Water Well Manual (USAID).pdf - The Water, Sanitation and Hygiene

dissolved solids content would therefore be expected to present rhe taste,
laxative and other problems associated with the individual minerals. Such
waters are usually corrosive to well screens and other parts of the well
structure.
pH is a measure of the hydrogen ion ccncentration in water and indicates
whether the water is acid or alkaline. It ranges in value from 0 to 14 with a
value of 7 indicating a neutral water, values between 7 and 0 increasingly acid
and between 7 and 14 increasingly alkaline waters. Most ground waters in the
United States have pH values ranging fr0.m about 5.5 to 8. Determination of
the pH value is important in the control of corrosion and many processes in
water treatment.
The dissolved ox-vgen content of ground wsters is usually low particularly
in waters found at great depths. Oxygen speeds up the corrosive attack of
water upon iron, steel, galvanized iron, and brass. The corrosive process is aLc,
more rapid when the pH is low.
Hydrogen sulfide is recognizable by its characteristic odor of rotten eggs.
It is very often found in ground waters which also contain iron. In addition to
the odor, which is noticeable at as low a concentration as 0.5 mg/l, hydrogen
;r sulfide combines with oxygen to produce a corrosive condition in wells and
also combines with iron to form a scale deposit of iron sulfide in pipes. Most
of the hydrogen sulfide can be removed from ground water by spraying it
into the air or allowing it to cascade in thin layers over a series of trays.
Carbon dioxide enters water in appreciable quantities as the water
percolates through soil in which plants are growing. Dissolved in water, it
forms carbonic acid which, together with the carbonates and bicarbonates,
controls the pH value of most ground waters. A reduction of pressure, such as
caused by the pumping of a well, results in the escape of carbon dioxide and
an increase in the pH value of the water. Testing of ground-water samples for
carbon dioxide content and pH, therefore, requires the use of special
techniques and should be done at the well site. The escape of carbon dioxide
from a water may also he accompanied by the settling out of calcium
carbonate deposits.
While the above list includes those chemical substances that are likely to
be of greatest general concern to owners of small wells, it is by no means an
exhaustive one nor intended to be such. Conditions peculiar to specific areas
may require analyses of ground waters for other substances. The group of
elements often referred to as the trace elements because of the very low
concentrations in which they are usually found in water are here worth
mentioning. Among these are arsenic, barium, cadmium, chromium, lead and
selenium, all of which are considered toxic to man at very low levels of intake
(the order of a fraction of 1 mg/l). Since the rate of passage of some of these
elements through the body is very slow, the effects of repeated doses are
additive and chronic poisoning occurs.
Trace elements generally are not present in objectionable concentrations in
ground waters but may be so in a few specific areas. It has been reported for
example, that arsenic has been found in sufficiently high concentrations in
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