EPA/600/A-97/019
FIELD ESTIMATION OF HYDRAULIC CONDUCTIVITY FOR
ASSESSMENTS OF NATURAL ATTENUATION
John T. Wilson, Jong S. Cho, and Frank P. Beck
National Risk Management Research Laboratory, U.S. EPA
R.S. Kerr Research Center, Ada, Oklahoma 74820
James A. Vardy
U.S. Coast Guard Support Center
Elizabeth City, North Carolina 27909
ABSTRACT: A Geoprobe is a sampling tool that drives hollow steel rods into the
earth to serve as a temporary ground water monitoring well. The rods are threaded
to allow them to be joined together, and the leading rod is slotted to admit the ground
water being sampled. A simple technique was developed by EPA staff that uses a
Geoprobe to estimate the hydraulic conductivity of the depth interval that provides
the water sample. The approach can be used where ground water can be sampled by
suction lift using a pump on the surface.
INTRODUCTION
Risk assessments of natural attenuation (intrinsic remediation) of organic
contaminants in ground water often require an accurate estimate of the residence
time of the contaminants along a flow path to a receptor. This is particularly true if
first-order rate constants for depletion of the contaminant are used to estimate
contaminant concentrations at the receptor or some point of compliance.
Traditionally, the time of travel from one monitoring location to another is inferred
from Darcv's law based on measured hydraulic gradients, the hydraulic conductivity
of the interval in the aquifer sampled by the monitoring wells, and an estimate of
effective porosity.
The Geoprobe and similar push technology is finding wide application as an
alternative to conventional wells. The hydraulic conductivity of the interval yielding
water to permanent monitoring wells can be estimated by pumping tests or slug tests
conducted in the well. However, no equivalent test exists for the Geoprobe or other
similar push technology.
MATERIALS AND METHODS
The test was developed using off-the-shelf Geoprobe tools and equipment.
Specific capacity refers to the flow of water yielded by a well at a particular
drawdown. The test is usually done by pumping from a well at a fixed rate and
monitoring the drop in the level of water in the well over time. We refer to the test
devised for the Geoprobe tools as an inverse specific capacity because the drawdown
is set at a predetermined level, and the yield at that predetermined level is measured.

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FIGURE 1. A test to measure the specific capacity of a Geoprobe well.
The tests were conducted as follows. A Geoprobe screened rod was driven
to the depth to be sample. The rods were screened over an interval of 1.5 feet (45
cm) with 0.020 inch (0.51 mm) slots. A polyethylene tube was inserted to the
bottom of the rods and pumped to remove all the sediment from the interior of the
rods. Occasionally sediment entered the screen when the drawdown in the well
exceeded 1.0 feet (30 cm). To remedy this, distilled water was poured into the rods
during pumping of the sediment to prevent excessive drawdown. The water level
inside the Geoprobe was allowed to come to equilibrium. A polyethylene tube was
inserted in the well with the tip at an elevation of 1.0 foot (30 cm) or 0.5 foot (15
cm) below the static water level. Water was pumped from the tube at a rate that
produced both water and air. This poised the level of water in the Geoprobe rods at
the predetermined level. The well was pumped until the flow rate came to
equilibrium, the time required to collect 100 ml was measured. If the yield was very
slow, the yield in five minutes was measured. Inverse specific capacity was

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calculated in milliliter per second per centimeter of drawdown. The specific
capacity was multiplied by an empirical calibration factor of 0.03 to estimate
hydraulic conductivity in centimeters per second. After the test for inverse specific
capacity, the tube was lowered and ground water was sampled for routine
parameters (FIGURE 1).
RESULTS AND DISCUSSION
Reproducibility. The reproducibility of the Geoprobe specific capacity test was
evaluated at a site on the North Beach area of the U.S. Coast Guard Support Center
at Elizabeth City, NC.
Log Specific Capacity (ml/sec.cm)
-6 -5 -4 -3 -2 -1 0
0
5
10
15
Depth
(feet) 20
25
30
35
FIGURE 2. Reproducibility of the test for specific capacity using a Geoprobe.
The diamonds are the logarithmic means of three independent tests. The bars
are the standard deviation of the logarithms of the independent tests.
At the test site, the shallow sediments are clay and silt, transitioning to silty
fine sand and fine sand at 15 feet (4.6 m) and back to silt at 25 feet (7.6 m). The
water table was 4.0 feet (122 cm) below the surface in clay and silt. FIGURE 2 plots
the mean of three tests conducted at thirteen separate depth intervals extending from
the water table to the bottom of the first semi-confined aquifer. As expected, the
specific capacity is low in the clay and silt extending to 15 feet (4.6 m) in depth, it
is about two orders of magnitude higher in the silty fine sand and fine sand, and
between one and two orders of magnitude lower in the deeper silty layer.



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FIGURE 2. plots the standard deviation of the common logarithm of the
means. In the interval of silty fine sand to fine sand, the widest standard deviation
corresponds to a factor of 1,8, and the average of nine standard deviations
corresponds to a factor of 1,4. The standard deviations are much wider in samples
across the transition zones at 15 feet (4.6 m) and 26 feet (7.9 m). This may reflect
natural heterogeneity in the aquifer, or more likely, error in the vertical position of
the Geoprobe screen. In uniform material with specific capacities ranging from
0.000366 ml/sec.cm up to 0.232 ml/sec.cm, the standard deviation of tests in general
corresponded to a factor of 2.0 or less. However, the standard deviation of the tests
conducted at 12.5 feet (3.8 m) increased to a factor of 5.4. The specific capacity of
this material was 0.0000237 ml/sec.cm. Apparently 0.000366 ml/sec.cm is the
effective lower limit for reproducibility in the test. As discussed below, this
corresponds to a hydraulic conductivity of 0.00001 cm/sec.
Calibration. The specific capacity of the Geoprobe wells was calibrated by
comparing the specific capacity to the hydraulic conductivity of a conventional
monitoring well 2.0 inches (5.1 cm) in diameter, or to the hydraulic conductivity of
a core sample subjected to a permeameter test (TABLE 1).
TABLE 1. Empirical calibration factors that estimate hydraulic
conductivity from the specific capacity of the Geoprobe well.
Location
Method
Hydraulic
Conductivity
(cm/sec)
Specific
Capacity
(ml/cm.sec)
Calibration
Factor
Elizabeth
City, NC
Pumping test
in 2 inch well
0.0246
0.70
0.035
Elizabeth
City, NC
Pumping test
in 2 inch well
0.00244
0.081
0.030
Eglin AFB
Florida
Slug test in 2
inch well
0.036
0.32
0.11
Eglin AFB
Florida
Permeameter
test on core
0.015
0.35
0.043
Plattsburgh
AFB, NY
Permeameter
test on core
0.0089
0.34
0.026
Pontotoc Co,
OK
Permeameter
test on core
0.0078
0.40
0.020
Pontotoc Co,
OK
Permeameter
test on core
0.000018
0.0044
0.004

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The calibrations at two sites at Elizabeth City, NC were conducted by
determining the average specific capacity of Geoprobe samples extending across the
interval sampled by an adjacent permanent monitoring well. Data for a well at the
U.S. Coast Guard Support Center in Elizabeth City, NC are illustrated in FIGURE
3. When a particular interval was not sampled by the Geoprobe, the specific capacity
was estimated by linear interpolation from the adjacent samples. The permanent
wells were 2.0 inches (5.1 cm) in diameter. They were pumped at a rate of 1.0 to 2.0
gallons (3.8 to 7.5 liters) per minute for twenty to thirty minutes. Drawdown in the
permanent well was used to estimate transmissivity using the equation of Jacob as
described in Appendix 16.D, page 1021 of Driscoll (1986). The calibration on
material from Eglin AFB, FL was conducted by comparing the specific capacity of
the Geoprobe wells to a permanent monitoring well that was 2.0 inches in diameter
(5.1 cm) with a 5.0 foot (152 cm) screen. The 1.5 feet (45 cm) of screen in the
Geoprobe rod was located at a depth adjacent to the center of the screen of the
permanent monitoring well.
Core samples were selected from sites that were known to be uniform over
the vertical interval sampled by the core. The calibrations on material from
Pittsburgh AFB, NY and Eglin AFB, FL were conducted by packing core material
from the same depth interval into a laboratory permeameter. The calibrations on
material from Pontotoc Co. OK were conducted by collecting a core in plastic sleeve
that was 1.5 inches (3.8 cm) in diameter, then conducting a permeameter test in the
field. The elevation of the cored interval corresponded within 1.0 inch (2.5 cm) with
the interval sampled by the Geoprobe.
In general the agreement between the empirical calibrations as listed in
TABLE 1 is good. Five of the calibrations involving two permanent monitoring
wells and three core samples produced empirical calibration factors that varied over
a small range, from 0.043 to 0.020. The hydraulic conductivity of these materials
varied from 0.0246 to 0.00244 cm/sec. At Eglin AFB, the Geoprobe yielded less
water than was expected from a slug test on the neighboring permanent monitoring
well. This may have resulted from the differences in the screened intervals. In the
less permeable material from Pontotoc Co, OK, the Geoprobe well yielded ten times
more water than would be expected from the permeameter test on the core sample.
The result is outside the expected standard deviations as determined in FIGURE 2.
The lower range for effective calibration is probably 0.0001 cm/sec. Below
that range the estimates should be considered accurate only within an order of
magnitude. The upper limit for effective calibration is controlled by the rate at which
the pump can pump water and air. The Masterflex pumps used at Kerr Research
Center have a maximum rate of about 300 ml/minute. The lowest imposed
drawdown that can be accurately measured is about 1.0 inch (2.5 cm), making the
upper limit that can be measured about 0.1 cm/sec.

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-25 "
Hydraulic Conductivity (cm/sec)
0.001	0.002 0.003
^sMonitoring
Well
0.004
Geoprobe
28CPT-J
FIGURE 3. Correlation between hydraulic conductivity as determined by a
pumping test in a permanent monitoring well and the specific capacity test in
temporary Geoprobe well.
DISCLAIMER
The views expressed in this abstract are those of the individual authors and
do not necessarily reflect the views and policies of the U.S. Environmental
Protection Agency (EPA). Scientists in EPA's Office of Research and Development
have prepared the EPA sections, and these sections have been reviewed in
accordance with EPA's peer review and administrative review policies and apposed
for presentation and publication. Mention of trade names or commercial products
does not constitute endorsement or recommendation for use.
ACKNOWLEDGMENTS
We thank David Ariail of EPA Region 4 and Barbara Wilson of the R.S. Ken-
Research Center for substantive help in experimental design and data collection.
REFERENCES
Driscoll, F.G. 1986. Groundwater and Wells: Second Edition. Johnson Division, St.
Paul, Minnesota 55112.

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TECHNICAL REPORT DATA
(PSecze read Instructions an the reverse before cample.
1, REPORT NO. 2
EPA/600/A-97/019
3.
4. TITLE AND SUBTITLE
Field Estimation of Hydraulic Conductivity for
Assesments of Natural Attenuation
I
5. REPORT OATS
6. PERFORMING ORGANIZATION COOS
7. AUTHOR(Sl
John T. Wilson, Jong S. Cho, and Frank P. Beck
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION name ano aooress
U.S. EPA, National Risk Management Laboratory
Subsurface Protection and Remediation Division
P.O. Box 1198
Ada, Oklahoma 74820
10. PROGRAM ELEMENT NO.
1 J. CONTRACT/GRANT NO.
In-House RPJW9
12. SPONSORING AGENCY NAME ANO AOORESS
U.S. EPA
NRMRL, SPRD
P.O. Box 1198
Ada, Oklahoma 74820
13. TYPE OF REPORT ANO PERIOD COVERED
Symposium Paper
14. SPONSORING AGENCY CODE
FP4 /Ann /i q
ts. supplementary notes
To be published in proceedings for the Fourth International Symposium On
In Situ and On-Site Bioremediation.
16. ABSTRACT
A Geoprobe is a sampling tool that drives hollow steel rods into the
earth to serve as a temporary ground water monitoring well. The rods are threaded
to allow them to be joined together, and the leading rod is slotted to admit the ground
water being sampled. A simple technique was developed by EPA staff chat uses a
Geoprobe to estimate the hydraulic conductivity of the depth interval that provides
the water sample. The approach can be used where ground water can be sampled by
suction lift using a pump on the surface.
1?. KEY WORDS ANO DOCUMENT ANALYSIS
a- DESCRIPTORS
b.lOENTIFIERS/OPEN ENO60 TERMS
c. COSATl Field. Croup



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