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

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MAJOR ISSUES AND FINDINGS

Geology and Climate Have Determined Natural Water Chemistry

Natural factors of geology and climate have affected the chemical composition of both surface and ground water. In the Nevada Basin and Range (NVBR) Study Unit, several chemical constituents were present in water, sometimes at levels that exceeded water-quality standards and criteria, because of geology and climate. Among these constituents were dissolved solids, arsenic, uranium, and radon.

Minerals in Rocks and Sediment Are Sources of Chemical Constituents

Certain rock types contain elements that may be harmful to humans or animals if they are released and concentrated in the water. In the NVBR Study Unit, arsenic, radon, and uranium were detected in some water samples at concentrations that exceeded drinking-water standards. The geology of the NVBR NAWQA Study Unit is quite diverse (fig. 2), including carbonate, metamorphic, sedimentary, granitic, and volcanic rocks, and unconsolidated deposits derived from the rocks [4]. Water in contact with each of these rock types can derive a chemistry specific to that particular rock type. In headwater areas, such as Carson Valley and the Reno-Sparks area, ground and surface water tend to be dilute (less than 1,000 mg/L of dissolved solids) and contain calcium and bicarbonate ions as the major dissolved constituents. In basin areas, such as Carson Desert and Las Vegas Valley, ground water tends to be dilute to briny (more than 35,000 mg/L of dissolved solids) and contain sodium, calcium, bicarbonate, and sulfate as the major dissolved constituents.

Dissolved Solids Are Contributed by Evaporite Minerals

In headwater areas (fig. 3), contact time between the water and rock, or sediment derived from the rock, is generally short, allowing minimal time for reaction with, or dissolution of, the minerals in the rock. In basin settings, Las Vegas Valley and Carson Desert, water tends to have higher concentrations of dissolved solids than in headwater areas owing to the longer time the water has to react with the rock and sediment. Sediments in the basin parts of the NVBR Study Unit also tend to be finer grained than those in headwater areas; thus, they have more surface area upon which the reactions can take place. Evaporite minerals, characteristically quite soluble, are present in parts of Las Vegas Valley and Carson Desert. If these minerals come into contact with water, they may dissolve, adding constituents including sodium, calcium, bicarbonate, and sulfate. In Las Vegas Valley and Carson Desert, the USEPA secondary maximum contaminant level (SMCL) of 500 mg/L for dissolved solids [7] is commonly exceeded.

Arsenic is Derived From Coatings on Sediment Grains

Surface-water samples from Carson Desert, mostly irrigation-return flow, and Steamboat Creek, in the Reno-Sparks area, had high arsenic concentrations. Agricultural drainwater and lake samples from Carson Desert had arsenic concentrations as great as 380 and 1,400 µg/L, respectively [8,9]. Steamboat Creek (fig. 3) had arsenic concentrations ranging from 44 to 120 µg/L during 1994 [10].

Ground water in several general areas of the Study Unit had arsenic concentrations that exceeded drinking-water standards. Arsenic concentrations in Carson Desert ground-water samples commonly were high with about 57 percent of the samples exceeding the USEPA maximum contaminant level (MCL) of 50 µg/L [7,11,12]. Previous reports have explained that the high arsenic concentrations are a result of dissolution of arsenic-bearing iron oxyhydroxide coatings on sediment grains [8,11,13] caused by a rise in the water table associated with large-scale irrigation. The ultimate source of arsenic probably is volcanic rocks and sediment derived from volcanic rocks.

Uranium is Dissolved From Granitic Rocks and Sediment

In surface waters within the NVBR Study Unit, uranium concentrations were high in Las Vegas Valley and Carson Desert. Las Vegas Wash (fig. 2), which carries urban runoff and drainage from the Las Vegas urban area, had a median uranium concentration of 24 µg/L during 1993-95 [10,14]. Shallow ground-water seepage into Las Vegas Wash is the most likely source. The proposed MCL for uranium is 20 µg/L [7]. Agricultural drains in Carson Desert had median uranium concentrations that ranged from 8 to 157 µg/L [9]. Water samples collected from five drain systems had uranium concentrations greater than 100 µg/L. Lico [8] noted uranium concentrations in drain-water samples ranging from 2.2 to 470 µg/L with a median concentration of 83 µg/L.

Ground water in Las Vegas Valley, Carson Desert, and Lake Tahoe Basin had uranium concentrations greater than the proposed MCL for drinking water. Five shallow monitoring wells in the water-table aquifer in the Las Vegas urban area had uranium concentrations that ranged from 7 to 56 µg/L [12]; the source of this uranium is not known. In the Sierra Nevada, uranium is dissolved from granitic rocks by water rich in carbon dioxide and oxygen [15]. Most of this released uranium is quickly removed from the water by attachment to fracture surfaces and fine-grained sediment and organic material. The resulting uranium-rich sediment grains and organic material have been transported to downstream valleys. Ground-water samples from deep wells on the southern side of Lake Tahoe (fig. 2) had high uranium concentrations ranging from less than 1 to 60 µg/L with a median concentration of 7.4 µg/L. Samples from shallow monitoring wells in the Lake Tahoe area, which had lower uranium concentrations than those from the deep wells, ranged from 0.5 to 16 µg/L with a median concentration of 1.4 µg/L.

Shallow (less than 50 ft) ground-water samples from irrigated areas in Carson Desert, collected during the NVBR study, had a median concentration of 40 µg/L [12], which is double the proposed MCL of 20 µg/L. Previous authors have suggested that uranium in Carson Desert ground water is due to the dissolution of uranium-rich sedimentary organic material and iron- and manganese-oxide coatings and was caused by a rise in the water table from large-scale irrigation in the area [11,15]. Ground water near the distal end of the flow system, in Carson Desert wetlands, had uranium concentrations ranging from 1.9 to 1,500 µg/L with a median value of 200 µg/L [8].

Radon is Derived From Granitic and Volcanic Rocks

The Sierra Nevada (fig. 2) has rock types that typically have high uranium content and high radon-generating potential. Radon is a radioactive gas derived from the decay of uranium-238 [16]. Granitic rocks have uranium concentrations that are high relative to most rocks. Radon has been reported at high concentrations in ground-water samples from western Nevada and eastern California [10,12,14,16,17,18], mostly where these rock types (fig. 2) and sediment from them are present. Ground-water samples from most of the study area, with the exception of Las Vegas Valley, had median values that exceeded the former proposed MCL [7] (currently under review for possible revision) of 300 picocuries per liter (pCi/L; see table below).

Geothermal Water Has High Concentrations of Arsenic, Boron, and Mercury

Geothermal systems in Carson Desert and Reno-Sparks area add arsenic and boron to ground- and surface-water systems. Active geothermal areas are present in the NVBR Study Unit (fig. 2). A representative water sample from a geothermal aquifer in Carson Desert was reported [22] as having high concentrations of arsenic (130 µg/L) and boron (18,000 µg/L) compared to other ground water in the area. In this area, deeper geothermal water moving upward may affect the quality of shallow ground water [23].

Geothermal systems in the Reno-Sparks area have an effect on the surrounding water quality--arsenic, boron, and mercury concentrations in ground-water samples commonly are high in comparison to other ground water in the area. Arsenic concentrations in water samples from wells have been reported as high as 640 µg/L and from springs as high as 4,000 µg/L [24]. The reported median arsenic concentration for all wells and springs was 970 µg/L [24]. Boron concentrations in water samples from wells and springs, ranged up to 127,000 µg/L with a median concentration of 22,000 µg/L.

Mercury is actively being deposited [24,25] as cinnabar (mercury sulfide), which is a possible source of mercury that could contaminate ground- or surface-water resources. Water samples collected at two sites on Steamboat Creek had a median mercury concentration of 3.2 µg/L [26] during 1977. The same report gives values of 1.7 and 2.0 µg/L of mercury in water samples collected from geothermal springs. Mercury concentrations in water and bottom-sediment samples were reported for three sites along Steamboat Creek [27]. Mercury was detected in 4 of 12 water samples, with a median concentration of 0.2 µg/L. The 17 bottom-sediment samples had a median concentration of 0.31 microgram per kilogram (µg/kg). Two bottom-sediment samples had mercury concentrations of 1.8 µg/kg, much higher than most of the values. Another possible source of mercury in Steamboat Creek was silver and gold ore processing in Washoe Valley during the late 1800s.

Median radon concentrations in ground-water samples from the NVBR Study Unit, 1986-95
[--, no sample collected; pCi/L, picocuries per liter]
 

Evapotranspiration Concentrates Dissolved Chemical Constituents

Nevada is the driest State in the Nation. Hot summer weather in parts of the Study Unit, namely Las Vegas Valley and Carson Desert, cause high rates of evaporation on open water bodies (lakes and streams) and transpiration by plants, especially when these high temperatures are accompanied by wind. Surface and ground water are affected by these processes directly and indirectly. Evapotranspiration (combined evaporation and transpiration) in the Carson and Truckee River Basins causes dissolved-solids concentrations to increase in a downstream direction. Headwater reaches of the Carson and Truckee Rivers, in the Sierra Nevada and adjacent areas, had median dissolved-solids concentrations of 123 and 86 mg/L, respectively.

Truckee River downstream from Lake Tahoe September 1992.  Photograph by Ronald P. Collins, U.S. Geological Survey.

Truckee River upstream from Pyramid Lake, September 1994.  Photograph by Stephen J. Lawrence, U.S. Geological Survey.

Downstream reaches of the rivers, in Carson Desert and near Pyramid Lake, had median concentrations of 604 and 454 mg/L, respectively--nearly a fivefold increase in dissolved solids. The median concentration in the Carson River in Carson Desert exceeds the 500-mg/L SMCL. Certainly, other factors contribute to this increase, but evapotranspiration is a major factor. Dissolved solids in ground water, especially near the water table, can be concentrated by high rates of evapotranspiration. In Carson Desert, large tracts of phreatophytes (plants that obtain much of their water from ground water) use large amounts of water and leave the dissolved solids behind. Median concentrations of dissolved solids in shallow ground water in Las Vegas Valley and Carson Desert (3,240 and 790 mg/L, respectively) exceeded the SMCL. Evapotranspiration increases the dissolved solids of water in some areas to concentrations greater than that of seawater.

U.S. Geological Survey Circular 1170

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