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Geohydrology of Grant and Stanton Counties

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Summary and Conclusions

Thick deposits of Pleistocene and Pliocene age underlie most of Grant and Stanton counties. These deposits are the principal aquifers that furnish water for irrigation in the area. Where these deposits do not exist or are not water-bearing, sandstone aquifers may supply water for irrigation.

Previously, geologists recognized that deposits of Pleistocene age were present in the area, but the meager data available made Pleistocene and Pliocene differentiation impractical. The detailed data in this report, including the information on fossils, have made possible the delineation of Pleistocene from Pliocene throughout most of the area.

Sample, electrical, and radioactivity logs from deep water-well tests and from oil and gas tests, have permitted an approximation of the thickness and areal extent of the Cretaceous and Triassic sandstone aquifers.

In 1959 there were about 135,000 acre-feet of ground water withdrawn for irrigation purposes in Grant County and about 113,000 acre-feet withdrawn in Stanton County. These withdrawal rates probably will vary considerably in accordance with the annual pumping rate for each well, which in turn depends upon the annual precipitation.

The coefficients of transmissibility obtained from the aquifer tests can be applied throughout the area, provided that the geology of the aquifers is known. The coefficients of storage obtained from the aquifer tests were too low, and this is believed to reflect the short period of time that the material at the water table had to drain during the aquifer tests. An areal drawdown coefficient of 32 percent was obtained by comparing the total amount pumped between 1939-42 and 1960 and the average water-level decline. This coefficient was later corrected for recharge from precipitation and for possible over-reporting of pumpage, which reduced the coefficient to 0.20.

Water levels in some areas declined as much as 42 feet between 1939-42 and 1960 and as much as 70 feet between 1939-42 and 1963. Most of this decline probably occurred after 1955. The weighted-average water level declined 8 feet and 18 feet, respectively, during the above time intervals. Water levels in small localized areas will fluctuate considerably from pumping seasons to off seasons. In general, water levels will decline over the area as shown by Figures 8 and 9, depending upon the rate of withdrawal.

The analysis of the water-table contour map indicated that about 58 mgd of ground water was flowing into the area from the west and about 86 mgd was flowing out of the area eastward. About 13 mgd seemed to be recharge from precipitation. The remaining increase of 15 mgd was from adjacent areas to the north and south.

Because the hydraulic gradients at the 3,240 and 2,830 water-table contours have not changed appreciably between 1939-42 and 1960 and because water levels in these areas have declined only slightly, the inflow and outflow of the area is assumed to be approximately the same in 1960 as in 1939-42; however, some change in the outflow was noted in January 1963. Therefore, it can be assumed that the amount of ground water pumped within the area during the last 20 years is predominantly from the storage within the area. This is further indicated by the decline of the weighted-average water level.

The recharge from precipitation was computed to be 0.013 mgd per square mile or 0.3 inches per year. This is about 2 percent of the annual precipitation or less than 10 percent of the present reported annual pumping rate.

In Grant and Stanton counties there is about 55 million acre-feet of ground water in storage. About 39 million acre-feet is in the unconsolidated aquifers, and 16 million acre-feet is in the consolidated aquifers. However, probably not more than half of this water can be economically recovered. The time that any well or well field would last should be considered on an individual basis.

The authors were unable to determine the geologic source of the water from various wells by the chemical constituents of the water. However, the chemical-quality program was useful in determining the suitability of water for irrigation.

Phreatophytes grow along the Cimarron River, North Fork Cimarron River, and Lakin Draw. These plants are difficult to control and use much water that might possibly be used more beneficially.

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Kansas Geological Survey, Geology
Placed on web July 24, 2007; originally published December 1964.
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