Properly Locating Shallow Wells for Water Supply

Properly Locating Shallow Wells for Water Supply

Must Reach Water Bearing Strata, Preferably of Sand or Gravel— Pollution Danger in Populated Areas—Consider Quantity and Quality

MANY cities rely upon shallow wells for their water supply, and the locations and sanitary conditions of these wells is a vital question which must be settled by each superintendent to his own and the city’s satisfaction. So many things enter into these problems—such as convenience and freedom from the danger of pollution, elimination of surface water and so on—that they are vexing in the extreme to the superintendent. The following paper will probably give him some excellent suggestions and hints as to the present condition and future location of such wells:

Shallow wells as sources of water supply undoubtedly cause water works officials and health officers more concern than any other type of water supply development. Fundamentally the reasons for the trouble so often experienced may be classified into two groups—faulty construction and improper locations of the wells. It is the purpose of this paper to outline briefly the principles which should be followed in the location and construction of shallow wells and to describe actual conditions where this type of water supply development is used for municipal water supplies.

How Wells are Defined

A shallow well is usually defined as one whose depth is less than 100 feet. This is not a rigid distinction but it serves fairly well for our purpose. Wells of this class may be either open (dug) or tubular {driven or bored). Open or dug wells very rarely exceed fifty feet in depth. Bored or drilled wells may be of varying depths within the limits mentioned. Both types are frequently used in developing municipal water supplies.

Open wells are of much larger diameter than the tubular wells. One of the advantages of this type is that the well forms a storage reservoir for a considerable amount of water, for example, a well 12 feet in diameter and piercing the water bearing stratum to a depth of 30 feet has a storage capacity of a little more than 25,000 gallons. When the pumps are operated at a variable rate the storage capacity tends to increase the real capacity’ of the well. Further in the operations of the pumps there are a number of advantages in being able to draw the entire supply’ from a single large well rather than from a number of small ones. If it becomes necessary to lower the pumps the large well permits the use of more economical pumping machinery. While the yield from a well of comparatively large diameter is somewhat greater than that which can be obtained from the small bore well, yet it does not increase directly with the diameter. Roughly, a well 10 feet in diameter will yield 125 per cent, more than a 6inch well under the same conditions.

Tubular wells are generally driven into the water bearing stratum. They are of small bore seldom exceeding 12 inches. Because of the small diameter which limits the storage volume, as well as the small area through which water flows into the well, the velocity of the inflowing water is considerably higher than that of water entering a large well which furnishes the same amount of water. This higher velocity’ tends to move particles of sand toward the well and causes trouble by clogging. Special strainers are used in tubular wells to prevent the entrance of sand. The cost of construction of tubular wells is less per unit of yield than for dug wells. In fact, it may be cheaper to put down several small diameter wells connected with the same suction line than it is to build the larger well. Economy’ in construction is the advantage which tubular wells have over open wells, while difficulty in operation constitutes the disadvantages.

Location of Wells

To furnish the large quantity of water needed for a municipal water supply a shallow well must reach a large and relatively porous water bearing formation. Wells of this type are not often found piercing a rock stratum (except in limestone country where underground streams are sometimes tapped). Strata of sand and gravel having large areas furnish the most uniform and constant sources of the underground water which can be reached by shallow wells. The finer sands and clay formations are not satisfactory because of their lower porosity and resulting frictional resistance to the movement of water through them.

The water table or surface of the underground water at any place is quite dependent on the topography. In a country that is rough, cut up into many hills and valleys by drainage channels the water table will lie at. some distance below the ground sur face on the high lands and much nearer to the ground level in the low lands. In localities which are old from a geological point of view where the drainage lines are relatively stable we often find extensive deposits of water bearing material in the valleys which will furnish plenty of water when lapped by shallow wells. Jn general, the water table has a surface which conforms with that of the ground surface but with more gentle slopes. Shallow wells? then, should be located on low lands if they are to furnish considerable quantities of water and at points where a good water bearing formation of considerable area can be pierced without going to great depths.

“To furnish the large quantity of water needed for a municipal water supply a shallow well must reach a large and relatively porous water bearing formation. Wells of this type are not often found piercing a rock stratum (except in limestone country where underground streams are sometimes tapped). Strata of sand and gravel having large areas furnish the most uniform and constant sources of the underground water which can be reached by shallow wells. The finer sands and clay formations are not satisfactory because of their lower porosity and resulting frictional resistance to the movement of water through them.”

“The well should be so located that it will furnish a water of satisfactory quality. It should be removed from all possible sources of contamination. Some judgment will be needed in obtaining a satisfactory location, for not only must present conditions be considered but thought must be given to the possible conditions which may arise as the town or city grows. Shallow wells should be developed out of the path of the natural drainage lines from the municipalities.”

Quality Must Also Be Considered

But the quantity of water to be obtained is not the only requisite necessary. The well should be so located that it will furnish a water of satisfactory quality. It should be removed from all possible sources of contamination. Some judgment will be needed in obtaining a satisfactory location, for not only must present conditions be considered but thought must be given to the possible conditions which may arise as the town or city grows. Shallow wells should be developed out of the path of the natural drainage lines from the municipality. It. is far better to locate wells of this type at some distance to one side of the main drainage axis of the city than it is to put them along this axis even though at. the time of development the distance from possible sources of contamination seems to be a very safe one. The writer has observed many instances where the municipality has grown toward the water works plant and as a result the quality of the water has become less safe year by year until finally the wells have been abandoned.

Surface Water Must be Kept Out

Shallow wells should be so constructed that surface water cannot get into them. The curb on ope 1 wells should be water tight to a depth of 10 or 12 feet and should extend above the ground surface so that the grading around the well will slope away in all directions. It is good practice to provide housing over large open wells. Tubular wells have the water-tight casing and should be given the same surface protection that has been mentioned for open wells The practice of partially digging a shallow well and then driving tubular wells in the bottom of the well has certain advantages in increasing the yield at less cost than would be incurred it the well were dug for the entire depth. The principal advantage is that the pumps may be lowered. The construction of manholes around the tops of tubular wells tor the purpose of protecting the wellhead from frost action should be carefully planned so that surface water will not collect in the manhole and overflow into the well. The importance of protecting wells from surface contamination cannot be too strongly emphasized.

With these points in mind let us consider briefly some actual instances of municipal water supplies which are obtained from shallow wells. These installations have been chosen not because they exemplify good practice but rather because in each case there is a lesson for those concerned with shallow wells and after all we may learn more from failures or partial failures than we do from successes.

Some Examples of Municipal Shallow Wells

A city of 2,000 population obtained its public water supply from five wells which were located on low ground within one block of the business district. These wells were of the dug type. One of them was located at the side of the main business street and protected by a cracked concrete cover, another was within fifteen feet of an artificial pond which received considerable street drainage in wet weather the other three were in a vacant lot which was considerably lower than the adjacent land. In every instance it was possible for surface drainage to enter the well. Each well was connected with a sump well at the pump house to which water flowed by gravity. The quality of the water was bad and the quantity insufficient to supply the needs of the city.

This installation is an example of poor location in the first place with the lack of proper protection from surface drainage adding to the hazard. Yet this community tolerated such a condition of affairs for many years. During the last few years the fact that the city water was not safe to drink was recognized and freely admitted by the citizens. It was used only for flushing and fire purposes. This meant that in many houses there were three sources of water supply, the public system for bath rooms, a cistern to collect rain water for laundry purposes and a private well which was the source of drinking water. It is not difficult to imagine that with three sources of water supply in the housea mistake might easily occur. About a year ago the well system was abandoned. The present supply is obtained by filtration of a surface water.

“Shallow wells should be so constructed that surface water cannot get into them. The curb on open wells should be water tight to a depth of 10 or 12 feet and should extend above the ground surface so that the grading around the well will slope away in all directions. It is good practice to provide housing over large open wells. Tubular wells have the watertight casing and should be given the same surface protection that has been mentioned for open wells.’

Well Located in Privy Infested District

A town of 900 population, located very near the drainage line between the Missouri and Mississippi Rivers, is built around a public square. About six years ago the citizens decided to build a public water supply system. Since shallow wells had furnished plenty of water at private houses it was thought that all that was necessary to secure a satisfactory supply for the proposed public system was to dig a large well. Evidently the only consideration given to the location was that it should be as close to the center of the town as possible so that the problem of distribution might be simplified and the expense of constructing the system be kept at a minimum, for the well, pumping plant and elevated tank were built within a half block of the public square. The well is 12 feet in diameter and 40 feet deep. There is no sewerage system in the town—open pit privies are used. With several of these unsanitary affairs within fifty feet of the well is it any wonder that the water was pronounced unsafe and tile well condemned by the board of health ?

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Properly Locating Shallow Wells

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In this particular case the surface protection of the well was fairly good, but the impossibility of obtaining a satisfactory public water supply from a well in this privy infested location would have been immediately recognized by any experienced water works man. The town in question is stlil wrestling with its water supply problem and judging froths further unfortunate experience with a deep w’ell 375 feet which furnishes a limited quantity of a highly mineralized water it has not yet realized the value of obtaining competent advice.

Gradual Increase of Pollution of Wells

The experience of a third town illustrates the point that a shallow well which has furnished a satisfactory supply of water for a public system may gradually become contaminated as the distribution system is extended. This is a case where the distribution system has been extended into a residential section which is not sewered. The result is that the underground water is receiving a contamination from cesspools that is increasing in volume every year.

The town in question has a population of 1400 and is divided by a river. The business district and larger residential district are located on the east side of the river while a smaller residential district containing about onethird of a total population is on the west side. As is often the case the river has a broad alluvium plain which is underlain by a sand and gravel formation of considerable area and depth. This is an excellent water bearing formation— the underground w’ater flowing toward the river. The city well which is 18 feet in diameter and 24 feet deep is located on the west bank about 75 feet from the stream. The general direction of ground water flow is southeast. There are now ten cesspools within two blocks—the nearest being about 250 feet from the well, in the territory northwest of the plant. While bacteriological pollution has not appeared, the chemical composition of the well water as determined Ey the examinations which have been made during the last eight years, shows evidences of the increasing pollution of the ground water which comes to the well. It seems quite probable that the water from this source will soon become unsafe if the pollution of the ground water is allowed to continue. The use of modern plumbing in the homes, made possible by a public water supply system, bids fair to become a boomerang in this city unless provision for the removal of waste water is made.

Careful Examination Showed Danger of Pollution

One of the most interesting examples of shallow wells as sources of municipal water supply was that which was found during the course of a sanitary survey made by the writer a few months ago. A town of some 900 population is located on the northern slope of a broad river valley. The business district is on low ground about three-quarters of a mile from the river. The residential sections are on the slopes which rise gradually from the center of the town toward the north and east so that the main drainage axis of the town for both surface and ground water runs south from the business district. The town water works plant is located on this axis and in the first block south of the commercial center.

Water is obtained from six driven wells six inches in diameter spaced approximately three feet apart. These wells are located in the basement of the city hall which was evidently built over the water works plant. The wells are cased with six inch galvanized iron pipe. A three inch suction pipe from each well is connected with the main suction pipe of the pump. The casing projects about three inches above the concrete floor of the basement. There is an annular opening between well casing and the suction pipe at each well-head. This is covered by a loose tin cap which possibly serves to keep out dirt but is not watertight. The basement in which the wells, pumps and pneumatic tanks are located was drained through a tile pipe which discharged into a ditch about one hundred feet back of the building. The difference in elevation between the basement floor and the outlet of this drain was about eighteen inches. No check valve was provided on this drainage line.

An unusually large rainfall early last spring flooded the ditch into which the drain discharged and caused surface water to flow back through the drainage line, flooding the basement to a depth sufficient to submerge the tops of the w’ells. Fortunately the tow’n officials recognized the seriousness of the situation and after issuing a warning to the public that the city water was unsafe, took immediate steps to clean out the wells, disinfect the entire system and fix the drainage system so that flooding of the W’ells might be prevented in the future. This is an example of a shallow w’ell development in which the protection of the wells from surface contamination would appear to the casual observer to be entirely adequate but where a careful examination of the situation would show its inadequacy.

Danger of Wells in Main Drainage Axis

This same development will serve to illustrate the danger of locating shallow w’ells on the main drainage axis of the town. Until this past year cesspools and privies have been the only means of disposing of sewage. (A sewerage system and sewage disposal plant were constructed in 1921 and put into service early in 1922.) There is very heavy contamination of the underground waters within two blocks north of the well. That this contamination affects the municipal wells is shown very clearly in the results of the few examinations that have been made during the last four years. In 1918 laboratory examinations of a sample of water from the municipal supply showed the following chemical composition: nitrites, .000; nitrates, 25.0, and chlorine, 72.0 parts per million. On a sample analyzed in June, 1922, the results obtained were as follows: nitrites, .008; nitrates, 30.0, and chlorine, 98.5 parts per million. These examinations, while too few’ to give conclusive evidence, certainly indicate gradually increasing contamination.

During the sanitary survey samples of water were also taken from private wells in different sections of the town. The results of laboratory examinations of these samples showed the influence of the density of population in the vicinity of the wells. Water from wells located near the center of the town gave results on chemical examination almost identical with those on water from the municipal wells. The following table gives results on well waters in different sections of the town.

Results of chemical examination of well waters in X:

“A”— Municipal well on main drainage axis.

“B”— Private well on main drainage axis below “A”.

“C Private well near w’estern edge of town.

“D”— Private w’ell near eastern edge of tow’n.

It will be noted that well “B” show’s results even more unsatisfactory than those obtained from well “A” while the results from examination of w’ater from wells “C” and “D ’ show that the ground w’ater at points located at some distance from the main drainage axis is of much better sanitary quality.

Summary

Where a suitable water bearing strata of considerable area can be reached within a depth of 100 feet shallow wells may be used. These may be either of the dug or tubular type. In locating the well care should be taken to choose a location removed from the main axis of the surface and underground drainage from the town. There is danger of underground as well as surface contamination; the former can only be eliminated by proper location of the well and the latter by proper construction.

(Excepts from paper read before the annual meeting of the Iowa Section of the American Water Works Association.)

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