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Scientific Investigations Report 2006-5127

Quality of Nevada’s Aquifers and their Susceptibility to Contamination, 1990-2004

By Thomas J. Lopes

Version 1.0


In 1999, the U.S. Environmental Protection Agency introduced a rule to protect the quality of ground water in areas other than source-water protection areas. These other sensitive ground-water areas (OSGWA) are areas that are not currently but could eventually be used as a source of drinking water. To help determine whether a well is in an OSGWA, the Nevada Division of Environmental Protection needs statewide information on the susceptibility and vulnerability of Nevada's aquifer systems to contamination. This report presents an evaluation of the quality of ground water and susceptibility of Nevada's aquifer systems to anthropogenic contamination.

Chemical tracers and statistical methods were used to assess the susceptibility of aquifer systems in Nevada. Chemical tracers included nitrate, pesticides, volatile organic compounds (VOCs), chlorofluorocarbons (CFCs), dissolved gases, and isotopes of hydrogen and oxygen. Ground-water samples were collected from 133 wells during August 2002 through October 2003. Logistic regression was done to estimate the probability of detecting nitrate above concentrations typically found in undeveloped areas. Nitrate is one of the most common anthropogenic contaminants that degrades ground-water quality, is commonly measured and is persistent, except in reducing conditions. These characteristics make nitrate a good indicator of aquifer susceptibility. Water-quality data for 5,528 wells were compiled into a database. The area around each well was characterized using information on explanatory variables that could be related to nitrate concentrations. Data also were used to characterize the quality of ground water in Nevada, including dissolved solids, nitrate, pesticide, and VOC concentrations.


Executive Summary


Purpose and Scope


Previous Studies of Ground-Water Quality

Nitrate and Synthetic Organic Compounds in the Environment

Aquifer Susceptibility and Vulnerability

Aquifer Systems in Nevada



Unsaturated Zone

Hydrogeologic Units

Ground-Water-Flow Paths and Velocities


Chemical Techniques


Dissolved Gases

Isotopes of Hydrogen and Oxygen

Well Selection

Sampling Procedures

Quality Assurance

Statistical Techniques

Correlations and Comparisons

Logistic Regression

Water-Quality Data Compilation

Geographic Information System Datasets

Ground-Water Quality

Dissolved Solids

Nitrate 26

Concentrations in Undeveloped Areas

Relations Between Nitrate and Explanatory Variables

Synthetic Organic Compounds

Co-Occurrence of Nitrate and Synthetic Organic Compounds

Aquifer Susceptibility

Chemical Results


Dissolved Gases

Stable Isotopes of Hydrogen and Oxygen

Logistic Regression




Figure 1. Nevada and locations of wells sampled for this study.
Figure 2. Deviation from mean annual precipitation for selected stations in Nevada.
Figure 3. Typical ground-water flow and recharge patterns perpendicular to the long axis of valleys in Nevada for mountain blocks with different permeability and annual precipitation.
Figure 4. Concentrations of soil organic material.
Figure 5. Ranges in the horizontal hydraulic conductivity of aquifers in Nevada.
Figure 6. Atmospheric mixing ratios of CFC-11, CFC-12, and CFC-113 for the northern hemisphere.
Figure 7. Locations of wells in Nevada with water-quality data.
Figure 8. Dissolved-solids concentrations in ground water, playas, and ground-water discharge areas in Nevada.
Figure 9. Nitrate concentrations for domestic monitoring, and production wells in undeveloped areas.
Figure 10. Concentrations of nitrate in ground water.
Figure 11. Nitrate concentrations versus well category.
Figure 12. Nitrate concentrations versus A, depth to water and B, well depth.
Figure 13. Nitrate concentrations versus well category in Eagle Valley.
Figure 14. Number of A, pesticides and B, volatile organic compounds detected versus well depth.
Figure 15. Apparent recharge date versus A, water level above the top of the screen and B, well depth.
Figure 16. Nitrate concentration versus apparent recharge date.
Figure 17. Apparent recharge dates versus aquifer type.
Figure 18. Apparent recharge dates versus hydrologic setting.
Figure 19. Recharge temperature estimated from dissolved gases versus estimated mean annual air temperature.
Figure 20. Locations of wells and springs where isotopes have been sampled.
Figure 21. Deuterium versus 18O measured in ground water and springs in Nevada.
Figure 22. Deuterium in ground water and springs versus A, latitude; B, well depth; and C, altitude.


Table 1. Range in soil permeability for descriptive categories of unconsolidated sediments and corresponding geomorphic features (from Maurer and others, 2004)
Table 2. Summary statistics of soil organic material in selected western states and the United States. Values are in percent by weight.
Table 3. Number of wells sampled in hydrologic landscape regions of Nevada
Table 4. Summary of water-quality data compilation
Table 5. Explanatory variables used in logistic regression
Table 6. Dissolved-solids concentrations in unconsolidated sediment and ground-water discharge areas
Table 7. Spearman rank correlations between nitrate, clay, and depth to water in selected areas
Table 8. Pearson correlations between binary nitrate data using a background concentration of 2 milligrams per liter and explanatory variables. Only statistically significant correlations (p<0.05) are shown

Supplemental Information

Worksheet 1. Station information
Worksheet 2. Chlorofluorocarbon data
Worksheet 3. Dissolved gas data
Worksheet 4. Isotope Data

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For more information about USGS activities in Nevada, visit the Nevada Water Science Center home page.

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Last modified: Thursday, December 01 2016, 07:13:37 PM
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