The High Plains aquifer, which underlies about 174,000 square miles in parts of eight States, is the principal source of water in one of the Nation's major agricultural areas. About 170,000 wells pump water from the aquifer to irrigate about 13 million acres in the High Plains. In 1978, the U.S. Geological Survey began a regional study of the High Plains aquifer to provide geohydrologic data and computer models of the aquifer needed to evaluate the effects of ground-water development. This report describes the geohydrology of the High Plains aquifer.The High Plains aquifer consists mainly of hydraulically connected geologic units of late Tertiary or Quaternary age. The upper Tertiary rocks include part of the Brule Formation, Arikaree Group, and Ogallala Formation. The Quaternary deposits included in the aquifer consist of alluvial, dune-sand, and valley-fill deposits.The Ogallala Formation, which underlies 134,000 square miles, is the principal geologic unit in the High Plains aquifer. The Ogallala consists of a heterogeneous sequence of clays, silts, sands, and gravels deposited by streams that flowed eastward from the mountains. Within the Ogallala, zones cemented with calcium carbonate are resistant to weathering and form escarpments that typically mark the boundary of the High Plains.The High Plains aquifer is, regionally, a water-table aquifer consisting mostly of near-surface sand and gravel deposits. The maximum saturated thickness of the aquifer is about 1,000 feet and averages 200 feet. Hydraulic conductivity and specific yield of the aquifer depend on sediment types, which vary widely both horizontally and vertically. Consequently, hydraulic conductivity and specific yield also are highly variable. Hydraulic conductivity ranges from less than 25 to 300 feet per day and averages 60 feet per day. Specific yield ranges from less than 10 to 30 percent and averages about 15 percent.Ground-water flow generally is from west to east, at an average rate of about 1 foot per day, and discharges naturally to streams and springs, and directly to the atmosphere by evapotranspiration. Precipitation is the principal source of recharge to the High Plains aquifer. Estimated recharge rates range from 0.024 inch per year in part of Texas to 6 inches per year in south-central Kansas. Typically, recharge estimates are greatest for sandy soils.The High Plains aquifer contains about 3.25 billion acre-feet of drainable water. About 66 percent of the water in storage is in Nebraska and about 12 percent is in Texas. New Mexico, the State with the smallest water resource in the High Plains, has only 1.5 percent of the volume of water in storage.

In addition, valuable information was provided by many other State and local agencies throughout the High Plains. Their contributions are an integral part of this investigation without which this study would not have been possible. The U.S. Geological Survey coordinated its investigation of the High Plains aquifer with a concurrent study by the Economic Development Administration of the Department of Commerce. The Six-State High Plains Ogallala Aquifer Area study conducted by the Economic Development Administration was authorized by Congress in 1976. This study was charged with the responsibility of examining the feasibility of increasing water supplies to insure the economic growth and vitality of the High Plains. Together, these two studies provide a comprehensive evaluation of the High Plains aquifer and the potential impacts of declining groundwater supplies on the region. The Economic Development Administration study has developed and proposed alternative strategies to alleviate or mitigate those impacts and the U.S. Geological Survey has provided hydrologic data and models needed to evaluate the effects of those strategies on the groundwater resource. CONVERSION FACTORS The following report uses inch-pound units as the primary system of measurements and metric units for water-chemistry measurements. The units commonly are abbreviated using the notations shown below in parentheses. Inch-pound units can be converted to metric units by multiplying by the factors given in the following list. Inch-pound unit Multiply by To obtain metric unit inch 2.540 X 101 millimeter foot (ft) 3.048 X 10'1 meter mile (mi) 1.609 kilometer acre 4.047 X 10" 1 hectare square mile (mi2) 2.59 square kilometer acre-foot (acre-ft) 1.233 X 10"3 cubic hectometer foot per day (ft/d) 3.048 X 10"1 meter per day cubic foot per second (ft3/s) 2.832 X 10~2 cubic meter per second

The study was charged with the responsibility of examining the feasibility of increasing water supplies to ensure the economic growth and vitality of the High Plains. Together, these two studies will provide a comprehensive evaluation of the High Plains aquifer and the potential impacts of declining groundwater supplies on the region. The Economic Development Administration study will develop and propose alternative strategies to alleviate or mitigate those impacts and the U.S. Geological Survey has provided hydrologic data and models needed to evaluate the effects of those strategies on the groundwater resource.

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