SOIL MOISTURE. A portion of the rainfall is consumed in keeping the soil moist. The ground can yield no water unless a portion of it is more than moist. It is not the amount of water a soil or stratum holds that is important, but the amount it will give up when penetrated. In recent years considerable work has been done in determining the capacity of different soils to hold moisture. This is an important investigation in connection with agriculture, and in calculating the percentage of run-off, but in connection with the problem of available underground water supply it is of less value, because it does not show the proportion of water that will be given up when the materials are punctured by the drill. In fact, the best character of ground for producing a good water supply is one which will not admit of a high percentage of soil moisture, but rather a soil that will readily yield almost all its moisture to the general water reservoir below. A sandy soil or a loose, open sandstone will gather large quantities of water from the rainfall and pass it down to the reservoirs below, where it is held available, while a soil with a greater capacity for holding moisture will yield less to the pump. AVAILABLE GROUND WATER. Throughout the humid regions of the world, with few exceptions, water can be found everywhere within a few feet of the surface, so that it is the common experience to obtain water for domestic uses by digging from 5 to 100 feet. This condition is so common that the masses of mankind throughout the humid areas of the world have come to look upon it as certain, and rarely give it any special consideration, and the industries and operations of civilized man are based upon a belief in the perpetual continuance of such condition. The available ground water in all localities and under all circumstances is the residue of the rainfall after the portions mentioned above the evaporation, the run-off, and the soil moisture-have been deducted. Each of them must be supplied, in whole or in part, before any available ground water can exist. In many places there is an underground movement, so that the available water under a given area may have fallen only in part as rain upon the surface at that place; yet somewhere and sometime it must have fallen as rain, some of which was evaporated, some carried off by the drainage, some held as soil moisture, and the remainder sunk below the surface, below the reach of growing plants, and held in an underground reservoir, inviting the spade or the drill to discover it and the pump to lift it to the surface. But even yet the wastes are not satisfied, for portions of this underground water are carried by underground drainage to the surfaces of ravines and bluffs, bursting forth as springs, and thus joining the general body of the run-off. GEOLOGIC CONDITIONS GOVERNING GROUND WATER. The stratified rocks of the earth are a heterogeneous mass of matter arranged in layers one above another. The strata are not coextensive with the surface of the earth, but some of them occur at one place and others at other places, each lapping under or over its neighbor, quite like shingles on a roof. Some of the strata are composed of loose, porous material, such as sandstone or badly fractured limestone, so that water can readily pass through them. Others are formed from the accumulation of finer sediments, such as clay and the finest of silt, while still other parts of the earth are composed of the crystalline rocks, such as granite, porphyry, and syenite. No substance known forming a constituent part of the earth is entirely impervious to water. The solid granite and the compact limestone and marble alike have moisture within them, commonly called "quarry sap," showing that water penetrates them. Compact, plastic clay is perhaps about as good a nonconductor of water as is known, while beds of sand and gravel are at the opposite extreme, allowing water to pass through them with relatively little resistance. The surface of the ground almost everywhere has a covering of residual soils, sands, and clays, varying from a few inches to many feet in thickness. This usually has high absorptive power for water, so that a large amount is received from the rains as they fall. When the surface water comes in contact with the porous strata it is absorbed and immediately begins moving downward, or as nearly in that direction as possible. Sooner or later it comes in contact with an impervious stratum below, and thereafter can only move laterally down the incline of that surface. The rapidity of motion will now depend principally upon two conditions-the angle of inclination of the impervious surface and the degree of porosity of the material through which the water moves. Should this be a mass of gravel or sand or porous sandstone, the motion will be sufficient to be easily detected, and somewhere farther down the water will reappear as springs or seeps, supplying the streams with "living" water. It matters not whether this porous stratum is on the surface of the ground in the form of a soil covering or whether it is deeply buried by impervious layers, the water movement within it will be practically the same. When the latter condition prevails and a sufficient head is produced, a well drilled through the upper and impervious layers allows the water to rise through the drill hole, and an artesian well results. Where the porous layer is on the surface, as is often the case on the Great Plains, no pressure or head can be produced, for the water is simply running down an inclined surface with nothing above to prevent it from rising, so that it would be comparable to water flowing down a wide open trough. A good illustration of this latter condition is found near the University of Kansas, at Lawrence. In 1893 the university authorities decided to own their own water supply. An investigation was therefore made to ascertain whether a sufficient supply could be had within a reasonable distance of the buildings. It was found that on the south side of the hill a large amount of débris produced by the decomposition of the limestone and shales of the hill had accumulated on the hillside, and that it was well charged with water. Fig. 1, drawn to scale, shows the conditions. The hill is composed principally of a fine-grained impervious shale, with a limestone mass (A) on top. At the boiler house, 300 feet south of the brow of the hill, the débris was found to be 40 feet deep. A well dug here (B) during the driest part of a dry year showed that the amount of water was not very considerable. At points farther down the hillside the water was more abundant. Finally, a large well was put down at the point C, 1,000 feet south of the brow of the hill, and galleries about 6 feet in height were run both east and west, just on top of the undecomposed shale, to intercept the FIG. 1.-Diagrammatic section of hill at Lawrence, Kansas. water as it moved down the slope and drain it into the well. It was reasoned that this greater distance from the summit of the hill was necessary because the gathering area above the boiler house was so limited that an insufficient amount of water would be obtained at that point, but that with the added distance to where the well was finally located a gathering area of sufficient extent was passed, considering that the average rainfall at Lawrence is a little more than 35 inches annually. Southward the thickness of the débris gradually decreases, so that a mile away it is only an ordinary soil above the undecomposed shale. Were the débris a mass of coarse sand, similar to that so often found in the western part of the State, without doubt the water would soon all run down the hillside and appear as springs in many places; but the débris from a mass of shale is principally a clay, which lets the water through it very slowly, and therefore its southward movement is so slow that little reaches the extreme southern limit of the débris. 19 Yet in the vicinity of the well a water supply is found sufficient to produce 5,000 gallons a day almost all the year, and 10,000 in wet weather, an amount which could be increased indefinitely by extending the east-west galleries. Here we have a good illustration of the underground water plane having a very concave surface facing upward. Instead of the water lying in the form of an underground lake, with a level surface, it is a mass of water held in the clay in such a manner that its upper surface is nearly parallel with the highly inclined surface of the ground. We may speak of the clay within the body of water as being more than saturated, using the term saturated to mean holding a water content just equal to the largest amount the clay can hold without being compelled to give up a part of it whenever an opening is made into it. When the clay is in this condition and more water is added to it, this extra amount will run out into the opening made. As the well at the point C was being dug it was noticed that the clay was moist almost from the surface, but that no water came into the well until it had reached to within about 6 feet of the undecom posed shale. Here the point of saturation was reached, and any greater depth passed clay which was more than saturated, that is, had more water within it than it could hold back from running into the well. This extra amount in excess of saturation is the available water in all cases. It is that which has an underground movement, and which is available in so many parts of the world as supply for man. rence. In the area under consideration, in the western part of Kansas, we find conditions remarkably similar to those just described for LawWe have a broad expanse of country on which rain falls, and has been falling since the close of Tertiary time, and possibly longer. The surface of the ground is usually well adapted for the absorption of large amounts of this rainfall. After absorption the water obeys the laws of gravity and moves downward, except such portions as are used by the growing vegetation and for moistening the soil. The remainder continues downward until it meets with a stratum which is so nearly impervious that it is almost entirely arrested, after which it moves slowly along the upper surface of the impervious floor in a manner similar to the water in the clays at Lawrence. This floor lies at varying depths in different parts of the Great Plains area, sometimes so deep that the upper surface of saturation can not be reached within 200 feet, while at other places it comes entirely to the surface of the ground. LOCATING GROUND WATER. The ability to locate ground water is a qualification desired by many and possessed by few who do not understand the principles governing ground-water movement. The "open sesame" of mythical times gave way to the wand of the wizard, an instrument still IRR 6-2 employed in many parts of the civilized world, usually in the form of a forked twig from the bough of a tree, but occasionally a branch, forked or otherwise, from the bough of some particular tree or shrub. In a majority of cases, predictions in humid climates made by the use of the wand prove to be correct unless impervious material is met with in digging, for the whole ground is more or less saturated with water. As above stated, the water which falls as rain or snow is partially absorbed by the surface materials and starts on its downward course under the influence of gravity. The laws governing its movements are identical with those which govern the movement of surface water. When impervious material is reached the water is arrested in its movements unless it can pass down an inclined plane, the surface of the impervious mass. If limestone or granite or other solid rock is reached, the water will follow the fissures in the rock, and will often ultimately be brought to the surface as springs along ravines and hillsides. The proper way of considering the matter is to look upon the whole of the subsurface part of the ground as containing available water except where impervious materials exist. If a heavy bed of shale is found which is close-grained and compact, it is useless to look for water within it. Many examples are met with in mining operations which illustrate this, a few of which may be cited. In mining for rock salt at Lyons, Kansas, a mass of loose surface material was found to extend downward for nearly 300 feet. This was so thoroughly charged with water, particularly near the bottom, that it interfered seriously with the mining. Below this a bed of fine-grained Permian shale was reached, in which the salt is found. This shale is particularly impervious to water, as is shown by the fact that no water has come into the shaft since the surface water was shut out, although the shaft is 1,000 feet deep. Similar conditions are found in drilling for oil and gas in Kansas and elsewhere. Often a heavy bed of shale or a solid body of limestone is met which has no water whatever within it. The gas fields of Indiana likewise have similar conditions. Here, after passing through a few hundred feet of water-bearing materials, a 300-foot bed of fine-grained solid shale is found lying immediately over the Trenton gas-bearing limestone. This shale is impervious to water, and lets none of the surface water pass downward through it, and none of the deeper-seated water pass upward. It is so dry that water has to be added while drilling in it. The existence of such shale beds, or beds of other impervious materials, can generally be recognized by the geologist by surface conditions if he is familiar with a sufficiently wide range of country; otherwise the drill is the only means of discovering it. Wherever large masses of such materials, or of granite or other solid and impervious crystalline rock, cover a wide extent of country, the only hope |