Geology: The Science of the Earth's Crust

by William J. Miller

Fig. 22.—Diagram illustrating a danger of contamination of wells by impure underground water. S, soil; B, bedrock; P, porous stratum. Impure water from cesspool moves through porous layer to bottom of well. (By the author.)

Finally, in this connection, it may be said that wells should be located in the light of the principles above explained, best of all upon the advice of some one with geological training, and that, to insure safety to health from the well water, sanitary analyses (at small cost) should be made once or twice a year. A bad well should be abandoned and a new one sunk.

A large amount of money has been wasted upon, and much mystery and superstition has surrounded so-called “water witches,” or those who claim some special or supernatural power of locating supplies of underground water. Most common of all devices used is the so-called “divining rod,” which is a forked stick of willow, witch-hazel, or other wood, according to the seemingly special requirement of the operator. Certain mechanical and electrical devices are also employed. With one fork of the divining rod grasped in each hand and the main part of the stick upright, the operator walks about until, due to some “mysterious” influence, a place is found where underground water pulls the upright portion of the stick downward in spite of the grasp of the holder. Some operators even claim to know just how deep a well must be sunk. Without any attempt to question the honesty of all operators, geologists are in full accord with the following quotation from a paper published by M. L. Fuller, for the United States Geological Survey: “The uselessness of the divining rod is indicated by the facts that it may be worked at will by the operator, that he fails to detect strong water currents in tunnels and other channels that afford no surface indications of water, and that his locations in limestone regions where water flows in well-defined channels are no more successful than those dependent upon mere guesses. In fact, its operators are successful only in regions in which ground water occurs in definite sheets in porous material, or in more or less clayey deposits, such as pebbly clay or till. In such regions (which are extremely common) few failures can occur, for wells can get water almost anywhere. Ground water occurs under certain definite conditions, and just as surface streams may be expected wherever there is a valley, so ground water may be found where certain rocks and conditions exist. No appliance, either mechanical or electrical, has yet been devised that will detect water in places where plain common sense will not show its presence just as well. The only advantage of employing a “water witch,” as the operator of the divining rod is sometimes called, is that crudely skilled services are thus occasionally obtained, since the men so employed, if endowed with any natural shrewdness, become, through their experience in locating wells, better observers of the occurrences and movements of ground water than the average person.”

It should not be assumed, however, from the above statement that the location or foretelling of underground water is mostly hopeless from a scientific point of view. In most regions the kinds of rocks which would be pierced by wells can be more or less accurately foretold by careful studies of the rocks exposed at the surface. But foretelling the underground water is often much more uncertain. Where the geologic structure or arrangement of rocks in a region is fairly regular, as in the case of most sedimentary rocks, and a few scattering deep wells have been drilled, with records preserved, the geologist, by combining such data with his surface studies, can do much toward putting the facts regarding the underground waters of the region on a scientific basis. There are many such regions, an excellent case in point being Iowa, regarding which State the United States Geological Survey has published a report containing data by the use of which it is possible to foretell almost exactly what formations would be pierced by drilling from 1,000 to 3,000 feet or more, the thickness of each, which ones are water-bearing and, in many cases, even the character of the mineralization of the water for almost any part of the State. Such knowledge, through the years, is worth untold millions of dollars to the State. Where the rocks are igneous and rather uniformly dense, usually little or nothing can be accurately foretold about the underground water supplies, because in such rocks the water follows exceedingly variable and irregular cracks and fissures. In metamorphic rocks the difficulties are usually about as great. In limestone regions, with humid climate, much water travels in channels underground, but these are so exceedingly irregular that there is no way of locating them by surface studies. In humid climates it seldom happens, however, that a well does not reach at least a fair supply of water within a few thousand feet even in rock formations in which the water travels along irregular cracks and channels.

Certain other important features of the geological work of underground water should be brought to the attention of the reader. One of these is its power to dissolve mineral substances of many kinds more or less rapidly. As already pointed out, limestone is especially susceptible to solution in water, both surface and underground.

The carbonate of lime taken into solution from limestone is the principal substance which causes so-called “hard water.” Most of the solution takes place in the upper part of the zone of fracture of the earth’s crust and the dissolved substances are carried along generally to the lower levels where they tend to deposit (and crystallize), filling fissures, cracks, and even tiny spaces between mineral grains. Cracks and fissures thus filled by mineral matter from solution are called “veins.” In many mining regions valuable ores and other substances have been deposited from underground water solutions and concentrated in veins. In many places underground waters with certain substances in solution travel through various rocks or encounter solutions of other substances and, as a result of chemical action, many new mineral combinations result. Such actions through the millions of years of geologic time have effected great changes in many rock formations. In the case of petrification, like that of petrified wood, the buried organism slowly decomposes cell by cell, and particle by particle it is replaced by mineral matter from underground water solutions. In this manner the remarkable so-called petrified forests (not really forests) of Arizona and the Yellowstone Park were formed, the petrifying material there having been the very common substance called silica which is the same in composition as the familiar mineral quartz. Mineral matter carried in solution in surface streams is derived from ground waters which reach the surface. An idea of the tremendous quantity of mineral matter thus removed may be gained from the statement that by careful determination the Mississippi River carries 120,000,000 tons in solution into the Gulf of Mexico each year.

Fig. 23.—Structure section and part of landscape in a limestone region showing how caves and natural bridges are formed by the dissolving action of underground water. AA, limestone; BB, sink holes; DD, caves and galleries; and an arch (natural bridge) which is the remnant of a large cave. (After Shaler, U. S. Geological Survey.)