Methods of Developing Underground Water Supply
First Problem Is the Rate and Direction of Flow and Quantity— Each Type of Well Has Advantages—How to Increase Yield of Wells
SO much depends upon the thorough understanding of all phases of the subject before choosing an underground water supply or the development of an additional supply, that the pronouncements of an expert on the subject are of considerable value. The following excellent paper gives some very important ideas on this matter that will be found of practical help to superintendents who are facing the problem:
The successful development of underground water for domestic and industrial supply is a problem which has engaged the attention of water supply officials and engineers for a good many years with varying degrees of success. Involving as it does, some knowledge of several closely related sciences, it has onlybeen in the last few years that special attention has been paid to the problem with the idea of using such information as is available in order to avoid previous mistakes and prevent further waste of money. We will pass the questions connected with the occurrence of water in the Earth’s crust, together with the questions concerning the amount and rate of flow, quality, etc., to the question of a development of an underground supply assuming that one is present.
First Problems to Be Investigated
The first problem is to investigate such questions as the rate and direction of flow, and quantity available in the water bearing beds, the danger of contamination, the nature of the beds to avoid localized supplies, the character of the material in the beds for their ability to deliver water, the action of the water when tapped, together with its static head, temperature, etc., keeping in mind the demands that will be made later, on the supply by the users. If it is found from these preliminary studies that it is feasible to go ahead with the development then the question of how to obtain the water presents itself.
By far the most common method of making an underground supply available for use is to sink one or more wells into the water bearing formation. There are other means such as collecting systems, infiltration galleries, impounding flowing springs, but these are special problems and are treated as such. The general use of wells though gives opportunity for the most study and also the most improvement in them as a means of obtaining underground water. Before undertaking to develop a well supply a report of the general geologic conditions together with a study of the existing wells in a locality is generally sufficient to determine the type of well construction best adapted to the situation. By type, the well builder refers to two general classes into which all wells are divided, namely, shallow or deep wells. Each have their advantages which should be carefully compared. The practice at present favors the small diameter cased well as it is known that in the final analysis no more water can be obtained from a well than the water bearing sands are able to pass up to it, provided the well is constructed in such a manner as to allow 100 per cent. of the water to enter the well.
Shallow Wells of Two Kinds
Shallow wells are of two kinds, large diameter or open wells and small diameter or cased wells. In some localities wells of both types can be obtained but generally the cost of deep well work makes the shallow well more economical and unless it is a case of necessity a shallow well usually is the best investment as well as the best water producer. It should be kept in mind that the waters nearest the surface are the softest and of lower temperature than deep waters.
The type of well being determined, the next questions are the method of construction, location, size or diameter and the probable yield to be obtained from the wells. Wells are sometimes classed according to the method of construction into dug, bored, drilled or driven wells. To the average person it would not seem that the method of sinking had any bearing on the capacity of the finished well, however, it is a well established fact that the method of sinking does play an important part in the future performance of a well. Recent experiments show that in stratified formations of clay, quick sand, sand and gravel, the greatest supply is obtained when the ground formations are least disturbed during well building. It is also important that an accurate record be kept showing the depth and kind of water bearing formations together with the head and quality of water in each water bearing layer.
Screen and Casing Play Important Part
Any kind of a well must be provided with openings in the casing through which the water may pass into the well. The size and kind of these openings must be such that they will allow the maximum amount of water to pass into the well with the least effort and without clogging, stopping up or weakening the screen and without passing particles of fine sand into the well which later on either fill it up or are pumped into the system damaging pump parts and stopping meters. Great care must be used in placing the screens in a well to insure that they are placed opposite the best water bearing formations. It is a recognized fact that practically all well troubles are screen troubles. The casing of the well also plays an important part and should be just as carefully installed as any other part of the well, special care being taken to see that it is intact when completed and plumb. It should be examined from time to time to insure that it is keeping out undesirable water and is not deteriorating.
“The whole subject of underground water development is comparatively in its infancy but the necessity and importance of considering the fundamentals underlying the subject are being more generally realized by water officials and many cities and towns are developing satisfactory underground supplies by applying these fundamentals to their water supply problems.”
Location an Item Often Neglected
The proper location of the well is perhaps as important as any other item but is the one most often neglected. It has long been the custom to sink a well at some spot previously selected on account of being near a good well or for most any reason except the most important one, namely, that it is in the best formation available. More than once complete water systems have been designed with the source of supply located convenient to the pumping plant and the pumping plant convenient to the railroad. However, it is becoming the practice to give more attention to the proper location of the source of supply.
Methods of Putting Down Test Holes
If it is not known where the best beds of water bearing material are, then some preliminary test holes if put down properly, will give sufficient information as to the underground conditions to properly locate the new wells. If deep wells are to be put down the geologist’s report is about the only information available and often times deep holes have to be abandoned when several hundred feet for lack of water. This can be prevented in wells of the shallow type. In putting down test holes the old style hydraulic method is not the best as the materials penetrated are washed out of the hole and become so badly mixed up that the samples when analyzed are not a true sample of the underground conditions. This is especially true of holes in sand and gravel, the coarser material separating from the finer which is washed out and lost. The percentage of fine particles being the determining factor in a formation as to its ability to transmit water, it is essential the samples be taken out as they exist in the ground and the thickness of each layer determined accurately. After comparing the results of several test holes it is not unusual to be able to pick a well location in which one well will furnish several times the amount of water that could be obtained from three or four wells put down hit of miss. Accurate testing not only insures the well in the right location but gives sufficient information to predict the amount of water a well will yield with reasonable accuracy.
Yield Follows Certain General Principles
The yield of wells follow certain general principles which you might say are fundamental. Having given a water bearing formation and a well sunk in this formation, the yield when a well is pumped continuously will be proportional to the distance the water is lowered below the normal ground water table. We find by experience that this holds good up to at least two-thirds of the depth of the water in the well before pumping, but that beyond the two-thirds point the draw down increases and the yield decreases. The yield is also proportional to the depth of the water bearing formations and for the same amount of draw down the yield will be larger the larger the diameter of the well but not in direct proportion. The increase varying as the log E. of R., R. being the radius of the screen surface. In other words, in two wells, one 20 feet in diameter and one 2 feet in diameter, all other things being equal the 20 foot one will only give about two times as much water as the smaller one. It will be seen then that the diameter of a shallow well depends largely on the amount of water available and the type of pump that will be used on the well. The diameter of deep wells is generally determined by the depth to be penetrated and the exigencies of drilling. In connection with the yield of wells the question of mutual interference should be kept in mind. We all know when water is pumped from a well that the level of the water in the well is lowered and that water flows into the well from the surrounding ground at an increased velocity and that the level of the water in the ground will take the form of a curve. If the well is built in a formation such that the water bearing sands are overlaid by impervious strata then there will be no circle of influence until the static head is removed but a curve of pressures can be plotted by measuring the decreasing head during pumping which closely follows the curve representing the ground water curve. The shape of this curve depends entirely on the formation and rate of pumping but it finally reaches a point at some distance from the well where there is no effect on the water table. The area in which the water table is lowered is called the circle of influence and after pumping is increased until the water level in the well can be lowered no farther and water is running into the well as fast as it is pumped out then there will be no further change in the circle of influence. If wells are located too near existing wells and these circles of influence overlap, their capacities will be cut down. In the same way, where more than one well is built at a time they should be spaced far enough apart that they can be pumped at their maximum capacity without effecting each other.
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Developing Underground Water Supply
(Continued from page 10)
Methods of Increasing Yield
There are a number of methods used to increase the yield of old wells such as cleaning or replacing the screens when possible, introducing gravel into and around the well, back blowing with air, steam or even using dynamite in the case of rock wells to loosen up the surrounding rock and form cavities. Nearly all wells in which the yield is decreasing have to be abandoned sooner or later and it is not good practice spending much money trying to reclaim old wells.
Before any well location is finally decided upon, careful attention should be paid to the sanitary features of the well and the quality of the water to be obtained from it. Most all surface waters or first waters are more or less contaminated and unreliable as a supply. They should he avoided if possible. Waters found under heavy beds of clay are nearly always good from a sanitary standpoint but may be poor chemically. Deep waters are also liable to be so highly mineralized as to be unfit for use. Some wells are contaminated by surface water seeping into them due to poor construction or location. Every precaution should be taken to insure a sanitary supply and keep it that way after being obtained.
The whole subject of underground water development is comparatively in its infancy but the necessity and importance of considering the fundamentals underlying the subject are being more generally realized by water officials and many cities and towns are developing satisfactory underground supplies by applying these fundamentals to their water supply problems.
(Excerpts from paper read before the annual convention of the Iowa Section, American Water Works Association.)
New Sellersville. Pa., Officers.—William Stauffer has been elected president of Sellersville, Pa., fire company and Frank Althouse, chief.