Principles of OccurrenceThis discussion of the principles governing the occurrence of ground water takes account of conditions in Seward County. Preparation of the discussion has been based chiefly on the authoritative and 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 groundwater occurrence, with special reference to Kansas, has been published by Moore (1940).
The rocks that make up the outer crust of the earth generally are not entirely solid, but have numerous openings, called voids or interstices, which may contain air, natural gas, oil, or water. The number, size, shape, and arrangement of the interstices in rocks depend upon the character of the rocks. The occurrence of water in any region is therefore determined by the geology.
The interstices or voids in rocks range in size from microscopic openings to the huge caverns found in some limestones. The open spaces generally are connected so that water may percolate from one to another, but in some rocks these open spaces are isolated and the water has little chance to percolate. In Seward County, the rocks from which most of the ground water is obtained are poorly consolidated sand and gravel. Generally the sand and gravel of the Tertiary and Quaternary deposits contain many interstices through which water percolates freely; locally these interstices may be filled with calcium carbonate, clay, or other materials that make the rock relatively impermeable; Much of the silt, sand, and gravel of the Tertiary and Pleistocene deposits is poorly sorted and the finer particles fill a part of the space between the larger particles, thereby decreasing the amount of space available to ground water. The sandstones of the Dakota formation are cemented with iron oxide or calcium carbonate. The cement occupies a part of the spaces between sand grains, but enough voids are left to contain some water.
The porosity of a rock is the percentage of the total volume of the rock that is occupied by interstices. A rock is said to be saturated when all its interstices are filled with water or other liquid and the porosity is then practically the percentage of the total volume of rock that is occupied by water. The porosity of a rock determines only the amount of water a given rock can hold, not the amount it may yield to wells. Some rocks may be highly porous, but will not yield an appreciable amount of water to. a well. The specific yield of a water-bearing formation is defined as the ratio of (1) the volume of water which, after being saturated, it will yield by gravity to (2) its own volume. It is a measure of the yield when it is drained by a lowering of the water table. The permeability of a water-bearing material is defined as its capacity for transmitting water under hydraulic head, and is measured by the rate at which it will transmit water through a given cross section under a given difference of head per unit of distance. A rock containing very small interstices may be very porous, but it would not be very permeable, whereas a coarser-grained rock, although it may have less porosity, generally is much more permeable. Some water is held in rocks by the force of molecular attraction, which, in fine-grained rocks, is sufficiently great to make the rock relatively impermeable.
Below a certain level in the earth's crust, the permeable rocks generally are saturated with water and are said to be in the zone of saturation. The upper surface of the zone of saturation is called the groundwater table, or simply the water table. All the rocks above the water table are in the zone of aeration, which ordinarily consists of three parts: the belt of soil water; the intermediate or vadose zone; and the capillary fringe.
In some rock formations, relatively permeable strata such as sand and gravel alternate with relatively impermeable strata such as clay or shale. If these beds have a slope, then water that falls on the area of outcrop of a permeable bed moves down the dip of the bed between the confining layers of relatively impermeable material and saturates the permeable stratum. Wells drilled to the saturated bed encounter water under artesian head. Artesian water has been encountered in many wells in Seward County but nowhere in this area has the artesian head been sufficient to cause them to flow.
Kansas Geological Survey, Seward County Geohydrology|
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Web version Sept. 2001. Original publication date March. 1948.