Especially is well or spring water desirable for private domestic use, since it is usually in a reasonable degree of purity and does not, therefore, require any artificial process for purification. The sources of water supply used for public purposes, made necessary because of the volume of water required, are not so satisfactory for a private or domestic supply because its purification would entail too great an expense to warrant the practicability on such a small scale. Therefore, the most desirable source when only a limited amount of water is required is the wells. Wells may be used for public water supply and are in some of the smaller towns, but they are not so practicable as their supply is likely to be inconstant. In other words, there is danger of the wells going dry, and since they are put down at a great expense, this would be too great a risk to take even though the water thus obtained is satisfactory from a hygienic standpoint. [Illustration: The above illustration shows the fluctuation of ground water. When the ground water is at the low mark the shallow well becomes dry; as the water rises it flows into the well. When the water has reached the high mark the intermittent spring becomes a flowing spring. The pressure of the ground water is great enough to produce flowing wells down on the level.] Not all wells, by any means, will furnish a pure water supply. There are many factors to consider from the viewpoint of the pollution of well water and these will be considered in due time. When water reaches an impervious strata it remains at that level and moves in a horizontal plane. Water may form in beds or be found in underground streams. It is only in the limestone regions that water forms in streams under the ground. [Illustration: Poorly constructed well, polluted by seepage thru limestone.] The depth at which water is found beneath the surface varies. The water directly beneath the surface, or that which is derived from surface drainage and the rainfall, is, by some, styled the ground water. This is the shallow water. That which is found at a lower level is called the lower subsoil water or underground water. The ground water does not provide a very desirable source of well water because there is danger of pollution from the surface. The degree of this impurity will depend largely upon the character of the soil. If it is sandy and thus provides a good filter medium there is little danger from pollution; but if the soil is of a limestone formation it makes a very poor filter and the shallow water is likely to be polluted from the surface drainage. The subsoil water moves in the direction of the nearest body of water. That is, if it is near a river or lake its movement will be in the direction of this river or lake. For this reason a well near the seashore will contain fresh water.