United States

Environmental Protection

Agency

EPA/600/R-92/002

January 1992

Robert S. Kerr

Environmental Research Laboratory

Ada, OK 74820

Research and Development

General Methods for

Remedial Operation

Performance Evaluations
i

General Methods for

Remedial Operations

Performance Evaluations

Project Officer

Randall R. Ross

Extramural Activities and Assistance Division

Robert S. Kerr Environmental Research Laboratory

Office of Research and Development

U.S. Environmental Protection Agency

Ada, Oklahoma 74820
ii

Notice

The information in this document has been funded wholly or in part by the United States Environmental Protection Agency

under Cooperative Agreement No. CR-812808 to the National Center for Ground-Water Research. It has been subjected to

the Agency’s peer review and administrative review, and it has been approved for publication as an EPA document. The

material presented in this document was extracted and edited by R.R. Ross and J.W. Keeley from a report entitled "Remedial

Operations Performance Evaluation Methodologies," written by Dr. J.F. Keely for EPA.

All research projects making conclusions or recommendations based on environmentally related measurements and funded

by the Environmental Protection Agency are required to participate in the Agency Quality Assurance Program. This project

did not involve environmentally related measurements and did not involve a Quality Assurance Project Plan. Mention of

trade names or commercial products does not constitute endorsement or recommendation for use.
iii

Foreword

EPA is charged by Congress to protect the nation's land, air and water systems. Under a mandate of national environmental

laws focused on air and water quality, solid waste management and the control of toxic substances, pesticides, noise and

radiation, the Agency strives to formulate and implement actions which lead to a compatible balance between human

activities and the ability of natural systems to support and nurture life.

The Robert S. Kerr Environmental Research Laboratory is the Agency's center of expertise for investigation of the oil and

subsurface environment. Personnel at the Laboratory are responsible for management of research programs to: (a) determine

the fate, transport and transformation rates of pollutants in the soil, the unsaturated and saturated zones of the subsurface

environment; (b) define the processes to be used in characterizing the soil and subsurface environment as a receptor of

pollutants; (c) develop techniques for predicting the effect of pollutants on ground water, soil, and indigenous organisms; and

(d) define and demonstrate the applicability and limitations of using natural processes, indigenous to soil and subsurface

environment, for the protection of this resource.

The pump-and-treat process, whereby contaminated ground water is pumped to the surface for treatment, is one of the most

common ground-water remediation technologies used at hazardous waste sites. In fact, pump and treat is a necessary

component of practically all ground water remediation systems, whether for restoration or for plume containment. However,

recent research has identified complex chemical and physical interactions between contaminants and the subsurface media

which may impose limitations on the extraction part of the process. This has raised questions about the effectiveness of

ground water restoration systems and methods for evaluating that effectiveness. This report was developed to summarize the

technical considerations involved in evaluating the performance of ground-water remediation activities.

Clinton W. Hall

Director

Robert S. Kerr Environmental Research Laboratory
iv
v

Table of Contents

FIGURES ................................................................................................................................................................................ iv

TABLES .................................................................................................................................................................................. iv

SUMMARY ............................................................................................................................................................................. v

I. INTRODUCTION .................................................................................................................................................... 1

Source Characteristics ........................................................................................................................................ 1

Hydrogeologic Complexities ............................................................................................................................. 2

Man-made Complexities .................................................................................................................................... 3

Pump-and-Treat Limitations .............................................................................................................................. 3

Innovations in Pump-and-Treat ......................................................................................................................... 5

II. COMPONENTS OF PERFORMANCE EVALUATIONS ....................................................................................... 5

Performance Evaluation Strategies .................................................................................................................... 5

1. Locations for Plume Monitoring ................................................................................................................ 7

2. Monitoring Criteria for Performance Evaluations ...................................................................................... 8

3. Strategies for Monitoring Locations and Criteria ....................................................................................... 9

4. Measures of Operational Effectiveness ...................................................................................................... 10

5. Measures of Operational Efficiency ........................................................................................................... 11

6. Strategies for Determination of Success/Closure ....................................................................................... 12

Data Collection Considerations ......................................................................................................................... 14

1. Purposes of Data Collection ....................................................................................................................... 14

2. Relevant Scale(s) of the Problem ............................................................................................................... 15

3. Actual Scale of the Measurements ............................................................................................................. 15

4. Data Quality vs. Quantity ........................................................................................................................... 17

Hydrogeologic Data ........................................................................................................................................... 17

1. Topographic and Geographic Data ............................................................................................................. 17

2. Geomorphologic and Geologic Data .......................................................................................................... 17

3. Flow Rates .................................................................................................................................................. 17

4. Hydraulic Parameters.................................................................................................................................. 18

5. Fluid-Behavior Data ................................................................................................................................... 18

6. Fluid Levels and Pressures ......................................................................................................................... 19

Chemical and Geochemical Data ....................................................................................................................... 20

1. Natural Ground-Water Chemistry Data ...................................................................................................... 20

2. Soil Chemistry Data .................................................................................................................................... 20

3. Contaminant Reaction Data ........................................................................................................................ 20

Biological Data .................................................................................................................................................. 20

Quality Assurance and Quality Control Data ..................................................................................................... 21

III. METHODS AND PROTOCOLS .............................................................................................................................. 21

Performance Evaluation Methods ...................................................................................................................... 21

1. Computational Methods ................................................................................................................................ 21

2. Statistical Methods ........................................................................................................................................ 22

3. Graphical Methods ........................................................................................................................................ 23

4. Theoretical .................................................................................................................................................... 25

General Protocols for Performance Evaluations ................................................................................................ 26

1. General Protocol for Selection of Monitoring Criteria and Reporting Requirements .................................. 26

2. General Protocol for Violations Reporting and Responses ........................................................................... 28

3. General Protocols for Design and Operation Modifications......................................................................... 29

REFERENCES ........................................................................................................................................................................ 30
vi

List of Figures

Figure 1. Pump-and-treat limitations may result in the rebound of

contaminant concentrations after the termination of pumping ......................................................... 3

Figure 2. Flow velocities developed during pumping may limit

desorption of contaminants and prevent removal of maximum concentrations ............................... 4

Figure 3. Site characterization efforts should be conducted throughout

the RI/FS process to allow data refinement during assessment and remediation............................. 6

Figure 4. Possible monitoring point locations in a ground-water

contamination plume undergoing pump-and-treat remediation ....................................................... 8

Figure 5. Concentration vs. time data plot and multiple regression line with

confidence bounds for judging persistence of the clean-up ............................................................. 13

Figure 6. The results of contouring a dataset by various mathematical

techniques may be significantly different......................................................................................... 24

Figure 7. Pattern diagram for selected VOCs .................................................................................................. 24

Figure 8. PERT Chart illustrating interdependency of tasks to produce outputs ............................................ 25

Figure 9. Outline of key efforts needed to select and use monitoring and

reporting requirements...................................................................................................................... 27

Figure 10. Key efforts needed to identify, verify, and correct inadequate

performance of the remediation........................................................................................................ 28

Figure 11. Key efforts needed to prepare for, implement, and

verify modifications of the remediation ........................................................................................... 29

List of Tables

1. Useful Statistical Methods for Performance Evaluations

Analysis of Variance (ANOVA) Techniques ................................................................................... 23
vii

Summary

This document was developed by an EPA-funded project to explain technical considerations and principles necessary to

evaluate the performance of ground-water contamination remediations at hazardous waste sites. This is neither a “cookbook,”

nor an encyclopedia of recommended field, laboratory, and data interpretation methods. Rather, this report presents and

discusses suggested generic principles for formulating site-specific performance evaluation strategies for ground-water

contamination remediations.

It is widely accepted that ground-water contamination problems cannot be adequately defined or addressed until the

governing physical, chemical, and biological processes which affect the fate and transport of contaminants are characterized

in detail. Recent research has led to a better understanding of these complex processes and how they control the movement of

contaminated ground water through the subsurface. This research has demonstrated that pump-and-treat remediations are far

more complicated than previously thought . Many of the complications result from the tortuosity of the ground-water

flowlines that are generated by the remediation wellfield and the re-distribution of contaminant pathways that occurs. An

immediate consequence of this re-distribution is that historical trends of contaminant concentrations at local monitoring wells

may no longer be useful for predictions and evaluations regarding the growth or reduction of the contaminant plume. Further,

the pattern of flow velocities and directions that resulted from the remediation wellfield may change substantially over time,

thus complicating attempts to evaluate the progress of the pump-and-treat remediation.

The increased ground-water flow velocities created by remediation wellfields may also introduce significant hydrodynamic

and chemical limitations to contaminant transport and subsequent remediation locally. Such limitations include

hydrodynamic isolation of portions of remediation wellfields, diffusively restricted movement of contaminants in lowpermeability subsurface materials, and failure to meet local chemical equilibrium requirements for maximal desorption and

liquid-liquid partitioning. For each well that is utilized to remove fluids from the subsurface, there is a stagnation zone

downgradient where contaminants do not receive active flushing while the well is pumping. Everywhere that water levels

have been lowered by the action of the remediation wellfield, there are sediments that may release additional contaminants

upon cessation of pumping and return of the water levels to normal levels. Throughout the sediments that overlie and

comprise the contaminated aquifer there may be inclusions of low-permeability sediments. Such inclusions may act as longterm contaminant reservoirs which release contaminants much more slowly (often, by diffusion only) than the surrounding

sediments where the vast majority of the ground water flows. The presence of nonaqueous phase liquids (NAPLs) may

represent one of the most significant limitations to pump-and-treat remediations. These immiscible liquids may leave a trail

of residual contaminants, trapped by capillary forces, as they move through the unsaturated and saturated zones. This residual

contaminant mass may serve as a long term source of ground-water contamination. Proper site-specific characterizations of

these limitations are essential in evaluating the performance of pump-and-treat remediation systems.

Emphasis is given to the need to develop a monitoring strategy at each site which ensures that sampling locations and

schedules are meaningful, not only as early-warning alarm systems, but in measuring progress toward remediation goals and

to make adjustments to improve performance. Emphasis is also given to the use of data reduction, presentation, and

interpretation techniques that may be used in evaluating the performance of the remediation. These nominally include

ground-water flow and contaminant transport models, statistical analyses, graphical techniques, and theoretical relationships.

The optimal effectiveness (extent and uniformity of cleansing) and efficiency (minimization of costs and duration) of a

remediation can often be obtained by managing the pump-and-treat system in terms of flowrates and extraction locations in

response to reductions in contaminant mass in portions of the plume. Active management of this kind must be supported by

the continuous gathering of key site characterization data.
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I. Introduction

Ground-water pump-and-treat systems are the most

common remedial technology used to remediate and/or

contain ground-water contaminants at hazardous waste sites.

Pump-and-treat remedial actions involve the extraction of

contaminated ground water and subsequent treatment for

contaminant removal. This report presents a review of the

factors and subsurface processes which may affect the

performance of pump-and-treat remediations. Also

discussed are the basic components of performance

evaluations, and suggested methods and protocols for

evaluating performance. It is assumed that the reader is

familiar with the basic concepts of hydrogeology. For

additional information refer to U.S. EPA, 1989a, 1989b, and

Mercer and others, 1990.

Pump-and-treat remediations are planned or have been

initiated at many ground-water contamination sites across

the country. Regulatory responsibilities require that

adequate oversight of these remediations be made to answer

the question: What can be done to demonstrate whether or

not a remediation is generating the desired control?

Recently, other questions have evolved with the realization

that many pump-and-treat remediations are not functioning

as well as had been expected:

1. What can be done to determine whether or not the

remediation will meet its proposed timelines? and,

2. What can be done to determine whether or not the

remediation will stay within budget?

It may be possible to better answer these questions with the

aid of sophisticated data analysis tools, such as

computerized mathematical models of ground-water flow

and contaminant transport.

Computer models can be used to make predictions about

future performance, but the results are highly dependent on

the quantity and quality of field and laboratory data utilized.

This is also true of models used for ongoing performance

evaluations of pump-and-treat remediations. They must be

supported by detailed site information that is continually

updated. In most instances, an accurate performance

evaluation cannot be made simply by comparing data

obtained from monitoring wells during remediation to the

water quality data generated prior to the onset of

remediation. Historical trends of contaminant levels at local

monitoring wells are often not useful for comparisons with

data obtained during the operation of pump-and-treat

remediations. This is a consequence of the complex

ground-water flow patterns produced by extraction and

injection wells as compared to the relatively simple natural

flowlines.

Detailed field investigations are required during remediation

to determine the locations of flowlines generated by a

pump-and-treat operation. It is likely that mathematical and

statistical models will be required to organize and analyze

this data. Ongoing site characterization efforts must include

examinations of the contaminant source characteristics, the

local and regional hydrogeologic complexities,

contaminant-specific behavior and plume dynamics.

Source Characteristics

There are many potential sources of ground-water

contamination at hazardous waste sites. These may be

categorized as either “point” or “nonpoint” sources,

depending on their geometry, and generally are engineered

structures with recognizable physical boundaries. Some

may release significant volumes of contaminants to the

environment during their operating history. Pits, ponds, and

lagoons, for example, constructed without lined bottoms or

sides are notorious for large fluid losses. Industrial and

municipal landfills are point sources that tend to generate

leachate by chemical reactions of the waste with infiltrating

precipitation. Such leachate very often contains high

concentrations of dissolved metals, inorganic salts, and

organic chemicals.

Historically, shallow injection wells have been used to

dispose of untreated stormwaters from city streets, parking

lots, highways, and airports and represent the most direct

avenue of contaminant introduction to ground water. Deep

injection wells continue to be used for the disposal of liquid

industrial waste. Point sources of ground-water

contamination also include leaks and spills at manufacturing

facilities such as refineries, pharmaceutical plants, fertilizer

plants, and other chemical production plants. Leaking

underground storage tanks and associated piping at gasoline

service stations and fuel depots also pose a serious point

source contamination problem. Some point sources, such as

abandoned wells, are very small and particularly difficult to

locate; whereas other point sources, such as mining

operations or municipal and industrial landspreading

operations are readily located, but their large size makes

identification of specific points of contaminant releases

difficult.

The more commonly recognized nonpoint sources of

ground-water contamination include agricultural

applications of fertilizers, herbicides, and pesticides. Other

traditional practices and activities that may result in

widespread releases of contaminants to underlying ground

water include the application (and storage) of salts to streets

and highways for winter de-icing, and the spraying of
2

waste-oils on unpaved roads for dust suppression. Small

spills that typically occur during dispensing or refilling

operations at transfer points and storage locations often go

unnoticed or are ignored because of their minor size. The

aggregate effect of many small spills may be equivalent to a

non-point source of contaminant releases to the underlying

ground water. It is generally easier to identify, control, and

eliminate point sources than nonpoint sources. The control

of nonpoint sources is often limited to prevention or

restriction of the activities which generate the contaminant.

The mechanism by which sources introduce contaminants to

ground water has a profound effect on the duration and areal

extent of the resulting contamination. The duration of a

source release may affect the extent of contamination. For

example, a small spill at the surface may be attenuated over

a short distance while contaminants introduced below the

surface, such as leaking tanks and pipelines, will have much

less attenuation. The hydraulic impacts of such sources may

impart a three-dimensional character to the local groundwater flow regime. Interactions with streams and other

surface water bodies may also impart three-dimensional

flow characteristics to contaminated ground water.

In ideal settings, the design of a pump-and-treat system

might be limited to the configuration of the extraction/

injection wellfield and the flowrates of the individual wells.

Unfortunately, there are few natural settings that appear to

be ideal at the scale of interest in pump-and-treat

remediations. Karst and fractured rock aquifers frequently

exhibit highly complex flow and contaminant transport

behaviors, as do many interbedded sand-and-gravel aquifers

(e.g., diffusively restricted flow in silt and clay strata)

(Barker and others, 1988; Feenstra and others, 1984;

Spayed, 1985; White, 1988). The general physiographic

setting may also be highly restrictive to pump-and-treat

remediation, such as in wetlands and wildlife sanctuaries.

The properties and size of the contaminant source and

resulting plume are also key features of the remedial action

setting. The amount of contaminant that may be readily

removed by pump-and-treat remediation in a reasonable

amount of time may be only a small portion of what is in the

subsurface (Hossain and Corapcioglu, 1988; Hunt and

others, 1988a and 1988b; Keely and others, 1986; Stover,

1989; Testa and Paczkowski, 1989).

Hydrogeologic Complexities

The geologic processes that shape the surface of the earth

are so varied it is easy to understand why most aquifers do

not exist as a single horizontal stratum of uniform

composition and thickness. As a result, the hydraulic impact

of extraction wells is often distorted, and if this occurs in the

zone of remediation, the system may not be as effective as

anticipated and additional pumping might be required. By

examining some of the more common hydrogeologic

complexities, it may be possible to anticipate these

additional requirements and account for them in the system

design.

Some of the most complex hydrogeologic settings for

pump-and-treat remediations are those that involve the

contamination of zones of low-permeability. These

formations can be categorized as active aquifers, despite

very low yields and high drawdowns. Remediations in

these types of settings may be further complicated by the

presence of limited occurrences of high permeability

sediments, such as sand and gravel, or fractures.

Pump-and-treat remediation in low-permeability formations

may be limited because yields are too low to allow

continuous pumping. Extraction wells may be designed to

operate on demand or, in some cases, it may be possible to

maintain continuous pumping by providing recharge over

the capture zone of each well.

Minor pump-and-treat problems can also be found in highpermeability formations. Flow net complications can result

from the minor occurrence of low-permeability strata. For

example, in formations such as alluvial floodplains and

glacial outwash deposits, it is common to find thin clay and

silt lenses extending tens to hundreds of feet in the direction

of flow.

In some geologic settings, most of the ground-water flow

occurs through fractures in low-permeability rock

formations (Barker and others, 1988; Feenstra and others,

1984; Spayed, 1985). Contaminant movement through

fractures may be primarily controlled by advective forces.

However, flow through the bulk matrix of many rocks is so

slow that contaminant movement may be dominated by

molecular diffusion.

Extensively fractured rock may exhibit hydraulic behavior

similar to porous media. In fractured rock aquifers

characterized by a very high degree of fracturing, many of

the equations developed for porous media may be applied

with reasonable confidence for the design of remediation

wellfields (Bear and Verruijt, 1987; de Marsily, 1986;

Mercer and Faust, 1981). When only a few fractures

dominate flow, the relationship of flowrate to drawdown

may be highly nonlinear and unpredictable. Equations have

been developed which allow prediction of drawdowns for

situations where a single horizontal fracture or a single

vertical fracture passes through a pumping well, but there

are no generically useful equations for multi-borehole or

multi-fracture situations. The usual design approach for

systems dominated by discrete fractures is to try to position

each extraction well such that it intersects as many fractures
3

as possible. In such cases it will probably be necessary to

conduct several pilot tests of extraction wells to develop

site-specific relationships of flowrate to drawdown. Such

empirical relationships may be highly nonlinear as a result

of changes in the fracture aperture during pumping due to

nonuniform drainage along the fracture face and the

siphoning effects of irregular fractures. In settings where

the flowrate-drawdown relationship at one extraction well

differs markedly from that of another, the wellfield design

strategy must default to well-by-well installation and

testing. Each extraction well should be tested singly, and in

concert with existing extraction wells to determine the

short-term effectiveness in providing the desired gradient

control. Supplementary extraction wells may be necessary

where additional control is needed.

Similar considerations and measures may be required for

hazardous waste sites located in karst terrains. Two flow

regimes, open-channel and pipe flow, occur in karst settings

that rarely occur elsewhere in the subsurface (White, 1988).

Because of these complexities, advection and dispersion of

contaminants through karst aquifers are not adequately

describable by Darcy’s Law and other porous media

concepts.

It has been shown that streams and springs control the flow

through karst terrain. Although dye tracers have proven

useful in studying fluid movement in karst aquifers, such

studies have not yielded relationships that can be transferred

from one site to another. While it may seem reasonable to

locate extraction wells on the basis of intersecting karst

streams, one should not expect a consistent response as the

drawdown and flowrate may vary substantially over short

periods of time.

Man-made Complexities

It is not uncommon for the construction of extraction wells

to be hampered by buried tanks and utility lines, streets,

sewers, and water and gas pipes. Railroads, canals, and

power transmission lines can present similar location

difficulties as well as add safety hazards. This lack of

access can create major difficulties for pump-and-treat

remediations, not only from the standpoint of well

locations, but the required piping network as well.

Seasonally operated pumps and on-demand pumping may

produce unanticipated hydraulic impacts locally. If a pumpand-treat remedy is designed without the knowledge of local

pumping wells, it may not overcome the hydraulic impact

sufficiently to maintain effective containment of

contaminants (Keely, 1984; Keely and Tsang, 1983; Ward

and others, 1987). Potential failure of the remediation by

this avenue is unlikely to be recognized unless

hydrodynamic monitoring is instituted for both horizontal

and vertical gradient control.

Drilling restrictions may be in effect for other purposes such

as water conservation, water rights, or to minimize upconing

of saline water. Fortunately, many institutional controls are

negotiable, particularly if the potential impacts of operation

of a pump-and-treat remediation can be shown to be

controllable or of only a temporary nature.

Pump-and-Treat Limitations

Conventional pump-and-treat remediations are based on the

operation of extraction and injection wells. The level of

contamination measured at monitoring wells may be greatly

reduced in a moderate period of time, but low levels of

contamination may persist indefinitely. The contaminant

mass removed may decline over time and gradually

approach a residual level in the latter stages (Figure 1). At

that point, large volumes of water are treated to remove

small amounts of contaminants. Depending on contaminant

residuals within the aquifer, this may cause a remediation to

be continued indefinitely. An increase in ground-water

contaminant levels may follow if the remediation is

discontinued prior to removal of all residual contaminants.

Several contaminant transport processes are potentially

responsible for the contaminant rebound effects and may

include:

(1) diffusion of contaminants in low-permeability

sediments,

(2) hydrodynamic isolation within wellfields,

(3) desorption of contaminants from sediments, and

(4) partitioning of immiscible fluids into ground water.

Figure 1. Pump-and-treat limitations may result in the rebound of

contaminant concentrations after the termination of

pumping (after Keely, 1989).

Cessation 

of Pumping

Time

Begin 

Pumping

Target

Contaminant

Concentration

Concentration

Apparent 

Residual 

Contamination
4

Local variations in ground-water flow results from the

heterogeneous nature of interlayered high-permeability and

low-permeability sediments (Anderson, 1979; Guven and

Molz, 1986; Keely, 1984; Keely and others, 1986; Matheron

and de Marsily, 1980; Molz and others, 1986 and 1988;

Osiensky and others, 1984; Satlin and Bedient, 1988).

When operating a remediation wellfield, these variations in

flow may result in relatively rapid cleansing of the higherpermeability sediments, which may conduct most groundwater flow. Contaminants are removed from the lowerpermeability sediments very slowly primarily by diffusion.

The specific rate at which this diffusive release occurs is

dependent on the concentration gradient within and external

to the low-permeability sediments. As the higherpermeability sediments are remediated, the concentration

gradient drawing contaminants from the lower-permeability

sediments is at its greatest and is exhausted only when the

chemical concentrations are at equilibrium.

Fine-grained sediments generally have lower permeabilities

and orders-of-magnitude greater surface areas than coarsegrained sediments (Bitton and Gerba, 1984; Corey, 1977;

Hillel, 1982; Keely and others, 1986; Mackay and others,

1986; Piwoni and Banerjee, 1989). Much greater amounts

of contaminants may accumulate in low-permeability

sediments by adsorption, ion-exchange, or other surface

chemical processes as compared with contaminant

accumulations in a like volume of high-permeability

sediments (Bouchard and others, 1989; Enfield and

Bengtsson, 1988). The occurrence of low-permeability

materials in higher-permeability formations may be a major

limitation to pump-and-treat remediations. Diffusive

transport of contaminants from the low-permeability stratum

to the surrounding high-permeability formation may dictate

the time necessary for complete remediation.

If substantial occurrences of low-permeability materials are

adequately characterized it may be possible to target these

selectively during the pump-and-treat remediation for

excavation, in-situ soil washing, hydrofracturing, or other

remedial methods. However, if low-permeability materials

occur as heterogeneously distributed microstrata that defy

physical removal or in-situ treatment, conventional pumpand-treat technologies may be implemented for the

containment of ground-water contaminants, rather than

remediation.

The operation of any wellfield results in the formation of

hydrodynamically isolated stagnation zones downgradient

of extraction wells and upgradient of injection wells. Mass

transport into or out of the isolated zone occurs mainly by

diffusion. Remedial action wells located within the bounds

of a contaminant plume may have stagnation zones which

are not effectively remediated. Flowline patterns must be

altered by changing the locations of pumping wells, or by

altering the balance of flowrates among pumping wells to

remediate the stagnation zones.

Adsorption and desorption of organic chemicals by

sediments are not always instantaneous processes as

hundreds of hours may be required for certain compounds to

reach chemical equilibrium (Bahr, 1989; Bouchard and

others, 1988; Brusseau and others, 1989; Curtis and others,

1986; Goltz and Roberts, 1986 and 1988; Lee and others,

1988; Mackay and others, 1986; Miller and others, 1985;

Nkedi-Kizza and others, 1985; Piwoni and Banerjee, 1989;

Valocchi, 1988; Woodburn and others, 1986). When a

pump-and-treat remediation is implemented, ground-water

flow velocities will be significantly increased and it may not

be possible for passing ground water to achieve or maintain

chemical equilibrium with the contaminants (Figure 2).

Consequently, large volumes of mildly contaminated water

may be produced over a long period of time.

Figure 2. Flow velocities developed during pumping may limit

desorption of contaminants and prevent removal of

maximum concentrations (after Keely, 1989).

Dissolved Contaminant 

Concentration

Slow Desorption

Equil. Conc.

Organic Carbon or 

Mineral Oxide Surface

Advection

Time

Initial Rapid 

Desorption
5

Immiscible, or nonaqueous phase liquids (NAPL), may

impose significant limitations on pump-and-treat

remediations. NAPLs are characterized as either light

(LNAPL) or dense (DNAPL) with respect to their density

relative to that of water. LNAPLs, often referred to as

“floaters,” may accumulate at the top of the saturated zone

while DNAPLs, often referred to as “sinkers,” are capable

of moving vertically through the unsaturated zone and

portions of the saturated zone.

Both light and dense immiscible liquids are subject to

capillary forces of sediments which may retain or trap

portions of the liquid in pore spaces (Huling and Weaver,

1991: Faust, 1985; Hinchee and Reisinger, 1987; Hunt and

others, 1988a and 1988b; Stover, 1989; Taylor, 1987). This

may be particularly troublesome when an extraction well is

utilized when a continuous phase (free product) LNAPL is

present as both the water and free product flow into the cone

of depression. The cone of depression may contain a

significant volume of trapped residual contaminant below

the water-table, which will serve as a continuous

contaminant source long after the extraction well stops

operating. The extent of contamination generated by the

residual contaminant source may exceed that generated by

the LNAPL pool prior to the onset of pumping.

Contaminant plumes composed of components of an

immiscible liquid are generated by the dissolution of the

NAPL into adjacent ground water, according to liquid-liquid

partitioning principles (Hunt and others, 1988a and 1988b;

Stover, 1989; Stumm and Morgan, 1981). Generally, the

lower the solubility, the slower the dissolution rate (Stumm

and Morgan, 1981; Weast, 1985). In many instances

involving pump-and-treat remediations, the contact time

between ground water and immiscible fluids may be too

short to allow maximum concentrations to develop.

Analogous to the limitation posed by slow chemical

equilibrium for some adsorptive contaminants, pumpinginduced limitation of liquid partitioning will generate large

volumes of mildly contaminated water under continuous

operation of the remediation wellfield.

There is evidence that DNAPLs are present at many

hazardous waste sites but they may largely go undetected

due to the numerous variables influencing their transport

and fate. As a result, the presence of DNAPLs may be a

limiting factor to ground-water pump-and-treat

remediations.

Innovations in Pump-and-Treat

To minimize the potential for poor contaminant removal

rates, innovative pumping strategies may be required.

These involve reconfiguring the remediation wellfield,

adopting unconventional pumping schedules, or integrating

pumping with other remediation technologies such as

subsurface barrier walls.

One innovation in pump-and-treat remediations is pulsed

pumping. Pulsed operation of hydraulic systems is the

cycling of extraction or injection wells in active and resting

phases. The resting phase of a pulsed-pumping operation

may allow sufficient time for contaminants to diffuse out of

low-permeability zones into adjacent high-permeability

zones and allow sorbed contaminants and NAPL residuals

sufficient time to reach equilibrium. Subsequent pumping

then removes maximum contaminants in a minimum

volume. Pulsed pumping will also bring zones of stagnation

into active flowpaths.

Other methods to enhance the efficiency of ground-water

remediations currently being pursued include the use of

surfactants, biorestoration, fracture enhancement, and

physical containment.

II. Components of

Performance Evaluations

Performance Evaluation Strategies

Performance evaluations of remedial actions may be

conducted at hazardous waste sites for a variety of reasons.

One of the most common reasons is to satisfy the

monitoring requirements established by negotiated or court

settlements, thus ensuring that the desired controls on

ground-water flow and contaminant transport are

maintained. Another reason may be to determine if the

remedial action objectives can be accomplished using the

selected remedial action alternative(s). The strategy

adopted for conducting a performance evaluation must focus

on the kinds of data that will be collected and how those

data will be presented for interpretation and decision

making purposes. It is best to start building a strategy,

therefore, by examining the data collection studies that have

been done in the past at a site.

The phases of work conducted in site investigations follow a

pattern of increasing detail and focus (Figure 3). This can

be viewed as a continuous data collection program that may

be extended into the remediation phase to benefit oversight

monitoring and operational management purposes alike. In

the Superfund site investigation process, the preliminary

assessment phase involves collection of readily obtainable

information which may include technical data for a

screening-level assessment of the potential need for further

investigation. Where the preliminary assessment indicates a

need for additional characterization, the next phase of work
6

Figure 3. Site characterization efforts should be conducted throughout the RI/FS process to allow data refinement during assessment and

remediation.

• Preliminary Assessment

• Site Inspection

Scoring of

RI/FS

Remedial Investigation

• Site 

Characterization

• Treatability 

Investigation

• Remedy Selection

• Record of Decision

• Remedial Design

• Remedial Action Feasibility Study

• Development/Screening 

of Alternatives

• Detailed Analysis 

of Alternatives

most often consists of site visits for visual appraisal and

limited sampling.

If these additional efforts indicate the need for further

characterization, then a preliminary assessment/site

investigation (PA/SI) is performed and the data used to

score the site using the Hazardous Ranking System. This

score determines whether the site will be listed on the NPL

and, consequently, whether Superfund resources can be

utilized. If so, a remedial investigation and feasibility study

(RI/FS) are undertaken. The remedial investigation is a

focused field and laboratory study of the distribution of

contaminants and potential transport and exposure

pathways. The feasibility study is a detailed examination of

the remedial action alternatives (U.S. EPA, 1988a).

The data collected during the RI/FS, including treatability

studies if warranted, should be such that the remediation

selection decision is supported and that pertinent parameters

can be determined including pumping rates, the number and

location of extraction wells, and other remediation features.

In some cases, however, additional information may be

required and collected during predesign. Experience has

shown that it may also be useful to conduct periodic

operational studies to support active management of a

remediation system (U.S. EPA, 1989a). Such studies may

consider the effects of changing flow rates, adding or

deleting wells, altering the treatment train or other changes

in the remedial action.

Strategies for performance evaluations should view data

gathering activities during the remediation as an extension

of the historical data gathering and characterization of the

site. For the sake of continuity and the ability to conduct

meaningful comparisons at future dates, the performance

evaluation program should (i) expand the collection of key

hydrodynamic and chemical data; and (ii) continue and

expand the site characterization efforts to refine the

databases regarding the hydrogeology and transport

processes, to the full extent that this is needed for both

oversight monitoring and operational management purposes.

Traditionally, the key controls on the form and quality of the

technical data obtained during remediations have been the

monitoring criteria that are selected and the monitoring

point locations at which those criteria are to be applied.

This traditional collection of ground-water samples for

chemical analyses and measurements of water level

elevations has not been adequate for detailed performance

evaluations. There is a need for ongoing improvement of

the site characterization database. This database may be

updated by periodic or opportunistic testing, which may

include analysis of chemical extracts of aquifer sediment

samples collected from new boreholes, and aquifer testing,

geophysical and geological logging of new wells. Such

information would allow refinement of ground-water

velocity and flowpath estimates under various pumping

conditions.

The collection of aquifer sediment samples for identification

and quantification of contaminant mass reserves is an

example of a technical activity that should be routine for

pre-operational site characterization studies, but which is

rarely done during operation of remedial actions. This is
7

plume. It is also important to monitor inside the plume

during remediation. Such actions may make it possible to

determine which parts of the plume are being remediated

effectively and efficiently. This facilitates management of

the remediation wellfield for best performance. For

example, this may be accomplished by reducing flowrates

from extraction wells producing water from relatively clean

zones and increasing the flowrates from wells producing

water from highly contaminated zones. By contrast, the

exclusive use of oversight monitoring points downgradient

of the plume boundary does not allow an understanding of

plume behavior during remediation, except to indicate when

contaminants have migrated beyond the zone of remedial

action.

There are many kinds of monitoring points which are

useful for performance evaluations. Natural water quality

or background sampling locations are the most widely

used monitoring points and are positioned outside of the

plume boundary (Figure 4). The location is chosen such

that:

(i) it is neither in the plume nor in adjacent areas that

may be affected by the remediation;

(ii) if downgradient, it is in an uncontaminated

portion of the aquifer through which the plume

would migrate if the remediation failed; and

(iii) its location minimizes the possibility of detecting

other potential sources of contamination (e.g.,

relevant to the target site only).

Data gathered from background monitoring points located

upgradient or cross-gradient of a contaminant plume may

also serve to indicate when contaminants have migrated

beyond the zone of remedial action. This latter might occur

when injection wells are used to return treated water to the

aquifer at the upgradient edge of the plume and

simultaneously have the unintended effect of forcing a

portion of the contaminant plume outward. The upgradient

and cross-gradient locations of background monitoring

points may also be useful in providing a continuous check

on the regional water quality, thus providing early warning

of previously undetected contaminants flowing into the area.

Public water supply wells located downgradient of a plume

are a second kind of beyond-plume monitoring point. These

wells were originally drilled in locations suitable for water

supply purposes and were never intended to serve as

monitoring wells. However, they may be sampled to assure

the quality of ground water that is delivered to consumers

using monitoring criteria that are related to specific

contaminants associated with a site and maintenance of

existing quality, or other health-based standards.

unfortunate because it may provide one of the few direct

means of evaluating the general effectiveness of the

remedial action (e.g., the magnitude of contaminant mass

reduction obtained during a given time period). However,

due to the inherent heterogeneous nature of the subsurface

this may only provide general indication of a remediation’s

effectiveness. Together with operational data, information

from the analyses of aquifer samples can be used to estimate

the future rate of contaminant mass reduction under the

existing wellfield configuration and pumping schedule.

With the same information, it should also be possible to

estimate the best performance of the remediation, which is

defined here as the lowest contaminant concentration profile

maintainable. However, this assumes that the contaminant

sources have been accurately delineated and that no

immiscible contaminant sources are present. Continuous

characterization of the site during remediation will provide

the basis for determining the optimal wellfield configuration

and pumping schedule for the next forecast period, and for

projecting the best performance during that period. One of

the best reasons for active remediation management is based

on economics. Pumping the minimum volume at the

highest concentrations and in the minimum time frame is the

prime avenue to cost minimization.

Note, however, that even if an initially optimized

remediation is operated continuously without any changes in

wellfield configuration or pumping schedule, occasional reestimation of its best performance may be necessary, due to

technological advances and because estimates of the

contaminant masses in reserve and their effective rates of

depletion may change with time. As a consequence, there is

a need for continuous collection of routine monitoring and

operational data during remediation. Operational data is

critical for system optimization, and must be obtained

regularly to achieve and maintain the most efficient and

effective remediation. Important data include:

(i) flow rates of extraction and injection

wells;

(ii) water-level elevations in all wells and piezometers;

and

(iii) the concentrations of contaminants in monitoring

wells, extraction and injection wells, and the

influent and effluent of each treatment process unit.

1. Locations for Plume Monitoring

It has long been recognized that it is important to provide

continuous information on what is happening downgradient

of the plume boundary, because this is the means by which

potential receptors may be warned of the advance of the
8

Figure 4. Possible monitoring point locations in a ground-water contamination plume undergoing pump-and-treat remediation.

Gradient-control is a third type of compliance which is

being used with increasing frequency. It is established

specifically for determinations of the directions and magnitudes of hydraulic gradients. Such a monitoring “point” is

comprised of a cluster of small diameter wells that have

very short screened intervals (e.g., piezometers), and is

usually located just outside the perimeter of the plume.

Water level measurements obtained from wells that are

screened over comparable stratigraphic intervals are used to

prepare detailed contour maps from which the directions

and magnitudes of local hydraulic gradients can be approximated. An evaluation of the vertical gradients can be made

in a similar fashion by comparing water level measurements

from adjacent wells screened at different depths. It is

usually just as important to determine the vertical hydraulic

gradients as the horizontal hydraulic gradients, because a

remediation wellfield may control only the uppermost

portions of a contaminant plume (e.g., when remediation

wells are too shallow or have insufficient flow rates).

Less often utilized than any of the foregoing are withinplume monitoring points. Most of these are installed during

the site investigation phase, prior to the remediation. These

wells are needed to monitor the progress of remediation.

These can be subdivided into on-site plume monitoring

points, located within the property boundary of the facility

that contains the source of the contaminant plume, and offsite plume monitoring points that are located beyond the

facility boundary.

2. Monitoring Criteria for Performance Evaluations

It is convenient to identify four categories of monitoring and

performance evaluation criteria: chemical, hydrodynamic,

treatment efficiency, and administrative control.

Although several federal statutes impose limitations on

chemical concentrations in fresh water, EPA’s 1985 National

Contingency Plan (NCP) revision and the Superfund

Amendments and Reauthorization Act (SARA) of 1986

provide the concept of applicable or relevant and

appropriate requirements (ARARs) that derive from any and

all federal statutes. ARARs also effectively include

regional, state, and local statutes, because EPA is required to

consider these (which a state may actively encourage by

withholding its concurrence signature in the site settlement).

Chemical monitoring criteria may be based on a

combination of:

(i) Maximum Contaminant Levels (MCLs) that

have been promulgated for drinking water

supplies under the Safe Drinking Water Act

of 1974, as amended by the SDWA

Amendments of 1986;

(ii) Alternate Concentration Limits (ACLs) that

are health/risk-driven and technology-driven

are site-specific to ground-water corrective

action plans (CAPs) under the Resource

M

Extraction Wells

Off-site Background

Quality Wells

Bldg 

2

Bldg 

1

Bldg 3

Gradient Control Points

M Monitoring Wells

Hospital 

Backup Well

Municipal

Wellfield

Potential

Receptors

Injection Wells

Source Areas

Ground Water

Flow Direction

Property Boundary

Pit 1

Contaminant Plume

M

M

M

M

M

M

M

M

M

M

M

M
9

Conservation and Recovery Act of 1976, as

amended by the Hazardous Waste

Amendments Act of 1984;

(iii) State Water Quality Standards that derive

from the Clean Water Act of 1972 and other

statutes that protect aquatic wildlife and their

habitat; and

(iv) county, municipal, or regional restrictions on

the quality of water that may be discharged to

local lands, or other requirements for the

return of pumped water in water-short areas.

The actual numerical levels of chemical oversight

monitoring criteria may be affected by detection limits, and

natural water quality. For example, the acceptable

concentration level for a given contaminant in health-based

terms may actually be much lower than can be physically

determined with routine instrumentation, or the natural

levels of certain contaminants (e.g., arsenic or selenium) are

greater than ARARs.

Hydrodynamic monitoring criteria include:

(i) prevention of infiltration through the unsaturated

zone;

(ii) maintenance of an inward hydraulic gradient at the

boundary of a plume of ground-water

contamination; and

(iii) providing minimum flows in streams or wetlands.

Vertical profiles of moisture content may be used to

determine whether there is effective prevention of

infiltration through the unsaturated zone (e.g., below a site

cap). The requirement that an inward hydraulic gradient be

maintained at the periphery of a contaminant plume

undergoing pump-and-treat remediation is imposed to

ensure that no portion of the plume migrates away from the

zone of remedial action.

Evaluations of the hydraulic gradients exerted by

remediation wellfields require many measurements. To

assess this performance adequately, the hydraulic gradients

must be measured accurately in three dimensions between

each pair of adjacent pumping or injection wells. Velocity

divides may occur in the horizontal plane near active wells

and may develop in the vertical dimension if the well

hydraulically influences only the upper portion of the

aquifer. The ground-water velocity is essentially zero at the

stagnation point which forms downgradient of pumping

wells.

The performance standards that are applied to the treatment

plant portion of a pump-and-treat remediation are most

often phrased in terms of treatment efficiency criteria.

Although simple comparisons of total organic chemical

loading in the influent and the effluent of the treatment train

give some idea as to overall treatment efficiency, it is

important to make a detailed examination of the influent and

effluent to each treatment unit.

In some cases, individual compounds present limitations to

the treatment process because of effluent quality standards

for discharging to a local stream (e.g., National Pollutant

Discharge Elimination System (NPDES) permit

requirements). In other cases, after some time of pumping

VOC-laden water for treatment solely by air-stripping, nonvolatiles may begin to arrive in the influent and require the

addition of carbon filtration to the treatment train. In still

other cases, there is a need for examination of key locations

within the treatment process to troubleshoot the effects of

certain contaminants on treatment units. Iron and calcium,

for example, may precipitate in an air stripper and

dramatically decrease throughput.

Administrative control monitoring criteria may be codified

governmental rules and regulations (e.g., for fire safety and

electrical hazards), but they also include:

(i) effective implementation/enforcement of drilling

bans and other access-limiting administrative

orders;

(ii) proof of maintenance of site security; and

(iii) reporting requirements, such as frequency and

character of operational monitoring reports.

Combinations of chemical, hydrodynamic, treatment

efficiency and administrative control monitoring criteria are

generally stipulated at monitoring points. The exact

combination for a specific monitoring point depends on its

location relative to the source of contamination and the

potential receptors.

3. Strategies for Monitoring Locations and Criteria

Each kind of monitoring point has a specific and distinct

role to play in evaluating the progress of a remediation. In

choosing chemical monitoring criteria, it is essential to

recognize the interdependency of the criteria at different

monitoring locations.

The monitoring point locations and criteria should be

selected initially on the basis of a detailed site

characterization, from which transport pathways prior to

remediation are identifiable, and from which the probable

transport pathways during remediation may be predicted. It

may be useful to complete pre-design studies and pilot
10

studies of remediations prior to selection of certain

monitoring point locations and criteria to allow consideration of the effects of system operation on flow and

transport pathways.

Specifically, monitoring point locations and criteria should

be keyed not only to the no-action transport pathways, but

also to the probable transport pathways during routine

operation and under partial operational failures. Generally,

complete failure may be presumed to be the same as noaction, though this may not be true if the pump-and-treat

remediation is augmented by other technology such as

subsurface barrier walls. Since the chemical composition

and flow pattern of the zone of remedial action may change

substantially as the remediation progresses, the monitoring

point locations and criteria may also need to change.

Assuming that a remediation wellfield is operated with

constant system configuration and pumping schedule, it will

still generate flow velocities that are highly variable

throughout the zone of remedial action. In part, this is due

to the heterogeneous nature of the subsurface sediments

(e.g., hydraulic conductivity may vary considerably over

short distances). This may also occur because each

pumping well causes local velocity variations.

There may be a need to relocate or add extraction or

injection wells to better remediate portions of the interior

core of the contaminant plume. These kinds of system

modifications underscore the need to anticipate changes in

chemical patterns at monitoring locations, by taking into

account the detailed flowlines generated by the wellfield,

and the changes in contaminant concentrations that might

result from variations in the influences of transport

processes along those flowlines.

One way to organize studies of possible monitoring point

locations is by mapping flowlines generated by remediation

wellfields in sufficient detail to characterize the capture

zones generated by extraction wells and mounding zones

produced by injection wells. For each remediation well, the

location of a stagnation point and the maximum width of the

capture zone should be identified. A different flowline plot

may be required for each change in key variables (e.g., well

flowrates and locations, natural hydraulic gradient, etc.).

Provided that the estimates of key parameters are reasonably

accurate, such flowline illustrations should form a sound

starting point for selections of monitoring point locations. It

is important to ensure that external stresses (e.g., local

wells) are accounted for when mapping flowlines.

4. Measures of Operational Effectiveness

At the heart of a remediation performance evaluation is the

need to evaluate its operational effectiveness, which may be

measured in a number of ways. One may view it as the

general degree of hydrodynamic control exerted and/or the

general degree of contamination clean-up achieved. To

quantify the general degree of hydrodynamic control

exerted, one may compute the mathematical average of

several estimates of the horizontal and vertical gradients

along the outer bounds of the contaminant plume. The

average value of the hydraulic gradients should be inward,

toward the core of the plume. To quantify the general

degree of contamination clean-up achieved, one may

compute the average value of the total contaminant loads

(e.g., total VOCs) at all monitoring locations and compare

this computed average with previous values.

The total contaminant mass present at any monitoring point

may be estimated by analyses of chemical extracts of

subsurface sediment samples. The physicochemical basis of

this estimation procedure is that chemical extraction

techniques recover all of the contaminant mass present in

the sediment sample, regardless of phase. Obviously, the

sampling technique, spatial coverage of sampling efforts,

and extraction efficiencies are quality control considerations

that must be considered for this technique to generate

reliable estimates. Under ideal hydrogeologic conditions

(e.g., homogeneous and isotropic sediments) the estimated

values should trend downward over time if the remediation

progresses satisfactorily.

However, there may be significant limitations to such

contaminant mass estimation methods. The degree of

heterogeneity of subsurface sediments will strongly

influence estimate reliability. Slight variations in vertical

and horizontal hydraulic conductivity may significantly

affect the migration of both aqueous phase and immiscible

phase contaminants. Furthermore, it is generally accepted

that the adsorptive capacity of sediments for organic

contaminants increases with decreasing grain size. As a

result, contaminant mass estimations using sediment

extraction techniques may only provide general indications

of the effectiveness of a remediation. As the degree of

hydrogeologic complexity increases, the likelihood of

obtaining replicable sampling and analytical results

decreases.

The total contaminant mass present at a monitoring location

has often been estimated from contaminant concentrations

in ground-water samples. This approach employs

adsorption isotherms to infer the concentration of

contaminant adsorbed by subsurface sediments from the

measured contaminant concentrations in ground-water

samples. Unfortunately, this approach may seriously

underestimate the mass present, because the assumptions on

which the method is based may be easily violated.

The total contaminant mass present at monitoring locations

has also been represented by the direct sum of the
11

concentration values of the contaminants found in groundwater samples. However, this is a near-meaningless

approach for many pump-and-treat remediations, primarily

because remediation wellfields cause increased flow

velocities locally and induce invasion of the plume by

uncontaminated water beneath and adjacent to the plume.

These and other factors serve to depress contaminant

concentrations in the ground-water, at least until pumping

ceases and chemical equilibrium can be re-established.

The general degree of hydrodynamic control or clean-up, as

represented by averaged values of hydraulic gradients or

total contaminant mass, is an informative and useful

measure of operational effectiveness, but it does not address

the variations in degree of control or clean-up. Therefore, a

second way one may view the operational effectiveness of a

pump-and-treat remediation is with regard to the spatial

uniformity of the control exerted and/or the clean-up level

achieved. In this context, hydrodynamic control should be

evident along the periphery of the plume at each location

where hydraulic gradients are determined from water-level

elevation measurements. Likewise, the total contaminant

load at each within-plume monitoring point should not vary

significantly from the average.

Operational effectiveness may also be viewed in terms of

the persistence of the desired effects of the remediation. Is

hydrodynamic control of a contaminant plume maintained

continuously, regardless of seasonal variations in the

recharge and flow rates of the regional system? Do

concentration profiles remain stable or are they affected by

residual contaminants as the water table rises and falls

throughout the year? Are concentration profiles affected by

increased infiltration due to aging, settling and slumping of

materials beneath an on-site cap? Any of the foregoing may

cause the desired effects of a pump-and-treat remediation to

be diminished over time. Persistence of desired effects is a

very real concern when evaluating operational effectiveness.

5. Measures of Operational Efficiency

While operational effectiveness may be the primary focus of

regulatory entities, operational efficiency is more often the

focus of those who bear the financial responsibility for the

remediation. There are a number of ways to measure

operational efficiency, and some are germane to both

regulatory and fiscal interests.

The minimization of total costs, while reaching and

maintaining the remediation targets, is the most

economically-oriented view of operational efficiency. Total

costs can be tracked on a quarterly or annual basis to

provide a relatively simple picture but, the operation of the

system may affect this picture positively or negatively. For

example, reduced downtime and stabilized operations

usually follow the initial shakedown period of operation;

automation of some manual operations may be possible as

experience is gained with the system or as technology

advances. The total costs per year may decline continuously

as the remediation progresses. However, the costs may also

be erratic, because of changes in the specific pattern of

contaminant removal. For example, portions of the plume

may not respond to remedial actions as predicted,

particularly if any previously undelineated sources are

present (e.g., NAPLs). Such sources may serve as longterm reservoirs and continue to release contaminants for a

very long time (U.S. EPA, 1989a, Mercer and others, 1990

and Huling and Weaver, 1991).

Unfortunately, the tracking of total costs also can be a

misleading measure of operational efficiency unless costs

are normalized for the effects of inflation, interest rates, and

depreciation of recoverable assets. Very real trends of

improved operational efficiency may be completely hidden

until costs are normalized. This can be a confusing task

because such factors may vary substantially from region to

region, and from small to large organizations.

A less complicated means of evaluating the operational

efficiency of a pump-and-treat remediation is to look for

maximization of contaminant removal per unit volume of

pumped and treated ground water. This approach focuses

on the mechanical efficiency, from which bottom-line

economics may be inferred without regard to market forces.

It is generally acknowledged that costs are inversely

proportional to the contaminant levels of the treated water

(e.g., it normally costs less on a per pound basis to remove

contaminants from a concentrated wastestream than from a

dilute wastestream). Hence, to evaluate the operational

efficiency by this approach, the concentration levels of the

contaminants in the pumped waters must be tracked. Under

idealized conditions the management of a remediation

system should be such that the most contaminated

wastestream is continuously produced. This concentration

would gradually decrease as the aqueous phase

contaminants are depleted from the subsurface. However,

the presence of residual contaminants adsorbed to aquifer

materials and/or immiscible contaminants (e.g., NAPLs)

may result in elevated contaminant concentrations for very

long periods of time.

One consequence of maximizing the wastestream

concentrations is that the minimum volume of contaminated

water may be produced over the lifetime of the remediation.

This is of interest particularly to regulators where ground

water is in short supply. The total volume of pumped and

treated ground water may therefore be a measure of

operational efficiency. Naturally, there are economic

attractions to minimizing the total volume of pumped and

treated ground water. It is generally accepted that costs are
12

directly proportional to the amount of material handled for a

given contaminant concentration level. It is also clear that the

electrical and maintenance costs of extraction and injection

well pumps depend directly on the volume of ground water

handled.

A concurrent benefit of minimizing the total volume of

pumped and treated ground water is that the time for

completion of the remediation may be minimized. This is of

interest to both regulators and the financial backers of

remediations, since they share sensitivity to public perceptions

of progress at the site. The time for completion of major

phases of the remediation may also be considered a measure

of operational efficiency. The obvious regulatory benefits of

minimization of time for completion of remediation are the

abbreviated costs of oversight efforts, and enhanced credibility

with the public. The economic benefits to the financial

backers of remediations may include reduced labor and

operational costs, and increased value of salvageable assets.

6. Strategies for Determination of Success/Closure

Measures of the operational effectiveness and efficiency of

a pump-and-treat remediation can be used to monitor the

progress of the remediation. Endpoints must be selected to

bring the remediation to termination. Such endpoints may

involve absolute or relative measures of success and may

include statistical considerations.

Absolute measures of the desirability of terminating the

operational phase of a pump-and-treat remediation can be as

simple as the removal of a specific mass of contaminants, such

as recovery of the total estimated volume of free product in

gasoline-contaminated sediments. Conversely, absolute

measures may be such things as achieving specific

contaminant concentration levels (e.g., MCLs). The former is

much less likely to be accepted by regulators than the latter,

primarily because it is often extremely difficult to accurately

estimate the total mass of contaminant present in the

subsurface. Consequently, pre-operational decisions to

remove only a specific mass or volume run the risk of

terminating the remediation without significant reduction of

risk to human health and the environment. Absolute measures

are more likely to meet opposition by those responsible for the

operation of the remediation because of the difficulties

associated with ensuring clean-up to specific levels. A similar

concern may be expressed by the regulators, since it is difficult

to foresee what technological innovations may develop during

the years of remedial operations. Fortunately, absolute

measures need not be chosen as stand-alone endpoints;

contingencies for demonstrated limitations or technological

innovations can be integrated into the phrasing of absolute

measures.

Relative measures of the success of remedial operations

specify that specific percentage reductions of contaminant

concentrations be achieved. These have not been used as

stand-alone criteria for termination of remediations, but are

often implemented as requirements such that the treatment

process operates continuously at a known efficiency of

contaminant removal. There is merit to considering relative

measures for a contaminant plume, since specific percentage

reductions of contaminant concentrations can be translated

into risk reductions to human health and the environment.

For all practical purposes, however, percentage reductions

that are applied to known pre-operational contaminant

concentration levels are equivalent to absolute measures

since the resulting concentrations are readily computed.

One major incentive for phrasing clean-up targets as

specified percentage reductions from pre-operational

concentration levels is the public relations benefits. Most

individuals can readily grasp the idea that 99.99% of the

contaminants were removed, whereas far fewer are schooled

in what specific concentration levels may mean.

Regardless of whether absolute or relative measures of the

success of a pump-and-treat remediation are employed, it is

clear that a single round of water-level measurements or

samples will not suffice to justify termination of a pumpand-treat remediation. At a minimum, successive sampling

on a monthly or quarterly basis is needed. The results are

examined to determine whether or not the remediation has

truly reached the specified targets. Such examinations of

the datasets inevitably reveal that the data values vary from

one sampling/measurement event to the next. While it is

possible to view these data value variations rigidly, such that

each value from each sampling/measurement event must be

at or below the target concentration level, this approach may

be unduly restrictive and may not be justified statistically.

The evaluation of pre-termination datasets with statistical

measures entails plotting the contaminant concentration

values versus time and fitting a multiple/nonlinear

regression line to the plotted values (Figure 5). Two kinds

of outcomes of the regression line fitting are acceptable

initial indications that the remediation may be ready for

termination. The first being that the contaminant

concentrations have leveled off and the regression line has a

slope equal to zero at an average contaminant concentration

value that is at or below the target concentration. The

second being that the regression line has a negative slope

and concentrations are below the target concentration.

The slope of a fitted regression line is an insufficient

statistical measure by itself, since data values inevitably

vary about this line. A Student’s t-test should be performed

to ascertain that the slope of the fitted regression line is not

greater than zero at a 95% level of confidence. Quality

control considerations also indicate that the vast majority of

the data values should fall within a 95% confidence interval
13

Figure 5. Concentration vs. time data plot and multiple regression line

with confidence bounds for judging persistence of the

clean-up.

1,400

1,200

1,000

800

600

400

200

*

*

*

*

*

* * *

*

*

* * * *

* * * *

UCL 95%

* * * *

* * * *

* * *

1981 1983 1985 1987 1989

Influent Concentration (ppb)

Target 

Concentration LCL 95%

about the regression line. Regulators are concerned only

with the upper 95% confidence interval boundary for most

parameters.

Following termination of the remediation, the sampling/

measurement events should be continued to assure

persistence of the effects of the remediation. A declining

frequency of sampling/measurement events is most

appropriate for this approach. For example, monthly

sampling events for the first three or four months

immediately following termination may be adequate. This

should be followed by quarterly sampling events until the

end of the second post-operational year, and annual

sampling events until the end of the post-operational

regulatory period. Data obtained during these sampling

events should be used to update regression plots. Reactivation of the remedial actions and/or re-evaluation of the

appropriateness of the remedy may be indicated when the

slope is no longer statistically at or below zero, or when two

or more successive data values are above the upper 95%

confidence interval bound of the regression line.

It may seem desirable to base the decisions to terminate or

re-activate a pump-and-treat remediation on examination of

a single regression plot that represents all monitoring point

locations. Technically, it may be possible to justify the

grouping of results from monitoring point locations of a

certain kind. For example, on-site monitoring point

locations may fall into this category. At downgradient

background monitoring point locations, however, grouping

of results may have undesirable sociopolitical consequences.

The idea that one potential receptor is better protected than

another is generally unacceptable—although it may be

acceptable in some cases if the natures of the potential

receptors differ substantially (such as a water-supply

wellfield versus an ecological habitat).

In those cases where grouping of like locations is

determined to be acceptable, there arises the question of

how to do the grouping: should individual regression plots

be made and their slopes averaged, or should a single

regression line be fitted to a dataset containing the values

from all of the monitoring point locations belonging to the

group? The former option is equivalent to the latter option

only when the population distribution forms of the datasets

from the individual wells have comparable statistical

behavior (e.g., variance and skewness) and are wholly

independent of each other.

If the slopes of individual regression plots are averaged

from datasets of different monitoring points, the computed

95% confidence interval will be largely independent and

insensitive to actual variations of the data in each of the

datasets. If a regression line is fitted to a super-dataset that

contains data from all monitoring locations, the computed

95% confidence interval will be dependent on the actual

variations of the data in all contributing datasets. The

confidence interval computed from the super-dataset may be

larger than the confidence interval computed by averaging

the slopes of individual regression plots. As a result,

regulators may prefer the former option (smaller 95%

confidence interval), and responsible parties may prefer the

latter option (larger 95% confidence interval). Neither is

more or less correct statistically.

The foregoing discussion presumes that a decision has been

made at each monitoring point regarding whether or not to

prepare regression plots of contaminant concentration levels

versus time for each contaminant, for groups of

contaminants, or for the total contaminant load. In

addressing this point, the methods for computing values of

95% confidence intervals may be considered in a manner

that is parallel to the foregoing discussion. One may either

compute the 95% confidence interval of measurements of

each contaminant at a given monitoring point location and

then average these results or, combine several (perhaps all)

data values into a single dataset and then compute its 95%

confidence interval. These computations must be based on

normalized data values (e.g., the results of dividing the raw

data values of each contaminant by its average value) or the

results will be biased toward the contaminant datasets with

the highest concentration levels. Again, regulators may

prefer the former option (a smaller 95% confidence interval

results), and responsible parties may prefer the latter option

(a larger 95% confidence interval results). The grouping of

the datasets from several contaminants at a given

monitoring point location may be desirable only if justified
14

by similarities in contaminant transport behaviors and the

health risks posed by the contaminants to be grouped.

Data Collection Considerations

Efforts made to characterize ground-water contamination

problems for remediation have traditionally focussed on the

quantification of contaminant levels rather than the potential

pathways and mechanisms that affect transport (Bear and

Verruijt, 1987; Boutwell and others, 1985; Faust and others,

1981; Keely and others, 1986; Konikow, 1986; Osiensky

and others, 1984), however, this bias is diminishing because

of a growing recognition of the limited usefulness of

chemical samplings of monitoring wells. It is clear that

periodic sampling can produce successive snapshots of the

distribution of contaminants at a site, but it is equally clear

that one can rarely expect to infer key contaminant transport

features and parameters from such snapshots. An approach

to site characterization is emerging wherein greater

emphasis is given to detailed characterization of transport

pathways and mechanisms, while retaining a commitment to

definition of the full extents and compositions of

contaminant plumes (Bouchard and others, 1989 and 1988;

Curtis and others, 1986; de Marsily, 1986; Goltz and

Roberts, 1986; Jorgensen and others, 1982; Mackay and

others, 1986; Major and others, 1988; Matheron and de

Marsily, 1980; Molz and others, 1986 and 1988; Scanlon,

1989; Usonoff and Guzman-Guzman, 1989).

1. Purposes of Data Collection

The objectives of collecting data are determined in large

part by the intended use (Boutwell and others, 1985; Davis,

1986; Huyakorn, 1984; Krabbenhoft and Anderson, 1986;

Loftis and others, 1987; Mercer and others, 1983; Ophori

and Toth, 1989). If data are to be used to assess the general

potential for contaminant releases and transport at a newly

discovered site, the objectives will be limited to problem

screening efforts for the time being. Based on the outcome

of the screening process, further data collection events may

be planned. This is an iterative process, which usually calls

for increasingly more detailed data collection which

culminates in a comprehensive characterization of the

physical extent and chemical makeup of the contamination

problem. Traditionally, the identification of transport

pathways has been limited to an understanding of the natural

flow system in such studies.

An understanding of contaminant migration pathways is

essential to the selection and design of an appropriate

remedy. Unfortunately, the objectives of past site

characterization studies have rarely included sufficiently

detailed testing for hydraulic parameters and have often

failed to identify key/limiting hydrogeologic processes or

features. As a result, the remedy selection process has often

proceeded without the benefit of information vital to the

proper evaluation of the success of various remedial actions.

Instead, such details were often addressed during the design

phase following the remedy selection, if at all.

With the benefit of hindsight and an acknowledgement of

the growing number of reported ineffective remediations, it

is clear that the site screening and site characterization

studies that precede remedy selection must include remedial

design parameters as primary data collection objectives.

This means that detailed hydraulic testing, bench treatability

studies, and pilot operations of individual extraction wells

must begin during the site characterization studies.

Fortunately, this has been recently acknowledged and since

1989 the EPA Regional Offices have supported this

approach (Cannon, 1989).

During the process of remedial design, it is essential to

formulate data objectives for the operational, closure, and

post-closure phases of the site work. While nominally

needed for regulatory compliance, data should also be

tailored to provide key information for active management

of the remediation (e.g., individual flows and chemical loads

of all extraction and injection wells, detailed water-level

measurements, treatment unit efficiencies). Data collections

for regulatory compliance purposes may include samplings

from upgradient and downgradient public supply wells,

private wells, and industrial supply wells, and from

monitoring wells placed close to particularly sensitive

ecological habitats.

The objectives of data collections for regulatory compliance

purposes extend beyond system effectiveness and efficiency

as operational considerations, to the driving force behind all

of the site efforts—protection of human health and the

environment. In regulatory terms, the effectiveness of the

remediation matters because of the potential that each

exposure to contaminants poses added, cumulative risk to

society. Likewise, the efficiency of the remediation matters

because the potential duration of exposure is limited

ultimately by the duration of the remediation. Efficiency

also matters because of the limited funds and human

resources that are available to conduct such work.

2. Relevant Scale(s) of the Problem

Perhaps the most subjective influence on data collection

strategies relates to the size of the problem. History

suggests that the larger the size of a contamination problem,

the more likely the problem will be addressed at a minimal

level in screening and site characterization efforts.

From a statistical viewpoint, the scale of the problem can be

addressed by breaking it down into definable regions

(Davis, 1986; Huntsberger and Billingsley, 1977;
15

Mendenhall, 1968; Steinhorst and Williams, 1985; Taylor,

1987; Ward and others, 1987). For example, the nominal

plume may encompass an area of a few hundred acres and

an average depth of a few tens of feet. A relatively dense

monitoring network is most appropriate within the plume

with each monitoring location consisting of a single well or

a cluster of monitoring wells. Each monitoring well is thus

constructed to sample a different specific zone and allow

vertical and horizontal delineation of the contaminant

plume.

By contrast, the areas that may be impacted if remedial

actions are not implemented may be many hundreds or

thousands of feet downgradient from the contaminant

source, and may encompass hundreds or thousands of acres.

The major data collection objective within the zone between

the contaminant plume and potential receptors is to provide

an early warning of failure of the remediation to contain the

plume. Consequently, it is important to have an

understanding of where the plume may migrate if it escapes

the remedial action zone. These data needs may decrease

with increasing distances down-gradient from the plume.

The monitoring well and hydrogeologic testing locations

should be skewed toward a dense pattern immediately

downgradient of the plume and a sparse pattern farther

downgradient.

It is important to understand the connection between the

local hydrogeologic setting and the regional flow system in

studies of subsurface contaminant transport (Mackay and

others, 1986; Osiensky and others, 1984; Scanlon, 1989;

Ward and others, 1987). It is often necessary to thoroughly

investigate the regional trends in water levels to formulate

appropriate boundary conditions and water balances for a

model of the local flow system.

3. Actual Scale of the Measurements

There are many ways to estimate the hydraulic conductivity

of subsurface materials (Cedergren, 1989; Corey, 1977;

Davis and DeWeist, 1966; Freeze and Cherry, 1979; Mercer

and others, 1982) and it is not uncommon for the various

methods to yield differing results when performed at the

same location. There are numerous reasons for this, but one

of the major factors relates to the scale of the measurements

(Anderson, 1979; Mercer and others, 1982). For example, it

has been shown that laboratory measurements of the

hydraulic conductivities of clay and silt samples may be as

much as two to four orders of magnitude lower than the

values determined by field tests and that such

determinations tend to have a much greater variability

(Anderson, 1979; Klotz and others, 1980; Mercer and

others, 1982; Molz and others, 1986a and 1988). Clay and

silt samples are often compacted during sampling operations

and laboratory analysis. Additionally, the sample volume

may be too small to intercept the macropores and other

preferential flow paths that conduct most of the flow

through the stratum.

Depending on the magnitude of hydraulic stress created, the

hydraulic conductivity values obtained from field tests may

vary considerably at the same location. Small-volume tests,

such as slug tests, affect only a few cubic feet of formation

materials surrounding the well screen. As a result, it is not

unusual for the hydraulic conductivity values obtained from

slug tests of closely spaced wells to differ by several orders

of magnitude. The results are highly sensitive to minor

variations in well construction details, especially filter pack,

screen position and grouting. The testing locations are

effectively isolated despite their proximity to one another.

This is underscored by the stochastic character of such

measurements. They often appear to be random and fully

independent in settings where trends in physiographic

features are clearly evident (Dagan, 1982a; de Marsily,

1986; Gelhar, 1986; Vomvoris and Gelhar, 1986).

Conversely, the hydraulic conductivity values obtained from

large-volume field methods, such as aquifer pumping tests

and two-well tracer tests, may affect thousands to millions

of cubic feet of subsurface materials. These field methods

often demonstrate considerable dependency and

autocorrelation. This is understandable given the potential

for physical overlap of the effective areas of testing with

large volume tests, but such character is observed even

when it is clear that overlaps do not occur. One reason is

that autocorrelation is endemic to the occurrence of

physiographic trends and the hydraulic forces that help to

shape strata. Dipping strata impose directional control on

natural flows, as do streambed sediments that are laid and

reworked into lenses parallel to the course of a river (Blatt

and others, 1980; Compton, 1962; Davis and DeWeist,

1966; Fetter, 1988; Freeze and Cherry, 1979; Heath, 1983;

Osiensky and others, 1984; Todd, 1980). These act as

intrinsic biases to flow direction, something that can be

quantified in terms of anisotropy. The large-scale continuity

of physiographic trends makes it highly probable that

hydraulic conductivity values determined by large-volume

field methods will autocorrelate.

While large-scale continuity is a hallmark of physiographic

features, it is by no means a hallmark of contaminant

distributions. Consequently, the questions of how to

interpolate between measured locations and how to

extrapolate contaminant trends in time emerge as a major

issue. All too often, these questions have been

misinterpreted to mean that sampling locations and

frequencies are the only concerns that merit attention. In

point of fact, however, the representativeness of each

sample is of equal or greater concern. An examination of

possible strategies for characterizing the reliability of
16

chemical measurements is warranted, and begins with an

examination of sampling techniques.

Bailers or other low-volume sampling devices are

commonly used to obtain samples from monitoring wells for

organic chemical analyses. Discharging wells such as the

extraction wells in a remediation wellfield may also be

sampled, but for such wells the contaminant concentration

levels are often a function of the duration of pumping. This

fact may be exploited by the collection of successive

samples as pumping begins and continues (e.g., chemical

time-series sampling) with each successive sample

representing ground water from a part of the aquifer farther

from the extraction well. (Keely, 1982). The overall effect

is similar to increasing the number of point-sampled

monitoring wells. The volume of the sample is important,

because point samples are tacitly presumed to represent the

average chemical quality of fairly large volumes of the

aquifer from which they were withdrawn; such as occurs in

numerical modeling efforts, where grid blocks of several

thousand cubic feet are assigned average concentration

values.

The use of chemical time-series sampling is quite similar to

the use of tracer tests to gain insights into preferential paths

of contaminant movement (Keely, 1982). Ground-water

tracers are defined as matter or energy transported by water

which will give information concerning the direction and

velocity of ground water and potential contaminants. Since

the time of release and the input concentration of intentional

tracers are known but the inputs are unknown for

contaminant studies, the interpretation of data from

chemical time-series sampling might initially seem to be

severely hampered. However, it is the pattern of relative

concentrations over time which is actually examined in

either case. The absolute values are neither required nor are

they readily obtainable under all but the very best controlled

field tests in relatively homogenous settings. There are

often too many avenues for tracer losses (sorption, ionexchange, biodegradation, etc.) to allow for rigorous mass

balances over reasonable distances.

Regardless of whether chemical time-series sampling results

in a pattern of chemical arrival that show definite trends or

are completely random, the data collected are useful to the

investigator in terms of the statistical uncertainties of

contaminant concentrations in the plume being defined.

Such data can be helpful in deciding when a model has been

adequately calibrated and when it is predicting ground-water

flow or contaminant transport with reasonable accuracy.

They can minimize the amount of adjusting of key

hydrogeologic parameters that often accompanies efforts to

calibrate a model to field data. Chemical time-series

sampling results may be used directly in modeling efforts to

generate estimates of uncertainties in the model results.

However, care must be taken to understand the

autocorrelation of such data, because successive samples are

not completely independent since they represent waters that

have passed through the zones occupied by previous

samples. The ground-water contaminant concentrations will

be affected by interactions with sediments and residual

contaminants present in those zones.

The simplest approach to predicting the arrival of

contaminants at a pumping well from a selected distance

involves the relationship of the total volume of water

pumped to the distance traveled. Assuming radial flow to a

well, the distance waters travel during specific times are

indicated by concentric circles. Obviously, samples

collected after several hours of pumping represent an

average water quality of a larger volume of the aquifer than

samples collected only a few minutes after the onset of

pumping.

4. Data Quality vs. Quantity

Regardless of the data objectives, there eventually comes a

point in planning where one faces a trade-off between

quality and quantity. In ground-water investigations, the

descent from high to poor quality data is easily made. It is

extremely difficult to plan for data of moderate quality—

sophisticated field methods are either performed properly or

they are not. The economics of bad data collections are

painfully obvious. Not only can the magnitude and extent

of the contamination problem be wholly misunderstood, but

the selected remedy can be so off target as to actually

worsen the problem. However, there are many field

methods which produce good relative information that may

prove valuable to site characterization. The usefulness of

such methods and their results should not be

underestimated.

Hydrogeologic Data

1. Topographic and Geographic Data

Topographic and geographic data are most commonly

available as elevation contour maps. These are available

from the U.S. Geological Survey (USGS), the U.S. Forest

Service, the U.S. Army Corps of Engineers, and various

State agencies. At the local/site scale, subdivision plats and

city planning maps provide contour spacings as small as

0.25 feet. Aerial photographs of many cities are available

from several government agencies at five to ten year

intervals starting in the late 1940’s. Remote sensing images

are also available from various government agencies. Such

maps and photos are useful for gaining an understanding of

the physiographical trends and structural controls on

ground-water movement.
17

It is essential to measure all depths to water against a

common datum and most topographic maps are not accurate

enough for this purpose. Differences of only a few feet may

be comparable to the seasonal changes in water levels at

many sites. The elevations of the tops of the monitoring

wells must be surveyed to an accuracy of better than 0.1

feet. Otherwise, the potential for misinterpretation of the

hydraulic gradient and direction of ground-water flow may

be significant. Surveyed elevations should be checked

periodically because of the potential for slumping and

settlement of capped waste and compaction of pumped

strata.

2. Geomorphologic and Geologic Data

Maps which describe the geomorphologic character of large

tracts of land are available from the USGS and State

geologic agencies. Specifically, it is common for basin

boundaries and structural features to be illustrated on maps

that cover an entire state or physiographic province. Fault

lines and fracture trace orientations are often detailed on

such maps, but these may have little direct applicability at

the local or site scale. Although geologic maps which

describe regional stratigraphic and lithologic features are

available, it is essential to have an experienced geologist

detail the local setting. Principally, the geologist will rely

on inspection of local exposures, drill cores, and

excavations. Geophysical techniques, particularly the

borehole methods, can be of great assistance to the geologist

in evaluating the nature and extent of specific strata, as well

as the general water quality.

3. Flow Rates

Rainfall events, streambed losses, and snowmelt provide the

majority of natural recharge to the subsurface. Precipitation

records are kept by the National Oceanic and Atmospheric

Administration. Estimates of effective recharge are made

by the U.S. Department of Agriculture for crop purposes,

and by the USGS and State geological agencies for water

supply purposes. However, these estimates are generally

based on basinwide studies and do not provide much detail

at the site level. Aquifers also may be recharged purposely

by infiltration basins and injection wells. These artificial

recharge mechanisms are manmade and directly measurable.

Soil scientists have devised a number of methods for

measuring the movement of water through the unsaturated

zone. However, most of these methods are intended

primarily for use in relatively shallow surface soils. The

most traditional methods include tensiometers, gypsum

blocks and collection pans. State-of-the-art methods for

determining soil moisture content include the use of neutron

probes, pressure transducers, and time-domainreflectometry (TDR). These sophisticated methods are

subject to interference and produce data which must be

properly interpreted to quantify fluid movement.

The flow rates of springs can be measured with ordinary

surface-water weirs or flumes. Likewise, baseflow

recession curves can be used to estimate the discharge from

an aquifer to streams. Aquifer discharges can also be

approximated by using estimates of the hydraulic

conductivity and hydraulic gradient in Darcy’s Law. The

cross-sectional area through which the aquifer is discharging

is also needed for this analysis. Errors in the cross-sectional

area are unlikely to be more than two- or three-fold. This is

negligible given the possible order(s)-of-magnitude

uncertainty that may be associated with hydraulic

conductivity estimates. Ground-water flownets have been

used traditionally to estimate aquifer flows, with reasonable

success at the basin level, but not necessarily at the local/site

level. However, flow nets can be useful for determining

ground-water flow direction, making time of travel

estimates and assessing ground-water vulnerability (U.S.

EPA, 1989b).

4. Hydraulic Parameters

There are several methods of estimating the hydraulic

conductivity of an aquifer. Hydraulic conductivity values

obtained from laboratory permeameter methods represent

point estimates only. Unconsolidated sands and gravels may

become disordered and compressed during extraction from

the field and emplacement in the permeameter columns,

which may significantly alter their hydraulic properties.

Clays may be extracted from the field with less disturbance,

but they often undergo compression during emplacement

into permeameter columns. Additionally, high hydraulic

gradients are required to induce flow through clay samples

in laboratory permeameter studies. From practical

experience, it is easy to see that laboratory estimates of the

hydraulic conductivities of aquifer materials may be

variable.

It may be difficult to take a sample that accurately

represents the amount of secondary permeability present in

the clay resulting from macropores or other preferential

flow paths. This lack of representativeness usually accounts

for the common observation that many clay units

determined to be impermeable from laboratory tests are

often characterized as relatively permeable units by field

tests. This takes on great importance in many settings

because estimates of the potential effectiveness of a

remediation often depend heavily on the properties of low

permeability strata.

Numerous small-volume field tests for determining

hydraulic conductivity have been described in the literature

(Bouwer and Rice, 1976; Papadopulos and others, 1973).
18

Most methods involve displacement of a fixed volume (slug)

of water in a piezometer and relating the observed hydraulic

response to the properties of the aquifer in the immediate

vicinity of the piezometer. Storage coefficients obtained from

slug tests may not be representative of the aquifer due to the

small volume of aquifer being tested. This is important

because both the transmissivity and the storage coefficient are

needed for the well hydraulics computations used to size and

position wells for remedial actions.

Recent developments in electromagnetic and thermal borehole

flowmeter technology allow the determination of horizontal

hydraulic conductivity values as a function of vertical position

within an aquifer (Young and Waldrop, 1989; Hess and Paillet,

1989). However, the hydraulic conductivity, transmissivity

and storage coefficient values obtained at most hazardous

waste sites are estimated using various mathematical analyses

of aquifer pumping test data. The average saturated thickness

of that portion of the aquifer affected by the pump test must be

determined from drilling records or estimated by geophysical

methods. If the formation is very fine-grained, the capillary

fringe above the water table may be large enough to comprise

a non-negligible portion of the effective saturated thickness.

In such cases, continuous coring may be extremely valuable

in defining the magnitude of the capillary rise locally. If the

formation is coarse-grained, the capillary fringe will comprise

a negligible portion of the effective saturated thickness.

In addition to the hydraulic conductivity, the porosity and

hydraulic gradient must be estimated in order to compute the

average rate of movement of the ground water. Tracer tests

may be used in the field to estimate the effective porosity, but

this is possible only if the hydraulic conductivity has been

defined with very high accuracy. In practical terms, it may not

be possible to achieve such accuracy because the variations in

hydraulic conductivity about the mean value at a given site

may have a range of two or more orders of magnitude. By

contrast, effective porosity values for most aquifers lie

between ten and forty percent and may be estimated within a

factor of two.

Tracer tests can be conducted by injection and subsequent

withdrawal of a tracer solution in a single well through an

isolated injection zone. The extreme sensitivity of single-well

tracer tests to well construction details and the small volume

of aquifer affected limits the applicability of this method for

site characterization studies. However, two- well tracer tests

have been devised to avoid most of these problems. Two basic

kinds of two-well tests include natural gradient and forced

gradient tracer tests. Natural gradient tracer tests are

conducted releasing a small volume of concentrated tracer

solution in an upgradient well and monitoring the concentration of the tracer over time in the downgradient well.

Forced gradient tracer tests involve injecting a dilute tracer

solution in one well, while the other well withdraws ground

water at an identical rate. While both natural gradient and

forced gradient tracer tests can ostensibly yield the same

information, they differ in their sensitivity to dispersive forces,

due to the radically different velocities that are involved.

Natural gradient tracer tests tend to be biased toward large

dispersion coefficients and forced gradient tracer tests tend to

be biased toward smaller dispersion coefficients. The

distinction is irrelevant for most pump-and-treat remediations,

however, because they generate such strongly advective flows

that dispersive forces are usually negligible by comparison.

Forced gradient tracer tests are intrinsically suited to provide

appropriate estimates of hydraulic parameters for pump-andtreat remediations, because of the obvious similarities in

hydraulic and hydrodynamic behavior.

5. Fluid-Behavior Data

Several important fluid-behavioral properties include

miscibility, density, solubility, and viscosity. Miscibility is the

property that describes the ability of one fluid to dissolve into

or completely commingle with another fluid. Miscible fluids

do not exhibit stratification or separation of distinct phases.

Familiar examples of organic solvents that are miscible in

water include ethanol, ethylene glycol, and acetone.

Immiscible fluids, or NAPLs, exhibit separation of fluids at a

distinct interface. Familiar examples of immiscible fluids are

gasoline, creosote, trichloroethylene (TCE) and ether.

Dense immiscible fluids (DNAPLs) exhibit fingering during

advancement, as well as loss of residuals by snap-off in small

pore-throats and trapping in dead-end pores. Because of the

existence of a wetting phase (water) on subsurface grain

surfaces prior to percolation of spills or subsurface releases

from pipelines and tanks, the penetration of TCE and other

DNAPLs into intergranular pores is highly sensitive to a

critical effective pore size and the acceptable surface tension

that results at the interface between the two fluids for that pore

size. This description presumes that the water film on the

grains is at residual levels after complete gravity drainage; if

water is present beyond that needed to satisfy the hygroscopic

force of the wetting tension, it will be present as a pendular

ring at the bottom face of the grain, and may coalesce with

heavy films from other grains and thereby gain sufficient mass

so that the acceleration of gravity overcomes the total wetting

tension and downward flow occurs. Because of the orders-ofmagnitude range of variability of effective pore sizes in most

settings, dense immiscible fluids such as TCE penetrate nonuniformly.

6. Fluid Levels and Pressures

Some of the most important measurements taken at groundwater contamination sites are water level elevations as

hydrogeologic features and hydraulic parameters can often
19

be estimated with such data. Many sites are located in

highly active hydrologic and hydraulic settings where large

daily water level fluctuations are possible, due to the

influence of local streams and industrial and municipal

wells, but hourly variations may occur also. Such variations

are not uniform regionally or locally, and tend to die out

logarithmically away from the source of the disturbance. It

is essential to collect consistent data, so that differences in

water level elevations recorded at various wells represent

the spatial distribution of water levels and not temporal

variations.

Wells of comparable construction (e.g., screen length) must

be used. Small diameter wells with short screens are the

most suitable for providing detailed information on

horizontal and vertical gradients. Clusters of such wells are

effective and efficient for delineating horizontal and vertical

gradients.

In terms of hydrostatic position, it is incorrect to group

water level measurements from wells whose screened

intervals occur at greatly differing elevations. This is

especially important at sites with significant vertical

hydraulic gradients. Otherwise, contour maps drawn from

the resulting data will present a picture that is an indefinable

mixture of the horizontal and vertical components of

hydraulic gradient. It may be important to measure water

pressure rather than water level elevations, especially where

there is the possibility of the contaminant plume being more

dense than the native formation water (e.g., plumes with

total dissolved solids in excess of 1,000 parts per million).

Water supply wells, irrigation wells, and industrial supply

wells are usually constructed with lengthy screens and often

without annular seals above the filter pack. Consequently,

measurements of their static water level elevations are

integrated averages of vertical head differences over a large

section of the aquifer. Such measurements should not be

combined with measurements from observation wells or

monitoring wells to produce contour maps.

Water level measurements obtained from water supply wells

or other production wells during operation are greatly

affected by the pressure drop that occurs as ground water

passes through the entrance openings of the wellscreen. The

magnitude of the effect varies from well to well, over time,

and as a function of the flow rate. It is very difficult to

accurately estimate water level elevations immediately

outside wellscreens.

There are numerous methods for measuring water-level

elevations. The most common are chalked steel measuring

tapes, electrical probes and pressure transducers. Chalked

tapes have been used for decades because of their simplicity,

reliability, and accuracy (about 0.01 ft). The method is

extremely slow and most suitable for static measurements.

Electric water level probes usually come with crimped foot

or meter markers, but may become unreliable due to the

tendency of the wire to stretch over time. A steel measuring

tape should be used to measure the length of electrical wire

lowered into the well when practical.

Pressure transducers utilize very sensitive strain gauges and

amplification circuitry to measure changes in hydrostatic

pressure, which may be converted to water level elevations.

Most pressure transducers incorporate an internal

atmospheric venting tube that prevents pressure

measurements from being affected by barometric

fluctuations. Pressure transducer measurements are easily

automated, so that no re-positioning need be done to acquire

successive measurements, and extremely rapid

measurements (greater than one per second) are possible.

The acquired measurements are stored in data loggers that

often allow real-time analyses of the data.

Chemical and Geochemical Data

1. Natural Ground-Water Chemistry Data

The goal of most aquifer remediations is to return ground

water to its beneficial uses where practicable; therefore, it is

necessary to characterize the chemical and biological

composition of nearby uncontaminated wells to determine

natural ground-water quality. Samples from

uncontaminated wells should be obtained with the same

rigor and care given to samples collected from suspected

zones of contamination. At a minimum, samples should be

analyzed for major cations and anions to allow the

computation of cation/anion balances. Descriptive graphical

analyses methods utilize such data, and are time-proven

methods of characterizing natural water quality (Cheng,

1988; Freeze and Cherry, 1979; Loftis and others, 1987;

Novak and Eckstein, 1988; Ophori and Toth, 1989; Rogers,

1989; Scanlon, 1989).

2. Soil Chemistry Data

For complete characterization and transport evaluation,

native soils must be collected and analyzed for major ion

chemistry (Corey, 1977; Hillel, 1982; Mercado, 1985;

Mercer and others, 1983; Osiensky and others, 1988;

Rogers, 1989). In addition, the organic carbon content of

the soil should be determined because of the significant role

it plays in the retardation of organic contaminants

(Bouchard and others, 1989; Lee and others, 1988; Mackay

and others, 1986; Nkedi-Kizza and others, 1985, 1987 and

1989; Piwoni and Banerjee, 1989).

Soils and subsoils can be easily sampled from the sidewalls

of excavated pits, but deeper sediments must be obtained
20

from cores obtained during drilling operations. Traditional

split-spoon and Shelby tube samplers may be used to obtain

core materials for use in chemical analyses, lithologic and

stratigraphic mapping, and laboratory batch studies. However,

special coring devices and modified laboratory glove boxes

purged with an inert gas may be used to retrieve cores used in

fate and transport evaluations, and microbiological studies

(Nkedi-Kizza and others, 1989; Scalf and others, 1981;

Siegrist and McCarty, 1987).

3. Contaminant Reaction Data

Contaminated soils are generally sampled similarly to

uncontaminated soils. However, air-tight sample containers

are necessary when sampling volatile organic contaminants.

Laboratory studies of contaminant transport behavior often

utilize soil columns packed with subsurface sediments (Bitton

and Gerba, 1984; Siegrist and McCarty, 1987). Columns

packed with uncontaminated soil samples are used to

determine retardation coefficients for specific contaminants.

Column studies may also be used to obtain estimates of the

number of pore volumes of water that must be flushed through

the material to reduce contaminant concentrations to specific

levels.

Batch, or flask studies may be utilized to determine site

specific distribution coefficients for contaminants and

sediments. These data may be used to predict the effects of

retardation on the transport of contaminants through the

subsurface. The fraction of organic carbon (foc) present in

subsurface materials will strongly influence the mobility of

many contaminants.

Biological Data

Until recently, it was presumed that the subsurface was largely

devoid of bacteria. This was shown to be untrue in the early

1980’s, when various microbial populations were found in

subsurface sediment samples (Scalf and others, 1981; Bitton

and Gerba, 1984). Since then, researchers have attempted to

identify which organisms might be responsible for the

degradation of specific organic chemical contaminants.

Presently, it is generally accepted that no single strain of

microbe is responsible; rather, consortiums of bacteria engage

in degradation activities concomitantly.

The population dynamics of subsurface biota depend heavily

on limiting nutrients (Bitton and Gerba, 1984; Bouwer and

Wright, 1988; Major and others, 1988; Molz and others, 1986;

Tinsley, 1979). The carbon atom skeleton of an organic

chemical contaminant represents a fuel source to biota, but

they cannot utilize it unless they are able to acquire sufficient

supporting nutrients, such as nitrogen and phosphate (Molz

and others, 1986; Roberts and others, 1989; Tinsley, 1979).

Threshold concentration effects have also been observed, so

that it is possible to have an adequate supply of nutrients but

still generate little degradation because of the low

concentration level of the fuel source, the contaminant

(Borden and others, 1989; Flathman and others, 1989; Jensen

and others, 1988; Roberts and others, 1989; Spain and others,

1989; Srinivasan and Mercer, 1988). Typically, total dissolved

carbon loads of less than 100 parts per billion represent an

inadequate source of energy to stimulate bioactivity.

Contaminants may be present in certain portions of a plume at

concentration levels that are toxic to the indigenous biota.

However, if the biota are able to survive and function

minimally, they may adapt to the contaminant and begin

degradation. Hence, the rate of transformation of specific

contaminants may change over time. Great interest has been

taken in the possibility of stimulating bio-activity by the

addition of nutrients and/or fuel sources (e.g., methanol or

ethanol) to subsurface sediments. Both laboratory and field

experiments are under way at a number of locations across the

nation to investigate this possibility (Bouwer and Wright,

1988; Major and others, 1988; Roberts and others, 1989; Spain

and others, 1989; Thomas and Ward, 1989). The laboratory

experiments are often conducted in flow columns.

Quality Assurance and Quality Control Data

There are a great number of texts available that describe

quality assurance and quality control methods (e.g., Davis,

1986; Huntsberger and Billingsley, 1977; Mendenhall, 1968;

SAS Institute, 1988c and 1989b). Quality control methods

presume that the data possess specific statistical properties and

yield results that must be narrowly interpreted. The

repeatability of a result from a measurement of any parameter

is termed the precision of the method of measurement. The

accuracy of a measurement of any parameter is the degree of

closeness of its value to the actual/real value. Accuracy and

precision do not adequately describe the appropriateness of a

measurement for its intended use. The degree of

representativeness is by far an overriding consideration. For

example, measurements of the concentration of a contaminant

in ground water removed from a monitoring well may be

obtained with extreme care, such that one concludes that they

are probably highly accurate, but there is no guarantee that

such measured values represent the contaminant levels 10 feet,

100 feet, or more away.

III. Methods and Protocols

Performance Evaluation Methods

The proper selection and use of computational, statistical,

graphical and other methods for performance evaluations of

pump-and-treat remediations are as important as the
21

strategies being pursued. This is because the inferences that

are drawn and the perceptions that are generated depend to a

great extent on the manner in which the data are processed

and presented. For detailed discussions of the methods

applicable to ground-water contamination data the reader is

referred to publications on methods of numerical analyses

(Bear and Verruijt, 1987; Burden and others, 1981; Chau,

1988; de Marsily, 1986; Faust and others, 1981; Gerald and

Wheatley, 1984; Javendel and others, 1984; Mercer and

Faust, 1981; Press and others, 1986; Remson and others,

1971; van der Heidje and others, 1985; Walton, 1985 and

1987; Wang and Anderson, 1982; Yeh and Han, 1989),

statistical analyses (Asbeck and Haimes, 1984; Davis, 1986;

Hallenbeck and Cunningham, 1986; Huntsberger and

Billingsley, 1977; Mendenhall, 1968; SAS Institute, 1988a,

1988c, 1988d, and 1989b; Taylor, 1987; Keely, 1989;

Vomvoris and Gelhar, 1986; Vouk and others, 1985),

graphical construction techniques and analyses (Cedergren,

1989; Cheng, 1988; GIS World, 1989; SAS Institute, 1989a

and 1989b; Tufte, 1983; van der Heijde and Srinivasan,

1983; Watson, 1984), and theoretical relationships (Bitton

and Gerba, 1984; Corey, 1977; Hillel, 1982; Krabbenhoft

and Anderson, 1986; Kueper and Frind, 1988; Stumm and

Morgan, 1981; White, 1988). Although space limitations

prohibit detailed discussions of the foregoing methods,

some general precautions are cited which may be valuable

to the reader in selecting and applying methods of data

presentation and analysis.

1. Computational Methods

Methods for the computation of key indicators of performance range from simple arithmetic to highly sophisticated

transport models incorporating chemical, physicochemical,

and biological reaction rates. The use of a particular method

should be justified by the conditions to which it is being

applied. Such justifications should be supported by the

appropriate quantity and quality of requisite data.

Subsurface contaminant transport models incorporate a

number of theoretical assumptions about certain natural

processes governing the transport and fate of contaminants.

However, these assumptions may be violated at many sites.

In order to make solutions tractable, simplifications are

made in applications of theory to practical problems. A

common simplification is the assumption that flow towards

a well is horizontal. This may then allow the application of

a two-dimensional model, rather than a more data intensive

three-dimensional model, to many sites. Two-dimensional

models typically do not represent complexities of threedimensional flow regimes. Most pump-and-treat

remediations use partially penetrating wells, which induce

significant vertical flow, whereas the two-dimensional

models implicitly assume that the remediation wells are

screened throughout the saturated thickness of the aquifer.

The majority of these models assume that the density and

viscosity of the fluid(s) are equal to that of pure water. It is

known that fluid density differences may cause threedimensional flow (e.g., high TDS leachate from landfills).

However, if the limitations of the models are realized and

accounted for, they may serve as useful tools to predict

ground-water flow and contaminant transport relatively

accurately at many sites.

It is essential to have appropriate field and laboratory

determinations of natural process parameters and variables.

These may be used to determine the validity and usefulness

of each modeling attempt. Errors arising from inadequate

data are not addressed properly by mathematical tests such

as sensitivity analyses, or by the application of stochastic

techniques for estimating uncertainty. Such tests and

stochastic simulations assume that the underlying conceptual basis of the model is correct (Andersen and others,

1984; Bear and Verruijt, 1987; Dagan, 1982b; El-Kadi,

1988; Konikow, 1986; Krabbenhoft and Anderson, 1986;

Smith and Freeze, 1979a and 1979b). A conceptual model

should be modified only if the data justify the change. The

high degree of hydrogeological, chemical, and microbiological complexity typically present in field situations

requires site-specific characterization of various natural

processes by detailed field and laboratory investigations.

It is also important that both the mathematics that describe

the models and the input parameters used by the models be

subjected to rigorous quality control measures. Otherwise,

results from field applications of models may be

qualitatively and quantitatively incorrect. If done properly,

mathematical modeling may be used to organize vast

amounts of disparate data into a sensible framework that

may provide realistic appraisals of the performance of

ground-water remediations.

Some of the more traditional appraisals conducted with

mathematical models include:

(i) evaluation of changes in size and relative

concentrations of conservative solute plumes, using

flow models;

(ii) evaluation of changes in size and relative

concentration of reactive solute plumes, using flow

models;

(iii) evaluation of changes in size and average

concentration of conservative plumes, using

contaminant transport models; and

(iv) evaluation of changes in size and average

concentration of reactive plumes, using

contaminant transport models.
22

Models may also be used to evaluate the effects of changes

in basic design and operating parameters, so that the most

effective and efficient remediation can be attained.

Geochemical speciation models and reaction rate models

can likewise be employed to improve the interpretation of

data that might otherwise be appraised by simple cation/

anion balances. The outputs of speciation and reaction rate

models may yield useful information concerning the release

rates of contaminants, treatment needs, and disposal options.

Quality control procedures have been developed for the

creation, documentation, and intercomparison of models

(Huyakorn, 1984; Mercer and Faust, 1981; Mercer and

others, 1982; Remson and others, 1971; van der Heijde and

others, 1985). These include:

(i) functionality testing of subroutines and linkages

between subroutines and the main driver of a

program;

(ii) double key entry of all test data, spell-checking,

automatic syntax checking, and debugging;

(iii) rate-of-convergence and other stability checks;

comparison of model outputs with known solutions

to analytical equations; and

(iv) partial validation by blind prediction of controlled

experiments.

The development of quality control procedures for

applications of models to field problems has not progressed

beyond the bounds of individual professional judgment.

2. Statistical Methods

The potential power of statistical methods has been tapped

infrequently in ground-water contamination investigations,

aside from their use in quality assurance protocols. There

are other important uses of statistical methods, however, as

shown in Table 1. While data interpretation and

presentation methods vary widely, most site documents lack

statistical evaluations. Many site reports present

inappropriate simplifications of datasets, such as grouping

or averaging broad categories of data, without regard to the

statistical validity of those simplifications.

Laboratory scientists use analysis of variance (ANOVA) and

covariance (ANCOVA) to analyze the sources of error in

sample analyses. Certain errors, such as those associated

with operator error, may be identified and segregated from

the irreducible errors associated with instrumental

limitations. It may then be possible to institute changes that

minimize and control such errors within specific confidence

Table I. Useful Statistical Methods for Performance Evaluations

Analysis of Variance (ANOVA) Techniques

ANOVA techniques may be used to segregate errors due to chemical

analyses from those errors that are due to sampling procedures and from

the intrinsic variability of the contaminant concentrations at each sampling

point.

Correlation Coefficients

Correlation coefficients can be used to provide justification for lumping

various chemicals together (e.g., total VOCs), or for using a single

chemical as a class representative, or to link sources by similar chemical

behavior.

Regression Equations

Regression equations may be used to predict contaminant loads based on

historical records and supplemental data, and may be used to test causeand-effect hypothesis about sources and contaminant release rates.

Surface Trend Analysis Techniques

Surface trend analysis techniques may be used to identify recurring and

intermittent (e.g., seasonal) trends in contour maps or ground-water

elevations and contaminant distributions, which may be extrapolated to

source locations or future plume trajectories.

bounds (Davis, 1986; Huntsberger and Billingsley, 1977;

SAS Institute, 1988c and 1989b; Taylor, 1987)

Multiple or linear regression plots may be used to examine

time-wise or space-wise trends in contaminant

concentrations and water-level elevations. When used in

conjunction with Student’s t-test and Fischer’s F-test,

regression equations can test hypotheses about cause-andeffect relationships (Huntsberger and Billingsley, 1977; SAS

Institute, 1988c, 1988d and 1989b). Correlation coefficients

may be computed for one variable versus another to study

the association between them, and to justify grouping,

averaging, or selection of class/category representatives.

Datasets that contain too few observations or wildly

fluctuating values may be analyzed with non-parametric

methods to gain some sense of average or expected value

and associated uncertainty.

The general linear model (GLM), auto-regressive moving

average (ARMA), and auto-regressive integrated moving

average (ARIMA) techniques may provide some additional

information about the direction in which the mean is

trending and its stability (SAS Institute, 1988c and 1988d).

Geostatistical methods, such as kriging, cokriging, and

conditional simulations, offer similar capabilities for twoand three-dimensional evaluations. The expected values at

key locations and the uncertainties associated with them
23

may be estimated at specified confidence levels (Steinhorst

and Williams, 1985; Usunoff and Guzman-Guzman, 1989).

Cluster analyses and principal component analyses may also

be used to examine two- and three-dimensional datasets to

identify dominant or recurring patterns.

3. Graphical Methods

Graphical methods of data presentation and analysis have

been used heavily in ground-water flow and geochemical

evaluations (Cedergren, 1989; Freeze and Cherry, 1979;

Javendel and others, 1984; Keely, 1984; Keely and Tsang,

1983; Watson, 1984; Cheng, 1988; Rogers, 1989; Stumm

and Morgan, 1981; Tinsley, 1979).

Flowline plots offer much information because velocity

computations are typically provided as a vehicle for

obtaining the flowlines. Most flowline plotting software

allows the user to create a flowline plot on which are

superimposed plots of injection fronts or arrival fronts for

specific durations of pumping. From analysis of such plots,

it is possible to estimate the number of pore volumes of

ground water that will be removed over a set period of time

of constant pumping, at different locations in the

contaminant plume.

Contour maps of contaminant concentrations or water-level

elevations are probably the most common graphics prepared

in site studies. When contour maps are prepared by hand,

the practitioner’s judgment is automatically applied to

shaping and smoothing contours. Beyond purely

mathematical decisions regarding the interpolation (e.g.,

linear, geometric, lognormal) of values at locations that lie

between measured points, there is the need to account for

the behavior of real physical boundaries (e.g., streams,

valley walls and bedrock).

When contour maps are prepared for identical data sets by

different automated mathematical routines dissimilar

outcomes are possible (Figure 6). The perceptions that may

be generated by examination of contour maps that have been

prepared by different automated mathematical interpolation

schemes can vary substantially because both the shapes of

the contours and the areas contained by specific contours

may vary considerably. It is clear that there are contouring

techniques, such as kriging, that allow measured data points

to be predicted precisely. However, even these are subject

to map-edge effects and often have difficulty with

boundaries. The shapes of the contours that are generated

by kriging depend on the kind of model that the data are

postulated to follow (e.g., spherical, parabolic). Moreover,

by varying the density of the grid of data or the number of

contours used to produce a contour map, major differences

in the locations and bounded areas of the contours may

occur.

Uni-directional plots, such as contaminant concentration

levels versus distance between source areas and potential

receptors, may also be used in performance evaluations.

These are constructed by plotting contaminant concentration

as a function of distance along a specific flowline. If the

flowline patterns shift due to the influence of pumping

wells, transport extrapolations will no longer be valid.

Extrapolations must be validated by proof of duration of

Figure 6. The results of contouring a dataset by various mathematical

techniques may be significantly different.

18500

19000

18500 20000 20500

20000

19500

20500

19000 19500

1

1

2

3

3

3

3

2

2

1

1

1

1

2

2

2

Inverse Distance Squared

18500

19000

18500 20000 20500

20000

19500

20500

19000 19500

Kriging (Universal)

0

2

1

1

2

0 1

3

2 1

2

2

0

0

2 1

0 3

0
24

Aside from the plotting of discrete quantitative values of

chemical concentrations or water-level elevations, plots of

qualitative information are useful. Soil survey maps,

drillers’ logs, geologic cross-sections, and fence diagrams of

sediment sequences are common qualitative methods of

presenting data. All are subject to major errors when data

points are sparse or inappropriately sampled; most tend to

be used in a manner that understates the true uncertainties

involved. For example, it may not be appropriate to detail

the possible linkages of strata denoted on the geologic log of

one location with that of another when the strata are known

to occur as remnants of stream channels and reworked

floodplain deposits. Inspection of raw datasets by those

whose judgment has been shaped by many hours at local

sites can provide valuable input for decisions on how

strongly to associate occurrences of similar sediments.

In addition to quantitative and qualitative plots and maps,

certain charts and illustrations can be prepared for use in

management of the remedial activities. Program evaluation

and review technique (PERT) charts may be used to map the

dependencies of each task on those that precede it, and can

be configured to facilitate analysis of the operations by the

critical pathway method (CPM), in which the chain of core

tasks that must be completed to lead to a specific output can

be identified and protected during modifications to the list

of tasks (Figure 8). Although this may seem trivial when a

single individual is charting tasks for a single project, it is

clearly more complex when coordinating the tasks of many

individuals or projects.

Other hierarchal action diagrams, such as sample selection

and routing protocols, are useful tools for improvement of

the efficiency and effectiveness of pump-and-treat

remediations. Database management systems, particularly

those that operate under a geographical information system

(GIS) are also extremely valuable for analysis and

presentation of hydrogeologic and ground-water

contamination data.

4. Theoretical

General indicators of physicochemical reaction potential

include the cation-exchange-capacity (CEC), alkalinity, and

acidity of subsurface samples. Theoretical relationships link

CEC, alkalinity, and acidity to the reserve of reactants, and

consequently to the potential products. Theoretical

equations have also been developed and tested for various

wastewater treatment unit designs.

Batch sorption isotherms and static microcosms can be used

to generate estimates of limiting reaction rates of specific

contaminants. Information on the dynamics of such

reactions is obtained from flow-through column studies

Figure 7. Pattern diagram for selected VOCs. utilizing labelled tracers. It is most common to evaluate

parts per billion

ETHANES ETHENES

Tetrachloro

Trichloro

1,2-Dichloro

1,1-Dichloro

Mono Chloro

105 104 103 102 101 100 101 102 103 104 105

transport to allow sufficient time for contaminants to

traverse the length of the flowline in a single orientation.

Time plots of contaminant concentration levels on a

monthly or quarterly basis may be prepared for specific

monitoring locations, but caution must be used to validate

decreasing or increasing trends of concentration levels.

Changes in contaminant concentration levels may occur as a

result of any combination of (i) source release

characteristics, (ii) depletion of the contaminants, and (iii)

flowline pattern shifts.

Stiff diagrams are multivariate plots of the milli-equivalence

values of the major cations and anions of water samples that

produce readily recognizable patterns of the chemical

composition of ground-water. The major ionic composition

of contaminated ground water may differ significantly from

the natural ground-water quality in adjacent areas, often

resulting in different Stiff diagram patterns. Water quality

specialists use the diagrams to differentiate zones of varying

water quality, such as leachate plumes emanating from

landfills and the upconing of saline water due to pumping.

Geochemical prospectors also use this and similar graphical

techniques to identify waters associated with mineral

deposits.

Recently, multivariate plots similar to Stiff diagrams have

been adapted to display organic chemical contaminants

(Lesage and Lapcevic, 1990). For example, a compound of

interest such as trichloroethene may be evaluated in terms of

its contribution to the total organic contaminant load, or

against other specific contaminants, so that some

differentiation of source contributions to the overall plume

can be obtained (Figure 7).
25

Water Table 

Measurements

Aquitard Head

Measurements

Artesian Head

Measurements

Excavation &

Drilling Logs

Surface 

Geophysics

Aerial

Photographs

Contour Maps of Static Water Levels

and Potentiometric Surfaces

Local (Hydro)Geology Cross-Sections & Maps

Used to Select Observation Well Locations

Pumping & Tracer Tests for

Aquifer Hydraulic Properties

Drilling of Observation Wells &

Collection of Sediment Samples

Lab Evaluation of Sediment

Porosity & Moisture Retention

Maps of Aquifer Properties &

Equilibrium Pumping Contours

HYDROGEOLOGIC FLOW MODEL

Figure 8. PERT Chart illustrating interdependency of tasks to produce outputs.

data from such experiments by comparison to an equation

that has been derived from theory, as opposed to the use of

empirical relationships.

The probable reaction rates and products of some

compounds may be predicted by quantitative structureactivity relationships (QSAR). These relationships typically

utilize information such as the physical and symmetry

properties of the molecule to draw analogies to the behavior

of other molecules.

General Protocols for Performance

Evaluations

The protocols presented here are generalized and should be

refined on a site-specific basis. The flowcharts that are

provided are neither exhaustive nor exclusive, but should

serve as reasonable skeletons on which to build site-specific

charts of key activities. The latter can be accomplished

most fruitfully by utilization of software that create and

allow interactive modification and automated updating of

PERT charts.

1. General Protocol for Selection of Monitoring

Criteria and Reporting Requirements

(1) Begin the selection of performance evaluation and

monitoring criteria by reviewing the data

collection history at the site to determine the

potential need for adoption of specific spatial or

temporal patterns of sampling to ensure continuity

with previous efforts.

(2) Pre-operational programs for sample collection

and analyses, quality control, and data interpretation will be required between the completion of

the RI/FS and effective start-up of the remediation.

These will be needed to provide current

information regarding the threat to human health

and the environment, and the size and orientation

of the zone of remedial action that should be

established to prevent loss of portions of the

contaminant plume to downgradient areas. Select

monitoring criteria appropriate to these data needs.

(3) Bench treatability studies and pilot plant

demonstrations may be called for by design

engineers prior to commencement of construction

of the final remedy.

(4) Consider conducting studies at flow rates

equivalent to those expected for different locations

within the proposed remediation wellfield.

(5) Obtain samples of subsurface sediments for

chemical extraction analyses of contaminant
26

operation should be accompanied by a statistical

evaluation of contaminant distribution at each

sampling point. Chart the estimated mass of

contaminants removed, both total and per location;

compare these estimates with the mass of

contaminants removed by the treatment system.

(14) Plot and examine water-level elevation contours to

determine the adequacy of hydrodynamic control

and to identify stagnation zones for subsequent

action.

(15) As the concentration-time plots begin to flatten out,

try varying the flowrates of a few extraction wells

at a time to determine the potential for

improvement with wellfield scheduling or

configuration changes.

(16) When all potential improvements have been

attempted and exhausted, prepare for termination

of the remediation by increasing the sampling

frequency to build a statistically significant

database for plotting regressions with 95%

confidence bounds so that success can be judged.

(17) Throughout the operational phase of the

remediation, utilize monitoring data as soon as they

become available, for active management

purposes; prepare comprehensive annual reports

for regulatory oversight purposes.

(18) If the regression plots indicate that the target

contaminant concentration levels have been

reached, propose termination of the remediation.

(19) If permission to begin termination is granted by the

regulators, continue the sampling program at the

same (high) frequency for up to two years to test

the need to re-activate and/or re-evaluate the

remediation. Prepare brief reports quarterly for

management and regulatory purposes during this

period.

(20) Subsequently reduce the frequencies of collecting

and reporting on samples and measurements to an

annual basis to monitor the persistence of the

effects of the remediation; continue at that level

until the end of the regulatory period.

2. General Protocol for Violations Reporting and

Responses

(1) Examine the data collected during each sampling

event for internal consistency prior to accepting the

accuracy or relevancy of anomalous values. If a

residuals and for key transport properties such as

total organic carbon during opportunities presented

by pre-construction activities (e.g., excavations,

trenching, drilling). Evaluate the potential for

NAPLs at the site.

(6) Overlay plots of all sampling events and sample

types to identify locations of inadequate sampling

and to set a schedule to fill such vacancies. Select

performance evaluation criteria appropriate to these

activities.

(7) Prepare brief reports of sample collection and

analyses as data becomes available, rather than

wait for all items to be synthesized into a major

report. This information can affect ongoing

preparations for commencement of operation.

(8) Modify the frequency and locations for sampling

during operation, as necessary. Specifically focus

on characterization of the flowlines that will result

from operation of the remediation wellfield, and on

tracking the transport of contaminants along the

flowlines. Anticipate flowline shifts due to partial

shutdowns or operational failures.

(9) Provide adequate coverage of monitoring wells and

piezometers beyond the horizontal and vertical

bounds of the plume, such that transgressions of

flowlines originating in the plume can be tracked to

ensure eventual capture by an extraction well, and

such that the contribution of uncontaminated

ground water to the total volume pumped can be

examined.

(10) Make provisions for measurements germane to

active management of the operation, such as flow

rates and contaminant concentration levels for (i)

individual extraction wells, (ii) treatment plant

influent, (iii) key treatment units, and (iv) the plant

effluent.

(11) Reduce the frequency of chemical sampling (but

not water-level elevation measurements) if stable

trends of contaminant concentration levels are

apparent, and provide contingencies for increased

sampling in response to those periods when the

data are highly erratic.

(12) Continue filling gaps in the site characterization

database with opportunistic and periodic testing.

(13) Evaluate the rate of depletion of the residual

contamination by collection of sediment samples

for chemical extraction and analysis. This
27

Install Additional Monitoring Wells

Perform All Repairs

and Maintenance

Poor Condition

Detailed Inspection of

Remediation Wellfield

Good 

Condition

Develop Baseline Chemical 

and Hydraulic Datasets

No Action 

Needed

OK

Evaluate Mass 

Recovery Rates

Control 

OK

Evaluate Effectiveness

of Hydraulic Control

Out of 

Control

Poor or

Declining

Evaluate Potential Effectiveness

of Alternate Wellfield Configurations

Reconfigure Remediation Wellfield

Wells Not

Needed

Evaluate Resulting Flowline Pattern

with Respect to Monitoring Well Locations

Additional Monitoring 

Wells Needed

Figure 9. Outline of key efforts needed to select and use monitoring and reporting requirements.

(3) If the suspect value is not rejected due to internal

inconsistency or due to local inconsistency, retain

the value in the dataset as valid and immediately

resample or remeasure at that location and at two

or more adjacent locations (for control). Evaluate

the resulting data in the same fashion as the initial

data.

(4) Regardless of the status (rejected or retained) of the

first such indication at a specific location, consider

successive indications to confirm the loss of

adequate control of the zone of remedial action.

(5) Study the wellfield scheduling to determine at

which wells the flowrates may be increased to

provide the degree of control needed.

single monitoring point indicates an unacceptable

level of a particular contaminant, check the

concentration levels of other contaminants at the

same location to see if they have approximately the

same relationship (ratio) to the suspect contaminant

as has been shown to occur generally for the

plume; if not, reject the suspect value as internally

inconsistent.

(2) If general agreement seems to be indicated, or

cannot be judged due to the small number of

contaminants associated with a particular plume

(e.g., predominantly TCE and nothing else), then

compare the suspect value with the concentration

levels found in nearby wells; if poor agreement is

indicated, reject considering the suspect value as

locally inconsistent.
28

(6) Expect that flowline patterns will shift with the

implementation of new pumping schedules, and

plan to evaluate the significance of the shift in

terms of the need to reposition sampling/

measurement points.

(7) Verify the operational viability (e.g., minimum

hydrodynamic control) of the modified wellfield by

increasing the frequency of sampling/measurement

for a period of three to six months.

(8) Utilize information obtained during the increased

sampling/measurement period to assess the impacts

of the correctional action in terms of changes in the

rate of depletion of the contaminant residuals.

3. General Protocols for Design and Operation

 Modifications

(1) Modifications of system design or operation may

be called for either as a result of checking the

remediation for readiness for termination or as a

result of violations. In either case, if a major

modification is anticipated, it is prudent to perform

a detailed inspection and analysis of the system

first. All piping, valves, and protective shielding

should be in good condition; leaks, indications of

corrosion or excessive wear, and poor electrical

grounds should be repaired. All gauges and

electronic monitors should be in calibration and

should provide appropriate responses to tests; if

not, the deficiencies should be remedied.

(2) A few sampling/measurement events should then

be undertaken to provide a baseline for comparison

to data to be collected after the modification, and to

provide immediate data for development of the

modification.

(3) Development of the modification should proceed

with the targeted hydraulic control and clean-up

levels still firmly in mind. If the remediation has

Install Additional Monitoring Wells and

Piezometers Needed to Improve Monitoring

Examine Monitoring Data for Anomalous 

Contaminant Levels & Hydraulic Gradients

No 

Anomalies Remediation Judged to be Operating

Within Bounds of Compliance Criteria

Anomalies Found

Examine the Dataset Containing Anomalous

Value(s), for Internal (Point) Consistency Inconsistent

Reject Anomalous Data and 

Re-sample/Remeasure for New Dataset

Examine the Dataset Containing Anomalous

Value(s), for Local (Area) Consistency

Inconsistent

Consistent

Consistent

Confirmed

Data Not Erratic

Retain Anomalous Data 

Re-sample & Remeasure for Confirmation

Not 

Confirmed Conduct Chemical Time-Series Testing of

Monitoring Wells Yielding Anomalous Data

Data Erratic

Design and Implement Modifications to

Flow Rates and/or Wellfield Configuration

Conduct Special Studies of Flow Rates

and Contaminant Removal Rates

Conduct New Chemical Samplings &

Hydraulic Gradient Measurements

Figure 10. Key efforts needed to identify, verify, and correct inadequate performance of the remediation.
29

been successful in depleting contaminants from the

outermost portions of the plume, it may be sensible

to shrink the remediation wellfield inward. If

additional control is needed, it may need to be

expanded outward.

(4) In these cases particularly, but also in the case

where the overall size remains unaffected and wells

are simply relocated within the existing wellfield, it

is essential to begin to anticipate the new

orientation of the flowline pattern.

(5) Monitoring point locations may need to be

changed, and new trends of contaminant transport

and recovery will have to be identified. Hence, an

increased frequency of sampling/measurement

events will be needed for three to six months

following establishment of routine operation of the

modified system.

(6) Verification of operational viability and

assessment of the impacts of the modification on

depletion rates of contaminant residuals then

follow.

Figure 12. Key efforts needed to prepare for, implement, and verify modifications of the remediation.

Use the Chemical and Hydraulic Data to

Evaluate Performance of the Remediation

Select Chemical Goals

to Satisfy ARAR's

Select Hydraulic Goals

to Control Source & Plume MODIFICATION OR TERMINATION

Identify Spatial Gaps & Temporal Trends

in Datasets from Existing Monitoring Wells

Continue Performance Evaluations & Use to

Determine Need for Changes or Termination

Install Wells to

Fill Spatial Gaps

Conduct Bench (Lab)

Treatability Studies

Finalize Operational Goals and Set

Termination & Post Operational Requirements

Design, Install, and Operate a Pilot

Remediation Wellfield & Treatment Plant

Reconfigure the Wellfield and Treatment

Train as Needed to Achieve Best Performance

Monitor Contaminant Concentrations and

Water Table (Potentiometric Surface) Elevations
30

Borden, R., M. Lee, J.M. Thomas, P. Bedient, and C.H.

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