Principles of Recovery
The discharge from a well is produced by a pump or some other lifting device or by artesian pressure (for a more detailed discussion of principles of recovery see Meinzer, 1923a, pp. 60-68). When water is standing in a well there is equilibrium between the pressure of the water inside the well and the pressure of the water outside the well. Whenever the pressure inside a well is reduced there is a resultant inward pressure and water moves into the well. The pressure on the inside of a well may be reduced in three ways: (1) by lowering the water level by a pump or other lifting device, (2) by removing the atmospheric pressure in a well pumped by suction, and (3) by relieving the pressure at the mouth of a well that discharges by artesian pressure. Whenever water is removed from a well there is a resulting draw-down or lowering of the water level, or, in a flowing artesian well, an equivalent reduction in artesian pressure.
When water is being discharged from a well at a given rate the water table is lowered in an area around the well to form a depression resembling somewhat an inverted cone. This depression of the water table is known as the cone of influence or cone of depression, and the surface area affected by it is known as the area of influence. The height of the cone of depression is equal to the draw-down. In any given well the greater the pumping rate the greater will be the draw-down, and the greater will be the diameter of the cone of influence and the area of influence.
The capacity of a well is the rate at which it will yield water after the water stored in the well has been removed. The capacity depends upon the quantity of water available, the thickness and permeability of the water-bearing bed, and the construction and condition of the well itself. The capacity of a well is generally expressed in gallons a minute. The known or tested capacity of a strong well is generally less than its total capacity, but some weak wells are pumped at their total capacity.
The specific capacity of a well is its rate of yield per unit of draw-down and is determined by dividing the tested capacity in gallons a minute by the draw-down in feet. Well 144 at Elkhart is reported to yield 120 gallons a minute with a draw-down of 30 feet. The specific capacity of that well, therefore, is 4 gallons a minute per foot of drawn-down, or simply 4.
When water is withdrawn from a well the water level drops rapidly at first and then more slowly, until it finally becomes nearly stationary. Conversely, when the withdrawal ceases the water level rises rapidly at first and then more slowly until it' eventually resumes its original position, or approximately its original position.
A dug well is one that is excavated with picks, shovels, spades, or by power machinery. They are generally between 2 and 10 feet in diameter and are comparatively shallow. They are most common in shallow-water areas such as river valleys, but may be found in the uplands where the water table is very deep. An old dug well on the upland east of Johnson, Stanton County, is reported to be 165 feet deep. Many of the earlier wells in Morton County were dug by hand, but almost all of them have been replaced by drilled wells. At present there are a few dug wells in the alluvium in the Cimarron River valley, in the Cockrum sandstone in the northwestern part of the county, and in the Ogallala formation in the northeastern part of the county. They range in depth from about 20 to 107 feet.
The dug wells in Morton County are curbed with stone, timber, barrels, or rims of tractor wheels. They are generally poorly sealed and hence may permit the entrance of surface waters. If such wells are situated near barnyards or privies, contamination of the water may result. Because of the difficulties of digging by hand below the water table, dug wells generally are excavated only a few feet below the water table. Therefore, dug wells are more likely to fail during a drought than are drilled wells, which generally extend many feet below the water table.
Bored wells are made by augers or post-hole diggers in loose un-consolidated sediments. A few shallow wells in the alluvium of Cimarron River were made in this way.
A drilled well is one that is excavated by means of a percussion or rotary drill. Most of the wells in Morton County were drilled by the percussion method by means of portable cable-tool drilling rigs. They range in diameter from 4.5 to 12.5 inches.
Drilled wells in consolidated deposits--About one-fourth of the wells in Morton County were drilled into consolidated deposits, chiefly sandstone and shale, after passing through the unconsolidated beds of the Ogallala formation. The wells are generally cased through the unconsolidated surface material and left open in the bedrock. Well 155, in the Cockrum sandstone, contains no casing and was drilled more than 30 years ago. In some of the wells casing is set in an upper sandstone to shut off hard water and the well is drilled down to a lower water-bearing bed that may contain softer water. If the well is cased its entire depth several reductions in the diameter of the casing may be necessary.
In Morton County the depth of the wells in consolidated sediments is generally less than 100 feet, but a few exceed 200 feet. More than 90 percent of the wells are cased with galvanized-iron casing and the wells range in diameter from 4.5 to 16 inches.
Drilled wells in unconsolidated deposits--More than three-fourths of the wells in Morton County obtain water from the unconsolidated sediments of the Ogallala formation. Most of these wells are cased all the way to the bottom of the hole with 4.5-inch or 5.5-inch galvanized-iron casing. A few wells have steel casing, especially those six inches or more in diameter. In some of these wells water may enter only through the open end of the casing, but in many of the wells the casing is perforated below the water table to provide greater intake area. Samples of the water-bearing sand or gravel should be examined so that the proper size of perforation may be used. The capacity of a well and even the life of a well may be determined by the size of the perforation, for if the perforation is too coarse the fine material may filter through and clog the well, and if the perforations are too small the water will be held back by unnecessary friction.
Some wells in unconsolidated sediments are equipped with well screens or strainers. It is common practice to select a slot size that will pass 30 to 60 percent of the water-bearing material, depending upon the texture and degree of assortment. Retention of the coarser particles around the screen forms a natural gravel packing that greatly increases the effective diameter of the well, increases its area of intake; and hence increases its capacity.
Gravel-wall wells are very effective for obtaining large supplies of water from relatively fine-grained unconsolidated deposits, and are widely used for irrigation. In constructing a well of this type, a hole of large diameter, 30 to 60 inches, is first drilled by the rotary method or by means of an orange-peel bucket and is temporarily cased. A well screen or perforated casing of a smaller diameter than the hole, 12 to 25 inches, is then lowered into place and centered opposite the water-bearing beds. Blank casing extends from the screen to the surface. The annular space between the inner and outer casings is then filled with carefully sorted gravel, preferably of a grain size just a little larger than the openings in the screen or perforated casing, and also slightly larger than that of the water-bearing material. In most wells of this type a medium- or coarse-grained gravel is used, but in very fine-grained deposits a fine-grained gravel or coarse-grained sand should be used. The outer casing is then withdrawn part way to uncover the screen and allow the gravel packing to come in contact with the water-bearing material. The gravel increases the effective diameter of the well and decreases the velocity at which the water enters the well, thus preventing fine sand from choking the well and injuring the pumping equipment. The gravel envelope reduces the entrance friction and the draw-down and hence increases the capacity of the well.
In deciding whether or not to employ gravel-wall construction it is important to know the character of the water-bearing material. If the material is a coarse gravel (as it is in parts of Morton County) it would be unprofitable and unnecessary to construct a gravel-wall well. Some wells have been walled with a gravel that was finer and less permeable than the water-bearing material it replaced, to the detriment of production.
According to McCall and Davison (1939, p. 29) draw-down can be kept to a minimum in several ways.
First, the well should be put down through all valuable water-bearing material. Secondly, the casing should be properly perforated so as to admit water to the well as rapidly as the surrounding gravel will yield the water. Third, the well should be completely developed so that the water will flow freely into the well. . . . Increasing the depth of a well will have a greater effect on reducing the draw-down than will increasing the diameter, so long as additional water-bearing formations are encountered.
A report (Davison, 1939) containing descriptions of different types of pumping plants, the conditions for which each is best suited, construction methods, and a discussion of construction costs is available from the Division of Water Resources, Kansas State Board of Agriculture, Topeka, Kan., and the reader is referred to this publication for additional details of well construction.
Methods of Lift and Types of Pumps
Almost all of the wells in Morton County, particularly those used for domestic and stock supplies, are equipped with lift or force pumps. The cylinders or working barrels in lift pumps and force pumps are similar and are placed at a level near that of the water table, but a lift pump is capable of discharging water only at the pump head, whereas a force pump can force water above this point, for example, to an elevated tank. Most of the pumps are operated by windmills, but a few are hand operated.
The pipe (1.5 to 3 inches in diameter) generally is clamped between two 4- by 4-inch boards that rest on the top of the casing. On some wells a circular piece of galvanized iron or steel is placed between the clamp and the casing to prevent small objects from falling into the well. In wells equipped with galvanized-iron casing the clamp may be supported by railroad ties in order to take the weight off the casing.
Most of the irrigation, public-supply, and railroad wells are equipped with power-operated centrifugal or turbine pumps, but in a few older wells power-operated force pumps are used. These pumps are driven by electric motors or by internal-combustion engines using gasoline, oil, or natural gas. Centrifugal pumps are mounted at the surface or in pits and can be used only where the depth to water plus the draw-down does not exceed the working suction limit. In wells in which the depth to water level or the draw-down is great deep-well turbine pumps generally are used. A turbine pump consists of a series of connected turbines called bowls or stages that are placed near or just below the water level and are connected by a vertical shaft to a vertical motor or pulley at the top. If there is a pulley at the top it is connected by a belt to a tractor motor, a combine engine, or an electric motor. Some turbines have gear heads for direct connection to the source of power.
Kansas Geological Survey, Morton County Geohydrology
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Web version Sept. 2004. Original publication date March 1942.