SOURCE AND MODE OF SUPPLY. Modes of supply are classed as gravity or pumping, and the latter mode is sub-classified as pumping direct to the distributing mains and consumers, to standpipe, to tank, or to reservoir. In seine cities a combination of two or all these modes is in use. Where there are marked varia tions in the levels of a city it is customary to divide the works into high and low and perhaps intermediate services. This saves strain on the street and house In in the lower levels. and where the water is pumped it may greatly re duce plumbing expenses. In addition, lower pres sures result in less leakage at defective joints and pumping. fixtures. Several pumps in one station may rai,c water to different levels, or the water !nay be p.111'1111)(1 for the higher districts. Whatever the source and mode of supply. an ample quantity of water should always be avail able near the centre of population and lire risk. To aecomplish this end. storage reservoirs, or in the case of pumping plants, duplicate pumps, may be required; and several independent lines of main pipes may also be essential.
The choice between the various classes of sup ply, such as springs, wells, streams, and lakes, will be governed by the relative availability of each possible source to meet the three essentials of quality, quantity, and pressure. For large cities surface supplies are most commonly the chief reliance, and in such cases, for all cities not on the larger rivers and lakes, impounding reser voirs are generally employed. Even relatively small supplies from wells are liable to yield less, after a few years, than their original rated ca pacity. In point of quality, underground waters,
except when drawn from wells in populous areas, are generally free from dangerous organic matter, but they may have, or give rise to. bad tastes and odors, and their water may be hard, or may contain iron in such large quantities as to make it unfit for use. Tastes and odors are liable to develop in water from underground sources when exposed to the light. This is due to the life processes of miscroscopic organic growths, mostly of vegetable origin. Water con taining iron compounds, on exposure to the air, may become fairly red with a precipitate re sembling iron rust. Surface waters are gen erally soft, unless from limestone drainage areas. They sometimes, on storage, foster odor and taste producing organisms. Their chief dan ger is from sewage pollution. Surface supplies are relatively warm, and underground waters cool. Surface supplies are also liable to be high in color and turbidity. Swampy or peaty gather ing grounds, heavily wooded areas, and improp erly cleaned sites for storage reservoirs give rise to color, while readily eroded clay or silt lands contribute turbidity. The Ohio, Mississippi, and many Southern and Western rivers are notable for their turbidity. Swamps may sometimes be drained, or cut out of the collecting area. Soils rich in organic matter may be removed from reservoir sites. See WATER SUPPLY.