Measuring water levels in boreholes
The most frequent field measurement that a hydrogeologist is likely to make is the water
level in a well, borehole or piezometer. Hydrogeologists often call all probes used to
measure well water levels a 'dipper' and a wide range is available. The better types
consist of a length of twin core cable with a pair of electrodes attached to one end. The
cable is typically formed into a flat tape graduated in metres and the tape is wound onto a
drum. When the electrodes touch the water surface, a circuit is completed activating
either a light or a buzzer. Both types of signal have their advantages. Many people
find the buzzer an easier signal to use but it may be difficult to hear in noisy environments
such as a pumping test site with a diesel generator in operation. The light signal is good
except in bright sun light when it is difficult to see. These dippers come in lengths from
about 30m up to 500m. Commonly 80m or 100m lengths are used.
The drawing shows the main features of a dipper. Readings can be taken quickly and easily
as the tape can be read to the nearest half centimetre.
Some dippers have ordinary round section cable, with the depth graduations marked by adhesive
bands. It is necessary to use a steel tape to measure the distance in centimetres (i.e. to
the nearest 0.01 metres) from the nearest metre mark, to obtain a more precise water level
reading. These instruments are not so easy to use as the type described above.
Using a dipper
Water-level measurements should be taken by lowering the probe down the well or borehole
until it hits the water, causing the buzzer to sound or the light to come on. When this
happens, pull the tape back slowly out of the water until the signal stops. Repeat the
exercise several times to enable you to 'feel' the water surface.
It is conventional to take the point where the buzzer stops or light goes off as being the
water level. Use your fingers to mark the position on the tape against a fixed datum point
such as the top of the casing, and then read off the level to the nearest centimetre. It is
good practice to note the reading and then check it again to make sure. This can't be done
during the early stage of a pumping test.
When using a dipper with a cable marked off in metres, use a steel tape to measure the distance
from your finger to the nearest metre mark below the place you are pinching, and add the two
values together.
If the dipper is of the 'lamp type', position it so that bright sunlight will not prevent
you from seeing when the indicator light comes on, perhaps testing the light before
starting to take readings make sure that no data are missed. In some makes, the electrical
circuit makes a 'click' as the light goes on and off, making a useful audible signal. On
the whole, fewer mistakes are made with audible dippers.
This Handy Hint is based on information in my book Field Hydrogeology.
Copyright © 2004 by Rick Brassington
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