Principles of OccurrenceThe following discussion on the occurrence of ground water has been adapted from Meinzer (1923, pp. 2-102), and the reader is referred to his report for a more detailed discussion of the subject. For a general discussion of the occurrence of ground water in Kansas the reader is referred to State Geological Survey Bulletin 27 (Moore, 1940).
Ground water, or underground water, is the water that supplies springs and wells. The rocks that form the outer crust of the earth are at very few places solid throughout, but contain numerous open spaces, called voids or interstices. These open spaces are the receptacles that hold the water that is found below the surface of the land and is recovered in part through wells and springs. There are many kinds of rocks, and they differ greatly in the number, size, shape, and arrangement of their interstices and, hence, in their properties as containers of water. Therefore, the character, distribution, and structure of the rocks of any region determine the occurrence of water.
The amount of water that can be stored in any rock depends upon the volume of the rock that is occupied by open spaces, that is, the porosity of the rock. The porosity is expressed as 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. The porosity of a sedimentary rock is controlled by (1) the shape and arrangement of its constituent particles, (2) the degree of assortment of its particles, (3) the cementation and compaction to which it has been subjected since its deposition, (4) the removal of mineral matter through solution by percolating waters, and (5) the fracturing of the rock, resulting in joints and other openings. Well-sorted deposits of unconsolidated silt, sand, or gravel have a high porosity regardless of the size of the grains. Poorly-sorted deposits have a much lower porosity because the small grains fill the voids between the large grains, thus reducing the amount of open space. The pore space in some well-sorted deposits of sand or gravel may gradually be filled with cementing material, thus gradually reducing the porosity.
The capacity of a rock to hold water is determined by its porosity, but its capacity to yield water is determined by its permeability. The permeability of a rock may be 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. Rocks that will not transmit water may be said to be impermeable. Some deposits, such as well-sorted silt or clay, may have a high porosity, but because of the minute size of the pores will transmit water only very slowly. Other deposits, such as well-sorted gravel containing large openings that are freely interconnected, will transmit water very readily.
Part of the water in any deposit is not available to wells because it is held against the force of gravity by molecular attraction--that is, by the cohesion of the water itself and by its adhesion to the walls of the pores. The ratio of the volume of water that a rock will yield by gravity, after being saturated, to its own volume is known as the specific yield of the rock.
Below a certain level, which in Finney and Gray counties ranges from the surface to about 210 feet below the surface, the permeable rocks are saturated with water under hydrostatic pressure. These saturated rocks are said to be in the zone of saturation, and the upper surface of this zone is called the water table. Wells dug or drilled into the zone of saturation will become filled with ground water to the level of the water table.
The permeable rocks that lie above the zone of saturation are said to be in the zone of aeration. As water from the surface percolates slowly downward to the zone of saturation, part of it is held in the zone of aeration by the molecular attraction of the walls of the open spaces through which it passes. In fine-grained material there is invariably a moist belt in the zone of aeration just above the water table, and this moist belt is known as the capillary fringe. Although water in the zone of aeration is not available to wells, much of the water in the upper part of the zone may be withdrawn by the transpiration of plants and by evaporation from the soil.
Sand and Gravel
In Finney and Gray counties more wells obtain water from sand and gravel than from any other source. Unconsolidated deposits of sand and gravel are found in the alluvium in all of the larger stream valleys, in the Pleistocene terrace deposits, in the undifferentiated Pleistocene deposits, and in the Ogallala formation.
Gravel is far superior to any other type of material in its ability to store and yield water. A great range in the water-bearing properties of various kinds of gravel is found, however. This difference is controlled by the degree of assortment and the degree of cementation of the gravel. Coarse, clean, well-sorted gravel has a high porosity, high permeability, and high specific yield. It has the ability to absorb water readily, to store it in large quantities, and to yield it to wells freely. In some deposits, however, clay, silt, or sand is mixed with the gravel, thus reducing its porosity, permeability, and specific yield. Most of the gravel deposits in Finney and Gray counties contain some silt and sand, but nevertheless yield water very freely. Some of the gravel deposits in the undifferentiated Pleistocene deposits and the Ogallala formation have been tightly cemented with a lime carbonate, thus making them worthless as producers of water. The tightly cemented zones are relatively thin, however.
Sand ranks next to gravel as a water bearer. The same factors causing variations in the water-bearing properties of gravels will cause variations in the water-bearing properties of sand. Sand differs from gravel in having smaller interstices; therefore, it will conduct water less readily and will give up a smaller proportion of its water to wells. Sand also consists of smaller particles, which are more readily carried by the water into the wells, thus causing difficult problems in connection with drilling and pumping. Proper well construction is especially important where the main source of water is from fine sand. A discussion of well construction is given on page 92.
The distribution, character, thickness, and water-yielding capacity of the sand and gravel deposits in this area are described under the chapter on geologic formations and their water-bearing properties.
The Dakota formation contains the only water-bearing sandstones tapped by shallow wells in this area. All of the wells tapping the Dakota formation in this area are in southeastern Gray County, and supply water for domestic and stock use. Sandstones also occur in the underlying Cheyenne sandstones, but no water wells are known to tap the Cheyenne in this area.
Sandstone ranks next to sand in its ability to store and transmit water. The factors determining the water-bearing properties of a sandstone are the size of grain, degree of assortment, and degree of cementation. A coarse-grained well-sorted sandstone generally will yield water freely, whereas an equally well-sorted very fine-grained sandstone holds a relatively large part of its water and surrenders the rest very slowly. A loosely cemented very fine-grained sandstone also is undesirable because of the tendency of the grains to enter wells, thus causing damage to the pumps and often clogging the wells (see page 93). The degree of assortment of the sand grains in a sandstone affects the water-bearing properties of the sandstone in the same way as in a gravel deposit. Fine sand, silt, or clay in a coarse-grained sandstone greatly decreases the porosity and permeability of the sandstone. The interstices of sandstone are small and are, therefore, easily closed by precipitates from percolating water. Many sandstones are so thoroughly cemented that they will not yield water from the original openings between their grains. Tightly cemented sandstone may, however, contain joints and fractures that hold water.
Sandstones in the Dakota formation range considerably in their capacity as water-bearers. A complete discussion of the character and water supply of the Dakota formation is given on pages 145-148 of this report.
Limestone, although inferior in this area to gravel, sand, or sandstone as a water bearer, supplies water to a few wells in this area where the more desirable types of rocks are absent or where they lie above the water table and are therefore barren of water. Such wells tap limestones in the Fort Hays limestone member of the Niobrara formation (p. 159). This limestone member also supplies water to several springs in the northeastern part of Finney County (p.160).
Water occurs in limestone in fractures or in solution openings that have been dissolved out of the rock by water containing dissolved carbon dioxide. The occurrence of fractures and solution openings is very irregular, making it difficult to predict where water will be found in a limestone. One well drilled to limestone may encounter water-filled fractures or solution openings and have a good yield, whereas another well drilled only a few feet from the first well may not encounter any fractures or solution openings and yield little or no water. In drilling for water in an area underlain by limestone, it is generally necessary to put down several test holes to locate water-bearing fractures or solution openings before the final well can be drilled.
Several wells in this area obtain all or a part of their water from shale. All of the wells known to obtain water from shale tap the Blue Hill shale member of the Carlile shale. Most of the wells have very small yields. Shales occur in all of the Cretaceous formations in this area. Shale is one of the most unfavorable of rocks from which to obtain water. Shale, if not too tightly indurated, may have a fairly high porosity and contain much water. The interstices between the individual particles are so small, however, that the water is held by molecular attraction and hence is not available to wells. Available water in shale is found only in joints and along bedding planes. Descriptions of various shales encountered by drilling in this area are given in the chapter on geologic formations and their water-bearing properties.
Kansas Geological Survey, Finney and Gray County Geohydrology|
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Web version April 2002. Original publication date Dec. 1944.