United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/S2-86/062 Sept. 1986
&EPA Project Summary
Performance and Analysis of
Aquifer Tracer Tests with
Implications for Contaminant
Transport Modeling
Fred J. Molz, Oktay Giiven, Joel G. Melville, and Joseph F. Keely
The scale-dependence of dispersivity
values used in contaminant transport
models to estimate the spreading of
contaminant plumes by hydrodynamic
dispersion processes was investigated
and found to be an artifact of conven-
tional modeling approaches (esp., verti-
cally averaged parameters in two-
dimensional plume simulations). The
work reported here shows that varia-
tions in hydraulic conductivity with
depth result in significant variations in
ground-water flow and contaminant
transport velocities; it is the resulting
velocity variations that, if vertically av-
eraged, give rise to apparent scale-
dependency of dispersion (e.g., in-
creased dispersion with increasing
travel distance). Special depth-selective
observation well designs are recom-
mended by the authors for use in tracer
tests, so that detailed estimates of the
variations in hydraulic conductivity,
and flow and transport velocities can be
obtained. Innovative modeling tech-
niques, that take advantage of the de-
tailed information obtainable from such
tests (by emphasizing advective trans-
port, as opposed to dispersive trans-
port), have been developed by the au-
thors. These modeling techniques are
shown to have an element of true pre-
dictive ability, being able to closely sim-
ulate actual results with little or no cal-
ibration.
This Project Summary was devel-
oped by EPA's Robert S. Kerr Environ-
mental Research Laboratory, Ada, OK,
to announce key findings of the re-
search project that is fully documented
in a separated EPA publication of the
same title (see ordering information at
back).
Introduction
Due to worsening national environ-
mental problems, hydrologists are be-
ing asked to identify, assess or even an-
ticipate situations involving ground
water contamination. Many of the U.S.
Environmental Protection Agency's reg-
ulatory activities relate to prevention or
remediation of such situations. In both
regulatory and assessment activities,
increasing use is being made of com-
plex mathematical models that are
solved with the aid of the digital com-
puter. Some of the principal areas
where mathematical models can be
used to assist in the management of
EPA's ground water protection pro-
grams are:
(1) appraising the physical extent,
and chemical and biological qual-
ity, of ground-water reservoirs
(e.g., for planning purposes),
(2) assessing the potential impact of
domestic, agricultural, and indus-
trial practices (e.g., for permit is-
suance, EIS's, etc.),
(3) evaluating the probable outcome
of remedial actions at hazardous
waste sites, and of aquifer
restoration techniques generally,
(4) providing exposure estimates and
risk assessments for health-
effects studies, and
(5) policy formulation (e.g., banning
decisions, performance stand-
ards).
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These activities can be broadly catego-
rized as being either site-specific or
generic modeling efforts, and can be
further subdivided into applications to
point-source or nonpoint-source prob-
lems. The success of these efforts de-
pends on the accuracy and efficiency
with which the natural process con-
trolling the behavior of ground water,
and the chemical and biological species
it transports, are simulated.
Models are collections of partial dif-
ferential equations that contain a num-
ber of parameters which represent aqui-
fer physical properties and must be
measured in the field. Of the various
parameters involved, the hydraulic con-
ductivity distribution is of major impor-
tance. Other parameters such as those
relating to sorption, hydrodynamic dis-
persion, and chemical/biological trans-
formation are important also, but
hydraulic conductivity is more funda-
mental because, combined with the hy-
draulic gradient and porosity, it relates
to where the water is moving and how
fast. Therefore, this communication is
devoted mainly to the conceptualization
and measurement of hydraulic conduc-
tivity distributions and the relationship
of such measurements to dispersion
(spreading) of contaminants in aquifers.
Discussion
For the most part, contemporary
modeling technology is built around
two-dimensional models having physi-
cal properties, such as hydraulic con-
ductivity, that are averaged over the
vertical thickness of the aquifer. In such
a formulation, the longitudinal disper-
sivity is forced to be the major aquifer
property related to contaminant spread-
ing. This is not due to any fundamental
theoretical limitation. The major limita-
tion is that dependable and economical
field approaches for measuring
vertically-variable hydraulic conductiv-
ity distributions are not available. In the
absence of such data, one has no choice
in a modeling sense but to use some
type of vertically-averaged advection-
dispersion approach built around full
aquifer longitudinal dispersivities.
In order to begin to overcome this
limitation, a series of single-well (Fig-
ure 1) and two-well (Figure 2) tracer
tests were performed at a field site near
Mobile, Alabama. A major objective of
this communication is to describe these
tracer tests and discuss some practical
Injection
J (t)
Withdrawal
Q = QOUT
I
Upper
/ /Sf S/SS/ / //// ///
Confining Layer
Injection-
Withdrawal
Well
Observation
Well
With
Multilevel
Samplers
Lower Confining Layer
Figure 1. Vertical cross-sectional diagram showing single-well test geometry.
implications of the results with regard
to modeling of contaminant dispersion
in aquifers. The tests utilized multilevel
sampling wells which had to be de-
signed and installed carefully.
The authors describe the design and
construction of a multilevel sampling
well system for use with chemical trac-
ers in a variety of confined and uncon-
fined aquifers. The actual sampling sys-
tem is not perfected and should be
viewed as a prototype. However, it ap-
peared to work in a satisfactory manner
at the Mobile site. As shown in Figure 3,
the screened portions of these multi-
level observation wells are composed
of three-foot long slotted sections alter-
nating with seven-foot long solid sec-
tions.
As also shown in Figure 3, a two-inch
diameter PVC insert was constructed
with slotted and solid portions that
matched with those of the observation
well screen. The insert was designed to
hold any wires, tubing, or instrumenta-
tion that ultimately would be placed in
an observation well. Composed of
threaded ten-foot long sections, the in-
serts extended all the way to the land
surface. In order to isolate the various
sampling zones, inserts were fitted ex-
ternally with cylindrical annular inflat-
able packers.
After the required probes, tubing and
wires were placed within the inserts, the
sampling sections were isolated inter-
nally with silicone rubber plugs. The
complete insert was constructed on the
surface, then placed in the well, using a
crane, positioned and the packers in-
flated. After installation, each isolated
sampling zone appeared as shown in
Figure 3. A conductivity probe was
placed near the zone center, and two
lengths of vacuum tubing connected
the sampling zone to the surface. This
tubing could be used with peristaltic
pumps to mix the contents of the sam-
pling zone and to obtain ground water
samples for data on the arrival of tracers
used in the experiments to simulate
contaminant movement.
In the recent past, some hydrologists
advocated the use of single-well or two-
well tracer dispersion tests as a means
of measuring full-aquifer longitudinal
dispersivity. However, analyses of
single- and two-well tests of the Mobile
site and at the Borden site in Canada
(both with stratified aquifers) indicated
that if this is done, the resulting number
will have little physical meaning. In the
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Injection Well
(Source)
Withdrawal Well
(Sink)
Multi-Level
Observation
Well
Plan View
x/xxxxxx
xxXXXXXXXXXxxx
Vertical Section in x-z Plane
Figure 2. Two-well test geometry in a stratified aquifer.
case of single-well tests, the full aquifer
breakthrough curves measured in ob-
servation wells are determined mainly
by the hydraulic conductivity profile in
the region between the injection-
withdrawal well and an observation
well if the travel distance between the
injection-withdrawal well and the ob-
servation well is typical of most test
geometries. Thus, information about
the hydraulic conductivity profile is nec-
essary for meaningful test interpreta-
tions. The relative concentration versus
time data recorded at the injection-
withdrawal well itself is primarily a
measure of the combined local and
semi-local dispersion that has taken
place during the experiment. The ef-
fects of such dispersion depend in part
on the hydraulic conductivity distribu-
tion in the aquifer, and in part on the
size of the experiment. As the size of the
experiment increases, the effects of
local vertical dispersion will become
large compared to the effects of local
horizontal (radial) dispersion.
The two-well simulations of experi-
ments conducted at the Mobile site
show that the concentration versus time
breakthrough curve measured at the
withdrawal well would be very sensitive
to variations of the hydraulic conductiv-
ity in the vertical. Without the use of the
kind of multilevel observation wells
used in the test, little useful information
about the hydraulic or dispersive char-
acteristics of the aquifer (e.g., aquifer
stratification or values of local disper-
sivities) would be obtained. Factors
such as the length of the injection pe-
riod, the use of recirculation, and the
physical size of the experiment all have
strong effects on the breakthrough
curve measured at the withdrawal well
(Figure 4) making the interpretation of
field results difficult, especially with
conventional modeling approaches
(Figure 5). This can be addressed more
satisfactorily if aquifer stratification
(Figure 6) is measured and properly
taken into account (Figure 7).
Conclusions
Based on the above observations and
the large values for full-aquifer disper-
sivities that consistently result from cal-
ibrated areal ground water transport
models, the authors believe that the fol-
lowing working conclusions are war-
ranted:
I. Local longitudinal hydrodynamic
dispersion plays a relatively unim-
portant role in the transport of
contaminants in aquifers. Differ-
ential advection (shear flow) in the
horizontal direction is much more
important.
II. The concept of full-aquifer dis-
persivity commonly used in
vertically-averaged (areal) models
will not be applicable over dis-
tances of interest in most contam-
ination problems. If one has no
choice but to apply a full-aquifer
dispersion concept, the resulting
dispersivity will not represent a
physical property of the aquifer.
instead, it will be an ill-defined
quantity that will depend on the
size and type of experiment used
for its supposed measurement.
III. Because of conclusion II, it makes
no sense to perform tracer tests
aimed at measuring full-aquifer
dispersivity. If an areal model is
used, the modeler will end up ad-
justing the dispersivity during the
calibration process anyway, inde-
pendent of the measured value.
IV. When tracer tests are performed,
they should be aimed at determin-
ing the hydraulic conductivity dis-
tribution. The theoretical and ex-
perimental work presented in this
report indicate that the variation
of horizontal hydraulic conductiv-
ity with respect to vertical position
is a key aquifer property related to
the spreading of contaminants.
V. Two- and three-dimensional mod-
eling approaches should be uti-
lized which emphasize variable
advection rates in the horizontal
direction and hydrodynamic dis-
persion in the transverse direc-
tions, along with sorption and
microbial/chemical degradation.
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2" PVC Removable Insert
Aquifer
70ft.
(21 m)
\
t
I
TT
M
I!
m
is
' t
3'
T
J_
(0.9 1 mf
(2.1 m)
Vacuum ^
Tubing
Plug
Figure 3. Diagram of a completed multilevel sampling well. This and similar systems were
used at the Mobile site.
25.0
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20.0
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15.0
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5.0
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40
120 200 280 360 440 520 600 680 760
0 SO 160 240 320 400 480 560 640 720
Time (hrs)
Figure 4. Measured tracer concentration versus time in the withdrawal well during the
two-well test.
the authors are suggesting, therefore, is
that the time may have arrived to begin
changing from a homogeneous to a
vertically-stratified concept when deal-
ing with contaminant transport, realiz-
ing fully that such an approach will be
interim in nature and not totally correct.
Performance and simulation of several
single- and double-well tracer tests sug-
gests that the stratified approach is
much more compatible with valid phys-
ical concepts, and at least in some
cases, results in a mathematical model
that has a degree of true predictive
ability. Nevertheless, real-world appli-
cations will undoubtedly require cali-
bration, which in the approach recom-
mended here would involve varying the
hydraulic conductivity distribution
rather than the longitudinal dispersivity.
The benefit is that when calibrating with
an estimated hydraulic conductivity dis-
tribution, one is dealing with the physi-
cal property that probably dominates
the dispersion process, rather than
dealing with a fitting parameter that has
little, if any, physical relationship to the
problem.
The change from a vertically-
homogeneous to a vertically-stratified
approach will not be easy from a field
measurement viewpoint, nor will it be
inexpensive. One obvious implication
of this study is that until better field
characterization tools are made rou-
tinely available, any type of ground
water contamination analysis and recla-
mation plan will be difficult, expensive
and possibly unable to meet all of the
desired objectives in a reasonable time
frame.
VI. In order to handle the more
advection-dominated flow sys-
tems described in conclusion V,
one will have to utilize or develop
numerical algorithms that are
more accurate than those utilized
in the standard dispersion-
dominated models.
Summary
Much of contemporary modeling
technology related to contaminant
transport may be viewed as an attempt
to apply vertically homogeneous aqui-
fer concepts to real aquifers. Real
aquifers are not homogeneous, but they
are not perfectly stratified either. What
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Figure 5.
80 160 240 320 400 480 560 640 720
Time (hrs)
Calculated tracer concentration versus time in the withdrawal well based on an
assumed homogeneous, isotropic aquifer with no local dispersion (circles} shown
together with the results of the present two-well test (full line).
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40
42
44
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-§. 48
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52
54
56
58
• 1 .2 .3 .4 .5 .6 .7 .8 .9 1.
Figure 6. Normalized hydraulic conductivity distribution inferred from travel times measured
during the two-well test.
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Fred J. Molz, Oktay Guven, and Joel G. Melville are with Auburn University,
Auburn, AL 36849; and Joseph F. Keelyfformerly the EPA Project Officer, see
below for present contact) is with Oregon Graduate Center in Beaverton,
Oregon. The complete report, entitled "Performance and Analysis of Aquifer
Tests with Implications for Contaminant Transport Modeling," fOrder No. PB
86-219 086/A S; Cost: $11.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
Inquiries should be directed to:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
Ada, OK 74820
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-86/062
2863
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