cleanup Criteria for Contaminated Soil
     and Groundwater. Air and Waste Management
     Association, pp. 1-18, November 6-9, 1990.

8.    Connor, J.A., C.J. Newell, O.K. Wilson, Assessment,
     Field Testing, Conceptual Design for Managing Dense
     Nonaqueous Phase Liquids (DNAPL) at a Superfund
     Site, in,  Proceedings of Petroleum Hydrocarbons and
     Organic Chemicals in Ground Water:  Prevention.
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9.    DiGiulio, D.C. and J.S. Cho, Conducting Field Tests for
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10.  Feenstra, S., Evaluation of Multi-Component DNAPL
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25.   Kemblowski, M.W. and C.Y. Chiang, Hydrocarbon
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26.   Kerfoot, H.B., Is Soil-Gas Analysis an Effective Means of
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27.   Kueper, B.H. and E.O. Frind, An Overview of Immiscible
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28.   Kueper, B.H., W. Abbott, and G. Farquhar, Experimental
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     pp. 83-95, 1989.
                                                        43

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29.  Lee, M.D., J.M. Thomas, R.C. Borden, P.B. Bedient, J.T.
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30.  Lindeburg, M.R., 1986, Civil Engineering Reference
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31.  Lyman, W.J.. W.F. Reehl, and D.H. Rosenblatt,
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32.  Mackay, D.M. and J.A. Cherry, Ground-Water
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35.  Marrin, D.L, Soil-Gas Sampling  and Misinterpretation,
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38.   Morrow, N.R., Interplay of Capillary, Viscous and
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41.  Sale, T., CH2M Hill, and Kuhn, B., Recovery of Wood-
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42.  Sale, T., K. Piontek, and M. Pitts, Chemically Enhanced
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43.  Schmidtke, K., E. McBean, and F. Rovers, Drawdown
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44.  Schmidtke, K., E. McBean, and F. Rovers, Evaluation of
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45.  Schwille, F.,  Groundwater Pollution in Porous Media by
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49.  Silka, L, Simulation of Vapor Transport Through the
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50.  Sitar, N., J.R. Hunt, and K.S. Udell, Movement of
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52.  Thompson, G., and Marrin,  D., Soil Gas Contaminant
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53.  Treiber, L.E., D.L. Archer, and W.W.  Owens, A
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     Prevention. Detection, and Restoration. A Conference
                                                        44

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     and Exposition, The Westin Galleria, Houston, Texas,
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55.  U.S. EPA,, Treatment of Contaminated Soils with
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56.  U.S. EPA, Handbook Remedial Action at Waste Disposal
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57.  U.S. EPA, RCRA Ground-Water Monitoring Technical
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58.  U.S. EPA, Microbial Decomposition of Chlorinated
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59.  U.S. EPA, Characterization and Laboratory Soil
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60.  U.S. EPA, Ground Water Modeling: An Overview and
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61.  U.S. EPA, Contaminant Transport in Fractured Media:
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63.  U.S. EPA, Bioremediation of Contaminated Surface
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64.  U.S. EPA, Performance Evaluations Of Pump And Treat
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65.  U.S. EPA, Subsurface Contamination Reference Guide,
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66.  U.S. EPA, Laboratory Investigation of Residual Liquid
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67.  U.S. EPA, Basics of Pump and Treat Ground  Water
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68.  U.S. EPA, Site Characterizations for Subsurface
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70.  Villaume, J.F., P.C. Lowe, and D.F. Unites, Recovery of
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72.   Waterloo Centre for Ground Water Research, University
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75.   Wisniewski, G.M., G.P. Lennon, J.F. Villaume, and C.L.
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     1985.
                                                       45

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Appendix B



DNAPL Bibliography
                        47

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                                                  49

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Anderson, W.G., 1986b. Wettability literature survey-part 2: Wettability measurement, Journal of Petroleum
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Blevins, T.R., J.R. Duerksen, and J.W. Ault, 1984. Light-oil steamflooding -- An emerging technology, Journal of
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Appendix C



DNAPL Workshop Agenda and Attendee Lists
                          71

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                                     DNAPL WORKSHOP
                                        Dallas, Texas
April 16,1991


INTRODUCTION

 1:00 p.m.             Opening Remarks                                    Scalf

 1:10 p.m.             OERR Perspective                                   Ankrum

 1:30 p.m.             DNAPLs: Statement of the Problem with Emphasis         Cherry
                              on Low Viscosity DNAPLs

 2:10 p.m.             State of the Practice: High Viscosity DNAPLs             Villaume

 2:40 p.m.             Current Regional Practices for Addressing DNAPLs         Willey

 3:10 p.m.             A Perspective for DNAPL Assessment and                M. Mercer
                     Remediation

 3:40 p.m.             BREAK

SITE CHARACTERIZATION: CURRENT STATE OF PRACTICE AND RESEARCH

 4:00 p.m.             How to Determine Presence or Absence of DNAPLs         Feenstra
                            in Groundwater

 4:40 p.m.             Monitoring and Modeling DNAPLs                      J. Mercer

 5:10 p.m.             Geophysical Methods for DNAPL Detection               Olhoeft

 5:40 p.m.             Delineation and Recovery of Mixed DNAPLs              Newell
                                          73

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April 17,1991

REMEDIATION

 8:00 a.m.

 8:30 a.m.

 9:00 a.m.

 9:30 a.m.

10:00 a.m.

10:30 a.m.

11:00 a.m.

11:30 a.m.

 1:00 p.m.

 3:00 p.m.

 3:30 p.m.


April 18,1991

 8:30 a.m.


12:00 Noon
Current Options: An Overview

Pumping, Drains, Sumps

Enhanced Recovery (High Viscosity)

Enhanced Recovery (Low Viscosity)

BREAK

Vacuum Extraction for DNAPL Remediation

Heat/Steam (Above and Below Water Table)

In Situ Bioremediation

Group Discussions-Site Characterization Opdons

BREAK

Group Discussions-Remediation Options
Continue Discussions
Writing and Review Assignments

ADJOURN
Cherry

McWhorter

Sale

Fountain



Johnson

L. Johnson

Sims

Enfield



Cherry
                                              74

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              DNAPL WORKSHOP ATTENDEE LIST

                         DALLAS, TEXAS
                         APRIL 16-18, 1991
Mr. Ted Ankrum
Dr. John A. Cherry
Ms. Kathy Davies
Ms. Lynn Deering
Dr. Ronald Drake
Mr. Neal Durant
Dr. Carl Enfield
Mr. Stan Feenstra
Dr. John C. Fountain
Dr. Michael Henson
Mr. Scott Ruling
Mr. Lyle Johnson
Dr. Rick Johnson
Dr. Dave McWhorter
Dr. James W. Mercer
Mr. Mark Mercer
Mr. Paul Nadeau
Dr. Charles Newell
Ms. Carolyn Offutt
Dr. Gary Olhoeft
Dr. Malcolm Pitts
Dr. Paul Roberts
Mr. Randall Ross
Mr. Tom Sale
Mr. Dick Scalf
Dr. Steve Schmelling
Dr. Ronald Sims
Dr. Nicholas Sitar
Ms. Jennifer Sutler
Dr. James Villaume
Dr. James Weaver
Mr. Dick Willey
Dr. John L. Wilson
USEPA OSWER/OERR
University of Waterloo
USEPA, Region III
USEPA, OSWER/OERR
Dynamac Corporation
USEPA, OSW/OSWER
USEPA, RSKERL
Applied GW Research, Ltd.
University of Buffalo
RMT, Inc.
USEPA, RSKERL
Western Research Institute
Oregon Graduate Institute
Colorado State University
GeoTrans, Inc.
USEPA, Hazardous Site Control Div.
USEPA, OSWER/OERR
Groundwater Services, Inc.
USEPA, OSWER/OERR
USGS, Denver
Surtek
Stanford University
USEPA, RSKERL
CHjM Hill, Inc.
USEPA, RSKERL
USEPA, RSKERL
Utah State University
University of California, Berkeley
USEPA, OSWER/OERR
Pennsylvania Power & Light
USEPA, RSKERL
USEPA, Region I
New Mexico Inst. of Mining & Tech.
                                75

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                    Attendee List
                 DNAPL Workshop
                     Dallas, TX
                September 5 & 6,1991
University

Dr. John A. Cherry
Dr. Dave McWhorter
Dr. John L. Wilson
Dr. Rick Johnson
Ms. Christina Hubbard
Ms. Sheryl Wilhelm

Consulting/Industry

Mr. Lyle Johnson
Dr. James Mercer
Mr. Tom Sale
Dr. Charles Newell
Dr. Malcolm Pitts
Dr. Michael Henson
Dr. James Villaume

U.S. EPA and USGS

Mr. Dick Willey
Ms. Kathy Davies
Ms. Lynn Deering
Mr. Mark Mercer
Ms. Jennifer Sutler
Mr. Peter Feldman
Ms. Penny Hansen
Mr. Randall Breeden
Ms. Carolyn Offutt
Mr. John Smith
Mr. Dick Scalf
Mr. Randall Ross
Dr. Carl Enfield
Dr. Steve Schmelling
Mr. Steve Acree
Dr. Dave Burden
Dr. Ellen Graber
Dr. Gary Olhoeft

Dynamac/RSKERL

Dr. Ronald Drake
University of Waterloo
Colorado State University
New Mexico Tech.
Oregon Graduate Institute
University of Waterloo
University of Waterloo
Western Research Inst.
Pres., GeoTrans, Inc.
CR,M-Hill, Inc.
Groundwater Services, Inc.
Surtek
RMT, Inc.
Pennsylvania Power & Light
Region I
Region III
OSWER/OERR
OSWER/OERR
OSWER/OERR
OSW/OSWER
OSWER/OERR
OSWER/OERR
OSWER/OERR
OSWER/OERR
RSKERL
RSKERL
RSKERL
RSKERL
RSKERL
RSKERL
Region VI
USGS
Dynamac
                       76

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Appendix D



Glossary of Terms
                           77

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The following terminology is intended to provide a consistent set of terms for use in discussions of investigations
and remediation at DNAPL sites. The use of this terminology may avoid some of the confusion that commonly
occurs when people from different backgrounds (hydrogeology, chemistry, reservoir engineering, regulatory, legal)
attempt to discuss DNAPL issues or problems. This set of terms will probably warrant some revision after it is
subjected to use in a multidisciplinary context.
Terminology for  DNAPL Sites
CUTOFF WALL - A low-permeability vertical barrier placed around a zone of contamination.  There are various
        types of walls, such as bentonite-soil slurry walls, concrete panel walls, plastic membrane walls, and
        scalable-joint steel sheet piling walls. The wall may be keyed into an aquitard (keyed wall) or not keyed
        (hanging wall).

DNAPL - An acronym for a dense nonaqueous phase liquid. It is synonymous with dense immiscible-phase liquid.
        The term has sometimes been used to refer to dissolved- or aqueous-phase contaminants, but to avoid
        confusion, the term DNAPL should be reserved exclusively for the nonaqueous phase liquid.

DNAPL ENTRY LOCATIONS - Locations where DNAPL has entered the subsurface by way of leaks, spills,
        discharges or any other escape.

DNAPL REMOVAL (or IMMISCIBLE-PHASE MASS REMOVAL) - Refers to removal of immiscible-phase
        residual, lens, or pools by technologies such as by free-phase pumping, water flooding, enhanced solubili-
        zation, vapor extraction or air sparging.

DNAPL SITE - A site where an immiscible liquid with a density greater than water has entered the subsurface and
        exists below the water table as a separate residual or immiscible phase.

DNAPL ZONE CONTAINMENT or IMMISCIBLE-PHASE ZONE CONTAINMENT - Refers to measures
        taken to control or prevent migration of dissolved-phase contaminants and immiscible-phase liquid
        (DNAPL).  Plume control and DNAPL zone containment may involve some of the same remedial actions
        such as pumping well networks or drainage trenches, but the are generally applied in different areas.

FREE-PHASE LIQUID - Immiscible liquid existing in the subsurface under positive pressure.  Free-phase liquid
        can flow into a well under the influence of gravity and can be mobilized by hydraulic forces.

GROUND-WATER REMEDIATION - A very general term referring to any or all activities taken to improve
        ground-water quality or control the spread of ground-water contamination, such as plume capture, or
        control, DNAPL removal or ground-water restoration.

GROUND-WATER RESTORATION - Refers to the removal of subsurface contamination to the degree necessary
        to achieve appropriate cleanup levels which protect public health and the environment.

GROUND-WATER ZONE - The zone below the water table or free water surface.

GROUND-WATER ZONE IMMISCIBLE PHASE - Immiscible phase in either the residual or free-phase state,
       present below the water table. Ground-water zone immiscible phase is particularly important at DNAPL
       sites because it is generally the primary cause of plumes of dissolved-phase chemicals. Permanent restora-
       tion of ground-water quality at DNAPL sites generally requires removal of essentially all the mass of
       ground-water zone immiscible phase.
                                              79

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IMMISCIBLE-PHASE ZONE or DNAPL ZONE - That portion of a site where immiscible-phase liquid exists as
        residual or free phase in the subsurface.  This zone will also include chemical mass in the dissolved and
        sorbed phases. The immiscible-phase zone usually has immiscible-phase liquid both above and below the
        water table.

IN SITU IMMISCIBLE-PHASE DESTRUCTION - Refers to the destruction of DNAPL residuals, lens, or pools
        in situ by means of chemical or microbiological processes.

IN SITU IMMISCIBLE-PHASE SOLIDIFICATION - Refers to the immobilization of DNAPL residual, lens, or
        pools by physicochemical processes that solidify the DNAPL within the geologic medium, thereby prevent-
        ing spread and/or dissolution of DNAPL in ground water.

LENS - A zone of free-phase liquid that rests on lower permeability strata of limited areal extent.  A lens is a small
        perched pool.  A "pool" and a "lens" are different only in scale.

PERMEABLE REACTION WALL - Is similar to a cutoff wall, but the wall is relatively permeable to allow
        unimpeded ground-water flow. The wall is partially constructed from reactive materials to destroy and/or
        sorb contaminants. A permeable reaction wall may be a passive alternative to pump-and-treat for plume
        control.

PLUME - The zone of ground-water contamination that exhibits dissolved-phase contaminants at concentrations
        above some specified concentration level, such as a drinking water limit, detection limit, or background.
        The volume of the subsurface that the plume encompasses includes the subsurface contaminant source
        (residual, lens, and pools of DNAPL). To  avoid confusion, the term "plume" should not be used to refer to
        a pool of free-phase immiscible liquid. Plume removal refers to the removal of the dissolved-phase mass.
        DNAPL removal refers to removal of the immiscible-phase liquid.

PLUME CONTROL (or PLUME MIGRATION CONTROL) - Refers to measures taken to cutoff or control
        dissolved-phase contamination so that the advance of the plume is restricted, thereby reducing or eliminat-
        ing the risk to receptors that would otherwise be impacted by dissolved-phase contaminants transported by
        ground water.

PLUME REMOVAL - Removal, generally, by pump and treat, of all or nearly all of a plume (except for dissolved-
        phase contamination in the immediate vicinity of DNAPL residuals, lens, or pools). Typically, at DNAPL
        sites, if the plume is removed and the  pump-and-treat system is shut down, the plume will reestablish itself
        by dissolution of DNAPL located below the water table. Therefore, plume removal at DNAPL sites
        without containment (or removal) of the DNAPL zone will not achieve "ground-water restoration."

POLISHING - Polishing is a cleanup phase applied to remove or destroy dissolved, sorbed, or residual contami-
        nants rather than immiscible-phase liquids in lens or pools. To achieve "ground-water restoration", an
        aquifer would generally need to be polished after other remedial measures have been taken to remove
        nearly all the mass of immiscible-phase (DNAPL) liquid.

POOL - A zone of free-phase immiscible liquid that resides at the bottom of an aquifer. A pool rests on top of an
        aquitard. A pool may be in an immobile state or it may be in a state of motion, depending on whether or
        not free-phase liquid is being added to the pool.

POOL or LENS REMOVAL -The removal of the free phase or otherwise movable immiscible liquid from a pool
        or lens by direct DNAPL pumping or enhanced mobilization.  Such removal leaves a residual mass of
        immiscible-phase liquid under capillary tension.
                                               80

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PUMP-AND-TREAT - This term is reserved for the removal of water containing dissolved-phase contaminants by
        means of wells or trenches.  The recovered water is treated at the surface.

RESIDUAL - Immiscible-phase liquid held in the pore spaces or fractures by capillary tension (negative immiscible-
        phase pressure). The immiscible-phase liquid in residual form cannot be mobilized by reasonable hydraulic
        forces.

SOURCE - This is a term meaning different things to different people. To some lawyers and administrators, source
        refers to surface or very near surface  causes or potential causes of ground-water contamination, such as
        drums,  sludge or contaminated surface soil. To scientists and engineers operating in the context of concep-
        tual models of DNAPL in the subsurface, a source of most significance is generally a mass of immiscible-
        phase liquid (residual, lens, or pool) located below ground surface.  To avoid confusion, the term "source"
        should be used with specific modifiers, such as below water table DNAPL source, surface source, etc.

VADOSE ZONE - The zone above the water table which may contain saturated or partially saturated sediments.

VADOSE ZONE IMMISCIBLE PHASE - Immiscible liquid present above the water table, generally as residual
        but occasionally as lenses of free-phase liquid perched on lower permeability strata.

VAPOR EXTRACTION - Refers to removal of DNAPL residuals, lenses, or pools by passage of air through vadose
        zone resulting in volatilization and gas transport.

VAPOR PLUME - Zone of vapor in the vadose zone.
                                               81                  *U.S. Government Printing Office: 1992—648-003/40719

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