Page 320 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 10
Water Quality Characteristics and Drinking Water Standards
Atmospheric moisture
Wind
Snow mantle
and radioactive fallout
Soil mantle:
gases, organic
Surface water; particulates, organic matter nitrates, phosphates,
matter
biocides (runoff)
Suring water; (see Ground water)
Groundwater: salts
Lake water: algae, odors, tastes
(calcium and Precipitation: gases vapors, particulates, including salt nuclei
Swamp water: color, odors, tastes
magnesium carbonates: River water: wastewaters, soil erosion
chlorides and sulfates),
iron, and manganese
Seawaters: salts
Figure 10.1 Characteristic properties of natural waters in the hydrological cycle (After Fair et al., 1971).
due to its low mineral content. Normally the fresh water of A spring is an opening in the ground surface from which
suitable quality from reservoirs and rivers is used for potable groundwater flows. Water may flow by force of gravity from
water production, under the conditions that the safe yield water table aquifers or be forced out by artesian pressure, as
(either impoundment safe yield or river safe yield, depend- shown in Fig. 10.1. When the water table or artesian pressure
ing on the source) will not be exceeded. Ocean water can be fluctuates, so does the flow of springs. When a source of
an important water supply source to many coastal regions groundwater is mixed with surface water, this groundwater
especially in the Middle East. Classification of brackish, is called the groundwater under direct influence of surface
saline, and sea water is based on total dissolved solids (TDS) water.
of 1,000–5,000, 5,000–30,000, and 30,000–35,000 mg/L,
respectively. 10.3 PUBLIC HEALTH ISSUES AND
Groundwaters absorb gases of decomposition and DRINKING WATER TREATMENT
degradable organic matter within the pores of the soil through
which they percolate. In the living earth rich in organic matter To slake man’s thirst, drinking water must be wholesome and
oxygen is removed from groundwaters and carbon dioxide is palatable. Accordingly it must not only be free from disease-
added. The pH is lowered and some of the soil minerals are producing organisms and toxic or otherwise physiologically
dissolved. Calcium and magnesium carbonates, sulfates, and undesirable substances, but also attractive to the senses. Most
chlorides enter the water and increase its hardness. Iron and dangerous, in the early history of communal water supply in
manganese, too, may be rendered soluble. Among gases of the industrializing countries of the world, were recurring epi-
decomposition in the pores of rich soils are hydrogen sulfide demics of enteric fevers traceable to unwholesome drinking
and methane as well as carbon dioxide. Natural filtration water. Most important, since drinking water has been made
of groundwaters removes organic matter and microbic life. microbiologically safe, has become the provision of water
Salts remain in solution. Groundwater moves within the that is also acceptable and generally useful in household and
aquifer although the movement is slow. The water table industry.
or artesian pressure surface slopes from areas of recharge However, even today, human and mechanical failures,
to areas of discharge. The pressure differences represented singly or in combination, occasionally lower the barriers to
by these slopes cause the flow of groundwater within the infection and allow contamination of water supplies that oth-
aquifer. erwise have had a long history of safety. Because this is
A well can be used to pump water from the groundwater so, water safety remains the unquestioned responsibility of
reservoir. If wells extract water from an aquifer over a period water authorities, their engineers, and their general personnel
of time at a rate such that the aquifer will become depleted down to the most recently hired workman. An example of
or bring about other undesired results, then the ground- inadequate water discipline is the outbreak of typhoid fever
water safe yield of the aquifer is exceeded. Under these at Croydon (London), England in 1937. There a workman
conditions, saltwater encroachment may occur where wells who happened to be a typhoid carrier failed to obey sanitary
are located near the seashore (Fig. 10.1), or surface water rules at work in a well shaft and contaminated the supply
may enter groundwater where wells are located near a river causing 341 cases of typhoid fever with 43 deaths. Another
or lake. example is the outbreak of typhoid fever at Rochester, NY,