SourceGround water, or underground water, is the water that supplies springs and wells. In Scott County, ground water is derived almost entirely from precipitation in the form of rain or snow. Part of the water that falls as rain or snow is carried away by surface runoff and is lost to streams; part of it percolates downward into the rocks until it reaches the water table where it joins the body of ground water known as the zone of saturation; and part of it may evaporate or be absorbed and transpired by the vegetation and thus returned directly to the atmosphere.
The ground water percolates slowly through the rocks in directions determined by the topography and geologic structure until it is discharged eventually through springs or wells, through seepage into streams, or by evaporation and transpiration in bottomlands adjacent to the streams. Most of the water obtained from shallow wells and springs in Scott County is obtained largely from precipitation in the general vicinity--that is, in Scott County and adjacent areas. The water in the Dakota formation under Scott County, however, is derived from precipitation and small streams in the areas of outcrop, which are mainly in southwestern Kansas and southeastern Colorado at higher altitudes.
Principles of OccurrenceHydrologic Properties of Water-bearing Materials
This discussion of the principles governing the occurrence of ground water takes account of conditions in Scott County. Preparation of the discussion has been based chiefly on the detailed treatment of the occurrence of ground water by Meinzer (1923), to which the reader is referred for more extended consideration. A general discussion of the principles of ground-water occurrence, with special reference to Kansas, has been made by Moore (1940).
Porosity--The rocks that form the crust of the earth are in general not solid throughout but contain numerous open spaces, called voids or interstices. It is in these spaces that water is found below the surface of the land and from which it is recovered in part through springs and wells. There are many kinds of rocks and they differ greatly in the number, size, shape, and arrangement of their interstices and in their water-holding capacities (Waite, 1942, p. 45). The occurrence of ground water in any region, therefore, is determined by the geology of the region.
The amount of water that can be stored in any rock depends upon the porosity of the rock. Porosity is expressed quantitatively as the percentage of the total volume of rock that is occupied by interstices. When all its interstices are filled with water a rock is said to be saturated. In a saturated rock the porosity is practically the percentage of the total volume of the rock that is occupied by water.
Specific yield--The specific yield of a water-bearing formation is defined by Meinzer (1923, p. 28) as the ratio of (1) the volume of water which, after being saturated, it will yield by gravity to (2) its own volume. This ratio is generally stated as a percentage. Thus if 100 cubic feet of saturated water-bearing material when drained will supply 20 cubic feet of water, the specific yield of the material is said to be 20 percent.
Permeability and transmissibility--The rate of movement of ground water is determined by the size, shape, quantity, and degree of interconnection of the interstices and by the hydraulic gradient from one point to another. The capacity of a water-bearing material for transmitting water under hydraulic head is its permeability. The coefficient of permeability may be expressed as the rate of flow of water, in gallons a day, through a cross sectional area of 1 square foot under a hydraulic gradient of 100 percent at a temperature of 60° F. (Meinzer's coefficient). The coefficient of transmissibility is a similar measure and may be defined as the number of gallons of water a day transmitted through each 1-foot strip extending the height of the aquifer under a unit gradient (Theis, 1935, p. 520). The coefficient of transmissibility may also be expressed as the number of gallons of water a day transmitted through each section 1 mile wide extending the height of the aquifer, under a hydraulic gradient of 1 foot to the mile.
The coefficient of transmissibility is equivalent to the coefficient of permeability (corrected for temperature) multiplied by the thickness of the aquifer.
The permeability of the water-bearing materials in Scott County was determined by four pumping tests using the recovery method involving the following special formula developed by Theis (1935, p. 522) and also described by Wenzel (1942, p. 94) for computing the transmissibility of an aquifer.
in which T = coefficient of transmissibility, q = pumping rate, in gallons a minute, t = time since lumping began, in minutes t1 = time since pumping stopped, in minutes, and s = residual drawdown at the pumped well, in feet, at time t1.
The residual drawdown (s) is computed by subtracting the static water level before pumping began from appropriate water levels taken from the recovery curve. The proper ratio
is determined graphically by plotting against corresponding values of s. By plotting t/t1 on the logarithmic coordinate of semi-logarithmic paper this procedure is simplified. For any convenient value of log10 (t/t1) the corresponding value of s may be obtained from the curve. Theoretically this curve is a straight line that passes through the origin. For all pumping tests, however, it does not do so and for some tests results are obtained that do not agree with results obtained by other pumping-test methods. Wenzel (1942, p. 96) has found that results generally consistent with other pumping-test methods can be obtained by applying a correction factor to the Theis formula to make the straight line pass through the origin. The Theis formula modified to include the empirical correction factor is
where c is a constant arbitrarily chosen so that the straight line determined by plotting
against s will pass through the t1 origin.
Pumping Tests--During the course of the investigation pumping tests on four irrigation wells in the Scott Basin were conducted by Melvin Scanlan of the Division of Water Resources, Kansas State Board of Agriculture, and Woodrow Wilson of the Federal Geological Survey. In each test the well was pumped for approximately three hours and measurements of the discharge of the well, using a Collins flow gage, were made periodically. Where possible, measurements of the drawdown in the pumped well were made at frequent intervals. The pump was then shut down and the water level in the well was allowed to recover. A series of water-level measurements, using a steel tape, were made periodically during the recovery period. Recovery curves were plotted for each of the tests, two of which are shown in Figure 6. The coefficient of transmissibility and thus the field coefficient of permeability may be computed from the recovery of the water level in the pumped well by using the Theis formula. The temperature of the water during the tests was 60° F.; hence no correction for temperature needs to be applied.
Figure 6--Drawdown and recovery curves for wells 91 and 137 during pumping tests on December 8, 1941, and May 19, 1942, respectively.
A pumping test on an irrigation well (172) owned by George Duff and situated in the NW 1/4 SW 1/4 sec. 12, T. 19 S., R. 33 W. was made on December 9, 1941. The pumping rate, depth to water level before, during, and after pumping, and values for t/t1 and for s (residual drawdown) are given in Table 2.
Table 2--Data on pumping test of well 172, Scott County, made on December 9, 1941.
The ratio t/t1 was determined graphically by plotting t/t1 against corresponding values of s on semi-logarithmic paper and using for the ratio the slope of the straight line drawn through the plotted points (Fig. 7). Thus
The transmissibility is computed to be 17,010 and the coefficient of permeability is determined by dividing the transmissibility by the thickness of the aquifer, or 280. The specific capacity of the well is computed to be 16.3 gallons per foot of drawdown.
Figure 7--Curves for pumping tests on well 172 owned by George Duff.
A pumping test on an irrigation well (91) owned by V. M. Harris and situated in the SE 1/4 NW 1/4 sec. 29, T. 18 S., R. 32 W. was made on December 8, 1941. The pumping rate, depth to water level before, during, and after pumping, and values for t/t1 and for s (residual drawdown) are given in Table 3.
Table 3--Data on pumping test of well 91, Scott County, made on December 8, 1941.
Using the Theis formula, the coefficient of transmissibility may be computed from the recovery of the water level in the pumped well. The ratio t/t1 was determined graphically by plotting t/t1 against corresponding values of s on semi-logarithmic paper and using for the ratio the slope of the straight line drawn through the plotted points. An empirical correction to make the straight line pass through the origin was applied. The correction c was determined by trial and error and found to be -135. Thus
The transmissibility is computed to be 46,270 and the coefficient of permeability is determined by dividing the transmissibility by the thickness of the aquifer, or 1,200. The specific capacity of the well is computed to be 26.8 gallons per foot of drawdown.
A pumping test on an irrigation well (139) owned by M. K. Armantrout and situated in the NE 1/4 NW 1/4 sec. 36, T. 18 S., R. 33 W. was made on May 22, 1942. The pumping rate, depth to water level before, during, and after pumping, and values for t/t1 and for s (residual drawdown) are given in Table 4.
Table 4--Data on pumping test of well 139, Scott County, made on May 22, 1942.
The ratio t/t1 was determined graphically by plotting t/t1 against corresponding values of s on semi-logarithmic paper and using the slope of the straight line drawn through the plotted points. Thus
The transmissibility is computed to be 10,300 and the coefficient of permeability is determined by dividing the transmissibility by the thickness of the aquifer, or 190. The specific capacity of the well is computed to be 20.1 gallons per foot of drawdown.
A pumping test on an irrigation well (137) owned by C. T. Hutchins and situated in the SE 1/4 SE 1/4 sec. 35, T. 18 S., R. 33 W. was made on May 19, 1942. The pumping rate, depth to water level before, during, and after pumping, and values for t/t1 and for s (residual drawdown) are given in Table 5.
Table 5--Data on pumping test of well 137, Scott County, made on May 19, 1942.
The ratio t/t1 was determined graphically by plotting t/t1 against corresponding values of s on semi-logarithmic paper and using the slope of the straight line drawn through the plotted points.
The transmissibility is computed to be 79,400 and the coefficient of permeability is determined by dividing the transmissibility by the thickness of the aquifer, or 850. The specific capacity of the well is computed to be 15.2 gallons per foot of drawdown.
Data on the four pumping tests in Scott County are listed in Table 6. As indicated in Table 6, the coefficient of permeability of the combined Pleistocene and Pliocene water-bearing formations penetrated by irrigation wells in Scott County is much greater than that of water-bearing formations of Pleistocene age only. Some of the largest yields obtained from irrigation wells in the county are believed to be derived from wells penetrating deposits of sand and gravel of the Ogallala formation of Pliocene age. One of the largest groups of successful irrigation wells obtaining water from this source is situated in an area covering several square miles about 3 miles west of Shallow Water.
Water in Sand and Gravel
Much of Scott County is underlain by unconsolidated deposits of sand and gravel of Pleistocene and Pliocene age. Sand and gravel are also found in the alluvium in several of the stream valleys. The history of their deposition is given under Geologic History, and their distribution, character, thickness, and water-yielding capacity are described under geologic formations and their water-bearing properties.
These stream deposits were subjected to the sorting action of water with the result that distinct beds of gravel, sand, silt, or clay were deposited. The source of material and degree of sorting determined the texture of this material, whether coarse or fine, some deposits being composed of clean well-sorted gravel while in others finer materials predominate. In some poorly sorted deposits finer materials occupy the pore spaces between the larger grains reducing the effective porosity. Coarse, clean, well-sorted gravel or sand has a high porosity and high permeability. Properly constructed wells in material of this type yield large quantities of water.
The sand and gravel deposits of the Ogallala formation constitute the principal source of water in Scott County. In the Scott Basin the sand and gravel deposits of Pleistocene age are also an important source of ground water. The sand and gravel deposits of the alluvium in the principal stream valleys, particularly in the valley of Beaver Creek, furnish water to many domestic and stock wells. The sand and gravel deposits of the Ogallala formation and the water-bearing sands and gravels of Pleistocene age furnish water to a great many domestic and stock wells and to many irrigation wells as well as to the public-supply wells at Scott City. Approximately 130 irrigation wells in the Scott Basin are supplied with water from these deposits.
Water in Chalk and Shale
Chalk and associated chalky shales are not important sources of water in Scott County, although many of the wells in the southeastern quarter of the county are known to end in either chalk or chalky shale.
Water occurs in limestone in fractures or in solution openings that have been dissolved out of the rock by water containing dissolved carbon dioxide. The occurrence of fractures and solution openings is very irregular, making it difficult to predict where water will be found in a limestone. One well drilled to limestone may encounter water-filled fractures or solution openings and have a good yield, whereas another well drilled only a few feet from the first well may not encounter any fractures or solution openings and yield little or no water. In drilling for water in an area underlain by limestone, it is generally necessary to put down several test holes to locate water-bearing fractures or solution openings before the final well can be drilled.
Shale is one of the most unfavorable of rocks from which to obtain water. Shale, if not too tightly indurated, may have a fairly high porosity and contain much water. The interstices between the individual particles are so small, however, that the water is held by molecular attraction and hence is not available to wells. Available water in shale is found only in joints and along bedding planes.
Kansas Geological Survey, Scott County Geohydrology|
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Web version March 2003. Original publication date July 1947.