The next subject to claim our attention is the protection of the sources
of drinking water. In the country water is, as a rule, obtained from wells
and springs. The important bearing upon well water of soil purity demands
a few explanatory remarks concerning the origin of well water. Wells
are excavations made into the ground to a variable depth until water is
reached. This water is denominated ground or subsoil water. Its origin
may be better understood if, for the moment, we conceive the surface of
the earth as more or less irregular and entirely impervious to water. The
rain would collect on this surface and form lakes, ponds, and streams,
according to the configuration of the surface. If, now, we conceive this
surface covered with sand or other porous earth to a greater or lesser
height, and the top of this be considered the earth's actual surface, the
water will remain in the same position, but it will be buried within and
fill the pores of the overlying soil as subterranean lakes, ponds, and
streams. In digging a well we remove this porous layer of earth until we
reach these subterranean streams or reservoirs of ground-water. If the
above description be thoroughly understood, the condition under which well
water may be obtained at different depths will become intelligible, and
it will also appear plain why ground-water may flow as any surface stream
and pick up on its way various substances which have percolated into the
ground.
When the bed of porous soil overlying the impervious layers is very deep,
wells will have to be dug down to a considerable depth to reach the
surface of the ground-water. Where this layer of pervious earth is of
slight thickness wells will be shallow, and the ground-water may appear on
the bottom of gullies, trenches, and wherever the porous layer has been
dug or washed away.
The movement of the ground-water depends on the inclination or slope of
the impervious strata, and has been observed to be quite rapid in some
instances. By adding common salt to the water in a well its detection
in other wells at a short distance has been found a guide in the
determination of the rapidity and direction of the underground current.
When the ground-water resting on the uppermost impervious layers is near
the surface, and therefore not safe or fit to use as drinking water,
it may be possible by digging below this layer to find another porous
bed containing water. This source will, in general, be much purer since
it is less exposed to pollution from above, and since the water has to
travel longer distances underground. Such a deep supply must, however, be
protected from the superficial supply by a water-tight wall extending to
the surface of the deep supply, otherwise the water from the upper layers
will simply drain into the well.