»EPA
United States
Environmental Protection
Agency
Office of
Research and
Development
Office of Solid Waste
and Emergency
Response
EPA/540/4-89/002
March 1989
Superfund
Ground Water Issue
Accuracy of Depth to Water Measurements
Jerry T. Thornhill
The Regional Superfund Ground Water Forum is a group of
ground-waterscientists, representing EPA's Regional Superfund
Offices, organized to exchange up-to-date information related
to ground-water remediation at Superfund sites.
Accuracy of depth to water measurements is an issue identified
by the Forum as a concern of Superfund decision-makers as
they attempt to determine directions of ground-water flow,
areas of recharge or discharge, the hydraulic characteristics of
aquifers, or the effects of manmade stresses on the ground-
water system.
Perhaps the most extensive investigation into methods for
measuring water levels in wells has been conducted by the U.S.
Geological Survey. The U.S.G.S., in conjunction with 32 other
federal agencies put together a "National Handbook of
Recommended Methods for Water-Data Acquisition," which
includes an entire section on water-level measurements. The
following discussion is based on that document.
For further information, contact Jerry T. Thornhill, RSKERL-
Ada, FTS 743-2310.
The graduated steel tape (wetted-tape method), the electrical
measuring line, and the air line are the most common tools for
manually measuring water level in nonflowing wells.
Graduated Steel Tape
The graduated steel tape method is considered to be the most
accurate for measuring the water level in nonflowing wells.
Steel surveying tapes in lengths of 100, 200, 300, 500 and
1,000 feet are commonly used. The tapes, up to 500-foot
lengths, are mounted on hand-cranked reels; the 1,000-foot
tapes usually required a motor-driven tape drive. A slender
weight, usually made of lead, is attached to the ring at the end
of the tape to insure plumbness and to permit some feel for
obstructions. The choice of a suitable weight, i.e., lead, stainless
steel, etc. is dictated by the water-quality parameters of interest
in a specific study. Lead weights are used so that if the weight
should fall off the tape in a well that has a pump, the soft lead
would be less likely to damage the pump. The weight is
attached in such a way that if it becomes lodged in the well, the
tape can still be pulled free.
The lower few feet of the graduated tape is chalked by pulling
a piece of blue carpenter's chalk across the tape. When the
chalk becomes wet, a line of color change between the dry and
wet chalk denotes the length of tape immersed in water. The
tape footage is read at the measuring point, and at the water
mark on the tape. The difference between these two readings
is the depth to water below the measuring point.
Submergence of the weight and tape may temporarily cause a
water-level rise in wells or piezometers having very small
diameters. This effect can be significant if the well is in materials
of very low hydraulic conductivity. Under dry surface conditions,
it may be desirable to pull the chalked part of the tape rapidly to
the surface before the wetted part of the tape dries and read the
water mark before rewinding the tape onto the reel. This is
accomplished by pulling the tape from the well by hand, being
careful not to allow it to become kinked. In cold regions, rapid
withdrawal of the tape from the well is necessary before the wet
part freezes and becomes difficult to read.
Garber and Koopman (1968, p. 3-6) describe corrections for
effects of thermal expansion of tapes and of stretch due to the
Superfund Technology Support Centers for Ground Water
Robert S. Kerr Environmental
Research Laboratory
Ada, OK
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suspended weight of the tape and plumb weight. Errors
resulting from these effects can become significant at high
temperatures and for measured depths in excess of 1,000 feet.
As a standard of good practice, the observer should make two
measurements. If two measurements of static water level made
within a few minutes do not agree within about 0.01 or 0.02 foot
(generally regarded as the practical limit of precision) in
observation wells having a depth to water of less than two
hundred feet, continue to measure until the reason for the lack
of agreement is determined or until the results are shown to be
reliable. Where water is dripping into the hole or covering its
wall, it may be impossible to get a good water mark on the
chalked tape.
Unless the well is equipped with an access pipe that is placed
to eliminate the possibility of lowering the tape into the pump
impellers, the graduated-tape method should not be used to
measure pumping levels in wells.
After each well measurement, the portion of the tape that was
wetted should be disinfected to avoid contamination of other
wells.
A simple and reliable method for measuring the depth to water
in observation holes between 1-1/2 and 6 inches in diameter is
a steel tape with a popper. The popper is a metal cylinder 1 to
1-1/2 inches in diameter and 2 to 3 inches long with a concave
undersurface and is fastened to the end of a steel tape. The
popper is raised a few inches and then dropped to hit the water
surface, where it makes a distinct "pop." By adjusting the length
of the tape, the point at which the popper just hits the surface is
rapidly determined (Bureau of Reclamation, 1977).
Electrical Methods
Many types of electrical instruments have been devised for
measuring water levels; most operate on the principle that a
circuit is completed when two electrodes are immersed in
water. Some instruments consist of a single conductor that is
lowered into the well where the metal well casing is used as the
second conductor. More commonly, a two-conductor cable and
special probe are used. Various forms of electrolytic cells using
two electrodes of dissimilar metals have been used, but current
is more commonly supplied by batteries.
Ordinarily, two-conductor electric tapes are 500-feet long and
are mounted on a hand-cranked reel that contains space forthe
batteries and some device for signaling when the circuit is
closed. Electrodes are generally contained in a weighted probe
that keeps the tape taut while providing some shielding of the
electrodes against false indications as the probe is being
lowered into the hole. The electric tapes generally are marked
at 5-foot intervals with clamped-on metal bands.
Before lowering the probe in the well, the circuitry can be
checked by dipping the probe in water and observing the
indicator. The probe should be lowered slowly into the well until
contact with the water surface is indicated. The electric tape is
marked at the measuring point and partly withdrawn; the
distance from the mark to the nearest tape band is measured
and added to (or subtracted from) the band reading to obtain the
depth to water. It is good practice to take a second or third check
reading before withdrawing the electric tape from the well.
The tape should not rub across the top of the casing because the
metal bands can become displaced; consequently, placement
of the bands should be checked frequently with a steel tape.
Electric tapes are more cumbersome and inconvenient to use
than the wetted-tape method, and they normally give less
accurate results. In some situations, however, they are superior.
Where water is dripping into the hole or covering its walls, it may
be impossible to get a good water mark on the chalked tape. In
wells that are being pumped, particularly with large-discharge
pumps, the splashing of the water surface makes consistent
results by the wetted-tape method impossible. Where a series
of measurements are needed in quick succession, such as in
aquifertests, electric tapes have the advantage of not having to
be removed from the well for each reading. Electric tapes are
also safer to use in pumping wells because the water is sensed
as soon as the probe reaches the water surface and there is less
danger of lowering the tape into the pump impellers.
Independent electric tape measurements of static water levels
using the same tape should agree within ± 0.04 feet for depths
of less than about 200 feet. At greater depths, independent
measurements may not be this close. For a depth of about 500
feet, the maximum difference of independent measurements
using the same tape should be within + 0.1 foot.
It is especially important to check the electric line length by
measuring with a steel tape after the line has been used for a
long time or after it has been pulled hard in attempting to free the
line. Some electric lines, especially the single line wire, are
subjectto considerable permanent stretch. In addition, because
the probe is larger in diameter than the wire, the probe can
become lodged in a well. Some operators attach the probe by
twisting the wires together by hand and using only enough
electrical tape to support the weight of the probe. In this manner,
the point of probe attachment is the weakest point of the entire
line. Should the probe become "hung in the hole," the line may
be pulled and breakage will occur atthe probe attachment point,
allowing the line to be withdrawn.
Air Line Methods
The air line method is especially useful in pumped wells where
waterturbulence may preclude using more precise methods. A
small diameter air-type tube of known length is installed from the
surface to a depth below the lowest water level expected.
Compressed air (compressor, bottled air, or tire pump) is used
to purge the water from the tube. The pressure, in pounds per
square inch (psi), needed to purge the water from the air line
multiplied by 2.31 (feet of water or one psi) equals the length in
feet of submerged airline. The depth to water below the center
of the pressure gage can be easily calculated by subtracting the
length of airline below the water surface from the total length of
the air line (assuming the air line is essentially straight). Accuracy
depends on the precision to which the pressure can be read.
The air line and any connections to it must be airtightthroughout
its entire length. If the line is broken or leaky, large errors may
occur. A long-term increase in air line pressure may indicate a
gradual clogging of the air line. A relatively sudden decrease in
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air line pressure may indicate a leak or break in the air line. Air
line pressures that never go above a constant low value may
indicate that the water level has dropped belowthe outlet orifice
of the air line. To minimize the effect of turbulence, the lower end
of the air line should be at least five feet above or below the
pump intake.
Frequency of Measurements
The frequency for measuring water levels in wells depends on
the nature ofthe aquifer under investigation, and on the problems
that are to be solved. Thus, the frequency of measurement
should be adjusted to the circumstances; i.e., water-level
measurements at a given location should be made at
approximately the same time of day whenever possible.
Atthe beginning of an investigation, when details ofthe ground-
water system are not yet known, water levels are commonly
made at regular intervals; for example, 10 times a month for
water-table conditions and 5 times a month for artesian conditions
at key observation sites (Ground-Water Studies, UNESCO).
The reader is referred to the references for detailed handling on
the subject of frequency of measurement in various applications.
References
National Handbook of Recommended Methods for Water-Data
Acquisition, Prepared under the sponsorship of the Office of
Water Data Coordination, Geological Survey, U.S. Department
ofthe Interior, Reston, Virginia, 1977.
Ground-Water Studies, An International Guide for Research
and Practice, UNESCO.
Ground Water Manual, Bureau of Reclamation, U.S. Department
ofthe Interior, 1977.
Garber, M.S. and Koopman, F.C., Methods of Measuring Water
Levels in Deep Wells; U.S. Geological Survey, TWRIL, Book 8,
Chapter A-1; Washington, U.S. Government Printing Office, 23
pp. 1968.
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