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

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