Ground Water, continued
RecoveryPrinciples of Recovery
When water is pumped from a well, a difference of head exists between the water in the well and the water outside the well for some distance from the well. The water table for some distance surrounding the well develops a cone of depression (Fig. 15). In any given well, the greater the rate of pumping, the greater the drawdown in the well and throughout the cone of depression. The character and thickness of the water-bearing materials have a direct bearing on the yield and drawdown of a well. Inasmuch as the specific capacity of a well is defined as the yield in gallons a minute per foot of drawdown, the character and thickness of the material surrounding a well have a direct bearing on its specific capacity. A well in coarse material will have a higher specific capacity than a well in fine-grained, poorly sorted material.
Figure 15--Diagrammatic section of a well that is being pumped, showing its drawdown, cone of depression, and radius of influence.
Types of Wells
Dug wells--Dug wells are excavated with pick and shovel or by machinery. Dug wells generally range in diameter from about 2 to 5 feet although they may have a larger diameter--for example, the well in Greensburg, Kiowa County, which has a diameter of 40 feet. Dug wells generally extend only a short distance below the water table.
Bored wells--Bored wells are made by augers or post-hole diggers in unconsolidated materials and generally are made in areas where the water table is shallow. Many wells in Reno County are constructed by this method.
Driven wells--Driven wells are wells constructed in unconsolidated material by driving a 1 1/4- or 1 1/2-inch pipe, equipped at the bottom with a screened drive point, below the water table. Wells generally can be driven only where the water-bearing material is sufficiently permeable to permit water to flow freely into the pipe, where the material is unconsolidated enough to permit a pipe to be driven, and where the depth to the water is not more than 20 feet or so below land surface. In Reno County where the depth to water exceeds 20 feet, the well generally is dug partway so that the distance from the pump cylinder, at the bottom of the dug part of the well, to the water table is less than 20 feet.
Drilled wells--Drilled wells are wells made by percussion or rotary machines and may be drilled either in consolidated or unconsolidated material. Generally, drilled wells in Reno County are 4 to 6 inches in diameter, but many public-supply wells and industrial wells are begun as holes 30 to 60 inches in diameter and finished as gravel-walled wells having a casing 12 to 20 inches in diameter. Most drilled wells in unconsolidated rocks are cased to the bottom of the well, the last few feet of casing being slotted to admit the water into the well. Wells in consolidated rocks generally have casing only to the top of the consolidated rock or in that part of the well that will not remain open without casing. Where large supplies of water are needed, the well should penetrate all the good water-bearing material, and the casing should be perforated so that water will enter the well as fast as the surrounding material will yield it.
Kansas Geological Survey, Reno County Geohydrology|
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Web version Feb. 2001. Original publication date Aug. 1956.