by Edwin D. Gutentag and Lloyd E. Stullken
Prepared by the Kansas Geological Survey and the U.S. Geological Survey
Originally published in 1976 as Kansas Geological Survey Irrigation Series 1. This is, in general, the original text as published. The information has not been updated. The publication is also available as an Acrobat PDF file (plates scanned separately).
Lane and Scott Counties comprise an area of 1,444 square miles (3,740 km2). Much of the area is underlain by saturated sand and gravel of Pliocene age (Ogallala Formation) and Pleistocene age (undifferentiated deposits) that form a major unconsolidated aquifer. The main body of the aquifer comprises about 34 percent of Lane County and about 81 percent of Scott County. A chalk aquifer, consisting of fractures and solution openings in the Niobrara Chalk of Late Cretaceous age, is being developed for irrigation in southeastern Scott County. A sandstone aquifer, consisting of Upper Jurassic and Lower Cretaceous rocks that underlie both counties, is a potential source of ground water.
Yields of wells in the unconsolidated aquifer range from 100 to 1,700 gallons per minute (6.3 to 110 L/s). Yields from 500 to 1,000 gallons per minute (32 to 63 L/s) are available from wells that tap fractures and solution openings in the chalk aquifer. The sandstone aquifer is tightly cemented in many places and wells in it may not yield sufficient water for irrigation.
The unconsolidated aquifer constitutes a groundwater reservoir in which inflow, outflow, and change in storage occur. These components of the groundwater flow system comprise many individual parameters that are difficult to separate. Estimates based on data for Scott County indicate that inflow to the ground-water reservoir was about 23,000 acre-feet (28 hm3) in 1971 and 33,000 acre-feet (41 hm3) in 1972. Outflow from the reservoir was about 124,000 acre-feet (153 hm3) in 1971 and 80,000 acre-feet (99 hm3) in 1972. The change in ground-water storage in Scott County, therefore, amounted to 101,000 acre-feet (125 hm3) in 1971 and 47,000 acre-feet (58 hm3) in 1972. Differences between the two years are attributed to pumpage in a dry growing season in 1971 and a wet growing season in 1972. Data are not available to permit similar estimates for Lane County.
In seven aquifer tests made for this study, the transmissivity of the unconsolidated aquifer ranged from 2,960 to 9,900 feet squared per day (275 to 920 m2/ day); the storage coefficient ranged from 0.001 to 0.25. Transmissivity values determined from aquifer and specific-capacity tests and estimated from logs of test holes were used in preparation of a map showing the areal distribution of transmissivity. The unconsolidated aquifer has a specific yield generally between 0.14 and 0.18. In the buried south-trending bedrock trough in central Scott County, where the water occurs under confined or semiconfined conditions, the specific yield probably ranges from 0.01 to 0.15. In Lane and Scott Counties, between 4 and 6 million acre-feet (4,900 and 7,400 hm3) of water is estimated to have been in storage in January 1973. Assuming that only 70 percent of the total volume of water is recoverable by wells, between 2.8 and 4.3 million acre-feet (3,500 and 5,200 hm3) of water is available for pumping.
Subsurface movement of water is predominantly in an easterly direction under a hydraulic gradient of about 10 feet per mile (1.9 m/km). The gradient is relatively flat in the vicinity of the buried bedrock trough in central Scott County, which probably reflects the higher transmissivity of the material that fills the trough.
Results of chemical analyses of water samples from typical wells in the three aquifers indicate that the concentration of dissolved fluoride and dissolved nitrate locally approaches or exceeds the limits recommended by the Kansas Department of Health and Environment for drinking water. Water in the unconsolidated and chalk aquifers generally is a calcium bicarbonate type and is suitable for most domestic, stock, and irrigation uses except where fluoride or nitrate concentrations are high. Water from the sandstone aquifer is a sodium bicarbonate type and is unsuitable for many uses.
Irrigation has been practiced in Scott County since about 1650 when the Taos Indians diverted water from Ladder Creek to irrigate crops. Pumping of ground water for irrigation began in 1888. By 1922, about 5,000 acres (2,020 hm2) were irrigated in Scott County by ground water, but irrigated acreage then declined to a low of 1,020 acres (410 hm2) in 1932. In 1945, a total of 129 wells supplied 18,400 acre-feet (23 hm3) of water to irrigate 21,000 acres (8,500 hm2), Irrigation development in Lane County in 1949 was less than 500 acres (200 hm2) irrigated by 3 irrigation wells. By 1972, about 100,000 acres (40,470 hm2) were irrigated by ground water in Scott County and about 20,000 acres (8,090 hm2) in Lane County.
As of January 1973, there were 164 large-capacity wells (more than 100 gpm) in Lane County and 717 large-capacity wells in Scott County. Analysis of the power used to pump water for irrigation indicates a total pumpage of 180,000 acre-feet (220 hm3) in 1971 and 120,000 acre-feet (150 hm3) in 1972. The greater amount pumped in 1971 and the lesser amount pumped in 1972 are attributed to below-normal and above-normal precipitation during the April-to-September growing season in respective years. The 1971 (dry year) gas-energy input for irrigation pumpage was 0.12 percent of the total natural gas produced in southwestern Kansas, which is equivalent to the production of about 6 typical gas wells. In 1972 (wet year), the gas-energy input for irrigation pumpage was 0.08 percent of the total natural gas production in southwestern Kansas, or the production of about 4.5 typical gas wells.
Water-level declines in the unconsolidated aquifer for the period from 1940-48 to 1973 ranged from less than 10 feet (3 m) to about 50 feet (15 m). Part of the decline in the south-trending trough represents a decline in artesian head, and part represents dewatering of the aquifer. Annual water-level measurements indicate that declines in 46 observation wells in Scott County averaged 1.15 feet per year (0.35 m/yr) during 1966-74. Assuming that the specific yield is about 15 percent for materials under unconfined conditions in most of the county and about 1 percent for materials under semiconfined and confined conditions in the central part, the annual reduction of ground-water storage in Scott County is estimated to be about 57,000 acre-feet (70 hm3) for the period 1966-74. This value is in agreement with the figures of 101,000 acre-feet (125 hm3) in 1971 to 47,000 acre-feet (58 hm3) in 1972 determined from the ground-water inventory of Scott County.
Depletion of ground water in storage in the unconsolidated aquifer is indicated by a 10- to 60-percent reduction in saturated thickness since 1940-48. In the area of the south-trending trough, the saturated thickness has been reduced about 11 percent.
The potential yield of the unconsolidated aquifer at a specific site can be estimated if the saturated thickness and effective thickness (water-yielding part of the saturated thickness) values are known. Potential yield to wells in the main body of the unconsolidated aquifer in the study area ranges from about 250 to 2,000 gallons per minute (16 to 126 L/s).
The upsurge in prices for grain in 1973 from longterm averages probably will result in an increase in irrigation in Lane and Scott Counties. Irrigation is an insurance against drought. The installation of new wells and increased ground-water withdrawals will accentuate the rate of depletion and the problem of mutual interference among wells. Because the average annual pumpage exceeds the average annual inflow to the ground-water reservoir, the mining of water in storage will continue to lower water levels and reduce well yields.
The newly formed Ground-Water Management District will have the responsibility to efficiently manage the ground-water reservoir. Some of the alternatives that should be considered to conserve the ground-water supply are: re-use of water from tailwater recovery pits, improving irrigation efficiency, most efficient cropping for the least amount of water applied, limiting further ground-water development in areas where saturated deposits are thin and where significant water-level declines have occurred, regulating well spacing to minimize the effects of mutual well interference, and weather modification and importation of water to supplement ground-water supplies.
Kansas Geological Survey, Geohydrology
Placed on web June 24, 2013; originally published 1976.
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