time, other minerals and chemicals are dissolved and carried along. It is now ground water in an
underground deposit and, although it may now become less contaminated or polluted, it is not
necessarily pure, and may contain disease organisms as well as harmful chemicals. In addition to the
impurities in water resulting from infiltration, many are contributed by an industrialized society.
Garbage, sewage, industrial waste, insect sprays, and fertilizers are examples of these. Impurities in
water are suspended or dissolved. The suspended impurities are usually more dangerous to health.
Suspended impurities include disease-producing organisms, mineral matter, silt, and algae. They must
be removed or destroyed in water that is to be consumed. Dissolved impurities include salts (calcium,
nitrogen).
3. Water Characteristics and Their Effects. Water takes on various characteristics and properties as
it passes over and through the earth. These characteristics and properties may vary and are dependent on
the materials encountered. They are classified according to means of detection, such as physical or
chemical. Physical properties are ones detected by one or more of the five human senses. The most
important physical characteristics are turbidity, color, odor, taste, and temperature. Chemical properties
are detected by chemical analysis. The most important chemical characteristics are acidity, alkalinity,
hardness, and corrosiveness. Sometimes these two types of characteristics overlap; for example, iron in
water is a dissolved mineral detectable by chemical analysis, yet its color and taste are also physical.
a. Physical Characteristics.
(1) Turbidity. Turbidity is a muddy or unclear condition of water, caused by particles of
sand, silt, clay, or organic matter being held in suspension. The faster water flows, the more material it
picks up and the larger the size of the pieces carried along. As water slows down, the larger particles
settle out. Clay and silt remain suspended in water the longest, because of their particle size and specific
gravity. The removal of turbidity is essential in the production of potable water. Such removal reduces
contamination, extends the time between backwashing of filters, decreases chlorine demand, improves
disinfection, and enhances the user acceptability of the finished product. The Army Preventive Medical
Branch requires turbidity removal because suspended particles often contains organisms which cause
such diseases as infectious hepatitis, cholera, bacillary dysentery, and a multitude of others.
(2) Color. Color in water is due to the presence of colored substances in solution, such as
vegetable matter dissolved from roots and leaves, and to humus, iron, and manganese salts. True color
is due to substances in true solution; apparent color includes true color and also that due to substances in
suspension. Water taken from swamps, weedy lakes, and streams containing vegetation is most likely to
be colored. Color may also be caused by industrial waste and turbidity. The latter is responsible for an
apparent color, rather than the true color, and is caused by materials of vegetable origin. Color as such
is harmless, but objectionable due to its appearance and to the taste and odors sometimes associated with
it.
(3) Odors and Taste. Taste and odors found in water are most commonly caused by alga,
decomposing organic matter, dissolved gases, or industrial waste. Mineral substances may also
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