Geology and ground-water resources of Washington County, Colorado

Washington County, in northeastern Colorado, has an area of 2,520 square miles. The eastern two-thirds of the county, part of the High Plains physiographic section, is relatively flat and has been moderately altered by the deposition of loess and dune sand, and by stream erosion. The western one-third is a part of the South Platte River basin and has been deeply dissected by tributary streams. The soils and climate of the county are generally suited for agriculture, which is the principal industry.

The rocks that crop out in the county influence the availability of ground water. The Pierre Shale, of Late Cretaceous age, underlies the entire area and ranges in thickness from 2,000 to 4,500 feet. This dense shale is a barrier to the downward movement of water and yields little or no water to wells. The Chadron Formation, of Oligocene age, overlies the Pierre Shale in the northern and central parts of the area. The thickness of the formation ranges from a few feet to about 300 feet. Small to moderate quantities of water are available from the scattered sand lenses and from the highly fractured zones of the siltstone. The Ogallala Formation, of Pliocene age, overlies the Chadron Formation and in Washington County forms the High Plains section of the Great Plains province. The thickness of the Ogallala Formation ranges from 0 to about 400 feet, and the yield from wells ranges from a few gallons per hour to about 1,500 gpm. Peorian loess, of Pleistocene age, and dune sand, of Pleistocene to Recent age, mantle a large pan of the county and range in thickness from a few inches to about 120 feet Although the loess and dune sand yield little water to wells, they absorb much of the precipitation and conduct the water to underlying formations. Alluvium, of Pleistocene and Recent age, occupies most of the major stream valleys in thicknesses of a few feet to about 250 feet. The yield of wells tapping the alluvium ranges from a few gallons per minute to about 3,000 gpm, according to the thickness of saturated material.

Development of ground water for irrigation has been generally restricted to the South Platte, Arikaree, and Beaver valleys. There were 134 irrigation wells, 3 industrial wells, and 10 municipal wells in the county in 1959. The annual ground-water pumpage from Washington County is estimated to be 18,000 acre-ft; about 10,000 acre-ft is from the High Plains ground-water province. Although some ground water enters the county as underflow, most of the recharge to ground-water reservoirs is from precipitation on the land surface. Recharge to the Ogallala Formation in the county is assumed to be approximately equal to the natural discharge from the county by underflow because ground-water withdrawals are from storage, and no other significant amount of natural discharge is apparent. Undertow in the Ogallala was calculated to be 83,000 acre-ft per year and the rate of recharge from precipitation to be about 0.95 inch per year. Neither recharge nor discharge was calculated for that part of the county in the South Platte River basin.

All ground water in Washington County has a high proportion of carbonate and is classed as hard to very hard. The sodium-adsorption-ratio for all samples analyzed was below the limit recommended for irrigation water. All the water from the Ogallala Formation and most of the water from the Chadron Formation is suitable for domestic use. Some water from the alluvial deposits overlying the Pierre Shale was exceptionally high in calcium, magnesium, and sodium sulfates.

Ground water has been heavily developed for irrigation in the South Platte valley and in some parts of the Beaver and Arikaree valleys. Some additional areas, however, could be developed in the latter two valleys. Large quantities of ground water in the Ogallala Formation are available for future development. The quantity of water in storage in the High Plains ground-water province in Washington County is about 6.5 million acre-f