United States
Environmental Protection
Agency
Office of
Research and
Davelopment
Office of Solid Waste
and Emergency
Response
EPA/540/4-89/005
EPA Ground Water Issue
Performance Evaluations of
Pump-and-Treat Remediations
Joseph F. Keely
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 ground-
water 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 rernediations. 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 rernediations 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
evaluate the progress of pump-and-treat rernediations. 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, site-
specific data for use in remediation design and management
efforts.
Introduction
Pump-and-treat rernediations of ground-water contamination
are planned or have been initiated at many sites across the
country. Regulatory responsibilities require that adequate
oversight of these rernediations 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 rernediations 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
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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 remediation 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 likelythat 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 leachate 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
boundary of the
capture zone
MODERATE
VELOCITY
FAST
MODERATE
COMPLETELY
FLLED PORES
EMPTY PORES
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 initially fill the upper most soil pores. As much as half
of the fluids remain in each pore after drainage.
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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 layer that is sandwiched between two sand
or gravel aquifers.
GREAT HYDRAULIC IMPACTS
FROM HIGH RATES OF RELEASE
Figure 3. Hydraulic impacts of contaminant 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 contaminant 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
major trend from
velocity distribution
minor tpreadng
by dispersion
Figure 5. Cross-sectional view of contaminant spreading.
Permeability differences between strata cause comparable
differences in advection and, hence, plume spreading.
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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
flow 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,
COLLODS
Figure 6. Contaminant transport facilitated by particles. Sorption
of organics (e.g., RGB'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.
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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.
r^r ''
{ PCB / > <
Figure 8. Conceptualization of transport by cosolvation. Insoluble
contaminants may dissolve in ground water that contains solvents
at high concentrations.
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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 oxidation-
reduction reactions (e.g., oxidation of iron II to iron III). 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.
floater movement controlled
by local gradents and capillarity
local depression of water*
tab)* by bulk effect
Figure 10. Dynamics of immiscible floater and sinker plumes.
Buoyant plumes migrate laterally on top of the water saturated
zone. Dense plumes sink and follow bedrock slopes.
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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 pump-
and-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
remediated and how quickly. This facilitates management of
the remediation 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 ground-
water 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 post-
operational 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.
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Public 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 established
is one for determinations 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 level 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.
[Internal] Plume 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.
interdependences 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 B 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.
apparent residual
contamination levsl
TIME
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 level 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,
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(3) resorption of contaminants from sediment
surfaces, and
(4) liquid-liquid partitioning of immiscible
contaminants.
target
concentration
TME
Figure 13. Contaminant increases after remediation stops. Con-
taminant levels may rebound when pump-and-treat operations
cease, because of contaminant residuals.
Advection vs. Diffusion
Localized 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 volume
of high permeability sediments. The thicker the low permeability
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 limit 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.
Hydro dynamic Isolation
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.
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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.
ADVECTION
^ORCUMC
: CARBON OR
IWERAL OXIDE SURFACE
EOUICONC,
^INITIAL RAPID
DESORPTON
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 (non-
pumping) 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 (MAPI'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 level 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
\
\ SOLUBILITY
LMTED
DIFFUSION
LIMITED
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 two-
fold, because much of the pumped water will contain contaminant
-------
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-and-
treat 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.
TTME -
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 levels 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-and-
treat 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-and-
treat 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 in-
situ 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 multilayer 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-and-
treat 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 site-
specific 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 qualitatively, as
well as quantitatively, incorrect, if done properly, however,
-------
contaminant concentration - ?
h 565 ft K - 668 ft/day
S - .0005 DOC 1 irtfl/L
T - 500,000 gal/day/ft
municipal
wdHleld
/ / / /V- /1
J I 1I rJ //
11 it I rr
I 11 I I
1
I
1
1
1
( 1
/
1
1
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-and-
treat 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) Techniques
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.
Correlation Coefficients
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 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 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
levels and contaminant distributions, which may be extrapolated to
source locations or future plume trajectories.
Table 1. Statistical methods useful in performance evaluations.
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
meq/l
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
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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 interns 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.
1.5 1.0 0.5 0 0.5 1.0 1.5
-HCO.
^:. so
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).
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 difficult
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 state-
of-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 low costs. It may not be possible to characterize
thoroughly the extent and probable behavior of a subsurface
contaminant plume with the conventional approach.
Table 2. Conventional approach to site characterization.
it will probably cost substantiality more to implement state-of-
the-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 level 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
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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 parallel to the axis of the valley), except in
localized areas around municipal and industrial wellfields.
The predominant direction of flow in the shallow zone is
trbutary
rlvw,
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 intercon-
nectivity 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
Availability 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
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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 likely 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.
trtoutary
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 likely 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 non-
volatile 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
MMitary
rlvary
38K0
J
phnobpfam.
<22» ,-
(gum 4 or IMP)
x/
* f*t"ly ivfl
' iw»» IW1
1175)
<«*M
(27S)j
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Figure 23. Moderate state-of-the-art remediation. 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 Is 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-the-
science 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).
tributary
Figure 24. Advanced state-of-the-art remediation. Subsurface
barrier walls or other technologies can be integrated with pump
and-treat operations.
tributary
riv«r
Figure 25. State-of-the-science remediation. Bioremediation and
other emerging technologies could be tested and Implemented with
reasonable certainty of effects.
16
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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 overtime,
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, 0. 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.
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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 High-
Density 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 AT. 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. Signer. 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. Wlson. 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., 0. 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, 0. 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
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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.
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Spain, J. C., J.D. Milligan, D.C. Downey, and J.K. Slaughter.
1989. Excessive Bacterial Decomposition of H202 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, CAand 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-
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Steinhorst, R.K. and R.E. Wlliams. 1985. Discrimination of
Groundwater Sources Using Cluster Analysis, MANOVA,
Canonical Analysis, and Discriminant Analysis. Water
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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
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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
idealized 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.
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