GWPD 4—Measuring water levels by use of an electric tape

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Courtesy of HERON Instruments Inc.

VERSION: 2010.1
PURPOSE: To measure the depth to the water surface below land-surface datum using the electric tape method.

Materials and Instruments

  1. An electric tape, double-wired and graduated in feet, tenths and hundredths of feet. Electric tapes commonly are mounted on a hand-cranked and powered supply reel that contains space for the batteries and some device (“indicator”) for signaling when the circuit is closed (fig. 1).
  2. An older model electric tape, also known as an “M-scope,” marked at 5-foot intervals with clamped-on metal bands (fig. 2) has been replaced by newer, more accurate models. Technical procedures for this device are available from the procedures document archives.
  3. A steel reference tape for calibration, graduated in feet, tenths and hundredths of feet
  4. Electric tape calibration and maintenance equipment logbook
  5. Pencil or pen, blue or black ink. Strikethrough, date, and initial errors; no erasures
  6. Water-level measurement field form, or handheld computer
  7. for data entry
  8. Two wrenches with adjustable jaws or other tools for removing well cap
  9. Key for well access
  10. Clean rag
  11. Cleaning supplies for water-level tapes as described in the National Field Manual (Wilde, 2004)
  12. Replacement batteries

Data Accuracy and Limitations

  1. A modern graduated electric tape commonly is accurate to +/– 0.01 foot.
  2. Most accurate for water levels less than 200 feet below land surface.
  3. The electric tape should be calibrated against an acceptable
  4. steel tape. An acceptable steel tape is one that is maintained in the office for use only for calibrating tapes, and this calibration tape never is used in the field.
  5. If the water in the well has very low specific conductance,
  6. an electric tape may not give an accurate reading.
  7. Material on the water surface, such as oil, ice, or debris, may interfere with obtaining consistent readings.
  8. Corrections are necessary for measurements made from angled well casings.
  9. When measuring deep water levels, tape expansion and stretch is an additional consideration (Garber and Koopman,
  10. 1968).

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