Ground Water, continued
The Water Table and Movement of Ground WaterShape and Slope
The water table is defined as the upper surface of the zone of saturation except where that surface is formed by an impermeable body (Meinzer, 1923a, p. 32). It may also be regarded as the boundary between the zone of saturation and the zone of aeration. The water table is not a level surface, but rather it is generally a sloping surface which shows many irregularities caused by differences in transmissibility of the water-bearing materials or by unequal additions of water to the ground-water reservoir at different places.
The shape and slope of the water table in Finney and Gray counties are shown on plate 1 by means of contour lines drawn on the water table. Each point on the water table along a given contour line has the same altitude. These water-table contours show the configuration of the water surface just as topographic contours show the configuration of the land surface. The direction of movement of the ground water is at right angles to the water-table contour lines--in the direction of the greatest slope.
Plate 1 shows that the ground water is moving through Finney and Gray counties in a general easterly direction, but that the direction of movement and the slope vary considerably in different places. The average gradient of the water table is about 7 feet to the mile. In the northwestern part of Finney County and the southeastern part of Gray County, the slope is much greater than this, being as much as 35 or 40 feet to the mile in northwestern Finney County and 25 or 30 feet to the mile in southeastern Gray County. In contrast to these two areas, the water table in the Finney basin in northern Finney County is nearly flat, sloping only about 3 feet to the mile.
The slope of the water table in any area in general varies inversely with the permeability of the water-bearing material; that is, the water assumes a steeper gradient in flowing through fine material than through coarse material, provided the same quantity of water is moving through both types of material. This explains in part the differences in the slope of the water table in different parts of this area. In the northwestern part of Finney County, where the water-table slope is steep, the water in the upper part of the zone of saturation occurs in chalk and limestone of the Niobrara formation. As the water moves eastward it enters the permeable unconsolidated sand and gravel of the undifferentiated Pleistocene deposits and the Ogallala formation, and the gradient becomes greatly reduced. The steeper gradients in southeastern Gray County seemingly also are caused by differences in the permeability of the water-bearing materials. Throughout most of Gray County the water in the upper part of the zone of saturation occurs in the permeable sands and gravels of the undifferentiated Pleistocene deposits or the Ogallala formation. As the water moves eastward in southern Gray County it enters the less permeable fine-grained sandstones of the Dakota formation (fig. 9 CC'), hence the gradient becomes steeper. Continuing southeastward the water again enters sands and gravels of the undifferentiated Pleistocene deposits, and the gradient becomes more nearly as it was before entering the Dakota formation.
The prominent ridge on the water table, shown by the downslope flexure of the contours in southeastern Gray County, also is caused by differences in permeability. In the western part of T. 29 S., R. 27 W., and in the eastern part of T. 28 S., R. 27 W., the water moves through sandstones of the Dakota formation. Between these two areas, however, the water moves through sand and gravel in the undifferentiated Pleistocene deposits. Because the permeability of the sand and gravel is much greater than the permeability of the sandstone, much of the water is deflected around the high sandstone areas and moves through the sand and gravel. This increase in the amount of water passing through the sand and gravel has caused a ridge to be formed on the water table.
Irregularities in the shape of the water table may also be caused by the unequal addition of water to the ground-water reservoir at different places. Plate 1 shows two other rather prominent ridges on the water table--one in T. 22 S., R. 34 W., indicated by the flexure of the contours toward the northeast, and one in T. 23 S., R. 34 W., indicated by the downslope flexure of the contours. Influent seepage from the two ephemeral streams (p. 77) that end in the northern part of T. 22 S., R. 34 W., and from the Great Eastern irrigation canal (p. 77) probably is responsible for the northernmost ridge on the water table. The southern ridge probably is caused by influent seepage from the Great Eastern irrigation canal alone. Plate 1 shows a mound on the water table in the northeastern part of T. 26 S., R. 27 W. This is a depression area that holds water after rains. Some of the water probably seeps downward to the underground reservoir, where it builds up a mound on the water table (p. 73). These ridges and mounds are formed by water percolating downward because the frictional resistance offered by the small openings in the water-bearing material prevents the water from spreading out rapidly as it would on the surface of a body of free water, such as a lake. As soon as the descending water reaches the water table it joins the main body of ground water.
The slight downslope flexure of the contours south of Arkansas River indicates that a very broad, low ridge has been formed on the water table beneath the sand hills. This is especially prominent southwest of Holcomb and south of Pierceville. Part of the water that falls on the sand hills during heavy showers seeps downward to the underground reservoir (p. 68), and has thus formed a broad ridge on the water table beneath the sand hills in the same manner that the smaller ridges north of Arkansas River were formed.
The slope of the water table and the direction of movement of the ground water also are influenced by the discharge of water into streams. The contours on plate 1 show that the water table slopes rather uniformly toward the Arkansas valley from both sides and also slopes downstream, indicating that water flows into the valley from both sides and thence down the valley. The contours also indicate that slight ground-water divides exist both north and south of the Arkansas valley throughout most of its course in this area.
Arkansas River in eastern Finney County and throughout Gray County is an effluent stream; that is, it is a perennial stream whose channel has been cut down below the water table and is thereby gaining water from the zone of saturation. This movement of water from the underground reservoir to the channel of the river has caused a trough to be formed on the water table that follows the course of Arkansas River, as indicated by the upstream flexure of the contours. It will be noted that the contours in Gray County and eastern Finney County change direction as they cross the channel of the river, but that from a point a few miles east of Garden City to the western Finney County line the contours do not change direction where they cross the river. The contours still flex upstream in this area, but they change direction in the valley either north or south of the river channel. Throughout this part of its course the water table was below the channel of the river at the time the water-level measurements were made for the water-table map. Under these conditions, this part of the river would furnish water to the underground reservoir, thereby forming a temporary ridge on the water table beneath the river channel. Heavy pumping for irrigation in this part of the valley, however, has caused a trench to be formed on the water table as indicated by the upstream flexure of the contours. If there were no heavy withdrawals of water from the underground reservoir beneath the valley, the contours probably would be straighter. A map of the water table between Garden City and Deerfield based on measurements made during the summer of 1904 shows the water-table contours crossing the Arkansas valley as nearly straight lines (Slichter, 1906, fig. 1). At the time this map was made the withdrawal from the underground reservoir beneath the Arkansas valley was relatively small compared with what it is now. It will be noted that no contours were drawn in the northeastern or "panhandle" part of Finney County. This area is underlain by Cretaceous shales and limestones and does not have normal water-table conditions. Wells in this area obtain water from limestone or shale, or from alluvium in the stream valleys.
Relation to Topography
The topography of the land surface is one of the chief factors controlling the depth to water level in Finney and Gray counties. A map (plate 2) has been prepared showing the depths to water level in Finney and Gray counties by the use of isobath lines--lines of equal depth to water level. In preparing this map the more general irregularities of the surface topography were taken into account by using aerial photographs and the available topographic maps. Some inaccuracy has been introduced by local irregularities of the land surface which are not shown on the topographic maps or aerial photographs.
As shown on the map, the depth to water level in this area ranges from less than 5 feet to about 200 feet. The relation between the water table and the land surface is shown in the four geologic sections in figures 8 and 9.
For purposes of detailed descriptions of the ground-water conditions, Finney and Gray counties may be divided into the following areas based upon the depth to water level: valley shallow-water areas, Finney basin shallow-water area, sand hills shallow-water area, upland areas of intermediate depth to water, and the Ensign deep-water area. Special ground-water conditions exist in the area of Cretaceous rocks in northeastern Finney County; therefore, it is described separately as the Pawnee area.
Valley shallow-water areas--The Arkansas valley is the most important shallow-water area in Finney and Gray counties. The width of the valley ranges from about 1 mile near Pierceville to about 3 1/2 miles near Garden City. In the Arkansas valley the measured depths to water level ranged from 3 to about 22 feet. Many domestic, stock, irrigation, public supply, and industrial wells are located in the valley. More water is pumped from wells in the valley than from any other area in the two counties. The greatest concentration of wells is in the vicinity of Garden City, where there are as many as 14 wells in a single section. Most of the irrigation wells obtain water from coarse gravel in the alluvium. Other wells are deeper and obtain water from either the undifferentiated Pleistocene deposits or the Ogallala formation.
Smaller and less important shallow-water areas occur in the narrow valleys of Crooked Creek and its tributaries in southern Gray County and Buckner creek in northeastern Gray County. The depth to water level in these smaller valleys ranges from about 10 feet in the bottoms of some of the valleys to 50 feet on the slopes bordering the valleys. A few wells in these areas obtain water from alluvium, but most of the wells tap undifferentiated Pleistocene deposits or the Ogallala formation. ground-water conditions in the valleys of Pawnee River and its tributaries are discussed under the heading Pawnee area on page 62.
Finney basin shallow-water area--This area of shallow water extends from the Arkansas valley in western Finney County northward to the Finney-Scott County line, and is continuous with the shallow-water basin in Scott County. Its eastern and western limits are roughly marked by the 50-foot depth-to-water line on plate 2. The surface topography of the Finney basin has been described on page 21. Measured depths to water level in this area ranged from 9 to 50 feet below the land surface. Domestic, stock, and irrigation wells in this area obtain water from undifferentiated Pleistocene deposits or from the Ogallala formation. Many wells, especially the deep irrigation wells, tap both the undifferentiated Pleistocene beds and the Ogallala. The zone of saturation in the Finney basin ranges from 100 to about 300 feet in thickness, and attains its greatest thickness in the southern part near the Arkansas valley. The possibilities of developing additional supplies of water for irrigation in this area are discussed on page 119.
Sand hills shallow-water area--The sand hills shallow-water area borders the Arkansas valley on the south and is that area lying between the Arkansas valley and the 50-foot depth-to-water line (pl. 2). In Gray County it is a narrow belt not more than 2 1/2 or 3 miles wide, but in Finney County it is as much as 5 miles wide. Because of local relief in the sand hills, the depth to water level varies greatly, and depends on the location of the well. The depth to water level in a well in a basin between dunes will be less than if the well were located on the top of a dune. It was for this reason that the 50-foot depth-to-water line was dashed in southern Finney County. The depth to water level in the wells visited in this area did not exceed 50 feet. The wells in the sand hills shallow-water area obtain water from the Pleistocene terrace gravels that underlie the sand hills or from the undifferentiated Pleistocene beds.
Upland areas of intermediate depth to water--In by far the greater part of Finney and Gray counties the depth to water level ranges from 50 to 150 feet below the surface. South of the Arkansas valley is a large area of intermediate depth to water, which includes Roost of the sand hills and uplands in central Gray County and all of southern Finney County. Wells in this large area tap the undifferentiated Pleistocene beds and/or the Ogallala formation, and the depth to water level in them ranges from 50 to 150 feet. The depth to the water table in the sand-hills part of this area may vary greatly within short distances because of local relief of the dune topography.
Two areas of intermediate depth to water level are found north of the Arkansas valley. The largest of the two includes all of northern Gray County, the southern part of the "panhandle" of Finney County, and all of central Finney County. Measured depths to water level in this area ranged from 57 to 130 feet. All of the wells derive water from the undifferentiated Pleistocene deposits or from the Ogallala formation or from both.
A third area of intermediate depth to water lies north of the Arkansas valley and west of the Finney basin shallow-water area. The depth to water level in this area ranges from 50 feet to a maximum known depth of 97 feet. Most of the wells tap the Ogallala formation, and a few may obtain water from undifferentiated Pleistocene beds. Wells 38 and 40, however, obtain water from the Fort Hays limestone member of the Niobrara formation (Cretaceous). In a small area surrounding these wells the Ogallala formation is thin and lies above the water table. In places in the extreme northwestern part of this area it is difficult to obtain any water, because Cretaceous limestones and shales are so near the surface.
Ensign deep-water area--The only area in Finney and Gray counties in which the depth to water level is greater than 150 feet is in east-central Gray County in the general vicinity of Ensign (pl. 2). Measured depths to water level in this area range from 150 to 207 feet. Most of the wells obtain water from undifferentiated Pleistocene beds or from the Ogallala formation. One well (500), however, obtains water from sandstone in the Dakota formation. This well is reported to be 420 feet deep and the measured depth to water level in 1940 was 207 feet.
Pawnee area--The ground-water conditions in the Pawnee River drainage basin in the "panhandle" of Finney County differ from those found elsewhere in Finney and Gray counties. Most of this area is underlain by limestone or shale of Cretaceous age. Where unconsolidated silt, sand, and gravel does occur, it is relatively thin and generally will not supply water to wells. A few wells obtain water from the Fort Hays limestone member of the Niobrara formation (wells 2, 7, and 11) and a few obtain water from the Blue Hill shale member of the Carlile shale (wells 1, 8, 46, and 57). Water is not found everywhere in limestone and shale (p. 49), and the wells that obtain water from such rocks have very small yields. For a complete description of these wells the reader is referred to the table of well records at the end of this report. Most of the wells in the Pawnee area are shallow and obtain water from alluvium in the valleys. The depth to water level in the valleys of Pawnee River and its tributaries generally is less than 25 feet. Springs occur in some of the valleys that have been cut below the water table (p. 87). The springs issue from limestone (Fort Hays limestone member of the Niobrara) or Pleistocene gravels where they are in contact with Cretaceous rocks.
Fluctuations in Water Level
General considerations--The water table in any area does not remain in a stationary position, but fluctuates up and down much like the water in a surface reservoir. If the inflow to the underground reservoir exceeds the draft, the water table will rise; conversely, if the draft exceeds the inflow the water table will decline. Thus, the rate and magnitude of fluctuation of the water table depend upon the net rate at which the underground reservoir is replenished or depleted.
The principal factors controlling the rise of the water table in Finney and Gray counties are the amount of precipitation that passes through the soil and descends to the water table, the amount of water added to the ground-water reservoir by seepage from Arkansas River and other streams, and the amount of water entering the counties beneath the surface from areas to the west. The principal factors controlling the decline of the water table are the amount of water pumped from wells, the amount of water lost through transpiration and evaporation where the water table is shallow, the discharge of water through springs, the amount of water discharged by effluent seepage into Arkansas River, and the amount of water leaving the county beneath the surface toward the east. The factors causing the water table to rise are discussed in detail in the chapter on ground-water recharge and the factors causing the water table to decline are discussed in the chapter on ground-water discharge.
In the fall of 1939, 27 wells in Finney County and 26 wells in Gray County were selected as observation wells, and monthly measurements of water level in them were begun in order to obtain information concerning the fluctuations in storage of the underground reservoir. Since July, 1936, the Division of Water Resources of the Kansas State Board of Agriculture has maintained an automatic water-stage recorder on well 269 in Finney County. Complete water-level records of this well have been made available by Mr. G.S. Knapp, Chief Engineer. The descriptions and the 1939 water-level measurements for all observation wells are given in the 1939 annual water-level report of the Federal Geological Survey (Meinzer and Wenzel, 1940, pp. 138-145, 158-161) and subsequent water-level measurements have been published in ensuing water-level reports (Meinzer and Wenzel, 1942, pp. 45-52, 61-66; 1943, pp. 60-65, 71-75). Tables 5 and 6 correlate the observation-well numbers used in this report with those given in Water-Supply Papers 886, 908, and 938. The location and description of each well appears in the table of well records at the end of this report.
Table 5--Numbers of Finney County observation wells used in this report and corresponding numbers given in Water-Supply Papers 886, 908, 938.
Table 6--Numbers of Gray County observation wells used in this report and corresponding numbers given in Water-Supply Papers 886, 908, 938.
Kansas Geological Survey, Finney and Gray County Geohydrology|
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Web version April 2002. Original publication date Dec. 1944.