Scientific Investigations Report 2006-5210
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As a part of the U.S. Geological Survey’s National Water-Quality Assessment Program, an effort to characterize the quality of major rivers and aquifers used as a source of supply to some of the largest community water systems (CWSs) in the United States has been initiated. These studies, termed Source Water-Quality Assessments (SWQAs), consist of two sampling phases. Phase 1 was designed to determine the frequency of detection and concentrations of about 260 volatile organic compounds (VOCs), pesticides and pesticide degradates, and other anthropogenic organic compounds in source water of 15 CWS wells in each study. Phase 2 monitors concentrations in the source water and also the associated finished water of CWSs for compounds most frequently detected during phase 1. One SWQA was completed in the Nevada Basin and Range area in Nevada. Ten CWS wells in Eagle Valley and five CWS wells in Spanish Springs Valley were sampled. For phase 2, two wells were resampled in Eagle Valley. Samples were collected during 2002–2004 for both phases.
Water use in Eagle Valley is primarily for domestic purposes and is supplied through CWSs. Ground-water sources provide about 55 percent of the public-water supply, and surface-water sources supply about 45 percent. Lesser amounts of water are provided by domestic wells. Very little water is used for agriculture or manufacturing. Spanish Springs Valley has water-use characteristics similar to those in Eagle Valley, although there is more agricultural water use in Spanish Springs Valley than in Eagle Valley.
Maximum contaminant concentrations were compared to two human-health benchmarks, if available, to describe the water-quality data in a human-health context for these findings. Measured concentrations of regulated contaminants were compared to U.S. Environmental Protection Agency and Nevada Maximum Contaminant Level (MCL) values. Measured concentrations of unregulated contaminants were compared to Health-Based Screening Levels, which are not regulatory standards and are not legally enforceable values. All of the contaminants detected in this study were found at concentrations less than available human-health benchmarks
In the source waters sampled in phase 1, 10 contaminants of the approximately 260 measured were detected in samples collected from Eagle Valley, and 4 contaminants were detected in samples from Spanish Springs Valley. The most frequently detected compounds in the Eagle Valley source water were chloroform (a disinfection by-product), which was detected in samples from four wells, and deethylatrazine (a degradation product of the herbicide atrazine), which was detected in samples from three wells. Each of the four contaminants detected in the Spanish Springs Valley source waters was detected in samples from one well.
The detection frequencies of VOCs and pesticides in samples from the SWQA wells were similar to those in samples from both shallow and deep monitoring wells in Carson City, Reno, and Spanish Springs. This indicates that the SWQA sampling is representative of the organic chemical compounds likely to be detected in the aquifers sampled. However, more organic compounds were detected at low frequencies and concentrations in samples from the monitoring wells than in samples from SWQA wells.
Three contaminants were detected in one finished-water sample collected from Eagle Valley. Comparison of SWQA results in the Nevada Basin and Range Study Unit to results of an SWQA in the larger urban area of Salt Lake City showed that fewer anthropogenic compounds were detected in Eagle and Spanish Springs Valleys and generally at lower concentrations than in the Salt Lake City study.
Purpose and Scope
Description of Study Area
Land and Water Use
Study Design and Methods
Regulatory and Non-Regulatory Human-Health Benchmarks
Occurrence of Anthropogenic Organic Compounds in Ground Water and Finished Water
Phase 1: Source Water
Spanish Springs Valley
Phase 2: Source and Finished Water
Comparison of Study Results to Other Community Water-Supply Well Data Sets
1–4. Maps showing:
1. Location of National Water-Quality Assessment Study Units, Nevada Basin and Range Study Unit, and Study Units that have completed or are conducting ground-water Source Water-Quality Assessments.
2. Nevada Basin and Range (NVBR) Study Unit
3. Eagle Valley well locations
4. Spanish Springs Valley well locations
5. Pie chart showing ground-water use in Nevada, in acre-feet and percent, 2000
6. Conceptual hydrologic block diagram of Eagle Valley
7. Photograph of a supply well in Spanish Springs Valley
For more information about USGS activities in Nevada, visit the USGS Nevada Water Science Center home page.
For more information about USGS National Water-Quality Assessment Program, visit the NAWQA Program home page.
|For additional information regarding Source Water-Quality Assessments, please contact:
U.S. Geological Survey
1608 Mt. View Road
Rapid City, South Dakota 57702
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