United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/625/R-95/005
July 1996
ERA Pump-and-Treat
Ground-Water Remediation
A Guide for Decision Makers
and Practitioners
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EPA/625/R-95/005
July, 1996
Pump-and-Treat Ground-Water Remediation
A Guide for Decision Makers and Practitioners
Prepared by
Eastern Research Group, Inc.
110 Hartwell Avenue
Lexington, MA
U.S. Environmental Protection Agency
Office of Research and Development
National Risk Management Research Laboratory
Center for Environmental Research Information
Cincinnati, Ohio
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Notice
The information in this document has been
funded wholly, or in part, by the U.S. Environmental
Protection Agency (EPA). This document has been
subjected to EPA's peer and administrative review
and has been approved for publication as an EPA
document. Mention of trade names or commercial
products does not constitute endorsement or recom-
mendation for use.
In September, 1995 the Office of Research and Development completed a reorganization of its Laboratories and
Centers. The former Risk Reduction Engineering Laboratory located in Cincinnati, Ohio, the Robert S. Kerr
Research Laboratory located in Ada, Oklahoma, the Air and Energy Research Laboratory, located in Research
Triangle Park, North Carolina, and the Center for Environmental Research Information located in Cincinnati, Ohio,
were merged into the National Risk Management Research Laboratory. No physical relocations were involved. The
documents referenced in this guide were published prior to the reorganization: therefore former laboratory/center
names are shown as they were at the time of publication.
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Foreword
The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation's land,
air, and water resources. Under a mandate of national environmental laws, the Agency strives to formulate
and implement actions leading to a compatible balance between human activities and the ability of natural
systems to support and nurture life. To meet this mandate, EPA's research program is providing data and
technical support for solving environmental problems today and building a science knowledge base neces-
sary to manage our ecological resources wisely, understand how pollutants affect our health, and prevent or
reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency's center for investigation of
technological and management approaches for reducing risks from threats to human health and the environ-
ment. The focus of the Laboratory's research program is on methods for the prevention and control of
pollution to air, land, water and subsurface resources; protection of water quality in public water systems;
remediation of contaminated sites and ground water; and prevention and control of indoor air pollution. The
goal of this research effort is to catalyze development and implementation of innovative, cost-effective
environmental technologies; develop scientific and engineering information needed by EPA to support
regulatory and policy decisions; and provide technical support and information transfer to ensure effective
implementation of environmental regulations and strategies.
This publication has been produced as part of the Laboratory's strategic long-term research plan. It is
published and made available by EPA's Office of Research and Development to assist the user community
and to link researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
iii
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IV
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Contents
List of Figures v
List of Tables xi
Acronyms and Abbreviations xii
Acknowledgments xiii
1. Introduction to Pump-and-Treat Remediation 1
2. Appropriate Use of Pump-and-Treat Technology 3
3. Smart Pump-and-Treat Techniques 5
3.1. Contaminant Removal/Control 5
3.2. Thorough Site Characterization 5
3.3. Dynamic Management of the Well Extraction Field 7
3.4. Realistic Cleanup Goals 10
4. Anticipating Tailing and Rebound Problems 19
4.1. Effects of Tailing and Rebound on Remediation Efforts 19
4.2. Contributing Factors 19
4.2.1. Non-Aqueous Phase Liquids (NAPLs) 19
4.2.2. Contaminant Desorption 20
4.2.3. Precipitate Dissolution 23
4.2.4. Matrix Diffusion 24
4.2.5. Ground-Water Velocity Variation 24
4.3. Assessing the Significance of Tailing and Rebound at a Site 26
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Contents (continued)
5. Effective Hydraulic Containment 28
5.1. Ground-Water Barriers and Flow Control 28
5.1.1. Horizontal and Vertical Capture Zones 28
5.1.2. Pressure Ridge Systems 31
5.1.3. Physical Barriers 31
5.2. Hydraulic Containment: Other Special Considerations 32
5.2.1. Effects of Anisotropy 32
5.2.2. Drawdown Limitations 32
5.2.3. Stagnation Zones 33
6. Pump-and-Treat System Design and Operation 36
6.1. Capture Zone Analysis and Optimization Modeling 36
6.2. Efficient Pumping Operations 40
6.3. Treating Contaminated Ground Water 42
6.4. Monitoring Performance 43
6.4.1. Hydraulic Head Monitoring for Containment 43
6.4.2. Ground-Water Quality Monitoring for Containment 44
6.4.3. Aquifer Restoration Monitoring 48
6.5. Evaluating Restoration Success and Closure 50
7. Variations and Alternatives to Conventional Pump-and-Treat Methods 53
7.1. Alternative Methods for Fluid Delivery and Recovery 53
7.2. Vadose Zone Source Control 54
7.3. Physical and Chemical Enhancements 54
7.3.1. Physical Enhancements 59
7.3.2. Chemical Enhancements 59
7.4. Biological Enhancements 59
7.5. Alternatives to the Pump-and-Treat Approach 61
7.5.1. Intrinsic Bioremediation 63
7.5.2. In Situ Reactive Barriers 63
VI
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Contents (continued)
8. References 66
9. EPA Publications Providing Further Information 70
List of Sidebars
1 Changing Expectations for the Pump-and-Treat Approach 2
2 MajorTypes of Hydrogeologic Settings 11
3 Computer Graphics as a Site Characterization Tool 13
4 The Effect of NAPL Phases on Ground-Water Contamination 16
5 Computer Modeling of Well Patterns Versus Hydrogeologic Conditions 38
vn
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Figures
1. Examples of hydraulic containment in a plan view and cross section using
pump-and-treat technology: (a) pump well, (b) drain, and (c) well within a
barrier wall system (after Cohen etal, 1994)
2. Contaminant plumes as a function of density and miscibility with ground water:
(a) light liquids (gasoline and methanol) create contaminant plumes that tend to
flow in the upper portions of an aquifer; (b) dense liquids (perchloroethylene
[PCE] and ethylene glycol) create a plume that contaminates the full thickness
of an aquifer (adapted from Gorelick et al., 1993) 6
3. This hollow-stem auger is fitted with a 5-foot sampling tube that collects a
continuous core as the auger advances, allowing detailed and accurate
observation of subsurface lithology. When drilling is completed, a monitoring
well also can be installed 8
4. Hydraulic or vibratory direct-push rigs can be installed on vans, small trucks,
all-terrain vehicles, or trailers and allow collection of continuous soil cores and
depth-specific ground-water samples for detailed subsurface mapping if
contaminants are generally confined to depths of less than 15 meters.
(Photo courtesy of Geoprobe Systems.) 9
5. Conceptual diagram of Dense Non-Aqueous Phase Liquid (DNAPL) Trichloro-
ethylene (TCE) based on soil and ground-water sampling in a heterogeneous sand
and gravel aquifer. The extreme difficulty in cleaning up this site, which includes
five distinct forms of TCE (vapors and residual product in the vadose zone; pooled,
residual, and dissolved product in the ground water) led to modification of the
pump-and-treat system for hydraulic containment rather than restoration
(adapted from Clausen and Solomon, 1994) 10
viii
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Figures (continued)
6. GEOS computer screen showing organic contaminant plume in relation
to subsurface stratigraphy 12
7. EPA's SITE3D software, under development at the Ada, Oklahoma,
laboratory, helps visualize in three dimensions a TCE contaminant plume at a
Superfund Site. Yellows and reds indicate zone with highest concentrations of
TCE in ground water. 14
8. Dark NAPL (Soltrol) and water in a homogenous micromodel after (a) the
displacement of water by NAPL and then (b) the displacement of NAPL
by water, with NAPL at residual saturation (Wilson et al., 1990) 17
9. Photomicrographs of (a) a single blob occupying one pore body, and
(b) a doublet blob occupying two pore bodies and a pore throat
(Wilson et al., 1990) 18
10. Concentration versus pumping duration or volume showing
tailing and rebound effects (Cohen et al., 1994) 20
11. Contaminants are mobilized when ground water that is undersaturated
with a contaminant comes in contact with a NAPL (a) or contaminant sorbed
on an organic carbon or mineral surface (b). High ground-water velocities
and short contact times will result in low contaminant concentrations, and
low velocities and long contact times will result in high contaminant
concentrations (c) (adapted from Gorelick et al., 1993) 21
12. Laboratory model of the transport of DNAPL contaminant through
an aquifer with varying permeability; note the concentration of
downward movement in fingers and the DNAPL pools above the low-
permeability zones (the horizontal discs).
(Source: U.S. EPA National Risk Management Research Laboratory.) 22
13. Dissolved contaminant concentration in ground water pumped from a
recovery well versus time in a formation that contains a solid-phase
contaminant precipitate (Palmer and Fish, 1992) 23
ix
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Figures (continued)
14. Changes in average relative trichloroethene (TCE) concentrations
in clay lenses of varying thickness as a function of time (NRC, 1994) 24
15. Tailing resulting from ground-water velocity variations: (a) horizontal
variations in the velocity of ground water moving toward a pumping well
(Keely, 1989) lead to (b) tailing as higher concentrations of ground water
in slower pathlines mix with lower concentrations in faster pathlines (Palmer
and Fish, 1992); (c) in a stratified sand and gravel aquifer, tailing occurs
at tl when clean water from the upper gravel strata mixes with still-
contaminated ground water in the lower sand strata (Cohen etal., 1994) 25
16. Zone of residuals created in former cone of depression after cessation of
LNAPL recovery system (Gorelick et al., 1993) 27
17. Plan view of a mixed containment-restoration strategy. A pump-and-treat
system is used with barrier walls to contain the ground-water contamination
source areas (e.g., where NAPL or waste may be present) and then collect
and treat the dissolved contaminant plume (Cohen etal., 1994) 29
18. In an isotropic aquifer, ground-water flow lines (b) are perpendicular
to hydraulic head contours (a). Pumping causes drawdowns and anew
steady-state potentiometric surface within the well's zone of influence
(c). Following the modified hydraulic gradients, ground water within
the shaded capture zone flows to the pumping well (d).
(Cohen etal., 1994, adapted from Gorelick etal., 1993) 30
19. Cross section showing equipotential contours and the vertical capture zone
associated with ground-water withdrawal from a partially penetrating well
in isotropic media (Cohen et al., 1994) 31
20. Effect of fracture anisotropy on the orientation of the zone of contribution
(capture zone) to a pumping well (Bradbury etal., 1991) 33
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Figures (continued)
21. Capture zone simulation of three pumping wells for an isotropic aquifer
(a) and anisotropy ratio of 10:1 (b) using the EPA Well Head Protection Area
(WHPA)code 34
22. Examples of stagnation zones (shaded where ground-water velocity is less than
4 L/T): (a) single pumping well and (b) four extraction wells with an injection
well in the center (Cohen etal., 1994) 35
23. Major types of pumping/injection well patterns (Satkin and Bedient, 1988) 39
24. Ground-water flow line in the vicinity of conceptual pumping centers at
Lawrence Livermore National Laboratory superimposed on an
isoconcentration contour map and showing areas of potential stagnation
(Cohen etal., 1994, after Hoffman, 1993) 41
25. Effect of adaptive pumping on cleanup time at Lawrence Livermore
National Laboratory Superfund site (Cohen et al, 1994, after Hoffman, 1993) 42
26. The pulsed pumping concept (Cohen et al., 1994, after Keely, 1989) 43
27. Nested piezometer hydrograph for 1992 at the Chem-Dyne Superfund site
(Cohen etal., 1994, after Papadopulos & Associates, 1993) 46
28. Ground-water flow between and beyond the extraction wells, resulting
even though hydraulic heads throughout the mapped aquifer are higher
than the pumping level (Cohen et al., 1994) 47
29. Example display of ground-water flow directions and hydraulic
gradients determined between three observation wells (Cohen etal., 1994) 48
30. Influent and effluent VOC concentrations (mg/L) at the Chem-Dyne
treatment plant from 1987 to 1992
(Cohen etal., 1994, after Papadopulos & Associates, 1993) 49
31. Cumulative mass of VOCs removed from the aquifer at the Chem-Dyne
site from 1987 to 1992 (Cohen et al., 1994, after Papadopulos & Associates, 1993) 50
xi
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Figures (continued)
32. Determining the success and/or timeliness of closure of a pump-and-treat
system (Cohen et al, 1994) 51
33. Stages of remediation in relation to example contaminant concentrations
in a well at a pump-and-treat site (U.S. EPA, 1992) 52
34. Some applications of horizontal wells: (a) intersecting flat-lying layers,
(b) intercepting plume elongated by regional gradient, (c) intersecting
vertical fractures, and (d) access beneath structures (U.S. EPA, 1994) 55
35. Two approaches using trenches or horizontal wells to intercept
contaminant plumes (U.S. EPA, 1994) 58
36. Process diagram for air sparging with (a) vertical wells, and (b) horizontal wells
[after National Research Council (NRC), 1994] 60
37. Schematic of chemical enhancement of a pump-and-treat system. Key areas of
concern are shown in boxes. In some cases, the reactive agent will be recovered
and reused (Palmer and Fish, 1992) 61
38. Two types of aerobic in situ bioremediation systems: (a) injection well with sparger,
(b) infiltration gallery (Sims et al., 1992, after Thomas and Ward, 1989) 62
39. Alternative ground-water plume management options: (a) pump-and-treat system,
(b) intrinsic bioremediation, (c) in situ reaction curtain, (d) funnel-and-gate system
(adapted from Starr and Cherry, 1994) 64
40. Funnel-and-gate configurations (Starr and Cherry, 1994) 65
Xll
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Tables
1. Categories of Sites for Technical Infeasibility Determinations (NRC, 1994) 15
2. Data Requirements for Pump-and-Treat Systems (Adapted from U.S. EPA, 1991) 37
3. Applicability of Treatment Technologies to Contaminated
Ground Water (U.S. EPA, 1991) 45
4. Issues Affecting Application of Alternative Methods for Delivery or Recovery
(U.S. EPA, 1994)... ..56
xin
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Acronyms and Abbreviations
ACL
CZAEM
DNAPL
DOD/ETTC
EPA
GAEP
LNAPL
LLNL
MCL
MCLG
NAPL
NRC
NTIS
ORD
OSWER
PCB
PCE
RCRA
SVE
TCE
TI
VOC
WHPA
Alternate concentration limit
Capture Zone Analytic Element Model
Dense non-aqueous phase liquid
Department of Defense Environmental Technology Transfer Committee
Environmental Protection Agency
Geographic Analytic Element Preprocessor
Light non-aqueous phase liquid
Lawrence Livermore National Laboratory
Maximum contaminant level
Maximum contaminant level goal
Non-aqueous phase liquid
National Research Council
National Technical Information Service
Office of Research and Development
Office of Solid Waste and Emergency Response
Polychlorinated biphenyl
Perchloroethylene
Resource Conservation and Recovery Act
Soil vapor extraction
Trichloroethylene
Technical impracticality
Volatile organic compound
Well Head Protection Area
xiv
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Acknowledgments
This document was prepared under Contract No.
68-C3-0315, Work Assignment No. 1-33, by Eastern
Research Group, Inc. (ERG), and under the sponsor-
ship of the U.S. Environmental Protection Agency.
xv
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Pump-and-treat is one of the most widely used
ground-water remediation technologies. Conven-
tional pump-and-treat methods involve pumping
contaminated water to the surface for treatment.
This guide, however, uses the term pump and treat
in a broad sense to include any system where
withdrawal from or injection into ground water is
part of a remediation strategy. Variations and
enhancements of conventional pump and treat
include hydraulic fracturing as well as chemical
and biological enhancements. The pump-and-treat
remediation approach is used at about three-
quarters of the Superfund sites where ground
water is contaminated and at most sites where
cleanup is required by the Resource Conservation
and Recovery Act (RCRA) and state laws [Na-
tional Research Council (NRC), 1994]. Although
the effectiveness of pump-and-treat systems has
been called into question (Sidebar 1), after two
decades of use, this approach remains a necessary
component of most ground-water remediation
efforts and is appropriate for both restoration and
plume containment.
This guide provides an introduction to pump-
and-treat ground-water remediation by addressing
the following questions:
When is pump and treat an appropriate
remediation approach?
What is involved in "smart" application of
the pump-and-treat approach?
What are tailing and rebound, and how can
they be anticipated?
What are the recommended methods for
meeting the challenges of effective hydraulic
containment?
How can the design and operation of a pump-
and-treat system be optimized and its perfor-
mance measured?
When should variations and alternatives to
conventional pump-and-treat methods be
used?
By presenting the basic concepts of pump-and-
treat technology, this guide provides decision-
makers with a foundation for evaluating the
appropriateness of conventional or innovative
approaches. An in-depth understanding of
hydrogeology and ground-water engineering is
required, however, to design and operate a pump-
and-treat system for ground-water remediation.
Readers seeking more information on specific
topics covered in this booklet should refer to the
U.S. Environmental Protection Agency (EPA)
documents listed at the end of this guide (Section
9).
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Sidebar 1
Changing Expectations for the Pump-and-Treat Approach
Pump-and-treat systems for remediating ground water
ame into wide use in the early to mid-1980s. By the
early 1990s, evaluations by EPA (Keely, 1989; U.S.
EPA, 1989; Haley et al, 1991) and others (Freeze and
Cherry, 1989; Mackay and Cherry, 1989) called into the
question the performance of pump-and-treat systems.
The general "failure" of the pump-and-treat approach
was identified as its inability to achieve "restoration"
(i.e., reduction of contaminants to levels required by
health-based standards) in 5 to 10 years, as anticipated
in the design phase of projects. Although a variety of
factors contributed to this shortcoming, tailing and
rebound (Section 4) represented the major barrier to
achieving remediation goals. Pump-and-treat systems
were criticized more pointedly by Travis and Doty
(1990), who asserted as a "simple fact" that "contami-
nated aquifers cannot be restored through pumping and
treating."
Expectations for the effectiveness of pump-and-treat
technology, however, may have been too high. Ground-
water scientists and engineers generally agree that
complete aquifer restoration is an unrealistic goal for
many, if not most, contaminated sites. Nonetheless,
further experience with pump-and-treat systems
indicates that full restoration at some sites with
relatively simple characteristics is possible; moreover, at
many sites, full restoration of ground-water quality can
be achieved for part of a site (NRC, 1994). For
example, Bartow and Davenport (1995), in a review of
37 applications of pump-and-treat systems in Santa
Clara Valley, California, found that one site had
achieved maximum contaminant levels (MCLs) for all
contaminants and about one-third achieved, or were
near, MCLs for one or more parameters. Bartow and
Davenport's conclusion that pump-and-treat systems hac
significantly reduced the mass of volatile organic
contaminants (VOCs) in the region's ground water
indicates how expectations regarding the technology
have changed.
Combining the pump-and-treat approach with in situ
bioremediation (see Section 7.4) provides further
opportunities for improving the effectiveness of ground-
water cleanup. For example, Marquis (1995) suggested
that in situ bioremediation used with the pump-and-treat
approach should always be considered as an option for
remediation of sand and gravel aquifers contaminated
with biodegradable organic compounds, especially
volatile aromatic and polyaromatic hydrocarbons.
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Pump-and-treat systems are used primarily to
accomplish the following:
Hydraulic containment. To control the
movement of contaminated ground water,
preventing the continued expansion of the
contaminated zone. Figure 1 illustrates three
major configurations for accomplishing
hydraulic containment: (1) a pumping well
alone, (2) a subsurface drain combined with a
pump well, and (3) a well within a barrier
wall system.
Treatment. To reduce the dissolved contami-
nant concentrations in ground water suffi-
ciently that the aquifer complies with cleanup
standards or the treated water withdrawn from
the aquifer can be put to beneficial use.
Although hydraulic containment and cleanup
can represent separate goals, more typically,
remediation efforts are undertaken to achieve a
combination of both. For example, if restoration is
not feasible, the primary objective might be
containment. In contrast, where a contaminated
well is used for drinking water but the contami-
nant source has not been identified, treatment at
the wellhead might allow continued use of the
water even though the aquifer remains contami-
nated.
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(a)
(b)
Figure 1.
Examples of hydraulic
containment in a plan
view and cross section
using pump-and-treat
technology: (a) pump well,
(b) drain, and (c) well
within a barrier wall
system (after Cohen et
al., 1994)
(c)
Upgradient
Barrier Wall
Plume
JDowngradient
Barrier Wall
Capture
Zone Limit
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A fundamental component of any ground-water
remediation effort using the pump-and-treat
approach is contaminant removal or control. Thus,
effective remediation of ground water using
pump-and-treat technology requires knowledge of
contaminants and site characteristics. Addition-
ally, the remediation plan should call for imple-
mentation of dynamic system management based
on a statement of realistic objectives (Hoffman,
1993).
3.1. Contaminant Removal/Control
Any ground-water cleanup effort will be
undermined unless inorganic and organic con-
taminant sources are identified, located, and
eliminated, or at least controlled, to prevent
further contamination of the aquifer. Toxic
inorganic substances may serve as a continuing
source of contamination through mechanisms
such as dissolution and desorption. At many
contaminated sites, organic liquids are a major
contributor to ground-water contamination.
Figure 2 illustrates four common types of con-
taminant plumes, each characterized by the
liquid's density relative to water and the degree to
which the liquid mixes with water. Even when the
organic liquid resides exclusively in the vadose
zone (i.e., the area between the ground surface
and the water table) it can serve as a source of
ground-water contamination. In such situations,
contamination occurs when percolating water
comes in contact with the liquid (sometimes
called product) or its vapors and carries dissolved
material to the ground water. Vapors also might
migrate to the water table and contaminate ground
water without infiltration.
Source removal is the most effective way to
prevent further contamination. Where inorganic or
organic contaminants are confined to the vadose
zone, removal is usually the preferred option.
When removal is not feasible, as is often the case
with dense non-aqueous phase liquids (DNAPLs)
residing below the water table, containment is an
essential initial step in remediation. In some
situations, containment can be achieved through
capping, which prevents or reduces infiltration of
rainfall through the contaminated soil. Capping
can be ineffective if water table fluctuations occur
within the zone of contamination or when NAPL
vapors are present.
3.2. Thorough Site Characterization
Comprehensive characterization of the contami-
nated site serves two major functions:
Accurately assessing the types, extent, and
forms of contamination in the subsurface
increases the likelihood of achieving treat-
ment goals. This requires an understanding of
the physical phases in which contaminants
exist (mainly sorbed and aqueous phases for
inorganic contaminants, and sorbed, NAPL,
aqueous, and gaseous phases for organic
liquids) and quantification of the distribution
between the phases. Indeed, inadequate site
characterization has undermined some pump-
-------
Immiscible
Ground Surface
ffis
Figure 2.
Contaminant plumes
as a function of
density and
miscibility with
ground water:
(a) light liquids
(gasoline and
methanol) create
contaminant plumes
that tend to flow in
the upper portions of
an aquifer; (b) dense
liquids (perchloro-
ethylene [PCE] and
ethylene glycol)
create a plume that
contaminates the full
thickness of an
aquifer (adapted
from Gorelick et al.,
1993).
(b)
Miscible
Soil
Methanol
Ground Surface
Ethylene Glycol
Ground Water
-------
and-treat efforts; for instance, when after a
few years greater quantities of contaminants
had been removed than were identified in the
initial site assessment.
A thorough, three-dimensional characteriza-
tion of subsurface soils and hydrogeology,
including particle-size distribution, sorption
characteristics, and hydraulic conductivity,
provides a firm basis for appropriate place-
ment of pump-and-treat wells. Such informa-
tion is also required for evaluating the extent
to which tailing and rebound may present
problems at a site (Section 4).
Three-dimensional characterization techniques
include primarily indirect observations, using
surface and borehole geophysical instruments and
cone-penetration measurements, and direct
sampling of soil and ground water. Important
advances in soil sampling technology have been
made relatively recently, such as continuous
samplers used with a hollow-stem auger (Figure
3) and smaller continuous-core, direct-push
equipment that also can be used to collect ground-
water samples without installing wells (Figure 4).
Vibratory drilling methods are another innovative
technique for collecting soil cores and ground-
water samples. Additionally, sensitive borehole
flow meters that allow measurement of vertical
changes in hydraulic conductivity in a borehole
represent an important recent development. These
techniques allow subsurface mapping to be
generated with a level of detail that generally
would be prohibitively expensive using conven-
tional drilling and sampling methods. Figure 5
presents a conceptual diagram of trichloroethene
(TCE) contamination at a complex site
(Sidebar 2) developed from extensive use of
direct-push sampling techniques.
Moreover, if sufficient data are obtained, the
interpretation of subsurface data can be enhanced
greatly by performing two- and three-dimensional
computer modeling of the subsurface. Figure 6
shows a conceptual model of a site developed by
combining contour visualization of a contaminant
plume of benzene with subsurface lithologic logs
(Sidebar 3). EPA's SITE3D software, being
developed by the National Risk Management
Laboratory's Subsurface Protection and Remedia-
tion Division, allows three-dimensional visualiza-
tion of contaminant plumes (Figure 7). Statistical
software developed by EPA such as Geo-EAS and
GEOPACK for geostatistical analysis and con-
touring of ground-water contaminant data and
GRITS/STAT for analysis of contaminant concen-
trations are among the many computer-based tools
available for analyzing subsurface data.
3.3. Dynamic Management of the
Well Extraction Field
To be effective, pump-and-treat efforts must go
beyond initial site characterization, using infor-
mation gathered after remediation operations are
under way to manage the well extraction field
dynamically. For instance, information collected
while drilling and installing extraction wells,
operating pumping wells, and tracking changes in
water levels in monitoring wells (Section 6.4) and
contaminant concentrations in observation wells
can refine the portrayal of the site.
Dynamic management of the well extraction
field based on more comprehensive information
can provide both economic and environmental
benefits. In general, additional information about
the site and the pump-and-treat effort allows
operators to make informed decisions about the
efficient use of remediation resources. More
specifically, this flexible site management ap-
proach may facilitate greater success in hydraulic
containment (Section 5). Ultimately, the time
required to achieve cleanup goals might be
minimized.
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Figures.
This hollow-stem
auger is fitted
with a 5-foot
sampling tube
that collects a
continuous core
as the auger
advances,
allowing detailed
and accurate
observation of
subsurface
lithology. When
drilling is
completed, a
monitoring well
also can be
installed.
-------
Figure 4. Hydraulic or vibratory direct-push rigs can be installed on vans, small trucks, all-terrain vehicles, or
trailers and allow collection of continuous soil cores and depth-specific ground-water samples for
detailed subsurface mapping if contaminants are generally confined to depths of less than 15 meters.
(Photo courtesy of Geoprobe Systems.)
A key component of the dynamic management
approach is the effective design and operation of
the pump-and-treat system. The following tech-
niques can be useful in this regard:
Using capture zone analysis, optimization
modeling, and data obtained from monitoring
the effects of initial extraction wells to
identify the best locations for wells (Section
6.1).
1 Phasing the construction of extraction and
monitoring wells so that information obtained
from operation of the initial wells informs
decisions about siting subsequent wells
(Section 6.2).
1 Phasing pumping rates and the operation of
individual wells to enhance containment,
avoid stagnation zones (Section 5.2.3), and
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'= Water Table
WEST
TCE Vapors
Area of Residual DNAPL
Above Water Table
Silt
EAST
Sand
Area of Residual DNAPL
Below Water Table \
365-
360-
355-
350-
345-
340-
335-
330-
325-
320-
315-
310-
305-
300-
295-
290-
285-
280-
Figure 5. Conceptual diagram of DNAPL (TCE) based on soil and ground-water sampling in a heterogeneous
sand and gravel aquifer. The extreme difficulty in cleaning up this site, which includes five distinct forms
of TCE (vapors and residual product in the vadose zone; pooled, residual, and dissolved product in the
ground water) led to modification of the pump-and-treat system for hydraulic containment rather than
restoration (adapted from Clausen and Solomon, 1994).
ensure removal of the most contaminated
ground water first (Section 6.3).
3.4. Realistic Cleanup Goals
Unrealistic expectations for the pump-and-treat
approach can lead to disappointments in system
performance (Sidebar 1). Indeed, a cleanup goal
that is realistic for one site may not be reasonable
elsewhere. The Committee on Ground Water
Cleanup Alternatives of the National Academy of
Sciences (NRC, 1994) has identified three major
classes of sites based on hydrogeology (Sidebar
2) and contaminant chemistry (see Table 1):
Class A. Sites where full cleanup to health-
based standards should be feasible using
current technology. Such sites include homo-
geneous single- and multiple-layer aquifers
involving mobile, dissolved contaminants.
10
-------
Sidebar 2
Major Types of Hydrogeologic Settings
The Committee on Ground Water Cleanup Alterna-
tives of the National Academy of Sciences has defined
three major hydrogeologic settings for evaluating the
technical feasibility of ground-water cleanup based on
the degree of uniformity of the aquifer material and
layering (Table 1).
Homogeneous aquifers consist of materials that do
not vary significantly in their water-transmitting
properties. Contaminant movement in homogeneous
aquifers is largely a function of the hydraulic
conductivity of the aquifer. For example, a homoge-
neous aquifer might comprise permeable, well-sorted
sands or gravels.
Heterogeneous aquifers consist of materials that vary
in their water-transmitting properties laterally,
vertically, or in both directions. Contaminants in
heterogeneous aquifers move preferentially in the
high-permeability zones, resulting in more rapid
transport than would be expected based on the
average hydraulic conductivity of the aquifer. A sand
aquifer with lenses of silts and clays is an example of
a heterogeneous aquifer.
Fractured aquifers typically consist of
low-permeability rock where most ground-water
flow is in joints and fractures.
Single-layered aquifers are less complex than
multiple-layered aquifers, which are separated by
less-permeable strata, because the possibility of cross
contamination between aquifers by either upward or
downward movement becomes a consideration. Note
that the term aquifer is used in this guide in a broad
sense to include any area within the saturated zone
where the presence of ground-water contamination is of
sufficient concern to require remediation. Figure 2
illustrates a homogenous, single-layer aquifer.
Figure 15c (described in Section 4.2.5) illustrates a
two-layered homogenous aquifer. Figure 5 illustrates a
multiple-layered heterogeneous aquifer. The challenge
of ground-water cleanup increases along with aquifer
complexity because of difficulties in delineating
contaminant sources and pathways and the increased
likelihood of tailing and rebound effects (Section 4).
Class B. Sites where the technical feasibility
of complete cleanup is likely to be uncertain.
This class includes a wide range of hydrogeo-
logic settings and contaminant types that do
not fall into classes A or C.
Class C. Sites where full cleanup of the
source areas to health-based standards is not
likely to be technically feasible. Such sites
include fractured-rock aquifers contaminated
by free-product light nonaqueous phase
liquids (LNAPL) or DNAPL and single- or
multiple-layered heterogeneous aquifers
contaminated by a free-product DNAPL
(Sidebar 4).
Typically, preliminary ground-water cleanup
efforts at contaminated sites are focused on
standards established for drinking water, such as
federal or state maximum contaminant levels
11
-------
SITE MAP CONTOURS
Figures.
GEOS computer
screen showing
organic
contaminant
plume in relation
to subsurface
stratigraphy (see
text for
discussion).
CROSS SECTION
riLLXTCPSOIL
SANDY/SILTY CU)Y
SAHDxSRAUEL*!
05^02/31
parasreter
Benzene
(MCLs) or nonzero MCL goals (MCLGs). EPA
has established procedures, however, by which
efforts can target alternative goals at Superfund
and RCRA sites using alternate concentration
limits (ACLs) where ground-water discharges into
nearby surface water (U.S. EPA, 1988) or demon-
strating the technical impracticality (TI) of
ground-water cleanup (U.S. EPA, 1993; Feldman
and Campbell, 1994). At DNAPL sites where the
TI of ground-water cleanup has been demon-
strated, the remedial strategy might call for
removal of as much of the DNAPL as is feasible,
containment of the remaining DNAPL, and
treatment of the aqueous contaminant plume
outside the containment area. Consequently, even
at Class C and Class B sites where restoration is
not feasible, application of some form of the
pump-and-treat approach may be required either
to help contain the contaminant source and
aqueous-phase plume or to clean up the contami-
nated ground water outside the containment area.
12
-------
Computer visualization can help focus attention on
the types of additional information needed when
characterizing a site before initiating a remediation
effort. For example, in Figure 6 the contoured benzene
data, collected from the sand/gravel # 1 aquifer (yellow
in the cross section in the upper right) shows two areas
of high concentration (i.e., MW-5 and MW-7). Does this
represent a contaminant plume from a single source, or
does it indicate two contaminant plumes from separate
sources? An examination of the cross section (upper
right of center) indicates that sand/gravel # 1 aquifer at
MW-7 has the lower "high" concentration of benzene,
suggesting that the two plumes might be related.
The cross section also shows, however, that the
aquifer is quite thin between the two monitoring wells.
Indeed, examination of monitoring well MP-10, which
lies near the cross section (see lower center log) reveals
that the aquifer is missing at this point. This suggests
the absence of a concentration gradient between the two
wells, indicating that two different sources may be
involved.
Further analysis of the spatial distribution of the sand/
gravel # 1 aquifer, including flow directions as indicated
by potentiometric heads, and possibly additional
sampling for benzene would be required to determine if
one or two sources are contributing to the contamina-
tion.
This particular example also cautions against relying
exclusively on computer-generated interpolations,
which can suggest features that are not actually present
(i.e., continuity of the aquifer between MW-5 and
MW-7).
13
-------
Figure 7. EPA's SITE3D software, under development at the Ada, Oklahoma, laboratory, helps visualize in three-
dimensions a TCE contaminant plume at a Superfund Site. Yellows and reds indicate zone with highest
concentrations of TCE in ground water.
14
-------
Table 1. Categories of Sites for Technical Infeasibility Determinations (NRC, 1994)
Contaminant Chemistry
Hydrogeology
Homogeneous,
single layer
Homogeneous,
multiple layers
Heterogeneous,
single layer
Heterogeneous,
multiple layers
Fractured
Mobile
Dissolved
(degrades/
volatilizes)
A
(1)
A
(1)
B
(2)
B
(2)
B
(3)
Mobile,
Dissolved
A
(1-2)
A
(1-2)
B
(2)
B
(2)
B
(3)
Strongly
Sorbed,
Dissolved
(degrades/
volatilizes)
B
(2)
B
(2)
B
(3)
B
(3)
B
(3)
Strongly
Sorbed,
Dissolved
B
(2-3)
B
(2-3)
B
(3)
B
(3)
B
(3)
Separate
Phase
LNAPL
B
(2-3)
B
(2-3)
B
(3)
B
(3)
C
(4)
Separate
Phase
DNAPL
B
(3)
B
(3)
C
(4)
C
(4)
C
(4)
Note: Shaded boxes at the left end (group A) represent types of sites for which cleanup of the full site to health-based standards should be feasible with
current technology. Shaded boxes at the right end (group C) represent types of sites for which full cleanup of the source areas to health-based
standards will likely be technically infeasible. The unshaded boxes in the middle (group B) represent sites for which the technical feasibility of
complete cleanup is likely to be uncertain. The numerical ratings indicate the relative ease of cleanup, where 1 is easiest and 4 is most difficult.
15
-------
The light (LNAPLs) and dense (DNAPLs) immiscible
nonaqueous phase liquids shown in Figure 2 pose the
most difficult problems for ground-water cleanup
because of their complex interactions with water and
solids in the subsurface. Figure 5 illustrates these
complexities when trichloroethene (TCE), a DNAPL has
moved through a heterogeneous, multiple-layered
alluvial aquifer (at a site in Tennessee). Four distinct
forms, or phases, of TCE are evident:
I The NAPL emits a vapor phase in the unsaturated
zone that moves by diffusion. DNAPL vapors tend to
sink until they reach impermeable layers (Figure 5) or
the water table. Even if the NAPL does not reach the
ground water, contamination can occur by dissolution
of the vapors directly into the ground water or by
water percolating through the unsaturated zone.
Residual NAPL remains after the free product has
moved through the subsurface by gravity or been
displaced by water (Figure 8a and b). Residual NAPL
exists as single- to complex-shaped blobs that fill
pore spaces (Figure 8b and 9). The amount of
residual NAPL remaining in the subsurface depends
on the subsurface material and the type of NAPL.
Residual saturation in the unsaturated zone typically
ranges from 10 to 20 percent of the subsurface
volume and in the saturated zone generally ranges
from 10 to 50 percent (Cohen and Mercer, 1993).
Figure 5 differentiates residual TCE above and
below the water table.
Free product exists where most of the pore space is
filled by the NAPL. It accumulates wherever a
barrier prevents downward movement. LNAPLs,
such as gasoline, tend to float on top of the water
table, whereas DNAPLS tend to sink until they reach
an impermeable layer (Figure 5).
Dissolved NAPL forms the aqueous contaminant
plume that moves in the direction of ground-water
flow. The residual NAPL and free product can serve
as a source of ground-water contamination as long as
they remain in the subsurface.
16
-------
Figure 8. Dark NAPL (Soltrol) and water in a homogenous micromodel after (a) the displacement of water by
NAPL and then (b) the displacement of NAPL by water, with NAPL at residual saturation (Wilson et al.
1990).
17
-------
Figure 9.
Photomicrographs of (a) a
single blob occupying one
pore body, and (b) a doublet
blob occupying two pore
bodies and a pore throat
(Wilson etal., 1990).
18
-------
The phenomena of tailing and rebound are
commonly observed at pump-and-treat sites.
Tailing refers to the progressively slower rate of
decline in dissolved contaminant concentration
with continued operation of a pump-and-treat
system (Figure 10). Rebound is the fairly rapid
increase in contaminant concentration that can
occur after pumping has been discontinued. This
increase may be followed by stabilization of the
contaminant concentration at a somewhat lower
level.
4.1. Effects of Tailing and Rebound
on Remediation Efforts
Tailing presents two main difficulties for
ground-water restoration:
Longer treatment times. Without tailing,
contaminants theoretically could be removed
by pumping a volume of water equivalent to
the volume of the contaminant plume (Figure
10). The tailing effect, however, significantly
increases the time pump-and-treat systems
must be operated to achieve ground-water
restoration goals. Indeed, pumping may need
to be conducted for hundreds of years rather
than tens of years.
Residual concentrations in excess of the
cleanup standard. When tailing occurs, often
initially the decline in the rate of contaminant
concentrations is fairly rapid, followed by a
more gradual decline that eventually stabilizes
at an apparent residual concentration level
above the cleanup standard (Figure 10).
Rebound is most problematic when a pump-
and-treat system attains the cleanup standard, but
concentrations subsequently increase to a level
that exceeds the standard.
4.2. Contributing Factors
The degree to which tailing and rebound
complicate remediation efforts at a site is a
function of the physical and chemical characteris-
tics of the contaminant being treated, the subsur-
face solids, and the ground water. Major factors
and processes that contribute to tailing and
rebound are discussed below.
4.2.1. Non-Aqueous Phase Liquids
Although immiscible LNAPLs and DNAPLs
tend to be relatively insoluble in water, unfortu-
nately they often are sufficiently soluble to cause
concentrations in ground water to exceed MCLs.
Consequently, residual and pooled free-product
NAPL will continue to contaminate ground water
that makes sufficient contact to dissolve small
amounts from the NAPL surface (Figure 1 la).
When ground water is moving slowly, contami-
nant concentrations can approach the solubility
limit for the NAPL (Figure lie). Although pump-
and-treat systems increase ground-water velocity,
causing an initial decrease in concentration, the
decline in concentration will later tail off until the
NAPL's rate of dissolution is in equilibrium with
the velocity of the pumped ground water. If
pumping stops, the ground-water velocity slows
and concentrations can rebound, rapidly at first
and then gradually reaching the equilibrium
19
-------
t
o
O
Pumping On
Theoretical Removal
Without Tailing
Apparent Residual
Contaminant
Concentration
Cleanup
Standard
Pumping Off
Rebound
X
Figure 10. Concentration versus pumping duration or volume showing tailing and rebound effects (Cohen et al.
1994).
concentration (Figure 1 Ic), unless pumping is
resumed.
As shown in Table 1, DNAPL contamination in
heterogeneous and fractured aquifers is the most
intractable. The reasons for this are
DNAPLs create an unstable wetting front in
the subsurface, with fingers of more rapid
vertical flow speeding the movement deeper
into the saturated zone (Figure 12). (This also
makes accurate delineation of zones of
residual contamination extremely difficult in
homogeneous aquifers.)
If the volume of DNAPL exceeds the residual
saturation capacity of the unsaturated and
saturated zones, the DNAPL will reach lower
permeability materials and form pools of free
product (Figure 5).
In heterogeneous aquifers, localized lenses of
low-permeability strata may cause pools of
free product to develop throughout the
saturated zone (Figure 12). Low-permeability
strata also may cause extensive lateral move-
ment of the DNAPL. DNAPL pools are
especially problematic because the contami-
nant will dissolve even more slowly than
residual DNAPL. It may take tens of years to
remove 1 cm of contaminant from a DNAPL
pool (NRC, 1994).
4.2.2. Contaminant Desorption
The movement of many organic and inorganic
contaminants in ground water is retarded by
sorption processes that cause some of the dis-
solved contaminant to attach to solid surfaces.
The amount of contaminant sorbed is a function
of concentration, with sorption increasing as
concentrations increase, and the sorption capacity
20
-------
Advection
Contaminant Free Product
Solid Grain
Liquid-Liquid Partitioning
(a)
Organic Carbon or
Mineral Oxide Surface
(b)
Desorption of
Adsorbed Contaminants
Figure 11.
Contaminants are mobilized
when ground water that is
undersaturated with a
contaminant comes in
contact with a NAPL (a) or
contaminant sorbed on an
organic carbon or mineral
surface (b). High ground-
water velocities and short
contact times will result in
low contaminant
concentrations, and low
velocities and long contact
times will result in high
contaminant concentrations
(c) (adapted from Gorelick et
al., 1993).
Equilibrium Concentration
Low Ground-Water Velocities and Long Contact
Times Produce High Contaminant Concentrations
(approaching equilibrium) in Ground Water
High Ground-Water Velocities and Short Contact Times
Produce Low Contaminant Concentrations in Ground Water
Contact Time Increases to Right
Ground-Water Velocity Increases to Left
(C)
21
-------
Figure 12. Laboratory model of the transport of DNAPL contaminant through an aquifer with varying permeability;
note the concentration of downward movement in fingers and the DNAPL pools above the low-
permeability zones (the horizontal discs). (Source: U.S. EPA National Risk Management Research
Laboratory.)
22
-------
of the subsurface materials. Sorbed contaminants
tend to concentrate on organic matter and clay-
sized mineral oxide surfaces (Figure 1 Ib). Sorp-
tion is a reversible process, however. Thus, as
dissolved contaminant concentrations are reduced
by pump-and-treat system operation, contami-
nants sorbed to subsurface media can desorb from
the matrix into ground water. Contaminant
concentrations resulting from sorption and
desorption show a relationship to ground-water
velocity and contact time similar to that of NAPLs
(Figure 1 Ic), causing the tailing of contaminant
concentrations during pumping as well as rebound
after pumping stops.
4.2.3. Precipitate Dissolution
As with sorption-desorption reactions, precipi-
tation-dissolution reactions are reversible. Thus,
large quantities of inorganic contaminants, such
as chromate in BaCrO4, may be found with
crystalline or amorphous precipitates in the
subsurface (Palmer and Fish, 1992). Figure 13
illustrates a tailing curve where the contaminant
concentration is controlled by solubility. In this
situation, if pumping stops before the solid phase
is depleted, rebound can occur.
1.2
o
"ro
o 0.8
o
O
T3
-5 0.6
CO
CO
b
cu
ฃ 0.4
0.2
Contaminant Concentration
Controlled by Solubility
Solid-Phase
Reserve Depleted
^fww^M^MM^sffi^K^HM!
Pumping Duration or Volume Pumped
Figure 13. Dissolved contaminant concentration in ground water pumped from a recovery well versus time in a
formation that contains a solid-phase contaminant precipitate (Palmer and Fish, 1992).
23
-------
4.2.4. Matrix Diffusion
As contaminants advance through relatively
permeable pathways in heterogeneous media,
concentration gradients cause diffusion of con-
taminant mass into the less permeable media
(Gillham et al., 1984). Matrix diffusion is most
likely to occur with dissolved contaminants that
are not strongly sorbed, such as inorganic anions
and some organic chemicals. During a pump-and-
treat operation, dissolved contaminant concentra-
tions in the relatively permeable zones are re-
duced by advective flushing, causing a reversal in
the initial concentration gradient and slow diffu-
sion of contaminants from the low to high perme-
ability media. Figure 14, based on theoretical
calculations of TCE concentrations in clay lenses
of varying thickness, shows that diffusion is a
slow process. For example, the figure indicates
that the time required to reduce the concentration
of TCE to 10 percent of the initial concentration
would be 6 years for a clay lens 1 foot thick, 25
years for a clay lens 2 feet thick, and 100 years
for a clay lens 4 feet thick. The significance of
matrix diffusion increases as the length of time
between contamination and cleanup increases. In
heterogeneous aquifers, matrix diffusion contribu-
tions to tailing and rebound can be expected, as
long as contaminants have been diffusing into
less-permeable materials.
4.2.5. Ground-Water Velocity Variation
Tailing and rebound also result from the vari-
able travel times associated with different flow
paths taken by contaminants to an extraction well
(Figure 15a-c). Ground water at the edge of a
capture zone created by a pumping well travels a
greater distance under a lower hydraulic gradient
Clay Lens Thickness = 1.2 Meters (4 ft)
100
Figure 14. Changes in average relative trichloroethene (TCE) concentrations in clay lenses of varying thickness
as a function of time (NRC, 1994).
24
-------
(a)
Moderate
Fast
Moderate
(b)
Figure 15.
Tailing resulting from
ground-water velocity
variations: (a) horizontal
variations in the velocity of
ground water moving
toward a pumping well
(Keely, 1989) lead to (b)
tailing as higher
concentrations of ground
water in slower pathlines
mix with lower
concentrations in faster
pathlines (Palmer and Fish,
1992); (c) in a stratified
sand and gravel aquifer,
tailing occurs attl when
clean water from the upper
gravel strata mixes with still-
contaminated ground water
in the lower sand strata
(Cohen etal., 1994).
~ro
ง 08
o
O
1
rl 02
n
\Tailing Due to
Different Travel
Times Alona Flow
\Paths to
Recovery Well
\/
\^
\
Pumping Duration or Volume Pumped
Stratified Sand-Gravel Aquifer
25
-------
than ground water closer to the center of the
capture zone (Figure 15a). Additionally, contami-
nant-to-well travel time varies as a function of the
hydraulic conductivity in heterogeneous aquifers
(Figure 15c).
4.3. Assessing the Significance of
Tailing and Rebound at a Site
Determining realistic objectives for apump-
and-treat system requires sufficient site character-
ization to define the complexity of the hydrogeo-
logic setting (Sidebar 2) and the subsurface
distribution of contaminants. Such information
makes it possible for the system operator to assess
whether conditions at the site will result in tailing
and rebound and to evaluate the extent to which
these conditions are likely to increase the time
needed to attain health-based cleanup standards.
The sorption characteristics of contaminants can
be assessed using batch sorption tests with aquifer
materials (Roy et al., 1992), although aquifer
heterogeneity increases the difficulty of interpret-
ing test results. For organics, the potential effects
of sorption can be assessed based on a literature
review of contaminant properties and on site-
specific data on organic carbon in aquifer materi-
als (Piwoni and Keely, 1990). Geochemical
computer codes can be used to assess the potential
for tailing and rebound effects from precipitation-
dissolution reactions.
Assessing the potential for removal or contain-
ment of free product may be the first priority at
NAPL-contaminated sites, followed by assess-
ment of the extent of residual NAPL contamina-
tion. For DNAPLs, residual saturation may extend
throughout the unsaturated and saturated zones
(Figure 5). Typically, for LNAPLs most residual
contamination is located in the vadose zone, but it
may also extend to the depth of the seasonal low
water table. As Figure 16 shows, pumping to
remove free LNAPL product can cause residual
NAPL to move deeper into the saturated zone.
Consequently, when removing free-product
LNAPL that is floating on the water table, steps
should be taken to avoid or minimize movement
of residual NAPL deeper into the saturated zone.
Berglund and Cvetkovic (1995) evaluated the
relative importance of the degree of heterogeneity
in hydraulic conductivity and mass transfer
processes and concluded that the rate of mass
transfer and the extent to which contaminants are
sorbed on aquifer solids are the most important
parameters that affect predicted cleanup time.
26
-------
Ground Surface
Zone of Residual LNAPL Contamination
After Initial Recovery Efforts Have Stopped
fies-duai
INAPL
Figure 16. Zone of residuals created in former cone of depression after cessation of LNAPL recovery system
(Gorelicket al., 1993).
27
-------
Hydraulic containment is a design objective of
nearly all pump-and-treat systems. Where restora-
tion of an aquifer to health-based standards is the
overall objective, the primary goal of containment
must be to prevent farther spread of the contami-
nant plume during restoration efforts. Where
NAPLs are present, containment using hydraulic
and physical barriers might be the primary
objective for cleanup efforts in the portion of the
aquifer contaminated by free product and residual
NAPL (Figures Ic and 17). In such situations a
conventional pump-and-treat system might be
used to restore the dissolved contaminant plume
(Figure 17).
Effective hydraulic containment using pumping
wells requires the creation of horizontal and
vertical capture zones that draw all contaminated
ground water to the wells (Section 5.1.1) or other
hydraulic barriers (Sections 5.1.2 and 5.1.3).
Failure to take aquifer anisotropy into account
(Section 5.2.1) or limitations in the ability to
create sufficient drawdown to establish capture
zones (Section 5.2.2) may allow contaminants to
escape from these systems. Additionally, stagna-
tion zones created by pumping operations or the
use of injection wells can reduce the effectiveness
of cleanup efforts (Section 5.2.3). The monitoring
of both hydraulic heads (Section 6.4.1) and
ground-water quality (Section 6.4.2) can provide
early indications that contaminants are not being
contained.
5.1. Ground-Water Barriers and Flow
Control
Hydraulic containment can be accomplished by
controlling the direction of ground-water flow
with capture zones (Section 5.1.1) or pressure
ridges (Section 5.1.2) or by using physical
barriers (Section 5.1.3). Figure 17 illustrates a
pump-and-treat system that uses all three types of
hydraulic controls: (1) the contaminant source
area is surrounded by a barrier wall, (2) extraction
wells around the margins of the dissolved plume
capture the contaminated ground water, and (3)
treated ground water is reinjected to create a
pressure ridge along the axis of the contaminant
plume. Note that the pressure ridge in Figure 17
serves the function of increasing pore-volume
exchange rates rather than functioning as a
barrier. Barrier pressure ridge systems are created
by placing injection wells along the perimeter of a
contaminant plume.
5.1.1. Horizontal and Vertical Capture
Zones
Pumping wells provide hydraulic containment
by creating a point of low hydraulic head to which
nearby ground water flows. The portion of an
aquifer where flow directions are toward a
pumping well is called a capture zone. In an
isotropic aquifer, where hydraulic conductivity is
the same in all directions, ground-water flow is
perpendicular to the hydraulic head contours, also
called equipotential lines (Figure 18b).
28
-------
Limit of Dissolved Plume
Contaminant
Source Area
Initial Ground-Water Flow Direction
Barrier Wall
Q Injection Well
9 Extraction Well
Figure 17. Plan view of a mixed containment-restoration strategy. A pump-and-treat system is used with barrier
walls to contain the ground-water contamination source areas (e.g., where NAPL or waste may be
present) and then collect and treat the dissolved contaminant plume (Cohen et al., 1994).
A pumping well creates a zone of influence
where the potentiometric surface has been modi-
fied (Figure 18c). The capture zone is the portion
of the zone of influence where ground water flows
to the pumping well (Figure 18d). Figure 15a
shows how a capture zone creates flow lines of
varying velocity. The size and shape of a capture
zone depend on the interaction of numerous
factors, such as
The hydraulic gradient and hydraulic conduc-
tivity of the aquifer.
The extent to which the aquifer is heteroge-
neous (Sidebar 2) or anisotropic (Section
5.2.1).
Whether the aquifer is confined or uncon-
fined.
The pumping rate and whether other pumping
wells are operating.
Whether the screened interval of the well
fully or partially penetrates the aquifer.
When the screened portion of a pumping well
fully penetrates an aquifer (Figure Ib), a two-
dimensional analysis to delineate the horizontal
29
-------
(a)
(b)
V \ \ M V
Zone of Influence \
(c)
(d)
Figure 18. In an isotropic aquifer, ground-water flow lines (b) are perpendicular to hydraulic head contours (a).
Pumping causes drawdowns and a new steady-state potentiometric surface within the well's zone of
influence (c). Following the modified hydraulic gradients, ground water within the shaded capture zone
flows to the pumping well (d). (Cohen et al., 1994, adapted from Gorelick et al., 1993).
30
-------
capture zone is usually sufficient. When a pump-
ing well only partially penetrates an aquifer.
however, vertical capture zone analysis also is
required to determine whether the capture zone
will contain a contaminant plume. Figure 19
shows a vertical capture zone for a partially
penetrating well. If the contaminant plume
extended to the base of the aquifer, some contami-
nants would bypass the well, despite the presence
of apparent upward gradients. In stratified aniso-
tropic media (Section 5.2.1), the vertical hydraulic
control exerted by a partially penetrating well will
be further diminished.
5.1.2. Pressure Ridge Systems
Pressure ridge systems are produced by inject-
ing uncontaminated water into the subsurface
through a line of injection wells located
upgradient or downgradient of a contamination
plume. The primary purpose of a pressure ridge is
to increase the hydraulic gradient and hence the
velocity of clean ground water moving into the
plume, thereby increasing flow to the recovery
wells, which serves to wash the aquifer.
Upgradient pressure ridges also serve to divert the
flow of uncontaminated ground water around the
plume, and downgradient pressure ridges prevent
further expansion of the contaminant plume.
Typically, treated ground water from extraction
wells within a contaminant plume supply the
upgradient or downgradient injection wells used
to create a pressure ridge.
5.1.3. Physical Barriers
Physical barriers are constructed of low-
permeability material and serve to keep fresh
ground water from entering a contaminated
aquifer zone. They also help prevent existing
areas of contaminant from moving into an area of
clean ground water or releasing additional con-
taminants to a dissolved contaminant plume. Most
systems involving physical barriers also require
ground-water extraction to ensure containment by
maintaining a hydraulic gradient toward the
contained area (see Figure Ic). The advantage of
physical barriers is that the amount of ground
water that must be extracted is greatly reduced
compared to the amount when using hydrody-
namic controls, as described in Sections 5.1.1 and
5.1.2. Major types of barriers include
Caps (or covers), which are made of low-
permeability material at the ground surface,
can be constructed of native soils, clays,
synthetic membranes, soil cement, bitumi-
nous concrete, or asphalt.
Slurry trench walls, excavated at the proper
location and to the desired depth while
Figure 19. Cross section showing equipotential contours and the vertical capture zone associated with ground-
water withdrawal from a partially penetrating well in isotropic media (Cohen et al., 1994).
31
-------
keeping the trench rilled with a clay slurry,
keep the trench sidewalls from collapsing and
backfilling with soil bentonite, cement
bentonite, or concrete mixtures.
Grout curtains are created by injecting
stabilizing materials under pressure into the
subsurface to fill voids, cracks, fissures, or
other openings in the subsurface. Grout also
can be mixed with soil using larger augers.
Sheet piling cutoff walls are constructed by
driving sheet materials, usually steel, through
unconsolidated materials with a pile driver or
more specialized vibratory drivers.
Knox et al. (1984) provide further information
on the design and construction of physical
ground-water barriers.
5.2. Hydraulic Containment: Other
Special Considerations
Certain site conditions can allow contaminants
to escape from a hydraulic containment system if
they are not characterized and anticipated.
5.2.1. Effects ofAnisotropy
In anisotropic aquifers, hydraulic conductivity
varies with direction. In flat-lying sedimentary
aquifers, hydraulic conductivity is often higher in
a horizontal than a vertical direction. In fractured
rock and foliated metamorphic rocks, such as
schist, the direction of maximum and minimum
permeability is usually aligned parallel and
perpendicular, respectively, to foliation or bedding
plane fractures (Cohen et al., 1994). Where
sedimentary strata and foliated media are inclined
or dipping, significant horizontal anisotropy may
be an aquifer characteristic. In anisotropic media,
the flow of ground water, as well as contaminants
moving with ground water, is usually not perpen-
dicular to the hydraulic gradient.
Figure 20 illustrates how horizontal anisotropy
in fractured rock can change the location of the
capture zone of a pumping well. In an aquifer that
is assumed to be isotropic, the general direction of
ground-water flow should be perpendicular to the
hydraulic gradient (Figure 20a). If fractures cause
hydraulic conductivity to be higher in a north-
south rather than an east-west direction, however,
the direction of ground-water flow will diverge
from the direction of the hydraulic gradient
(Figure 20b). In this example, siting a pumping
well based only on the hydraulic gradient (Figure
20a) would result in its failure to capture any
portion of a contaminant plume, except in the
immediate vicinity of the well.
A contaminant plume that does not follow the
hydraulic gradient may indicate that anisotropy is
influencing the direction of ground-water flow.
Aquifer heterogeneities, such as buried stream
channels that have a different direction than the
hydraulic gradient, also may allow the direction
of contaminant travel to diverge from the hydrau-
lic gradient. Computer programs, such as EPA's
Well Head Protection Area (WHPA) code, can be
useful for evaluating the potential effects of
anisotropy on well capture zones. Figure 21
shows such a simulation for three pumping wells.
In this case, with a vertical to horizontal anisot-
ropy ratio of 10:1, the orientation of the capture
zones shifts from northwest-southeast (isotropic)
to east-west (anisotropic).
5.2.2. Drawdown Limitations
Under some conditions creating and maintain-
ing an inward hydraulic gradient for a contami-
nant plume is problematic. In such situations,
injection wells may be required to create pressure
ridges (Section 5.1.2) or physical barriers may
need to be installed (Section 5.1.3). Site condi-
tions that might indicate the need for such mea-
sures include (Cohen et al., 1994)
Limited saturated thickness of the aquifer
Relatively high initial hydraulic gradient
32
-------
(a) Isotropic Aquifer
(b) Anisotropic Aquifer
Water-
Table
Contours
Water-Table
Contours
A} Pumping
s? Well
Figure 20. Effect of fracture anisotropy on the orientation of the zone of contribution (capture zone) to a pumping
well (Bradbury etal., 1991).
Sloping aquifer base
Very high aquifer permeability
Low aquifer permeability
Where these conditions exist and hydraulic
containment is planned, particular care should be
taken during site characterization and pilot tests to
assess drawdown limitations.
5.2.3. Stagnation Zones
Stagnation zones develop in areas where pump-
and-treat operations create low hydraulic gradi-
ents and, consequently, low ground-water veloci-
ties. The stagnation zone associated with a single
extraction well is likely to be located
downgradient from the well (Figure 22a). A
stagnation zone can develop upgradient from an
injection well, however, and form in low-perme-
ability zones, regardless of hydraulic gradient.
When multiple extraction or injection wells are
involved, a number of stagnation zones may
develop (Figure 22b). Stagnation zones caused by
low hydraulic gradients can be identified by
measuring hydraulic gradients, tracer movement,
and ground-water flow rates using downhole
flowmeters and through modeling analysis.
Stagnation zones within a contaminant plume can
reduce the efficiency of a pump-and-treat system;
thus, minimizing stagnation is an important
objective of capture zone analysis and optimiza-
tion modeling (Section 6.1).
33
-------
(ft)
Isotropic
9000
7200
5400
3600
1800
I
2100 4200 6300
(a)
8400
(ft)
10500
(ft)
Anisotropic
Figure 21.
Capture zone
simulation of
three pumping
wells for an
isotropic aquifer
(a) and
anisotropy ratio of
10:1 (b) using the
EPA WHPA code.
9000
7200
5400
3600
1800
I
(ft)
2100 4200 6300
(b)
8400
10500
34
-------
Hydraulic Head
Contour Map
I I I I I I 1 I 1
CO
LO
x \ T
11 10 9
\
Ground-Water Velocity
Contour Map
Hydraulic Head
Contour Map
O
o
(b)
O
1
Ground-Water Velocity Contour Map
Figure 22. Examples of stagnation zones (shaded where ground-water velocity is less than 4 L/T): (a) single
pumping well and (b) four extraction wells with an injection well in the center (Cohen et al., 1994).
35
-------
The basic operating principle of a pump-and-
treat system calls for locating a well (or wells)
and then pumping at rates that cause all water in a
contaminant plume to enter the well rather than
continue traveling through the subsurface. Table 2
lists types of data required for evaluating the
feasibility of using the pump-and-treat approach
at a contaminated ground-water site and then
designing an appropriate system. This section
describes the key aspects of designing and
operating a pump-and-treat system for optimal
performance.
6.1. Capture Zone Analysis and
Optimization Modeling
In recent years, numerous mathematical models
have been developed or applied to compute
capture zone, ground-water pathlines, and associ-
ated travel times to extraction wells or drains. For
relatively simple hydrogeologic settings (homoge-
neous isotropic aquifers), analytical equations
solved manually, using graphical techniques or
computer codes based on analytical solutions,
may be adequate. For more complex sites, nu-
merical computer models may be required. These
models provide insight to flow patterns generated
by alternative pump-and-treat approaches and to
the selection of monitoring points and frequency.
The WHPA model (Blandford and Huyakorn,
1991) and Capture Zone Analytic Element Model
(CZAEM) (Haitjema et al., 1994; Stock et al.,
1994) developed by EPA are examples of rela-
tively simple computer software based on analyti-
cal equations (WHPA) and the innovative analytic
element method (CZAEM) that allows capture
zone and ground-water pathline analysis. The
numerical MODFLOW and MODPATH models
developed by the U.S. Geological Survey are
commonly used to model more complex hydro-
geologic settings. Cohen et al. (1994) identify a
number of computer codes of potential value for
capture zone analysis. More detailed information
about specific models and EPA guidance on the
use of models are available in references on
Ground-Water Modeling at the end of this guide
(Section 9). Sidebar 5 summarizes the results of
computer modeling performed to evaluate the
effect of different hydrogeologic conditions on the
effectiveness of different types of well patterns.
In addition, optimization programming methods
are being used increasingly to improve pump-and-
treat system design (Gorelick et al., 1993). As
applied to the design of pumping systems, optimi-
zation involves defining an objective function,
such as minimizing the sum of pumping rates
from a number of wells. A set of restrictions, or
constraints, specify various conditions, such as
maximum pumping rates and minimum hydraulic
heads at individual wells, that must be satisfied by
the optimal solution alternative. Hydraulic
containment of a contaminant plume usually
requires only linear optimization methods, but
when contaminant concentrations are specified as
constraints, nonlinear methods are often required
(Rogers et al., 1995). At the Lawrence Livermore
36
-------
Table 2. Data Requirements for Pump-and-Treat Systems (Adapted from U.S. EPA, 1991)
Data Description Purpose(s)
Source(s)/Method(s)
Hydraulic conductivities and
storativities of subsurface
materials
Contaminant concentrations
and areal extent
Contaminant/soil properties
(density, aqueous solubility,
octanol-water/carbon
partitioning coefficient, soil
organic carbon content,
sorption parameters)
Types, thicknesses, and extent
of saturated and unsaturated
subsurface materials
Depth to aquifer/water table
Ground-water flow direction
and vertical/horizontal gradients
Seasonal changes in ground-
water elevation
NAPL density/viscosity/
solubility; residual saturation of
vadose zone and saturated zone
Ground-water/surface water
connection
Precipitation/recharge
Locations, screen/open interval
depths, and pumping rates of
wells influenced by site
To determine feasibility of
extracting ground water;
applicability of pump-and-treat
approach
To determine seriousness of the
problem; existence of NAPL;
applicability and evaluate
effectiveness
To determine mobility
properties; applicability of
pump-and-treat approach
To develop conceptual design;
applicability/considerations for
implementation
To select appropriate extraction
system type; consideration for
implementation
To determine proper well
locations/spacing considerations
for implementation
To locate wells and screened
intervals; considerations for
implementation
To predict vertical distribution
of contamination; consideration
for implementation and
evaluating effectiveness
To determine impacts of surface
water
To calculate water balance;
consideration for implementing
and evaluating effectiveness
To determine
impacts/interference;
considerations for implementing
and evaluating effectiveness
Pumping test, slug tests,
laboratory permeability tests
Soil and water quality sampling
data
Published literature, laboratory
tests
Hydrogeologic maps, surficial
geology maps/reports, boring
logs, geophysics
Hydrogeologic maps,
observation wells, boring logs,
piezometers
Water level data,
potentiometric maps
Long-term water level
monitoring
Literature, laboratory
measurements
Seepage measurements,
stream gaging
NOAA reports, local weather
bureaus; onsite measurements
Well inventory, pumpage
records
37
-------
Sidebar 5
Computer Modeling of Well Patterns Versus Hydrogeologic Conditions
Satkin and Bedient (1988) used the U.S. Geological
Survey MOC model to evaluate the effectiveness of
seven different well patterns (Figure 23) for restoring
contaminated ground water under eight generic
hydrogeologic conditions. The hydrogeologic settings
were defined as various combinations of three major
factors: maximum drawdown (high > 10 ft; low < 5 ft),
hydraulic gradient (high = 0.008; low = 0.0008), and
longitudinal dispersivity (high = 30 ft; low =10 ft).
Because the contaminants were assumed to not interact
with aquifer solids, tailing and rebound effects were not
a consideration in the study. Major conclusions of the
computer simulations include the following:
Significant differences in cleanup time were observed
using various well locations for a given well pattern.
The three-spot, doublet, and double-cell well
patterns are effective under low hydraulic gradient
conditions. These well patterns minimize cleanup
time, volume of water circulated, and volume of
water treated.
The three-spot well pattern performed better than
any of the other well patterns studied under a high
hydraulic gradient, high drawdown, and either a low
or high dispersivity.
None of the well patterns investigated was able to
contain and clean up the contaminated plume in a
setting with high gradient, low drawdown, and high
dispersivity.
The centerline well pattern is effective in achieving
up to 99 percent contaminant reduction under both
low and high gradient conditions, but it may present
a water disposal problem.
The five-spot well pattern was the least effective of
the well patterns studied.
38
-------
Single
Doublet
Centerline
(..Pumping Well
X
X
3-Spot
X..Injection Well
Figure 23.
Major types of
pumping/injection
well patterns
(Satkin and
Bedient, 1988).
X X
X X
5-Spot
X
X X
Double Triangle
X
X
Double Cell
39
-------
National Laboratory (LLNL) site, Rogers et al.
(1995) applied an innovative nonlinear optimiza-
tion approach, using artificial neural networks and
a genetic algorithm, to evaluate more than 4
million pumping patterns for the project's 28
extraction and injection wells. The three top-
ranked patterns required 8 to 13 wells, with
projected costs estimated at $41 to $53 million
over the 50-year project life. Using these pumping
patterns was estimated to cost from one-third to
one-quarter the cost of using all 28 wells at an
estimated cost of $155 million.
6.2. Efficient Pumping Operations
Removal of contaminated ground water should
be a dynamic process that uses information on the
response of the ground-water system to improve
the efficiency of pumping operations (Section
3.3). Elements of efficient pumping operations
can include
Combined plume containment and source
remediation, which can be achieved through
the design of the initial pumping flow field.
For example, at the LLNL site a line of
extraction wells at the downgradient margins
of the plume were established to prevent the
movement of contaminants toward municipal
water-supply wells, while other wells were
located in the source areas where the contami-
nant concentrations were highest. This limited
the area requiring remediation and maximized
contaminant removal (Hoffman, 1993).
Phased construction of extraction wells,
which allows data on the monitored response
of the aquifer to pumping operations to be
used in siting subsequent wells.
Adaptive pumping, which involves designing
the well field such that extraction and injec-
tion can be varied to reduce zones of stagna-
tion. Extraction wells can be periodically shut
off, others turned on, and pumping rates
varied to ensure that contaminant plumes are
remediated at the fastest rate possible. Figure
24 illustrates stagnation zones that would
develop at the LLNL site if a fixed pumping
well configuration were used. With this
approach, remediation at the site would take
about 100 years (Figure 25). Computer
modeling of adaptive pumping indicates that
this technique should make it possible to
reduce the time required for site cleanup to
about 50 years (Figure 25). Further refine-
ments in design might shorten the time even
further (Hoffman, 1993).
Pulsed pumping, which has the potential to
increase the ratio of contaminant mass
removed to ground-water volume where mass
transfer limitations restrict dissolved contami-
nant concentrations. Figure 26 illustrates the
concept of pulsed pumping. During the
resting phase of pulse pumping, contaminant
concentrations increase due to diffusion,
desorption, and dissolution in slower moving
ground water (Figure 11). Once pumping is
resumed, ground water with a higher concen-
tration of contaminants is removed, thus
increasing mass removal during pumping.
Special care must be taken to ensure that the
hydraulic containment objective is met during
pump rest periods. Bartow and Davenport
(1995) have reported that about 19 percent of
the pump-and-treat systems in Santa Clara
Valley, California, use some form of pulsed
pumping. A recent study by Harvey et al.
(1994), however, on the effects of physical
parameters (e.g., the mass transfer rate
coefficient) concluded that pulsed pumping
provides little if any advantage over continu-
ous pumping at an average rate.
40
-------
LEGEND
^. Ground-water
flow Line
O Extraction
location
'i^SHy- Areas ฐf potential
'J|||p'; ground-water
stagnation
*1"~ Total VOC (ppb)
A 3 isoconcentration
~-*i oCK- conto u rs ,
ฐ.^QGQ""" dashed where inferred
Scale : Feet
0 500 1000
All wells within VOC plumes are
contoured without regard to depth
&
Union Pacific
- . . . , 1 1, ,711 ,-t ' *-*'
/ / Pattsrson Pas
i/X ''^K /
x S / .Wkt ^X -*S X- J-..
s
Rhonewood Subdivision
Figure 24. Ground-water flow line in the vicinity of conceptual pumping centers at Lawrence Livermore National
Laboratory superimposed on an isoconcentration contour map and showing areas of potential
stagnation (Cohen et al., 1994, after Hoffman, 1993).
41
-------
1000
Figure 25.
Effect of adaptive
pumping on
cleanup time at
Lawrence
Livermore
National
Laboratory
Superfund site
(Cohen et al.,
1994, after
Hoffman, 1993).
10 15 20 25 30 35 40 45 50
Time (yr)
6.3. Treating Contaminated Ground
Water
Once extraction wells have brought contami-
nated water to the surface, treatment is relatively
straightforward, provided that appropriate meth-
ods have been selected and the capacity of the
treatment facility is adequate. Table 3 summarizes
the applicability of various treatment technologies
to ground water contaminated by any of the major
categories of inorganic and organic contaminants.
U.S. EPA (1995) describes conventional technolo-
gies that have evolved from industrial wastewater
treatment and that have been implemented at full
scale for treatment of contaminated ground water.
These methods fall into two main categories:
Biological. Biological treatment methods use
microorganisms to degrade organic com-
pounds and materials into inorganic products.
The methods may be applicable for treatment
of ground water contaminated by organic
compounds if concentrations are low enough
and the biological processes are not inhibited.
The best established biological treatment
methods include (1) activated sludge systems,
(2) a sequencing batch reactor, (3) powdered
activated carbon in activated sludge (bio-
physical system), (4) rotating biological
contactors, and (5) an aerobic fluidized bed
biological reactor.
42
-------
Figure 26.
The pulsed
pumping concept
(Cohen etal., 1994,
after Keely, 1989).
Time
Physical/Chemical. Physical, chemical, or a
combination of physical and chemical meth-
ods can be used to remove contaminants from
ground water. The most commonly used
methods include (1) air stripping, (2) acti-
vated carbon, (3) ion exchange, (4) reverse
osmosis, (5) chemical precipitation of metals,
(6) chemical oxidation, (7) chemically
assisted clarification, (8) filtration, and (9)
ultraviolet (UV) radiation oxidation.
Various emerging and innovative treatment
technologies, such as electrochemical separation
and wet air oxidation, are being tested. The EPA
reference sources identified for ground-water
treatment methods at the end of this guide (Sec-
tion 9) provide additional information on estab-
lished and innovative treatment technologies.
6.4. Monitoring Performance
An appropriately designed monitoring program
is essential for measuring the effectiveness of a
pump-and-treat system in meeting hydraulic
containment and aquifer restoration objectives. In
general, containment monitoring involves (1)
measuring hydraulic heads to determine if the
pump-and-treat system creates inward gradients
that prevent ground-water flow and dissolved
contaminant migration across the containment
zone boundary, and (2) ground-water quality
monitoring to detect any contaminant movement
or increase of contaminant mass across the
containment zone boundary. Aquifer restoration
monitoring mainly involves measurement of
contaminant concentrations in pumping and
observation wells to determine the rate and
effectiveness of mass removal. Cohen et al.
(1994) provide more detailed guidance on moni-
toring the performance of pump-and-treat sys-
tems.
6.4.1. Hydraulic Head Monitoring for
Containment
In general, the number of observation wells
needed for monitoring inward hydraulic gradients
in a containment area increases with site complex-
ity and with decreasing gradients along the
containment perimeter. Strategies for adequately
monitoring inward gradients and hydraulic
containment include (Cohen et al., 1994)
43
-------
1 Measuring hydraulic heads in three dimen-
sions using nested piezometers for detecting
vertical gradients. As shown in Figure 19,
partially penetrating wells may not create an
adequate vertical capture zone. Where leaky
confining layers separate aquifers, hydraulic
gradients should be toward the contaminated
zone. Figure 27 illustrates observations from
a nest of piezometers at the Chem-Dyne
Superfund site in Ohio. Water levels from the
deep piezometer are consistently about a foot
higher than in the intermediate and shallow
piezometers, indicating an upward gradient.
1 Monitoring water levels in observation wells
intensively during system startup and equili-
bration to determine an appropriate measure-
ment frequency. This may involve using
pressure transducers and dataloggers to make
near-continuous head measurements for a few
days or weeks, then switching sequentially to
daily, weekly, monthly, and possibly quarterly
monitoring. Data collected during each phase
should provide the justification for any
subsequent decrease in monitoring frequency.
1 Making relatively frequent hydraulic head
measurements when the pumping rates or
locations are modified, or when the system is
significantly perturbed in a manner that has
not been evaluated previously. Significant
new perturbations can arise from, for ex-
ample, unusual recharge, flooding, drought,
and new off site well pumping.
1 Measuring hydraulic head as close to the
same time as possible when monitoring
inward hydraulic gradients or a potentiomet-
ric surface so that data are temporally consis-
tent. This ensures that differences in ground-
water elevation within a network represent
spatial rather than temporal variations.
Supplementing hydraulic head data with flow-
path analysis using potentiometric maps or
particle tracking computer codes. Figure 28
shows that ground water can flow between
and beyond recovery wells even though
hydraulic heads throughout the mapped
aquifer are higher than the pumping level.
Conducting an analysis to determine if
containment is threatened or lost when
hydraulic head data do not indicate a clear
inward gradient. Rose diagrams can be
prepared to display the variation over time of
hydraulic gradient direction and magnitude
based on data from at least three wells (Figure
29). Even when the time-averaged flow is
toward the pump-and-treat system, contain-
ment can be compromised if contaminant
escapes from the larger capture zone during
transient events or if a net component of
migration away from the pumping wells
occurs overtime.
6.4.2. Ground-Water Quality Monitoring
for Containment
Monitor well locations and completion depths
should be selected to provide a high probability of
detecting containment system leaks in a timely
manner. Consequently, monitor wells with
relatively close spacing are usually located along
or near the potential downgradient containment
boundary. Ground-water quality sampling usually
is performed less frequently than the measuring of
hydraulic head because contaminant movement is
a slower process. Because ground-water quality
monitoring is more expensive than hydraulic head
monitoring, designing a cost-effective monitoring
plan requires special care. Strategies that may
help reduce costs without compromising the
integrity of the program include (Cohen et al.,
1994)
44
-------
Table 3. Applicability of Treatment Technologies to Contaminated Ground Water (U.S. EPA, 1991)
Contaminants
Metals
Heavy metals
Hexavalent chromium
Arsenic
Mercury
Cyanide
Corrosives
Volatile organics
Ketones
Semivolatile organics
Pesticides
Dioxins
Oil and grease/floating
products
(
eutralizatiol
-z.
X
X
X
X
X
X
X
X
X
X
recipitation
Q_
0
X
X
X
0
0
(5
3
D)
ro
o
o
c
n
oprecipitatil
O
X
X
X
X
X
0
0
V/Ozone
^>
X
X
0
X
X
0
o
X
c
o
"(0
-Q
hemical Ox
O
X
X
0
X
X
X
eduction
or
O
X
X
X
X
X
X
X
X
istillation
Q
X
X
X
X
X
O
ir Stripping
<
X
X
X
X
X
X
X
X
X
D)
c
'o.
Q.
&
E
ro
-2
OT
X
X
X
X
X
X
0
X
c
o
_Q
ctivated Ca
<
O
O
0
X
X
X
X
vaporation
LU
X
X
X
X
X
0
0
0
g
"ro
ro
Q.
CD
X
0
o
X
X
X
X
0
0
0
c
o
'-*-!
ro
"5
LL
X
X
X
X
X
X
X
X
0
0
0
*
c
o
U-*
ro
L_
CO
Q.
-------
568
567
B 566
g
>
.S?
LU 565
564
563
562
565.36
Jan Feb ' Mar ' Apr ' May ' June ' July ' Aug ' Sept ' Oct
1992
Nov Dec
Explanation
Water-Level Measurement in Deep Piezometer
Water-Level Measurement in Intermediate Piezometer
Water-Level Measurement in Shallow Piezometer
Daily Average Water Level From Hourly Data Recorded
in Shallow Piezometer
Time-Averaged Water Level in Deep Piezometer
Time-Averaged Water Level in Intermediate Piezometer
Time-Averaged Water Level in Shallow Piezometer
Figure 27. Nested piezometer hydrograph for 1992 at the Chem-Dyne Superfund site (Cohen et al., 1994, after Papadopulos &
Associates, 1993).
46
-------
Figure 28.
Ground-water
flow between and
beyond the
extraction wells,
resulting even
though hydraulic
heads throughout
the mapped
aquifer are higher
than the pumping
level (Cohen et
al., 1994).
Elevation in
Each Well Is
360.0ft
Sampling more frequently and performing
more detailed chemical analyses in the early
phase of the monitoring program, and using
the information gained to optimize sampling
efficiency and reduce the spatial density and
temporal frequency of sampling in the later
phases.
Monitoring ground-water quality in perimeter
and near-perimeter leak detection wells more
frequently than in wells that are at a greater
distance from the contaminant plume limit.
Specifying sampling frequency based on
potential containment failure migration rates
that factor in hydraulic conductivity and
effective porosity of the different media as
well as the maximum plausible outward
hydraulic gradients. Consider more frequent
sampling of more permeable strata in which
migration might occur relatively quickly as
compared to the sampling frequency for less
permeable media.
1 Focusing chemical analyses on site contami-
nants of concern and indicator constituents
after performing detailed chemical analyses
during the remedial investigation or the early
phase of a monitoring program. Conduct
more detailed chemical analyses less fre-
quently or when justified based on the results
of the more limited analyses.
47
-------
6.4.3. A quifer Restoration Monitoring
Aquifer restoration monitoring consists of three
main elements:
Ground-water sampling from all extraction
wells and selected observation wells within
the contaminant plume to interpret cleanup
progress. Parameters analyzed should include
(1) the chemicals of concern, (2) chemicals
that could affect the treatment system, such as
iron, which can precipitate and clog treatment
units if ground water is aerated, and (3)
chemicals that may indicate the occurrence of
other processes of interest, such as dissolved
oxygen, carbon dioxide, and biodegradation
products. These sampling data are important
for making adjustments for efficient well
operation (Section 6.2).
1 Periodic sampling and chemical analysis of
aquifer materials from representative loca-
tions in the contamination zone to measure
removal of nondissolved contaminants.
North
West
Figure 29.
Example display of ground-water
flow directions and hydraulic
gradients determined between
three observation wells (Cohen et
al., 1994).
East
South
48
-------
1 Regular sampling and analysis of treatment
system influent and effluent to assess (1)
treatment system performance, (2) change in
influent chemistry that may influence treat-
ment effectiveness, and (3) dissolved con-
taminant concentration trends. Figure 30
shows influent and effluent VOC concentra-
tions for the first 6 years of operation at the
Chem-Dyne Superfund site treatment plant.
Influent concentrations data showed a large
drop in the first year, and then a more gradual
decline over the next 5 years due to tailing
effects (Section 4).
The simplest indicator of progress in removing
ground-water contaminants is a plot of the
cumulative mass removed from the aquifer as
measured by influent concentrations to the
treatment system. Figure 31 shows the cumulative
mass of VOC removal at the Chem-Dyne site.
Approximately 27,000 pounds of VOCs have been
removed since the system became operational. As
is apparent from both Figures 30 and 31, however,
the rate of removal slowed significantly in the
sixth year. Consequently, removal of the remain-
ing one-third of the in-place mass will take much
longer than 6 years.
12
1987
1988
1989
1990
1991
1992
Year
Influent
O Effluent
Figure 30. Influent and effluent VOC concentrations (mg/L) at the Chem-Dyne treatment plant from 1987 to 1992
(Cohen etal., 1994, after Papadopulos & Associates, 1993).
49
-------
30,000
.Q
.E 25,000
O
O
-t-J
m
"3
20,000
Q- 15,000
>,
^ 10,000
o
5,000
1987
1988
1989
Year
1990
1991
1992
Figure 31. Cumulative mass ofVOCs removed from the aquifer at the Chem-Dyne site from 1987 to 1992 (Cohen
etal., 1994, after Papadopulos & Associates, 1993).
6.5. Evaluating Restoration Success
and Closure
Ground-water restoration, as operationally
defined, is achieved when a predefined cleanup
standard is attained and sustained. Figure 32
outlines procedures for determining the success
and/or timeliness of closure of a pump-and-treat
system. U.S. EPA (1992) defines six stages of
remediation using water quality data from a single
well (Figure 33):
Stage 1. Site evaluation to determine the need
for and conditions of a remedial action; define
cleanup standard.
1 Stage 2. Operation of the remediation system,
during which contaminant concentrations
decline.
1 Stage 3. Conclusion of treatment after con-
taminant concentrations have remained below
the cleanup standard for a sufficient period of
time based on expert knowledge of the
ground-water system and data collected
during pump-and-treat operations.
1 Stage 4. Post-termination monitoring of water
levels and contaminant concentrations to
determine when the ground-water flow
system is reestablished.
50
-------
Define Attainment Objectives and
Cleanup Standard
Develop Sampling and Analysis
Plan for Performance
Monitor System
Performance
No
Continue/
Modify
Treatment
Is the Cleanup
Standard Reached?
Demonstration of
Technical
Impracticability
Modify
Remedial
Action
Objectives
Allow System to Reach
Steady-State
Assess and Revise
Treatment Design as
Necessary
Verify the Attainment of
Cleanup Standard
Figure 32.
Determining the
success and/or
timeliness of
closure of a
pump-and-treat
system (Cohen et
al., 1994).
Is the Cleanup
Standard Maintained
Over Time?
Yes
Monitor as
Necessary
51
-------
1.2 T
Start
Treatment
g
'4-J
ro
End Sampling
Declare Clean or
Contaminated
o
O
o
CD
a)
ro
0.6
0.4 . .
0.2 -
Date
Figure 33. Stages of remediation in relation to example contaminant concentrations in a well at a pump-and-treat
site (U.S. EPA, 1992).
Stage 5. Sampling to assess attainment of the
cleanup standard. If the treatment standard is
not met, the treatment design may need to be
assessed and revised (Figure 32).
Stage 6. Declaration that the aquifer is clean
or still contaminated based on data collected
during Stage 5.
Cohen et al. (1994) and U.S. EPA (1992)
address in more detail the types of statistical
techniques that are required to analyze short-term
and long-term trends in contaminant concentra-
tions.
52
-------
Numerous variations and enhancements of
pump-and-treat systems are possible. Major types
include
Using trenches or drains in combination with
or to replace vertical pumping wells (Section
7.1). Where site conditions are favorable (i.e.,
shallow contamination), trenches are a
commonly used method for intercepting
contaminated ground water.
Using horizontal wells or trenches to replace
or complement vertical wells (Section 7.1).
Recent developments in directional drilling
technology make the use of horizontal or
inclined wells an attractive alternative ap-
proach.
Inducing fractures in the subsurface to
improve the yield of wells (Section 7.1).
Although widely used by the petroleum
industry, the use of induced fractures is
considered an emerging technology in
ground-water remediation with applications
limited to contaminated ground water in low-
permeability materials.
Implementing vadose zone source control and
remediation, often as a necessary adjunct to
ground-water cleanup (Section 7.2).
Making chemical enhancements, which can
have the potential to accelerate aquifer
remediation (Section 7.3).
Making biological enhancements, which can
present opportunities for eliminating or
reducing the requirements for surface treat-
ment of contaminated ground water (Section
7.4).
Notably few alternatives to pump-and-treat
systems are without requirements for continuous
energy input for pumping fluids (Section 7.4).
7.1. Alternative Methods for Fluid
Delivery and Recovery
Conventional pump-and-treat systems usually
involve extraction wellsand possibly injection
wellsplaced vertically in an aquifer. Alternative
methods of delivery and recovery of contaminated
ground water might enhance the performance of a
pump-and-treat system, especially while interim
measures are undertaken, by improving the
effectiveness of containment. These methods also
might augment the performance of a variety of
remedial actions selected as possible long-term
remedies. Major alternatives include
Interceptor Trenches. After vertical wells,
trenches are the most widely used method for
controlling subsurface fluids and recovering
contaminants. They function similarly to
horizontal wells, but also can have a signifi-
cant vertical component, which cuts across
and can allow access to the permeable layers
in interbedded sediments. For shallower
applications, trenches can be installed at
relatively low cost using conventional equip-
ment. Recent innovations combine trench
excavation and well screen installation into a
53
-------
single step for depths up to 20 feet (U.S. EPA,
1994). Where depth is not a constraint,
interceptor trenches are generally superior to
vertical wells. In such situations, they are
especially effective in low-permeability
materials and heterogeneous aquifers.
Horizontal and Inclined Wells. Relatively
recent advances in directional drilling tech-
nology, which use specialized bits to curve
bores in a controlled arc, have revolutionized
the field of well design. Directional drilling
methods can create wellbores with almost any
trajectory. Wells that curve to a horizontal
orientation are especially suited to environ-
mental applications (Figure 34).
Induced Fractures. EPA research has shown
that petroleum engineering technology used
to induce fractures for increased productivity
of oil wells also can improve the performance
of environmental wells. Induced fractures are
used mainly where low-permeability aquifer
materials create problems for the recovery of
contaminants.
Table 4 rates the potential applications of
alternative methods for delivery or recovery of
subsurface fluids in relation to (1) access, (2)
depth, (3) recovered phases, (4) geology, and (5)
availability. Figure 35 illustrates two ways in
which horizontal wells or trenches can be used to
intercept a contaminant plume. In many applica-
tions, deciding between use of a trench or a
horizontal well hinges on economic rather than
technical issues, with trenches generally being
more cost effective at depths less than 20 feet and
horizontal wells being generally more cost
effective at depths greater than 20 feet. Cost
savings can be substantial compared to vertical
well systems. For example, initial remediation
plans at a site in North Carolina called for 100
vertical wells to recover a hydrocarbon plume at
an estimated cost of $1 million. Instead, a con-
tinuous excavation and completion system was
installed for less than $350,000 (U.S. EPA, 1994).
EPA's Manual Alterative Methods for Fluid
Delivery and Recovery (U.S. EPA, 1994) provides
more detailed information on design consider-
ations and applications of these methods.
7.2. Vadose Zone Source Control
Removal of contaminants from the vadose
(unsaturated) zone is an essential part of any
remedial action plan to clean up contaminated
ground water. Major methods include
Capping to reduce infiltration of precipitation.
Excavation to remove contaminated soil for
ex situ treatment, which is most commonly
used where contaminants have not penetrated
deeply into the subsurface.
Soil vapor extraction (SVE), which is used to
extract volatile organic contaminants by
flushing with air, and bioventing, a SVE
system in which the addition of nutrients
further enhances the biodegradation of
organic contaminants. Both techniques,
considered innovative technologies a few
years ago, are widely used.
In situ thermal technologies to enhance the
mobility of volatile and semivolatile organic
contaminants; for example, steam-enhanced
extraction and radio frequency heating are
promising innovative technologies.
7.3. Physical and Chemical En-
hancements
Physical and chemical enhancements to pump-
and-treat systems primarily function by enhancing
the mobility of contaminants, thus increasing their
recovery in ground water that has been pumped to
the surface for treatment. Some chemical en-
hancements transform contaminants in place in
the subsurface to reduce toxicity.
54
-------
(a)
(b)
(c)
(1)
Figure 34. Some applications of horizontal wells: (a) intersecting flat-lying layers, (b) intercepting plume elongated
by regional gradient, (c) intersecting vertical fractures, and (d) access beneath structures (U.S. EPA,
1994).
55
-------
Table 4. Issues Affecting Application of Alternative Methods for Delivery or Recovery (U.S. EPA, 1994)
Issue Horizontal Well Induced Fracture
Trench
Access
Fragile structures
over target
Poor access over
target
Depth
<6m
6-20 m
>20 m
Recovered Phase
Aqueous
LNAPL
Minimal surface disturbance
Standoff required
1 m minimum depth
Cost of guidance system
increases at >6 m
No depth limit within
environmental applications
Requires accurate drilling;
best if water table fluctuations
are minor
Evaluate effects of surface
displacement
Possible with horizontal
well
1-2 m minimum depth
No depth limit within
environmental applications
Best with access to
individual fractures
Excavation expected to be
infeasible
Excavation expected to be
infeasible
Installation with common
equipment
ฎ Excavation costs increase
with depth
* Specialized excavation
methods required
Widely used to ensure
capture; accommodates
water table fluctuations
DNAPL
Requires accurate drilling and
site characterization
Caution; steeply dipping
fractures may cause
downward movement
Assuming mobile phase
present and accurately
located
Vapor
Consider omitting gravel pack
to save costs
Best with access to
individual fractures
Requires tight seal on top
of trench
Geology
Normally
consolidated clay
Swelling clay
Silty clay till
Stratified sediment
or rock
ฎ Smearing of bore wall may
reduce performance
ฎ Smearing of bore wall may
reduce performance
ฎ Smearing of bore wall may
reduce performance
* Anisotropy may limit vertical
influence of well
* Induced fractures may be
vertical and limited in size
Relatively large, gently
dipping fractures expected
Relatively large, gently
dipping fractures expected
ฎ Stratification may limit
upward propagation and
increase fracture size
Large discharge expected
relative to alternatives
Large discharge expected
relative to alternatives
Large discharge expected
relative to alternatives
Good way to access many
thin beds or horizontal
partings
(Continued)
56
-------
Table 4. (Continued)
Issue
Horizontal Well
Induced Fracture
Trench
Vertically fractured
sediment or rock
Coarse gravel
Thick sand
Orient well normal to
fractures when possible
Possible problems with hole
stability; penetrating cobbles
May be difficult to access top
and bottom of formation; hole
stability problems
Good where induced
fractures cross-cut natural
fractures
(overconsolidated
sediment and rock)
Permeability enhancement
may be unnecessary
Permeability enhancement
may be unnecessary
Orient trench
perpendicular to natural
fractures when possible
Stability a concern during
excavation
Stability a concern during
excavation
Rock
Feasible, but drilling costs
more in rock than in sediment
Widely used in oil, gas,
and water wells drilled in
rock
Excavation difficult but
blasting possible to make
trench-like feature
Availability
Current Experience
(Approximate)
10 to 20 companies with capabilities;
nationwide coverage but may require
equipment mobilization
150 to 250 wells at 50 to 100 sites
Several companies offer service;
nationwide coverage with
equipment mobilization
200 to 400 fractures at 20 to 40
sites
Shallow trench (<6 m)
installation widely available
from local contractors; deep
trench will require mobilization
1,000+ trenches at many
hundreds of sites
Key
Good application
ฎ Moderately good
* Fair, with possible technical difficulties
Poor; not recommended using available methods
57
-------
Horizontal Well
Capture Zone
Source
(b)
Figure 35. Two approaches using trenches or horizontal wells to intercept contaminant plumes (U.S. EPA, 1994).
58
-------
7.3.1. Physical Enhancements
Air sparging, also known as in situ aeration, is
an approach that is similar to soil vapor extrac-
tion except that air is injected into the saturated
zone rather than the vadose zone (Figure 36). Air
sparging systems can effectively remove a
substantial amount of volatile aromatic and
chlorinated hydrocarbons in a variety of geologic
settings, but significant questions remain about
the ability of this technology to achieve health-
based standards throughout the saturated zone
(NRC, 1994). Thermal enhancements, such as
steam and hot-water flooding, increase the
mobility of volatile and semivolatile contami-
nants. Use of induced fractures (Section 7.1) is
another form of physical enhancement to pump-
and-treat systems.
7.3.2. Chemical Enhancements
Chemically enhanced pump-and-treats systems
require use of injection wells to deliver reactive
agents to the contaminant plume and extraction
wells to remove reactive agents and contaminants
(Figure 37). The major types of chemical en-
hancements are
Soil flushing, which enhances recovery of
contaminants with low water solubility, free-
product and residual NAPLs, and sorbed
contaminants. Two major types of chemical
agents can be used: (1) cosolvents, which,
when mixed with water, increase the solubil-
ity of some organic compounds, and (2)
surfactants, which may cause contaminants
to desorb and may increase NAPL mobility
by lowering the interfacial tension between
the NAPL and water, increasing the solubil-
ity. Soil flushing is one of the most promis-
ing innovative technologies for dealing with
separate phase DNAPLs in the subsurface
(NRC, 1994).
In situ chemical treatment, which involves
reactive agents that oxidize or reduce con-
taminants, converting them to nontoxic forms
or immobilizing them to minimize contami-
nant migration. This innovative technology is
still in the early stages of development.
The EPA report Chemical Enhancements to
Pump-and-Treat Remediation (Palmer and Fish,
1992) provides additional information on techni-
cal issues related to this topic.
7.4. Biological Enhancements
Biological enhancements to pump-and-treat
systems stimulate subsurface microorganisms,
primarily bacteria, to degrade contaminants to
harmless mineral end products, such as carbon
dioxide and water. In situ bioremediation of
certain types of hydrocarbons (primarily petro-
leum products and derivatives), encouraged by
addition of oxygen and nutrients to the ground
water, is an established technology. Other readily
biodegradable substances, such as phenol, cresols,
acetone, and cellulosic wastes, are also amenable
to aerobic in situ bioremediation. Key elements in
such a system are delivery of oxygen and nutri-
ents by use of an injection well (Figure 38a) or an
infiltration gallery (Figure 38b). A limitation of in
situ bioremediation is that minimum contaminant
concentrations required to maintain microbial
populations may exceed health-based cleanup
standards, particularly where heavier hydrocar-
bons are involved.
In situ bioremediation of chlorinated solvents is
less well demonstrated because metabolic pro-
cesses for their degradation are more complex
than those for hydrocarbon degradation (NRC,
1994). Nonetheless, methanotrophs are able to
degrade some chlorinated solvents under aerobic
conditions if methane is supplied as an energy
source. Also, the ability of anaerobic bacteria to
degrade a variety of chlorinated solvents is well
59
-------
(a)
Air Compressor
Vent to
Atmosphere
Surface Soil/Cap
Vapor
Treatment
Unsaturated Zone
Saturated Zone
Streamtubes of Air
Direction of Ground-
Water Flow
(b)
Figure 36.
Process diagram
for air sparging
with (a) vertical
wells, and (b)
horizontal wells
(after NRC,
1994).
Injection Point for Flushing Gas
I f^- Extraction of Contaminated Gas
Y I
A A A A A A A A A A
Surface Soil/Cap
Unsaturated Zone
Saturated Zone
60
-------
Injection
of Reactive
Agents
DISPOSAL
| Recovery of |
Reactive Agent |
|
Treatment
Extraction of
Reactive Agents
and Contaminants
Figure 37.
Schematic of chemical
enhancement of a pump-and-
treat system. Key areas of
concern are shown in boxes.
In some cases, the reactive
agent will be recovered and
reused (Palmer and Fish,
1992).
Delivery > Reaction > Removal-
documented. Two major obstacles to the use of
anaerobic processes for in situ bioremediation are
that (1) hazardous intermediate degradation
products can accumulate, and (2) undesirable
water quality changes, such as dissolution of iron
and manganese, can occur.
EPA reference sources identified at the end of
this guide (Section 9) that are particularly relevant
to in situ bioremediation include Norris et al.
(1993), Sims et al. (1992), and U.S. EPA (1993,
1994).
7.5. Alternatives to the Pump-and-
Treat Approach
Nearly all approaches to ground-water cleanup
involve some degree of ground-water pumping.
Even when containment is the primary objective,
low-flow pump-and-treat systems are usually
required to prevent the escape of contaminated
water from the confined area. Two remediation
approaches that eliminate pumping as a compo-
nent of the system are (1) intrinsic bioremedia-
tion, and (2) in situ reactive barriers. Although
both of these methods show promise, they are still
61
-------
(a)
To Sewer or
Recirculate
Sparger
(b)
Air Compressor or
Hydrogen Peroxide
Tank
Nutrient Addition
Infiltration Gallery
Figure 38.
Two types of
aerobic in situ
bioremediation
systems: (a)
injection well with
sparger, (b)
infiltration gallery
(Sims et al.,
1992, after
Thomas and
Ward, 1989).
Trapped Hydrocarbons ,., .
Table
Recirculated Water
and Nutrients
Recovery Well
Monitoring Well
62
-------
in development and their effectiveness remains to
be demonstrated.
7.5.1. Intrinsic Bioremediation
Intrinsic bioremediation relies on indigenous
microbes to biodegrade organic contaminants,
without human intervention in the form of supply-
ing electron acceptors, nutrients, and other
materials. The processes that occur are the same
as those in engineered bioremediation systems,
but they occur more slowly. A decision to refrain
from active site manipulation does not eliminate
the need to conduct ground-water sampling within
the contaminant plume to document that biodeg-
radation is occurring. Moreover, sampling would
still need to be performed outside the contami-
nated area to identify any offsite migration of
contaminants that might require initiation of more
active remedial measures (Figure 39b). There is a
greater risk of failure with intrinsic bioremedia-
tion compared to engineered bioremediation
because no active measures are used to control the
contaminant plume. The possible perception that
intrinsic bioremediation is the equivalent to doing
nothing is also a barrier to its acceptance (NRC,
1994).
7.5.2. In Situ Reactive Barriers
The concept of using permeable in situ reactive
barriers to treat a contaminant plume as it moves
through an aquifer under natural hydraulic
gradients (Figure 39c and 39d) was first suggested
by McMurty and Elton (1985), but it has only
recently begun to receive significant attention
from the research community (Starr and Cherry,
1994). The funnel-and-gate concept, which
combines impermeable barriers to contain and
channel the flow of the contaminant plume toward
the reactive barrier has received the most attention
because numerous possible configurations can be
developed to address different types of contami-
nant plumes and geologic settings (Figure 40).
Depending on the contaminants present in the
plume, the reactive zone uses a combination of
physical, chemical, and biological processes.
The great promise of in situ reactive barriers is
that they will require little or no energy input once
installed, yet provide more active control and
treatment of the contaminant plume than intrinsic
bioremediation. The main engineering challenges
involve provision of suitable amounts of reactive
materials in a permeable medium and proper
placement to avoid short-circuiting the contact
between the gate and the cutoff wall.
63
-------
Contaminant Source
Zone
Extraction Well
(b)
In Situ Reaction Curtain'
Remediated
Plume
(d)
In Situ Reactor
'Gate1
Figure 39. Alternative ground-water plume management options: (a) pump-and-treat system, (b) intrinsic
bioremediation, (c) in situ reaction curtain, (d) funnel-and-gate system (adapted from Starr and
Cherry, 1994).
64
-------
(a)
Single Gate System
(b)
Multiple Gate System Multiple Reactor Systems
Fully Penetrating Gate
Hanging Gate
Figure 40. Funnel-and-gate configurations (Starr and Cherry, 1994).
65
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Bartow, G. and C. Davenport. 1995. Pump-and-
Treat Accomplishments: A Review of the
Effectiveness of Ground Water Remediation
in Santa Clara Valley, California. Ground
Water Monitoring and Remediation
15(2): 140-146.
Berglund, S. andV. Cvetkovic. 1995. Pump-and-
Treat Remediation of Heterogeneous Aqui-
fers: Effects of Rate-Limited Mass Transfer.
Ground Water 33(4):675-685.
Blandford, T.N. and P.S. Huyakorn. 1991. WHPA:
Modular Semi-Analytical Model for the
Delineation of Wellhead Protection Areas,
Version 2.0. Office of Ground Water Protec-
tion; Available from EPA Center for Subsur-
face Modeling Support, Ada, OK. Version 1.0
was released in 1990 [Four modules:
MWCAP, RESSQC, GPTRAC, MONTEC;
most current disk version is 2.1]
Bradbury, K.R., M.A. Muldoon, A. Zaporozec,
and J. Levy. 1991. Delineation of Wellhead
Protection Areas in Fractured Rocks. EPA/
570/9-91-009. Office of Water, Washington,
DC. 144 pp.
Cohen, RM. and J.W Mercer. 1993. DNAPL Site
Evaluation. EPA/600/R-93/002 (NTIS PB93-
150217). R.S. Kerr Environmental Research
Laboratory, Ada, OK. [Also published by
Lewis Publishers as C.K. Smoley edition,
Boca Raton, FL. 384 pp.]
Cohen, R.M., A.H. Vincent, J.W. Mercer, C.R.
Faust, and C.P. Spalding. 1994. Methods for
Monitoring Pump-and-Treat Performance.
EPA/600/R-94/123. R.S. Kerr Environmental
Research Laboratory, Ada, OK. 102 pp.
Clausen, J.L. and D.A. Solomon. 1994. Character-
ization of Ground Water Plumes and DNAPL
Sources Using a Driven Discreet-Depth
Sampling System. Ground Water Manage-
ment 18:435-445 (Proc. of 8th Nat. Outdoor
Action Conf. on Aquifer Remediation,
Ground Water Monitoring and Geophysical
Methods).
Feldman, PR and D.J. Campbell. 1994. Evaluat-
ing the Technical Impracticality of Ground-
Water Cleanup. Ground Water Management
18:595-608 (Proc. of 8th Nat. Outdoor Action
Conf. on Aquifer Remediation, Ground Water
Monitoring and Geophysical Methods).
Freeze, RA. and J.A. Cherry. 1989. What Has
Gone Wrong? Ground Water 27(4):458-464.
Gillham, R.W., E.A. Sudicky, J.A. Cherry, and
E.O. Frind. 1984. An Advective-Diffusion
Concept to Solute Transport in Heterogeneous
Unconsolidated Geologic Deposits. Water
Resour. Res. 20(3):369-378.
Gore lick, S.M., R.A. Freeze, D. Donohue, and
J.F. Keely. 1993. Groundwater Contamina-
tion: Optimal Capture and Containment.
Lewis Publishers: Boca Raton, FL. 416 pp.
66
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Haitjema, H.M., J. Wittman, V. Kelson, and N.
Bauch. 1994. WhAEM: Program Documenta-
tion for the Wellhead Analytic Element
Model. EPA/600/R-94/210, 120 pp. Available
from EPA Center for Subsurface Modeling
Support, Ada, OK. [Includes Geographic
Analytic Element Preprocessor (GAEP) and
Capture Zone Analytic Element Model
(CZAEM)]
Haley, J.L., B. Hanson, C. Enfield, and J. Glass.
1991. Evaluating the Effectiveness of Ground
Water Extraction Systems. Ground Water
Monitoring Rev. 11(1): 119-124. [Summary of
U.S. EPA (1989)]
Harvey, C.F., R. Haggerty, and S.M. Gorelick.
1994. Aquifer Remediation: A Method for
Estimating Mass Transfer Rate Coefficients
and an Evaluation of Pulsed Pumping. Water
Resour. Res. 30(7): 1979-1991.
Hoffman, F. 1993. Ground-Water Remediation
Using "Smart Pump and Treat." Ground
Water31(l):98-106.
Keely, J.F. 1989. Performance Evaluation of
Pump-and-Treat Remediations. Superfund
Issue Paper. EPA/540/8-89/005. RS. Kerr
Environmental Research Laboratory, Ada,
OK. 14 pp.
Knox, R.C., L.W. Canter, D.F. Kincannon, E.L.
Stover, and C.H. Ward. 1984. State-of-the Art
of Aquifer Restoration. EPA/600/2-84/
182a&b (National Technical Information
Service [NTIS] PB85-181071 andPB85-
181089). RS. Kerr Environmental Research
Laboratory, Ada, OK.
Mackay, D.M. and J.A. Cherry. 1989. Groundwa-
ter Contamination: Pump-and-Treat Remedia-
tion. Environ. Sci. Technol. 23(6):630-636.
Marquis, Jr., S. 1995. Don't Give Up on Pump
and Treat: Enhance It with Bioremediation.
Soils & Groundwater Cleanup, August-
September, pp. 46-50.
McMurty, D.C., and RO. Elton. 1985. New
Approach to In-Situ Treatment of Contami-
nated Groundwaters. Environ. Progress
4(3): 168-170.
National Research Council (NRC). 1994. Alterna-
tives for Ground Water Cleanup. National
Academy Press. 336 pp.
Norris, R.D. et al. 1993. In-Situ Bioremediation
of Ground Water and Geological Material: A
Review of Technologies. EPA/600/R-93/124
(NTIS PB93-215564). R.S. Kerr Environmen-
tal Research Laboratory, Ada, OK. [13
authors; see also Norris et al., 1994]
Palmer, C.D. and W. Fish. 1992. Chemical
Enhancements to Pump-and-Treat Remedia-
tion. Ground Water Issue Paper. EPA/540/S-
92/001. R.S. Kerr Environmental Research
Laboratory, Ada, OK. 20 pp.
Papadopulos & Associates, Inc. and Conestoga-
Rovers & Associates Ltd. 1993. Chem-Dyne
Site Trust Fund; 1992 Annual Report. Chem-
Dyne Site, Hamilton, OH. April.
Piwoni, M.D. and J.W. Keeley. 1990. Basic
Concepts of Contaminant Sorption at Hazard-
ous Waste Sites. Ground Water Issue. EPA/
540/4-90/053. R.S. Kerr Environmental
Research Laboratory, Ada, OK. 7 pp.
Rogers, L.L., R.U. Dowla, and V.M. Johnson.
1995. Optimal Field-Scale Groundwater
Remediation Using Neural Networks and
Genetic Algorithm. Environ. Sci. Technol.
29(5): 1145-1155.
67
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Roy, W.R., I.G. Krapac, S.F.J. Chou, and R.A.
Griffin. 1992. Batch-Type Procedures for
Estimating Soil Adsorption of Chemicals.
EPA/530/SW-87/006F (NTIS PB92-146190).
Risk Reduction Engineering Laboratory,
Cincinnati, OH. 100 pp.
Satkin, RL. and P.B. Bedient. 1988. Effectiveness
of Various Aquifer Restoration Schemes
Under Variable Hydrogeologic Conditions.
Ground Water 26(4):488-498.
Sims, J.L., J.M. Suflita, and H.H. Russell. 1992.
In-Situ Bioremediation of Contaminated
Ground Water. Ground Water Issue Paper.
EPA/540/S-92/003. R.S. Kerr Environmental
Research Laboratory, Ada, OK. 11 pp.
Starr, R.C. and J.A. Cherry. 1994. In Situ Reme-
diation of Contaminated Ground Water: The
Funnel-and-Gate System. Ground Water
32(3):465-476.
Stock, O.D.L. et al. 1994. CZAEM User's Guide:
Modeling Capture Zones of Ground-Water
Wells Using Analytic Elements. EPA/600/R-
94/174, 58 pp. Available from EPA Center for
Subsurface Modeling Support, Ada, OK. [See
also, Haitjema et al., 1994]
Thomas, J.D., and C.H. Ward. 1989. In Situ
Biorestoration of Organic Contaminants in
the Subsurface. Environ. Sci. Technol.
23:760-786.
Travis, C.C. and C.B. Doty. 1990. Can Contami-
nated Aquifers at Superfund Site Be
Remediated? Environ. Sci. Technol.
24(1): 1464-1466.
U.S. Environmental Protection Agency (EPA).
1988. Guidance on Remedial Actions for
Contaminated Ground Water at Superfund
Sites. EPA/540/G-88/003. Office of Solid
Waste and Emergency Response (OSWER)
Directive 9283.1-2 (NTIS PB89-184618).
Office of Solid Waste and Emergency Re-
sponse, Washington, DC.
U.S. Environmental Protection Agency (EPA).
1989. Evaluation of Ground-Water Extraction
Remedies: Volume 1, Summary Report (EPA/
540/2-89/054, NTIS PB90-183583, 66 pp.);
Volume 2, Case Studies 1-19 (EPA/540/2-89/
054b); and Volume 3, General Site Data Base
Reports (EPA/540/2-89/054c). Office of Solid
Waste and Emergency Response, Washington,
DC.
U.S. Environmental Protection Agency (EPA).
1991. Handbook: Stabilization Technologies
for RCRA Corrective Actions. EPA/625/6-91/
026. Center for Environmental Research
Information, Cincinnati, OH. 62 pp.
U.S. Environmental Protection Agency (EPA).
1992. Methods for Evaluating the Attainment
of Cleanup Standards, Volume 2: Ground
Water. EPA/230/R-92/014. Office of Solid
Waste and Emergency Response, Washington,
DC.
U.S. Environmental Protection Agency (EPA).
1993. Guidance for Evaluation the Technical
Impracticability of Ground-Water Restora-
tion. EPA/540/R-93/080, OSWER 0234.2-25
(NTIS PB93-963507). Office of Solid Waste
and Emergency Response, Washington, DC.
U.S. Environmental Protection Agency (EPA).
1994. Manual: Alternative Methods for Fluid
Deliver and Recovery. EPA/625/R-94/003.
Center for Environmental Research Informa-
tion, Cincinnati, OH. 87 pp.
68
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U.S. Environmental Protection Agency (EPA). Wilson, J.L., S.H. Conrad, W.R. Mason, W.
1995. Manual: Ground-Water and Leachate Peplinski and E. Hagen. 1990. Laboratory
Treatment Systems. EPA/625/R-94/005. Investigation of Residual Liquid Organics.
Center for Environmental Research Informa- EPA/600/6-90/004. R.S. Kerr Environmental
tion, Cincinnati, OH. 119 pp. Research Laboratory, Ada, OK. 267 pp.
69
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The EPA publications listed below provide more
detailed information on the subjects discussed
in this document. Publications and additional
copies of this brochure can be obtained at no
charge (while supplies are available) from the
following sources:
EPA/625-series documents: Office of Research
and Development (ORD) Publications, P.O.
Box 19968, Cincinnati, OH 45219-0968;
phone 513 569-7562, fax 513 569-7562.
Other EPA documents: National Center for
Environmental Publications and Information
(NCEPI), 11029 Kenwood Road, Cincinnati,
OH 45242; fax 513 891-6685.
Other documents, for which an NTIS acquisition
number is shown can be obtained from the
National Technical Information Service
(NTIS), Springfield, VA 22161; 800 336-
4700, fax 703/321-8547.
Contaminant Transport and Fate
Huling, S.G. 1989. Facilitated Transport. Ground
Water Issue. EPA/540/4-89/003. R.S. Ken-
Environmental Research Laboratory, Ada,
OK. 5 pp.
Huling, S.C. and J.W. Weaver. 1991. Dense
Nonaqueous Phase Liquids. Ground Water
Issue. EPA/540/4-91/002. R.S. Kerr Environ-
mental Research Laboratory, Ada, OK. 21 pp.
McLean, J.E. and B.E. Bledsoe. 1992. Behavior
of Metals in Soils. Ground Water Issue. EPA/
540/S-92/018. R.S. Kerr Environmental
Research Laboratory, Ada, OK. 25 pp.
Palmer, C.D. and R.W. Puls. 1994. Natural
Attenuation of Hexavalent Chromium in
Ground Water and Soils. Ground Water Issue.
EPA/540/S-94/505. R.S. Kerr Environmental
Research Laboratory, Ada, OK. 13 pp.
Piwoni, M.D. and J.W. Keeley. 1990. Basic
Concepts of Contaminant Sorption at Hazard-
ous Waste Sites. Ground Water Issue. EPA/
540/4-90/053. RS. Kerr Environmental
Research Laboratory, Ada, OK. 7 pp.
Sims, J.L., J.M. Suflita, andH.H. Russell. 1991.
Reductive Dehalogenation of Organic Con-
taminates in Soils and Ground Water. Ground
Water Issue. EPA/540/4-91/054. RS. Ken-
Environmental Research Laboratory, Ada,
OK. 12 pp.
Wilson, J.L., S.H. Conrad, W.R Mason, W.
Peplinski, and E. Hagen. 1990. Laboratory
Investigation of Residual Liquid Organics.
EPA/600/6-90/004. RS. Kerr Environmental
Research Laboratory, Ada, OK. 267 pp.
Site Characterization
U.S. Environmental Protection Agency (EPA).
1991. Site Characterization for Subsurface
Remediation. EPA/625/4-91/026. Center for
70
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Environmental Research Information, Cincin-
nati, OH. 259 pp.
U.S. Environmental Protection Agency (EPA).
1992. Estimating the Potential for the Occur-
rence of DNAPL at Superfund Sites. OSWER
Publication 9355.4-07/FS. Office of Solid
Waste and Emergency Response, Washington,
DC.
U.S. Environmental Protection Agency (EPA).
1993. Evaluation of the Likelihood of
DNAPL Presence atNPL Sites. EPA/540/R-
93/002 (OSWER 0355.4-13). Office of Solid
Waste and Emergency Response, Washington,
DC.
U.S. Environmental Protection Agency (EPA).
1993. Use of Airborne, Surface and Borehole
Geophysical Techniques at Contaminated
Sites: A Reference Guide. EPA/625/R-92/007.
Center for Environmental Research Informa-
tion, Cincinnati, OH.
U.S. Environmental Protection Agency (EPA).
1993. Subsurface Characterization and
Monitoring Techniques: A Desk Reference
Guide; Vol. I: Solids and Ground Water; Vol.
II: The Vadose Zone, Field Screening and
Analytical Methods. EPA/625/R-93/003a&b.
Center for Environmental Research Informa-
tion, Cincinnati, OH.
Pump-and-Treat Systems
Cohen, R.M., A.H. Vincent, J.W. Mercer, C.R.
Faust, and C.P. Spalding. 1994. Methods for
Monitoring Pump-and-Treat Performance.
EPA/600/R-94/123. R.S. Kerr Environmental
Research Laboratory, Ada, OK. 102 pp.
Keely, J.F. 1989. Performance Evaluation of
Pump-and-Treat Remediations. Superfund
Issue Paper. EPA 540/8-89/005. R.S. Kerr
Environmental Research Laboratory, Ada,
OK. 14 pp.
Mercer, J.W., D.C. Skipp, and D. Giffm. 1990.
Basics of Pump-and-Treat Ground-Water
Remediation Technology. EPA/600/8-90/003.
R.S. Kerr Environmental Research Labora-
tory, Ada, OK. 58 pp.
Palmer, C.D. and W. Fish. 1992. Chemical
Enhancements to Pump-and-Treat Remedia-
tion. Ground Water Issue Paper. EPA/540/S-
92/001. R.S. Kerr Environmental Research
Laboratory, Ada, OK. 20 pp.
Repa, E. and D.P. Doerr. 1985. Leachate Plume
Management. EPA/540/2-85/004 (NTIS
PB86-122330). Hazardous Waste Engineering
Research Laboratory, Cincinnati, OH.
U.S. Environmental Protection Agency (EPA).
1989. Evaluation of Ground-Water Extraction
Remedies: Volume 1, Summary Report (EPA/
540/2-89/054, NTIS PB90-183583, 66 pp.);
Volume 2, Case Studies 1-19 (EPA/540/2-89/
054b); and Volume 3, General Site Data Base
Reports (EPA/540/2-89/054c). Office of Solid
Waste and Emergency Response, Washington,
DC.
U.S. Environmental Protection Agency (EPA).
1992. Evaluation of Ground-Water Extraction
Remedies, Phase II. Oswer Publication
9355.4-05, Vols. 1-2. Office of Solid Waste
and Emergency Response, Washington, DC.
U.S. Environmental Protection Agency (EPA).
1992. General Methods for Remedial Opera-
tions Performance Evaluation. EPA/600/R-92/
002. R.S. Kerr Environmental Research
Laboratory, Ada, OK. 37 pp.
U.S. Environmental Protection Agency (EPA).
1993. Guidance for Evaluating the Technical
71
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Impracticability of Ground-Water Restora-
tion. EPA/540/R-93/080, OSWER 0234.2-25
(NTIS PB93-963507). Office of Solid Waste
and Emergency Response, Washington, DC.
U.S. Environmental Protection Agency (EPA).
1993. Bioremediation Resource Guide. EPA/
542/B-93/004. Office of Solid Waste and
Emergency Response, Washington, DC.
[Includes annotated list of more than 80
significant references]
U.S. Environmental Protection Agency (EPA).
1994. Bioremediation in the Field. EPA/540/
N-94/501. Office of Solid Waste and Emer-
gency Response, Washington, DC. [Periodi-
cally updated; latest issue No. 11, July, 1994]
U.S. Environmental Protection Agency (EPA).
1994. Manual: Alternative Methods for Fluid
Delivery and Recovery. EPA/625/R-94/003.
Center for Environmental Research Informa-
tion, Cincinnati, OH. 87 pp.
U.S. Environmental Protection Agency (EPA).
1995. In Situ Remediation Technology Status
Report: Hydraulic and Pneumatic Fracturing.
EPA/542/K-94/005. Office of Solid Waste and
Emergency Response, Washington, DC. 15
pp.
Ground-Water Treatment Methods
Canter, L.W. and R.C. Knox. 1986. Ground Water
Pollution Control. Lewis Publishers: Chelsea,
MI. 526 pp. [Contains mostly same material
as Knox etal. (1984)]
Knox, R.C., L.W. Canter, D.F. Kincannon, E.L.
Stover, and C.H. Ward. 1984. State-of-the Art
of Aquifer Restoration. EPA 600/2-84/
182a&b (NTIS PB85-181071 and PB85-
181089). RS. Kerr Environmental Research
Laboratory, Ada, OK. [See also Canter and
Knox (1985)]
McArdle, J.L., M.M. Arozarena, and W.E.
Gallagher. 1987. A Handbook on Treatment
of Hazardous Waste Leachate. EPA/600/8-87/
006 (NTIS PB87-152328). Hazardous Waste
Engineering Research Laboratory, Cincinnati,
OH.
U.S. Department of Defense Environmental
Technology Transfer Committee (DOD/
ETTC). 1994. Remediation Technologies
Screening Matrix and Reference Guide. EPA/
542/B-94/013 (NTIS PB95-104782). Office
of Solid Waste and Emergency Response,
Washington, DC.
U.S. Environmental Protection Agency (EPA).
1994. Ground-Water Treatment Technology
Resource Guide. EPA/542/B-94/009. Office
of Solid Waste and Emergency Response,
Washington, DC. [Includes annotated list of
more than 60 significant references]
U.S. Environmental Protection Agency (EPA).
1994. Innovative Treatment Technologies
Annual Status Report, 6th ed. EPA/542/R-94/
005. Office of Solid Waste and Emergency
Response, Washington, DC.
U.S. Environmental Protection Agency (EPA).
1994. Superfund Innovative Technology
Evaluation Program: Technology Profiles, 7th
ed. EPA/540/R-94/526. Risk Reduction
Engineering Laboratory, Cincinnati, OH. 499
pp.
U.S. Environmental Protection Agency (EPA).
1995. Manual: Ground-Water and Leachate
Treatment Systems. EPA/625/R-94/005.
Center for Environmental Research Informa-
tion, Cincinnati, OH.
72
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In Situ Ground-Water Treatment
Norris, R.D. et al. 1993. In-Situ Bioremediation
of Ground Water and Geological Material: A
Review of Technologies. EPA/600/R-93/124
(NTIS PB93-215564). R.S. Kerr Environmen-
tal Research Laboratory, Ada, OK. [13 au-
thors; see also Norris et al., 1994]
Norris, R.D. et al. 1994. Handbook of Bioreme-
diation. Boca Raton, FL: Lewis Publishers.
272 pp. [Contains same material as Norris et
al., 1993]
Sims, J.L., J.M. Suflita, and H.H. Russell. 1992.
In Situ Bioremediation of Contaminated
Ground Water. Ground Water Issue Paper.
EPA/540/S-92/003. R.S. Kerr Environmental
Research Laboratory, Ada, OK. 11 pp.
U.S. Environmental Protection Agency (EPA).
1995a. In Situ Remediation Technology
Status Report: Thermal Enhancements. EPA/
542/K-94/009. Office of Solid Waste and
Emergency Response, Washington, DC. 22
pp.
U.S. Environmental Protection Agency (EPA).
1995b. In Situ Remediation Technology
Status Report: Surfactant Enhancements.
EPA/542/K-94/003. Office of Solid Waste and
Emergency Response, Washington, DC. 22
pp.
U.S. Environmental Protection Agency (EPA).
1995c. In Situ Remediation Technology
Status Report: Cosolvents. EPA/542/K-94/
006. Office of Solid Waste and Emergency
Response, Washington, DC. 6 pp.
U.S. Environmental Protection Agency (EPA).
1995d. In Situ Remediation Technology
Status Report: Electrokinetics. EPA/542/K-
94/007. Office of Solid Waste and Emergency
Response, Washington, DC. 20 pp.
U.S. Environmental Protection Agency (EPA).
1995e. In Situ Remediation Technology
Status Report: Treatment Walls. EPA/542/K-
94/004. Office of Solid Waste and Emergency
Response, Washington, DC. 26 pp.
Ground-Water Modeling
Bear, J., M.S. Beljin, and RR Ross. 1992.
Fundamentals of Ground-Water Modeling.
Ground Water Issue. EPA/540/S-92/005. R.S.
Kerr Environmental Research Laboratory,
Ada, OK. llpp.
Schmelling, S.G. and R.R. Ross. 1989. Contami-
nant Transport in Fractured Media: Models
for Decisionmakers. Ground Water Issue.
EPA/540/4-89/004. RS. Kerr Environmental
Research Laboratory, Ada, OK. 8 pp.
U.S. Environmental Protection Agency (EPA).
1988. Selection Criteria for Mathematical
Models Used in Exposure Assessments:
Ground-Water Models. EPA/600/8-88/075
(NTIS PB88-248752). Office of Health and
Environmental Assessment, Washington, DC.
[Contains summary tables and descriptions of
63 analytical solutions and 49 analytical and
numerical codes for evaluating ground-water
contaminant transport]
U.S. Environmental Protection Agency (EPA).
1994. Assessment Framework for Ground-
Water Model Applications. EPA/500/B-94/
003 (OSWER Directive 9029.00). Office of
Solid Waste and Emergency Response,
Washington, DC. 41 pp.
van der Heijde, P.K.M. 1994. Identification and
Compilation of Unsaturated/Vadose Zone
Models. EPA/600/R-94/028 (NTIS PB94-
157773). R.S. Kerr Environmental Research
Laboratory, Ada, OK.
73
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van der Heijde, P.K.M. and O.A. Einawawy. 1993. Ada, OK. [Summary information on models
Compilation of Ground-Water Models. EPA/ for porous media flow and transport,
600/R-93/118 (NTIS PB93-209401). R.S. hydrogeochemical models, stochastic models,
Kerr Environmental Research Laboratory, and fractured rock]
74
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