By Howard G. O'Connor
(Although the ground-water section of this report is a cooperative product of the State Geological Survey of Kansas and the United States Geological Survey, the stratigraphic nomenclature used is that of the State Geological Survey of Kansas.)
Ground water is the water below the surface of the land that supplies water to wells and springs. It is derived largely from precipitation falling as rain or snow. Part of the water falling as rain or snow enters the soil, either directly or from streams, and percolates downward to the zone of saturation, part of the water after entering the soil is returned to the atmosphere by evaporation and transpiration without becoming a part of the ground-water body, and part of the water is carried away by streams as surface runoff. The ground water percolates slowly through the rocks in a direction determined by the topography and geologic structure until it is discharged through wells, through springs or seeps, or by evaporation and transpiration in the valley areas.
Principles of Occurrence
Basic principles of the occurrence of ground water were described by Meinzer (1923) and the reader is referred to this publication for more detailed discussion of ground-water principles. A general discussion of ground-water occurrence in Kansas has been made by Moore and others (1940).
The rocks that form the outer crust of the earth are not solid but contain numerous voids or interstitial openings that may contain air, water, or other fluid. The number, size, shape, and arrangement of these openings in different rock types vary greatly, and the water-bearing characteristics of rocks vary accordingly. With respect to water supplies, the original or primary interstices, are of primary importance in the unconsolidated rocks and certain types of consolidated rocks. Secondary interstices, such as joints, other fracture openings, and solution openings, are of greater importance in many of the consolidated rocks. Thus, the geology is a primary factor in the mode of occurrence of ground water in any area.
The amount of water that can be stored in a rock is determined by the porosity, or the percent of the rock occupied by voids. The amount of water that a saturated rock will yield by gravity is termed its specific yield; the amount retained or held in the voids of the rock is termed the specific retention. Thus, if 10 cubic feet of saturated rock yields 2 cubic feet of water, the specific yield is 20 percent. If 1 cubic foot of water is retained in the rock, the specific retention is 10 percent. The porosity is the sum of the specific yield and specific retention, or 30 percent.
The permeability of a water-bearing rock is its capacity to transmit water under hydraulic head and is a function of the size, shape, arrangement, and degree of interconnection of the voids, rather than the porosity.
Below a certain level in the earth's crust the permeable rocks generally are saturated with water under hydrostatic pressure and are said to be in the zone of saturation. The upper surface of the zone of saturation is called the water table. The rocks above the zone of saturation are in the zone of aeration, which ordinarily consists of three parts: the belt of soil water just below the surface, the intermediate or vadose zone, and the capillary fringe.
Unconfined, or free ground water, is water in the zone of saturation that does not have a confining or impermeable body restricting its upper surface. The upper surface of unconfined ground water is called the water table. Changes in permeability and amounts of recharge to and discharge from the ground-water reservoir cause irregularities in the water table which, on a reduced scale, are generally similar to those of the surface topography. The height to which small openings in rocks above the water table are filled with capillary water is generally several inches to several feet in fine-grained granular materials but much less where it occurs in fissures, fractures, or solution channels.
Ground water is said to be confined if it occurs in permeable zones between relatively impermeable confining beds. Slightly permeable confining beds are probably much more abundant than impermeable confining beds and considered over a wide area probably no bed is strictly impermeable.
In areas where water is confined by alternating permeable and impermeable beds, a well may pass through several beds in the zone of saturation that have sufficient permeability to yield water to the well.
Artesian conditions--The level at which water stands in a well is called the piezometric, or pressure indicating, surface. Where the top of the zone of saturation is in permeable material, the piezometric surface coincides with the water table and the water is said to be under normal pressure. If water is confined between relatively impermeable beds, the level at which water will stand in a well may be above or below the top of the zone of saturation. Confined water is said to be under artesian pressure if the water level stands above the water table or under subnormal pressure if the water level in a well is lower than the water table. If the water has sufficient pressure, or head, to flow at the surface of a well it is a flowing artesian well.
Many wells obtaining water from the Pennsylvanian and Permian limestones and sandstones in Lyon County obtain artesian water, but no areas were found in which the head was sufficient to cause the wells to flow at the surface.
Kansas Geological Survey, Lyon County Geohydrology
Web version Sept. 2001. Original publication date March 1953.
Comments to firstname.lastname@example.org
The URL for this page is http://www.kgs.ku.edu/General/Geology/Lyon/pt2_reso.html