Principles of OccurrenceThe fundamental principles of the occurrence and movement of ground water have been given by Meinzer (1923), and a general discussion of the occurrence of ground water with special reference to Kansas has been given by Moore (1940). The reader is referred to these publications for a more detailed discussion of the occurrence of ground water.
The rocks that make up the outer crust of the earth generally are not solid but have numerous openings, called voids or interstices. The number, size, and shape of these openings depend upon the character of the rocks; therefore, the occurrence of ground water in any region is determined by the geology of that region.
The interstices or voids in rocks range in size from microscopic openings in clay to huge caverns in limestones. The openings generally are connected so that water may move from one void to another, but in some rocks the voids are isolated so that there is little or no movement of the water. Several common types of interstices or voids, and the relation of texture to porosity, are shown in Figure 12.
Figure 12--Diagram showing several types of rock interstices.
Below a certain level in the earth's crust the rocks generally are saturated with water and are said to be in the zone of saturation (Fig. 13). The upper surface of the zone of saturation is called the ground-water table or the water table. The rocks above the water table are in the zone of aeration. This zone generally consists of three parts: the belt of soil water at the top, the intermediate vadose zone, and the capillary fringe at the bottom.
Figure 13--Diagram showing divisions of subsurface water.
The belt of soil water lies just below the land surface and normally contains water held by molecular attraction. During periods of ground-water recharge this zone contains water in excess of the amount that can be held by molecular attraction, and the excess percolates downward to the water table. The thickness of the belt of soil water depends upon the soil, the precipitation, and the vegetation.
The intermediate belt of vadose water lies between the soil belt and the capillary fringe. In this zone the interstices in the rocks contain water held by molecular attraction, and at times of ground-water replenishment they contain also water that is moving downward to the water table. The intermediate zone may be absent or may be several hundred feet thick, depending on the local geology, topography, and climate. In Reno County the intermediate zone is absent in some areas and is nowhere more than 60 feet thick.
The capillary fringe lies directly below the intermediate belt and over the water table and is formed of water held up from the zone of saturation by capillary force. The water in this zone is not available to wells, which must be deepened to the zone of saturation to obtain water. The capillary fringe may be absent or very thin in coarse-grained materials, but it may be several feet thick in fine-grained materials.
The porosity of a rock aggregate is its property of containing interstices. Porosity is expressed as the percentage of the total volume occupied by the interstices.
The moisture equivalent of a water-bearing material is expressed as a ratio of (1) the weight of water that the material, after saturation, will retain against a centrifugal force 1,000 times greater than the force of gravity, to (2) the weight of the dry material. To convert this figure to percentage of volume, the moisture equivalent is multiplied by the apparent specific gravity of the dry material.
The specific retention of a rock or soil, with respect to water, is the ratio of (1) the volume of water which, after being saturated, it will retain against the pull of gravity to (2) its own volume. It is stated as a percentage and may be expressed by the formula R = 100 (r/v), where R is the specific retention, r is the volume of water retained by the rock or soil against the pull of gravity, and v is the volume of the rock or soil.
The specific yield of a water-bearing formation is the ratio of the volume of water a saturated material will yield to gravity in proportion to its own volume (Meinzer, 1923, p. 28). The specific yield is equal to the porosity minus the specific retention. The specific yield of a formation is needed to estimate the quantity of water available to wells and to estimate the quantity of water represented by a rise or decline in the water table during periods of recharge or discharge.
Physical and Hydrologic Properties of Water-bearing MaterialsThe quantity of water an aquifer will yield to wells depends upon the physical and hydrologic properties of the materials composing the aquifer. Geologic descriptions of the materials penetrated by test holes and wells are useful in making estimates of the quantity of water an aquifer will yield. A more precise estimate of the amount of water that an aquifer will yield can be obtained from field or laboratory tests of the water-bearing materials.
Samples of water-bearing materials were collected for analysis in the hydrologic laboratory of the Geological Survey in Lawrence. These studies included mechanical (particle-size) analyses and permeability determinations. Some of the samples were collected in the fall of 1945 during an investigation of the ground water in the Arkansas River valley in the vicinity of Hutchinson (Williams, 1946). In November 1949, samples were collected from six test holes that were drilled in the county, and mechanical analyses and permeability determinations were made on a part of these samples (Table 3).
A mechanical, or particle-size, analysis of materials consists of separating into groups the grains of different size and determining the percentage by weight of each size group. Results of the analyses are shown in Table 3.
Laboratory determinations of porosity and specific yield were not made on any of the samples from test holes in Reno County, but such determinations were made on some well cuttings from wells in the Wichita well field, which is a few miles east of Reno County. The porosity ranged from 24.1 to 60.2 percent. The specific yield averaged 26.8 percent (Williams and Lohman, 1949). The water-bearing materials near Hutchinson in Reno County in the Arkansas River valley are very similar to those in the Wichita well field and probably have about the same porosity and specific yield.
The permeability of water-bearing material generally is expressed as a coefficient of permeability. The coefficient of permeability is defined as the number of gallons of water a day at a temperature of 60°F that will be conducted through each mile of the water-bearing bed under investigation, measured at right angles to the direction of flow, for each foot of thickness of the bed and for each foot per mile of hydraulic gradient (Meinzer's coefficient, or meinzer).
The quantity of water that will percolate through a given cross section of water-bearing material under a known hydraulic gradient is directly proportional to the coefficient of permeability. Thus, to compute the quantity of water that will percolate into or out of a given area the permeability must be determined.
Coefficients of permeability have a wide range in value. Clay and silt, which are fine grained, may have high porosity, but very low permeability; a coarse-grained sand may have a lower porosity, but a high permeability, owing to the greater ability of the coarse-grained material to transmit water. Coefficients of permeability of less than 100 are considered low, coefficients of 100 to 1,000 are medium, and those more than 1,000 are considered high.
Permeability tests were made on samples of water-bearing materials collected near Hutchinson in Reno County. The permeability ranged from 10 for fine sand mixed with silt to 4,400 for coarse and medium gravel. The permeability of most samples ranged from 1,000 to 3,000, except in the sand-dune area (Table 8). Permeability of the silts and clays was not tested, but generally the permeability of these materials is very low.
Kansas Geological Survey, Reno County Geohydrology|
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Web version Feb. 2001. Original publication date Aug. 1956.