Principles of OccurrenceThe following discussion of the occurrence of ground water has been adapted from Meinzer (1923) and the reader is referred to his report for a more detailed discussion. A general discussion of the principles of ground-water occurrence with special reference to Kansas has been presented by Moore and others (1940).
Hydrologic Properties of Water-Bearing MaterialsThe rocks that make up the crust of the earth generally are not solid but have many openings, called voids or interstices, which may contain air, natural gas, oil, or water. The many kinds of rocks differ greatly in the number, size, shape, and arrangement of their interstices; therefore, the occurrence of water in any region is determined by the geology of the region.
The interstices or voids in rocks range in size from microscopic openings to the huge caverns found in some limestones. The porosity of a rock is expressed quantitatively as the percentage of the total volume of the rock that is occupied by interstices or that is not occupied by solid rock material. Uncemented deposits of gravel having a uniform grain size have greater porosity than deposits made up of a mixture of sand, clay, and gravel, in which the smaller particles occupy space between adjacent large particles. Relatively soluble rock such as limestone, though originally dense, may become cavernous as a result of the removal of part of its substance through the solvent action of percolating water. Hard, brittle rock may acquire large interstices through fracturing that results from shrinkage or deformation of the rocks or through other agencies.
The permeability of a rock is its capacity for transmitting water under pressure and is measured by the rate at which the rock will transmit water through a given cross section under a given difference of head per unit of distance. The permeability of water-bearing material generally is expressed as a coefficient of permeability, which is commonly defined by the U.S. Geological Survey as the number of gallons of water per day at a temperature of 60 deg. 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. 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 moderate porosity but only slight permeability; a coarse-grained sand may have less porosity but much greater permeability, i.e., a greater ability to transmit water. Coefficients of permeability of less than 100 gallons per day per square foot are regarded as low, coefficients of 100 to 1,000 are medium, and those of more than 1,000 are high.
The coefficient of transmissibility is equal to the field coefficient of permeability (same as the coefficient defined above, except that it is for the prevailing temperature of the ground water) multiplied by the saturated thickness of the aquifer (water-bearing material) in feet. The coefficient of transmissibility and the coefficient of permeability are discussed further in the section on aquifer tests.
The specific yield of a rock or soil is the ratio of (1) the volume of water it will yield by gravity after being saturated to (2) its own volume. This ratio is stated as a percentage. The specific retention of a rock is the ratio of (1) the volume of water it will retain against the pull of gravity after being saturated to (2) its own volume.
Classification of Subsurface WaterThe permeable rocks that lie below a certain level are generally saturated with water under hydrostatic pressure, and such rocks are said to be in the zone of saturation (Fig. 4). The zone of saturation ordinarily extends down to a depth much greater than is reached by modern drilling methods. The term ground water is used to designate that part of the subsurface water within the zone of saturation. The upper surface of the zone of saturation, where not formed by an impermeable body, is called the water table. In most places there is only one zone of saturation, but in certain localities the water may be hindered in its downward course by an impermeable or nearly impermeable bed to such an extent that it forms an upper zone of saturation, or perched water body, which is not associated with the lower zone of saturation.
Figure 4--Diagram showing divisions of subsurface water (after O.E. Meinzer).
Subsurface water above the water table is in the zone of aeration, which ordinarily consists of three parts: the belt of soil water, the intermediate belt, and the capillary fringe.
Soil water, which is water held by molecular attraction, lies just below the land surface and extends down to the maximum depth to which evaporation and plant action are effective. The soil water is not available to wells but is of the utmost importance to agriculture. Before any water can percolate downward to the water table through this belt, the amount of water present must exceed that which will be held by adhesion. The thickness of the belt of soil water is determined by the texture of the rock or soil and by the character of the vegetation.
The intermediate belt, which lies between the belt of soil water and the capillary fringe, is thick where the depth to the water table is great but may be absent where the water table is at or near the land surface. In this belt the interstices in the rocks contain some water held by molecular attraction but also may contain appreciable quantities of water that is moving downward from the belt of soil moisture to the water table.
The capillary fringe lies directly above the water table and contains water held above the zone of saturation by capillary force. The water in the capillary fringe is not available to wells, which must be deepened to the zone of saturation before water will enter them. The capillary fringe may be very thin in coarse-grained sediments, in which capillary action is negligible, or it may be several feet thick in fine-grained sediments.
Shape and SlopeThe water table has been defined as the upper surface of the zone of saturation. The water table is not a static, level surface; generally it is a sloping surface having many irregularities and constantly changing. The irregularities are caused chiefly by local differences in geology and topography, and the fluctuations are due to gain or loss of water within the zone of saturation.
The generalized shape of the water table in Sumner County is shown in Plate 2 by contour lines. All points along a contour line have the same altitude, and the shape and slope of the water table are shown by the lines as the land surface is shown by topographic contours. Water moves downslope in a direction at right angles to the contour lines. The movement is very slow because of the frictional resistance offered by the small interstices through which the water must pass. The shape of the water table in Sumner County conforms in general to the land surface, but relief is much more subdued. In areas where conditions are suitable for rapid recharge, water may percolate down to the water table faster than it can spread laterally, thus a mound or ridge is formed in the water table. Conversely, if water is withdrawn from the zone of saturation faster than it can flow in laterally, the water is lowered locally, and a cone or trough is formed. The permeability of the water-bearing material has a significant effect upon the slope of the water table. To produce a given rate of flow, the slope of the water table must be much steeper in a fine-grained deposit having slight permeability than in a coarse-grained permeable deposit.
The slope of the water table in Sumner County ranges from considerably less than 10 feet per mile in the extremely permeable alluvium of Arkansas River to at least 40 feet per mile in the relatively impermeable Wellington Formation and Ninnescah Shale. Ground water in general moves toward the major streams. The water-table contours in Plate 2 are much more generalized in the areas where the Wellington Formation or the Ninnescah Shale is the chief aquifer, because fewer wells were inventoried in these areas. In the area west of Conway Springs the water table is in a sense perched, in that the water accumulates in the permeable sand and gravel faster than it can percolate downward through the relatively impermeable Ninnescah Shale. The underlying Ninnescah Shale, however, probably is completely saturated.
An observation-well program was started in the Arkansas River valley near Mulvane in 1954; water-level measurements in these wells are shown in Table 4. Hydrographs showing fluctuations in several of the wells are given in Figures 5 and 6. T. Max Reitz has made periodic measurements in his irrigation well 31-2E-20acc since 1935, and fluctuations in water level in this well are shown in Figure 7 and Table 5.
Figure 5--Hydrographs showing fluctuations of water level in five wells in Mulvane area.
Figure 6--Hydrographs showing fluctuations of water level in three wells in Mulvane area, and monthly precipitation and cumulative departure from normal precipitation at Wichita.
Figure 7--Hydrograph showing fluctuations of water level in well 31-2E-20acc. A larger version of this figure is available.
Kansas Geological Survey, Sumner County Geohydrology|
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Web version January 2003. Original publication date August 1961.