Fig. 20.—Ideal section illustrating the chief requisite conditions of artesian wells. A, a porous stratum; B and C, impervious beds below and above A, acting as confining strata; F, the height of the water level in the porous bed A, or, in other words, the height of the reservoir or fountainhead; D and E, flowing wells springing from the porous water-filled bed A. (After U. S. Geological Survey.)
Most wells are simply dug to depths a little below the water table. In humid climate regions the depths seldom exceed fifty feet. The water encountered in such wells is rarely under pressure. In some regions of deep soils or loose formations, wells are actually bored with an auger to depths of as great as 200 feet. Deep wells in relatively hard rocks are always drilled to depths of even thousands of feet. In such cases the purpose is to strike either a porous rock layer charged with water, or a crack or fissure filled with water, the water almost always being under pressure (sometimes very great), under such conditions. These are called artesian wells whether the water under pressure actually flows out at the surface or not.
We may now inquire as to the necessary conditions for artesian wells. This may best be done by the aid of diagrams. [Figure 20] illustrates a very common case where a porous layer, lying between impervious layers, passing under a valley, comes to the surface of the hills on each side where the water enters the porous layer. On sinking a well to the water-charged layer, the water rushes through the hole to a greater or less distance above the surface. In [Figure 21] the porous and impervious layers are simply tilted, and the water under pressure rises through the free opening to the surface. Wells of this kind are also common in the Atlantic Coastal Plain of the United States. In another case, less comprehensible to the layman, the porous water-bearing stratum curves downward under a hill or mountain, water entering it where it is exposed on each side. Under such conditions a flowing artesian well cannot be drilled at or near the summit, but since the water is under pressure it will rise in the hole to a level approaching that of the lowest part of the outcrop of the porous layer on either side of the hill or mountain. This is essentially the condition of things toward the interior of Iowa, where water from the deeper wells rises 2,000 feet or more in the holes, but does not reach the surface.
Fig. 21.—Section illustrating the thinning out of a porous water-bearing bed. A, inclosed between impervious beds B and C, thus furnishing the necessary conditions for an artesian fountain at D. (After U. S. Geological Survey.)
The drilling of deep wells, where records, including samples of rock materials brought up, have been kept, has been a great aid to the geologist in determining, or rendering more precise, the knowledge of not only the kinds of rocks underground, but also the thicknesses and structural relations of the formations.
In yet another way deep wells are of special significance, that is in regard to the light which they throw upon the subterranean temperature of the earth. Very recently the deepest well in the world was drilled near Fairmont, West Virginia, to a depth of 7,579 feet, in quest of oil or gas. At a depth of 7,500 feet, the temperature was found to be 168 degrees F. Allowing for a near-surface temperature 50 degrees, this means an average rate of increase downward of 1 degree in 62 feet. The second deepest well is near Clarksburg, West Virginia, sunk to a depth of 7,386 feet, with a temperature of 172 degrees at the 7,000-foot level, or at the rate Of 1 degree in 57 feet, allowing for a near-surface temperature of 50 degrees. It is a remarkable fact, that little or no water was encountered all the way down. A well 7,348 feet deep in southeastern Germany gave a temperature of 186 degrees at the bottom, or a rate of increase of 1 degree in 54 feet. These three records are about the average for the deep holes of the world. Next to the deepest mining shafts in the world are in the copper mining region of northern Michigan, where over 5,000 feet (counted vertically, not down the slope) down the temperature is nearly 90 degrees the year round. The rate of increase is here less than in most wells of such depth, because of the cooling air currents. Many years ago a rather remarkable experiment in well drilling was tried by the city of Budapest, Hungary, the attempt being to get a supply of water at the brewing temperature of 176 degrees in order to encourage the manufacture of beer. After getting a good supply of water at a depth of 3,120 feet and a temperature of 158 degrees, work was stopped. In building the two great tunnels (St. Gotthard and Simplon) through portions of the Alps, such high temperatures were encountered that work was continued only under great difficulties. In the famous Comstock gold and silver mine of Nevada, over forty years ago, temperatures as high as 157 degrees were encountered in the shafts at a depth of only 2,000 feet or a little over, the exceptional temperature for such depth no doubt being due to occurrence of the ores in geologically recent igneous rocks which have not yet cooled to the normal temperature for the depth of 2,000 feet.
From the sanitary standpoint, wells are of very great significance, especially in view of the fact that such a large proportion of people depend upon well water. It is generally understood that typhoid fever is more common in the country than in cities, in spite of what might reasonably be expected. What are some of the causes leading up to such a situation? The idea that water purifies itself after flowing a relatively short distance is, in many cases, far from being true, especially when we are dealing with underground water. Actual observations prove that germ-laden water may travel surprisingly far underground. Germ-laden water from barns, cesspools, or outhouses spreads notably on sinking to the water table and it is easy to see how so many wells become contaminated. On general principles, a geologist is especially wary of water from a well in a barn yard. The well for human use at least should be located out of reasonable range of such contamination. Under the condition of the diagram a well or spring some distance down the side of the hill may actually be unfit for use, though a serious situation is much less likely to develop there. Nor should one assume that by locating the well on the uphill side of the house, and the outhouses or cesspool on the downhill side, safety is assured. From what we have learned in regard to earth movements, and the tilting of strata from their original positions, we know how the movement of water in the saturated zone near the surface may be downhill roughly following the hill slope, while in a tilted porous layer of rock farther below the surface the movement of water may be in just the opposite direction. A well drilled into the solid rock for safety on the uphill side of a house might derive its water from this very same porous layer, whose water has been contaminated from a cesspool or other source down the side of the hill. Such a case is by no means a theory or a rarity. There is also real danger of contamination in cases where the water flows more like streams underground through cracks or fissures in hard or dense rocks, or through channels developed by solution in limestone. It may happen that water becoming contaminated from barn sites, cesspools, or outhouses finds its way along such a channel to the side or bottom of a well. The author well remembers the case of a farmer whose house, barn, and well were close together on a little limestone terrace and who continued to use the well water although he complained of its disagreeable taste, especially after a rain when he could “taste the barn in it.”