Geologic Basis for Artesian Prediction
Many persons do not know the condition of occurrence of underground waters, but have vague ideas of subterranean rivers and lakes. They do not realize that there are water-bearing deposits which are just as definite in their geologic relations as the strata which carry coal and most other mineral products. The study of underground water problems is a special branch of geology which has had rapid development during recent years, and now the prediction of artesian and other ground waters is on a sound geologic basis. This later statement is substantiated by the fact that a very large number of predictions have been verified by notably successful wells. . The subject of underground waters is an extensive one because of the diversity of geologic conditions in different parts of the country, but on this occasion I can only point out some fundamental principles which guide the geologist in making artesian predictions.
One of the most important considerations is the fact that a large part of the earth’s crust consists of thick layers of alternating sandstones, limestones, shales and other rocks often lying nearly horizontal, but mo>e frequently flexed into extensive arches – and troughs. Many districts are underlain by crystalline rocks, such as granites and gneiss, while on the surface, especially along the river valleys and in lake basins and in the glaciated province, •here are deposits of gravels, sands and clays. Some regions are covered by sheets of lava. In these features we have most of the artesian water problems, but naturally with great diversity of local conditions. One of the most important conditions, and the one to which I shall devote the greater part of this paper, is a widespread sheet of sandstone overlain bv clay, shales or limestone sloping into a shallow basin or dipping gently in one direction. 1 his condition is shown in the following diagram.
Most sandstones are porous and the water which they absorb from rainfall and sinking of streams in the surface outcrops, flows slowly underground through the interstices. the ground is higher on one side of the basin than on the other, of course, the water will flow slowly from the higher to the lower side, provided the standstone is covered by clay other impervious or less pervious material prevent or impede leakage. This condition also shown in Fig. 3. That the waters flow slowly underground through sheets of sandstone is demonstrated in most instructive manner in the great artesian basin of the Dakotas. The water passes into the sandstones in their elevated outcrop zone along the foot of the Black hills and Rocky mountains and flows east through the permeable rocks finally to escape in springs in the lower Missouri valley. The sandstones are overlain by a thick mass of clay, and as the intake zone has an altitude of 4,000 ft. or more, and the region in which the principal wells have been sunk is only from 1,200 to 1,400 ft. above sea level the water is under great pressure. The existence of thi-. pressure is the strongest evidence we possess that the waters flow underground for many hundreds of miles. Several of the wells showsurface pressure of over 175 lb. to the square inch, and two are slightly over 200 lb., the latter indicating a pressure of 780 lb. to the inch at the bottom of the well. Some flows are from 1,000 to 4,000 gal. per minute. Such pres sures can only be explained by the hydrostatic influence of a column of water extending to high altitude at the intake zone. If it were not for the outflow of the water in springs to the east, the pressures would be much higher. Owing, however, to this leakage, the intial head is not maintained and there is a gradual diminution of head to the east, known as “hydrostatic grade,” a slope sustained by the great friction of the water in it flow through the relatively small interstices of the sandstone. In Fig. 2 is shown an apparatus which illtts trates these conditions and affords a basis for ascertaining the prospects for artesian flows.
In the two following figures there is shown the practical application of this principle to artesian well prediction.
In Fig. 3 is shown the reason for flow in well at (a) and for lack of flow in a boring at (b).
In big. 4 there is given a simplified map of typical artesian region in central South Dakota, similar to the one shown in profile and Fig. 3, showing the manner in which the area of flow is platted. The “head” of the water diminishes to the east, owing (o outflow in the southeastern corner of the State, as shown in F’ig. 3. The waterbearing stratum underlies all of the region at depths of 2,000 ft. or less and reaches the surface along the outcrop zone, shown by the line of X near the top of the diagram. The altitude in the valley along this outcrop is 3,300 feet, and that is the initial “head” of the water. A well reaching this stratum in the southeastern corner of the diagram at an altitude of 2.770 ft. has a closed pressure of 100 lb. to the square inch, which indicates a head of 3,000 ft., or, in other words, the water has pressure sufficient to rise in a tube 230 ft. above the mouth of the Now on the principle illustrated in Fig. 2 this indicates a regular gradient from 3,300 ft. at outcrop zone to 3,000 ft. at the well. Accordingly head lines for 3,100, 3.200 and 3,300 ft. arc drawn equidistantly on the contour map, as shown by the broken lines in Fig. 3. Having the altitude of head and altitude of the land on the map, it is a simple matter to plat the “hydrostatic” grade by a line which delimits the flow area. This line crosses the 3,200-ft. contour line at the intersection of the 3,200 “head” line, and so on. The figure shows that a projected well at A may expect a flow, while B lies slightly too high for one. A few years ago the writer predicted flows in central South Dakota on this basis and recently several succcssful wells sunk along the new railroad lines have verified the prediction in most gratifying manner. The pressure to be expected at A may be calculated easily. Suppose the altitude of the land is 2,950 ft., as shown by the contour lines, and the head is 3,100 ft., the height to which the water would rise above the surface is 150 ft., equivalent to a pressure of slightly over 55 lb. to the square inch. Good contour maps are important in underground water predictions, but in their absence suitable lines of levels can be run which will afford basis for “head” determinations.
The determination of depths to deep artesian waters in stratified rocks sometimes can be readily made from few data, but it frequently happens that it must be based on an extended examination of the local geologic conditions. The principal basis is knowledge of the thickness of the strata and while for many regions the facts artalready available in cithers it is necessary to trace the strata to thVir surface mierops which often arc many miles from the locality in question. The structure of the strata in the intervening country also has to be carefully considered. The records of borings anywhere in the neighborhood throw much light on underground relations, although in many cases they are so poorly kept that they are highly misleading. Samples of strata penetrated are much more valuable, especially if they have been carefully collected and labelled. An ideal condition is shown in the following section:
1A column of water 2.3 feet high, and 1 inch in cross section weighs approximately 1 pound, therefore. pounds pressure y 2.3 altitude of land _ altitude of head.
Suppose that a boring is desired at A. I’he geologist, from an examination of the country from A to 15, which may be a distance of many miles, concludes that the only promising waterbearing rock is the stratum outcropping at C. liy carefully measuring clips of the strata from C to A, especially if aided by a distinct bed as at D, he can construct a cross section, such as the one given in the figure. On this section he can base a prediction that at A the top of the water-bearing stratum may be expected at a depth of 700 ft., providing the strata do not thicken or thin materially in the distance An interesting illustration of such a prediction is at K.dgcmont, S. Dak., where tbe writer estimated that the sandstone would be found at a depth of about 0,000 ft. Recently the C. St Q. R R struck it at 2,005 ft., and obtained a large flow.
In the case of granites and other crystalline rocks the artesian problem is very different from the one above outlined. Ordinarily such rocks arc not underlain by a porous stratum and the fresh rock is too compact to carry any water supply. They are, however, usually broken by joint planes and often deeply disin tegrated so that more or less water is stored in their upper portion. Some of the crevices ex tend for long distances and pass under clays or other deposits in lower lands so that “head” is established. When a crevice of this kind is struck by a well, such as the one shown in big. an artesian flow is obtained. The occurrence of water under such conditions is difficult for the geologist to predict, but in some places the rock structure is so evident that it may guide to a successful forecast.
Some of the negative features of underground water arc of great importance. In many localities it is evident from the geologic conditions that no water supply can be obtained and in such places it is possible to avoid the great waste of expense of a deep boring which cannot succeed. In some cases this con dition is evident from geologic facts at the surface, while in others it can be inferred from the samples of boring after certain beds have been penetrated. We have frequent instances of deep borings made in compact granites other crystalline rocks which could not succeed. or in shales which we know arc so thick that underlying strata could not be reached by the means available. It is probable that in the aggregate the warnings against hopeless borings have been even more valuable than the predictions that water would be found. These warnings have saved the waste of a very large amount of money, but sometimes they will not deter tbe driller who has some notion of his own which he believes is of greater value than the scientific deduction of the geologist.