United States Office of Office of Solid Waste EPA/540/4-89/005

Environmental Protection Research and and Emergency

Agency Davelopment Response

E P A Ground Water Issue

Performance Evaluations of

Pump-and-Treat Remediations

Joseph

One of the most commonly used ground-water remediation

technologies is to pump contaminated waterto the surfacefor

treatment, Evacuating the effectiveness of pump-and-treat

rernediations at Superfund sites is an issue identified by the

Regional Superfund Ground Water Forum as a concern of

Superfunddecision-makers. The Forum is a group of groundwater scientists, representing EPA’s Regional Superfund

Offices, organized to exchange up-to-date information related

to ground-water remediation at Superfund sites.

Recent research has led to a better understanding of the

complex chemical and physical processes controlling the

movement of contaminants through the subsurface, and the

abilitytopump such contaminantsto the surface. Understanding

these processes permits the development and use of better

site characterization technology and the design and

implementation of more effective and efficient site remediation

programs.

This document is an interim product of a research project that

is developing a protocol for evacuating the effectiveness of

ground-water remediations. It has been reviewed by members

of EPA’s Robert S. Kerr Environmental Research Laboratory.

For further information contact Marion R. (Dick) Scalf, Chief,

Applications and Assistance Branch, RSKERL-Ada, FTS

743-2312, or Randall R. Ross, Project Officer, RSKERL-Ada,

FTS 743-2355.

Summary

Pump-and-treat remediations are complicated by a variety of

factors. Variations in ground-water flow velocities and

directions are imposed on natural systems by remediation

wellfields, and these variations complicate attempts to

F. Keely

evaluate the progress of pump-and-treat remediations. This

is in part because of the tortuosity of the flowlines that are

generated and the concurrent re-distribution of contaminant

pathways that occurs. An important consequence of altering

contaminant pathways by remediation wellfields is that historical

trends of contaminant concentrations at local monitoring

wells may not be useful for future predictions about the

contaminant plume.

An adequate understanding of the true extent of a contamination

problem at a site may not be obtained unless the site’s

geologic, hydrologic, chemical, and biological complexities

are appropriately defined. By extension, optimization of the

effectiveness and efficiency of a pump-and-treat remediation

may be enhanced by the utilization of sophisticated site

characterization approaches to provide more complete, sitespecific data for use in remediation design and management

efforts.

Introduction

Pump-and-treat remediations of ground-water contamination

are planned or have been initiated at many sites across the

country. Regulatory responsibilities require that adequate

oversight of these remediations be made possible by structuring

appropriate monitoring criteria for monitoring and extraction

wells. These efforts are nominally directed at answering the

question: What can be done to show whether a remediation

is generating thedesired control of the contamination? Recently,

other questions have come to the forefront, brought on by the

realization that many pump-and-treat remediations may not

function as well as has been expected: What can be done to

determine whether the remediation will meet its timelines?

and What can be done to determine whether the remediation

will stay within budget?

Superfund Technology Support Centers for Ground Water

Robert S. Kerr Environmental

Research Laboratory

Ada, OK
Conventional wisdom has it that these questions can be

answered by the use of sophisticated data analysis tools,

such as computerized mathematical models of ground-water

flow and contaminant transport. Computer models can

indeed be used to make predictions about future performance,

but such predictions are highly dependent on the quality and

completeness of the field and laboratory data utilized. This

is also true of models used for performance evaluations of

pump-and-treat remediations. In most instances an accurate

performance evaluation can be made simply by comparing

data obtained from monitoring wells during rernediation to

thedatagenerated priortothe 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 complex flow patterns produced locally by

the extraction and injection wells, where previouslythere was

a comparatively simple flow pattern.

Complex ground-waterflow patterns present great technical

challenges in terms of characterization and manipulation

(management) of the associated contaminant transport

pathways. In Figure I, for example, waters moving along the

flowline that proceeds directly into a pumping well from

upgradient are moving the most rapidly, whereas those

waters at the limits of the capture zone move much more

slowly. One result is that certain parts of the aquifer are

flushed quite well and other parts poorly. Another result of

the pumpage is that previously uncontaminated portions of

the aquifer at the outer boundary of the contaminant plume

may become contaminated by the operation of an extraction

well that is located too close to the plume boundary, because

the flowline pattern extends downgradient of the well.

The latter is not a trivial situation that can be avoided without

repercussions by simply locating the extraction well far

enough inside the plume boundary so that its flowline pattern

does not extend beyond the edge of the plume. Such actions

would result In very poor cleansing of the aquifer between the

extraction well and the plume boundary, because of the

stagnation of flow that occurs downgradient of the well.

Detailed field investigations are required during remediation

to determine the locations of the various flowlines generated

byapump-and-treat operation. Consequently, there maybe

a need for more data to be generated during the site remediation

(especially inside the contamination plume boundaries) than

were generated during the site investigation, and for

interpretations of those data to require highly sophisticated

tools. For most settings, it is Iikelythat interpretations of the

data that are collected during a pump-and-treat remediation

will require the use of mathematical and statistical models to

organize and analyze those data.

Contaminant Behavior and Plume Dynamics

Ground water flows from recharge zones to discharge zones

in response to the hydraulic gradient (the drop in hydraulic

pressure) along that path. The hydraulic gradient may be

obtained from water-level elevation contours for ground water

that has constant fluid density, but it must be obtained from

water pressure contours when the fluid density varies. This

is because hydraulic pressure is created by the combined

effects of elevation, fluid density, and gravity. Additions to the

dissolved solids content of a fluid increase its density. For

example, synthetic seawater can be prepared by adding

mineral salts to fresh water. Landfill Ieachate is often so laden

with dissolved contaminants that its density approaches that

of seawater.

As ground water flows through the subsurface It may dissolve

some of the materials it contacts and may also transport

viruses and small bacteria. This gives rise to natural water

quality -- a combined chemical, biological, and physical state

that may, or may not, be suitable for man’s uses. Brines and

brackish waters are examples of natural ground waters that

are unsuitable for man’s use. It is this same power of water

to solubilize minerals and decayed plant and animal residues

that causes contamination when ground water Is brought into

contact with manmade solids and liquids (Figure 2). Once

contaminated, ground water also provides a medium for

 .

. ,

F

Figure 1. Flowline pattern generatad by an extraction well.

Ground-water within the bold line will be captured by the well.

Prior to pumping, the flowlines were straight diagonals.

Figure 2. Above-ground spill of chemicals from storage drums.

Spilled fluids initiaily fill the upper most soil pores. As much as half

of the fluids remain in each pore after drainage.

2
potentially destructive interactions between contaminants

and subsurface formations, such as the dissolution of limestone

arid dolomite strata by acidic wastewaters. Contaminated

ground water is a major focus of many hazardous waste site

cleanups, At these sites, a large number of EPA’s Records of

Decision (ROD’s) call for pump-and-treat remediations.

The mechanism by which a source introduces contaminants

to ground water has a profound effect on the duration and

areal extent of the resulting contamination. Above-ground

spills (Figure 2) are commonly attenuated over short distances

by the moisture retention capacity of surface soils. By

contrast, there is much less opportunityfor attenuation when

the contaminant is introduced below the surface, such as

occurs through leaking underground storage tanks, injection

wells, and septic tanks.

The hydraulic impacts of some sources of ground-water

contamination, especially injection wells and surface

impoundments, may impart a strongly three-dimensional

character to local flow directions. The water-table mounding

that takes place beneath surface impoundments (Figure 3),

for instance, is often sufficient to reverse ground-water flow

directions locally and commonly results in much deeper

penetration of contaminants into the aquifer than would

otherwise occur. Interactions with streams and other surface

water bodies may also impart three-dimensional flow

characteristics to contaminated ground water (e.g., a losing

stream creates local mounding that forces ground-waterflow

downward). In addition, contaminated ground water may

move from one aquifer to another through a leaky aquitard,

such as a tight silt Iayer that is sandwiched between two sand

or gravel aquifers.

II GREAT HYDRAULIC IMPACTS

FROM HIGH RATES OF RELEASE

Figure 3. Hydraulic impacts of contarninant sources. Injection

wells and surface impoundments may release fluids at a high rate,

resulting in local mounding of the water table.

As ground water moves, contaminants are transported by

advection and dispersion (Figure 4). Advection, or velocity,

estimates can be obtained from Darcy’s Law, which states

that the amount of water flowing through porous sediments

in a given period of time is found by multiplying together

values of the hydraulic conductivity of the sediments, the

cross-sectional area through which flow occurs, and the

hydraulic gradient along the flowpath through the sediments.

The hydraulic conductivities of subsurface sediments vary

considerably over small distances. It is primarily this spatial

variability in hydraulic conductivity that results in a corresponding

distribution of flow velocities and contaminant transport rates.

travel by advection alone

additionel spreading caused by dispersion

Figure 4. Bird’s-eye view of contarninant plume spreading. Advection

causes the majority of plume spreading in most cases Dispersion adds

only marginally to the spreading.

The plume spreading effects of spatially variable velocities

can be confused with hydrodynamic dispersion (Figure 5), If

the details of the velocity distribution are not adequately

known. Hydrodynamic dispersion results from the combination

of mechanical and chemical phenomena at the microscopic

level.

The mechanical component of dispersion derives from velocity

Figure 5. Cross-sectional view of contaminant spreading.

Permeability differences between strata cause comparable

differences in advection and, hence, plume spreading.

3
variations among water molecules traveling through the

pores of subsurface sediments (e.g., the water molecules

that wet the surfaces of the grains that bound each pore move

little or not at all, whereas water molecules passing through

the center of each pore move most rapidly) and from the

branching of flow into the accessible pores around each grain.

By contrast, the chemical component of dispersion is the

result of molecular diffusion. At modest ground-water flow

velocities, the chemical (or diffusive) component of dispersion

is negligible and the mechanical component creates a small

amount of spreading about the velocity distribution. At very

slow ground-waterflow velocities, such as occur in clays and

silts, the mechanical component of dispersion is negligible

and contaminant spreading occurs primarily by molecular

diffusion.

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

occurs through fractures in low permeability rock formations.

The flow in the fractures often responds quickly to rainfall

events and other fluid inputs, whereas the flow through the

bulk matrix of the rock is extremely slow -- so slow that

contaminant movement by molecular diffusion may be much

quicker by comparison. On the other end of the ground-water

fiow velocity spectrum is the flow in karst aquifers, since it may

occur mostly through large channels and caverns. In these

situations, ground-water flow is often turbulent, and the

advection and dispersion of dissolved contaminants are not

adequately describable by Darcy’s Law and other porous

media concepts. Dye tracers have been used to study

contaminant transport in fractured rock and karst aquifers,

but such studies have yet to yield relationships that can be

transferred from the study site to other sites.

Regardless of the character of ground-water flow, contaminants

may not be transported at the same rate as the water itself.

Variations in the rate of contaminant movement occur as a

result of sorption, ion-exchange, chemical precipitation, and

biotransformation. The movement of a specific contaminant

may be halted completely by precipitation or biotransformation,

because these processes alter the chemical structure of the

contaminant. Unfortunately, the resulting chemical structure

may be more toxic and more mobile than the parent compound,

such as in the anaerobic degradation of tetrachloroethene

(PCE), which yields, successively, trichloroethene (TCE),

dichloroethene (DCE), and monochloromethene (vinyl chloride).

Sorption and ion-exchange (Figure 6), conversely, are

completelyreversibie processes that release the contaminant

unchanged after temporarily holding it on or in the aquifer

solids. This effect is commonly termed retardation and is

quantified by projecting or measuring the mobility of the

contaminant relative to the average flow velocity of the

ground water. Projections of retardation effects on the

mobility of contaminants are baaed on equations that incorporate

physical (e.g., bulk density) and chemical (e.g., partition

coefficients) attributes of the real system. Direct measurement

of the effective mobility of contaminants can be made by

observations of plume composition and spreading overtime.

Alternatively, samples of soils or sediments from the

contamination site may be used in laboratory studies to

determine the effective partitioning of contaminants between

mobile (water) and immobile (solids) phases.

Retardation effects can be short-circuited by facilitated transport,

Figure 6. Contaminant transport facilitated by particles. Sorption

of organics (e.g., PCB’S) or metals (e.g., Pb) onto particles may

be effective in increasing their transport.

a term that refers to the combined effects of two or more

discrete physical, chemical, or biological phenomena that act

in concert to materially increase the transport of contaminants,

Examples of facilitated transport include particle transport,

cosolvation, and phase shifting.

Particle transport (Figure 7) involves the movement of colloidal

particles to which contaminants have adhered by sorption,

ion-exchange, or other means. Contaminants that otherwise

exhibit moderate to extreme retardation may travel far greater

distances than projected from their nominal retardation values.

Pumping often removes many colloidal particles from the

subsurface. This fact can complicate remediations, and is

also relevant to public water supply concerns.

Figure 7. Retardation of metals by ion exchange. Metal ions

carrying positive charges are attracted to negatively charged

surfaces, where they may replace existing ions.

4
Cosolvation is the process by which the volubility and mobility

of one contaminant are increased by the presence of another

(Figure 8), usually a solvent present at levels of a few percent

(note: 1 percent= 10,000 parts per million). Such phenomena

are most likely to occur close to contamination sources,

where pure solvents and high dissolved concentrations are

often found.

Figure 8. Conceptualization of transport by cosolvation. Insoluble

contaminants may dissolve in ground water that contains solvents

at high concentrations.

Those who design treatment strategies should anticipate the

need to remove from ground water certain contaminants that

are normally immobile, if the groundwater is to be extracted

in areas that are close to a source of contamination. Those

who make health risk estimates should attempt to factor in the

increased mobility and exposure potential generated by

cosolvation.

Shifts between chemical phases (Figure 9) involve a large

change in the pH or redox (reaction) potential of water, and

can increase contaminant solubilties and mobilities by ionizing

neutral compounds, reversing precipitation reactions, forming

complexes with other chemical species, and limiting bacterial

activity. Phase shifts may occur as the result of biological

depletion of the dissolved oxygen normally present in ground

water, or as the result of biological mediation of oxidationreduction reactions (e.g., oxidation of iron Il to iron Ill). Phase

shifts may also result from raw chemical releases to the

subsurface.

Some ground-water contaminants are conponents of immiscible

solvents, which may be either floaters or sinkers (Figure 10).

The floaters generally move along the upper surface of the

saturated zone, although they may depress this surface

locally, and the sinkers tend to move downward under the

influence of gravity. Both kinds of immiscible fluids leave

residual portions trapped in pore spaces by capillary tension.

This is particularly troublesome when an extraction well is

utilized to control local gradients such that free product

(drainable gasoline) flows into its cone of depression.

Figure 9. Facilitated transport by phase diagram shifts, Releases

of acidic contaminants, or depletion of oxygen by biota, may

solubilize precipitated metals or ionize organics.

Figure 10. Dynamics of immiscible floater and sinker plumes.

Buoyant plumes migrate Iaterally on top of the water saturated

zone. Dense plumes sink and follow bedrock slopes.

5
The point of concern is that the cone of depression will contain

trapped residual gasoline below the water-table (Figure 11).

That residual will become a continuous some of contamination,

which will persist even when the extraction well is turned off.

The extent of the contamination that is generated by the

residual gasoline in the cone of depression may exceed that

generated by the gasoline resting in place above the saturated

zone prior to the onset of pumping.

Figure 11. Zone of contaminant residuals caused by pumping.

Pumping creates a cone of depression to trap gasoline for removal

but also leaves residues below the water table.

Reliable prediction of the future movement of contaminant

plumes under natural flow conditions is difficult because of the

need to evaluate property the many processes that affect

contaminant transport in a particular situation. Remediation

evacuations are even more difficult because of extensive

redirection of pre-remediation transport pathways by pumpand-treat wellfields. Hence, to prepare for remediation, it is

important to determine the potential transport pathways during

the site investigation.

Monitoring for Remediation Performance

Evaluations

Ground-water data are collected during rermediations to evaluate

progress towards goals specified in a ROD. The key controls

on the quality of these data are the monitoring criteria that are

selected and the locations at which those criteria are to be

applied. Ideally, the criteria and the locations would be

selected on the basis of a detailed site characterization, from

which transport pathways prior to remediation could be identified,

and from which the probable pathways during remediation

could be predicted.

The monitoring criteria and locations should also be chosen in

such a way as to provide information on what is happening

both downgradient of the plume boundary and inside the

plume. Monitoring within the plume makes it possible to

determine which parts of the plume are being effectively

rernediated and how quickly. This facilitates management of

the rernediation wellfield for greatest efficiency; for example,

by reducing the flowrates of extraction wells that pump from

relatively clean zones and increasing the flowrates of extraction

wells that pump from zones that are highly contaminated. By

contrast, the exclusive use of monitoring points downgradient

of the plume boundary does not allow one to gain any

understanding about the behavior of the plume during

remediation, except to indicate out-of-control conditions when

contaminants are detected.

There are many kinds of monitoring criteria and locations in

use today. The former are divided into three categories:

chemical, hydrodynamic, and administrative control. Chemical

criteria are based on standards reflecting the beneficial uses

of ground water (e.g., MCL’S or other health-based standards

for potential drinking water). Hydrodynamic monitoring criteria

are such things as:

(1) prevention of infiltration through the

unsaturated zone,

(2) maintenance of an inward hydraulic gradient

at the boundary of a plume of groundwater contamination, and

(3) providing minimum flows in a stream.

Administrative controls maybe codified governmental rules

and regulations, but also include:

(1) effective implementation of drilling bans

and other access-limiting administrative

orders,

(2) proof of maintenance of site security, and

(3) reporting requirements, such as frequency

and character of operational and postoperational monitoring.

Combinations of chemical, hydrodynamic, and administrative

control criteria are generally necessary for specific monitoring

points, depending on location relative to the source of

contamination.

Natural Water Quality Monitoring Points

Natural water quality (or “background”) sampling locations

are the most widely used monitoring points, and are usually

positioned a short distance downgradient of the plume. The

exact location is chosen so that:

(1) it is neither in the plume nor in adjacent

areas that may be affected by the

remediation,

(2) it is in an uncontaminated portion of the

aquifer through which the plume would

migrate if the remediation failed, and

(3) its location minimizes the possibility of

detecting other potential sources of

contamination (e.g., relevant to the target

site only).

Data gathered at a natural water quality monitoring point

indicate out-of-control conditions when a portion of the plume

escapes the remedial action. The criteria typically specified

for this kind of monitoring point are known natural water

quality concentrations, usually established with water quality

data from wells located upgradient of the source.

6
Publlc Supply Monitoring Points

Public water supply wells located downgradient of a plume are

another kind of monitoring point. The locations of these points

are not negotiable; they have been drilled in locations that are

suitable for water supply purposes, and were never intended

to serve as plume monitoring wells. The purpose of sampling

these wells is to assure the quality of water delivered to

consumers, as related to specific contaminants associated

with the target site. The criteria typically specified for this kind

of monitoring point are MCL’S or other health-based standards.

Gradient Control Monitoring Points

A third kind of off-plume monitoring point frequently estabiished

is one for deterrninations of hydraulic gradients. This kind may

be comprised of a cluster of small diameter wells that have

very short screened intevals, and iS usually located just

outside the perimeter of the plume. Water ievel measurements

are obtained from wells that have comparable screened

intervals and are then used to prepare detailed contour maps

from which the directions and magnitudes of local horizontal

hydraulic gradients can be determined. it is equally important

to evaluate vertical gradients, by comparison of water level

measurements from shallow and deeper screened intevals,

because a remediation wellfield may control only the uppermost

portions of a contaminant plume if remediation wells are too

shallow or have insufficient flow rates.

[Internai] Piume Monitoring Points

Less often utilized is the kind of monitoring point represented

by monitoring wells located within the perimeter of the plume.

Most of these are installed during the site investigation phase,

prior to the remediation, but others maybe added subsequent

to implementation of the remediation; they are used to monitor

the progress of the remediation within the plume. 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 off-site plume monitoring

points located beyond the facility boundary, but within the

boundary of the contamination plume.

interdependencies of Monitoring Point Criteria

Each kind of monitoring point has a specific and distinct role to

play in evaluating the progress of remediation. The information

gathered is not limited to chemical identities and concentrations,

but includes other observable or measurable items that relate

to specific remedial activities and their attributes. in choosing

specific locations of monitoring points, and criteria appropriate

to those locations, it iS essential to recognize the interdependency

of the criteria for different locations.

In addition to the foregoing, one must decide the following:

Should evacuations of monitoring data incorporate allowances

for statistical variations in the repotted values? if so, then

what cut-off (e.g., the average value plus two standard deviations)

should be used? Should evaluations consider each monitoring

point independently or use an average? Finally, what method

should be used to indicate that the maximum clean-up has

been achieved? The zero-slope method, for example, holds

that one must demonstrate that contaminant levels have

stabilized at their lowest values prior to cessation of remediation

. . and that they will remain at that level subsequently, as

shown by a fiat (zero-slope) plot of contaminant concentrations

versus time,

Limitations of Pump-and-Treat Remediations

Conventional remediations of ground-water contamination

often involve continuous operation of an extraction-injection

wellfield. In these remedial actions, the level of contamination

measured at monitoring wells maybe dramatically reduced in

a moderate period of time, but low levels of contamination

usually persist. In parallel, the contaminant load discharged

by the extraction wellfield declines over time and gradually

approaches a residual level in the latterstages (Figure 12). At

that point, large volumes of water are treated to remove small

amounts of contaminants.

Figure 12. Apparent dean-up by pump-and-treat remediation.

Contamination concentrations in pumped water decline overtime,

to an apparently irreducible level.

Depending on the reserve of contaminants within the aquifer,

this may cause a remediation to be continued indefinitely, or

it may lead to premature cessation of the remediation and

closure of the site. The latter is particularly troublesome

because an increase in the Ievel of ground-water contamination

may follow (Figure 13) if the remediation is discontinued prior

to removal of all residual contaminants,

There are several contaminant transport processes that are

potentially responsible for the persistence of residual

contamination and the kind of post-operational effect depicted

in Figure 13. To cause such effects, releases of contaminant

residuals must be slow relative to pumpage-induced water

movement through the subsurface.

Transport processes that generate this kind of behavior

during continuous operation of remediation wellfield include:

(1) diffusion of contaminants in low permeability

sediments,

(2) hydrodynamic isolation (’dead spots’) within

wellfields,

7
(3) resorption of contaminants from sediment

surfaces, and

(4) Iiquid-liquid partitioning of immiscible

contaminants.

Figure 13. Contaminant increases after remediation stops. Contaminant levels may rebound when pump-and-treat operations

cease, because of contaminant residuals.

Advection vs. Diffusion

Lccalized variations in the rate of ground-waterflow (advection)

arise in heterogeneous settings because of interlayering of

high-and low-permeabiiity sediments. When operating a

remediation wellfield, these advection variations result in

rapid cleansing of the higher permeability sediments, which

conduct virtually all of the flow (Figure 14). By contrast,

contaminants are removed from the lower permeability

sediments very slowly, by diffusion. The specific rate at

which this diffusive release occurs is dependent on the

difference in contaminant concentrations within and external

to the low permeability sediments.

When the higher permeability sediments are cleaned up, the

chemical force drawing contaminants from the lower

permeability sediments is at its greatest. This force is

exhausted only when the chemical concentrations are nearly

equal everywhere.

Low permeability sediments have orders-of-magnitude greater

surface area per volume of material than high permeability

sediments. Much greater amounts of contaminants may thus

accumulate in a given volume of low permeability sediments,

as compared with contaminant accumulations in a like voiume

of high permeability sediments. The thicker the low permeabillty

stratum, the more contaminant reserves it can hold, and the

more diffusion controls contaminant movement overall. Thus

the majority of contaminant reserves in heterogeneous settings

may be available only under just such diffusion-controlled

conditions.

Figure 14. Permeability variations Iimit remediations. High

permeability sediments conduct moat of the flow; low permeability

sediments act as leaky contaminant reservoirs.

The situation iS similar, though reversed, for in-situ remediations

that require the injection and deliveryof nutrients or reactants

to the zone of intended action: access to contaminants in low

permeability sediments is restricted to that provided by

diffusion.

Hydrodynamic lsolation

The operation of any wellfield in an aquifer containing moving

fluid results in the formation of stagnation zones downgradient

of extraction wells and upgradient of injection wells. The

stagnation zones are hydrodynamically isolated from the

remainder of the aquifer, so mass transport into or out of the

isolated water may occuroniy by diffusion. If remedial action

wells are located within the bounds of a contaminant plume,

such as for the removal of contaminant hot-spots, the portion

of the plume lying within their associated stagnation zone(s)

will not be effectively remediated. The flowline pattern must

be altered radically, by major changes in the locations of

pumping wells, or by altering the balance of flowrates among

the existing wells, or both, if the original stagnation zone(s)

are to be remediated.

Another form of hydrodynamic isolation is the physical

creation of enlarged zones of residual hydrocarbon (Figure

11). This occurs when deep wells are used to create cones

of drawdown into which underground storage tank and

pipeline leaks of gasoline can flow, so that skimmer pumps

can remove the accumulated product. When the deep water

pump is turned off, the watertable will rise to its pre-pumping

position. This will allow the aquifer waters that then fill the

cone of depression, and any subsequent ground-waterflow

through the former cone of depression, to become highly

contaminated with BTEX compounds (benzene, toluene,

the ethyl benzenes, and the xylenes) as a result of contact

with the gasoline remaining there on the aquifer solids.

Gasoline in residual saturation may occupy as much as 40

percent of the pore space of the sediments.

8
Sorption Influences

The number of pore volumes of contaminated water to be

removed during a remediation depends on the sorptive

tendencies of the contaminant. The number of pore volumes

to be removed also depends on whether ground-waterflow

velocities during remediation are too rapid to allow contaminant

levels to build up to equilibrium concentrations locally (Figure

15). If insufficient contact time is allowed, the affected water

is advected away from sorbed contaminant residuals prior to

achieving a state of chemical equilibrium and is replaced by

fresh water from upgradient. 

TIME

Figure 15. Sorption limitation of pump-and-treat remediations.

Increased flow velocities caused by pumpage may not allow for

sufficient time to reach chemical equilibrium locally.

Hence, continuous operation of pump-and-treat remediations

may result in steady releases of contaminants at substantially

less than their chemical equilibrium levels. With less

contamination being removed per volume of water brought

into contact with the affected sediments, it is clear that large

volumes of mildly contaminated water are recovered, where

small volumes of highly contaminated water would otherwise

be recovered.

Unfortunately, this is all too likely to occur with conventional

pump-and-treat remediations and with those in-situ remediations

that depend upon injection wells for delivery of nutrients and

reactants. This is because ground-waterflow velocities within

wellfields may be many times greater than natural (nonpumping) flow velocities. Depending on the sorptive tendencies

of the contaminant, the time to reach maximum equilibrium

concentrations in the ground water may simply be too great

compared with the average residence time in transit through

the contaminated sediments.

Liquid-Liquid Partitioning

Subsequent to gravity drainage of free product that has been

discharged to the subsurface, immiscible or non-aqueous

phase liquids (NAPL’s) remain trapped in the pores of

subsurface sediments by surface tension to the grains that

bound the pores. Liquid-liquid partitioning controls the

dissolution of NAPL residuals into ground water.

As with sorbing compounds, flow rates during remediation

may be too rapid to allow aqueous saturation Ievel of

partitioned NAPL residuals to be reached locally (Figure 16).

If insufficient contact time is allowed, the affected water is

advected away from the NAPL residuals prior to reaching

chemical equilibrium and is replaced by fresh water from

upgradient. “

ADVECTION

GROUND-WATER VELOCITY

Figure 16. Partitioning limits pump-and-treat effectiveness. Less

than solubility levels of contaminants maybe released from trapped

solvents if pumpage increases flow velocities.

Again, this process generates large volumes of mildly

contaminated water where small volumes of highly contaminated

water would other wise result, and this means that it will be

necessary to pump and treat far more water than would

otherwise be the case. The efficiency loss Is generally twofold, because much of the pumped water will contain contaminant

9
concentrations that are below the level at which optimal

treatment is obtained.

Design and Analysis Complications

Contaminant concentrations and ground-waterflow velocities

can be highly variable throughout the zone of action, which is

that portion of an aquifer actively affected by the remediation

wellfield. Consequently, monitoring strategies should be

focussed on detection of rapid, sporadic changes in contaminant

concentrations and flow velocities at any specific point in the

zone of action. In practice, this means that tracking the

effectiveness of pump-and-treat remediations by chemical

samplings is quite complicated.

Decisions regarding the frequency and density of chemical

samplings should take into account the detailed flowpaths

generated by the remediation wellfield, including changes in

contaminant concentrations that result from variations In the

influences of transport processes along those flowpaths. The

need to reposition extraction wells occasionally, to remediate

portions of the contaminated zone that were previously subject

to slow flowlines, means that the chemical samplings may

generate results that are not easily understood. It also means

that it may be necessary to move the chemical monitoring

points during the course of a remediation.

Evaluations of the hydrodynamic performance of remediation

wellfields are also data intensive. For example, It is usually

required that an inward hydraulic gradient be maintained at

the periphery of a contaminant plume undergoing pump-andtreat remediation. This requirement Is imposed to ensure that

no portion of the plume is free to migrate away from the zone

of action. To assess this performance adequately, the

hydraulic gradient should be measured accurately in three

dimensions between each pair of adjacent pumping or injection

wells. The design of an array of piezometers (small diameter

wells with very short screened Intervals, that are used to

measure the hydraulic head of selected positions in an

aquifer) for this purpose is not as simple as one might first

imagine. Many points are needed to define the convoluted

water-table surface that develops between adjacent pumping

or Injection wells. Not only are there velocity divides in the

horizontal dimension near active wells, but in the vertical

dimension, too, because the hydraulic Influence of each well

extends to only a limited depth in practical terms.

Innovations in Pump-and-Treat Remediations

One of the promising innovations in pump-and-treat remediations

is pulsed pumping. Pulsed operation of hydraulic systems is

the cycling of extraction or injection wells on and off in active

and resting phases (Figure 17). The resting phase of a

pulsed-pumping operation can allow sufficient time for

contaminants to diffuse out of low permeability zones and into

adjacent high permeability zones, until maximum concentrations

are achieved In the higher permeability zones. For sorbed

contaminants and NAPL residuals, sufficient time can be

allowed for equilibrium concentrations to be reached In local

ground water. Subsequent to each resting phase, the active

phase of the cycle removes the minimum volume of

contaminated ground water, at the maximum possible

concentrations, for the most efficient treatment. By occasionally

cycling only select wells, stagnation zones may be brought

into active flowpaths and remediated.

Figure 17. Pulsed pumping removal of residual contaminants.

Repetitive removal of pulses of highly contaminated water ensures

effective depletion of contaminant residuals.

Pulsed operation of remediation wellfields incurs certain

additonal costs and concerns that must be compared with its

advantages forsite-specific applications. During the resting

phase of pulsed-pumping cycles, peripheral gradient control

may be needed to ensure adequate hydrodynamic control of

the plume. In an ideal situation, peripheral gradient control

would be unnecessary. Such might be the case where there

are no active wells, major streams, or other significant hydraulic

stresses nearby to influence the contaminant plume while the

remedial action wellfield is In the resting phase. The plume

would migrate only a few feet during the tens to hundreds of

hours that the system was at rest, and that movement would

be rapidly recovered by the much higher flow velocities back

toward the extraction wells during the active phase.

When significant hydraulic stresses are nearby, however,

plume movement during the resting phase may be unacceptable.

Irrigation or water-supply pumpage, for example, might cause

plume movement on the order of several tens of feet per day.

it might then be impossible to recover the lost portion of the

plume when the active phase of the pulsed-pumping cycle

commences. In such cases, peripheral gradient control

during the resting phase would be essential. If adequate

storage capacity is available, it may be possible to provide

gradient control in the resting phase by injection of treated

waters downgradient of the remediation wellfield. Regardless

of the mechanics of the compensating actions, their capital

and operating expenses must be added to those of the

primary remediation wellfield to determine the complete cost.

Pump-and-treat remediations are underway today that

Incorporate some of the principles of pulsed pumping. For

instance, pumpage from contaminated bedrock aquifers and

other low permeability formations results in Intermittent wellfield

operations by default; the wells are pumped dry even at low

flow rates. In such cases, the wells are operated on demand

with the help of fluid-level sensors that trigger the onset and

cessation of pumpage. This simultaneously accomplishes

the goal of pumping ground water only after it has reached

10
chemical equilibrium, since equilibrium occurs on the same

time frame as the fluid recharge event in low permeability

settings. in settings of moderate to high permeability, the

onset and cessation of pumpage could be keyed to contaminant

concentration Ievels in the pumped water, independent of

flow changes required to maintain proper hydrodynamic

control. Peripheral hydrodynamic controls mayor may not be

necessary during the resting phase of the cycle.

Other strategies to improve the performance of pump-andtreat remediations include:

(1) scheduling of wellfield operations to satisfy

simultaneously hydrodynamic control and

contaminant concentration trends or other

performance criteria,

(2) repositioning of extraction wells to change

major transport pathways, and

(3) integration of wellfield operations with other

subsurface technologies, such as barrier

walls that limit plume transport and minimize

pumping of fresh water, or infiltration ponds

that maintain saturated flow conditions for

flushing contaminants from previously

unsaturated soils and sediments.

Flexible operation of a mediation wellfield, such as occasionally

turning off individual pumps, allows for some flushing of

stagnant zones. That approach may not baas hydrodynamically

efficient as one that involves permanently repositioning or

adding pumping wells to new locations at various times

during remediation. Repositioned and new wells require

access for drilling, however, and that necessarily precludes

capping of the site until after completion of the pump-andtreat operations. The third approach cited above, combining

pump-and-treat with subsurface barrier walls, trenching, or

in-situ techniques, ail of which may occur at any time during

remediation, may also require postponement of capping until

completion of the remediation.

The foregoing discussion may raise latent fears of lack of

control of the contaminant source, something almost always

mitigated by isolation of the contaminated soils and subsoils

that remain long after manmade containers have been removed

from the typical site. Fortunately, vacuum extraction of

contaminated air/vapor from soils and subsoils has recently

emerged as a potentially effective means of removing volatile

organic compounds (VOC’S). Steam flooding has shown

promise for removal of the more retarded organics, and insitu chemical fixation techniques are being tested for the

isolation of metals wastes.

Vacuum extraction techniques are capable of removing

several pounds of VOC’S per day, whereas air stripping of

VOC’S from comparable volumes of contaminated ground

water typically results in the removal of only a few grams of

VOC’S per day because VOC’S are so poor insoluble in water.

Similarly, steam flooding is an economically attractive means

of concentrating contaminant residuals, as a front leading the

injected body of steam. Steam flooding or chemical fixation

have potential for control of fluid and contaminant movement

in the unsaturated zone and should thus be considered a

potentially significant addition to the list of source control

options. in addition, soils engineering and landscape

maintenance techniques can minimize infiltration of rainwater

in the absence of a multilayer RCRA-style cap.

In terms of evacuation of the performance of a remediation,

the presence of a multliayer RCRA-styled cap could pose

major limitations. The periodic removal of core samples of

subsurface solids from the body of the plume and the source

zone, with subsequent extraction of the chemical residues on

the solids, is the only direct means of evacuating the true

magnitude of the residuals and their depletion rate. Since this

must be done periodically, capping would conflict unless

Postponed until closure of the site. Ifcappingcan be postponed

or foregone, great flexibility for management of pump-andtreat remediations (e.g., concurrent operation of a soil vapor

extraction wellfield, and sampling of subsoils) can be used to

improve effectiveness.

Modeling as a Performance Evaluation Technique

Subsurface contaminant transport models incorporate a number

of theoretical assumptions about the natural processes governing

the transport and fate of contaminants, in order for solutions

to be made tractable, simplifications are made in applications

of theory to practical problems. A common simplification for

wellfield simulations is to assume that air flow is horizontal, so

that a two-dimensional model can be applied, rather than a

three-dimensional model, which is much more difficult to

create and more expensive to use. Two-dimensional model

representations are obviously not faithful to the true complexities

of real world pump-and-treat remediations since most of

these are in settings where three-dimensional flow is the rule.

Moreover, most pump-and-treat remediations use partially

penetrating wells, which effect significant vertical flow

components, whereas the two-dimensional models assume

that the remediation wells are screened throughout the entire

saturated thickness of the aquifer, and therefore do not cause

upconing of deeper waters.

Besides the errors that stem from simplifying assumptions,

applications of mathematical models to the evacuation of

pump-and-treat remediations are also subject to considerable

error where the study site has not been adequately characterized.

It is essential to have appropriate field determinations of

natural process parameters and variables (Figure 18), because

these 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, contrary to popular beliefs, because such tests

and stochastic simulations assume that the underlying

conceptual basis of the model is correct. One cannot properly

change the conceptual basis (e.g., from an isolated aquifer to

one that has strong interaction with a stream or another

underlying aquifer) without data to justify the change. The

high degree of hydrogeological, chemical, and microbiological

complexity typically present in field situations requires sitespecific characterization of various natural processes by

detailed field and laboratory investigations.

Hence, both the mathematics that describe models and the

parameter inputs to those models should be subjected to

rigorous quality control procedures. Otherwise, results from

field applications of models are likely to be quatitatively, as

well as quantitatively, incorrect, if done properly, however,

11
Figure 18. Grid of points for a contaminant transport modal.

Known values of water level and other inputs are used to predict

concentration changes at each grid intersection (node).

mathematical modeling may be used to organize vast amounts

of disparate data into a sensible framework that will provide

realistic appraisals of which parts of a contaminant plume are

being effectively cleansed, when the remediation will meet

target contaminant reductions, and what to expect in terms of

irreducible contaminant residuals. Models may also be used

to evaluate changes in design or operation, so that the most

effective and efficient pump-and-treat remediation can be

attained.

Other Data Analysis Methods for Performance

Evacuations

Mathematical models are by no means the only methods

available for use in evaluating the performance of pump-andtreat remediations. Two other major fields of analysis are

statistical methods and graphical methods. The potential

power of statistical methods has been tapped infrequently in

ground-water contamination investigations, aside from their

use in quality assurance protocols. The uses are many,

however, as shown in Table 1. While data interpretation and

presentation methods vary widely, many site documents lack

statistical evaluations; some also present inappropriate

simplifications of datasets, such as grouping or averaging

broad categories of data, without regard to the statistical

validities of those simplifications.

Graphical methods of data presentation and analysis have

been used heavily in both ground-waterflow problems (e.g.,

flowline plots and flownets) and water chemistry problems

(e.g., Stiff kite diagrams). Figure I, for example, is a flowline

plot for a single well. From analysis of such plots, it is possible

to estimate the number of pore volumes that will be removed

over a set period of time of constant pumpage, at different

locations in the contaminant plume. Figure 9 is a chemical

phase diagram for iron, which may be used to relate pH and

redox measurements to the most stable species of iron.

Figure 19 presents one means of producing readily recognizable

patterns of the major ion composition of a water sample, so

that it maybe differentiated from other water types. At sites

Analysis of Variance (ANOVA) Teohniques

ANOVA techniques maybe 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.

Oorrelation Coefflcients

Correlation coefficients can be used to provide justification for

lumping various chemicals together (e.g., total VOC’S), or for using a

single chemical as a class representative, or to Iink sources by similar

chemical behavior.

Regression Equatlons

Regression equations may be used to predict contaminant loads

based on historical records and supplemental data, and may be used to

test cause-and-effect hypotheses about sources and contaminant

release rates.

Surface Trend Analysis Technique

Surface trend analysis techniques maybe used to identify recurring

and intermittent (e.g., seasonal) trends in oontour maps of ground-water

Ievels and contaminant distributions, which may be extrapolated to

source locations or future plume trajectories.

Table 1. Statistical methods useful in performance evaluatlons.

of subsurface contamination, the major ion composition often

differs greatly from the natural quality of water in adjacent

areas. Water quality specialists have used such plots for

decades to differentiate zones of brackish water from zones

of potable water, in studies of saltwater intrusion into coastal

aquifers, and in studies of the upconing of saline water from

shales during pumping of overlying sandstone aquifers.

TOTAL

Figure 19. Pie chart of major ions in aground-water sample.

The mini-equivalence values of the major ions are computed

and plotted to generate patterns specific to the source.

12
Figure 20 illustrates another means of producing readily

recognizable patterns of the milli-quivalence values of major

cations and anions in aground-water sample. Geochemical

prospectors have used this graphical technique as an aid in

the identification of waters associated with mineral deposits.

These graphical presentation techniques have been adapted

recently to the display of organic chemical contaminants. For

example, a compound of interest such as trichloroethene

(TCE) maybe evaluated interms of its contribution to the total

organic chemical contamination load, or against other specific

contaminants, so that some differentiation of some contributions

to the overlll plume can be obtained.

(a)

(b)

Figure 20. Stiff diagrams of major ions in two samples.

The concentrations of the ions are plotted in the manner shown

in (a); the uniqueness of another water type is shown by (b).

Perspectives for Site Characterizations

Concepts pertinent to investigating and predicting the transport

and fate of contaminants in the subsurface are evolving.

Additional effort devoted to site-specific characterizations of

preferential pathways of contaminant transport and the natural

processes that affect the transport behavior and ultimate fate

of contaminants may significantly improve the timeliness and

cost-effectiveness of remedial actions at hazardous waste

sites.

Characterzation Approaches

To underscore the latter point, it is useful to examine the

principal activities, benefits, and shortcomings of increasingly

sophisticated levels of site characterization approaches:

conventional (Table 2), state-of-the-art (Table 3), and stateof-the-science (Table 4). The conventional approach to site

characterizations is typified by the description given in Table

2.

Each activity of the conventional approach can be accomplished

with semi-skilled labor and off-the-shelf technology, with

moderate to iow costs. It may not be possible to characterize

thoroughly the extent and probable behavior of a subsurface

contaminant plume with the conventional approach.

Key management uncertainties regarding the degree of health

threat posed by a site, the selection of appropriate remedial

action technologies, and the duration and effectiveness of the

remediations all should decrease significantly with the

implementation of more sophisticated site characterization

approaches.

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

Actions Typically Taken

install a few dozen shallow monitoring wells

Sample ground-water numerous times for 129+

priority pollutants

Define geology primarily by driller’s logs and drill

cuttings

Evaluate local hydrology with water level contour

maps of shallow wells

Possibly obtain soil and core samples for chemical

analyses

Benefits

Rapid screening of the site problems

Costs of investigation are moderate to low

field and laboratory techniques used are standard

Data analysis/interpretation is straightforward

Tentative identification of remedial alternatives is

possible

Shortcomings

True extent of site problems maybe misunderstood

Selected remadial alternatives may not be appropriate

Optimization of final remediation design may not be

possible

Clean-up costs remain unpredictable, tend to excessive

levels

Verification of compliance is uncertain and dlfficult

Table 2. Conventional approach to site characterization.

it will probably cost substantiality more to implement state-ofthe-art and state-of-the-science approaches in site

characterizations, but the increased value of the information

obtained is likely to generate offsetting cost savings by way of

improvements in the technical effectiveness and efficiency of

the site clean-up.

Obviously, it is not possible to test these conceptual relationships

directly, because one cannot carry site characterization and

remediation efforts to fruition along each approach

simultaneously. One can infer many of the foregoing discussion

points, however, by observing the changes in perceptions,

decisions, and work plans that occur when more advanced

techniques are brought to bear on a site that has already

undergone a conventional Ievel of characterization. The latter

situation is a fairly common occurrence, because many first

attempts at site characterization turn up additional sources of

contamination or hydrogeologic complexities that were not

suspected when the initial efforts were budgeted.

Hypothetical Example

It is helpful to examine possible scenarios that might result

from the different site investigation approaches just outlined.

13
●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

Recommended Actions

Install depth-specific clusters of monitoring wells

Initially sample for 129+ priority pollutants, be selective

subsequently

Define geology by extensive coring/sediment samplings

Evaluate local hydrology with well clusters and

geohydraulic teats

Perform limited tests on sediment samples (grain size,

day content, etc.)

Conduct surface geophysical surveys (resistivity,

EM, ground-penetrating radar)

Benefits

Conceptual understandings of the site problems are

more complete

Prospect are improved for optimization of remedial

actions

Predictability of remediation effectiveness is increased

Clean-up costs maybe lowered, estimates are more

reliable

Verification of compliance is more soundly based

shortcomings

Characterization costs are higher

Detailed understandings of site problems are still

difficult

Full optimization of remediation is still not likely

Field tests may create secondary problems (disposal

of pumped waters)

Demand for specialists is increased, shortage is a

key limiting factor

Table 3. State-of-the-art approach to site characaterizatlon.

Figure 21 depicts a hypothetical ground-water and soil

contamination situation located in a mixed residential and

light industry section of a town in the Northeast. As illustrated,

there are three major plumes: an acids plume (e.g., from

electrolytic plating operations), a phenols plume (e.g., from a

creosoting operation that used large amounts of

pentachlorophenol), and a volatile organics plume (e.g., from

solvent storage leaks). in addition, soils onsite are heavily

contaminated in one area with spilled pesticides, and in

another area with spilled transformer oils that contained

PCB’S in high concentrations.

The hydrogeologic setting for the hypothetical site is a productive

alluviai aquifer composed of an assortment of sands and

gravels that are interfingered with clay and silt remnants of old

streambeds and floodplain deposits. The latter have been

continually dissected by a central river as the valley matured

over geologic time. The deeper portion of the sediments is

highly permeable and is the zone most heavily used for

municipal and industrial supply wells, whereas the shallow

portion of the sediments is only moderately permeable since

it contains many clay and silt lenses. The predominant

ground-water flow direction in the deeper zone parallels the

river (which is also parallei to the axis of the valley), except in

Iocallzed areas around municipal and industrial wellfields.

The predominant direction of flow in the shallow zone is

Figure 21. Hyothetical contamination site in an alluvial setting.

Ground-water contamination at the site includes a variety of plumes;

the setting is complex geologically and hydrologically.

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

●

Idealized Approach

Assume state-of-the-art as starting point

Conduct soil vapor surveys for volatile% fuels

Conduct tracer teak and borehole geophysical surveys

(neutron and gamma)

Conduct karat stream tracing and recharge studies, if

appropriate to the setting

Conduct bedrock fracture orientation and interconnectivity studies, if appropriate

Determine the percent organic carbon and cation exchange

capacity of solids

Measure redox potential, pH, and dissolved oxygen level

of subsurface

Evaluate sorption-desorption behavior by laboratory

column and batch studies

Assess the potential for biotransformation of specific

compounds

Benefits

Thorough conceptual understandings of site problems

are obtained

Full optimization of the remediation is possible

Predictability of the effectiveness of remediation is

maximized

Clean-up cost maybe lowered significantly,

estimates are reliable

Verification of compliance is assured

Shortcoming

Characterization costs may be much higher

Few previous applications of advanced theories

and methods have been completed

field and laboratory techniques are specialized

and are not easily mastered

Avallabiiity of specialized equipment is low

Need for specialism is greatly increased (it may

be the key limitation overall)

Table 4. state-of-the-science approach to site characterization.

14
seasonally dependent, having the strongest component of

flow toward the river during periods of low flow in the river, and

being roughly parallel to the river during periods of high flow

in the river.

Strong downward components of flow carry water from the

shallow zone to the deeper zone throughout municipal and

industrial weilfields, as well as along the river during periods

of high flow. Slight downward components of flow exist

elsewhere due to local recharge by infiltrating rainwaters.

Conventional Characterization Scenario

A conventional site characterization would define the horizontal

extent of the most mobile, widespread plume. However, it

would provide only a superficial understanding of variations in

the composition of the sediments. An average hydraulic

conductivity would be obtained from review of previously

published geologic reports and would be assumed to represent

the entire aquifer for the purpose of estimating flow rates. The

kind of clean-up that would Iikely result from a conventional

site investigation is illustrated in Figure 22. The volatile

organics plume would be the most important to remediate,

since it is the most mobile, and an extraction system would be

installed. Extracted fluids would be air-stripped of volatiles

and then passed through a treatment plant for removal of

non-volatile residues, probably by relatively expensive filtration

through granular activated carbon.

Extraction wells would be placed along the downgradient

boundary of the VOC plume to withdraw contaminated ground

water. A couple of injection wells would be placed upgradient

and would be used to return a portion of the extracted and

treated waters to the aquifer. The remainder of the pumped

and treated waters would be discharged to the tributary under

an NPDES permit.

Figure 22. Conventional dean-up of the hypothetical site. EW’s

are extraction wells. IW’s are injection wells; all are screened at

the same elevation and have identical flowrates.

information obtained from the drilling logs and samples of the

monitoring wells wouldbe inadequate to do more than position

all of the screened sections of the remediation wells at the

same shallowdepth. The remediation wellfield would operate

for the amount of time needed to remove a volume of water

somewhat greater than that estimated to reside within the

bounds of the zone of contamination, allowing for average

retardation values (from the scientific literature) for contaminants

found at the site. The PCB-laden soils would be excavated

and sent to an incinerator or approved waste treatment and

disposal facility. The decision makers would have based their

approval on the presumption that the plume had been adequately

defined, and that if it had not, that the true magnitude of the

problem does not differ substantially, except for the possibility

of perpetual care.

State-of-the-Art Characterization Scenario

incorporation of some of the more common state-of-the-ail

site investigation techniques, such as pump tests, installation

of vertically-separated clusters of monitoring wells (shallow,

intermediate, and deep) and river stage monitors, and chemical

analysis of sediment and soil samples would Iikely result in

the kind of remediation illustrated in Figure 23. Since a

detailed understanding of the geology and hydrology would

be obtained, optimal selection of well locations, well screen

positions and flowrates (the values in the parentheses in

Figure 23, in gallons per minute) for the remediation wells

could be determined, A special program to recover the acid

plume and neutralize itwould be instituted. A special program

could also be instituted for the pesticide plume. This approach

would probably lower treatment costs overall, despite the

need for separate treatment trains for the different plumes,

because substantially lesser amounts of ground water would

be treated with expensive carbon filtration for removal of nonvolatile contaminants.

The screened intervals of the extraction wells would be

placed at deeper positions towards the river, if water quality

data from monitoring well clusters show that the plume is

migrating beneath shallow accumulations of clays and silts to

Figure 23. Moderate state-of-the-art remediatlon. Clusters of

vertically-separated monitoring wails and an aquifer test are used

to tailor the remedy to the site hydrogeology.

15
the deeper, more permeable sediments. All or most of the

extracted and treated ground water could be returned to the

aquifer through injection wells. The well screens would need

to be positioned (e.g., deeper) to avoid diminishing the

effectiveness of nearby extraction wella. As in the conventional

remedy, the remediation weilfield would operate for the

amount of time needed to remove a volume of water that is

determined from average contaminant retardation values

and the rate of flushing of groundwater residing in the zone

of contamination. The detailed geologic and hydrologic

information acquired would result in an expectation of more

rapid cleansing of portions of the contaminated zone than

others.

One could conclude that this remediation is optimized to the

point of providing an effective clean-up, and decision makers

would be reasonably justified in giving their approval. One

should note, however, that the efficiency (esp., duration) of

the remediation maybe less than optimal.

Advanced State-of-the-Art Characterization

Scenario

if ail state-of-the-art investigation too ls were used at the site,

there would be an opportunity to evaluate the desirability of

using a subsurface barrier wall to enhance remediation

efforts (Figure 24). The wall would not entomb the plumes,

but would limit pumping to contaminated fluids, rather than

having the extracted waters diluted with fresh waters, as was

true of the two previous approaches. The volume to be

pumped could be lowered because the barrier wall will

increase the drawdown at each well by hydraulic interference

effects, thereby maintaining the same effective hydrodynamic

control with less pumping (note the lower flowrates for each

well in Figure 24). Treatment costs should be less, also,

because the pumped waters should contain higher

concentrations of contaminants and treatment efficiencies

are often greatest at moderate to high concentrations. soil

washing techniques could be used on the pesticide contaminated

area minimize future source releases to ground water.

Both the effectiveness and the efficiency of this remediation

might therefore appear to be optimizable, but that is a

perception that is based on the presumption that the

contaminants will be released readily. Given the potential

limitations to pump-and-treat remediations discussed previously,

it is doubtful that even this advanced state-of-the-art site

investigation precludes further improvement. Much attention

could be devoted to the chemical and biological peculiarities,

just as has been given to the geologyand the hydrology. For

example, detalledevaluation of sorption or other contaminant

retardation processes at this site, rather than the use of

average retardation values from the literature, should generate

additional improvements ineffectiveness and efficiencyof the

remediation. Likewise, detailed examination of the potential

for biotransformation would be expected to lead to improved

effectiveness and efficiency.

State-of-the-Science Characterization Scenario

At the state-of-the-science level of site characterization,

tracer tests could be undertaken which would provide good

information on the potential for diffusive restrictions in low

permeability sediments and on anisotropic biases in the flow

regime, Sorption behavior of the VOC’S could be evaluated

in part by determining the total organic carbon content of

selected subsurface sediments. Similarly, the cation exchange

capacities of subsurface sediment sampes could be determined

to obtain estimates of release rates and nobilities of toxic

metals. The stabilities of various possible forms of elements

and compounds could be evaluated with measurements of

pH, redox potential, and dissolved oxygen. Contemporary

research indicates that the acids plume and the phenols

plume might be better addressed with such measurements

(e.g., chemical speciation modeling). Finally, if state-of-thescience findings regarding potential biotransformations could

be taken advantage of, it might be possible to effect in-situ

degradation of the phenols pluma, and remove volatile residues

too (Figure 25).

Figure 24. Advanced state-of-the-art remediation. Subsurface

barrier walls or other technologies can be integrated with pump

and-treat operations.

16

Figure 25. State-of-the-science remediafion. Bioremediation and

other emerging technologies could be tested and Implemented with

reasonable oertainty of effects.
Additional Considerations

The foregoing discussion highlights genetic gains In effectiveness

and efficiency of remediation that should be expected by

better defining ground-water contamination problems and

using that information to develop site-specific solutions.

Because the complexities of the subsurface cannot fully be

delineated even with “state-of-the-science” data collection

techniques and many of these techniques are not fully developed

nor widely available at this time, it Is important to proceed with

remediation in a phased process so that information gained

from initial operation of the system can be incorporated into

successive stages of the remedy. Some considerations that

may help to guide this process include the following:

1. In many cases, it maybe appropriate to initiate a

response action to contain the contaminant plume

before the remedial investigation is completed.

Containment systems (e.g., gradient control) can

often be designed and implemented with less information

than required for full remediation. In addition to

preventing the contamination from migrating beyond

existing boundaries, this action can provide valuable

Information on aquifer response to pumping.

2. Early actions might also be considered as a way of

obtaining information pertinent to design of the final

remedy. This might consist of installing an extraction

system in a highly contaminated area and observing

the response of the aquifer and contaminant plume

as the system is operated.

3. The remedy itself might be Implemented In a staged

process to optimize system design. Extraction wells

might be installed incrementally and observed for

a period of time to determine their range of influence,

This will help to identify appropriate locations for

additional wells and can assure proper sizing of

the treatment systems as the range of contaminant

concentrations in extracted ground water is

confirmed.

4. In many cases, ground water response actions

should be initiated even though it is not possible

to assess the restoration time frames or ultimate

concentrations achievable. After the systems

have been operated and monitored over time,

it should be possible to better define the final

goals of the action.

Selected References

Abriola, L.M. and G.F. Pinder. 1985. A Multiphase Approach

to the Modeling of Porous Media Contamination by Organic

Compounds. Water Resources Research 21(1):11-18.

Baehr, A, L., G.E. Hoag, and M.C. Marley. 1989. Removing

Volatile Contaminantsf rom the Unsaturated Zone by Inducing

Advective Air-Phase Transport. Journal of Contaminant

Hydrology 4(1 ):1 -26.

Barker, J. F., G.C. Patrick, and D. Major, 1987. Natural

Attenuation of Aromatic Hydrocarbons in a Shallow Sand

Aquifer. Ground Water Monitoring Review 7(1):64-71.

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

1989. In Situ Measurement and Numerical Simulation of

Oxygen Limited Biotransformation. Ground Water Monitoring

Review 9(1):63-91.

Bouchard, D.C., A.L. Wood, M.L. Campbell, P. Nkedi-Kizza,

and P.S.C. Rae. 1988. Sorption Nonequllibrium during

Solute Transport. Journal of Contaminant Hydrology2(3):209-

223.

Chau, T.S. 1988. Analysis of Sustained Ground-Water

Withdrawals by the Combined Simulation-optimization

Approach. Ground Water 26(4):454-463.

Cheng, Songlin. 1988. Computer Notes - Trilinear Diagram

Revisited: Application, Limitation, and an Electronic spreadsheet

Program. Ground Water 26(4):505-510,

Curtis, G. P., P.V. Roberts, and Martin Reinhard. 1986. A

Natural Gradient Experiment on Solute Transport in a Sand

Aquifer: 4. Sorption of Organic Solutes and its Influence on

Mobility. Water Resources Research 22(13):2059-2067.

Enfield, C.G. and G. Bengtsson. 1986. Macromolecular

Transport of Hydrophobic Contaminants in Aqueous

Environments. Ground Water 26(l) :64-70.

Faust, C.R. 1985. Transport of Immiscible Fluids Within and

Below the Unsaturated Zone: A Numerical Model. Water

Resources Research 21(4):587-596.

Feenstra, S., J.A. Cherry, E.A. Sudicky, and Z. Haq. 1984.

Matrix Diffusion Effects on Contaminant Migration from an

Injection Well in Fractured Sandstone. Ground Water22(3):307-

316.

Flathman, P., D. Jerger, and L. Bottomley. 1989. Remediation

of Contaminated Ground Water Using Biological Techniques.

Ground Water Monitoring Review 9(1 ):105-1 19.

Gelhar, L.W. 1986. Stochastic Subsurface Hydrology from

Theory to Applications. Water Resources Research 22(9):135S

145S.

Goltz, M.N. and P.V. Roberts. 1986. Interpreting Organic

Solute Transport Data from a Field Experiment using Physical

Nonequilibrium Models. Journal of Contaminant Hydrology

1 (1/2):77-94.

Goltz, M.N. and P.V. Roberts. 1988. Simulations of Physical

Nonequilibrium Solute Transport Models: Application to a

Large-Scale Field Experiment. J. Contaminant Hydrology

3(1):37-64.

Guven, O. and F.J. Molz. 1986. Deterministic and stochastic

Analyses of Dispersion in an Unbounded Stratified Porous

Medium. Water Resources Research 22(1 1):1565-1574.

17
Hinchee, R. and H.J. Reisinger. 1987. A Practical Application

of Multiphase Transport Theory to Ground Water Contamination

Problems. Ground Water Monitoring Review 7(1):84-92.

Hossain, M.A. and M.Y. Corapcioglu. 1988. Modifying the

USGS Solute Transport Computer Model to Predict HighDensity Hydrocarbon Migration. Ground Water 26(6):717-

723.

Hunt, J. R., N. Sitar, and K.S. Udell. 1988. Nonaqueous

Phase Liquid Transport and Cleanup: 1. Analysis of Mechanisms.

Water Resources Research 24(8):1 247-1258.

Hunt, J. R., N. Sitar, and K.S. Udell. 1988. Nonaqueous

Phase Liquid Transport and Cleanup: 2. Experimental Studies.

Water Resources Research 24(8):1 259-1269.

Jensen, B. K., E. Arvln, and A.T. Gundersen. 1988.

Biodegradation of Nitrogen- and Oxygen-Containing Aromatic

Compounds in Groundwater from an Oil-Contaminated Aquifer.

Journal of Contaminant Hydrology 3(1):65-76.

Jorgensen, D.G., T. Gogel, and D.C. Signor. 1982.

Determination of Flow in Aquifers Containing Variable-Density

Water. Ground Water Monitoring Review 2(2):40-45.

Keely, J.F. 1984. Optimizing Pumping Strategies for

Contaminant Studies and Remedial Actions. Ground Water

Monitoring Review 4(3):63-74.

Keely, J. F., M.D. Piwonl, and J.T. Wilson. 1986. Evolving

Concepts of Subsurface Contaminant Transport. Journal

Water Pollution Control Federation. 58(5):349-357.

Keely, J. F.and C. F. Tsang. 1983. Velocity Plots and Capture

Zones of Pumping Centers for Ground Water Investigations.

Ground Water 22(6):701 -714.

Kipp, K. L., K.G. Stollenwerk, and D.B. Grove. 1986.

Groundwater Transport of Strontium 90 in a Glacial Outwash

Environment. Water Resources Research 22(4):519-530.

Konikow, L.F. 1986. Predictive Accuracy of a Ground-Water

Model - Lessons from a Postaudit. Ground Water24(2):173-

184.

Kueper, B.H. and E.O. Frind. 1988. An Overviewof Immiscible

Fingering in Porous Media. Journal of Contaminant Hydrology

2(2):95-110.

Macalady, D. L., P.G. Tratnyek, and T.J. Grundl. 1986.

Abiotic Reduction Reactions of Anthropogenic Organic

Chemicals in Anaerobic Systems:A Critical Review. Journal

of Contaminant Hydrology 1(1/2):1-28.

Mackay, D. M., W.P. Ball, and M.G. Durant. 1986. Variability

of Aquifer Sorption Properties in a Field Experiment on

Groundwater Transport of Organic Solutes: Methods and

Preliminary Results. Journal of Contaminant Hydrology 1(1/

2):119-132.

Mackay, D. M., D.L. Freyberg, P.V. Roberts, and J.A. Cherry.

1986, A Natural Gradient Experiment on Solute Transport in

a Sand Aquifer 1. Approach and Overview of Plume Movement.

Water Resources Research 22(1 3):201 7-2029.

Major, D.W., Cl. Mayfield, and J.F. Barker. 1988.

Biotransformation of Benzene by Denitrification In Aquifer

Sand. Ground Water 26(l):8-14.

Mercado, Abraham. 1985. The Use of Hydrogeochemical

Patterns in Carbonate Sand and Sandstone Aquifers to

Identify Intrusion and Flushing of Saline Water. Ground

Water 23(5):635-645.

Molz, F.J., O. Guven, J.G. Melville, and J,F. Keely. 1986.

Performance and Analysis of Aquifer Tracer Tests with

implications for Contaminant Transport Modeling. U.S. EPA

Office of Research and Development: EPA/600/2-86-062.

Molz, F.J., O. Guven, J.G. Melville, J.S. Nohrstedt, and J.K.

Overholtzer. 1988. Forced-Gradient Tracer Tests and Inferred

Hydraulic Conductivity Distributions at the Mobile Field Site.

Ground Water 26(5):570-579.

Molz, F.J., M.A. Widdowson, and L.D. Benefield. 1986.

Simulation of Microbial Growth Dynamics Coupled to Nutrient

and Oxygen Transport in Porous Media. Water Resources

Research 22(8):1207-1216.

Novak, S.A, and Y. Eckstein. 1988. Hydrogeochemical

Characterization of Brines and Identification of Brine

Contamination in Aquifers. Ground Water 26(3):317-324.

Osiensky, J. L., K.A. Peterson, and R.E. Williams. 1988.

Solute Transport Simulation of Aquifer Restoration after In

Situ Uranium Mining. Ground Water Monitoring Review

8(2):137-144.

Osiensky, J.L., G.V. Winter, and R.E. Williams. 1984. Monitoring

and Mathematical Modeling of Contaminated Ground-Water

Plumes in Fluvial Environments. Ground Water 22(3):298-

306.

Ophori, D. U., and J. Toth. 1989. Patterns of Ground-Water

Chemistry, Ross Creek Basin, Alberta, Canada. Ground

Water 27(1):20-26.

Pinder, G.F. and L.M. Abriola. 1986. On the Simulation of

Nonaqueous Phase Organic Compounds in the Subsurface.

Water Resources Research 22(9):109S-1 19S.

Pollock, D.W. 1988. Semianalytical Computation of Path

Lines for Finite Difference Models. Ground Water26(6):743-

750.

Roberts, P.V., M.N. Goltz, and D.M. Mackay. 1986. A

Natural Gradient Experiment on Solute Transport In a Sand

Aquifer: 3. Retardation Estimates and Mass Balances for

Organic Solutes. Water Resources Research 22(13):2047-

2058.

Satlin, R.L. and P.B. Bedient. 1988. Effectiveness of Various

Aquifer Restoration Schemes Under Variable Hydrogeologic

Conditions. Ground Water 26(4):488-499.

Siegrist, H. and P.L. McCarty. 1987. Column Methodologies

for Determining Sorption and Biotransformation Potential for

Chlorinated Aliphatic Compounds in Aquifers. Journal of

Contaminant Hydrology 2(1 ):31 -50.

18
Spain, J. C., J.D. Milligan, D.C. Downey, and J.K. Slaughter.

1989. Excessive Bacterial Decomposition of H2O2 During

Enhanced Biodegradation. Ground Water 27(2):163-167.

Spayed, S.E. 1985. Movement of Volatile Organics Through

a Fractured Rock Aquifer. Ground Water 23(4):496-502.

Srinivasan, P. and J.W. Mercer. 1988. Simulation of

Biodegradation and Sorption Processes in Ground Water.

Ground Water 26(4):475-487.

Staples, CA and S.J. Geiselmann. 1988. Cosolvent Influences

on Organic Solute Retardation Factors. Ground Water

26(2):192-198.

Starr, R.C., R.W. Gillham, and E.A. Sudicky. 1985. Experimental

Investigation of Solute Transport in Stratified Porous Media:

2. The Reactive Case. Water Resources Research 21(7):1043-

1050.

Steinhorst, R.K. and R.E. Williams. 1985. Discrimination of

Groundwater Sources Using Cluster Analysis, MANOVA,

Canonical Analysis, and Discriminant Analysis. Water

Resources Research 21(8):1 149-1156.

Stover, Enos. 1989. Co-produced Ground Water Treatment

and Disposal Options During Hydrocarbon Recovery Operations.

Ground Water Monitoring Review 9(1):75-82.

Sudicky, E.A., R.W. Gillham, and E.O. Frind. 1985. Experimental

Investigation of Solute Transport in Stratified Porous Media:

1. The Nonreactive Case. Water Resources Research

21 (7):1 035-1041.

Testa, S. and M. Paczkowski. 1989. Volume Determination

and Recoverability of Free Hydrocarbon. Ground Water

Monitoring Review 9(1):120-128.

Thomsen, K., M. Chaudhry, K. Dovantzis, and R. Riesing.

1989. Ground Water Remediation Using an Extraction,

Treatment, and Recharge System. Ground Water Monitoring

Review 9(1):92-99.

Thorstenson, D.C. and D.W. Pollock. 1989. Gas Transport

in Unsaturated Porous Media: The Adequacy of Fick’s Law.

Reviews of Geophysics 27(1):61-78.

Usunoff, E.J., and A. Guzman-Guzman. 1989. Multivariate

Analysis in Hydrochemistry: An Example of the Use of Factor

and Correspondence Analyses. Ground Water27(l):27-34.

Valocchi, A.J. 1988. Theoretical Analysis of Deviations from

Local Equilibrium during Sorbing Solute Transport through

ideallzed Stratified Aquifers. Journal of Contamination Hydrology

2(3):191-208.

Watson, [an. 1984. Contamination Analysis - Flow Nets and

the Mass Transport Equation. Ground Water 22(1):31-37.

Zheng, C,, K.R. Bradbury, and M.P. Anderson. 1988. Role

of Interceptor Ditches in Limiting the Spread of Contaminants

in Ground Water. Ground Water 26(6):734-742.

19