casing. In general, the wetted tape method is the only method for water level

measurement that is consistently accurate to 0.01 foot (Dalton et al., 1991).

Water level measurements from boreholes, piezometers, or monitoring wells

used to define the water table or a single potentiometric surface should be

made within 24 hours. In certain situations, water level measurements should

be made within an even shorter time interval. These situations typically

include:

tidally influenced aquifers;

aquifers affected by river stage, bank storage, impoundments,

and/or unlined ditches;

aquifers stressed by intermittent pumping of production irrigation

or supply wells; and

aquifers being actively recharged because of recent precipitation.

Water level measurement equipment should be constructed of materials that are

chemically inert and not prone to sorption or desorption.

Water level measurement equipment should be decontaminated prior to use at

each well to ensure sample integrity and to prevent cross contamination of

ground water.

Measuring tapes and marked cables that are used to measure water levels

should be periodically checked for stretch.

Well depth should be measured each time ground water is sampled. Well depth may

be measured using a weighted tape measure or marked cable constructed of materials that are

chemically inert and not prone to sorption or desorption. The weight should be heavy enough

to keep the tape measure straight and blunt enough so that it will not penetrate soft materials

on the bottom of the well. The deeper the well, the heavier the weight has to be to "feel" the

bottom of the well. Standing water level measuring devices are generally not appropriate for

making well depth measurements. Equipment used to measure well depth should be

decontaminated prior to use at each well. The measuring tape or marked cable used to

measure well depth should be periodically checked for stretch.

November 1992

7 5