United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Ada, OK 74820
Research and Development
EPA/60Q/SR-98/Q58
August 1998
d
Steven C. Young, Hank E. Julian, Hubert S. Pearson,
Fred J. Molz and Gerald K. Boman
Interpretation and prediction of
contaminant transport in the saturated
zone requires knowledge of the hydraulic
conductivity distribution in granular
media and flowpath distribution in
fractured media. Spatial variability of
saturated zone hydraulic properties has
important implications with regard to
design of monitoring wells for sampling
water quality parameters, use of con-
ventional methods to estimate trans-
missivity, and remedial system design.
Characterization of subsurface hetero-
geneity requires an effective technique
for measuring spatial variations in
physical properties. Sensitive vertical-
component borehole flowmeters are
effective tools for measuring vertical
variations in ground-water flow within a
well or borehole and evaluating hydro-
stratigraphy from these data. This report
describes an electromagnetic (EM)
borehole flowmeter, developed by the
Tennessee Valley Authority (TV A), which
is based on Faraday's law of induction
and produces a voltage proportional to
the velocity of water passing through the
central cylindrical channel of the meter.
The threshold velocity for a prototype
instrument is less than 8.8 +/- 0.9 cm/
min. This meter has the sensitivity,
precision, and physical dimensions
necessary for application in many
hydrogeologic settings. The report
describes methodology and application
of the EM flowmeter to the characteri-
zation of the vertical distribution of
ground-water flow to a well and estima-
tion of hydraulic conductivity distribution
from this information under appropriate
conditions.
This Project Summary was developed
by EPA's National Risk Management
Research Laboratory's Subsurface
Protection and Remediation Division,
Ada, OK, to announce key findings of the
research project that is fully documented
in a separate report of the same title
Project Report ordering information at
back),
Introduction
Underestimation of subsurface hetero-
geneity may significantly contribute to
improper design and, consequently,
inadequate performance of many remedi-
ation systems. Characterization of hydraulic
structure requires an effective method for
measuring vertical variation in hydraulic
properties. Alternative methods for
measuring vertical variation of hydraulic
conductivity in the saturated zone include
small-scale tracertests, multi-level slug tests,
laboratory permeameter tests, equations
based on grain-size distributions, and
borehole flowmetertests. Of these methods,
the borehole flowmeter offers one of the
more direct and versatile techniques for
estimating variation in subsurface hydraulic
properties.
Various types of flowmeters have been
developed and a few ground-water appli-
cations have been reported. Impellermeters
have been used for several decades in the
petroleum industry. However, historical
limitation to widespread use of borehole
flowmeters in the ground-water/environ-
mental area has been the relative lack of
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commercially available instruments of
sufficient sensitivity and precision. Within
the last ten years, three different types of
flowmeters have been developed and used
in such applications. These meters have
been based on improved impeller, heat-
pulse, and electromagnetic technologies.
An EM flowmeter with high sensitivity has
been developed based on Faraday's law of
induction by the TVA Engineering Laboratory
in Norris, Tennessee. This vertical-
component flowmeter can operate in both
the high and low flow rate ranges required
for many ground-water studies and has a
durable construction without any moving
parts. Commercialization of this technology
has recently been completed.
Objectives of this project included
refinement of a compact, reliable, and
versatile EM borehole flowmeter; develop-
ment of techniques for meter application
and data analysis; and application of the
meter in various hydrogeologic settings.
The reportdescribesthe operation and utility
of the prototype EM borehole flowmeter,
including theory, design, calibration, basic
field applications, data analysis procedures,
and potential effects of various well
construction and development procedures
on flowmeterdata. In addition, case studies
describing test objectives, designs, and
results are also discussed. Information in
the report is of interest to investigators
planning hydrogeologic characterization
studies.
Electromagnetic
Flowmeter
The flowmeter (Figure 1) consists of an
electromagnet and a pair of electrodes
mounted 180° apart at right angles to pole
pieces of the magnet and cast in a durable
epoxy. The epoxy is molded in a cylindrical
shape to minimize turbulence associated
with channeling water through the hollow
core of the meter. The flowmeter operates
accordingto Faraday's lawof induction which
states that the voltage induced across a
conductor moving at right angles through a
magnetic field is directly proportional to the
velocity of the conductor. The electromagnet
creates a strong magnetic field across the
flow passage. As water (electrical conductor)
flows through the magneticfield, avoltage is
generated which is proportional to average
water velocity across the magnetic field.
Induced voltage across the electrodes is
measured by the electronics package.
Polarity of the generated voltage is
dependent on direction of flow.
The prototype EM flowmeters described
in the report are 30-cm long with an outer
diameter of 4.8 cm and 1.27-cm or2.54-cm
inner diameters (ID). The 1.27-cm and
2.54-cm ID flowmeters are typically used to
measure low flow rates and high flow rates,
respectively. The threshold average
discharge velocity for the 1.27-cm ID
flowmeter is less than 8.8 +/- 0.9 cm/min
which translates to a flow rate of approx-
imately 10 ml/min and is measured with a
precision of approximately 10%. Linear
response ranges of the 1.27-cm ID and
2.54-cm ID meters are approximately 30 ml/
min to 10 l/min and 100 ml/min to over 40 I/
min, respectively. Both flowmeters are
designed to operate under a maximum
hydraulic head not exceeding 600 m.
In wells or boreholes with relatively large
diameters, sensitivity of the flowmeter
diminishes due to increased flow around the
meter which is not detectable using a probe
of this design. In order to direct flow through
the inside of the flowmeter in a large-diameter
well, a packer assembly is used. Both a
mechanical collar consisting of a rubber
gasket held between two plexiglass rings
and an inflatable packer have been
30cm
Magnetic Coil
Iron Core
4.8cm
Figure 1, Schematic diagram of the
Tennessee Valley Authority
electromagnetic borehole
flowmeter.
developed for use in wells with diameters of
approximately 20 cm or less.
Above-ground electronics include the
electromagnet drive, circuitry to measure
voltage generated by flow through the meter,
andcomputerhardware/software. Thesignal
that is produced is in the microvolt range
and will typically be several orders of
magnitude less than background noise. Due
to the high noise level, synchronous
demodulation is used to extract the signal.
With additional amplification and filtering, a
direct current signal proportional to water
velocity through the flowmeter is generated.
The electronics collect and process signals
from the flowmeter every second. Attheend
of a pre-set time interval or upon keyboard
command, signals are averaged and the
standard deviation is calculated. This
average flow rate and standard deviation
are displayed, stored on disk, and printed.
Collection and Analysis of
Flowmeter Logs
Borehole flowmeter tests using a tool of
this design focus on measuring the vertical
distribution of horizontal flow into a
screened well or open hole. Such tests
are useful for characterizing both granular
and fractured media. Data obtained in the
field are vertical ground-water flow rates
measured at selected elevationswithinthe
well or borehole. Measurements are
typically performed underambient(natural)
and induced flow (pumping) conditions.
Flow profiles obtained under ambient
conditions provide information on the
magnitude and direction of the vertical
hydraulicgradient. Flow profiles measured
during ground-water extraction show the
proportion of total discharge provided by
each monitored interval. If certain
conditions are met, these data may be
used to estimate relative differences in
hydraulic conductivity of selected aquifer
intervals in granularmaterials. Flowprofiles
in fractured rock provide information
regarding the locations of hydraulically
active features.
Single well tests may be performed as
illustrated in Figure 2. First, a caliper log is
recorded or examined to ascertain that the
borehole/screen diameter is constant. If it is
not constant, variations must be taken into
account during data analysis. Flowmeter
measurements are then recorded atselected
intervals prior to pumping to measure any
vertical water movement within the well under
ambient conditions. Following recording of
the ambient flow distribution, ground water
is pumped from the well at a constant rate.
After a stable flow condition is achieved, the
flowmeter is lowered to the bottom of the
well and used to measure cumulative
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discharge at positions where ambient flow
rates were measured. This results in a
series of data points describing cumulative
discharge from the aquifer as a function of
depth (Figure 3). Such tests may be
performed in confined and unconfined
aquifers. Alternative test designs involving
injection of water instead of ground-water
extraction and monitoring flow distribution
in an observation well during ground-water
extraction from another well are also
possible.
Well Construction and
Development
Studies were conducted to investigate
effects of various well construction and
development methods on flow distributions.
In an investigation performed near
Columbus, Mississippi, three well installa-
tion techniques (i.e., a modified rotary wash
method/installation using a natural filter
pack, use of a 19.4-cm hollow-stem auger/
installation using a natural filter pack, and
use of a 27.0-cm hollow-stem auger/
installation using an artificial filter pack)
were evaluated. A second study of well
development effects was conducted in
sedimentarydepositsnearMobile, Alabama,
using wells constructed with natural filter
(Discharge from Pump)
Pump
Screen
or
Borehole
Borehole—
Flowmeter
packs in boreholes drilled using mud rotary
techniques. In both studies, well develop-
ment was conducted in stages with flowmeter
tests performed between each stage.
Each well development stage was
approximately twenty minutes in length and
included air development at the Mobile test
site and overpumping, backwashing, and
mechanical surging at the Columbus test
site. Analyses included comparison of
drawdown response, ambient flow profiles,
and induced flow profiles after successive
well development. Test results demon-
strated that the importance of well
development is site dependent butthatthe
effect may be significant. The most
dramatic difference occurred between pre-
development and the first well development
stage. Changes in direction and magnitude
of ambient flow profiles between these two
stages were not uncommon. Significant,
but less notable changes occurred in the
drawdown response to pumping and
induced flow profiles.
Flowmeterresultssuggesta convergence
to a stabilized flow profile under pumping
conditions for all wells. The amount of well
development required to obtain stability
appeared to be influenced by characteristics
of the aquifer materials with greater
percentage of fine-grained materials
To Flowmeter Logger (Q)
- Casing
Figure 2. Apparatus and geometry associated with a borehole flowmeter test.
3
increasing development required for
stabilization. Aquifer heterogeneity at the
Columbus test site obscured sensitivity of
flowmeter results to well type. An additional
observation was that wells installed with a
rotary wash method had the least sensitivity
to well development beyond initial phase of
development. Results from these study
sites demonstrate that well development is
an important aspect of characterization using
the borehole flowmeter. The optimum level
ofwell developmentwill depend on flowmeter
test objectives, well design/construction, and
properties of aquifer materials.
Electromagnetic Flowmeter
Applications
The EM borehole flowmeter has been
used in hydrogeologic characterization at
several sites. The first major application of
this flowmeter in a granular aquifer was at
Columbus Air Force Base in Columbus,
Mississippi. The test site, which is
approximately one hectare in area, overlies
highly heterogeneous, unconsolidated,
unconfined fluvial deposits. The aquifer is
composed of approximately 11 m of terrace
deposits consisting of poorly to well sorted
sandy gravel and gravelly sand that often
occur in irregular lenses and layers
containing significant amounts of clay.
Numerous pumping tests, flowmeter tests,
and recirculating tracer tests had been
performed to study site characterization
techniques. Several EM flowmeter tests
were performed in each well to characterize
the horizontal hydraulic conductivity
distribution. Detailed hydraulic conductivity
fields generated from results of these tests
provided sufficient data to delineate sand
and gravel beds of two former river channels.
The flowmeter has been used to charac-
terize ground-waterflow patterns in fractured
bedrockatsites including OakRidge National
Laboratory in eastern Tennessee. Geologic
units at this site consist of sequences of
calcareous shale, siltstone, shaley lime-
stone, and limestone. Ground-water flow
paths are predominantly through bedrock
joints and fractures. Flow rate profiles
observed in wells differed amongthevarious
geologic units and areas of the site. In some
regions, a relatively permeable zone located
nearthe top of bedrock appears to serve as
a pathway for significant shallow ground-
water flow to nearby streams. Flowmeter
results from a few open boreholes were
used to identify zones with hydraulically
active fractures for isolation using packers
and subsequent ground-water quality
sampling.
The versatility of the EM flowmeter and
the value of even limited testing was
demonstrated at three additional field sites
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I
I
80
100
50
100
150
200
120
-0.012 -0.008 -0.004 0
Ambient Flow(Us)
250
(b)
-0.04 -0.03 -0.02 -0.01 0
Induced Flow (Us)
Figure 3, Diagrams illustrating the ambient flow distribution in a well and the flow distribution
induced by pumping.
funded underthis project. Geologic settings
at these sites included sandstone, uncon-
solidated glacial deposits, and fractured
igneous/metamorphic rock. Forthe majority
of wells at each test site, the range in flow
rates induced by pumping and, corres-
pondingly, hydraulic conductivity among the
tested intervals was two to three orders of
magnitude or more. The report addresses
the potential importance of flowmeter test
results for subsurface remedial design and
monitoring at each site. Additional case
studies describing test objectives, designs,
and results using borehole flowmeters are
also discussed in the report.
Conclusions
In many geologic settings, the borehole
flowmeter offers one of the most direct
techniques available for developing
information regarding the horizontal
hydraulic conductivity distribution in granular
media and ground-water flowpaths in
fractured media. Such information is vital
at many sites for conceptualization of
contaminanttransport/fate and development
of effective and efficient remediation
systems. Techniques for flowmeter use
described in this report may be viewed as an
extension of a standard pumping test. In a
pumping test, only total discharge rate is
measured, whereas both the vertical flow
rate distribution within the screened interval
and the total discharge rate are recorded
during a flowmeter test. The EM flowmeter
developed by TVA appears to be a versatile
tool with high sensitivity and precision.
Applications attest sites located in various
hydrogeologic settings indicate the tool has
the potential to significantly enhance site
characterization through delineation of
subsurface heterogeneity and preferential
ground-water flow paths. Preliminary studies
of the effects of various drilling technologies,
well constructions, and well development
techniques on hydraulic properties of
materials adjacentto the well screen indicate
appropriate methodologies depend to some
degree on aquifer material properties.
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Steven C. Young, Hank E. Julian, and Hubert S. Pearson are with the Tennessee
Valley Authority, Engineering Laboratory, Norris, TN 37828. Fred J. Molz, previously
with Auburn University, Auburn, AL, is currently with Clemson University, Clemson,
SC 29634. Gerald K. Boman is with Auburn University, Auburn, AL 36849.
Steven D. Acree is the EPA Project Officer (see below).
The complete report, entitled "Application of the Electromagnetic Borehole Flowmeter"
(Order No. PB98- : Cost: , 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:
U.S. Environmental Protection Agency
National Risk Management Research Laboratory
Subsurface Protection and Remediation Division
P.O. Box 1198
Ada, OK 74820
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