Water percolating through the soil passes downward by gravity until it reaches an impervious stratum. The surface of this underground sheet of water is technically called "water table" or ground-water level. The water is not at rest, but has a slow and well-defined motion, the rate of which depends upon the porosity of the soil and also upon the inclination or gradient of the water table. A shallow well may be either excavated or driven into this subsoil sheet of water. In populous districts, in villages, towns, but also near habitations, the soil from which water is obtained must, of necessity, be impregnated with organic waste matter. If, in such a surface well, the level of the water is lowered by pumping, the zone of pollution is extended laterally in all directions. Ordinary shallow well water should always be considered "suspicious water." There are two distinct ways in which surface wells are contaminated: one is by leakage from cesspools, sewers, privies, etc.; the other, just as important and no less dangerous, by direct contamination from the surface. The latter danger is particularly great in wells which are open at the surface, and from which water is drawn in buckets or pails. A pump well is always the safer of the two. Frogs, mice, and other small animals are apt to fall into the water; dust and dirt settle into it; the wooden curb and the rotten cover also contribute to the pollution; even the draw-buckets add to it by reason of being often handled with unclean hands.
Always avoid, in the country, drinking water from farmers' wells located near cesspools or privies. Such shallow wells are particularly dangerous after a long-protracted drought. It is impossible to define by measurement the distance from a cesspool or manure pit at which a well can be located with safety, for this depends entirely upon local circumstances. Contamination of shallow wells may, in exceptional cases, be avoided by a proper location of the well with reference to the existing sources of impurity. A well should always be placed above the source of pollution, using the word "above" with reference to the direction in which the ground water flows.
Precautions Regarding Wells
Other precautions to be observed with reference to surface wells are the following:
Never dig a well near places where soil contamination has taken or is taking place. Line the sides of the well with either brick, stone, or tile pipe, cemented in a water-tight manner to a depth of at least twenty feet from the surface, so that no water can enter except from the bottom, or at the sides near the bottom.
Raise the surface at the top of the well above the grade; arrange it so as to slope away on all sides from the well; cover it with a flagstone, and cement the same to prevent foreign matters from dropping into the well; make sure that no surface water can pass directly into the well; make some provision to carry away waste water and drippings from the well.
Shallow wells made by driving iron tubes with well points into the subsoil water are preferable to dug wells. Use a draw-pump in preference to draw buckets.
When a well is sunk through an impervious stratum to tap the larger supply of water in the deeper strata, we obtain a "deep well." Water so secured is usually of great purity, for the impurities have been filtered and strained out by the passage of the water through the soil. Moreover, the nature of the construction of deep wells is such that they are more efficiently protected against contamination, the sides being made impervious by an iron-pipe casing. In some rare cases, even deep wells show pollution due to careless jointing of the lining, or water follows the outside of the well casing until it reaches the deeper water sheet. Deep wells usually yield more water than shallow driven wells, and the supply increases perceptibly when the water level in the well is lowered by pumping. While surface wells draw upon the rainfall percolating in their immediate vicinity, deep wells are supplied by the rainfall from more remote districts. Deep wells are either non-flowing or flowing wells. When the hydrostatic pressure under which the water stands is sufficient to make it flow freely out on the surface or at the mouth of the well, we have a flowing, or true artesian well.
Character of Water From Deep Wells
Water from deep wells is of a cool and even temperature. It is usually very pure, but in some cases made hard by mineral salts in the water. Sulphur is also at times present, and some wells on the southern Atlantic coast yield water impregnated with sulphur gases, which, however, readily pass off, leaving the water in good condition for all uses. In many cases the water has a taste of iron. No general rule can be quoted as to the exact amount of water which any given well will yield, for this depends upon a number of factors. Increasing the diameter of very deep wells does not seem to have any marked effect in increasing the supply. Thus, a two-foot well gives only from fifteen to thirty per cent more water than a three-inch-pipe well. This rule does not seem to apply to shallow wells of large diameter, for here we find that the yield is about in proportion to the diameter of the well.