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The U.S. Geological Survey National Water-Quality Assessment Program performed a regional study in the Southwestern United States (Southwest) to describe the status and trends of dissolved solids in basin-fill aquifers and streams and to determine the natural and human factors that affect dissolved solids. Basin-fill aquifers, which include the Rio Grande aquifer system, Basin and Range basin-fill aquifers, and California Coastal Basin aquifers, are the most extensively used ground-water supplies in the Southwest. Rivers, such as the Colorado, the Rio Grande, and their tributaries, are also important water supplies, as are several smaller river systems that drain internally within the Southwest, or drain externally to the Pacific Ocean in southern California. The study included four components that characterize (1) the spatial distribution of dissolved-solids concentrations in basin-fill aquifers, and dissolved-solids concentrations, loads, and yields in streams; (2) natural and human factors that affect dissolved-solids concentrations; (3) major sources and areas of accumulation of dissolved solids; and (4) trends in dissolved-solids concentrations over time in basin-fill aquifers and streams, and the relation of trends to natural or human factors.
Dissolved-solids concentrations of ground water in the basin-fill aquifers of the Southwest ranged from less than 500 milligrams per liter near basin margins where ground water is recharged from nearby mountains to more than 10,000 milligrams per liter in topographically low areas of some basins or in areas adjacent to specific streams or rivers in the Basin and Range and Rio Grande aquifer systems. The area of the basin-fill aquifer systems with dissolved-solids concentrations less than or equal to 500 milligrams per liter was about 57 percent for the Rio Grande aquifer system, 63 percent for the Basin and Range basin-fill aquifers, and 44 percent for the California Coastal Basin aquifers. At least 70 percent of the area of these three basin-fill aquifer systems had dissolved-solids concentrations less than or equal to 1,000 milligrams per liter.
Dissolved solids in streams were described on the basis of median daily concentration, median annual load, and median annual yield data for 420 surface-water-quality monitoring sites. The time period with dissolved-solids data for individual sites varied but was at least 10 or more years between 1974 and 2003. Median daily dissolved-solids concentrations vary substantially among the sites in the Southwest, ranging between 22 and 13,800 milligrams per liter, and also vary between different sites on the same stream. Median daily concentrations generally increased in a downstream direction for sites on the Rio Grande, Colorado River, Yampa River, White River, Green River, San Juan River, Gila River, Bear River, and Sevier River. Median annual dissolved-solids loads ranged from 60 tons per year for a site on Elk Creek, a headwater tributary to the Colorado River, to 7.86 million tons per year at Colorado River below Hoover Dam, Arizona-Nevada. Typically, streams with the highest flows have the highest dissolved-solids loads. Median annual loads for sites on these rivers generally increased in the downstream direction, except where streamflow decreased substantially due to diversions and (or) streambed infiltration, typically in the downstream part of the river system. Median annual yields ranged from 0.69 to 7,510 tons per year per square mile, and the mean for all 420 sites was 125 tons per year per square mile. Most (104 of 112) sites with median annual yields greater than 100 tons per year per square mile were in the Colorado River basin upstream from Lees Ferry and in the Bear and Great Salt Lake hydrologic subregions.
A conceptual model was developed for the effects of natural and human factors on dissolved-solids concentrations in basin-fill aquifers and streams. Factors affecting concentrations in streamflow of upland mountain
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USGS Numbered Series
Dissolved Solids in Basin-Fill Aquifers and Streams in the Southwestern United States