1

EPA/540/S-97/504

September 1997

United States

Environmental Protection

Agency

Office of Solid Waste

and Emergency

Response

Office of

Research and

Development

Ground Water Issue

Design Guidelines for Conventional

Pump-and-Treat Systems

National Risk Management Research Laboratory

Subsurface Protection and Remediation Division

Robert S. Kerr Environmental Research Center

Ada, Oklahoma

Superfund Technology Support Center for Ground Water

Technology Innovation Office

Office of Solid Waste and Emergency

Response, US EPA, Washington, DC

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

Director

1 GeoTrans, Inc., Sterling, VA 2 National Risk Management Research Laboratory, Office of Research

and Development, U.S. EPA, Ada, OK

Robert M. Cohen1, James W. Mercer1, Robert M. Greenwald1, and

Milovan S. Beljin2

The RCRA/Superfund Ground-Water Forum is a group of

scientists representing EPA’s Regional Superfund Offices,

committed to the identification and resolution of ground-water

issues affecting the remediation of Superfund sites. Design of

conventional ground-water extraction and injection (i.e., pumpand-treat) systems has been identified by the Forum as an issue

of concern to decision makers. This issue paper focuses on

design of conventional ground-water extraction and injection

systems used in subsurface remediation.

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

Randall Ross (405) 436-8611 at the Subsurface Remediation

and Protection Division of the National Risk Management

Research Laboratory, Ada, Oklahoma.

Introduction

Containment and cleanup of contaminated ground water are

among the primary objectives of the CERCLA (Comprehensive

Environmental Response, Compensation, and Liability Act; also

known as Superfund) and RCRA (Resource Conservation and

Recovery Act) remediation programs. Ground-water

contamination problems are pervasive in both programs; over

85 percent of CERCLA National Priority List sites and a substantial

portion of RCRA facilities have some degree of ground-water

contamination (U.S. EPA, 1993a). A common approach to deal

with contaminated ground water is to extract the contaminated

water and treat it at the surface prior to discharge or reinjection

as illustrated in Figure 1. This is referred to as conventional

pump-and-treat (P&T) remediation.

Conventional pump-and-treat is an applicable component of

many remedial systems. However, such a system will not be

appropriate to achieve restoration in portions of many sites due

to hydrogeologic and contaminant-related limitations such as

those presented by significant accumulations of DNAPLs

(denser-than-water nonaqueous phase liquids) trapped below

the water table. Such limitations will directly impact the

effectiveness of P&T at many sites and the selection of remedial

actions. Detailed discussion of the contaminant transport and

fate processes that limit the potential for subsurface restoration

using P&T and their characterization is beyond the scope of this

document.

Figure 1. Example of a P&T system (after Mercer et al., 1990).

Pumping Wells

Treatment Facility

Landfill

Injection

Well

Clay Cap

Clay Liner

Overburden Sand Silt Bedrock Flow Line

Water Table

Under Pumping

Conditions

Unconfined

Confined

SemiConfined

Unsaturated

Clay

• COMMUNICATION • TRAINING •

TECHNOLOGY SUPPORT

Technology

Support

Center
2

Inadequate design and implementation also may severely impact

the performance of a P&T system. Examples of design

inadequacies include too few recovery wells, insufficient pumping

rates, deficient well locations or completion intervals, and failure

to account for complex chemistry of contaminants. Similarly,

poor system operation, exemplified by excessive downtime or

failure to manipulate pumping schemes to limit ground-water

stagnation, will restrict P&T effectiveness. This document

provides guidance on designing conventional ground-water

P&T systems. Chemical enhancements to P&T and immiscible

contaminant recovery methods are addressed elsewhere (e.g.,

American Petroleum Institute, 1989, 1992; Palmer and Fish,

1992; U.S. EPA, 1992a, 1995; Grubb and Sitar, 1994; NRC,

1994).

P&T Remediation Strategies

In order to determine an appropriate strategy to manage

contaminated ground water, it is necessary first to evaluate site

conditions and define remediation goals. Historically, the goal

of ground-water remediation has been to protect human health

and the environment and to restore ground water to beneficial

uses where practicable. For ground water that is or may be used

for drinking, clean-up goals under CERCLA and RCRA generally

are set at drinking water standards such as Maximum

Contaminant Levels (MCLs) established under the Safe Drinking

Water Act. Other clean-up requirements may be appropriate

for ground water that is not used for drinking.

It has long been recognized that chemical transport from

contaminant source/release areas, such as abandoned landfills

and leaking tanks, contaminates ground water and other media

in downgradient areas (e.g., OTA, 1984). As such, a common

strategy for managing contaminated ground water has been to

remove or contain contaminant sources (e.g., by excavation,

construction of physical barriers, and/or pumping) and to address

downgradient contamination using P&T technology.

Strategies for managing ground-water contamination (Figure 2)

using P&T technology include: (1) hydraulic/physical

containment, (2) ground-water quality restoration, and (3) mixed

objective strategies. Several innovative technologies, such as

air sparging, engineered bioremediation, and permeable

treatment walls, can be used in conjunction with P&T, or alone,

to address these ground-water remediation objectives. At some

sites, natural attenuation processes may limit the need for P&T.

The management strategy selected depends on site-specific

hydrogeologic and contaminant conditions, and remediation

goals.

Hydraulic Containment

P&T systems are frequently designed to hydraulically control

the movement of contaminated ground water in order to prevent

continued expansion of the contamination zone. At sites where

the contaminant source cannot be removed (e.g., a landfill or

bedrock with DNAPLs), hydraulic containment is an option to

achieve source control. Hydraulic containment of dissolved

contaminants by pumping ground water from wells or drains has

been demonstrated at numerous sites. The concept is illustrated

in Figure 3. Properly controlled fluid injection using wells,

drains, or surface application (e.g., along the downgradient

periphery of the proposed containment area) and physical

containment options (e.g., subsurface barrier walls and surface

covers to limit inflow) can enhance hydraulic containment

systems by reducing the pumping rate required to maintain

containment. In many cases, hydraulic containment systems

are designed to provide long-term containment of contaminated

ground water or source areas at the lowest cost by optimizing

well, drain, surface cover, and/or cutoff wall locations and by

minimizing pumping rates.

Cleanup/Restoration

For sites where the contaminant source has been removed or

contained, it may be possible to clean up the dissolved plume.

P&T technology designed for aquifer restoration generally

combines hydraulic containment with more aggressive

manipulation of ground water (i.e., higher pumping rates) to

attain clean-up goals during a finite period. Ground-water

cleanup is typically much more difficult to achieve than hydraulic

containment. Hydrogeologic and contaminant conditions

favorable to cleanup (e.g., degradable dissolved contaminants

in uniform, permeable media) are summarized in Figure 4.

Mixed Objective Strategies

At many sites, P&T systems can be used to contain contaminant

source areas and attempt restoration of downgradient dissolved

plumes (Figure 2). A mixed P&T strategy is appropriate,

therefore, at sites where different portions of the contaminated

Figure 2. Several ground-water contamination management

strategies using P&T technology (after NRC, 1994;

Cherry et al., 1992).

Plume Containment

TODAY FUTURE

Plume Cut-Off

Aquifer Restoration

Downgradient Aquifer Restoration

Dissolved Plume Dissolved Plume

Pumping Well Pumping Well

Source

Area

Source

Area

Pumping Wells for Cleanup

Pump Well for

Source Containment

Pump Well for

Source Containment

Restored Aquifer

Restored Aquifer

Restored Aquifer

Containment Pumping

and Cut-Off Wall

Containment Pumping

and Cut-Off Wall

Downgradient Aquifer Restoration

Pumping Wells for Cleanup

Source Removed

Natural Attenuation
3

region are amenable to remediation using different methods. At

sites contaminated with LNAPLs (lighter-than-water NAPLs),

for example, a mixed remedial strategy may include: (1)

vacuum-enhanced pumping to recover free product, affect

hydraulic containment, and stimulate bioremediation in the

LNAPL release area; and (2) restoring downgradient ground

water via natural attenuation, P&T, and/or air sparging.

Characterizing Sites for P&T Design

The main goal of site characterization should be to obtain

sufficient data to select and design a remedy (NRC, 1994). This

is accomplished by investigating: (1) the nature, extent, and

distribution of contaminants in source areas and downgradient

plumes; (2) potential receptors and risks posed by contaminated

ground water; and (3) hydrogeologic and contaminant properties

that affect containment, restoration, and system design in

different site areas. Categories of data used to formulate a site

conceptual model for remedy evaluation are identified in Figure

5. The conceptual model is used to formulate remedial strategies

such as restoration and/or containment.

Inadequate site characterization can lead to flawed P&T design

and poor system performance. A complete understanding of a

contamination site is unobtainable, however, due to subsurface

complexities and investigation cost. Thus, characterization

efforts must develop sufficient data to select and design an

effective remedy while recognizing that significant uncertainties

about subsurface conditions will persist.

Site characterization for remedial design is an extensive subject,

key aspects of which are addressed briefly below. Additional

information regarding procedures and strategies for investigating

contamination sites is provided by U.S. EPA (1988a, 1991a,

1993b), Nielsen (1991), Cohen and Mercer (1993), Sara (1994),

CCME (1994), and Boulding (1995).

Using a Phased and Integrated Approach

Due to slow contaminant transport and interphase transfer,

many P&T systems will operate for decades to contain and

clean up contaminated ground water. Data collected during

investigation and remediation should be reviewed periodically

to refine the site conceptual model and identify modifications

that will improve P&T system performance. Thus, as depicted

in Figure 6, a phased and integrated approach should be taken

to site characterization and remediation. For example, given

significant uncertainty regarding well locations and pumping

rates needed to achieve remedial objectives, it may be prudent

to initiate pumping at several locations and then determine

system expansion requirements based on performance

monitoring data. This phased approach to system installation

may be more cost effective than grossly overdesigning the

system to account for uncertainty in subsurface characterization

at many sites.

During the initial phase of site investigation, prior studies and

background information are reviewed to identify likely

Figure 3. Examples of hydraulic containment in plan view and

cross section using an extraction well (a), a drain (b),

and a well within a barrier wall (c).

Figure 4. Generalized ground-water restoration difficulty scale

(modified from U.S. EPA, 1993a).

 Plume

Static Water Table

Capture

Zone Limit

(a) 993

 992

 991

 996

 995

 994

 998

 997

Static Water Table

Capture

Zone Limit

 996

 998

 997

 994

 993

 992

 991

 995

Drain

 991

Drain

Upgradient

Barrier Wall

Capture

Zone Limit

(c)

 997

 991

 992

Downgradient

Barrier Wall

 999

 991

 996

 995

 994

 998 998

 994

 993

Barrier

Wall

 993

 Plume

 Plume

 Plume

 992

 Plume

 Plume

(b)

Biotic/Abiotic Decay Potential High Low

Volatility High Low

Contaminant Sorption Potential Low High

CONTAMINANT PROPERTIES

CONTAMINANT AND

HYDROGEOLOGIC

CHARACTERISTICS

GENERALIZED RESTORATION

DIFFICULTY SCALE

 Increasing Difficulty

 SITE USE

Small volume Large volume

Nature of Release Short duration Long duration

Slug release Continual

CONTAMINANT DISTRIBUTION

Contaminant Phase

Volume of Contaminated Media Large

Contaminant Depth Deep

Aqueous, Gaseous

Small

Shallow

LNAPL

GEOLOGIC CONDITIONS

Stratigraphy Complex

Unconsolidated Media Texture Fine-grained

Degree of Heterogeneity High

Simple

Coarse-grained

Low

GROUNDWATER FLOW PARAMETERS

Hydraulic Conductivity High Low

(> 0.01 cm/s) (< 0.0001 cm/s)

Temporal Variation Little High

Vertical Flow Little High downward

DNAPL
4

contaminant sources, transport pathways, and receptors. Based

on this initial conceptualization, a data collection program is

devised to better define the nature and extent of contamination

and provide information (i.e., hydraulic conductivity distribution,

aquifer boundary conditions, and initial hydraulic gradients) for

remedy design. Contaminant source and downgradient dissolved

plume areas should be delineated early during the

characterization process to clarify site management strategies.

P&T systems can often be designed to contain source and

downgradient plume areas based on data acquired during the

early and intermediate phases of investigation. Additional

studies, including monitoring of actual P&T performance, are

usually required, however, to assess the potential to restore

ground-water quality in different site areas.

Mathematical models representing aspects of the site conceptual

model should be used to evaluate alternative extraction/injection

schemes, perform sensitivity analysis, and identify additional

data needs. Integrating P&T operation and monitoring data can

lead to model refinements and design enhancements.

P&T performance is typically assessed by measuring hydraulic

heads and gradients, ground-water flow directions and rates,

pumping rates, pumped water and treatment system effluent

quality, and contaminant distributions in ground water and

porous media. Guidance on methods for monitoring P&T

performance is provided by Cohen et al. (1994). Careful

examination of system performance, considering transient

effects, is commonly warranted during the first months after

start-up, and after subsequent major changes to P&T operation.

Remediation, therefore, should be considered part of site

characterization, yielding data that may lead to improved P&T

system design and operation.

In recognition of inherent uncertainty and the potential for

phased remediation, a reasonable degree of flexibility should

be incorporated in P&T design to accommodate modifications.

This may involve overdesign of certain system components

(e.g., pipe or electric wire size), use of modular equipment (e.g.,

package treatment units), and strategic placement of junction

boxes. Overdesign may allow system modifications such as

Figure 5. Types of data used to develop a site conceptual model

for remedy assessment (modified from U.S. EPA,

1993a).

Figure 6. Iterative phases of site characterization and remediation

(modified from U.S. EPA, 1993a; NRC, 1994).

•

•

•

•

Nature and Extent of

Contamination

Spatial distribution of

subsurface contaminants

Types and concentrations of

contaminants

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

 Contaminant Sources

Location and characteristics of

continuing near-surface

releases and sources

Location of subsurface sources

(NAPL pools, residual NAPL,

metal precipitates, etc.)

 Restoration Potential

Estimates of contaminant mass

Temporal trend in contaminant

concentration

Sorption data

Contaminant transformation

processes and rates

Contaminant migration rates

NAPL properties

Other characteristics that affect

transport and fate

Hydrogeologic Setting

Contaminant receptors

Description of regional and site

geology

Stratigraphy (thickness and

lateral extent of units,

preferential pathways, etc.)

Depth to ground water

Hydraulic gradients

Hydraulic conductivity, storage

coefficient, porosity distribution

Temporal variation in water

levels

Ground-water recharge and

discharge

Ground water / surface water

interactions

 contamination defined

 • Containment systems

 designed

 removed

 • Near-surface

 • Nature and extent of

 contaminant sources

Excavation and

capping of lagoon

Drum and soil

removal

Monitoring

systems

Source and

plume

containment

pumping

Cleanup pilot

studies

Full-scale P&T

implementation

 • Restoration potential

 evaluated for site

 subareas

 • Remediation systems

 designed and installed

 • Remediation system

 performance monitored

 • System adjusted as

necessary

 • Subsurface contaminant

 sources identified and

 contained

 • Pilot studies conducted

?

?

?

•

• Preliminary site

 conceptual model

• Preliminary site

 investigation

history review

Site background and
5

incorporation of additional extraction wells or higher flow rates

at relatively minimal expense. The degree of overdesign

required as a contingency for uncertainties in subsurface

conditions will be site specific and largely dependent on the

level of site characterization performed prior to design. Estimates

of potential ranges of required flowrates may be obtained at

many sites during design-stage ground-water flow modeling.

Contaminant Characterization

Contaminant characterization is a key element of remedial

evaluations. The nature, distribution, and extent of contamination

will influence the selection of remedial actions and specific

system designs. Contaminant characterization data needed to

select and design a P&T system are listed in Figure 5. Important

goals include: (1) delineating contaminant source areas and

release characteristics; (2) defining the nature and extent

(horizontal and vertical) of contamination; (3) characterizing

contaminant transport pathways, processes, and rates; (4)

estimating risks associated with contaminant transport; and (5)

assessing aquifer restoration potential (see below). Contaminant

characterization efforts generally involve document review,

indirect and direct field characterization methods (e.g., soil, soil

gas analysis and ground-water sampling), and data analysis.

Assessing Potential Limitations to P&T

Monitoring contaminant concentrations in ground water with

time at P&T sites often reveals “tailing” and “rebound”

phenomena. “Tailing” refers to the progressively slower rate of

dissolved contaminant concentration decline observed with

continued operation of a P&T system (Figures 7 and 8). The

tailing contaminant concentration may exceed clean-up

standards. Another problem is that dissolved contaminant

concentrations may “rebound” if pumping is discontinued after

temporarily attaining a clean-up standard (Figure 7).

If aquifer restoration is a potential remediation goal, then site

characterization should investigate the physical and chemical

phenomena that cause tailing and rebound. At many sites, most

of the contaminant mass is not dissolved in ground water, but is

present as NAPL, adsorbed species, and solids. Slow mass

transfer of contaminants from these phases to ground water

Figure 7. Concentration versus pumping duration or volume

showing tailing and rebound effects (modified from

Keely, 1989).

Figure 8. Hypothetical examples of contaminant removal using

P&T (modified from Mackay and Cherry, 1989). Black

indicates NAPL; stippling indicates contaminant in

dissolved and sorbed phases (with uniform initial

distribution); and arrows indicate relative groundwater velocity. Ground water is pumped from the well

at the same rate for each case. The dotted lines in (a)

represent the volume of water that would have to be

pumped to flush slightly retarded contaminants from

the uniform aquifer.

Pumping Duration or Volume Pumped

Cleanup standard

Apparent

residual

concentration

Theoretical

removal without

tailing

Removal

with tailing

Rebound

Relative Concentration

Pump on Pump off

during P&T will cause tailing and prolong the clean-up effort.

Physical causes of tailing include ground-water velocity and

flowpath variations, and the slow diffusion of contaminants from

low permeability zones during P&T operation. These phenomena

are briefly discussed in Appendix A.

Tailing and rebound patterns associated with different physical

and chemical processes are similar. Multiple processes (i.e.,

dissolution, diffusion, and desorption) will typically be active at

a P&T site. Diagnosis of the cause of tailing and rebound,

therefore, requires careful consideration of site conditions and

usually cannot be made by examining concentration-versustime data alone. Quantitative development of the conceptual

model using analytical or numerical methods may help estimate

the relative significance of different processes that cause tailing

and rebound. Knowledge of the potential limitations at each site

may allow more detailed analyses of the potential effectiveness

of different P&T remediation strategies and different system

configurations.

t2

t0

t1

t0

t1

t0

t1

t0

t0 t1 Time

t0 t1

t0 t1

t0 t2

Contaminant concentration in

extracted water

(a) Uniform sand-gravel aquifer

(c) Clay lens in uniform sand-gravel aquifer

(b) Stratified sand-gravel aquifer

(d) Uniform sand-gravel aquifer

NAPL
6

Hydrogeologic Characterization

Components of hydrogeologic investigation needed for P&T

design are listed in Figure 5. Care must be taken to avoid

exacerbating the contamination problem as a result of field work

(e.g., inducing unwanted migration via drilling or pumping), or

performing investigations not needed for risk or remedy

assessment. Characterizing ground-water flow and contaminant

transport is particularly challenging in heterogeneous media,

especially where contaminants have migrated into fractured

rock. Methods for characterizing fractured rock settings include

drilling/coring, aquifer tests, packer tests, tracer tests, surface

and borehole geophysical surveys, borehole flowmeter surveys,

and air photograph fracture trace analysis (Sara, 1994). At the

scale of many contaminated sites, complete characterization of

fractured rock (and other heterogeneous media) may be

economically infeasible (Schmelling and Ross, 1989), and not

needed to design an effective P&T system (NRC, 1994). The

appropriate characterization methods and level-of-effort must

be determined on a site-specific basis.

Long-term aquifer tests and phased-system installations are

often cost-effective means for acquiring field-scale hydrogeologic

and remedial design data. Aquifer tests should be conducted to

acquire field-scale measurements of hydrogeologic properties,

such as formation transmissivity and storage coefficient, that

are critical to extraction system design. Test results are used to:

(1) determine well pumping rates and drawdowns; (2) assess

well locations and pumping rates needed for full-scale operation;

(3) evaluate the design of well and treatment system components;

and (4) estimate capital and O&M costs. Recommended

procedures for conducting aquifer tests are described by

Osborne (1993) and others.

The number and duration of tests required to obtain sufficient

data to design a P&T system depends on many factors, including

plume size, the distribution of hydrogeologic units, their hydraulic

properties, and hydrogeologic boundary conditions. In general,

multiple tests are warranted at large and heterogeneous sites.

Test design parameters (including specification of observation

well locations, test duration, and pumping rate) can be assessed

using well hydraulics solutions, ground-water flow models, and/

or by conducting short-term step tests.

Observation wells should be located close enough to the pumping

well to ensure adequate responses to pumping stress.

Drawdowns will depend on site-specific hydrologic conditions

that influence ground-water elevations during the test. Wells

should also be located so that data may be used to evaluate

heterogeneity and anisotropy, if warranted.

Although reasonable estimates of formation transmissivity can

generally be obtained using data acquired during the first

several hours of pumping (if observation wells are close to the

pumping well), it may be advisable to extend aquifer tests to

days or weeks to evaluate capture zones, boundary conditions,

and ground-water treatability issues. Slug tests can also be

used to augment aquifer test results. However, short-term

aquifer and slug tests generally are not as reliable indicators of

system performance as long-term aquifer tests.

Disposal options for aquifer test water are subject to site

conditions and regulations but may include: discharge to a

storm or sanitary sewer, discharge to the ground, discharge to

surface water, reinjection to the subsurface, and transport to an

off-site disposal facility. Regulatory agencies should be contacted

to determine disposal requirements.

Ground-Water Treatability Studies

Treatability data needed for design of ground-water treatment

systems generally should be acquired by conducting chemical

analyses and treatability studies on contaminated ground water

extracted during aquifer tests. Analysis of water samples

obtained at different times during an aquifer test often will

provide data regarding the initial range of contaminant

concentrations in influent water to the treatment plant. Benchand pilot-scale treatability studies are valuable means for

determining the feasibility of candidate processes for treating

contaminated ground water (U.S. EPA, 1989, 1994a). Laboratory

bench-scale tests use small quantities of extracted ground

water to provide preliminary data on various treatment processes,

pretreatment requirements, and potential costs. During pilotscale tests, skid-mounted or mobile pilot equipment is operated

to study the effect of varying system parameters (e.g., flow rate)

on treatment results and to identify potential problems, such as

chemical precipitation of dissolved iron (Fe) and manganese

(Mn) in an air stripper.

Air stripping and granular activated carbon (GAC) units may be

used to remove organic compounds from ground water during

aquifer tests; ion exchange/adsorption can be used to remove

most metals (U.S. EPA, 1996). Air stripping is generally more

cost-effective than GAC for treating volatile organic compounds

when flow rates exceed 3 gpm (Long, 1993), but may require

additional vapor phase treatment.

Potential for Fluid Injection

Artificial fluid injection/recharge is used to enhance hydraulic

control and flushing of contamination zones. Treatment plant

effluent or public supply water can be injected above or below

the water table via wells, trenches, drains, or surface application

(sprinkler, furrow, or basin infiltration). The applied water can be

amended to stimulate bioremediation or to minimize well and

formation clogging problems. Recharge is typically controlled

by maintaining the water level in injection wells or drains or by

pumping at specified rates. Regulatory agencies should be

contacted to determine injection permit requirements. Potential

problems with the use of injection include undesired horizontal

or vertical contaminant migration due to the increased hydraulic

gradients. Sites where injection is to be used should be

carefully characterized and monitored to ensure that

environmental problems are not exacerbated.

Aspects of site characterization critical to fluid injection design

include determination of: (1) site stratigraphy and permeability

distribution, (2) hydrogeologic boundary conditions, (3) possible

injection rates and resulting hydraulic head and ground-water

flow patterns, and (4) the potential for well and formation

clogging due to injection.

Hydraulic parameters estimated from analysis of standard aquifer

tests are often used to design injection systems. Constanthead, constant-rate, and stepped rate or head injection tests

can also be conducted to evaluate hydraulic properties and

injection potential using standard aquifer test procedures

(Driscoll, 1986; Kruseman and deRidder, 1990). More discrete
7

techniques (e.g., packer tests, borehole flowmeter surveys)

may be desirable to identify high permeability zones. Hydraulic

heads and ground-water flow patterns resulting from injection

can be examined and predicted using well or drain hydraulics

equations and ground-water flow models. Such analysis can

also be used to determine potential injection rates, durations,

and monitoring locations for injection tests. In addition to

helping estimate formation hydraulic properties, injection tests

provide information on water compatibility and clogging issues

that are critical to injection design.

The most common problem associated with fluid injection is

permeability reduction due to clogging of screen openings. This

causes a decline in injection rates. Clogging results from

physical filtration of solids suspended in injected water, chemical

precipitation of dissolved solids, and the excessive growth of

microorganisms (also known as biofouling). Less frequently,

well or formation damage results from air entrainment, clay

swelling, and clay dispersion due to injection. In general, the

injection capacity of a system is often overdesigned by a

significant factor (e.g., 1.5 to 2) to account for loss of capacity

under operating conditions due to such problems as permeability

reduction and the temporary loss of capacity during well

maintenance. The optimal degree of overdesign is site specific

and will depend on such factors as the rate at which clogging

occurs and the cost of maintenance.

The potential for well clogging and mitigative measures can be

examined by analysis of the injected fluid and bench scale

testing. In general, injection water should contain: (1) no

suspended solids to minimize clogging; (2) little or no dissolved

oxygen, nutrients, and microbes to minimize biofouling; and (3)

low concentrations of constituents that are sensitive to changes

in pH, redox, pressure, and temperature conditions (e.g., Fe

and Mn) to minimize precipitation. Column permeameter tests

can be conducted to examine changes in hydraulic conductivity

resulting from injection. Due to the potential significance of

many hydrogeologic, physical, and chemical factors, however,

fluid injection is best evaluated by conducting extended

injection tests during which injection rates and hydraulic heads

are monitored carefully. Results of field tests help define

formation hydraulic properties, potential injection rates, injection

well spacings, mounding response, and clogging potential.

Dissolved or suspended solids may need to be removed from

water by aeration, flocculation, and filtration prior to injection.

Similarly, nutrients and/or dissolved oxygen may need to be

removed to prevent biofouling. Water should be injected below

the water table through a pipe to prevent its aeration in the well.

Injecting warm water can also promote biofouling. Clogging

problems can be minimized by overdesigning injection capacity

(e.g., by installing more wells, longer screens, etc.) and

implementing a regular well maintenance program.

Extraction and injection rate monitoring and well inspection,

using a downhole video camera or other means, can help

identify wells in need of treatment or replacement. Periodic

rehabilitation of wells or drains (by surging, jetting, chlorination,

or acid treatment) may be required to restore declining injection

rates (Driscoll, 1986). Chemical incrustation can be addressed

by acid treatment, backwashing, mechanical agitation (with a

wire brush or surge block), and pumping. Strong oxidizing

agents, such as a chlorine solution, can be used in conjunction

with backwashing, mechanical agitation, and pumping to treat

wells damaged by slime-producing bacteria. Acidification and

chlorination, however, may interfere with interpretation of

ground-water chemistry data. Fine particles can be removed

(to some extent) using standard well development techniques.

Experienced well drillers should be contacted for advice on

rehabilitation methods. These potential problems need to be

considered when projecting P&T costs. Significant maintenance

may be required at many sites to retain desired injection

capacity. More detailed discussions of the engineering aspects

of water injection are provided by Pyne (1995).

Data Presentation

Complete discussion of methods for characterization and

remedial design analyses and supporting data is beyond the

scope of this document. In general, such information should be

presented graphically and accompanied by supporting

calculations and analyses. Tools for electronic storage,

manipulation, analysis, and display of data and designs are

generally available and often provide a convenient format for

storage and access of this information (e.g., database, CAD,

and/or GIS programs). Characterization data such as threedimensional contaminant distribution are best presented on site

maps and in representative cross sections. Hydraulic properties

and hydraulic head data may also be presented in similar

fashion. Pertinent features such as well locations (i.e., monitoring,

production, injection), surface water bodies, potential source

areas, and relevant structures should be included, as appropriate.

Supporting data should be provided in tabular or spreadsheet

form and accompany the maps and cross sections.

Capture Zone Analysis for P&T Design

P&T design is refined by performing field tests, modeling

alternative injection/extraction schemes, and monitoring system

performance. The first step in establishing design criteria, after

characterizing pre-remedy ground-water flow patterns and

contaminant distributions, is to determine the desired

containment and/or restoration area (two-dimensional) and

volume (three-dimensional). These should be clearly specified

in the remedial design and monitoring plans. After defining the

proposed containment area, a capture zone analysis is conducted

to design the P&T system and a performance monitoring plan is

developed based on the predicted flow field.

The capture zone of an extraction well or drain refers to that

portion of the subsurface containing ground water that will

ultimately discharge to the well or drain (Figures 3 and 9). It

should be noticed that the capture zone of a well is not coincident

with its drawdown zone of influence (ZOI) (Figure 9). The extent

of the ZOI depends largely on transmissivity and pumping rate

under steady-state conditions. However, the shape of the

capture zone depends on the natural hydraulic gradient as well

as pumping rate and transmissivity. Relatively high natural

hydraulic gradients result in narrow capture zones that do not

extend far in the downgradient direction. Therefore, some

sidegradient and downgradient areas within the ZOI of a recovery

well will be beyond its capture zone, and “rules-of-thumb”

regarding overlapping drawdown zones should not be used to

determine well spacings or pumping rates for P&T design.

In recent years, many mathematical models have been

developed or applied to compute capture zones, ground-water

pathlines, flushing rates, and associated travel times to extraction
8

wells or drains (Javandel et al., 1984; Javandel and Tsang, 1986;

Shafer, 1987a,b; Newsom and Wilson, 1988; Fitts, 1989,1994;

Strack, 1989; Bonn and Rounds, 1990; Bair et al., 1991;

Rumbaugh, 1991; Bair and Roadcap, 1992; Blandford et al.,

1993; Gorelick et al., 1993; Pollock, 1994; Strack et al., 1994).

These models provide insight into flow patterns generated by

alternative P&T schemes and the selection of monitoring locations

and frequency. Additionally, linear programming methods are

being used to optimize P&T design (Ahlfeld and Sawyer, 1990;

Gorelick et al., 1993; Hagemeyer et al., 1993) by specifying an

objective function subject to various constraints (e.g., minimize

pumping rates but maintain inward hydraulic gradients).

Model selection for P&T design analysis depends on the

complexity of the site, available data, and the familiarity of the

analyst with different codes. In general, the simplest tool

applicable to site conditions and the desired degree of uncertainty

should be used in design. However, conditions at many sites

will be sufficiently complex that screening level characterizations

and design tools will result in significant uncertainty. Regardless

of the design tools which are used, capture zone analysis

should also be conducted, and well locations and pumping rates

optimized, by monitoring hydraulic heads and flow rates during

aquifer tests and system operation. Conceptual model

refinements gained by monitoring lead to enhanced P&T design

and operation. In some cases, these refinements are

incorporated in a mathematical model that is used to reevaluate

and improve system design.

Capture Zone Analysis Tools

Many types of tools are available for capture zone analysis and

system design (Table 1). Graphical methods are useful screening

level design tools in many situations. Based on this approach,

the simple graphical method shown in Figure 9 can be used to

locate the stagnation point and dividing streamlines, and then

Drawdown Contour

Capture Zone

Stagnation

Zone

K=0.02

K=10

Ground Surface

Partially-Penetrating

Trench Drain

Q

Ti

w

Q

2 Ti π

y

x

stagnation

point

w = Q/2Ti

w = -Q/2Ti

Q

2Ti

Vertical Capture Zone

Cross Section

(a) (b)

(c) (d)

Figure 9. (a) Illustration of drawdown contours (i.e., zone of influence) and the capture zone of a single pumping well in a uniform

medium. Equations for the dividing streamlines (w = Q/2Ti) that separate the capture zone of a single well from the rest of

an isotropic, confined aquifer with a uniform regional hydraulic gradient are given in (b) where T = transmissivity, Q = pumping

rate, and i = initial uniform hydraulic gradient. Simplified capture zone analysis methods may provide misleading results

when applied to more complex problems, such as those dealing with heterogeneous media, as depicted in (c) where K =

relative hydraulic conductivity, and three-dimensional flow (d).
9

Table 1. P&T Design Tools (modified from van der Heijde and Elnawawy, 1993)

Method Example Description

Aquifer Tests and Controlled and monitored pilot tests are conducted to assist P&T design.

Pilot Testing Suggested operating procedures for aquifer tests and analytical methods

are described by Osborne (1993) and many others. Test results should be

used to improve P&T design modeling, where applicable.

Graphical - (Javandel and A simple graphical method can be used to determine minimum pumping rates and

Capture Zone Tsang, 1986) well spacings needed to maintain capture using 1, 2, or 3 pumping wells along a line

Type Curves perpendicular to the regional direction of ground-water flow in a confined aquifer.

Semi-analytical WHPA These models superposition analytic functions to simulate simple or complex

Ground-Water (Blandford et al., aquifer conditions including wells, line sources, line sinks, recharge, and regional

Flow and Pathline 1993) flow (Strack, 1989). Advantages include flexibility, ease of use, speed, accuracy,

Models and no model grid. Generally limited to analysis of 2-D flow problems.

WHAEM

(Strack et al.,

1994; Haitjema et

al., 1994)

Numerical MODFLOW Finite-difference (FD) and finite element (FE) ground-water flow models have been

Models of (McDonald and developed to simulate 2-D areal or cross-sectional and quasi- or fully- 3-D, steady

Ground-Water Harbaugh, 1988) or transient flow in anisotropic, heterogeneous, layered aquifer systems. These

Flow models can handle a variety of complex conditions allowing analysis of simple and

complex ground-water flow problems, including P&T design analysis. Various preand post-processors are available. In general, more complex and detailed site

characterization data are required for simulation of complex problems.

Pathline and MODPATH These programs use particle tracking to calculate pathlines, capture zones, and

Particle Tracking (Pollock, 1994) travel times based on ground-water flow model output. Programs vary in assumptions

Post-Processors and complexity of site conditions that may be simulated (e.g., 2-D or 3-D flow,

heterogeneity, anisotropy).

GPTRAC

(Blandford et al.,

1993)

Numerical MT3D These models can be used to evaluate aquifer restoration issues such as changes

Models of (Zheng, 1992) in contaminant mass distribution with time due to P&T operation.

Ground-water

Flow and MOC

Contaminant (Konikow and

Transport Bredehoeft, 1989)

Optimization MODMAN Optimization programs designed to link with ground-water flow models yield

Models (Greenwald, answers to questions such as: (1) where should pumping and injection wells be

1993) located, and (2) at what rate should water be extracted or injected at each well?

The optimal solution maximizes or minimizes a user-defined objective function and

satisfies all user-defined constraints. A typical objective may be to maximize the

total pumping rate from all wells, while constraints might include upper and lower

limits on heads, gradients, or pumping rates. A variety of objectives and constraints

are available to the user, allowing many P&T issues to be considered.

Software is available from a variety of sources including the Center for Subsurface Modeling Support at the U.S. EPA’s Robert S. Kerr

Environmental Research Center in Ada, Oklahoma (405-436-8594).
10

sketch the capture zone of a single well in a uniform flow field.

This analysis is extended by Javandel and Tsang (1986) to

determine the minimum uniform pumping rates and well spacings

needed to maintain a capture zone between two or three

pumping wells along a line perpendicular to the regional direction

of ground-water flow. Their capture zone design criteria and

type curves can be used for capture zone analysis, but more

efficient P&T systems can be designed with nonuniform pump

well orientations, spacings, and extraction rates. The extent to

which the results of these simple models represent actual conditions

depends on the extent to which the assumptions vary from actual

site conditions.

More complex tools are often necessary to optimize P&T design

and reduce uncertainty. Several semianalytical models employ

complex potential theory to calculate stream functions, potential

functions, specific discharge distribution, and/or velocity

distribution by superimposing the effects of multiple extraction/

injection wells using the Thiem equation on an ambient uniform

ground-water flow field in a two-dimensional, homogeneous,

isotropic, confined, steady-state system (e.g., RESSQC,

Blandford et al., 1993). Streamlines, flushing rates, and capture

zones associated with irregular well spacings and variable

pumping rates can be simulated by these models. Many of

these models support reverse and forward particle tracking to

trace capture zones and streamlines. For example, reverse

particle tracking is implemented in RESSQC to derive steadystate capture zones by releasing particles from the stagnation

point(s) of the system and tracking their advective pathlines in

the reversed velocity field. Similarly, time-related captures

zones (Figure 10) are obtained by tracing the reverse pathlines

formed by particles released around each pumping well (Shafer,

1987a; Blandford et al., 1993).

Application of semianalytical models to field problems requires

careful evaluation of their limiting assumptions (e.g., isotropic

and homogeneous hydraulic conductivity, fully-penetrating wells,

no recharge, no vertical flow component, and constant

transmissivity). Several analytical models relax these restrictive

assumptions by superposition of various functions to treat

recharge, layering, heterogeneity, three-dimensional flow, etc.

Examples of two-dimensional time-related capture zones

determined using TWODAN (Fitts, 1994; 1995) are shown in

Figure 10. Given their ease of use and inherent uncertainties

regarding the ground-water flow field, the more robust

semianalytical models are ideal tools for evaluating alternative

injection/extraction well locations and pumping rates at many

sites. Where field conditions do not conform sufficiently to

model assumptions, the simulation results will be invalid.

Numerical models are generally used to simulate ground-water

flow in complex three-dimensional hydrogeologic systems (e.g.,

MODFLOW, McDonald and Harbaugh, 1988; and SWIFT/486,

Ward et al., 1993). For example, the benefits of using partiallypenetrating recovery wells to minimize pumping rates and

unnecessary vertical spreading of contaminants can be examined

using a three-dimensional flow model. Numerical flow model

output is processed using reverse or forward particle-tracking

software such as MODPATH (Pollock, 1994), GWPATH (Shafer,

1987b), and PATH3D (Zheng, 1990) to assess pathlines and

capture zones associated with P&T systems at sites that cannot

be adequately modeled using simpler techniques. Solute

transport models are primarily run to address aquifer restoration

issues such as changes in contaminant mass distribution with

time due to P&T operation.

Ground-water flow models can be coupled with linear

programming optimization schemes to determine the most

effective well placements and pumping rates for hydraulic

containment. The optimal solution maximizes or minimizes a

user-defined objective function subject to all user-defined

constraints. In a P&T system, a typical objective function may

be to minimize the pumping rate to reduce cost, while constraints

may include specified inward gradients at key locations, and

limits on drawdowns, pumping rates, and the number of pumping

wells. Gorelick et al. (1993) present a review of the use of

optimization techniques in combination with ground-water

models for P&T system design. Available codes include AQMAN

(Lefkoff and Gorelick, 1987), an optimization code that employs

the Trescott et al. (1976) two-dimensional ground-water flow

model, and MODMAN (Greenwald, 1993), which adds

optimization capability to the three-dimensional USGS

MODFLOW model (McDonald and Harbaugh, 1988). A case

study of optimization code use to assist P&T design is given by

Hagemeyer et al. (1993).

Techniques have been presented in the literature for combining

nonlinear optimization methods with contaminant transport

simulation models (Gorelick, 1983; Wagner and Gorelick, 1987;

Ahlfeld et al., 1988). These techniques are intended to provide

solutions to problems formulated in terms of predicted

concentrations (e.g., minimize pumping such that TCE is below

the required clean-up level within five years at target locations).

However, such analysis requires the use of a solute transport

model and solution of a relatively difficult nonlinear problem. As

a result, computation effort is large and uncertainty in results is

high compared to optimization based on ground-water flow.

Nonlinear optimization methods using solute transport models

have not yet been packaged into commercial software and have

rarely been applied to ground-water contamination problems.

Extraction / Injection Scheme Design

For a successful hydraulic containment, contaminants moving

with ground water in the desired containment zone must follow

pathlines that are captured by the P&T system. An appropriate

remedial objective might be to minimize the total cost required

to maintain perpetual containment and satisfy regulatory

requirements. Given this objective, installing low permeability

barriers (Figure 3c) to reduce pumping rates might be costeffective. At sites with an objective of contaminant mass

removal (i.e., where the containment area size may be diminished

or P&T discontinued if clean-up goals are met), a more complex

cost-effectiveness trade-off exists between minimizing hydraulic

containment costs and maximizing contaminant mass removal

rates.

Unless natural attenuation mechanisms are being relied upon to

limit plume migration, hydraulic containment is generally a

prerequisite for aquifer restoration. Restoration P&T design will

typically reflect a compromise among objectives that seek to:

(1) reduce contaminant concentrations to clean-up standards,

(2) maximize mass removal, (3) minimize clean-up time, and (4)

minimize cost. Due to the limitations described in Appendix A,

P&T for aquifer restoration requires a high degree of performance

monitoring and management to identify problem areas and

improve system design and operation.

Restoration P&T ground-water flow management involves

optimizing well locations, depths, and injection/extraction rates

to maintain an effective hydraulic sweep through the
11

Figure 10. Hydraulic head contours and capture zones simulated using TWODAN (Fitts, 1995) for several extraction/injection schemes

in an aquifer with a uniform transmissivity of 1000 ft 2/d, and an initial hydraulic gradient of 0.01. Pathline time intervals of

one year are marked by arrows. Note the stagnation zones that develop downgradient of extraction wells and upgradient of

injection wells.

70 gpm extraction

3 wells

23 gpm extraction per well

(a) (b)

(c) (d)

(e) (f)

1000 Feet 1000 Feet

1000 Feet 1000 Feet

1000 Feet 1000 Feet

7 wells

10 gpm extraction per well

3 wells

23 gpm extraction per well

6 wells

23 gpm extraction from 3 wells

10 gpm injection via 3 wells ( )

3 wells

23 gpm extraction per well

30 gpm injection via trench
12

contamination zone, minimize stagnation zones, flush pore

volumes through the system, and contain contaminated ground

water. Wells are installed in lines and other patterns to achieve

these objectives (Figure 10). Horizontal wells and drains are

constructed to create ground-water line sinks and mounds, and

thereby affect linear hydraulic sweeps.

Pore Volume Flushing

Restoration requires that sufficient ground water be flushed

through the contaminated zone to remove both existing dissolved

contaminants and those that will continue to desorb from porous

media, dissolve from precipitates or NAPL, and/or diffuse from

low permeability zones. The sum of these processes and

dilution in the flow field yields persistent acceptable groundwater quality at compliance locations.

The volume of ground water within a contamination plume is

known as the pore volume (PV), which is defined as

 PV = I A bn dA (1)

where b is the plume thickness, n is the formation porosity, and

A is the area of the plume. If the thickness and porosity are

relatively uniform, then

 PV = BnA (2)

where B is the average thickness of the plume.

Assuming linear, reversible, and instantaneous sorption, no

NAPL or solid contaminants, and neglecting dispersion, the

theoretical number of PVs required to remove a contaminant

from a homogeneous aquifer is approximated by the retardation

factor, R, which is the ground-water flow velocity relative to

velocity of dissolved contaminant movement. An example of

the relationship between the number of PVs and R, that also

accounts for dispersion, is demonstrated by a numerical model

used to evaluate a P&T design at the Chem-Dyne site in Ohio

(Ward et al., 1987). Due to simulation of linear sorption, a nearly

linear relationship was found to exist between retardation and

the duration of pumping (or volume pumped) needed to reach

the ground-water clean-up goal. Batch flush models (e.g., U.S.

EPA, 1988b; Zheng et al., 1992) often assume linear sorption to

calculate the number of PVs required to reach a clean-up

concentration, Cwt in ground water as a function of the retardation

factor, R, and the initial aqueous-phase contaminant

concentration, Cwo:

 No. of PVs = -R ln (Cwt / Cwo) (3)

Though useful for simple systems, the representation of linear,

reversible, and instantaneous sorption in contaminant transport

models can lead to significant underestimation of P&T clean-up

times. For example, the desorption of most inorganic

contaminants (e.g., chromium and arsenic) is nonlinear. In

addition, much of the pore space in aquifer materials may not be

available for fluid flow. In such situations, flushing is not efficient

and removal of a greater number of pore volumes of water will

be required.

Kinetic limitations often may prevent sustenance of equilibrium

contaminant concentrations in ground water (Bahr, 1989; Brogan,

1991; Haley et al., 1991; Palmer and Fish, 1992). Such effects

occur in situations where contaminant mass transfer to flowing

ground water is slow relative to ground-water velocity. For

example, contaminant mass removal from low permeability

materials may be limited by the rate of diffusion from these

materials into more permeable flowpaths. In this situation,

increasing ground-water velocity and pore volume flushing

rates beyond a certain point would provide very little increase in

contaminant removal rate. Kinetic limitations to mass transfer

are likely to be relatively significant where ground-water velocities

are high surrounding injection and extraction wells.

The number of PVs that must be extracted for restoration is a

function of the clean-up standard, the initial contaminant

distribution, and the chemical/media phenomena that affect

cleanup. Screening-level estimates of the number of PVs

required for cleanup can be made by modeling and by assessing

the trend of contaminant concentration versus the number of

PVs removed. At many sites, numerous PVs (i.e., 10 to 100s)

will have to be flushed through the contamination zone to attain

clean-up standards.

The number of PVs withdrawn per year is a useful measure of

the aggressiveness of a P&T operation. Many current systems

are designed to remove between 0.3 and 2 PVs annually. For

example, less than 2 PVs per year were extracted at 22 of the

24 P&T systems studied by U.S. EPA (1992b) and reviewed

by NRC (1994). Low permeability conditions or competing

uses for ground water may restrict the ability to pump at higher

rates. As noted above, kinetic limitations to mass transfer

also may diminish the benefit of higher pumping rates. The

potential significance of such limitations should be evaluated

prior to installation of aggressive systems designed for

relatively high flushing rates. If limiting factors are not

present, pumping rates may be increased to hasten cleanup.

The time required to pump one pore volume of ground water

from the contaminated zone is a fundamental parameter that

should be calculated for P&T systems. NRC (1994), however,

determined that the number of PVs withdrawn at P&T sites is

rarely reported. Restoration assessments should include

estimates of the number of PVs needed for cleanup. However,

it must be noted that such analyses generally oversimplify

highly complex site conditions. It may often be impracticable to

characterize the site in sufficient detail to reduce uncertainty in

estimates of restoration time frames to insignificant levels.

Uncertainty in these estimates should be considered during

remedial evaluations.

Poor P&T design may lead to low system effectiveness and

contaminant concentration tailing. Poor design factors include

low pumping rates and improper location of pumping wells and

completion depths. A simple check on the total pumping rate is

to calculate the number of PVs per year. Inadequate location or

completion of wells or drains may lead to poor P&T performance

even if the total pumping rate is appropriate. For example, wells

placed at the containment area perimeter may withdraw a large

volume of clean ground water from beyond the plume via

flowlines that do not flush the contaminated zone. Similarly,

pumping from the entire thickness of a formation in which the

contamination is limited vertically will reduce the fraction of

water that flushes the contaminated zone. In general, restoration

pumping wells or drains should be placed in areas of relatively

high contaminant concentration as well as locations suitable for

achieving hydraulic containment.
13

Well placement can be evaluated by: (1) using ground-water

flow and transport models; (2) comparing contaminant mass

removed to contaminant mass dissolved in ground water; and

(3) applying expert knowledge. P&T system modifications should

be considered if any of these methods indicate that different

pumping locations or rates will improve system effectiveness.

Minimize Ground-Water Stagnation

Ground-water flow patterns need to be managed to minimize

stagnation during P&T operation. Stagnation zones develop in

areas where the P&T operation produces low hydraulic gradients

(e.g., downgradient of a pumping well and upgradient of an

injection well) and in low permeability zones regardless of

hydraulic gradient. Ground-water flow modeling can be used to

assess ground water and solute velocity distributions, travel

times, and stagnation zones associated with alternative pumping

schemes. During operation, stagnation zones can be identified

by measuring hydraulic gradients, tracer movement, groundwater flow rates (e.g., with certain types of downhole flowmeters

or in situ probes), and by modeling analysis. Low permeability

heterogeneities should be delineated as practicable during the

site characterization and P&T operation. Stagnation zones

associated with different pumping schemes are evident in

Figure 10.

Once identified, the size, magnitude, and duration of stagnation

zones can be diminished by changing pumping (extraction and/

or injection) schedules, locations, and rates. Again, flow modeling

based on field data may be used to estimate optimum pumping

locations and rates to limit ground-water stagnation. An adaptive

pumping scheme, whereby extraction/injection pumping is

modified based on analysis of field data, should result in more

expedient cleanup.

Guidance from Modeling Studies

Several modeling studies have been conducted to examine the

effectiveness of alternative extraction and injection well schemes

with regard to hydraulic containment and ground-water cleanup objectives (e.g., Freeberg et al., 1987; Satkin and Bedient,

1988; Ahlfeld and Sawyer, 1990; Tiedeman and Gorelick, 1993;

Marquis, Jr. and Dineen, 1994; Haggerty and Gorelick, 1994).

Although the optimum extraction/injection scheme depends on

site-specific conditions, objectives, and constraints,

consideration should be given to guidance derived from

simulation studies of P&T performance.

A conceptual modeling analysis using FTWORK (Faust et al.,

1993) of three alternative pumping strategies for an idealized

site with a uniform medium, linear equilibrium sorption, a single

non-degrading contaminant, and a continuing release is

presented in Figure 11. The plume management strategies

include: (1) downgradient pumping, (2) source control with

downgradient pumping, and (3) source control with mid-plume

and downgradient pumping. As shown, downgradient pumping

by itself allows and increases the movement of highly

contaminated ground water throughout the flowpath between

the release area and the downgradient recovery well. This

alternative results in expansion of the highly contaminated

plume and makes it more difficult to achieve cleanup. The

importance of source control is clearly demonstrated by

comparing the management alternatives. Source control

pumping prevents continued offsite migration and thereby

facilitates downgradient cleanup of contaminated ground water.

The combined source control, mid-plume, and downgradient

pumping alternative reduces the flowpath and travel time of

contaminants to extraction wells and diminishes the impact of

processes which cause tailing. As such, with more aggressive

P&T, cleanup is achieved more quickly and the volume of

ground water that must be pumped for cleanup is less than for

the other alternatives.

The effectiveness of seven injection/extraction well schemes

shown in Figure 12 at removing a contaminant plume was

evaluated by Satkin and Bedient (1988) using the MOC transport

model (Konikow and Bredehoeft, 1989). The performance of

each scheme was assessed for eight different hydrogeologic

conditions, which were simulated by varying maximum

drawdown, dispersivity, and regional hydraulic gradient.

Effectiveness was judged based on simulated cleanup, flushing

rate, and the volume of water requiring treatment. Findings of

this study include (Satkin and Bedient, 1988): (1) multiple

extraction wells located along the plume axis (the center line

scheme) reduce clean-up time by shortening contaminant travel

paths and allowing higher pumping rates; (2) the three-spot,

double-cell, and doublet schemes were effective under low

hydraulic gradient conditions, but require onsite treatment and

reinjection; (3) the three-spot pattern outperformed the other

schemes for simulations incorporating a high regional hydraulic

gradient; and, (4) the center line pattern was effective under all

simulated conditions. Andersen et al. (1984) and Satkin and

Bedient (1988) showed that the five-spot pattern (Figure 12)

may be a relatively inefficient scheme for cleanup.

Brogan (1991) and Gailey and Gorelick (1993) used simulations

to demonstrate that the best single recovery well location is

somewhat downgradient of a plume’s center of mass. The

optimum location (requiring the lowest pumping rate) for a

single extraction well to remediate a plume within a given time

period increases in distance downgradient from the center of

contaminant mass with increasing remediation time (Gailey and

Gorelick, 1993; Haggerty and Gorelick, 1994). Thus, optimum

pumping locations and rates depend on the specified clean-up

time frame.

The relative merits of conventional extraction/injection well

schemes, in-situ bioremediation, and P&T enhanced by injecting

oxygenated water to stimulate biodegradation for containing

and cleaning up a hypothetical naphthalene plume in a uniform

aquifer were examined by Marquis and Dineen (1994). Nineteen

remediation alternatives were modeled using BIOPLUME II

(Rifai et al., 1987), a modified version of the MOC code (Konikow

and Bredehoeft, 1989) that simulates oxygen transport and

oxygen-limited biodegradation. Key findings made by Marquis

and Dineen (1994) include the following: (1) ground-water

extraction was more effective at preventing offsite migration

than bioremediation; (2) P&T enhanced by injecting highly

oxygenated water (with 50 mg/L dissolved oxygen) provided the

most effective plume control and cleanup; (3) greater contaminant

mass reductions occurred when extraction or injection wells

were located in the more contaminated portions of the plume;

(4) cleanup is hastened by minimizing the distances that

contaminants must travel to extraction wells or that dissolved

oxygen must travel to reach degradable contaminants; (5) to

maximize containment, P&T schemes should be designed to

produce convergent flow toward a central extraction location

and to minimize divergent flow along the plume periphery; and

(6) extraction/injection schemes should be designed to minimize

the presence of upgradient and intraplume stagnation areas.
Figure 11. Results of FTWORK (Faust et al., 1993) simulation analysis of three P&T alternatives for an idealized site (with uniform me

 linear equilibrium sorption, and a single non-degrading contaminant) showing dissolved contaminant concentrations with tim pumping.

Duration of Ground-Water Extraction

31 gpm

16 gpm

 26 gpm 16 gpm

 26 gpm 21 gpm16 gpm

Downgradient Pumping

Source and Downgradient Pumping

Source, Mid-Plume, and

Downgradient Pumping

Simulated Potentiometric Surface

for each Pumping Alternative

5 Years 9 Years 19 Years 29 Years 

31 gpm

Dissolved

Contaminant

Concentration

Pre-Pumping Dissolved Contaminant

Concentration Distribution

(after 5 years of plume growth)

Coordinates in feet

Uniform Pre-Pumping

Ground-Water Flow

Direction

Continuing Contaminant

Source Area

14
15

Pulsed Pumping

Pulsed pumping, with alternating pumping and resting periods

as illustrated in Figure 13, has been suggested as a means to

address tailing, flush stagnation zones by selective well cycling,

and increase P&T efficiency (Keely, 1989; Borden and Kao,

1992; Gorelick et al., 1993). Dissolved contaminant

concentrations increase due to diffusion, desorption, and

dissolution in slower-moving ground water during the resting

phase of pulsed pumping. Once pumping is resumed, ground

water with higher concentrations is removed, thus increasing

the rate of mass removal during active pumping. Due to slow

mass transfer from immobile phases to flowing ground water,

however, contaminant concentrations decline with continued

pumping until the next resting phase begins.

Several simulation studies have been conducted to evaluate the

effectiveness of pulsed pumping (Powers et al., 1991; Brogan,

1991; Borden and Kao, 1992; Armstrong et al., 1994; Rabideau

and Miller, 1994; and Harvey et al., 1994). Harvey et al. (1994)

found that: (1) for equal volumes of ground water extracted,

pulsed pumping does not remove more contaminant mass than

pumping continuously at the lower equivalent time-averaged

rate; (2) if the resting period is too long, pulsed pumping will

remove much less mass than pumping continuously at an

equivalent time-averaged rate; and, (3) if pulsed and continuous

pumping rates are the same, pulsed pumping will take longer to

achieve clean-up goals, but will require significantly less time of

pump operation. At many sites with significant tailing and

rebound, it will be preferable, therefore, to pump continuously at

a lower average rate than to initiate pulsed pumping. Cost

savings associated with less time of pump operation, however,

may make pulsed pumping advantageous.

If used, pulsed pumping schedules can be developed based on

pilot tests, modeling analysis, or ongoing performance monitoring

of hydraulic heads and contaminant concentrations. The

pumping period should be long enough to remove most of the

contaminant mass in the mobile ground water. The resting

period should not be so long that the dissolved concentration in

mobile ground water exceeds 70% to 90% of its equilibrium

value. Additional resting becomes inefficient as equilibrium is

approached because the rate of mass transfer from immobile to

mobile phases is driven by the concentration gradient. Care

must be taken to ensure that the hydraulic containment objective

is met during pump rest periods. Further guidance on interpreting

field data to designate pulsed pumping parameters is provided

by Harvey et al. (1994). Simulation results showing the sensitivity

of pulsed pumping performance to rest period duration are

shown in Figure 13.

Figure 12. Well schemes evaluated by Satkin and Bedient (1988).

Figure 13. Effects of varying pulsed pumping parameters (after

Harvey et al., 1994). The fraction of total mass removed

with time is shown in (a) and (d); pumping well

concentrations are shown in (b), (c), (e), and (f). Dashed

lines represent equivalent constant pumping rates.

Black bars at top of figures represent pumping periods

and white bars represent rest periods.

Dealing with Multiple Contaminant Plumes

Multiple contaminants that migrate at different velocities in

ground water are commonly encountered at contamination

sites. Compounds that partition more strongly to the solid phase

are transported more slowly, remain closer to source areas, and

are more difficult to extract from the subsurface by pumping

than the more mobile compounds. Thus, a P&T design that is

ideal for a single contaminant plume might perform poorly at a

site with multiple contaminants.

Haggerty and Gorelick (1994) used a solute transport model

and optimization analysis to examine the ability of five pumping

schemes to simultaneously remediate three contaminant plumes

that were chromatographically separated during ground-water

transport. The simulated problem and alternative extraction

schemes are shown in Figure 14.

In the single well scheme, one well is placed along the plume

axis at one of the indicated locations. For the other schemes,

Single Doublet Centerline 3-Spot

Double

Cell

Double

Triangle 5-Spot

Injection

Well

Extraction

Well

2470

days

867 days

Sensitivity to Rest Period

0 1000 1500 2000 500

Short Rest Period

Long Rest Period

99% of equilibrium

concentration

Long

Rest

Short

Rest

867 days

2470

days

High Pumping Rate

Low Pumping Rate

High Q

Low Q

Pulsed

Pumping

Constant

Pumping

Sensitivity to Pumping Rate

Time (days)

50

0

0.6

0 100 1500

0.2

0.4

0

0.6

0.4

0.2

0.2

0.4

0

0.6

0.8

1.0

Pump

off Pump

on (b)

(a)

(c)

(d)

(e)

(f)

Time (days)

39% of equilibrium

concentration

Concentration Mass Fraction Removed Concentration
16

wells can be placed at any number of the sites shown. The

optimum number, location, and pumping rates of wells in each

scheme were determined using the optimization model to achieve

cleanup at the lowest possible pumping rate within a specified

remediation period. Sensitivity analyses were conducted to

examine the influence of mass transfer rate limitations on

contaminant mobilization and removal. Findings presented by

Haggerty and Gorelick (1994) for each pumping scheme are

summarized in Figure 15.

For the smallest mass transfer rate parameter, ξ = 0.005 day -1,

none of the schemes can achieve cleanup within three years

regardless of pumping rate due to mass transfer rate limitations.

Assuming that the site is cleaned up everywhere with no dilution

caused by mixing with uncontaminated ground water, then the

minimum remediation time due to mass transfer limitations can

be calculated as,

 tmin = - ( ρb λk / ξ ) ln ( sk

*/sk’ ) (4)

where ρb is the formation bulk density (M/L3),λk is the distribution

coefficient for compound k (L3/M), ξ is a first-order mass transfer

rate parameter (1/T), sk

*

 is the immobile domain concentration

standard (M/M), and sk

’

 is the initial maximum immobile

concentration of contaminant k found at the site (M/M). Ratelimited mass transfer hinders short-term cleanup, but may have

negligible impact on long-term P&T. Desorption or diffusion rate

limitations may make it impossible to achieve cleanup within a

short time.

For the combination scheme shown in Figure 14, seven or eight

wells are optimal to achieve cleanup within three years to

sufficiently reduce the distance contaminants must travel within

the short remediation period. Ground water is pumped at the

highest rates along the plume axis and in the location of the most

retarded compounds to compensate for their low velocities.

The combination scheme essentially reduces to an individual

downgradient well design for longer remediation periods. Only

two or three wells along the plume axis are needed for cleanup

and the ideal well locations approximate those of the individual

downgradient scheme (e.g., one well cleans up the most retarded

plume and the other cleans up the more mobile, downgradient

plumes). The individual downgradient scheme, which requires

fewer wells, therefore, is well-suited for longer-term P&T efforts.

For fast cleanup, the hot spot scheme requires less pumping

than all but the combination scheme. More pumping, however,

is required using the hot spot wells for a 15-year clean-up period

compared to the individual downgradient scheme. This is

because individual downgradient wells take advantage of the

plume migration via slow regional ground-water flow during the

longer clean-up period.

The classic downgradient scheme (Figure 14) is the least

desirable alternative shown for attaining cleanup because the

contaminants must travel completely across the multiplume site

to reach the recovery wells. As a result, the more retarded

contaminant plumes are smeared to the wells, an excessive

volume of ground water must be extracted for cleanup, and

short-term cleanup is infeasible. The single recovery well option

also has significant drawbacks. It will generally require pumping

more ground water and result in more contaminant smearing

than all of the other schemes except the classic downgradient

design.

A good P&T design must address mobile, weakly-sorbed and

slow-moving, highly-sorbed contaminants to be effective at

Figure 14. Map view of five pumping schemes studied by Haggerty and Gorelick (1994) overlain on the initial 5 μg/L contours of

simulated CCl4, DCA, and THF plumes. Many of the possible well locations were not used because the optimization analysis

determined pumping at some locations to be 0 liters/sec. Only the optimum single well location was used for pumping under

the single well scheme (modified from Haggerty and Gorelick, 1994).

 300 meters

Hot-Spot Scheme Combination Scheme

Single Well Scheme Individual Downgradient Scheme Classic Downgradient Scheme

Flow Flow Flow

Flow Flow

CCL4

DCA THF

CCL4

DCA THF

CCL4

DCA THF

CCL4

DCA THF

CCL4

DCA THF

CCL4 = carbon tetrachloride

DCA = 1,2, dichloroethane

THF = tetrahydrofuran

Initial 5 g/L contours shown

Possible well location
17

cleanup. Substantial pumping should occur in the upgradient

portion of a multiplume site to minimize both the smearing of

strongly sorbed contaminants and the total volume of ground

water that must be extracted for cleanup.

Other Considerations

Cyclic water-level fluctuations — Ground-water levels near

surface water respond to changes in surface water stage.

Cyclic stage fluctuations occur in tidal waters and in some

streams that are regulated by pumping or discharge control.

Where the surface water fluctuates as a harmonic motion, as

occurs due to tides, a series of sinusoidal waves is propagated

into the aquifer (Ferris, 1963). The amplitude of each transmitted

wave decreases and the time lag of a given wave peak increases

with distance from the surface water. Hydraulic gradients

between contamination sites and nearby tidal water bodies,

therefore, increase at low tide and decrease (or may be locally

reversed) at high tide. As a result, these cyclic water-level

fluctuations tend to enhance ground-water capture during high

tide periods and inhibit capture during low tide periods. The

impact of cyclic water-level fluctuations can be examined using

analytical solutions (Jacob, 1950; Ferris, 1963) or numerical

models with highly refined time steps and boundary conditions.

At contamination sites that are influenced by cyclic water-level

fluctuations, consideration should be given to adopting a variable

rate pumping schedule, with higher extraction rates during low

stage periods, to provide cost-effective hydraulic containment

throughout the surface water stage cycle.

Dewatering — Water flushing will be limited to infiltration rates

where P&T operation has lowered the water table and partially

dewatered contaminated media. As a result, dissolved

contaminant concentrations may rebound when the water table

rises after pumping is reduced or terminated. Water can be

injected or infiltrated, and pumping locations and rates can be

varied, to both minimize this potential problem and increase the

rate of flushing. Where injection is not feasible, soil vapor

extraction or other vadose zone remedial measures might be

needed to remove contaminant mass above the water table.

Drawdown limitations — Under some conditions, hydraulic

containment cannot be maintained unless barrier walls are

installed and/or water is injected (or infiltrated) downgradient of,

or within, the contaminated zone. Limited aquifer saturated

thickness, a relatively high initial hydraulic gradient, a sloping

aquifer base, and low permeability are factors that can prevent

hydraulic containment using wells or drains (Saroff et al., 1992).

Where these conditions exist and hydraulic containment is

planned, particular care should be taken during pilot tests and

monitoring to assess this limitation.

Fractured and karst media — Fractured and solution-channeled

geologic materials often represent highly heterogeneous and

anisotropic systems to which techniques developed for

characterization and evaluation of porous media are not readily

applicable. Characterization techniques in such systems are an

area of continuing research and beyond the scope of this

document. Contaminant transport and P&T design/operation

will be largely controlled by such factors as orientation, density,

and connectivity of transmissive fracture systems. Techniques

used to evaluate potential capture zones and remedial time

frames based on porous-media assumptions often will not be

applicable. Evaluations of capture zones will generally be

based on site-specific characterization of the fractured or karst

system and may involve use of tracer tests, observations during

aquifer tests, and other specialized techniques such as borehole

flowmeter investigations to define transmissive fracture systems

and evaluate connectivity.

Information required for extraction well design will include

characterization of transmissive, contaminated areas and

intervals in fractured/karst systems and characterization of flow

and transport parameters in any overlying porous materials

(e.g., overburden, saprolite). At some sites where overburden

and fractured rock are contaminated, extraction wells screened/

Figure 15. Optimal pumping rates for each well scheme (Figure 14) showing the minimum rate needed to capture and clean up the

contaminants for the 3-year and 15-year pumping periods and mass transfer rates ranging from infinite (at equilibrium) to

0.005/day (modified from Haggerty and Gorelick, 1994).

Mass Transfer Rate = Indefinite (equilibrium) 0.06/day 0.02/day 0.005/day

Infeasible

Infeasible

Infeasible

Infeasible

Infeasible

Infeasible Infeasible Infeasible

Infeasible

0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25

0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25

3-Year Cleanup 15-Year Cleanup

Total Pumping Rate (liters/sec)

Total Pumping Rate (liters/sec)

Single Well

Classic Downgradient

Individual Downgradient

Hot-Spot

Combination

Single Well

Classic Downgradient

Individual Downgradient

Hot-Spot

Combination
18

open across both units may be acceptable with adjustments in

filter pack/screen specification for each unit. Conversely, it may

be practical to screen wells only in the more transmissive unit to

capture contaminants in both units. Such determinations depend

on the distribution of hydraulic parameters in each affected unit.

Ultimately, pilot testing of wells with careful monitoring generally

will be required to evaluate the effectiveness of such systems.

In some situations, rock units may be sufficiently fractured as to

approximate porous media behavior (de Marsily, 1986) allowing

use of more traditional design evaluations discussed elsewhere

in this document. In other situations, contaminants may be

moving only in very discrete fracture systems rendering

characterization difficult and necessitating careful delineation

of dominant fractures and design of wells with very discrete

screen/open intervals for optimum operation. The usual design

approach in this situation is to locate and screen wells to

intersect as many contaminated, transmissive fractures as

possible (Gorelick et al., 1993). Testing of each well will be

required to determine specific drawdown/flowrate relationships

and evaluate potential gradient control.

The optimal well design for each of these situations will depend

on the site-specific distribution of contaminants and hydraulic

properties of each rock and overburden unit. However, similar

design principles apply to fractured systems as to heterogeneous

porous media. Design should be based on three-dimensional

contaminant distribution and three-dimensional analysis of

hydrologic properties of each unit within the system. In general,

there still will be a significant degree of uncertainty associated

with determinations of flow/transport in fractured/karst systems

at most sites due to the impracticability of defining contaminant

distribution and transport parameters in sufficient detail using

available characterization techniques. A flexible design

approach and performance monitoring can be used to minimize

the effect of these uncertainties.

Highly permeable and heterogeneous media — In highly

permeable media, high pumping rates are usually required to

attain demonstrable hydraulic containment. Barrier walls and

low-permeability surface covers installed to reduce the rate of

pumping needed for containment also facilitate demonstration

of inward hydraulic gradients (Figure 3). Hydraulic containment

and site characterization can also be enhanced in heterogeneous

media by installing barrier drains and walls, particularly if done

in a manner that allows subsurface examination during

construction.

Horizontal anisotropy — Significant horizontal anisotropy may

be present at some sites, particularly where strata are inclined

or fractured. The directions of maximum and minimum

permeability are usually aligned parallel and perpendicular,

respectively, to foliation or fractures. In anisotropic media, the

flow of ground water (and contaminants moving with ground

water) is offset from the hydraulic gradient in the direction of

maximum permeability. Interpretation of hydraulic head data

and capture zone analysis must account for anisotropy to

evaluate extraction/injection wellfield effectiveness. Various

well hydraulics equations (Papadopulos, 1965; Kruseman and

deRidder, 1990) and numerical models can be employed to

account for anisotropic conditions during P&T design.

Injection/extraction cells — Recharging upgradient of the

contaminant plume and flushing the contaminant toward a

downgradient extraction well can be designed to create a

ground-water recirculation cell that isolates the plume from the

surrounding ground water (Figure 16). Injection and extraction

rates and locations can be adjusted to minimize the volume of

ground water requiring treatment, increase flushing rates through

the contamination zone (thereby reducing the flushing time),

and provide additional containment (Wilson, 1984). If permitted

and properly designed, water injection can greatly enhance

hydraulic control and contamination zone flushing. Of course,

due to water balance considerations (i.e., recharge from the

land surface), it is generally not possible to reinject and recapture

all of the extracted ground water. Poorly designed and

inadequately monitored injection can lead to unintended

horizontal and/or vertical contaminant migration.

Partial penetration — Construction of wells that only partially

penetrate the aquifer may be desirable or undesirable in different

situations. Contaminated ground water emanating from shallow

source areas frequently is limited to the upper portion of a

hydrogeologic unit. For this case, partially-penetrating recovery

wells should be constructed to limit the downward spread of

contaminants and the extraction of clean deep ground water. In

situations where extraction wells or drains partially penetrate a

contaminant plume capture may not extend to the lower limits of

the plume. Three-dimensional data (e.g., hydraulic head,

hydraulic conductivity distribution, contaminant distribution) are

required to evaluate and monitor three-dimensional capture. In

such situations, construction of wells or drains that fully penetrate

the contaminated interval may reduce uncertainty and costs

associated with monitoring vertical capture.

Physical barriers — Physical barriers to ground-water flow (e.g.,

slurry walls, grout curtains, sheet piling, etc.) reduce inflow into

the system and often allow use of lower ground-water extraction

and treatment rates to achieve a particular hydraulic head

distribution (e.g., inward hydraulic gradient or significant

dewatering). Surface caps may also be used to reduce infiltration

and further reduce extraction requirements. In addition, use of

such barriers and maintenance of an inward hydraulic gradient

will generally reduce the complexity of adequately monitoring

capture zones.

Figure 16. Plan view of a single-cell hydraulic containment,

showing flow lines and a hatched contaminant

plume (modified from Wilson,1984).

Injection

Well

Containment

Cell

Extraction

 Well

Ground-water

Flow
19

Situations in which use of physical barriers may be advantageous

or cost effective include sites where treatment capacity for

extracted ground water is limited, reductions in treatment costs

outweigh barrier construction costs, and heterogeneous sites or

sites with relatively high pre-design hydraulic gradients where

uncertainty in capture zone determinations is high. Additional

details regarding design and construction aspects of physical

barriers may be found in U.S. EPA (1984), Evans (1991), Grube

(1992), and Rumer and Ryan (1995).

Although these features may be used as enhancements to a

P&T system, they often will not be appropriate replacements for

P&T. Physical barriers without the use of P&T to lower hydraulic

head within the enclosure will generally result in increasing

hydraulic head within the wall. This may result in leakage over

the wall, under the wall, or through relatively minor imperfections

in the wall.

Physical constraints — Many ground-water contamination sites

are located in developed areas where the presence of roads,

buildings, and other structures constrain the placement of P&T

components (i.e., wells, pipelines, and treatment plants). Such

constraints should be identified early in the design process and

incorporated into the analysis of feasible remedies. In some

cases, it will be necessary to assess potential for subsidence

that may result from pumping.

Surface-water interactions — Streams, rivers, lakes, and other

surface water bodies frequently act as discharge boundaries to

local and regional ground-water flow systems (and dissolved

contaminants migrating therein). A variety of complex leakage

and discharge relationships, however, exist spatially and

temporally between surface water and ground water. Interaction

between ground water and surface water may help or hinder

P&T operations. At some sites, P&T design can take advantage

of induced infiltration along stream line sinks to enhance hydraulic

containment and flushing rates. Elsewhere, it may be desirable

to limit streambed leakage (e.g., using physical barriers) to

minimize requisite pumping rates or the inflow of surface water

that has been contaminated at upstream locations. Consideration

should also be given to potential hydraulic benefits of discharging

treated ground water at alternative stream locations. Relatively

complex interactions between surface water and ground water

can best be analyzed by numerical model analysis and monitoring

system performance.

Timeliness of remedial action — Research has shown that

contaminants that have been in contact with porous media for

long times are much more resistant to desorption, extraction,

and degradation (Brusseau, 1993). As the residence time of a

contaminant plume increases, so do potential contaminant

tailing and rebound problems associated with sorption/desorption

and matrix diffusion. Old plumes are likely to exhibit significant

nonideal behavior, making cleanup difficult. Remedial efforts

should be implemented as soon as practicable following a

release to limit the difficulty of removing contaminant mass from

low permeability zones and sorbed phases.

Well completion interval — Well completion intervals are selected

based on site conditions and P&T strategy. Maximum well yield

can generally be obtained by screening 80 percent to 90 percent

of the thickness of a confined aquifer. In an unconfined

formation, the screen should be placed low enough in the

contaminated section so that the pumping level is not drawn into

the screen. This will prevent aeration of the screen and extend

the service life of the screen and pump. Longer screens may be

needed in thick contamination zones and in low permeability

formations to achieve an acceptable yield.

An individual well (with zone-dependent screen and sandpack

characteristics) may be completed in multiple transmissive

zones and hydrogeologic units if such a construction will not

exacerbate vertical contaminant migration or prevent the costeffective cleanup of individual layers. In general, (1) screens

should not be constructed to hydraulically connect transmissive

zones across an aquitard; (2) it is undesirable to pump ground

water directly from uncontaminated intervals; and (3) partiallypenetrating recovery wells can be used to limit the downward

contaminant spreading and recovery pumping rates at sites

where contaminants are limited to the upper portion of a thick

hydrogeologic unit. Open-hole bedrock well completions are

usually acceptable, but care must be taken to not promote

contaminant migration (e.g., by completing an open-hole well

across an effective aquitard).

Site characterization activities (such as interval-specific packeraquifer tests, borehole flowmeter testing, and ground-water

sampling) and three-dimensional simulation analysis can be

used to help evaluate complicated cost-benefit trade-offs

between alternative well designs in vertically heterogeneous

media.

P&T Components

Ground-water extraction/injection systems are tailored to sitespecific conditions and remediation goals. As a result, system

component combinations yield a large variety of P&T

configurations. A conceptual process flow diagram for a typical

P&T system where volatile organic contaminants are removed

from ground water by air stripping (and carbon adsorption

polishing, as needed) is shown in Figure 17. Selected P&T

system components are described below and in Table 2. Specific

guidance regarding component selection and monitoring

treatment system discharge compliance with appropriate

regulations is beyond the scope of this document. Guidance

regarding monitoring system effectiveness with respect to

remedial design objectives is provided in Cohen et al. (1994).

Vertical Wells

Vertical wells are integral components of most P&T systems.

Extraction wells are intended to capture and remove

contaminated ground water; injection wells are used to enhance

hydraulic containment and ground-water flushing rates. Basic

component considerations include drilling/installation method,

well diameter, screen and casing specifications, completion

depth interval, and pump specifications. Detailed guidance on

well drilling, construction, and development methods is provided

by Repa and Kufs (1985), Driscoll (1986), Bureau of Reclamation

(1995), and others.

Well yield and efficiency are of prime concern when designing

extraction and injection wells. Yield is the rate at which

ground water can be pumped under site-specific conditions

(e.g., desired drawdown limits). Well losses caused by poor

design or construction decrease well efficiency and result in

increased drawdown within the well to maintain a particular

yield. This is one reason that hydraulic head measurements

taken in a pumping well are often poor indicators of hydraulic

head in the formation immediately adjacent to the well. Within
20

Table 2. Appurtenant Pump-and-Treat Equipment

 Equipment Description

Piping Conveys pumped fluids to treatment system and/or point of discharge. Piping materials will dictate if the system may

be installed above or below grade with or without secondary containment measures. Piping materials (i.e., steel,

HDPE, PVC, etc.) are selected based on chemical compatibility and strength factors.

Flowmeters Measures flow rate at given time and/ or the cumulative throughput in a pipe. Typically installed at each well, at major

piping junctions, and after major treatment units. Some designs allow for the instrument to act as an on/off switch

or flow regulator. Many different types are available.

Valves The primary use of valves (i.e., gate, ball, check, butterfly) is to control flow in pipes and to connections in the pipe

manifold. Valves may be operated manually or actuated by electrical or magnetic mechanisms. Check valves are

used to prevent backflow into the well after pumping has ceased and siphoning from tanks or treatment units. Other

uses for valves include sample ports, pressure relief, and air vents.

Level Switches Float, optical, ultrasonic, and conductivity switches/sensors are used to determine the level of fluids in a well or tank.

Sensors Used to actuate or terminate pumping and to indicate or warn operators of rising or falling fluid levels in wells and

tanks.

Pressure Switches Used to shut off pumps after detecting a drop in discharge pressure caused by a loss in suction pressure.

Pressure and Used to measure the pressure in pipes, across pipe connections, and in sealed tanks and vessels.

Vacuum Indicators

Control Panels Device which provides centralized, global control of P&T system operation and monitors and displays system status.

Control panels are typically custom designed for specific applications.

Remote Monitoring, Provides interactive monitoring and control of unattended P&T systems. Allows for real-time data acquisition. Alerts

Data Acquisition, & operators to system failures and provides an interface for remote reprogramming of operations. Remote monitoring

Telemetry Devices devices should also be accessible from the Control Panel.

Pull and Junction Above and/or below grade installations that allow access to connections in the piping manifold, electric wiring, and

Boxes system controls. Strategic placement provides flexibility for system expansion.

Pitless Adaptor Unit Allows for the transfer of extracted ground water from the well to buried piping outside of the well casing.

Well Cover Available with padlock hasps, with and without a connection to the electrical conduit for submersible pumps.

Figure 17. Example conceptual treatment diagram for a P&T system using air-stripping and optional granular activated carbon polishing

treatment of liquid and vapor phase effluent from the air stripper.

GAC

1

GAC

2

GAC

1

GAC

2

Air

Stripper

Vapor

Discharge to

Atmosphere

Contingent

Vapor

Phase

Carbon

Treatment

Vapor

Discharge to

Atmosphere

Contingent

Vapor

Phase

Carbon

Treatment Solids

Filter

Liquid Discharge

Air

Blower

Pump

Pre-Treatment Units

(i.e., oil/water

separation, solids

removal, metals

treatment, chlorination,

heat exchanger)

Flowmeter

Wells

Valve

Storage

and Flow

Equilization

Tank
21

limits, such parameters as screen diameter, screened interval,

and screen open area are specified to optimize yield and

maximize efficiency. Increased yield results in minimizing the

number of wells required to attain specific system design

objectives.

The well diameter must be large enough to accommodate the

pump and other downhole instrumentation. The size of the

pump required to obtain the desired yield, within the sitespecific hydrogeologic limits, will determine the size of the

casing. Although several different types of pumps may be

selected, standard electric submersible pumps are most

commonly used for extracting ground water at contamination

sites. A close fit between the pump and casing promotes

cooling of the submersible pump motor; but can lead to insertion

or removal difficulties. Commonly, the casing diameter is sized

two standard pipe sizes larger than the pump. For example, a

4-inch diameter pump is set in a 6-inch diameter well. The well

diameter generally should be no less than one pipe size larger

than the nominal pump diameter. Except for well point systems,

pumping wells are usually at least four inches in diameter. The

casing size should also be selected to ensure that the uphole

velocity during pumping is <5 ft/sec (Driscoll, 1986) to prevent

excessive head losses.

Pump selection depends on the desired pumping rate, well

yield, and the total hydraulic head lift required. Designers

should consult performance curves and data provided by

pump manufacturers. Pneumatic pumps are used in some

P&T applications, particularly at sites where providing electrical

service is problematic, combustible vapors are present, or

excessive drawdown might damage electric submersible

pumps. Electric and pneumatic pumps that extract total fluids

or separate liquid phases (e.g., LNAPL, water, and DNAPL)

are readily available.

Extraction wells may be driven (or jetted) well points, naturally

developed wells, or filter-packed wells. Screens and filter packs

should be appropriately sized to the native media. Grain-size

analyses of unconsolidated formation samples are highly

recommended to determine appropriate slot and sandpack

sizes. Wells can often be developed with natural packs in areas

where the formation materials are permeable and relatively

coarse grained. In naturally developed wells, the slot size is

chosen so that most fines adjacent to the borehole are pumped

through the screen during development. Custom screen design

using sections with different slot sizes based on the grain size

distribution of the different materials in the screened interval

may be useful at sites where the highest possible specific

capacity is desired.

Use of an artificial filter pack is advantageous when the geologic

materials are highly laminated; highly uniform, fine-grained

deposits; or in situations where a small screen slot size (e.g.,

<0.010 inch) dictated by natural pack criteria would significantly

reduce the water transmitting capability of the screen (Driscoll,

1986). Filter pack materials generally are composed of clean,

well-rounded, uniformed-sized, siliceous grains and designed

to retain most of the natural formation materials. The screen slot

size is then typically selected to retain 90 percent of the

sandpack. Grading of the filter pack is based on the finestgrained layer in the screened interval. Such a design generally

does not restrict flow from coarser-grained layers as the hydraulic

conductivity of the filter pack is significantly higher than the

conductivity of these layers.

Filter packs mechanically retain formation particles. The factor

controlling formation retention is the ratio of pack grain size to

that of the formation, not pack thickness. Pack thickness

recommendations from the literature for production wells

range from approximately 3 inches to 8 inches (U.S. EPA,

1975). Pack thickness in the lower end of the recommended

range will often be required to allow sufficient development for

maximum well efficiency. Two common errors in filter packed

wells that lead to low yields are use of a standard filter pack

regardless of formation characteristics and use of screens

with improper slot sizes for given filter pack characteristics

(Driscoll, 1986).

Bedrock wells can be completed as open-holes; but screen

and sandpack may be desirable to prevent caving and limit

sand pumping. Well development by surging, jetting,

backwashing, and pumping improves well efficiency. Driscoll

(1986) provides a comprehensive treatise on well design,

construction, and development and should be consulted prior

to design.

Well screen and casing are frequently constructed of black lowcarbon steel, Type 304 or Type 316 stainless steel, and PVC.

Although low-carbon steel is frequently used for well casing,

serious iron oxidation problems may occur when sodium

hypochlorite is used to redevelop the wells. Iron flaking may

cause clogging in injection wells. Manufacturers can provide

advice on material compatibility with ground water and

contaminants regarding the potential for corrosion, incrustation,

and chemical attack. Material compatibility guidance is also

available in various documents (e.g., Driscoll, 1986; McCaulou

et al., 1995). The physical strength of the screen and casing

materials is a concern for very deep wells. PVC casing may not

be suitable for depths exceeding 300 feet, especially for largediameter wells. Screens do not need to be as strong as casing

because their openings relieve hydrostatic pressure, but must

be able to withstand stresses associated with well installation

and development. Screens are more susceptible to corrosion

failure than steel casing. Whereas casing can suffer substantial

corrosion and still function, minor screen corrosion can enlarge

slot openings and result in severe sand pumping. This accounts

for the use of stainless steel screens in conjunction with mild

steel casing. For an economical well installation that resists

degradation due to high concentrations of organic chemicals,

stainless screen and casing can be threaded to PVC riser above

the water table. Properties and dimensions of selected well

casing products are highlighted in Table 3.

Generally, well screen diameter is selected to provide sufficient

open area so that the velocity of water entering the screen is less

than 0.1 ft/s to minimize friction losses, corrosion, and incrustation

(Driscoll, 1986). Screen diameter influences well yield but to a

lesser extent than does screen length. The potential increase

of well yield with increasing screen diameter depends on sitespecific conditions. Potential increases in some situations

may be relatively insignificant. For example, in relatively

conductive material where yields are high, increasing the

screen diameter from 6 inches to 12 inches may only result in

yield increases of several percent. In materials with low hydraulic

conductivity, potential yield increases resulting from increased

screen diameter may be significant and should be considered.

Open area of the screen affects entrance velocity and well

efficiency. Limited open area limits well development and

results in increased drawdown within the well for a specific yield.
22

Open area for different slot configurations (e.g., machine slotted

vs. continuous slot) varies significantly. Continuous slot screens

have significantly more open area per foot of screen than other slot

configurations. Manufacturers should be consulted regarding

open area of their screens. If the entrance velocity is calculated to

be too high (i.e., > 0.1 ft/s), longer screens with greater open area,

or larger diameter screens, where practical, should be considered.

The pump intake generally should not be placed in the well

screen. Such placement may result in high screen entrance

velocity, increased incrustation or corrosion rates, sand pumping,

or dewatering of the screen (Driscoll, 1986). In general, the

pump intake position does not greatly affect the relative volumes

of water produced by different formation materials in the screened

interval. In most situations, this distribution is predominantly

controlled by the hydraulic properties (e.g., hydraulic conductivity)

of the various materials.

Due to their tendency to clog, injection wells are typically

overdesigned in terms of well diameter or screen length to

reduce maintenance activities. Rather than using vertical wells,

artificial recharge may be better accommodated by surface

spreading, infiltration galleries, trenches, or horizontal wells.

These options have greater surface area and are less likely to

clog than vertical wells.

Vertical Well Point Systems

Well point systems are comprised of multiple closely spaced

wells that are connected via a main pipe header to a suction lift

pump. Suction lift systems are limited to pumping shallow

ground water at depths of less than approximately 25 feet. Such

systems are based on construction dewatering technology.

Well point systems are often used at sites where the hydraulic

conductivity of aquifer materials is relatively low and large

numbers of wells would be required to meet design objectives,

particularly hydraulic containment or dewatering objectives.

Where applicable, such systems may be more cost-effective

than conventional wells. Well point systems are described in

more detail in Bureau of Reclamation (1995).

Horizontal and Slant Wells

During recent years, directional drilling rigs from the utility,

mining, and petroleum industries have been adapted to install

horizontal wells at contaminated sites (Kaback et al., 1989;

Table 3. Properties and Dimensions of Selected Well Casing Products

Wall Collapse

Casing Size OD ID thickness Weight Strength

Material (in.) (in.) (in.) (in.) (lb/ft) (psi) Comments

Black Steel Thin 4 4.500 4.216 0.142 6.60 + Stronger, more rigid, and less temperature

Wall Water sensitive than PVC.

Well Casing 6 6.625 6.249 0.188 12.9 1030 + Much less expensive than stainless steel.

- Rusts easily, providing sorptive and reactive

Black Steel 4 4.500 4.026 0.237 10.8 capacity for metals and organic chemicals.

Schedule 40 - Subject to corrosion (given low pH, high

6 6.625 6.065 0.280 19.0 2286 dissolved oxygen, H2S presence, >1000 mg/L

TDS, >50 mg/L Cl-).

PVC 4 4.500 4.026 0.237 2.0 158 + Lightweight, easy workability, inexpensive.

Schedule 40 + Completely resistant to galvanic and

(PVC 12454) 6 6.625 6.065 0.280 3.6 78 electrochemical corrosion.

+ High strength-to-weight ratio.

+ Resistant to low concentrations of most organic

PVC 4 4.500 3.826 0.337 2.8 494 contaminants.

Schedule 80 - Poor chemical resistance to high concentrations

(PVC 12454) 6 6.625 5.761 0.432 5.4 314 of aromatic hydrocarbons, esters, ketones, and

organic solvents.

- Lower strength than steel; may not be suitable

for very deep applications.

Stainless Steel 4 4.500 4.334 0.083 3.9 315 + Stronger, more rigid, and less temperature

Schedule 5 sensitive than PVC.

6 6.625 6.407 0.109 7.6 129 + Good chemical resistance to organic

chemicals.

+ Resistant to corrosion and oxidation.

Stainless Steel 4 4.500 4.026 0.237 10.8 2672 - Expensive.

 Schedule 40 - May corrode if exposed to long-term corrosive

6 6.625 6.065 0.280 19.0 1942 conditions.
23

Karlsson and Bitto, 1990; Langseth, 1990; Kaback et al., 1991;

Morgan, 1992; Conger and Trichel, 1993; WSRC, 1993; U.S.

EPA, 1994b; and CCEM, 1995). As of January 1996, more than

370 horizontal wells had been drilled at contamination sites in

the United States for ground-water extraction (33%), soil vapor

extraction (35%), air sparging (21%) and other purposes (11%)

including petroleum recovery, ground-water infiltration, and

bioventing (CCEM, 1996). Most of these wells (73%) were

installed less than 26 feet deep by utility type contractors.

Horizontal wells can be drilled in soil or rock as continuous holes

with surface access at each end or as blind holes (Figure 18).

Slant wells are completed in straight angle borings. As shown

in Figure 18, slant or horizontal wells can be strategically

installed to: (1) allow injection or extraction in inaccessible

areas such as beneath buildings, ponds, or landfills; (2) intercept

multiple vertical fractures; and (3) provide hydraulic control

along the leading edge of a plume or elsewhere by creating

hydraulic line sinks (extraction) or pressure ridges (injection)

without the need to excavate trenches. Horizontal wells with

long screens may be more cost-effective than vertical wells,

particularly at sites where contaminated ground water is extensive

horizontally, but not vertically. The higher cost of horizontal

wells, compared to vertical wells, may be offset by savings

derived from more efficient remediation, drilling fewer wells, the

purchase of fewer pumps, etc.

Horizontal well construction methods are described by U.S.

EPA (1994b), CCEM (1995), and in drilling contractor literature.

Continuous holes are typically drilled as inverted arcs from a

surface entry point to a surface exit point. Using an adjustable

angle, slant rotary drill rig, a pilot boring may be advanced at an

angle to the desired subsurface elevation, directed along the

completion path using a steerable drill head and a walkover

radio-frequency (or other) guidance system, and then angled

upward to exit the ground. Following completion of the pilot

hole, the boring is cut to the final diameter using reamers. Rock

holes may be advanced using a steerable tungsten carbide bit

with a downhole air hammer, air rotary, or mud motor assembly.

Drilling fluids, consisting of water, air, bentonite slurries, or

polymeric solutions, are typically recycled through a closedloop system to remove cuttings from the borehole. Well screen,

casing, risers, filter fabric and/or pre-packed filter media are

assembled at the exit end of a continuous hole and pulled into

place behind the reamer. Pre-packed stainless steel screens

have been selected for wells requiring a filter pack, but less

expensive materials, including PVC, HDPE, steel, and fiberglass

have also been used for well screen. Similar to conventional

water wells, horizontal well screen and filter pack sizes are

designed to optimize well yield and limit fine particle entry.

Much greater compressive and tensile strength are required of

horizontal well materials, however, to prevent failure during

emplacement or wellbore collapse. Pre-packed filter materials

are used due to the difficulty of placing sand, plugs, and grout

in a horizontal well bore. Horizontal wells may be developed

by pumping, swab/surging, and jetting. For blind wells, a

washover bit and pipe are used to allow horizontal well

construction within a temporary casing.

Trench Drain Systems

Trench drain systems are typically constructed perpendicular to

the direction of ground-water flow to cut off and contain

contaminant migration by creating a continuous hydraulic sink

(Figure 19). A trench drain installed along the plume axis,

however, will provide more effective contaminant mass removal

(but may not provide complete containment). Designers should

assess the potential for downgradient mounding of ground

water transmitted along the length of a drain system, particularly

if the drain is not oriented perpendicular to the natural flow

direction. It may be appropriate to construct segmented drains

to restrict flow along the drain length.

Trench drains are typically constructed using a backhoe to

shallow depth in heterogeneous, low permeability media where

many wells would be needed to obtain the required yield for

capture of a specific area, but may also be suitable in moderate

and high permeability soil. Although the depth limit for

conventional excavation techniques is about 20 feet, specialized

equipment can be used to install trench drains as deep as

approximately 70 feet or deeper. The saturated zone of the

trench is backfilled with a highly-permeable granular material

such as sand or gravel. Geotextile filter fabric is placed around

the permeable backfill to prevent fine particles from clogging the

drain system. The upper few feet of the trench should be

backfilled with low permeability material to reduce infiltration.

Ground water that enters the granular backfill flows through the

fill, and/or through perforated pipe installed near the trench

bottom, to an extraction sump or sumps pumped to maintain a

hydraulic sink along the drain.

Consideration should be given to pipe cleanout access and the

installation of monitor wells along the drain length. At some

locations, it may be advantageous to install an impermeable

synthetic membrane on the downgradient side of a cutoff trench

to prevent fluid bypass. Trench drain systems can also be used

to inject treated or clean water to create pressure ridges and

Figure 18. Several applications of horizontal wells (modified from

U.S. EPA, 1994b).

Building, landfill,

lagoon, etc.

(a) Continuous horizontal well for access

(c) Continuous horizontal well to intersect vertical fractures

Leaking UST

(b) ‘Blind’ horizontal well to intercept a flat-lying plume
24

thereby enhance hydraulic containment and flushing rates.

Drain depth, spacing, location, and other design criteria can be

assessed using the computational tools described in Table 1

and various analytical solutions (Cohen and Miller, 1983).

The cost of excavating drains into bedrock is usually prohibitive.

Drain construction also may be impractical due to access

restrictions, building stability concerns, and costs associated

with excavating large quantities of contaminated materials.

Potential excavation stability problems can be addressed by

using a trench box or other shoring methods, minimizing both

the time that excavated sections are kept open and the length

of open sections, and/or by use of guar guam or other gels (U.S.

EPA, 1992c). Alternatively, ‘one pass’ trenching techniques

may be applicable. For example, a ‘one-pass’ trencher can be

used to excavate a 12-inch wide trench to a maximum depth of

22 feet, install a HDPE perforated collection pipe, and place

granular backfill in a simultaneous operation (Gilbert and Gress,

1987). This method minimizes contaminant exposure during

trenching, quantities of contaminated material requiring

disposition, and stability problems. Additional information on

trench drain systems is provided by Repa and Kufs (1985),

Meini et al. (1990), Day (1991), and U.S. EPA (1991b, 1994b).

Treatment Technology Selection

Ground-water treatment technologies rely on physical, chemical,

and/or biological processes to reduce contaminant

concentrations to acceptable levels. Presumptive treatment

technologies include use of: air stripping, granular activated

carbon (GAC), chemical/UV oxidation, and aerobic biological

reactors for dissolved organic contaminants; chemical

precipitation, ion exchange/adsorption, and electrochemical

methods for treatment of metals; and a combination of

technologies to treat ground water containing both organic and

inorganic constituents (U.S. EPA, 1996). Widely-used groundwater treatment technologies that are available as package

plants are described in Appendix B .

The evaluation and selection of treatment alternatives for a

particular P&T system is based on technical feasibility and costs

(capital and operational) of achieving remediation goals. Key

parameters that influence treatment design and efficacy include

flow rate, ground-water constituents requiring treatment

(including naturally occurring dissolved metals that may foul or

interfere with a treatment system), influent concentrations, and

discharge requirements. Relationships between these

parameters and treatment design are discussed briefly below,

and in more detail by AWWA (1990), Nyer (1992), U.S. EPA

(1994a), WEF (1994), and Noyes (1994).

The treatment flow rate, influent concentrations, and desired

effluent concentrations influence the applicability of potential

treatment methods. Flow rate is usually based on a projection

of the pumping rate needed to achieve remediation goals.

Treatment plant capacity may need to be increased where

effluent is reinjected or where aggressive P&T is employed to

hasten cleanup. The degree of contaminant concentration

reduction required for each constituent is crucial to treatment

design. For example, although GAC adsorption may reduce the

concentration of a particular contaminant more than air stripping,

depending on the discharge requirements, air stripping may be

utilized as the sole technology, as pretreatment to GAC, or not

at all. The discharge requirements often depend on the final

disposal method for the treated water. Options include discharge

to surface water, reinjection, discharge to another treatment

system, or direct use. Regulations may preclude some options

due to effluent concentrations, flow rate, or potential impacts to

the ground water. Discharge to an existing treatment system

(POTW or industrial treatment system) is generally the least

restrictive option, but each system will have specific flow rate

and concentration requirements. Effluent discharge to surface

water and reinjection below the water table require permits.

The first step in selecting a treatment strategy is to exclude

methods that are not implementable based on contaminant

type, concentrations, flow rate, and site characteristics. Where

multiple contaminants are present, some technologies may be

excluded as complete solutions, but considered as a pretreatment

or polishing step in a ‘treatment train’. Thus, to effectively use air

stripping for volatile organic contaminants, it may be necessary to

pretreat the influent by chemical precipitation to remove dissolved

metals that could foul the stripping unit. Examples of unit processes

and sequences in ground-water treatment trains are listed in Table

4. At some sites, it may be beneficial to split ground water that is

extracted from different areas into more than one treatment train

(e.g., highly contaminated water from a source area may be treated

differently than dilute downgradient ground water).

The technically implementable methods are then assessed with

regard to effectiveness, relative implementability, and cost. An

evaluation of effectiveness should consider the projected rate

and duration of flow, the level of treatment required for each

constituent, and the reliability of each method. Reliability may

be difficult to assess for innovative technologies based on

readily available data. In the absence of adequate performance

data, treatability and pilot-scale testing should be conducted to

yield critical data for use in technology selection, design of fullscale facilities, estimating costs, and identifying potential

problems. The time element of treatment can be addressed

during pilot studies by appropriate scaling of treatment units,

flow rates, and concentrations (e.g., smaller capacity GAC units

can be used to determine constituent breakthrough times more

quickly). An evaluation of relative implementability should consider

technical and administrative aspects, including permits, space

limitations, storage and disposal options, availability of equipment,

availability of skilled workers to implement the technology, visual

impacts, and community relations. Cost estimates should include

capital costs, annual costs, and an estimate of treatment duration;

and cost comparisons should incorporate a discount rate for future

costs and a cash flow analysis.

Figure 19. A trench drain constructed perpendicular to the

direction of ground-water flow may provide more

effective containment than extraction wells (e.g., for

shallow contamination in heterogeneous, lowpermeability media).

Source

Area Drain
25

Treatment strategies should be designed and implemented in a

manner that will accommodate changing conditions over the life

cycle of a P&T project. At many sites, modifying treatment

capacity or methods to match changing influent chemistry or flow

rate over time can improve system performance and reduce cost. As

with pumping, treatment optimization requires ongoing monitoring.

Proposed designs (e.g., extraction/injection well construction

and placement, piping diagrams, treatment system design)

should be presented in drawings and accompanied by detailed

text discussion with appropriate tables. Discussion should

include such topics as materials selection, proposed processes,

and installation procedures. Rationale for design choices

should also be discussed with supporting calculations presented

and supporting data presented or referenced.

Technology Integration

Under favorable conditions (Figure 4), P&T technology can

achieve clean-up goals. However, most, if not all, remedial

methods will have difficulty rapidly restoring ground-water quality

to meet low concentration standards in the presence of highly

sorbing contaminants, NAPL, and heterogeneous media. In

these cases, remedial performance may be improved by

integrating P&T operation with other clean-up technologies.

This integration can occur spatially (e.g., where P&T is applied

to the dissolved plume and other technologies are applied to

source areas) and temporally (e.g., where multiple technologies

are applied in series).

Remedial technology integration has occurred at many sites

contaminated with petroleum product LNAPLs. Although mobile

LNAPL may be pumped via extraction wells, immobile product

will remain in the subsurface. Excavation is a candidate

technology to remove shallow LNAPL. Due to their volatility and

degradability, many petroleum products, such as gasoline, can

also be remediated using SVE and enhanced bioremediation.

Alternatively, natural attenuation may be demonstrated to be an

effective petroleum contaminant management strategy at some

sites.

Table 4. Common Treatment Train Unit Processes 1 and Sequence (modified from U.S. EPA, 1996)

Ground-Water Treatment Train Unit Processes

Solid or Liquid Separation Primary Treatment Effluent Polishing Vapor Phase Treatment

Technologies Technologies Technologies2 Technologies3

• Oil/grease separation4 For Organics: • Activated carbon • Activated carbon

• Filtration5 • Air stripping • Ion exchange • Catalytic oxidation

• Coagulation or flocculation5 • Granular activated carbon • Neutralization • Thermal incineration

• Clarification or sedimentation5 • Chemical/UV oxidation • Acid gas scrubbing

• Aerobic biological reactors • Condensation

For Metals:

• Chemical precipitation

• Ion exchange/adsorption

• Electrochemical methods

General Sequence of Ground-Water Treatment Train Unit Processes

Sequence Unit Treatment Process Treatment Stage

Begin Equalize inflow Pre-treatment

Separate solid particles Pre-treatment

Separate oil/grease (NAPLs) Pre-treatment

Remove metals Treatment

Remove volatile organic contaminants Treatment

Remove other organic contaminants Treatment

Polish organics2 Post-treatment

Polish metals Post-treatment

 End Adjust pH, if required Post-treatment

Notes: 1 Technologies that may be required for treatment residuals, such as spent carbon, are not listed.

 2 Effluent polishing technologies are used for the final stage of treatment prior to discharge, and can include pH

adjustment (neutralization) as well as additional removal of aqueous constituents.

 3 Vapor phase contaminants released during water treatment may need to be contained and treated. These include

organic contaminants volatilized during air stripping, from biological treatment, or other gases released from chemical

oxidation, reduction, or biologic processes (e.g., hydrochloric acid, hydrogen sulfide, methane, etc.).

 4 Methods for separating oil and/or grease from water include, but are not limited to, gravity separation and dissolved air

floatation. These methods can be used to remove NAPLs from extracted water.

 5 These technologies can be used to remove solid particles at the start of the treatment train or to remove other solids

resulting from chemical precipitation, chemical/UV oxidation, or biological treatment.
26

These same technologies (extraction, excavation, SVE, air

sparging and, to a lesser extent, bioremediation) have also

been applied to DNAPL source areas where the chemicals

have the appropriate properties. Additional technologies are

being evaluated for NAPL recovery (i.e., surfactant flushing,

steam flushing, alcohol flooding, hot water flooding, and

surfactant-enhanced solubilization). Except for excavation,

however, there are no proven technologies to remove sufficient

DNAPL to fully restore a DNAPL-contaminated aquifer (U.S.

EPA, 1992a). Therefore, hydraulic containment will remain

an important management option for the DNAPL-contaminated

portion of the subsurface.

Away from source areas, bioremediation also can be combined

with P&T. Various solutions, including dissolved oxygen and

nutrients, can be injected upgradient or within the contaminant

plume to enhance biodegradation. At some sites, natural attenuation

may be used in conjunction with or following ground-water extraction.

Natural attenuation refers to natural biological, chemical, and

physical processes that reduce contaminant concentrations

and mass. Also known as intrinsic remediation, it includes

destructive chemical transformation processes (radioactive

decay, biodegradation, and hydrolysis) and nondestructive

partitioning and dilution processes (sorption, volatilization, and

dispersion). At many sites, contaminant plume growth is

restricted by biodegradation, partitioning, and/or dilution. For

example, the limited mobility of many soluble petroleum

hydrocarbons, such as BTEX compounds, in ground water due

to biodegradation has been particularly well-documented (e.g.,

Barker et al., 1987).

Natural attenuation processes may be significant factors in

contaminant removal and limitations to aqueous-phase

contaminant migration at many sites. Field evaluation of such

processes and rates is an area of continuing research. Proposed

methodologies for evaluating natural attenuation of fuel

contaminants are discussed in Wiedemeier et al. (1995) and

McAllister and Chiang (1994).

Potentially cost-effective, innovative enhancements and

alternatives to P&T (NRC, 1994) are being pilot-tested at many

contamination sites. Permeable treatment walls using the

funnel-and-gate approach are leading candidate remedial

technologies (Starr and Cherry, 1994). These systems are

designed to reduce contaminant concentrations in ground water

that is passively funneled through a permeable reaction wall,

which contains abiotic or biologically reactive media, an air

sparge system, or some other enhancement.

Design of Operations and Maintenance

Detailed plans for evaluation of maintenance requirements

should be established prior to installation. Establishment of the

plan during the design stage allows for incorporation of features

to simplify maintenance procedures (e.g., access ports for

cleaning distribution piping in infiltration galleries). Maintenance

such as pump replacement and well development may be

performed on an as needed basis. The required frequency will

depend on site conditions and equipment. Equipment manuals

may be consulted regarding maintenance requirements for

specific system components.

The major causes of decreased well performance include

reduction in yield due to incrustation or biofouling of the screen

or adjacent materials, formation plugging by fine-grained

materials, corrosion or incrustation resulting in increased water

velocity and sand pumping, structural failure of the casing or

screen, and pump damage (Driscoll, 1986). Periodic monitoring

of total depth, pumping rate, drawdown, specific capacity, and

efficiency may be used as indicators of maintenance

requirements for extraction or injection wells. Injection wells and

galleries are particularly susceptible to blockage or fouling and

may require frequent maintenance. Maintenance schedules

should be sufficiently frequent so as not to compromise system

performance with respect to the established design objectives

(e.g., maintain capture, maintain specified pore volume flushing

rates). Additional discussion of operation and maintenance issues

is provided in Driscoll (1986) and Bureau of Reclamation (1995).

Performance Monitoring

P&T performance is monitored by measuring hydraulic heads

and gradients, ground-water flow directions and rates, pumping

rates, pumped water and treatment system effluent quality, and

contaminant distributions in ground water and porous media.

These data are evaluated to interpret P&T capture zones,

flushing rates, contaminant transport and removal, and to

improve system operation. Detailed guidance on methods for

monitoring P&T performance is provided by Cohen et al. (1994).

Restoration progress can be assessed by comparing the rate of

contaminant mass removal (e.g., plotted as cumulative mass

removed) to estimates of the dissolved and/or total contaminant

mass-in-place. If the rate of contaminant mass extracted

approximates the rate of dissolved mass-in-place reduction,

then the contaminants removed by pumping are primarily derived

from the dissolved phase. Conversely, a contaminant source

(i.e., NAPL, sorbed contaminant, or a continuing release) is

indicated where the mass removal rate greatly exceeds the rate

of dissolved mass-in-place reduction. Site hydrogeology and

contaminant properties should be evaluated to determine if

source removal or containment, or P&T system modifications,

could improve P&T performance.

The time needed to remove dissolved mass-in-place can be

projected by extrapolating the trend of the mass removal rate

curve or the cumulative mass removed curve. Future

concentration tailing, however, may extend the extrapolated

clean-up time. If the mass removal trend indicates a significantly

greater clean-up duration than estimated originally, system

modification may be necessary. The effect (or lack of effect) of

P&T system modifications will be evidenced by the continuing

mass removal rate and cumulative mass removed trends.

Progress inferred from mass removal rates can be misleading,

however, where NAPL and solid phase contaminants are present

(e.g., the mass removed will exceed the initial estimate of

dissolved mass-in-place). Interpretation suffers from the high

degree of uncertainty associated with estimating NAPL or solid

contaminant mass-in-place. Stabilization of dissolved

contaminant concentrations while mass removal continues may

be an indication of NAPL or solid phase contaminant presence.

Methods for evaluating the potential presence of NAPL are

provided by Feenstra et al. (1991), Newell and Ross (1992), and

Cohen and Mercer (1993).

Mass removal rates are also subject to misinterpretation where

dissolved contaminant concentrations decline rapidly due to:
27

(1) mass transfer rate limitations to desorption, NAPL or

precipitate dissolution, or matrix diffusion; (2) dewatering a

portion or all of the contaminated zone; (3) dilution of

contaminated ground water with clean ground water flowing to

extraction wells from beyond the plume perimeter; or (4) the

removal of a slug of highly contaminated ground water.

Contaminant concentration rebound will occur if pumping is

terminated prematurely in response to these conditions.

The projected restoration or clean-up time is site specific and

varies widely depending on contaminant and hydrogeologic

conditions and the clean-up concentration goal. Estimating

clean-up time is complicated by difficulties in quantifying the

initial contaminant mass distribution and processes that limit

cleanup. Guidance for estimating ground-water restoration

times using batch and continuous flushing models is provided

by U.S. EPA (1988b). The batch flushing model is based on a

series of consecutive discrete flushing periods during which

contaminated water in equilibrium with adsorbed contaminants

is displaced from the aquifer pore space by clean water. Values

of contaminant concentration in soil and water are calculated

after each flush. An example of an analogous method (and

corrections) to this batch flushing model are provided by Zheng

et al. (1991, 1992). The batch and continuous models assume

that: (1) zero-concentration influent water displaces

contaminated ground water from the contamination zone by

simple advection with no dispersion; (2) the clean ground water

equilibrates instantaneously with the remaining adsorbed

contaminant mass; (3) the sorption isotherm is linear; and (4)

chemical reactions do not affect the sorption process. Care

must be taken to avoid relying on misleading estimates of

restoration time that may be obtained by using these simplified

models. Although more sophisticated modeling techniques are

available (i.e., contaminant transport models), their application

often suffers from data limitations, resulting in uncertain

predictions. Nevertheless, clean-up time analyses are useful

for assessing alternative remedial options and determining

whether or not clean-up goals are feasible.

Disclaimer

The U.S. Environmental Protection Agency through its Office of

Research and Development partially funded and collaborated

in the research described here under Contract No. 68-C4-0031

to Dynamac Corporation. It has been subjected to the Agency’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

recommendation for use.

Quality Assurance Statement

All research projects making conclusions or recommendation

based on environmentally related measurements and funded by

the Environmental Protection Agency are required to participate in

the Agency Quality Assurance Program. This project did not involve

physical measurements and as such did not require a QA plan.

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33

Appendix A

Limitations for Conventional P&T Technology

Widespread experience with P&T systems during the past 15

years indicates that their ability to reduce and maintain dissolved

contaminant concentrations below clean-up standards in

reasonable time frames is hindered at many sites due to

complex hydrogeologic conditions, contaminant chemistry

factors, and inadequate system design (Keely, 1989; Mercer et

al., 1990; U.S. EPA, 1993a; NRC, 1994; and Cohen et al. 1994).

Hydrogeologic conditions that confound ground-water cleanup

include the presence of complex sedimentary deposits, low

permeability formations, and fractured bedrock. Chemical

processes that cause contaminant concentration tailing and

rebound during and after P&T operation, respectively, and

thereby impede complete aquifer restoration, include: (1) the

presence and slow dissolution of nonaqueous phase liquids

(NAPLs); (2) contaminant partitioning between ground water

and porous media; and (3) contaminant diffusion into low

permeability regions that are inaccessible to flowing ground

water. These limitations may render restoration using only

conventional P&T technology impracticable at some sites.

NAPL Dissolution

NAPLs that are denser than water (DNAPLs), in particular,

exacerbate ground-water restoration efforts. This is due to their

prevalence at contamination sites, their complex subsurface

migration behavior and distribution, their low aqueous solubility,

and limits to DNAPL removal using available technologies (U.S.

EPA, 1992a; Grubb and Sitar, 1994; and Pankow and Cherry,

1995). Greater success has been achieved remediating

petroleum hydrocarbon LNAPLs using conventional methods

and enhanced technologies such as soil vapor extraction,

bioremediation, and air sparging.

Subsurface NAPL trapped as ganglia at residual saturation or

contained in pools can be a long-term source of ground-water

contamination, as illustrated in Figure 8-d, due to its limited

aqueous solubility that may greatly exceed drinking water

standards. At many sites, NAPL pools will continue to

contaminate ground water long after residual fingers and ganglia

have dissolved completely (Cohen and Mercer, 1993). If

NAPLs are not removed (e.g., by excavation) or contained (as

depicted in Figure 2), then tailing and rebound will occur during

and after P&T operation, respectively, in and downgradient of

the NAPL zone. Above residual saturation, NAPL will flow

unless it is immobilized in a stratigraphic trap or by hydrodynamic

forces. NAPL movement can greatly expand the subsurface

volume where restoration is impractical. A critical element of

site characterization, therefore, is to delineate the nature and

extent of mobile and residual NAPL so that these source areas

can be removed or contained. Detailed guidance on NAPL site

characterization is provided by American Petroleum Institute

(1989), U.S. EPA (1992a), Cohen and Mercer (1993), and

Newell et al. (1995).

Contaminant Sorption and Desorption

Sorption/desorption also cause tailing, concentration rebound,

and slow ground-water restoration. As dissolved contaminant

concentrations are reduced by P&T operation, contaminants

sorbed to subsurface media desorb from the matrix and dissolve

in ground water. The volume of ground water that must be

passed through a contamination zone to attain clean-up

standards increases with contaminant sorption and kinetic

limitations to the rate of desorption.

Equilibrium contaminant partitioning between porous media

and ground water can be described by the Langmuir or Freundlich

isotherms (Figure A-1). For the linear isotherms (N = 1) and for

limited ranges of Cw (particularly at low concentration) where N

≠ 1, the Freundlich constant, Kf

, can be identified as a soil-water

distribution coefficient, Kd:

Kd = Cs / Cw (A-1)

where Cs and Cw are the equilibrium contaminant concentrations

in soil and water, respectively.

Figure A-1 The Langmuir and Freundlich adsorption isotherms

(modified from Palmer and Fish, 1992).

Contaminant Kd values must be characterized to predict groundwater restoration times for different P&T schemes or for natural

ground-water flushing. By assuming that sorption is

instantaneous, reversible, and linear, Kd values can be used to

estimate: (1) the retardation factor, R,

R = 1 + Kd ρb / n (A-2)

and (2) the equilibrium distribution of contaminant mass between

the solid and aqueous phases

Csmax

Langmuir Isotherm:

Cs = Csmax K Cw

1+ K Cw

Adsorbate Concentration (Cs)

(mass sorbate / mass sorbent)

Aqueous Concentration (Cw) Aqueous Concentration (Cw)

Cs = Kf Cw

N

Freundlich

Isotherm:

N > 1

N = 1

N < 1
34

fw = CwVw / [(CwVw )+(Cs Ms )] = Vw / (Vw + Kd Ms ) (A-3)

where ρb is the dry bulk density of the media, n is the media

porosity, Vw is the volume of water in the total subject volume,

Ms is the mass of solids in the total subject volume, and fw is the

fraction of mass residing in the aqueous phase.

Although the ratio of bulk density to porosity is typically within a

range of four to six, Kd

 values for different contaminants vary

over orders-of-magnitude (e.g., Montgomery and Welkom, 1990).

Thus, contaminant velocity and P&T restoration time are

particularly sensitive to soil-water partitioning (Kd values) of

ground-water contaminants.

The nonlinearity of contaminant desorption and difficulty of

contaminant removal appear to increase with the duration of

contaminant presence in the subsurface (Brusseau, 1993).

Thus, old plumes are likely to exhibit significant nonideal behavior.

Conversely, ground-water cleanup may be simplified if remedial

efforts are undertaken quickly after the occurrence of a

contaminant release.

Sorption and retardation values vary between different

contaminants at a given site and between different sites for a

given contaminant (Mackay and Cherry, 1989). As depicted in

Figure 8, desorption and retardation increase the volume of

ground water that must be pumped to attain dissolved

contaminant concentration reductions. Tailing and rebound

effects will be exacerbated where desorption is slow relative to

ground-water flow and kinetic limitations prevent sustenance of

equilibrium contaminant concentrations in ground water (Bahr,

1989; Brogan, 1991; Haley et al., 1991; and Palmer and Fish,

1992). This concept is illustrated in Figure A-2.

Figure A-2. Relationship between ground-water velocity and the

concentration of dissolved contaminants that (a)

desorb from porous media, (b) dissolve from

precipitates, or (c) dissolve from NAPL (modified from

Keely, 1989). Kinetic limitations to dissolution

exacerbate tailing.

Solids Dissolution

Important physicochemical processes that affect the solubility,

reactivity, mobility, and toxicity of inorganic contaminants include:

(1) chemical speciation, (2) oxidation/reduction, (3) dissolution/

precipitation, (4) ion exchange and sorption, and (5) particle

transport (Palmer and Fish, 1991). Inorganic contaminants

occur in many different chemical forms or “species.” Knowing

the total concentration of an inorganic element in ground water

or soil, as commonly provided by laboratory analysis, may be of

little value (alone) in assessing its subsurface behavior. Rather,

it is often more important to determine contaminant speciation

which depends on several factors including pH, Eh, ion and gas

concentrations, and temperature. For example, metal cations

combine with different anions to form aqueous complexes that

increase the solubility, mobility, and risk associated with

potentially toxic metals such as chromium and arsenic.

Given the complex interaction between solid minerals, inorganics,

and environmental factors (such as Eh-pH relations), computer

codes are used to assess the solubility and geochemical behavior

of inorganic species in ground water. Codes used to evaluate

mineral solubility, saturation, and chemical speciation include

WATEQ (Truesdell and Jones, 1974), SOLMINEQ88 (Kharaka

et al., 1988) and mass transfer codes, such as PHREEQE

(Parkhurst et al., 1980), EQ3/EQ6 (Wolery, 1979), and MINTEQ

(Felmy et al., 1984), that can also be used to deduce equilibrium

chemical reactions. Data requirements for these codes typically

include field analysis of ground-water samples for pH,

temperature, Eh or dissolved oxygen, and alkalinity, and a

complete inorganic chemical analysis for all major and minor

ions and all metals and anions under investigation.

Besides conducting thorough chemical analyses for speciation

studies, investigations should be conducted to delineate

inorganic contaminant plumes and estimate plume migration

rates. Mineralogical characterization efforts can be used to

identify solid phases (e.g., clay minerals and Fe and Mn

oxyhydroxides) that control inorganic contaminant partitioning

(EPRI, 1989). Sorption-desorption and other tests can be

conducted to assess inorganic contaminant partitioning,

solubility, and mobility as a function of pH and other factors, and

the potential for aquifer restoration. Additional information

relevant to assessing inorganic ground-water contamination is

given by EPRI (1989), Domenico and Schwartz (1990), Palmer

and Fish (1991), Stumm (1992), Fetter (1993), Runnells (1993),

and Allen et al. (1993).

Ground-Water Velocity Variation

Tailing and rebound also result from variable travel times

associated with different flowpaths taken by contaminants to

extraction wells. Ground water at the edge of a capture zone

travels a greater distance under a lower hydraulic gradient than

ground water closer to the center of the capture zone. Travel

times also vary as a function of initial contaminant distribution

and hydraulic conductivity differences. If pumping is stopped,

rebound will occur wherever the resulting flowpath modification

diminishes contaminant dilution. Permeability and contaminant

distributions should be characterized to facilitate analysis of

ground-water stagnation and velocity variations that would be

induced by alternative pumping schemes.

Matrix Diffusion

As contaminants advance through relatively permeable pathways

in heterogeneous media, concentration gradients cause diffusion

of contaminant mass into the less permeable media and thereby

retard solute velocity relative to ground water (Gillham et al.,

1984). During a P&T operation, dissolved contaminant

concentrations in the relatively permeable zones may be quickly

Equilibrium Concentration

Ground-water Velocity

Contact Time

Dissolved Concentration

Kinetic limitations limit

dissolved concentration

Long contact time

produced equilibrium

partitioning concentration
35

reduced by advective flushing relative to the less permeable

zones. This causes a reversal in the initial concentration

gradient and the slow diffusion of contaminants from the low to

high permeability media. This slow process can cause longterm tailing and rebound after the termination of pumping.

Matrix diffusion may dictate the time necessary for complete

remediation, particularly in heterogeneous and fractured media

where transport via preferential pathways results in large

concentration gradients (Grisak and Pickens, 1980; McKay et

al., 1993; and Parker et al., 1994). For example, consider a

sand aquifer with clay lenses that was contaminated for a long

time before commencing P&T operation. Advective transport

induced by pumping may quickly reduce contaminant

concentrations in the sand. Concentrations in the clay lenses,

however, will decrease slowly as contaminants slowly diffuse

from the clay to the sand. The areal extent of the clay is such

that an approximation of one-dimensional diffusion out of each

lens can be used to estimate the time needed to reduce

contaminant concentrations in the clay. If (Co) is a uniform initial

contaminant concentration in a clay lens of thickness m, and

that P&T maintains a very low concentration in the sand, then

the time required for diffusion to reduce the average relative

contaminant concentration (C/Co) in a clay lens can be estimated

by (Carslaw and Jaeger, 1959, p. 97):

C 

C 0 

 = 

8 C 0 

π 2 

4 

3 

n = 0 

1 

á 2 n + 1 é 2 exp ä 

ã - D 0 

α Rm 2 á 2 n + 1 é 2 

 π 2 t ë

í

 (A-4)

where R is the retardation factor, α is tortuosity (typically = 1.6

to 1.3 in granular media; Bear, 1972), Do is the free water

diffusion coefficient, and t is time. Considering typical free water

diffusion coefficients for organic contaminants (1 x 10 -5 to 1 x

10 -6 cm 2/sec), changes in C/Co in clay lenses of different

thickness are shown as a function of time in Figure A-3, and

indicate that matrix diffusion can greatly increase aquifer cleanup time.

Figure A-3. Changes in average relative contaminant concentration in clay lenses of specified thickness due to

diffusion to adjacent clean zones during P&T (based

on typical diffusion coefficient and tortuosity values).

1.0E-06 sq. cm/s

1.0E-05 sq. cm/s

Clay lens thickness

given in meters

1.2 m

0.6 m

1.2 m 0.6 m 0.3 m

Free Liquid Diffusion Coefficients

1

0.8

0.6

0.4

0.2

0

0 10 20 30 40

Relative Concentration

Time (years)

The potential for matrix diffusion to cause tailing and rebound

can be assessed based on (1) knowledge of the contaminant

concentration history in the subsurface, (2) site stratigraphy, (3)

chemical analyses conducted on vertical core samples taken

from low-permeability matrix material, (4) diffusion modeling,

and (5) review of P&T monitoring data. Estimates of water

diffusion coefficients for various contaminants and media are

available in the literature (Parker et al., 1994) or can be, but

rarely are, measured in a laboratory (Myrand et al., 1992).
36

Appendix B

Selected Ground-Water Treatment Technologies Available as Package Plants

(modified from U.S. EPA, 1994a; Boulding, 1995)

Advantages and Limitations

Air Stripping

Widely used to

remove volatile

contaminants from

ground water.

Volatile contaminants are transferred from water to gas phase by

passing air or steam through water

in a tall packed tower, shallow tray

tower, or stripping lagoons. The air

stream containing volatile contaminants may require treatment (e.g.,

with vapor-phase carbon).

Stripping with steam may be costeffective for water containing a mix

of relatively nonvolatile and volatile

compounds, particularly at

industrial facilities where steam is

readily available.

Method Process Description Package Plant Components

and Sizes

(Dimensions are for overall plant envelope)

Package plants include tall packed

tower or compact low profile diffuser

tray units, feed pump, air blower,

and effluent pump. Flow meters for

influent and air flow are required.

An influent throttle valve and blower

damper are required to adjust the

air/water ratio. Acid or chlorine is

used to wash the tower packing

(e.g., of Fe precipitates). Heights

are for packed tower units.

1-10 gpm — 4’x4’x20' — 2 HP

10-50 gpm — 6’x8’x25' — 5 HP

50-100 gpm — 7’x10’x30' — 8 HP

100-400 gpm— 8’x12’x40' —20 HP

Effective for VOCs. Equipment is

relatively simple. Startup and

shutdown can be accomplished

quickly. Modular design is well-suited

for contaminant P&T. Package

systems widely available.

Dissolved Fe and Mn can be

precipitated and foul the packed media

resulting in headloss and reduced

system effectiveness. Pretreatment

(oxidation, precipitation, sedimentation)

of influent may be required. Biological

fouling may also occur (requiring

cleaning via chlorination or a biocide).

Sensitive to pH, temperature, and flow

rate. May be cost-prohibitive at temperatures below freezing (may need to

heat). May need GAC polishing of

water effluent and treatment of air

stream.

Granular Activated

Carbon (GAC)

Adsorption

Widely used to

remove metals,

volatile and semivolatile organics,

pesticides, PCBs,

etc. from ground

water and leachate.

Aqueous contaminants are sorbed

to GAC or synthetic resin packed

in vessels in parallel or series.

Used sorbent is regenerated or

replaced. Extent of adsorption

depends on strength of molecular

attraction, molecular weight of

contaminants, type and

characteristics of adsorbent, pH,

and surface area.

Package systems include 1 to 3

pressure vessels on a skid, interconnecting piping, a feed pump,

optionally a backwash pump,

pressure gauges, differential pressure gauges, influent flow meter,

backwash flow meter, and control

panel. Spent adsorbers are

disconnected and sent to

regeneration centers or landfills.

1-10 gpm - 12’x8’x8' - 2 HP

10-50 gpm - 14’x8’x8' - 7 HP

50-100 gpm - 20’x10’x8' - 10 HP

100-200 gpm - 20’x20’x8' - 20 HP

Effective for low solubility organics.

Useful for a wide range of

contaminants over a broad

concentration range. Not adversely

affected by toxics.

High O&M costs. Intolerant of

suspended solids (will clog).

Pretreatment required for oil and

grease greater than 10 mg/L.

Synthetic resins intolerant of strong

oxidizing agents.

Chemical

Precipitation,

Flocculation,

Sedimentation

Widely used to

remove metals from

contaminated ground

water and landfill

leachate.

Metals are precipitated to insoluble

metal hydroxides, sulfides, carbonates, or other salts by the addition

of a chemical (e.g., to raise pH),

oxidation, or change in water

temperature. Flocculent aids may

be added to hasten sedimentation.

Package plants include a rapid-mix

tank, flocculation chamber, and

settling tank. Inclined plate gravity

separation or circular clarifiers are

used for settling. Typical equipment

includes a rapid mixer, flocculator

and drive, feed pump, sludge pump,

acid and caustic soda pumps for pH

control, and a polymer pump.

1-10 gpm - 8’x4’x9' - 3 HP

10-50 gpm - 10’x4’x13' - 5 HP

50-100 gpm - 11’x6’x14' - 7 HP

Useful for many contaminated groundwater streams, particularly as a

pretreatment step.

Effectiveness limited by presence of

complexing agents in water.

Precipitate sludge may be a hazardous

waste.
37

UV Oxidation

Used increasingly to

remove organic

contaminants from

ground water and

other wastewaters.

Ultraviolet (UV) oxidation involves

adding an oxidant, such as hydrogen peroxide, to contaminated

water and then irradiating the

solution with UV light. This splits

the hydrogen peroxide, producing

hydroxyl radicals which react with

organic contaminants, causing

their breakdown to non-toxic

products (e.g., low weight

aldehydes, carbon dioxide and

water).

An oxidant (hydrogen peroxide) is

injected upstream of the reactor

vessel and mixed with the contaminated water in line. The fluid then

flows sequentially through 1 or

more reactors containing UV lamps

where treatment occurs.

1x10 kW - 2’x6’x6'

1x30 kW - 4’x4’x8'

4x30 kW - 12’x5’x8'

UV oxidation can treat a broad range of

soluble organics and is particularly

effective for destroying chloroalkanes

such as TCE and vinyl chloride and

aromatic compounds such as benzene

and toluene.

Pretreatment may be needed to

remove suspended solids, NAPL, and

iron concentrations > 100 mg/L.

Treatability studies needed.

Filtration

Widely used to

remove fine

suspended solids

from ground water

and landfill leachate.

A fixed or moving bed of media

traps and removes suspended

solids from water passing through

the media. Monomedium filters

usually contain sand, while multimedia filters include granular

anthracite over sand possibly over

very fine garnet sand. Filters are

used upstream of other treatment

processes.

Package filters consist of one or

more pressure vessels on a skid. A

feed pump, backwash pump, piping,

and valves complete the system.

1-10 gpm - 10’x4’x8' - 2 HP

10-50 gpm - 14’x6’x8' - 3 HP

50-100 gpm - 18’x8’x8' - 5 HP

100-250 gpm - 24’x10’x8' - 15 HP

Reliable and effective means of removing low levels of solids. Equipment is

readily available and easy to operate

and control.

Filters clog if suspended solids concentration is high. Backwash water

requires further treatment.

Ion Exchange

Widely used to

remove metal

cations, TDS, and

anions (e.g., nitrate,

sulfate, chromate)

from drinking water

and for various other

applications.

Ion exchange is an adsorption

process that uses a resin media to

remove dissolved ion contaminants

(by exchanging sorption sites held

by harmless ions). Cation resins

adsorb metals while anion resins

adsorb such contaminants as

nitrate and sulfate. Systems consist of pressure vessels containing

beds of resin pellets and strainers

to retain the pellets. The resin bed

is regenerated by flushing with

acid and/or caustic soda.

Package plants include resin-filled

pressure vessels, regeneration

chemical tanks, and waste brine

storage tanks. Acid and caustic

soda solution pumps are provided

to regenerate the resin. Resins can

be selected that are ion-specific.

1-10 gpm - 8’x3’x6' - 3 HP

10-50 gpm - 14’x5’x8' - 10 HP

50-100 gpm - 17’x6’x10' - 12 HP

Removes a broad range of ionic

species. Units are compact and not

energy intensive.

Must monitor effluent for contaminant

breakthrough. High concentrations of

Fe and Mn, hardness cations (Ca and

Mg), suspended solids, and certain

organics will foul ion exchangers.

These constituents are often present at

much higher concentration than the

targeted contaminants. One option is

to use ion-specific resins to remove

heavy metals in the presence of Ca and

Mg.

Reverse

Osmosis (RO)

Widely used for

removing dissolved

solids from drinking

water and other

applications.

RO is a separation process that

uses selective semipermeable

membranes to remove dissolved

solids from water. A high-pressure

pump forces the water through a

membrane, overcoming the natural

osmotic pressure, to divide the

water into a dilute (treated) stream

and a concentrated (residual brine)

stream.

RO package plants include

cartridge prefilters, a high-pressure

feed pump, RO modules, pressure

vessels, and a backpressure valve.

1-10 gpm - 8’x3’x6' - 13 HP

10-50 gpm - 12’x6’x6' - 35 HP

50-100 gpm - 14’x12’x8' - 85 HP

Can reduce both inorganic and organic

dissolved solids.

Some brine must flow out of the RO

module to remove concentrated contaminants. This rejected flow may be

10% to 50% of the feed flow. Units are

subject to chemical attack, fouling, and

plugging. Pretreatment needs (e.g., to

remove Fe, Ca, Mg) may be great.

Appendix B (continued)

Method Process Description Advantages and Limitations Package Plant Components

and Sizes

(Dimensions are for overall plant envelope)
38

Fluidized Bed

Biological Reactor

(FBR)

Widely used to

remove soluble

organics (e.g., BTEX,

aromatics, halogenated aliphatics,

etc.) from ground

water, but not landfill

leachate.

An aerobic FBR is a fixed-film

biological treatment technology

using microbes grown on GAC or

sand media. Dedicated pumps

provide desired fluidization and

control the reactor internal flux.

Influent enters the reactor bottom.

The media bed expands as the

biofilm grows thicker and reduces

the media density. An internal

growth control system intercepts

the rising bed at a desired height,

removes most biomass from the

media, and returns the media to

the reactor. Aerobic GAC FBR

integrates biological removal with

GAC sorption.

Package plants include an enclosed

vertical cylindrical vessel, influent

pump, air compressor or blower, air

diffuser, effluent recycle pump, and

media/biomass separation tank.

1-10 gpm - 12’x7’x15' - 7 HP

10-50 gpm - 18’x10’x15' - 10 HP

50-100 gpm - 18’x12’x15' - 12 HP

100-400 gpm - 18’x16’x15' - 40 HP

Expected to have a high process and

mechanical reliability. Single or dual

reactor design provides on-line

flexibility. GAC FBR provides stable

performance under fluctuating loading

conditions.

NAPL may pass through or cover the

biofilm surface. Iron levels > 20 mg/L

may require pretreatment to avoid

plugging. Ca and Mg may cause

scaling problems. Not designed for

removing suspended solids. GAC FBR

is not efficient for low-yield, nonbiodegradable organics because it is often

operated as a high loading system with

a short retention time.

Activated Sludge

System

Widely used to

remove biodegradable organic contaminants and

inorganic nutrients

(e.g., N and P) from

landfill leachate, but

not from ground

water.

This is a suspended-growth,

biological treatment system that

uses aerobic microbes to biodegrade organic contaminants.

Influent is pumped into an aeration

tank, mixed with bacteria, and kept

in suspension. In the presence of

oxygen, nutrients, organic compounds, and acclimated biomass,

organic contaminants are biodegraded. After a treatment period,

the fluid and biomass are passed

to a settling tank, where cells are

separated from treated water. A

portion of the settled cells are

recycled to the next treatment

batch and the remaining sludge is

disposed.

Package plants include cylindrical

or rectangular aeration tanks and

clarifiers, positive displacement

blower, air diffusers, sludge recycle

pump, sludge waste pump,

chemical feed pumps, and control

panel.

1-10 gpm - 23’x12’x12' - 5 HP

10-50 gpm - 45’x24’x12' - 15 HP

50-100 gpm - 45’x50’x12' - 25 HP

100-200 gpm- 45’x100’x12'- 47 HP

Effective and reliable if there are no

shock loads. Technology is highly

developed. Can tolerate higher organic

loads than most biological treatment

processes. High degree of flexibility.

High capital costs. Generates sludge

that may be high in metals and

refractory organics. Sensitive to high

concentrations of heavy metals or toxic

organics. Fairly energy intensive. Has

difficulty with low concentrations of

contaminants, relatively long time

needed for organism acclimation. Long

detention times for some complex

contaminant degradation.

Sequencing Batch

Reactor (SBR)

Widely used to

remove biodegradable organics and

inorganic nutrients

from LF leachate, but

not from ground

water.

The SBR is a periodically

operated, suspended growth,

activated sludge process. It is

different from the continuous

activated sludge process in that

the treatment steps are carried out

in a single reactor tank in

sequential steps.

Package plants include 1 or 2

rectangular or cylindrical SBR

tanks, blowers, air diffusers, influent

pumps, waste sludge pump, effluent

pump, and chemical pumps. A

floating decanter removes clear

water from the reactor water at the

end of the treatment cycle.

1-10 gpm - 20’x10’x12' - 7 HP

10-50 gpm - 30’x15’x14' - 40 HP

50-100 gpm - 40’x20’x14' - 80 HP

See above.

By using a single tank, SBR saves land

requirements and provides flexibility in

changeable time and mode of aeration

in each stage.

Appendix B (continued)

Method Process Description Advantages and Limitations Package Plant Components

and Sizes

(Dimensions are for overall plant envelope)