United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas NV 89114
Research and Development
EPA/600/S2-86/111 Apr. 1987
Project Summary
-AM
Borehole Sensing Methods for
Ground-Water Investigations at
Hazardous Waste Sites
Stephen W. Wheatcraft, Kendrick C. Taylor, John W. Hess, and
Thomas M. Morris
The complex nature of the ground-
water contamination problem requires
the collection of extensive amounts of
data in order to understand the problem
well enough to recommend and execute
the appropriate remedial action. As the
complexity and consequences of ground-
water contamination increase, geo-
physical methods are becoming a cost-
effective approach to providing answers
to hydrogeologic questions associated
with ground-water contamination.
Geophysical methods applicable to
hazardous waste site investigations can
be broken down into two categories:
surface and subsurface methods. Sur-
face methods offer the advantage of
relatively little capital investment at the
site (no borehole is required), and rapid
collection of data over a horizontal area.
However, the interpretation is often
ambiguous and limited in vertical reso-
lution. Subsurface methods require a
borehole and can only investigate an
area immediately around the borehole.
However, subsurface methods provide
excellent information and resolution for
vertical changes in measured param-
eters. Also, a synergistic effect is
achieved when certain logs run together,
potentially providing unambiguous in-
terpretation of hydrogeologic param-
eters, especially in the vertical
dimension.
This Project Summary was developed
by EPA's Environmental Monitoring Sys-
tems Laboratory, Las Vegas, NV, to
announce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
The complex nature of the ground-
water contamination problem requires
the collection of extensive amounts of
data in order to understand the problem
well enough to recommend and execute
the appropriate remedial action. Because
it is nearly impossible to collect adequate
amounts of data using traditional hy-
drogeologic methods, new methods must
be developed.
Geophysical methods have been widely
used in oil and mineral exploration since
the 1920's. However, due to their cost
and the relative simplicity of most pre-
vious ground-water problems, geophysical
methods have not commonly been used
for ground-water investigations. As the
complexity and consequences of ground-
water contamination increase, geophysics
is becoming a more cost-effective ap-
proach to answer the hydrologic ques-
tions associated with ground-water
contamination.
Geophysical methods applicable to
hazardous waste site investigations are
of two types: surface and subsurface
methods. Surface methods offer the ad-
vantages of relatively little capital in-
vestment at the site (no borehole is
required) and rapid collection of data over
a horizontal area. However, interpretation
is often ambiguous and limited in vertical
resolution. Subsurface methods require
a borehole and can only be used to in-
vestigate an area immediately around
the borehole. However, these methods
provide excellent information on vertical
changes in measured parameters. A suite
of complementary logs has the potential
to provide unambiguous interpretation of
hydrogeologic data, especially in the
vertical dimension.
-------
The two approaches complement each
other very well. The subsurface methods
provide the necessary vertical detail for a
small area, and the surface methods ex-
tend this detail horizontally between
boreholes. In this research effort, prob-
lems of site characterization, contaminant
plume detection and monitoring of con-
taminant plumes are addressed using
borehole geophysics.
Our primary research effort concen-
trated on evaluating and selecting a suite
of borehole sensing tools and to design
an integrated interpretation strategy for
the use of these tools for ground-water
investigations at hazardous waste sites.
These techniques are meant to be used
in conjunction with surface geophysical
methods; the down hole methods pro-
viding vertical resolution, and the surface
methods extending the information
horizontally.
Borehole Sensing Methods
Borehole methods fall into five major
categories: acoustical, electromagnetic,
nuclear, flow and dimension, and thermal.
Major applications of these techniques
include: lithologic correlation, lithology,
rock density, fractures, porosity, per-
meability, flow, water level, water quality,
temperature gradient and hole diameter.
Table 1 is a summary of borehole objec-
tives and the methods used to achieve
them.
Hardware for borehole geophysical
logging consists of similar basic compo-
nents for all the different tools, consisting
of sensor, signal conditioners, and a
recorder. The sensor or sonde receives
power and transmits the signal to the
surface through a conducting cable, which
also serves to position the tool in the hole
by means of a winch. Electronic controls
at the surface regulate logging speed and
direction, power to the downhole elec-
tronics, signal conditioning, and recorder
responses. The return signal from the
probe is a function of lithologic, fluid, and
borehole parameters and is recorded and
analyzed later with a computer.
Limitations of Borehole Methods
for Hydrogeologic Hazardous
Waste Site Investigations
Borehole logging methods have been
developed primarily by and for the petro-
leum industry. Logging tools are designed
to be used in uncased, large diameter,
deep holes. Several logging tools are
usually attached to one downhole sonde
that can be as much as 5-m in length.
Tab/0 1.
Objective
Borehole Sensing Methods
Borehole Methods
Location of Zones of Saturation
Physical and Chemical Characteristics
of Fluids
Stratigraphy and Porosity
Flow and Direction
Electric log
Temperature log
Neutron log
Gamma-gamma log
Electric log
Temperature log
Fluid conductivity log
Spontaneous potential log
Specific ion electrodes
Fiber optics
D.O., Eh, pH probes
Formation resistivity log
Induced polarization log
Natural gamma log
Spectral gamma log
Thermal neutron log
Cross borehole radar
Cross borehole shear
Resistance log
Acoustic -
Transit time log
Acoustic -
Wave form log
Neutron log
Induction log
Spontaneous potential log
Flow meter
Tracer
Differential temperature log
Water level
Interpretation schemes have traditionally
been used to obtain subsurface data of in-
terest in petroleum reservoir engineering.
The typical borehole at (or near) a
hazardous waste site is shallow (probably
less than 100-m), narrow diameter (5-
cm) and cased, usually with polyvinyl
chloride (PVC), Teflon (TM), or some other
plastic. None of the borehole tools
designed for the petroleum industry are
usable in such an environment. A 5-m-
long downhole sonde could barely fit into
a 50-m-deep hole, even if the hole dia-
meter was large enough to accept the
sonde. None of the open-hole logging
tools (such as electric logging) can be
used in the PVC cased holes. Because
most downhole tools are designed for
high-temperature, high-pressure environ-
ments, they would be over-designed for
the typical shallow monitoring well
around hazardous waste sites. Moreover,
in monitoring wells near hazardous waste
sites, the tools may be subjected to
hazardous chemical environments that
they are not designed to withstand.
The interpretation schemes developed
for the petroleum industry are designed
to remove effects of drilling fluid from the
data. Logging is normally done before, or
just after, hole completion, and holes are
almost never relogged, especially after
casing has been set. For hazardous waste
site investigations, borehole logging is
commonly done after PVC casing has
been set, and it is desirable to relog holes
regularly to monitor for changes in
formation-fluid chemistry and ground-
water velocity.
The borehole logging parameters that
are of interest to the hydrogeologist
investigating ground-water contamina-
tion are quite different from the param-
eters commonly sought by the petroleum
reservoir engineer. As a result of the
above considerations, it is of primary
importance to develop a new borehole
logging strategy that is designed to pro-
vide the information sought by the hy-
drogeologist for hazardous waste site
investigations. Table 2 summarizes the
kinds of environments in which various
types of logging tools are used.
Borehole Logging Interpretation
Strategy For Hydrogeologists
The vertical variation in hydraulic pa-
rameters within an aquifer is recognized
-------
Table 2.
Borehole Sensing Techniques Applicable To Various Borehole Environments
Logging Techniques
Single Well
Cross Borehole
In Situ
PVC*
Cased
Steel
Cased
PVC"
Cgsed
Steel
Cased
Uncased
PVC*
Cased
Steel
Cased
Uncased
WET DRY WET DRY WET DRY WET DRY WET DRY WET DRY WET DRY WET DRY WET DRY
ACOUSTIC
ELECTRIC
INDUCTION
NUCLEAR
FLOW
TEMPERATURE
CHEMICAL
•
• • •
•
•
•
•
• • • •
• • • • •
•
• •••••
• • •
• • •
"Note. PVC is shown but other plastic casings fie, teflon) behave similarly.
to be of primary importance in deter-
mining the fate and transport of con-
taminants in ground-water systems.
Traditionally, the process of hydrodynamic
dispersion has been thought to be the
dominant process causing contaminant
mixing. Macroscale heterogeneity and
vertical stratification induce large varia-
tions in the advective flow rate of the
groundwater. This process has been
termed macroscopic dispersion, and it is
the dominant mechanism controlling
contaminant mixing and transport in
many aquifers.
Largely because of macroscopic dis-
persion, traditional ground-water flow
equations are inadequate to describe
contaminant transport in aquifers. Al-
though it is important to account for
vertical variation in hydraulic parameters,
there has been little effort to develop
adequate borehole methods that would
provide such parameters.
If borehole methods are to be of use for
hydrogeologists, it is essential that they
answer questions of hydrologic signific-
ance. In particular the strategy outlined
in this report describes how the following
parameters vary with depth: porosity;
hydraulic conductivity; lithology; ground-
water velocity; cation exchange capacity
of the formation; and electrical conduc-
tivity of the pore fluid.
Hazardous waste sites are located in
every conceivable geologic setting. Each
one is unique and relationships developed
for one site cannot be considered valid
elsewhere. It is essential that relation-
ships used in interpretations be based on
data collected at the site under study. To
do this, it is necessary to drill a char-
acterization hole at each site.
The characterization hole should be
drilled with a technique that allows good
core samples to be taken. These cores
will be analyzed for lithology, hydraulic
conductivity and cation exchange capacity.
This information will be combined with
the well logs of the hole to provide the
necessary site specific relationships for
interpretation of the other wells from
which cores are not available. Although
the characterization well does not provide
an absolute calibration of the logging
tools, it permits the tool response to be
related to the local conditions.
The interpretation strategy combines
geophysical information from the well
logs and geologic information from the
characterization well to answer the hy-
drologic questions of interest. Figure 1
shows a block diagram of the strategy.
This strategy assumes that the site
specific relationships obtained from the
calibration well hold throughout the site.
Although different relationships could be
developed for different formations it is
assumed that these relationships are valid
throughout the formation for which they
were developed. In unusual cases, it is
possible that the presence of the con-
taminant could alter these relationships
and invalidate the interpretation. Because
these relationships are based on fairly
simple physical and chemical principles,
a review of the literature, along with an
understanding of the mechanisms in-
volved, may make it possible to identify
conditions where the contaminant might
be altering the relationships used in the
interpretation.
Use of a Borehole Thermal Flow
Meter For Determination of
Ground-Water Velocity and
Hydraulic Conductivity
The traditional way of determining
ground-water velocity is to calculate it
using Darcy's Law and regional or local
piezometric head gradient information.
This is an indirect measurement, and
does not take into account velocity varia-
tions in the vertical dimension. A more
desirable method to obtain velocity in-
formation in principle would be to directly
measure it in a borehole. One way of
doing this is with a thermal ground-
water flow meter.*
The probe itself consists of a central
heat source surrounded by five pairs of
thermistors. The basic principle of opera-
tion is that the central heat source gen-
erates a pulse of heat energy. This pulse
diffuses radially from the center of the
probe by heat diffusion and is advected
by the ambient groundwater. The direc-
tion and relative magnitude of the advec-
tive ground-water velocity can be
determined by measuring the temperature
difference between opposite pairs of
thermistors (see Figure 2).
The flow meter is 4.4-cm in diameter
and can be used with a simple end cap
packed with glass beads in a 5-cm well.
The glass beads are packed around the
thermistors and heat source in order to
minimize heat convection and to ensure
a continuous porous medium from the
aquifer into the borehole for more ac-
curate velocity measurements. A diagram
of the 5-cm end cap and flow meter
probe is shown in Figure 3.
This particular model of the flow meter
is designed to be used primarily with 5-
cm well casing, but the manufacturer
provides two different packer configura-
tions to allow the flow meter to be used
in 10-cm well casings. These two packers
are shown in Figure 4. The first is a
pneumatic packer, consisting of inflatable
tubes above and below the thermistor
* It would be more appropriate to call the instrument
a ground-water velocity meter, but the term flow
meter is in widespread use, so we use the same
terminology
-------
1
Illlllllll l.\
Gamma rtM_
Density <
1
1
1
Natural ^
Gamma , \
TTT7~r / rTT\
\
,,„,,„,.!
[Induction L }
''''"''' i
j 1 1 1 1 1 1 1 1 1 \
"""•" ,
l
i
Matrix
Density
\
Porosity L» Site Specific _^
vs. Porosity Relationship
Site Specific — J Lithology \
Litnology
Cross Plot
Site Specific Cation
vs. CEC Relationship Capacity
. L . ..
k 1 Conductivity Model
j (Waxman/Smits)
Selection of
i Perforated Zone
T
Ground- Water
* velocity
(Flow Meter)
Regional
Gradient
1
Hydraulic 1 ^1 Parry'? 1 ».
Conductivity \ \ Law \
1 i_ _ r
Site Specific i Effective '
Total vs. Effective ~*]_ Porosity j
Porosity Relation
1 ^ Pore Fluid I
j Conductivity |
i
1
1
j
t
Ground- Water
Velocity
Indication of
Anisotropy
~~~~~~ Desired Information
' Empirical Relation
From Literature
Site Specific Data
Hydrologic Judgment
Field Data
Figure 1. Interpretation strategy.
No Flow Condition
Flow Condition
Direct/on
/max
Thermistors
Heat Source
Figure 2. Operating principle of the Boreho/d Thermal Flow Meter (from KV-Associates).
array. A nylon mesh sock is installed
between the tubes which contains the
glass beads and thermistor array. This
sock expands outward to grip the inner
sides of the well casing when the tubes
are inflated, in theory providing a con-
tinuous porous medium in the borehole,
similar to the 5-cm end cap arrangement.
The second packer shown in Figure 4
is somewhat simpler in design than the
pneumatic packer. It is referred to as the
"fuzzy packer," and consists of a simple
cylinder with an outside diameter equal
to the inside diameter (or slightly less) ol
a 10-cm well casing. The fuzzy packer is
filled with glass beads and the probe is
screwed into it by means of an adapter.
The fuzzy packer must fit into the well
casing very tightly in order to achieve the
continuous porous medium arrangement
of the 5-cm end cap and the pneumatic
packer.
In practice, the probe is lowered down
the borehole to the level at which the
ground-water velocity is to be measured
opposite a screened or slotted section.
The submerged probe creates a short
duration point source of heat. After a
period of time, the relative thermal dif-
ferences between each of the five pairs
of thermistors are displayed using a rotary
switch which selects the pairs to be read.
4
-------
o
D
Probe
Pour in Sand
Figure 3.
End Cap
Diagram of the 5-cm end cap
and probe
The information is then used to calculate
the ground-water speed and direction.
Laboratory attempts to calibrate the
instrument for velocity, including a
specially designed sandbox, were not en-
tirely successful. The data generated
clearly indicated that the flow meter using
the fuzzy packer is inaccurate for velocity
as well as direction and should not be
used.
Because of poor calibration compari-
sons in the laboratory experiments, study
was initiated to develop a way to directly
measure velocity magnitude. Equations
were developed that express aquifer fluid
velocity as a function of the fluid velocity
in the borehole packer, hydraulic con-
ductivity and porosity of the borehole
packer and the hydraulic conductivity of
the aquifer. With some modification to
the thermal flow meter, it is theoretically
possible to directly measure these pa-
rameters. Therefore, the aquifer fluid
velocity can be directly calculated, thus
eliminating the need for the questionable
calibration procedure. Although these
equations are theoretically correct, they
are untested. Considerable experimental
work will be necessary to determine how
Pneumatic Packer
Tube to Air Pump
Exterior
Elastic
Bag
Expansion
Bladder
Stiffening
Ribs
\
\
"Fuzzy" Packer
Thermal
Flow
Meter
Interior Fine Mesh
Bag Filled with
Glass Beads
— 4" Well Screen
— Wire Mesh Screen
Fuzzy Material
(Similar to that
on a Paint Roller)
Inflation Unit
Figure 4. Diagram of the packers for the 10-cm borehole.
well they work with the thermal flow
meter.
The most significant result of the
theoretical work is that the hydraulic
conductivity of the aquifer can be cal-
culated with the thermal flow meter. This
method requires two measurements to
be taken with the flow meter, with two
different packers of different hydraulic
conductivity. These two measurements
provide two equations and two unknowns,
the aquifer hydraulic conductivity and the
aquifer specific discharge. If the aquifer
porosity is known from other borehole
logs, then the aquifer fluid velocity can be
calculated as a function of aquifer depth.
Conclusions
Severe limitations exist for using tradi-
tional borehole interpretation methods
and tools. These methods and tools have
been developed for petroleum industry
applications. In the petroleum industry,
the boreholes typically are deep, uncased,
with a large diameter. Several tools are
attached to one sonde, which may be 5-m
long. The formations to be evaluated are
primarily lithified sequences of sandstone,
shale and limestone. For the typical
hazardous waste site, the boreholes are
shallow, cased and small diameter. The
tools must be capable of fitting down
5-cm boreholes and be attached to short
sondes to allow complete borehole pene-
tration. The formations in and around
hazardous waste sites are typically un-
consolidated, heterogeneous alluvial
material.
An interpretation strategy is proposed
which has, as input, gamma density,
natural gamma, induction, televiewer and
horizontal borehole flow meter. Using
these tools together with selected site
specific input, the following hydrogeologic
parameters can be determined as a func-
tion of depth: effective porosity, hydraulic
conductivity tensor, ground-water velo-
city, and pore fluid conductivity.
Laboratory testing of the thermal
ground-water flow meter shows that
directional accuracy is acceptable in a
5-cm borehole for ground-water velocities
greater than 0.5 m/d. Experimental re-
sults show that laboratory calibration of
instrument readout to ground-water
velocity is highly questionable. However,
theoretical work demonstrates that the
laboratory calibration may not be neces-
sary. Theory is developed that allows
direct calculation of both ground-water
specific discharge and aquifer hydraulic
conductivity. The instrument will require
modification for this procedure, but the
method shows great promise.
-------
S. W. Wheatcraft, K. C. Taylor. J. W. Hess, and T. M. Morris are with the
Desert Research Institute, University of Nevada System. Reno, NV 89506.
Leslie G. McMillion is the EPA Project Officer (see below).
The complete report, entitled "Borehole Sensing Methods for Ground-Water
Investigations at Hazardous Waste Sites, "(Order No. PB87-132 783/AS; Cost:
$13.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89114
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-86/111
0000329 PS
U S ENVIR PROTECTION AGENCY
REGION 5 LIBRARY
230 S DEARBORN STREET
CHICAGO IL 60604
------- |