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Ground Water
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Ground WaterSourceThe following discussion on the source and occurrence of ground water has been adopted from Meinzer (1923, pp. 2-102) and the reader is referred to his report for a more complete discussion of the subject. A summary of groundwater conditions in Kansas has been made by Moore (1940).Water that occurs in the pores or openings of the rocks and within the zone of saturation is called ground water. The amount of ground water that can occur below any region and the manner and rate of its movement to wells or springs is largely controlled by the character of the rocks. In Thomas County, as in other parts of the High Plains, 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 may evaporate or be absorbed by vegetation and transpired into the atmosphere. The part that escapes surface runoff, evaporation, and transpiration percolates downward through the soil and underlying strata until its reaches the water table, where it joins the body of ground water in the zone of saturation. In the southern High Plains the average amount of rainfall entering the groundwater body each year has been determined by two different investigations as about one-fourth inch (Frye, 1942, p. 66) and as about one-half inch (Theis, Burleigh, and Waite, 1935, pp. 1-4). The geologic conditions in Thomas County are somewhat different from those in the two areas farther south, but the average annual rainfall is about the same and the amount of rainfall reaching the groundwater body in this area is probably of the same order of magnitude. Although this is a small percentage of the annual rainfall it should be noted that one-fourth inch of water entering the groundwater reservoir under 1 square mile amounts to 4,344,674 gallons or 13.3 acre feet, and one-half inch of rainfall over 1 square mile amounts to 8,689,348 gallons. Ground water moves slowly through the rocks in directions determined by the shape and slope of the water table, which is controlled by topography, local variations in the quantity of recharge or discharge, and the stratigraphy and structure of the rocks. It is eventually discharged through springs or wells, through seeps into streams, or by evaporation or transpiration in bottom lands adjacent to streams. Most of the water obtained from wells in Thomas County comes from precipitation in the general vicinity and adjacent areas to the west. OccurrenceNearly all rocks that underlie the surface of the earth at depths shallow enough to be penetrated by drills contain various percentages of open spaces. These voids or interstices range in size from microscopic openings to the large caverns developed in limestone regions. The percentage of the volume of the rock mass consisting of such open spaces determines the porosity. Therefore, if we know the porosity of any rock we can determine the amount of water that it can hold. Although it is desirable when considering problems of groundwater supply to know the porosity of the strata under an area it is the permeability of the material that influences the amount of ground water which can move through it toward a pumping well. The permeability of a rock is determined by the size, shape, and arrangement of the openings. For instance, a bed of fine silt or clay might have a relatively high porosity, but because of the size of the particles each opening is very small. As the force known as molecular attraction holds a very thin layer of water on the surface of each grain, these layers of water (that are not free to move) might fill or almost fill all the openings in such a fine-textured sediment; thus the permeability, or water yielding capacity of the rock, is very low even though its porosity, or water holding capacity, is quite high. Likewise, larger openings that are not connected or are poorly connected might produce a high porosity and a low permeability. Water moves most freely through a rock that has relatively large and well-connected openings. Several common types of openings or interstices and the relation of texture to porosity are shown in figure 5.Figure 5--Diagram showing several types of rock intensities and the relation of rock texture to porosity. A. Well-sorted sedimentary deposit having high porosity; B. poorly sorted sedimentary deposit having low porosity; C. Well-sorted sedimentary deposit consisting of pebbles that are themselves porous, so that the deposit as a whole has a very high porosity; D. well-sorted sedimentary deposit whose porosity has been diminished by the deposition of mineral matter in the interstices; E. rock rendered porous by solution; F. rock rendered porous by fracturing. (From O.E. Meinzer.)
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Kansas Geological Survey, Thomas County Geohydrology Comments to webadmin@kgs.ku.edu Web version Nov. 2001. Original publication date Dec. 1945. URL=http://www.kgs.ku.edu/General/Geology/Thomas/05_gw.html | ||
