WATERS WITHIN THE EARTH
It has been estimated that approximately 1,500 cubic miles of water
fall upon the surface of the United States each year. About one-half
of this goes back into the atmosphere by evaporation; about one-third
of it flows away in surface streams; and the remaining one-sixth
enters the crust of the earth. Considerable water which enters the
earth returns to the surface as springs, by capillarity of soils and
rocks, or by being drawn up into plants and evaporated. Some idea of
the amount of ground water may be gleaned from the statement, based
upon a careful estimate, that all the water in the rocks and soils of
the first 100 feet below the surface of the United States would make a
layer seventeen feet thick. In most humid regions the soils and loose
rock formations are saturated with water at greater or less depths
(usually less than 100 feet) below the surface. The surface of this
saturated layer is called the ground-water level, or more familiarly
the "water table." The water table shifts up and down more or less
according to variation in rainfall.
In addition to the water held in the loose rocks and soils near the
earth's surface, large quantities occur in definite layers (usually
strata) of porous rocks which very commonly extend at various angles,
hundreds or even thousands of feet into the earth. A very fine
illustration of this principle is the case of the Dakota sandstone
formation of Nebraska. Almost anywhere across the State a well drilled
through a bed of clay and into the porous sandstone layer encounters
water. (Figure 19.) Another principle is also well illustrated,
namely, that water in such a porous layer may actually travel hundreds
of miles, water obtained from a well sunk to the Dakota sandstone
having actually traveled under the surface of the State all the way
from the eastern face of the Rocky Mountains, where rain and melting
snow entered the upturned and exposed porous rock layer. Another good
example is Iowa, where certain porous rock layers outcropping in the
northwestern and northeastern corners of that and adjacent States
gradually bend down under the State, reaching the greatest depths (up
to 3,000 feet) far in the interior. From wells 3,000 feet deep near
Boone, Iowa, it is, therefore, a fact that some of the water pumped
out of the earth actually traveled underground all the way from beyond
the corners of the State. This sort of travel of underground water is
common in many parts of the world. It should be clearly understood
that such water does not flow freely as in a pipe along subterranean
passageways, but rather it slowly works its way between the grains of
porous rock. Where such water moves distinctly downward, and the porous
layer has both above and below it an impervious rock layer like shale
or clay, it gradually gets under greater and greater pressure. In some
cases such pressure has actually been found by deep drilling to be
equivalent to that of a column of water several thousand feet high. The
rate of motion of water in porous underground rock layers is very slow,
data from various sources indicating a rate of speed of not more than
one-fifth of a mile a year in coarse porous sandstone, while in many
rocks it cannot be more than ten to fifty feet per year.
[Illustration: Fig. 19.--Vertical (structure) section from the Rocky
Mountains to Omaha, Nebraska, illustrating a widespread underground
porous rock layer (the Dakota sandstone) charged with water under
pressure, the clay formation acting as a cap rock. (After Darton, U. S.
Geological Survey.)]
We still have to consider a third mode of occurrence of waters within
the earth. Many formations, like granite and other types of crystalline
rocks are neither in definite layers, nor are they sufficiently porous
to allow water to really flow through them. Where such rocks extend
far down from or near the surface, how does rain water descend? It
does so along cracks or fractures (both joints and faults) which we
have learned are almost universally abundantly present in all hard
rocks in the upper (or zone of fracture) portion of the earth's crust.
Joint cracks are generally very irregular in direction and spacing,
while fault fractures are usually fairly regular and straight. Many
cracks are not wide enough to allow anything like good passageways for
water, while others are sufficiently open to allow water to travel
along them for hundreds, or even thousands of feet. In canyons of the
West, springs not rarely emerge from the bottoms of great, nearly
vertical ledges of granite and other hard crystalline rocks, the waters
certainly having entered the rocks hundreds, or even some thousands of
feet, higher. In rocks of the kind here considered it is evident, then,
that the movements of subterranean waters must be mostly exceedingly
irregular and usually not in great quantities. In many deep mines of
the world, underground water causes little or no trouble except often
near the surface. Occasionally a shaft or tunnel strikes a prominent
joint or fault fracture filled with water.
What we might really call underground streams may occur only under
exceptional conditions in rocks other than limestone, but in limestone
they are not uncommon because the slow solubility of the rock allows
underground waters to slowly enlarge the passageways to form distinct
channels. Echo River, which flows through Mammoth Cave, is a fine case
in point.
Most water by far which emerges as springs, was at one time surface
water. A simple, but common case is where rain water soaking through
porous soil (e.g., sand) or rock, sinks to the top of an underlying
impervious layer (e.g., clay) along whose surface it flows until it
reaches the side of a valley where a spring results. In fact, wherever
the water table is crossed by the surface of the ground, water must
either seep or flow out. Where underground streams which are common in
limestone regions reach the surface on hill or valley sides, springs
result. Another source of springs is where under proper conditions of
slope a porous rock layer, charged with water well below the surface,
appears at a lower level than its source of water. Still another type
of spring is where a fissure or fracture crosses a water-bearing layer
in which the pressure is great enough to cause the water to rise to the
surface along the relatively open fissure or fracture.
In various localities we hear of springs in seemingly paradoxical
situations on tops of hills and even mountains. Such a mystery is not
difficult to clear up. In the first place, such springs are rarely at
the summit of the hill or mountain. A case well known to many persons
is the small, but never-failing spring a little below the summit of
Mount Whiteface, a peak in the Adirondacks, rising 3,000 feet above the
general level of the immediately surrounding country. In this case, a
mass of highly fractured rock, subjected to much rainfall and lying
above the level of the spring, is sufficiently large easily to contain
and give forth enough water to account for several such springs. In
rare cases, however, springs or flowing wells are located on summits,
and in such places it is only necessary to bear in mind some of the
principles above set forth, but mainly the facts that water may travel
under pressure long distances underground, and that the point of
emergence may be on a hill which is actually lower than the source of
the water far away.
The economic significance of underground waters is forcibly brought
to our attention when we realize that 75 per cent of the people of
the United States depend upon wells for their water supply. Many city
supplies, most farm supplies, and much irrigation water come from
wells. The 3,000,000 people of Iowa, for example, are dependent upon
underground waters from wells varying in depth from a few feet to
several thousand feet.
[Illustration: 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.
[Illustration: 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."
[Illustration: 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.
[Illustration: 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.)]
One interesting effect of the dissolving power of underground water
in limestone regions is the development of caves or caverns. Most
remarkable of all is Mammoth Cave, Kentucky, with its hundreds of
miles of passageways and galleries. This marvelous work of nature is
all a result of the action of underground water which has dissolved
and carried away vast quantities of limestone. Echo River, which flows
through the cavern, is still carrying on the work aided by various
underground tributaries. The stalactites and stalagmites, which are
so strikingly displayed in many caves, as at Luray, Virginia, in
which water with carbonate of lime drips or oozes from the roof and,
due mainly to evaporation, deposits the lime. Many wonderful and
fantastic effects are thus produced. Where part of the roof of a cave
is dissolved out, or falls in, a "sink hole" results. Where all but a
portion of the roof of a cave or underground channel has fallen in,
a natural bridge, like the famous one in Virginia, results, though
natural bridges are also formed by other means.
In concluding this chapter we shall briefly discuss hot underground
waters, hot springs, and geysers. There are two well-known ways by
which underground waters may become heated. One is by the movement of
water downward into the normally heated portion of the earth, the rate
of increase downward being, as above stated, 1 degree F. for about
50 to 60 feet. Water descending two miles would, therefore, attain a
temperature of about 200 degrees F. In some regions such a temperature
may be reached at depths considerably less. Such water (under pressure)
taking a short course to the surface (forming springs) at a lower
level would retain much of its heat taken up far below the surface.
In regions where there are great down-folds of the strata (i.e.,
synclines), as in the central to southern Appalachians, conditions
appear to be favorable for such warm or hot springs, as, for example,
at Hot Springs, Virginia. A second cause of the heating of underground
water is by the descent of surface waters into contact with masses of
still hot igneous rock of relatively recent geologic age. In some such
cases the water does not go more than some hundreds of feet down and
when, under proper conditions, it returns to the surface hot and even
boiling springs may result.
[Illustration: Plate 7.--An Upbend Fold (_anticline_) in the
Appalachian Mountain Strata Near Hancock, Maryland. The strata were
deposited in horizontal layers upon the sea bottom, covering the region
many millions of years ago in middle Paleozoic time. At the time of
the Appalachian Mountain revolution, near the end of Paleozoic time,
this and many other folds developed well below the surface. Removal
of overlying material by erosion has laid bare the fold as we see it
to-day. (_Photo by Russell, U. S. Geological Survey._)]
[Illustration: Plate 8.--(_a_) A Ledge of Igneous Rock (Granite) in
Northern New York. This illustrates so-called "joints" or natural
cracks, commonly separating most hard rock masses into more or less
prismatic blocks. (_Photo by the author._)]
[Illustration: Plate 8.--(_b_) A Fault Fracture in a Ledge at East
Canada Creek in the Mohawk Valley, New York. The Ordovician limestone
formation in thin layers on the right has sunk hundreds of feet along
vertical fault to the left of middle, bringing it sharply against the
older (Cambrian) massive formation on the left. The hole is artificial.
(_Photo by Darton, U. S. Geological Survey._)]
Geysers are periodically eruptive hot springs found only in a few of
the volcanic regions of the world. They are most wonderfully displayed
in the Yellowstone National Park, where they send columns of hot
water to all heights up to 250 feet at various intervals of time.
Almost incredible amounts of hot water are sent into the air every
day in the geyser basins of Yellowstone Park. The single geyser "Old
Faithful," which erupts at intervals of about seventy minutes, sends
a column of water several feet in diameter to heights of from 125 to
150 feet. During each eruption about 1,500,000 gallons of water are
sent forth, or every day enough to supply the need of a fairly large
city. A very brief explanation of the cause of geyser eruptions may
be stated as follows: The very irregular, narrow, geyser tube extends
nearly vertically downward into yet uncooled lava. The tube is more or
less rapidly filled by underground water. The bottom, or near-bottom,
portion of the water gradually becomes heated by the lava until finally
the boiling point is reached for that depth. But, because of the
pressure of the overlying water column, the boiling point at that depth
is considerably greater than for the surface. A little steam develops
far down and this causes the whole column of water above it to lift
slightly, thus relieving the pressure on the superheated water far
down. This relief of pressure allows much of the superheated water far
down to flash into steam, which violently forces the column of water
out of the geyser tube.