Popular Science Monthly/Volume 42/March 1893/Artesian Waters in the Arid Region
|ARTESIAN WATERS IN THE ARID REGION.|
By ROBERT T. HILL.
THE United States Government expends annually over twenty million dollars, mostly in the Eastern half of the country, for the improvement of its rivers, harbors, and other surface waters. The Western half of our domain, which with the exception of the upper coast of the Pacific is known as the arid region, possesses no superabundance of surface waters to improve, but, upon the contrary, the scarcity of water for ordinary domestic and agricultural uses prevents the settlement and utilization of the remaining portion of the public lands. Even the semi-humid or Great Plains region, east of the Rocky Mountain front, has been retarded in development by this scarcity of surface water; and many settlers, who purchased alleged agricultural lands from the Government in this region, are begging Congress to apportion for the investigation of its underground resources a sum at least as large as that given for the smallest creek upon the River and Harbor Bill. Our national legislators have not been entirely neglectful in the matter, however. The rivers of the arid region have been gauged, and the rainfall ascertained, with the disheartening conclusion that, could every drop of the rainfall be utilized, it would still be insufficient to water the fertile lands. Large sums of money have also been wasted in vainly bombarding the skies for rain, contrary to every known law of Nature. The Department of Agriculture has investigated the underground waters of the Great Plains region east of the Rocky Mountains, but the underground supply of the true American Desert lying between the Rockies and the Sierras has been little studied.
Fig. 1—Prevalent Conception of Ordinary Artesian Conditions.Drawn from a geological report.
This section includes one fifth the total area of the United States and most of the great central plateau of Mexico. It is marked by peculiar geographic, geologic, and climatic phenomena altogether different from those of the rest of the country, chief of which is the absence of surface water. Streams are rare even in the mountains, and, with the exception of the Colorado, the Snake, and the Rio Grande, not a drop of its surface water reaches the sea, so great is, the evaporation and the capacity of the porous desert soils for absorption. Almost any Eastern State has a greater area of surface water than has all the arid region; and the smallest New England brook, could it be transported West, would be a great blessing. In this arid section there are many thousand square miles without a drop of water even for drinking purposes. Nearly every available stream has been appropriated for irrigation by the present population, and all improvement in the water supply must come from underground sources.
It is wrong to encourage anticipations of enormous supplies of underground water where rainfall is so slight; but when we remember that in this region water is of greater value than land, or rather that land is worthless without water, the procurement of even small supplies, sufficient for stock, for irrigating small areas, or for supplying the thirsty locomotive, will be of great value. In view of these facts it is well to understand the laws of the occurrence and availability of underground waters, for not only have large sums been wasted in boring in unfavorable localities, but impracticable notions have been obtained from scientific treatises on this subject.
The laws of the distribution and utilization of underground water are as simple as those controlling the surface supply, but the popular fallacies concerning them are appalling. The most prevalent of these is that the waters originate at some remote point from their outlet, and flow in subterranean streams like the "blood in the human body," as a farmer once said, and that these streams must be tapped by the well borer or digger before water can be obtained. In nearly every community is some person supposed to possess the art of locating the exact spot above these currents by means of a switch called the divining rod. It is also a current fallacious belief that all underground water is due to rain which falls on the more or less distant mountains, and especially is this true in the region between the Rocky Mountains and the Mississippi, where every spring and well, even on the Texas
Fig. 2.—Favorable Structure for Artesian Water, in which the Receiving Area is a Valley.
coastal plain a thousand miles distant, is commonly explained upon the hypothesis that the water comes from this lofty range.
These prevalent impressions in the minds of those untrained in geology are more excusable than the widely prevalent idea conveyed by cuts in geological text-books that the usual and ordinary conditions for artesian wells are in great synclinal areas in which the strata can be seen markedly dipping from two including mountain borders against which their edges are upturned as shown in the following figure.
While there is no theoretical objection to this ideal conception, the conditions it represents seldom occur in Nature; on the contrary, as will be shown later, mountain rocks are not the source of great artesian wells; neither do they usually occur in synclinal valleys, but the most favorable conditions are gently sloping monoclinal plains in which the receiving areas, instead of being the upturned mountain rocks, are, in fact, the escarpment valleys of the plains. (See Fig. 2.)
To understand the distribution of earth water, it is necessary to be familiar with the true laws of its occurrence. The rainfall is the source of all underground water, and with the exception of certain deep-seated artesian wells the source is always the rain which falls in the immediate vicinity, as the physician knows when called to treat disease caused by seepage of the adjacent surroundings into the family well.
Part of the rainfall is quickly drained away by the surface channels, a part is evaporated, and a third, and for our consideration the most important part, is imbibed by the rocks and soil. The proportional disposition of the rainfall in the above manner varies with the climate and geologic conditions, but so far as underground waters are concerned it is necessary to consider only the water which sinks into the ground.
That portion of the earth visible to human inspection, known as the crust, is more or less saturated with water. In times of drought and in the arid region this is not always evident at the immediate surface, where evaporation is taking place, but a post hole, a plow furrow, a blast in a quarry, or a newly dug well reveals the dampness of the rock material. This moisture is sometimes invisible to the eye, but in general its quantity varies in proportion to the compactness or porosity of the rocks, the number of joints, fissures, or other crevices, and the topographic situation which controls the drainage.
If rainfall be long continued, the portion of the crust upon which it falls becomes completely saturated. Upon cessation of the rain, evaporation or drying begins at the surface, causing the line of saturation to sink deeper and deeper. Thus it is that in the Eastern States, where rainfall is excessive and evaporation slow, the line of saturation usually coincides with the surface, while in the arid regions it is often several hundred feet below. In this section, holes three hundred feet deep are often drilled through soil and rock as dry as powder without reaching the line of saturation, while on the East, as for example in New Orleans, water is so near the surface that dry graves can not be dug for the dead.
If the earth were of uniform porosity, temperature, and composition the water it contains would be uniformly distributed through it, as is the water in a well-soaked sponge. But this is not the case, for the outer portion of the globe consists of rocks of much less density than are those of the interior, while the downward percolation of water in some instances encounters the superheated mass of the earth's interior, and is forced back to the surface as steam, as in geysers and volcanoes, or enters into mineral combinations. Hence the available water is confined to that portion of the earth's crust between the lines of heated interior and surface evaporation. Even in this narrow belt the distribution of water is very irregular.
Inasmuch as there is a great diversity of geologic structure, the possibility of securing water at any given point must be determined by the local formations. All rocks imbibe moisture in proportions varying with their physical structure, a fact which can be demonstrated experimentally by saturating familar types of rocks. Glass is similar in water capacity to large areas of volcanic and other igneous rocks, and will absorb no perceptible amount of moisture; marble will drink in only a slight quantity; while chalk, sand, and brick will absorb nearly their own weight of water. The manner in which rocks absorb water is simple.
Fig. 3.—Mesa Structure of Llano Estacado.
In most rocks, however compact to the eye, there exist interstices, cavities, and other spaces in which water may enter and be stored. This is especially true of all sedimentary rocks, which comprise ninety-nine per cent of the earth's crust. A fine sandstone whose grains and intervening spaces are indistinguishable to the eye, when placed under the microscope resembles a mass of cobblestones in which the spaces occupy as much of the aggregate area as the solid particles. Into a gallon measure of dry pebbles varying in size may be poured half a gallon of water. The consolidated rocks which compose most of the mountain masses are more compact and less adapted for the storage and passage of water than the sedimentary rocks. Nearly all the minerals which compose them are impervious, as is readily seen in a large crystal of quartz, feldspar, or mica. The rocks of valleys and plains usually consist of detrital material less hardened by mountain-folding, and hence more pervious.
Rocks which have imbibed all the moisture they can contain are in a condition of saturation, and all water in excess of this
Fig. 4.—Artesian Functions of Impervious and Pervious Strata.
amount will pass off by gravity or evaporation. The excess above the water of saturation is available as the source of springs, but the supply of wells is from the water of saturation.
Each kind of rock has an individual capacity for the transmission of the water which it has imbibed, and this is entirely distinct from its capacity for imbibition. If the component particles of a rock—for instance, the quartz pebbles of a loose conglomerate or the grains of a sandstone—present an impervious surface, water will cohere to the individual surfaces until the entire specimen is enveloped in a coat of water. If the interstices are smaller than the average drop of water, the resistance of cohesion to the transmission of water will be greater; hence a chalk or a fine-grained brick will drink in much water, but will transmit it slowly, while water will pass rapidly through coarse gravel. The capacity for transmission in variously grained rocks and the accompanying cohesion is similar to that seen in passing water through sieves of different mesh. Thus, some sandstones of exactly the same capacity for imbibition as chalk transmit water six hundred times faster.
The rock materials of the earth with these different capacities for imbibition and transmission have been sorted into definite sheets or strata by the water which deposited them, and thus another important fact in the question of underground water is introduced—the stratification or arrangement of the rocks relative to one another. Earth water percolates downward through a
Fig. 5.—Unfavorable Conditions for Artesian Water.
porous stratum until an impervious one is reached, while an impervious stratum at the surface will prevent the saturation of a pervious one below. Stratification performs the important function of controlling the distribution of earth water, of resistance, transmission, and storage. If the surface rock stratum is pervious and horizontal, it simply serves as a sponge to hold the water until disturbed by evaporation or seepage, unless the supply is constantly renewed by rainfall. (See Fig. 3.)
If an impervious sheet is above an inclined outcropping porous stratum (Fig. 4), it opposes the tendency of water to rise by hydrostatic pressure and retains it in the porous sheet. If an impervious stratum is beneath a porous one, it prevents the water of the latter from percolating to greater depths. If vertically arranged from folding, the including strata cut off the horizontal transmission of underground water. (Fig. 5.)Water is transmitted by gravity in the direction of the inclination of the strata—i. e., with the dip; and if the topographic conditions are favorable, flowing wells can be obtained at lower points more or less distant from the outcrop. If the strata
Fig. 6—Typical scene in Arid Region, Trans Pecos, Texas.
incline in a direction opposite to the general slope of the country, no matter how favorable the conditions, they will furnish no flowing artesian supply, for water can not rise above the height of the receiving area. (Fig. 5.)
If strata are excessively inclined, as in most mountain regions, artesian wells are improbable if not impossible over any wide area, for the strata soon dip below all available borings; hence the generally accepted idea that artesian wells are peculiar to regions of great stratigraphic dip is fallacious. A dip of one per cent is scarcely visible to the eye, but it will carry a stratum downward 52·8 feet per mile; a dip of ten per cent is hardly noticeable, but will carry a stratum 528 feet in a mile; a dip of forty-five degrees will carry a stratum deeper in a mile than any drill has yet penetrated.
If the earth's surface were level, and a homogeneous mass, earth water would be at a uniform depth throughout, as in an undrained field. But the surface is broken into mountains and plains, and scored by valleys, and the line of saturation sinks toward the level of these, where springs are often found escaping at the level of the streams. There are in Nature two kinds of valleys: (1) Unfinished, or active valleys, which are in the process of being cut out at the present time by the streams seeking base level; and (2) finished, or ancient valleys, which originated in past geologic time, and have been partially refilled with the débris of the adjacent region. All the valleys in the mountains proper, and of the eastern United States, belong to the first class, which may be called stream valleys, and their function is to furnish a channel for the passage of the surface waters to the sea. The valleys of the second class, or basin valleys are characteristic of the great arid region, and, with one or two exceptions, they are void of running surface water.
In mountains the surface and underground water is constantly seeking the level of the surrounding valleys, owing to the action of gravity. In general, mountains owe their existence to the superior hardness and imperviousness of their strata, and are of little importance to the problems of underground water.
Basin plains surrounded by the great areas of mountain surface are more favorably situated for the occurrence of underground water in quantity than those with a smaller surrounding area of mountain slopes, for impervious mountains serve to concentrate the rain-water which runs down their slopes upon the pervious valleys, thereby increasing the available water supply beneath the latter. (Fig. 6.)
The water of saturation in buttes and mesas, which usually consist of horizontal strata, is reduced by gravity toward the level of the surrounding plain, or, when alternations of pervious and impervious strata occur, the water seeps out as springs at their contact. The Llano Estacado, or great Staked Plain of Texas and New Mexico, is the largest of all the American mesas in area. Its geological structure is practically that shown in Fig. 3, consisting of a pervious surface formation, averaging three hundred feet, resting upon a foundation of impervious clays and other rock. The upper formation readily imbibes all the surface rainfall; hence the region is void of running streams.
Throughout this large area, once considered hopelessly void of water, good non-flowing wells are now everywhere obtained by boring to the lower depths of the saturated, sponge-like surface formation, while springs occasionally break out at the margin of the plains where the two formations are in contact.
While water-bearing strata should always be porous, and usually are but slightly if at all consolidated, the degree of consolidation has but little bearing upon the retaining function of impervious strata. Soft clay shale is practically as impervious as hard slate. In the West many people discredit the possibility of artesian water in many favorable localities, because of the absence of consolidated strata which they suppose are necessary to constitute the impervious stratum above the one containing the water. In fact, the less consolidated the rocks of a region are, the more favorable are the artesian conditions; and, inasmuch as the older formations of the earth are more consolidated, metamorphosed, and disturbed by greater tilting, faults, and folds, they are least favorable for the occurrence of artesian water. Upon the other hand, the later formations present the opposite and more favorable conditions, and with few exceptions the great artesian wells of the world are found in them. These later rocks play an important part in the geology of the arid region.
Fig. 7.—Sections from the rocky mountains to the gulf.
Fig. 8.—Basin and Mountain Scenery of Arid Region, Southwest New Mexico. From Mexican Boundary Report.
of alternations of porous and impervious strata of the later geological ages, dipping at an almost imperceptible angle toward the sea and accompanied by slight scarp valleys along their western outcrop, which are the receiving areas for the artesian waters. The Atlantic coast plain from New Jersey to the Rio Grande nearly everywhere presents similar conditions, and abundant artesian wells have been obtained. This group of rocks rests upon another series of older rocks (b, c), which presents negative conditions for artesian water, owing to their inclination in a direction opposite to that of the topographic slant. No artesian wells of large flow have been, or are apt to be, obtained in this region. Above the west part of this series is the great mesa of the Llano Estacado (d), the non-flowing wells of which have been explained. A second negative area is shown in the portion of the diagram in northeastern New Mexico (d), where the inclination of the strata is again opposite to that of the topographic slant. Where the front of the Rocky Mountains appears (e), the principle that the mountain rocks are unfavorable for artesian conditions is shown by the faulting and excessive dip of the strata.
Let us now briefly examine the bearing of the foregoing principles on the question of underground water in the great arid
Fig. 9.—Filled-in Valley of Arid Region.
region proper, west of the Rocky Mountain front. Topographically this country, from the union of the Cordilleras in southern Mexico to the British boundary, consists in alternations of mountain and desert plain (Fig. 8). The mountains are isolated masses of hard, impervious rock, broken by faults, and dipping at angles which render the strata unpropitious for artesian exploitation. The wide areas of desert plain separating the mountain masses are of the older type of valleys described on a previous page, which are now filled to a depth of two thousand feet by the detrital deposits from the adjacent mountains (Fig. 9). The original valley floor, consisting of mountain rook, is entirely obscured by these deposits, and of no value to the artesian possibilities. The rainfall upon the mountains is rapidly shed by canon-streams and arroyos to the level of the adjacent valleys, where it sinks into the ground, owing to the thirsty character of the valley formation, and gravitates downward toward the lower and usually central depths of the deposit, the underlying floor of mountain rock serving as a bottom for the retention of the water in the valley deposits. So great is the capacity for imbibition of these desert plains that every drop of rain upon them, except such as is evaporated, is quickly drunk in, and all the mountain streams of the vast region, with three exceptions, completely disappear upon reaching them, and are known as "lost rivers" in the parlance of the West. Under the old erroneous idea that the mountain rocks contained the artesian waters, many hundred futile and costly experiments have been made by the Government, railroad corporations, and private individuals, in boring wells at the margins of these deserts, where the mountain rock was seen disappearing beneath the valley deposit, instead of seeking the lowest topographic point in the
Fig. 10.—Desert Travellers.
plains, and relying upon the unconsolidated formation of the desert as the probable source of water.
Within the past few years there have been many accidental demonstrations of this principle; and when it is generally understood, it is probable that in nearly every one of these now useless waste places at least a small quantity of water will be secured, and in many instances good flowing artesian wells. Wells of this character have been procured in great number in California, at Riverside and in the San Joaquin Valley, not one of which penetrates to the underlying floor of impervious mountain rock, but are all secured in the detrital valley deposit. Similar wells have been found in numbers in the Great Salt Lake Desert of Utah, and a few in the deserts of Nevada. In Mexico they are obtained in the valley deposits of the city of Mexico, the Lake Chapala basin of Jalisco, and could be secured at many other points. The grandest demonstration of the principle, however, is found in the famous San Luis Park of Colorado. It is true that this valley, owing to the large area of mountains surrounding it, has an unusual number of "lost rivers" supplying water to the valley deposits from which over thirty-seven hundred flowing wells have been obtained. Less than ten per cent of these attain a depth of seven hundred feet, and barely fifteen per cent reach four hundred feet. On the great La Noria Desert, stretching for one hundred miles northeast of El Paso, Texas, between the Organ and Franklin ranges, which is void of a drop of surface water, abundant supplies of underground water are now procured at a depth of two hundred feet, and are pumped by windmills for irrigating purposes.
The wells of all these localities were drilled haphazard, without reference to the geological principle we have endeavored to describe. If this principle could be made known that the underground waters of the arid region are stored in the desert deposits, and not in the mountain rocks, there is no reason to doubt that wells could be procured in most of these innumerable wastes of the arid region, which would at least suffice for the passing traveler, and in many cases supply water for live stock and irrigation, sufficient to supply the necessaries of life to the mining populations of the adjacent mountain regions.