Studies of geology and hydrology in the Basin and Range Province, Southwestern United States, for isolation of high-level radioactive waste: Characterization of the Sonoran region, Arizona

Abstract

The Sonoran region, southwestern Arizona, of the Basin and Range province, is south and east of the Colorado River. The structural basins typically are filled with 200 to 1,500 meters of clastic material with thick evaporite sections present locally. Basins of the region generally trend north-northwest. Relief between the valleys and adjacent mountains generally is 300 to 1,300 meters. Surface drainage is to the Colorado River, major tributaries being the Bill Williams and Gila Rivers. A few basins have interior or poorly integrated drainage. Bedrock is exposed in about one-fourth of the region. Bedrock consists of Precambrian metamorphic and plutonic rocks, Paleozoic sedimentary rocks, Mesozoic volcanic and sedimentary rocks, and middle to late Tertiary volcanic and plutonic rocks. In addition to igneous tectonism, the rocks have been repeatedly deformed and moved along slight and steep-angle faults. Present topography was largely shaped by Tertiary basin-and-range faulting, erosion, and deposition. 1 Potential host media for isolation of high-level radioactive waste in the Sonoran region, Arizona, include intrusive rocks, tuffaceous rocks, basaltic rocks, laharic breccias, and salt. Basin-fill deposits, and possibly other rock types, have potential as host media in the unsaturated zone. Quaternary tectonism in the region is indicated in the northwest and southern part of the region by relatively substantial strain release, epicenters of few earthquakes greater than magnitude 4 (Richter scale, surface wave), and vertical crustal movement. Quaternary volcanic rocks may be present in the late Cenozoic volcanic field in the southeastern part of the region. The Sonoran region is arid with annual precipitation less than 200 millimeters throughout most of the region and annual potential evaporation greater than 2.5 meters. Recharge of ground water occurs principally in areas of higher altitude where bedrock is permeable and by infiltration of mountain runoff in the basin areas. Discharge of ground water occurs by seepage to gaining streams, evapotranspiration, springs, and by withdrawal from wells. In most of the region, the concentration of dissolved solids in ground water is less than 500 milligrams per liter. Ground water near the Gila River and the Colorado River contains 1,000 milligrams per liter or more dissolved solids. The ground water is mostly of the calcium mangesium or sodium bicarbonate type. Sulfate and chloride type waters occur in and near large discharge areas and in some playas. Mineral development in the 19th century was confined to precious-metal lodes, base-metal vein and replacement ores, and placer deposits. More recent development has focused on large scale open-pit copper mines, which produce substantial quantities of precious metals as byproducts.