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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**

###  EXECUTIVE SUMMARY 

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.

## Contents

Executive Summary

Introduction

Purpose and Scope

Acknowledgments

Previous Studies of Ground-Water Quality

Nitrate and Synthetic Organic Compounds in the Environment

Aquifer Susceptibility and Vulnerability

Aquifer Systems in Nevada

Precipitation

Recharge

Unsaturated Zone

Hydrogeologic Units

Ground-Water-Flow Paths and Velocities

Methods

Chemical Techniques

Chlorofluorocarbons

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

Chlorofluorocarbons

Dissolved Gases

Stable Isotopes of Hydrogen and Oxygen

Logistic Regression

Summary

References

### Figures

| Figure 1.                                                                                                                                                                     | Nevada and locations of wells sampled for this study. |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------|
| Deviation from mean annual precipitation for selected stations in Nevada.                                                                                                     |
| 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. |
| Concentrations of soil organic material.                                                                                                                                      |
| Ranges in the horizontal hydraulic conductivity of aquifers in Nevada.                                                                                                        |
| Atmospheric mixing ratios of CFC-11, CFC-12, and CFC-113 for the northern hemisphere.                                                                                         |
| Locations of wells in Nevada with water-quality data.                                                                                                                         |
| Dissolved-solids concentrations in ground water, playas, and ground-water discharge areas in Nevada.                                                                          |
| Nitrate concentrations for domestic monitoring, and production wells in undeveloped areas.                                                                                    |
| Concentrations of nitrate in ground water.                                                                                                                                    |
| Nitrate concentrations versus well category.                                                                                                                                  |
| Nitrate concentrations versus A, depth to water and B, well depth.                                                                                                            |
| Nitrate concentrations versus well category in Eagle Valley.                                                                                                                  |
| Number of A, pesticides and B, volatile organic compounds detected versus well depth.                                                                                         |
| Apparent recharge date versus A, water level above the top of the screen and B, well depth.                                                                                   |
| Nitrate concentration versus apparent recharge date.                                                                                                                          |
| Apparent recharge dates versus aquifer type.                                                                                                                                  |
| Apparent recharge dates versus hydrologic setting.                                                                                                                            |
| Recharge temperature estimated from dissolved gases versus estimated mean annual air temperature.                                                                             |
| Locations of wells and springs where isotopes have been sampled.                                                                                                              |
| Deuterium versus 18O measured in ground water and springs in Nevada.                                                                                                          |
| Deuterium in ground water and springs versus A, latitude; B, well depth; and C, altitude.                                                                                     |

### Tables

| Table 1.                                                                                                                                                                                                  | Range in soil permeability for descriptive categories of unconsolidated sediments and corresponding geomorphic features (from Maurer and others, 2004) |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------|
| Summary statistics of soil organic material in selected western states and the United States. Values are in percent by weight.                                                                            |
| Number of wells sampled in hydrologic landscape regions of Nevada                                                                                                                                         |
| Summary of water-quality data compilation                                                                                                                                                                 |
| Explanatory variables used in logistic regression                                                                                                                                                         |
| Dissolved-solids concentrations in unconsolidated sediment and ground-water discharge areas                                                                                                               |
| Spearman rank correlations between nitrate, clay, and depth to water in selected areas                                                                                                                    |
| 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 |

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## Supplemental Information

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

**This report is contained in the following files:**

| -  **Scientific Investigations Report 2006-5127.pdf** (5.3 MB)
- Worksheet1.pdf (65 kb) 
- Worksheet2.pdf (472 kb) 
- Worksheet3.pdf (153 kb) 
- Worksheet4.pdf  (24 kb) | **For viewing and printing upon download.***(This version of the report is accessible as defined in Section 508 of the Rehabilitation        Act of 1973, Amendments of 1998.)* |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|

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#####  Send questions or comments about this report to the Public Information Assistant, GS-W-NVpublic-info@usgs.gov, 775-887-7649.

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

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