Read the following text and say what natural sources of water are and what requirements they must satisfy.
Text D. Natural Water Sources and Their Use for Water Supply Purposes
“When the well is dry, we know the worth of water.” BENJAMIN FRANKLIN (1706-1790)
The choice of a water source is one of the most responsible tasks in designing a water supply system. The source determines to a considera- ble degree the type of the water supply system itself, the necessity of certain facilities and, therefore, the cost of its construction and mainte- nance.
A water supply source must satisfy the following requirements:
a. it must provide the acquisition of adequate quantities of water with a glance of a prospective increase in water consumption;
b. it must provide continuity of water supply;
c. it must provide the water of such quality that meets the demands of water consumers by means of reasonably priced treatment;
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d. 
it must enable water transmission at the lowest cost;
e. it must guarantee water acquisition without ecological disturbance*.
Natural sources of water include:
· surface sources (oceans, seas, lakes, reservoirs, rivers, streams, tanks and ponds);
· underground sources (ground water, artesian [confined] water, shallow wells, deep wells and springs).
Natural sources such as rivers, lakes, impounding reservoirs, etc. are sources of surface water. Water is withdrawn from them through in- takes. The simplest intakes are pipes extending from the shore into deep water.
Water obtained from subsurface sources, such as sands and gravels and porous or fractured rocks, is called ground water. The flow of ground water takes place in river valleys and, in some areas, along the seacoast in water-bearing strata known as aquifers. Groundwater is ac- cessed through a bore.
For the community’s needs groundwater is more suitable. However, for the supply of water to large inhabited localities groundwater sources are often insufficient, and acquisition of a considerable quantity of water from them is unprofitable.
For the supplying of big cities and industrial enterprises with water, therefore, surface sources of fresh water are mainly used.
Sources of Drinking Water. Drinking water is water intended pri- marily for human consumption, either directly, as supplied from the tap, or indirectly, in beverages or food prepared with water. It should contain no harmful concentrations of chemicals or pathogenic microorganisms, and ideally it should be aesthetically pleasing in regard to appearance, taste and odour.
Drinking water comes from both surface and groundwater sources. Surface water (rainfall and its runoff into streams and rivers) normally contains suspended matter, pathogenic organisms, and organic substanc- es. Groundwater (water that has collected in aquifers) normally contains dissolved minerals and gases. Both require treatment. Water suppliers access this water, treat it and distribute it to consumers.
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The amount of water on our planet that is suitable and available for drinking is very small. Across the globe, population growth, urban de- velopment and environmental degradation pose an ever-increasing threat
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to freshwater supplies. Today, 4 out of every 10 people live in areas that are experiencing water scarcity, and nearly 50% of the world’s popula- tion is likely to face severe water shortages by 2025.
* ecological disturbance – нарушение экологического баланса
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