CHAPTER IV.
THE WORK OF GROUND- (UNDERGROUND) WATER.
Many familiar facts demonstrate the general presence of abundant water beneath the surface of the land. The thousands of wells in regions peopled by civilized races, and the countless springs which issue from the sides of mountains and valleys are a sufficient proof both of the wide distribution of ground-water and of its great abundance.
Certain well-known facts make it clear that ground-water is intimately connected with rainfall. In a dry season the level of the water in wells commonly sinks, and after a heavy rain it rises ([p. 71]); and the amount of sinking is greater when the drought is long, and the rise is most notable when the rainfall is heavy. Many springs which discharge large quantities of water during a wet season flow with reduced volume, or cease to flow altogether in periods of drought. Furthermore, the water of springs and wells has the properties which rain-water would possess after sinking beneath the surface and dissolving mineral substances. Rain-water is seen to sink beneath the surface with every shower, and since this source seems altogether adequate for ground-water, and since no other source is known whence any considerable amount of ground-water might come, it is concluded that atmospheric precipitation is its chief source.
Water gets beneath the surface by processes which are readily seen. Wherever the soil is porous some of the rain which falls upon it is absorbed. Sinking through the soil to the solid rock it finds cracks and pores through which it descends to great depths. Nowhere are the rocks beneath the mantle rock so compact and so free from cracks, when any considerable area is considered, as to prevent the percolation of water through them.
Conditions influencing descent of rain-water.—There are several conditions which influence not only the amount of water which sinks beneath the surface in a given area, but the proportion of the precipitation which follows this course. These are as follows: (1) Amount of precipitation.—In a general way it is true that the greater the amount of precipitation the greater the amount of water which will sink beneath the surface. Other things being equal, a region of heavy precipitation is a region where wells are easily obtained and springs common. (2) Rate of precipitation.—A given amount of precipitation may be concentrated in a short interval, or distributed through a considerable period of time. In the latter case more of the water sinks beneath the surface; in the former, a larger proportion runs off over the surface. The reason is readily seen. Water passes through small spaces, such as those of soil, slowly, and its rate of passage decreases rapidly with decreasing size of the passageways. When rain falls rapidly on a surface of even moderately close texture, the uppermost layer of soil is promptly filled with water, and since the water passes downward slowly, the uppermost saturated part of the soil becomes virtually impervious. While in this condition, the water which falls on it will run off if there be slope, and stand if there be none. In the latter case it will sink slowly as the water in the soil passes down to lower levels. If precipitation takes place no faster than the water can sink through the soil, all the water may become ground-water. (3) The topography of the surface has much to do with determining the proportion of rainfall which becomes ground-water. If the surface be flat, more will sink in; if it be sloping, more of it will run off before it has time to sink. Other things being equal, the steeper the slope the larger the proportion of the rainfall which will run off over it. (4) The texture of the soil, or other material on which the rain falls, helps to determine what proportion of it sinks beneath the surface. If the surface materials be porous, the water sinks readily; if of close texture, it finds less ready ingress. Other things being equal, the closer the texture of the soil the less the proportion of the rainfall which will enter it. (5) The texture and structure of the rock beneath the surface have some influence on the amount of ground-water. The rock may be stratified or massive; it may be abundantly or sparsely jointed; it may be porous or compact. On the whole, stratified rock is more favorable for the entrance of water than unstratified, partly because of its greater average porosity, and partly because the planes of division between beds often allow the passage of water. If the beds of stratified rock are vertical or inclined, water finds its way into them more readily than if they are horizontal, in so far as it descends along stratification planes. Horizontally bedded rock, or rock which is not bedded at all, may be so much jointed, and the joints so open, as to allow the water to enter readily.
The conditions favorable to the sinking of abundant water below the surface are therefore heavy precipitation, falling slowly on a surface with little relief, a soil of open texture underlain by rock which is porous, or affected by vertical or highly inclined planes of cleavage. The annual discharge of water by rivers is estimated to be about 22 percent. of the rainfall on the land.[93]
Supply of ground-water not altogether dependent on local rainfall.—The amount of ground-water in a given region is not always entirely dependent on the local rainfall. Ground-water is in constant movement, and entering the soil or rock at one point it may, after a long subterranean journey, reach a point far from that at which it entered. Thus beneath the Great Plains of the West there is much subterranean water which fell on the eastern slopes of the mountains to the west. It has flowed beneath the surface to the Plains, where some of it is now withdrawn for the purposes of irrigation in regions where rainfall is deficient. The accompanying diagram ([Fig. 199]) illustrates the flow here described.
Fig. 199.—Diagram illustrating the general point that ground-water is not dependent entirely upon local supply.
Fig. 200.—Diagram illustrating the position of the ground-water surface (the dotted line) in a region of undulatory topography.
The ground-water surface. Water table.—The water table has already been defined ([p. 71]) as the upper surface of the ground-water. In a flat region of uniform structure the ground-water surface is essentially level, but rises and falls with the rainfall. Where the topography of a region is not flat, the ground-water surface is not level. As a rule it is higher, though farther below the surface, under an elevation than under surrounding lowlands, as illustrated by [Fig. 200]. The explanation is not far to seek. If a hill of sand be exposed to rainfall, most of the water falling on its porous surface will sink into it. If the precipitation continues long enough, as in a protracted rain, the hill of sand will be filled with water, the water occupying the interstices between the grains. If the sand of the hill could be removed, leaving the water which it contains on the same area, it would constitute a mound perhaps a third or a fourth as high as the hill itself. If unsupported, this mound of water would spread promptly in all directions until its surface was level. While the sand remains, the water in it constitutes a mound, and has a tendency to spread. It does in fact spread, but since the process involves great friction the spreading is slow. With the spreading the surface of the water in the sand sinks, and sinks fastest at the center where it is highest (b, [Fig. 201]). If the process were not interrupted the surface of the water in the hill would, in time, sink approximately to the level of the water in the surrounding land (d, [Fig. 201]); but at every stage preceding the last, the surface of the water would be higher beneath the summit of the hill than elsewhere, though farther from the surface. In regions of even moderate precipitation the water surface beneath the hills rarely sinks to the level of that in the lowlands adjacent, before being raised by further rains.
Fig. 201.—Diagram to illustrate the relations of ground-water to the surface.
The water-level beneath the lowlands also sinks. Some of it finds its way into valleys, some of it sinks to greater depths, and some of it evaporates; but since the water surface beneath the elevation sinks more rapidly than that beneath the lowland, the two approach a common level. Their difference will be least at the end of a drought, and greatest just after heavy rains.
Depth to which ground-water sinks.—The depth to which ground-water penetrates has not been determined empirically. No borings or excavations of any sort have been made to such depths as to indicate that its limit was being approached, though some of them are a mile or more deep. There is a popular belief that water sinks until it reaches a temperature sufficient to convert it into steam, but except for special localities where hot lava lies near the surface, this belief is not well founded. In the first place, it is not known at what temperature water below the surface would be converted into steam. While water boils at sea-level at a temperature of 212° (Fahr.) a higher temperature would be necessary below that level.
Assuming the temperature of water sinking beneath the surface to be 50° Fahr., its temperature must be raised 162° to bring it to the temperature at which it would boil at sea-level. On the above assumption of initial temperature, the following table shows the depths at which water would reach a temperature of 212° Fahr. under various assumptions as to the rate of increase of temperature. It shows also the pressure in atmospheres which would exist at these several depths if the overlying rock were full of water.
| Rate of Increase of Temperature. | Depth at which Temperature of 212° would be reached. | Equivalent Pressure in Atmospheres. | ||
|---|---|---|---|---|
| 1° for 50 feet | 8,100 | feet | 238 | (approximately) |
| 1° for 60 “ | 9,720 | “ | 285 | “ |
| 1° for 70 “ | 11,340 | “ | 333 | “ |
With an initial temperature of 80°, corresponding to that of the warmer parts of the earth’s surface, instead of 50°, the table would be as follows: | ||||
| 1° for 50 feet | 6,600 | feet | 194 | (approximately) |
| 1° for 60 “ | 7,920 | “ | 214 | “ |
| 1° for 70 “ | 9,240 | “ | 272 | “ |
The temperature at which water boils increases with the pressure. A pressure of about 200 atmospheres is the critical pressure for water; that is, the pressure which, if increased, will prevent boiling altogether. The depth at which a pressure of 200 atmospheres would be reached, supposing the upper rock to be full of water, is about 6800 feet. The temperature of the water at this depth, under various assumptions as to initial temperature and rate of increase of heat, is shown in the following table:
| Initial Temperature. | Rate of Increase of Temperature. | Temperature at a Depth of 6,800 Feet. |
|---|---|---|
| 50° | 1° for 50 feet | 186° Fahr. |
| 50° | 1° for 60 “ | 163° “ |
| 50° | 1° for 70 “ | 147° “ |
| 80° | 1° for 50 “ | 216° “ |
| 80° | 1° for 60 “ | 193° “ |
| 80° | 1° for 70 “ | 177° “ |
Only one of these temperatures reaches the boiling-point of water at sea-level. It is therefore clear that at this depth water has not even closely approached the boiling temperature for this depth, and since this is the depth of the critical pressure, it is clear that it cannot boil at any greater depth. The descent of water is therefore not stopped, under normal conditions of crustal temperature, because it reaches its boiling-point. Locally, as in the vicinity of active or recently extinct volcanoes, the case may be different.
It is conceivable that water may descend until it reaches its critical temperature (somewhere between 610° and 635° Fahr.). The depth at which the critical temperature would be reached, under various assumptions, is shown in the following table:
| Initial Temperature. | Rate of Increase of Temperature. | Depth of Critical Temperature. |
|---|---|---|
| 50° | 1° for 50 feet | 28,000 to 29,250 feet |
| 50° | 1° for 60 “ | 33,600 to 35,100 “ |
| 50° | 1° for 70 “ | 39,200 to 40,950 “ |
| 80° | 1° for 50 “ | 26,500 to 27,750 “ |
| 80° | 1° for 60 “ | 31,800 to 33,300 “ |
| 80° | 1° for 70 “ | 37,100 to 38,850 “ |
There is good reason, in the increasing density beneath the surface, for believing that the rate of increase of temperature decreases with depth, and therefore that the rate of 1° for 50 feet for the depths concerned is too high. The greater depths of the table above are therefore believed to more nearly represent the truth than the lesser ones. (See discussion of underground temperature in [Chapter XI].)
If descending water attained its critical temperature, the extent to which the resulting water-gas might be absorbed is not known. So far as limited by temperature, therefore, it is not possible to assign a limit to the descent of water under average conditions of crustal temperature.
Other considerations seem to place a limit to the descent of water. Rock, solid and unyielding as it seems, is yet plastic when under sufficiently great pressure. The cracks and cavities affecting it are believed to descend a distance which is but slight in comparison with the radius of the earth. Even if openings were once formed at greater depths, they could not persist, for the adjacent rock, under the pressure which there exists, would “flow” in, in effect (though perhaps not in principle) much as stiff liquid might, and close them. The outer zone of the earth where cavities may exist is known as the zone of fracture.[94] The depth of the zone of fracture differs for different rocks, but is not believed to extend below some such depth as five or six miles, even for the most resistant.[95] It is to be noted that these depths are less than those at which the critical temperature of water would be reached under most of the conditions, including all the more probable ones, specified in the above table.
Let it be assumed that water descends through openings in the rock to a depth of six miles. At this depth it would, under the various assumptions specified in the first and second columns of the following table, have the temperature indicated in the third column:
| Initial Temperature. | Rate of Increase of Temperature. | Temperature at Depth of Six Miles. |
|---|---|---|
| 50° | 1° for 50 feet | 683° Fahr. |
| 50° | 1° for 60 “ | 578° “ |
| 50° | 1° for 70 “ | 502° “ |
| 80° | 1° for 50 “ | 713° “ |
| 80° | 1° for 60 “ | 608° “ |
| 80° | 1° for 70 “ | 532° “ |
In two of these cases, namely, those in which the assumed rate of increase of temperature is highest, the temperature of the water at the assumed lower limit of the zone of fracture is above the critical temperature of water. If the assumptions involved in these two cases be correct, water might descend to the point where it would be converted into water-gas, and in this condition it might be occluded by the hot rock. In the other cases, involving the more probable assumptions, the critical temperature is not reached at a depth of six miles. If pores and cracks do not extend to greater depths, liquid water could not; and since the water at this depth has probably not reached its critical temperature, it cannot exist as water-gas. If it does not exist in the form of water-gas, its occlusion by the hot rock substance would not be probable. It would seem, therefore, that the descent of water under ordinary conditions is much more likely to be limited by the zone of fracture, than by temperature.
Movement of ground-water.[96]—Ground-water is in more or less continual movement. If all the water be pumped out of a well it soon fills up again to its normal level by inflow from all sides. Springs and flowing wells also demonstrate the movement of ground-water. Near the surface the movement of ground-water is primarily downward if the medium through which it passes is equally permeable in all directions; but so soon as the descending water reaches the water surface, its descent is checked and its movement is partly lateral.
The commonest sort of movement of ground-water is that exemplified as the water sinks beneath the surface, namely, slow percolation through the pores and cracks of the soil and rock. Ground-water is not generally organized into definite streams, though underground streams, mostly small, are sometimes seen in caves and crevices, and sometimes issue as springs. Most underground streams which issue as springs probably have definite channels for short distances only before they issue. It is probable that ground-water frequently flows in considerable quantity along somewhat definite planes, without having open channels. Thus every porous bed of rock is likely to serve as the pathway along which subterranean drainage passes. This is especially true where the porous bed is underlain by an impervious one. The “reservoirs” from which artesian wells draw their supply are not usually streams or lakes, but porous beds of rock through which abundant water passes. As the supply is drawn off at one point, it is renewed by water entering elsewhere. Since the freedom of movement of ground-water is notably influenced by the porosity of the rock, and since the rock is, on the average, most porous and the pores largest near the surface, the movement of ground-water is, on the average, greatest near the surface, and least at its lower limit. In general the decrease of movement is much more rapid than the decrease in the size of the pores. It follows that while the upper part of the ground-water, especially that above ground-water level, moves somewhat freely, the lower part moves much more slowly. It is probable, indeed, that the movement in the lower part of the subterranean hydrosphere is extremely slight.
The amount of ground-water.—The porosity of surface rocks varies widely, and the porosity of but few has been determined.[97] Such determinations as have been made are chiefly on building stones, in which the range of porosity varies from a fraction of one percent., in the case of granite, to nearly 30 percent. in the case of some sandstones. Building stone is perhaps more dense than the average surface rock. Furthermore, such tests as have been made do not take account of the larger cracks and openings of rock, for these would not appear in the specimens tested; nor of the mantle rock, which generally contains a large amount of water. From such determinations as have been made it is estimated that the average porosity of the outer part of the lithosphere is somewhere between 5 and 10 percent. If the porosity diminishes regularly to a depth of six miles, where it becomes zero, the average porosity to this depth would be half the surface porosity.[98] An average porosity of two and one-half percent. would mean that the rock contains enough water to form a layer nearly 800 feet deep. With an average porosity of 5 percent., this figure would be doubled.[99] While these figures are not to be regarded as measurements, they perhaps give some idea of the amount of ground-water. It is this sphere of ground-water which justifies the term hydrosphere, as applied to the waters of the earth.
Fate of ground-water.—Most of the water which sinks into the earth reaches the surface again after a longer or shorter journey. Some of it is evaporated from the surface directly; some of it is taken up by plants and is passed by them into the atmosphere; some of it issues in the form of springs; some of it seeps out; some of it is drawn out through wells; and much of the remainder finds its way underground to the sea or to lakes, issuing as springs beneath them. A small portion of the descending waters enters into permanent combination with mineral matter. Many minerals are known to take up water, being changed thereby from an anhydrous to a hydrous condition. It does not necessarily follow, however, that the total supply of water is thereby decreasing. Minerals once hydrated may be dehydrated subsequently, the water being set free. Furthermore, considerable quantities of water in the form of vapor issue from volcanoes, and volcanic vents often continue to steam long after volcanic action proper has ceased. It is probable that some, and perhaps much of the water issuing from these vents has never been at the surface before, and it is not now possible to affirm that the supply from this source does not offset, or even surpass, the depletion of the hydrosphere resulting from mineral hydration.