In small communities the householder usually receives his supply of water from a well near the house. To understand how a well obtains its water, it is necessary to recall what has been said concerning the structure of the earth.* The earth is made of layers of sand, chalk, clay, etc. Water will soak through sand and chalk thoroughly, but clay, on the contrary, will not allow water to pass through it. As the result of various upheavals of the earth, the layers are not horizontal but are tipped at angles and are often exposed to the surface of the earth. When a layer (stratum) of sand is supported by a layer of clay, the water is held up as in a basin. Therefore, when a hole is dug into the ground through the sand and not below the clay, water is usually found to have percolated through the ground and settled in the sand. Figure 206 shows a section of the earth and illustrates how flowing wells are formed. The upper and third layers of earth are porous, the second and fourth non-porous. The force of gravity causes the water to sink into the lower porous layer and remain at this level.
* Pages 150-151.
Fig. 206. - Section of Earth showing Conditions which Produce Artesian Wells.
This water comes from the rain, ice, or snow and takes up considerable impurities as it passes along the ground. The sand usually filters or removes these impurities, although sometimes it fails to do so completely. This is most frequently the case in a shallow well. A well should be at least 150 ft. from the nearest known source of pollution.
The water is raised from wells or from the source of supply to reservoirs by means of pumps.