United States

Environmental Protection

Agency

EPA Superfund

Ground Water Issue

Superfund Technology Support Centers for Ground Water

Jerry T. Thornhill

Accuracy of Depth to Water Measurements

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

The Regional Superfund Ground Water Forum is a group of

ground-water scientists, 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 groundwater 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, RSKERLAda, 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

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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 for the

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

aquifer tests, 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

subject to 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 at the probe attachment point,

allowing the line to be withdrawn.

Air Line Methods

The air line method is especially useful in pumped wells where

water turbulence 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 air line 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 airtight throughout

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 below the 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 of the 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.

At the beginning of an investigation, when details of the groundwater 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

of the Interior, Reston, Virginia, 1977.

Ground-Water Studies, An International Guide for Research

and Practice, UNESCO.

Ground Water Manual, Bureau of Reclamation, U.S. Department

of the 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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