Ground Water, continued
Recovery of Ground Water
Principles of RecoveryThe following discussion on the principles of recovery of ground water has been adopted in part from Lohman (1938, pp. 54-56).
When water is withdrawn from a well there is a difference in head between the water inside the well and the water in the surrounding material for some distance from the well. The water table in the vicinity of the well develops a cone of depression (fig. 9). In any given well a higher pumping rate produces a greater drawdown (depression of the water level, commonly expressed in feet), and the diameter of the cone of influence and of the area of influence will be greater.
The specific capacity of a well is its rate of yield per unit of drawdown, and is generally stated in gallons a minute per foot of drawdown. When a well is pumped the water level drops rapidly at first and then more slowly, and it may continue to decline for several hours or days. In testing the specific capacity of a well, therefore, it is important to continue pumping until the water level remains approximately stationary. When the pump is stopped the water level rises rapidly at first, then more slowly, and may continue to rise long after pumping has ceased (fig. 10).
Figure 10--Recovery curves of wells 14, 31, 33, and 36, based on measurements made at termination of pumping tests. Rate and period of pumping are given in table 4. A large version of this figure is available.
Table 4--Pumping tests of irrigation wells in Thomas County. Pumping tests by Howard Palmer and Allan Graffham, Division of Water Resources, Kansas State Board of Agriculture, in cooperation with State and Federal Geological Surveys.
The character and thickness of the water-bearing materials have a definite bearing on the yield and drawdown of a well and hence on its specific capacity. Drawdown increases the height that water must be lifted in pumping, thus increasing the cost of pumping. If the water-bearing material is coarse and of fairly uniform size it will readily yield large quantities of water to a well with a minimum drawdown; if the water-bearing material is fine and poorly sorted it will offer more resistance to the inflow of water, thereby decreasing the yield and increasing the drawdown. Other things being equal, the drawdown of a well varies inversely with the permeability of the water-bearing materials.
Dug WellsDug wells are excavated with picks, shovels, spades, or by power machinery. They generally are between 2 and 10 feet in diameter and are quite shallow. Many of the early wells in Thomas County were dug by hand, but most of these have since been replaced by drilled wells. A few dug wells are now in use in the major valleys. Of the 114 wells listed in table 9 only three are dug wells, two are bored wells, and 109 are drilled wells.
Bored and Driven WellsBored wells are made by augers or post-hole diggers. Some wells are bored to the water-bearing formation by this method and a well point is driven into the sand or gravel from which the water is obtained. In some shallow water areas well points are driven from the surface without recourse to boring. These are called driven wells. Bored and driven wells are in use in Thomas County in the valley areas and at some places on the uplands.
Drilled WellsA drilled well is one that is excavated by means of a percussion or rotary drill. Most of the wells in Thomas County are drilled wells. The drilled domestic and stock wells generally are 6 inches in diameter and those used for irrigation and public supply purposes generally are 14 to 18 inches in diameter.
All the wells in Thomas County obtain water from relatively unconsolidated deposits. Wells in such deposits generally are cased nearly to the bottom of the hole with galvanized or wrought-iron casing. In some wells the water may enter only through the open end of the casing, but to provide greater intake facilities in many wells a strainer or well screen is used or the casing is perforated below the water table. The size of the perforations is an important factor in the construction of a well and the capacity or even the life of the well may be determined by it. If the perforations are too large the fine material may filter through and Jill the well; if the perforations are too small they may become clogged so that water is prevented from entering the well freely.
Some wells in unconsolidated sediments are equipped with well screens or strainers. It is good practice to select a slot size that will pass 30 to 60 percent of the water-bearing material, depending on the texture and degree of sorting. The coarser particles that remain around the screen form a natural gravel pack which increases the effective diameter and therefore the capacity of the well.
Gravel-wall wells generally are effective for obtaining large supplies of water from relatively fine-grained unconsolidated deposits and have been used in many public supply and irrigation wells. In such wells a large-diameter hole is first drilled with a rotary drill or excavated by a hoist and orange-peel bucket and the hole generally is temporarily cased with unperforated casing. A well screen or perforated casing is then centered in the hole opposite the water-bearing material and enough blank casing added to reach the surface. The space between the two casings is filled with carefully screened gravel and all but 20 or 30 feet of the outer casing is pulled from the hole. A well-sorted medium to coarse gravel packing generally gives the best results, but where the water-bearing material is extremely fine a coarse sand or fine gravel mat give best results. The slots in the screen or slotted casing should be as large as possible yet small enough to keep out the gravel used for packing the well. If the water-bearing formation consists of well-sorted coarse gravel the capacity of the well probably will not be increased by addition of a gravel pack around the screen.
McCall and Davison (1939, p. 29) stated that drawdown can be kept at 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 drawdown than will increasing the diameter, so long as additional water-bearing formations are encountered."A report (Davison, 1989) 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, Kansas, and the reader is referred to this publication for additional details of well construction.
Methods of Lift and Types of PumpsWater from many of the domestic and stock wells in Thomas County is obtained by windmill-operated lift or force pumps. The cylinder or working barrels in lift pumps and force pumps are similar and are located below the land surface, either above or below the water surface, but a lift pump is capable of discharging water only at the pump head, whereas a force pump can raise water above this point--such as to an elevated tank. Pitcher pumps are used on a very few dug or bored wells in the shallow water areas of the major valleys. Most of the pitcher pumps and a few of the lift and force pumps are hand operated.
Several types of power-driven pumps are in use on the irrigation and city wells in the county. For the most part these are turbine pumps and are powered by electric motors, stationary gasoline engines, and tractor engines. Data concerning such wells are given in table 9. One irrigation well is equipped with a centrifugal pump powered by a stationary gasoline engine. Several of the city wells at Colby and several of the railroad supply wells in the county are equipped with double action plunger pumps, and one railroad well is powered by a steam engine.
Kansas Geological Survey, Thomas County Geohydrology|
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Web version Nov. 2001. Original publication date Dec. 1945.