Water Quality in the Las Vegas Valley Area and Carson and Truckee River Basins, Nevada and California, 1992-96

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SUMMARY OF MAJOR ISSUES AND FINDINGS

This report summarizes the major findings of the National Water-Quality Assessment (NAWQA) Program for the Las Vegas Valley area and the Carson and Truckee River Basins in Nevada and California. These areas are the Nevada Basin and Range (NVBR) NAWQA Study Unit. The statements on major findings below are expanded in the section on Major Issues and Findings of this report. Comparisons of data for this Study Unit with data from all 20 NAWQA Study Unit investigations that began in 1991 are presented in the section on Water-Quality Conditions in a National Context. Although this report is part of a national program, it also is intended as a resource for anyone interested in water-quality conditions in the NVBR Study Unit.

Geology and Climate Have Caused High Concentrations of Dissolved Solids, Arsenic, Uranium, and Radon in Some Areas

• Median concentrations of dissolved solids (see Glossary for definitions of selected terms) for the Carson River in Carson Desert and in shallow ground-water samples from Las Vegas Valley and Carson Desert exceeded the U.S. Environmental Protection Agency (USEPA) secondary maximum contaminant level (SMCL) for drinking water. Dissolved-solids concentrations are increased by evapotranspiration in these areas.
• Median concentrations of arsenic and uranium in ground-water samples from Carson Desert exceeded the USEPA maximum contaminant level (MCL) and the proposed MCL, respectively, for drinking water. Arsenic and uranium are dissolved from coatings on sediment grains.
• Median concentrations of radon in ground-water samples from all parts of the Study Unit approached or exceeded the former proposed MCL (currently under review). Radon is a decay product of uranium-238, which is found in granitic rock.

Urban Activities Have Been Primary Sources of Nutrients, Synthetic Organic Compounds, and Trace Elements

• The annual total-nitrogen load of Las Vegas Wash downstream from discharge of treated sewage increased from 750 tons in 1974 to 2,400 tons in 1988 because of a rapidly increasing population in the Las Vegas urban area. Most of the load was ammonia.
• The median ammonia concentration in NAWQA samples from Las Vegas Wash was nearly 300 times the NAWQA national median and all samples exceeded the USEPA aquatic-life criterion for un-ionized ammonia. Subsequent samples collected by other agencies indicate that the criterion probably is no longer being exceeded because of improved tertiary treatment of sewage.
• Nine percent of NAWQA samples from the Truckee River downstream from discharge of tertiary-treated sewage from the Reno-Sparks urban area exceeded the aquatic-life criterion for un-ionized ammonia but nitrate concentrations were low, compared to the national NAWQA median, because of tertiary treatment of sewage effluent.
• The median nitrate concentration in NAWQA samples from shallow monitoring wells (not used for water supplies) in the Las Vegas urban area was more than twice the national NAWQA median; 12 percent equaled or exceeded the MCL. The MCL was not exceeded for any samples collected from water-supply wells.

• Twenty-four pesticides were detected in NAWQA samples from Las Vegas Wash downstream from the Las Vegas urban area. The most commonly detected pesticides were simazine, prometon, diuron, DCPA, diazinon, and malathion. USEPA aquatic-life criteria for the insecticides diazinon and malathion were exceeded in 47 and 25 percent of the samples, respectively.
• Ten pesticides were detected at low levels in NAWQA samples from the Truckee River downstream from the Reno-Sparks urban area during May-August 1994, but only one was detected upstream. No available MCLs or aquatic-life criteria were exceeded.
• At least one pesticide was detected at a low level in 28 percent of the shallow monitoring wells sampled by NAWQA in the Las Vegas urban area and in 68 percent of those in the Reno-Sparks urban area. Two or more pesticides were detected in 39 percent of the samples from the Reno-Sparks area. No available MCLs were exceeded.
• At least one pesticide was detected at a low level in 14 percent of the water-supply wells sampled by NAWQA in the Las Vegas urban area and in 44 percent of those in the Reno-Sparks urban area. Two or more pesticides were detected in 22 percent of the samples from the Reno-Sparks area. No available MCLs were exceeded.

• Low levels of trihalomethanes (THMs) and of methyl tert-butyl ether (MTBE), a gasoline oxygenate, were detected in NAWQA samples collected from Las Vegas Wash downstream from the Las Vegas urban area. No available MCLs were exceeded.
• More than 50 percent of the shallow monitoring wells sampled by NAWQA had detections of VOCs; chloroform, a THM that is a product of chlorination, was detected in 68 percent of the samples from the Las Vegas urban area and in 21 percent of the samples from the Reno-Sparks urban area. VOC concentrations exceeded MCLs for tetrachloroethylene (three samples) and trichloroethylene (one sample) and the drinking water advisory for MTBE (two samples).
• VOCs were detected in 45 percent of the water-supply wells sampled by NAWQA in the Las Vegas and Reno-Sparks urban areas; chloroform was the most common. Combined concentrations of THMs did not exceed the MCL. Tetrachloroethylene exceeded the MCL in one water-supply well sample in the Las Vegas urban area.

• An urban trace-element index computed from concentrations of cadmium, chromium, copper, lead, and zinc indicated that these elements were enriched at NAWQA sites downstream from the Las Vegas and Reno-Sparks urban areas.
• Organochlorine compounds were not detected in 11 of 13 NAWQA sites sampled in the Study Unit.
• Semivolatile organic compounds (PAHs, phenols, and phthalate esters) were commonly detected at NAWQA sites downstream from the Las Vegas and Reno-Sparks urban areas. The concentrations exceeded the national NAWQA 75th percentile.
• A study by the NAWQA Program in cooperation with the National Park Service and U.S. Fish and Wildlife Service found evidence of disruption of carp endocrine systems in Las Vegas Wash and Las Vegas Bay of Lake Mead. Partly in response to this study, the Nevada Division of Environmental Protection instituted the Lake Mead Water Quality Forum.

Agricultural Activities Have Contributed Dissolved Solids, Trace Elements, Nutrients, and Pesticides

• Aquatic-life criteria for dissolved solids, arsenic, boron, and molybdenum in Carson Desert wetlands were commonly exceeded. These constituents are leached from desert soils by irrigation in Carson Desert.
• Concentrations of orthophosphate in shallow ground water beneath agricultural areas were greater than those beneath urban areas but concentrations of nitrate were about the same for samples collected in both areas during October 1969-April 1990.

• Pesticides were detected in 17 of 19 NAWQA samples collected from irrigation drains in the Carson Desert agricultural area in 1995. Atrazine, simazine, and prometon were most commonly detected.
• Pesticides were detected in 27 percent of shallow monitoring wells sampled in the Carson Valley and Carson Desert agricultural areas. Atrazine and simazine were the most commonly detected pesticides.

Urban and Agricultural Activities Have Affected Water Resources Differently

• Ammonia concentrations in NAWQA samples were higher in streams draining urban areas than in streams draining agricultural areas. Orthophosphate concentrations were higher in shallow ground-water and surface-water samples from agricultural areas than from urban areas.
• The most commonly detected pesticides in Las Vegas Wash downstream from the Las Vegas urban area were simazine, prometon, diuron, DCPA, diazinon, and malathion. Atrazine, prometon, and simazine were the most commonly detected pesticides in samples from shallow monitoring wells in the Las Vegas and Reno-Sparks urban areas. Atrazine, simazine, and prometon were the most commonly detected pesticides in samples collected from irrigation drains in Carson Desert. Atrazine and simazine were the most commonly detected pesticides in samples from shallow monitoring wells in the Carson Valley and Carson Desert agricultural areas.
• Concentrations of cadmium, chromium, copper, lead, and zinc generally were enriched at NAWQA sites downstream from the Las Vegas and Reno-Sparks urban areas; whereas, arsenic, boron, and molybdenum were leached from desert soils by irrigation in Carson Desert, and commonly exceeded aquatic-life criteria.
• NAWQA samples from Las Vegas Wash had detections of VOCs (THMs and MTBE). More than 50 percent of the shallow monitoring wells and 45 percent of the water-supply wells sampled in the Las Vegas and Reno-Sparks urban areas had detections of VOCs; chloroform was the most common. SVOCs (PAHs, phenols, and phthalate esters) in bottom sediment exceeded the national NAWQA 75th percentile downstream from the Las Vegas and Reno-Sparks urban areas.

Historical Mining Activities Have Released Trace Elements to the Carson River

• Aluminum, arsenic, chromium, copper, lead, mercury, and nickel concentrations were enriched in bottom sediment of the East Fork Carson River downstream from the confluence of Bryant Creek, compared to those at an upstream background site. The source of these trace elements probably is acidic drainage and eroded mine tailings from Leviathan Mine and other mines.
• Silver and mercury concentrations in Carson River bottom sediment downstream from the Comstock Lode mining area ranged from nearly 3 to 25 times higher than those at an upstream background site. Historical ore processing during the Comstock mining boom (1860-80) is the principal source of enriched silver and mercury in Carson River bottom sediment.

U.S. Geological Survey Circular 1170

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