National Water-Quality Assessment Program (NAWQA) -- Nevada Basin and Range Study Unit

The Nevada Basin and Range (NVBR) study unit (Fig. 1A-D) includes the Truckee River (Figure 1B) and Carson Basins (Figure 1C) in northwestern Nevada and northeastern California, and the Las Vegas Valley area in southeastern Nevada (Figure 1D). These two areas represent many of the diverse environments found in the Basin and Range physiographic province, which is characterized by high mountains surrounding valleys underlain by thick, unconsolidated deposits (Covay and others, 1996). The north and south parts of NVBR belong to the Basin and Range alluvial and carbonate-rock aquifers, which is ranked third out of the 71 principal aquifers/aquifer systems in the U.S. based on 1990 use for drinking-water supply.

In the south, mountains are primarily carbonate rocks. Snowmelt and storm runoff rarely flows to the valleys, which receive less than 4 inches of annual precipitation. Las Vegas Wash, the only perennial stream in the southern part of the study unit, is sustained by urban drainage and treated sewage effluent and flows into Lake Mead. Las Vegas is in hydrologic landscape (HL) 14, which encompasses much of the desert southwest. HL 14 is characterized by low total flatland, medium relief, medium aquifer and soil permeability, and low precipitation minus potential evaporation (PMPE). Other metropolitan statistical areas (MSAs) overlying the Basin and Range principal aquifer and in HL 14 are Salt Lake City, Utah, and Tucson and Yuma, Arizona.

In the north, mountains are primarily granitic and volcanic rocks that exceed 10,000 feet and receive more than 20 inches of annual precipitation, mostly as snow during winter. Snowmelt runoff recharges aquifers and sustains streamflow in the Truckee and Carson Rivers, which drain to hydrologically closed lakes and wetlands. The northern part of NVBR is a mix of several HLs that occur throughout the Basin and Range Province. Reno, the largest MSA in northern NVBR, is in HL 15. Other MSAs overlying the Basin and Range principal aquifer and in HL 15 are Salt Lake City and Provo/Orem, Utah. In general, this region is colder, PMPE is higher, and bedrock permeability is lower than HL 14.

The Basin and Range is one of the most desolate areas of the U.S., so it may seem surprising that urbanization is the primary water-quality issue for NVBR. More than 90 percent of Nevada’s population lives in either the Las Vegas (1.4 million) or Reno/Sparks (320,000) metropolitan areas. Las Vegas has been the fastest growing metropolitan area in the U.S. during the past decade. Between 1990 and 1999, Clark County, dominated by Las Vegas, grew by 64 percent and Washoe County, dominated by Reno/Sparks, grew by 26 percent. Similar growth rates are occurring in towns surrounding these metropolitan areas. For example, Pahrump, west of Las Vegas, grew by 67 percent and Fernley/Dayton, east of Reno and Carson City, grew by 57 percent. In comparison, the average growth rate of most western metropolitan areas was 10.6 percent in central cities and 16.2 percent in areas surrounding city centers.

In the Reno and Carson City areas, septic tanks have contaminated domestic wells using relatively shallow ground water (Seiler and others, 1999), and cities that have sewage treatment plants are increasingly using the effluent to irrigate parks and golf courses. Septic systems are commonly used in the rapidly growing communities surrounding the central metropolitan area and in towns throughout Nevada and Utah (Geoff Freethey, Utah District, oral communication). Homes are encroaching on the foothills because valleys are mostly developed and homeowners want hilltop views. Principal aquifers used by municipalities could eventually be degraded because the foothills are recharge areas for deep aquifers. In addition to concerns for ground-water quality, urbanization affects the water quality and ecosystems of Lake Mead (Bevans and others, 1996) and the Truckee and Carson Rivers. Altered habitats, eutrophication, elevated temperatures, and toxic compounds from the Reno metro area could be significant barriers to USFWS efforts to restore the endangered Lahontan Cutthroat Trout in the Truckee River Basin, a major new effort for the U.S. Fish and Wildlife Service. Urbanization will be the primary theme that NVBR addresses during Cycle II.

Naturally occurring trace elements and radionuclides is another important water-quality issue in NVBR. Many of the public water supplies (PWSs) in Nevada exceed the proposed maximum contaminant level (MCL) for arsenic (10 mg/L), and several Cycle I wells exceeded the proposed MCL for uranium (20 mg/L). About 50 percent of wells sampled during Cycle I and municipal wells in the Reno area exceeded the formerly proposed MCL for radon (300 pCi/L). Nationally, NVBR ranked 11 out of the 35 91’ and 94’ SUs for radon (75th percentile concentration 920 pCi/L). Mercury and other trace elements from historic mining in the Carson River Basin is another major water-quality issue.

More than 85 percent of the water use in Las Vegas is for PWS (Table 1). Lake Mead is the source for almost all of the water use and also is a PWS for millions of people in Arizona and southern California. Water from Lake Mead is treated and directly input to the distribution system or stored by artificial recharge through wells in the deep aquifer underlying Las Vegas. Las Vegas obtains about 20 percent of its PWS from ground water; however, some of the pumpage is the artificially recharged water from Lake Mead. In the near future, the Las Vegas metro area will depend more on ground water because current demand consumes almost all of the allocation of Colorado River water.

About 35 percent of the water use in the Truckee and Carson Basins is for PWS (Table 1). Reno obtains most of its PWS from the Truckee River and supplements diversions with ground water. About 85 percent of the water diverted from the Truckee River for municipal use occurs at the Chalk Bluff treatment facility, which started operation in 1994. The remaining 15 percent of the water is diverted at the Glendale treatment facility in Sparks, which was the primary treatment facility prior to 1994. Carson City obtains about 50 percent of its PWS from ground water, 20 percent from induction wells under the Carson River, and 30 percent from surface water diversions.

Ground water is the primary source of public and domestic water supply in surrounding towns. In recent years, domestic use of ground water has increased as undeveloped and irrigated lands are converted to residential developments.

Table 1. Water use in the NVBR in the year 2000.

Las Vegas Basin
Carson/Truckee Basins
Category of use
Surface Water
Ground Water
Surface Water
Ground Water
Public Supply

Bevans, H.E., Goodbred, S.L., Miesner, J.F., Watkins, S.A., Gross, T.S., Denslow, N.D., and Schoeb, Trenton, 1996. Synthetic organic compounds and carp endocrinology and histology in Las Vegas Wash and Las Vegas and Callville Bays of Lake Mead, Nevada, 1992 and 1995. U.S. Geological Survey Water-Resources Investigation Report 96-4266, 12 p.

Covay, K.J., Banks, J.M., Bevans, H.E., Watkins, S.A., 1996, Environmental and hydrologic settings of the Las Vegas Valley area and the Carson and Truckee River Basins, Nevada and California: U.S. Geological Survey Water-Resources Investigation Report 96-4087, 72 p.

Seiler, RL, Zaugg, D, Thomas, JM, Z, Howcroft, DL, 1999, Caffeine and pharmaceuticals as indicators of waste water contamination in wells: Ground Water, v. 37, no. 3, p. 405-410.

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