Natural water level fluctuations necessitate a longer screen length;
The interval monitored is slightly greater than the appropriate screen length
(e.g., the interval monitored is 12 feet thick); or
The aquifer monitored is homogeneous and extremely thick (e.g., greater than
300 feet), thus a longer screen (e.g., a 20 foot screen) represents a fairly
discrete interval.
The head measured in a well with a long screened interval is a function of all of the different
heads over the entire length of the screened interval. Care should be taken when interpreting
water levels collected from wells that have long screened intervals (e.g., greater than 10 feet).
Hydrostratigraphic relationships should be determined by a qualified ground water
scientist when obtaining and evaluating water level data. Unqualified individuals may
confuse a potentiometric surface with the water table in areas where both confined and
unconfined aquifers exist. In all cases, well or piezometer screen placement should be based
on the detailed boring log, and the well or piezometer screen should not intercept
hydraulically separated zones of saturation.
At sites where the hydraulic gradient is so small that the error introduced by
measuring water levels in crooked or out of plumb wells will produce an inaccurate
determination of hydraulic gradient or flow direction, a deviation survey should be performed
on all wells. If a well is out of plumb and/or not straight (crooked), the information gathered
from the deviation survey should be used to correct water level elevations measured in the
well. A deviation survey will determine whether the wells are in vertical alignment (i.e.,
straight) and are plumb. Several instruments and methods have been designed for this
purpose; a good description of these instruments and methods is provided by Driscoll (1986).
A proper deviation survey will consider both magnitude of well deviation and direction of
deviation. If a well is out of plumb and/or not straight (crooked), the information gathered
from the deviation survey should be used to correct water level elevations measured in the
well, because the depth to ground water measured in an out of plumb or crooked well will be
greater than the depth to ground water measured in a straight well. A correction can be
accomplished easily by first graphing the actual vertical configuration of the well, and then by
establishing a relationship between a measured water level elevation in the crooked and/or
out of plumb well and the water level elevation in an imaginary straight and plumb well at
the same location. A method for graphing the actual vertical configuration of an out of 
plumb and/or crooked well is provided by Driscoll (1986).
November 1992
4 41






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