Chapter 5 SURFACE WATER - WHO

1 Chapter 5 SURFACE WATER SURFACE WATER originates mostly from rainfall and is a mixture of SURFACE run-off and ground WATER . It includes larges rivers, ponds and lakes, and the small upland streams which may originate from springs and collect the run-off from the watersheds. The quantity of run-off depends upon a large number of factors, the most important of which are the amount and intensity of rainfall, the climate and vegetation and, also, the geological, geographi- cal, and topographical features of the area under consideration. It varies widely, from about 20 % in arid and sandy areas where the rainfall is scarce to more than 50% in rocky regions in which the annual rainfall is heavy.

2 Of the remaining portion of the rainfall. some of the WATER percolates into the ground (see "Ground WATER ", page 57), and the rest is lost by evaporation , transpiration and absorption. The quality of SURFACE WATER is governed by its content of living organisms and by the amounts of mineral and organic matter which it may have picked up in the course of its formation. As rain falls through the atmo- sphere, it collects dust and absorbs oxygen and carbon dioxide from the air. While flowing over the ground, SURFACE WATER collects silt and particles of organic matter, some of which will ultimately go into solution.

3 It also picks up more carbon dioxide from the vegetation and micro-organisms and bacteria from the topsoil and from decaying matter. On inhabited watersheds, pollution may include faecal material and pathogenic organisms, as well as other human and industrial wastes which have not been properly disposed of. In rural areas, WATER from small streams draining isolated or uninhabited watersheds may possess adequate bacteriological and che- mical quality for human consumption in its natural state. However, in most instances SURFACE WATER is subject to pollution and contamination by pathogenic organisms and cannot be considered safe without treatment.

4 It should be remembered that clear WATER is not necessarily fit for human consumption and that one cannot depend wholly on self-purification to produce potable WATER . Collection of Rain- WATER in Cisterns This system is used by individual families and farms in areas where there is no ground WATER or where the quality of such WATER is objectionable for domestic use. Rain- WATER may also be collected to provide a supply of soft WATER , for laundry purposes, for example, in places where the avail- 162 WATER SUPPLY FOR RURAL AREAS able WATER is hard, , possessing a high content of mineral salts.

5 The collection of rain- WATER requires clean roofs. an even roof overhang, col- lection troughs, and storage tanks. Paved SURFACE may also be used. In Bermuda and the Middle West of the USA, cisterns are commonly employed as a source of domestic WATER -supply. The quality of rain- WATER is affected by the nature and the degree of maintenance of the collecting surfaces. Certain wood or paint materials used on roofs, and dead leaves which u~ually accumulate in the troughs, are capable of imparting taste and colour to cistern WATER ; rough roof surfaces, such as those made of thatch, are likely to retain wind-blown dust which is later collected by rain- WATER .

6 Galvanized-iron roofs, which are extensively used in the tropics, provide excellent and smooth surfaces for the collection of rain- WATER . To strain out suspended matter, sand filters may be built at the entrance to storage tanks or cisterns. Such filters cannot be depended upon, how- ever, to remove bacteria and to produce safe WATER . Consequently, cistern WATER should always be boiled or otherwise disinfected before human consumption. In the storage tank itself, the chemical as well as bacteriological quality of the rain- WATER may deteriorate. For example, the mortar employed for the construction of unlined masonry walls may yield substantial amounts of mineral substances to the rain- WATER .

7 Also, the number of bacteria may considerably increase because of the decomposition of organic matter held in suspension or in solution in the WATER stored for long periods, or by passage through sand filters which are not properly maintained. Most of these difficulties can be avoided, however, by suitably construct- ing and locating the cisterns and by providing for their thorough cleansing at regular intervals. Cisterns should be located in such a manner as to prevent any possibility of pollution of their contents by SURFACE WATER or by sewage.

8 For this reason they should preferably be sited on higher ground than the surrounding area and higher than, and at least 3 m (10 ft) away from, sewage-disposal installations. For convenience and economy, how- ever, they are frequently built underground. This has the further ad- vantage of keeping the stored WATER cool in warm climates and of protecting it against freezing in cold regions. Cisterns may be built of brick or stone masonry, or of reinforced concrete. The latter material is to be preferred, especially for under- ground cisterns, since it provides watertight walls in addition to necessary structural strength.

9 A concrete mixture of 1 part cement to 2 parts of sand and 3 of gravel, and containing 23 litres (6 US gal., approximately) of WATER per bag of Portland cement will be satisfactory in most cases. A well-constructed concrete tank needs no SURFACE treatment to make it suitable for WATER storage. However, in most cases a coating of silicate of soda or bituminous paint is recommended. Masonry walls should INSTALLATION OF WATER -SUPPLY SYSTEMS 163 be carefully built with strong cement mortar joints, and the interior surfaces should receive two ( v2-in.)

10 Plaster coats of 1 : 3 (cement : sand) mortar to provide for waterproofing. Manhole openings should have edges which project about 10 cm (4 in.) above the level of the surround- ing SURFACE , and their covers should be tight to prevent the entrance of light, dust, and other pollutional material. An overflow or air-vent provided with screen should be installed. The outlet should be located slightly above the bottom of the tank, and a clean-out pipe flush with the bottom and provided with a drain valve should be installed. Fig. 64 shows typical designs of filters and cisterns.