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

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WATER-QUALITY CONDITIONS IN A NATIONAL CONTEXT

Stream Quality || Ground-water quality

Comparison of Ground-Water Quality in the Nevada Basin and Range Study Unit with Nationwide NAWQA Findings

Five Major Water-Quality Characteristics Were Evaluated for Ground-Water Studies in Each NAWQA Study Unit

National map Ground-water resources were divided into two categories: (1) drinking-water aquifers, and (2) shallow ground water underlying agricultural or urban areas. Summary scores were computed for each characteristic for all aquifers and shallow ground-water areas that had adequate data. Scores for each aquifer and shallow ground-water area in the Nevada Basin and Range Study Unit were compared with scores for all aquifers and shallow ground-water areas sampled in the 20 NAWQA Study Units during 1992-95. Results are summarized by percentiles; higher percentile values generally indicate poorer quality compared with other NAWQA ground-water studies. Water-quality conditions for each drinking-water aquifer also are compared to established drinking water standards and criteria for protection of human health. (Methods used to compute rankings and evaluate standards and criteria are described by Gilliom and others [58].)
 
 
Dissolved Solids in Las Vegas Valley Water-Supply Wells Were Greater Than the National 75th Percentile 

Dissolved-solids concentrations in deeper supply wells and shallow monitoring wells in Las Vegas Valley and shallow monitoring wells in Carson Desert and Reno-Sparks area were greater than the 75th percentile of NAWQA sites nationwide. Dissolved-solids concentrations in supply wells in Carson Valley and the Reno-Sparks area were less than the national median. The SMCL (500 mg/L) was exceeded in some samples from all areas except Carson Valley supply wells. Shallow monitoring wells in basin areas (Las Vegas Valley and Carson Desert) had the highest frequency of exceedence (97 and 70 percent, respectively); these high concentrations were caused by dissolution of evaporite minerals (salts) and their concentration in ground water by evapotranspiration.

Dissolved solids map explanation
 
 
nitrate map Nitrate in Samples From Water-Supply Wells Approached the National Median 

Nitrate concentrations in Study Unit water-supply wells were close to the national median for NAWQA drinking-water supply wells. Shallow monitoring wells in agricultural areas (Carson Valley and Carson Desert) had nitrate concentrations less than the national median, and those in urban areas (Las Vegas and Reno-Sparks) had concentrations near or greater than the national median. In the NVBR Study Unit, high nitrate concentrations were uncommon and possibly were due to fertilizer application, septic systems, or leaking sewer pipes in urbanized areas. Concentrations of nitrate did not exceed the MCL for any NAWQA samples collected from water-supply wells in the Study Unit.

 
 
Pesticides in the Reno-Sparks Urban Area Were Detected More Frequently Than the National Median 

The frequencies of pesticide detections in samples from water-supply and shallow monitoring wells in the Reno-Sparks urban area were between the national NAWQA median and the 75th percentile. Pesticides used on urban landscapes are leached into ground water by irrigation. In all areas sampled, except the Reno-Sparks urban area, pesticides were detected in ground-water samples less frequently than the national median. 

pesticides map
 
 
VOC map Volatile Organic Compounds (VOCs) in the Las Vegas and Reno-Sparks Urban Areas Were Detected More Frequently Than the National 75th Percentile 

The frequencies of detection for VOCs in the urban areas of Las Vegas and Reno-Sparks (both water-supply wells and shallow monitoring wells) were greater than the 75th percentile of NAWQA wells nationwide. Trihalomethanes (THMs), chlorinated solvents, and a gasoline oxygenate (MTBE) were the most frequently detected VOCs. Concentrations of several VOCs in water from supply wells in the Las Vegas urban area and from shallow monitoring wells in Las Vegas and Reno-Sparks urban areas exceeded MCLs (the chlorinated solvents--tetrachloroethylene and trichloroethylene) or a drinking water advisory (MTBE). Water-supply wells and shallow wells in agricultural areas had less frequent detections of VOCs than the 25th percentile of NAWQA wells nationwide. 

 
 
Radon in Carson Valley and Reno-Sparks Urban Area Water-Supply Wells was Greater Than the National 75th Percentile 

Radon, a decay product of uranium in granitic rocks in the Study Unit, occurs naturally in soils. Radon concentrations in water from supply and shallow monitoring wells in the Las Vegas urban area were less than the national median. Concentrations in water from supply wells in Carson Valley and the Reno-Sparks area and shallow monitoring wells in Carson Valley exceeded the 75th percentile of NAWQA sites nationwide. Shallow monitoring wells in the Reno-Sparks area had radon concentrations greater than the national median. Carson Valley and Reno-Sparks area soils were derived, in part, from granitic rocks in the Sierra Nevada. 
 
 

Radon map
 
 
 
CONCLUSIONS
Compared to other NAWQA Study Units: 
  • The frequencies of pesticide detections in water-supply and shallow monitoring wells in the Reno-Sparks urban area exceeded the national NAWQA median, probably because of landscape pesticide applications that subsequently were leached by irrigation into the ground water.
  • Dissolved-solids concentrations in water from wells in basin areas (Las Vegas Valley and Carson Desert) exceeded the 75th percentile for the NAWQA Program because of the dissolution of evaporite minerals and the concentration of dissolved constituents by evapotranspiration.
  • Nitrate concentrations did not exceed the MCL in any of the samples from water-supply wells.
  • The frequencies of VOC detections in ground water beneath urban areas were among the highest sampled by the NAWQA Program.
  • Ground water in Carson Valley and Reno-Sparks had radon concentrations greater than the national NAWQA median; concentrations in most samples were greater than 75 percent of the national NAWQA samples. Radon occurs naturally in soils throughout the Study Unit, but it is especially high in soils derived from granitic rocks of the Sierra Nevada.
 
 
well installation photo A shallow monitoring well being installed in the Las Vegas urban area, August 1993. Photograph by Kenneth J. Covay, U.S. Geological Survey. 
 
 

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Suggested citation:
Bevans, H.E., Lico, M.S., and Lawrence, S.J., 1998, Water Quality in the Las Vegas Valley Area and the Carson and Truckee River Basins, Nevada and California, 1992-96, on line at <URL: https://water.usgs.gov/pubs/circ1170>, updated 19 March 1998

This page is a subpage of <URL:http://water.usgs.gov/pubs/circ1170>
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Last modified: Tue Mar 17 17:53:42 1998