Ground-water geology of the Bruneau–Grand View area, Owyhee County, Idaho

The Bruneau-Grand View area is part of an artesian basin in northern Owyhee County, Idaho. The area described in this report comprises about 600 square miles, largely of undeveloped public domain, much of which is open, or may be opened, for desert-entry filing. Many irrigation-entry applications to the Federal Government are pending, and information about ground-water geology is needed by local citizens and well drillers, by Federal agencies that have custody of the land, and by local and State agencies that administer water rights. The areal geology and ground-water conditions in the Bruneau-Grand View area seemingly typify several basins in southwestern Idaho, and this study is a step toward definition and analysis of regional problems in ground-water geology and the occurrence and availability of ground water for irrigation or other large-scale uses.

Owyhee County is subdivided physiographically into a plateau area, the Owyhee uplift, and the Snake River valley. The Bruneau-Grand View area is largely within the Snake River valley. The climate is arid and irrigation is essential for stable agricultural development. Nearly all usable indigenous surface water in the area is appropriated, including freshet flow in the Bruneau River, which is used for power generation at the C. J. Strike Dam. However, with storage facilities additional land could be irrigated, and some land may be irrigated with Snake River water if suitable reclamation projects are constructed.

Sedimentary and igneous rocks exposed in the area range in age from Miocene to Recent. The igneous rocks include silicic and basic intrusive and extrusive bodies, and the sedimentary rocks are compacted stream and lake sediments. The rocks contain economically important artesian aquifers; the principal ones are volcanic rocks in which ground water is imperfectly confined beneath sediments of the Idaho formation, thus forming a leaky artesian system. The altitude of the piezometric surface of the artesian water does not exceed about 2,700 feet above mean sea level. In some areas, where the land surface is below that altitude, the artesian system discharges water through springs and seeps and locally causes waterlogging and development of alkali soil.

In chemical quality much of the water is unsuitable for irrigation and domestic use. The water contains a relatively moderate amount of dissolved solids, but the percent sodium and the concentration of fluoride are excessive for some uses. The quality of the water for irrigation ranges from excellent in the southern part of the artesian system to unsuitable in the northern part. All the artesian ground water that was sampled contained excessive amounts of fluoride.

There is a substantial supply of undeveloped artesian water in the area, but sustained use of the water for irrigation may not be feasible unless provisions can be made for adequate soil drainage and soil amendment, because of the high percentage of sodium in the water. Detailed hydrologic and geologic study of the area should precede development.