Some gages measure feet of water in addition to air
pressure; if available, use a gage that indicates feet of water. To determine the
distance below the air gage at which the water stands in the well, subtract the
calculated height of water above the bottom of the air line from the known length of the
air line below the well top. This value is the static level.
Start the well pump and observe the air gage until the
reading no longer changes, pumping in additional air to make up for any leakage.
Convert this pressure reading to feet. This measurement is the height at which water
stands in the well above the bottom of the air line during pumping. Deduct this value
from the length of the air pipe below the well top to get the pumping level. The
drawdown is the difference between the static level and the pumping level.
Maintain careful records of static level and drawdown correlated
with the capacity or pumping rate. This information can help you anticipate difficulties
and provide data for proper maintenance measures:
A falling static level indicates gradual lowering of the water
An increased drawdown may indicate receding groundwater
level, well interference, or leaky casing or delivery pipes. Increased drawdown may
also suggest a clogged, scaled, or corroded well screen; a sand- or silt-packed gravel
area and adjacent stratum; or a cave-in of the water-bearing stratum.
Take measurements periodically to develop a well chart like the
one shown in Figure 6 (Section 6). The pumping rate can be kept constant using a
valve in the discharge line. Start the test with the valve one-half to three-fourths open
at the desired flow rate. Compare well charts with prior test results to detect changes
that require attention.
Checking for Safe Pumping Yield. Typically, the measured drawdown is
compared to the depth of the top of the well screen or artesian aquifer. To prolong the
life of the well, especially a gravel-packed well, and to decrease maintenance, operate
the well at a pumping rate that achieves 50 percent of the maximum drawdown. If the
desired yield cannot be obtained at this pumping rate, you may need to perform
maintenance on the well to restore well yield.
Disinfecting Wells. Major pollution of aquifers may take years to
overcome. Minor pollution may be overcome by chlorinating the water. Disinfect wells,
springs, infiltration galleries, and radial collectors as a normal maintenance procedure.
Disinfect deep wells after original development, each time the pump is removed, each