Handbook of preventive medicine.

  • Great Britain. Air Ministry.
Date:
1959
Catalogue details

Licence: Public Domain Mark

Credit: Handbook of preventive medicine. Source: Wellcome Collection.

    48/556 (page 12)
    them. Although some natural purification may occur through the settle- ment of solids, by oxygenation and by sunlight, lake water should not be used for drinking without sterilization. In times of heavy rain, streams, rivers and lakes become even more polluted, as impurities are washed from the banks. by the increased flow of water. 16. Ponds. Water from ponds is heavily contaminated and must be treated. Many dry up completely in summer and are not, in consequence, reliable sources. 17. Rain Water. Water collected from roofs contains impurities ab- sorbed from the air and washed from the collecting surface. It is flat and unpalatable but soft and very suitable for laundry and washing purposes. If the collecting surface is protected and if proper storage tanks are provided, the water would, in theory, be safe for drinking untreated. But in practice sterilization is always advisable. 18. Shallow Wells. Rain soaks into the ground until it reaches a stratum, such as clay or rock, through which it cannot pass. The porous earth above the impervious stratum becomes saturated with water; a well sunk into it will fill up to the level of saturation. This is called a shallow well, irrespective of its depth. The water from it is little protected from surface pollution, so is unfit for domestic purposes without previous sterilization. 19. Springs. If the impervious stratum, following a level or downward trend, emerges on the side of a hill, a surface spring may be found at the outcrop. As the water is derived from above the impervious stratum, it is subject to pollution in the same way as in a shallow well. 20. Deep Wells. A deep well extends down into a water-bearing layer below the uppermost impervious stratum. Consequently the water in the deep well is drawn from that flowing below an impervious stratum, which in the absence of fissures protects it from immediate pollution. This water has come from comparatively distant surface sources and any possible remote contamination will have been removed by filtration through the earth and by storage. An artesian well taps water which is held down by an overhead impermeable layer so that it is under pressure and rises in the tube of the well. Although the water from deep sources may be safe for drinking purposes, deep wells must be properly con- structed and maintained to exclude contamination. The bore should have a watertight lining, extending from about a foot above the surface of the ground to the first impervious stratum. The ground, for six feet around the well, should be concreted to exclude polluted surface water. A watertight and dustproof cover is necessary and should always be kept. locked. Water should be drawn from the well by means of a pump, the suction pipe of which is a permanent fixture. Use of the old oaken bucket (or any bucket) is a method certain to cause contamination sooner or later. An area around the head of the well should be fenced off to prevent the entry of animals. No possible source of contamination, such as a manure heap or a cesspool, should be allowed within 100 feet of a well. Distribution of Water 21. Water is brought from the source of supply to the household tap by the distribution system, which is made up of :—