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.