The ground-water system in southeastern Laramie County, Wyoming

Abstract

Increased development of irrigation wells in southeastern Laramie County, Wyo., has caused concern about the quantity of water available. Ground water from approximately 230 large-capacity wells is used to irrigate most of the 18,165 acres under irrigation. The purpose of this study is to provide more knowledge about the character of the aquifers, quantity of water in storage, rate of withdrawal, and the effect of withdrawals on streamflow. The area studied consists of about 400 square miles in southeastern Laramie County in the extreme southeast corner of Wyoming. The White River Formation of Oligocene age and alluvium of Quaternary age are the principal aquifers. The White River Formation is made up primarily of clay, silt, and fine sand. Secondary permeability in the White River Formation accounts for it being an important aquifer. The alluvium, which Includes terrace and flood-plain deposits, consists of sand and gravel that contain some lenses of silt and clay. Existence of secondary permeability in the White River Formation has been accepted for some time although the nature of the secondary permeability has been disputed. Examination of downhole conditions with a television camera during this study revealed openings in the formation that appeared to be similar to tubes or caverns. The openings were of various sizes and shapes but only a few appeared to be associated with fracturing. Solution activity in the formation probably is an important factor in the development of secondary permeability. The study area was divided into the Pine Bluffs-Egbert area and the Carpenter area. Ground-water movement in the Pine Bluffs-Egbert area is generally eastward into Nebraska; in the Carpenter area, movement is generally southward into Colorado. Pumpage from large-capacity wells in the Pine Bluffs-Egbert area was estimated to be about 21,790 acre-feet in 1971. Water levels exhibited a declining trend annually in some areas during the period of record. Data indicate that pumpage in the Pine Bluffs-Egbert area probably is the cause of decreased base flow in Lodgepole Creek since approximately 1961. Increased pumpage, above that in 1971, will result in further reduction of discharge of Lodgepole Creek into Nebraska. In the Carpenter area, it was estimated that the terrace deposit contained about 1 million acre-feet of saturated sediments in March 1971. The amount of ground water in storage in the White River Formation in this area is unknown. Pumpage from large-capacity wells in the Carpenter area was estimated to be about 7,090 acre-feet in 1971. Recharge to the area was not estimated but there was no indication of a net decline in water levels between September 1970 and September 1971.