National Water-Quality Assessment (NAWQA) Project

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Regional Assessment of Groundwater Quality in the Basin and Range, Rio Grande, Coastal Basins, and Central Valley Aquifer Systems of the Southwestern United States

Basin-fill aquifers are an important source of groundwater for many cities and agricultural communities in the arid and semiarid southwestern United States. The quality of groundwater is of concern because of the general scarcity of renewable water supplies in the region. To address this concern, the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey is conducting a regional analysis of water quality in the principal aquifer systems (U.S. Geological Survey, 2003) in the Southwest: the Basin and Range basin-fill aquifers in Nevada, Utah, and Arizona; the Rio Grande aquifer system in New Mexico and Colorado; and the Coastal Basin aquifers and the Central Valley aquifer system in California. These Southwest principal aquifers (SWPA) consist primarily of unconsolidated basin-fill deposits with similar land- and water-use practices, and water-quality issues. About 46.6 million people live in the SWPA study area and the basin-fill aquifers accounted for about one quarter of the total withdrawals from all aquifers in the United States in 2000 (Maupin and Barber, 2005). Information collected by the NAWQA Program on natural contaminants in groundwater, such as dissolved solids (salinity) and arsenic, and contaminants related to human activities, such as nitrate and pesticides, is available for many basins in the Southwest.

Topics selected for the assessment of water quality:

  • Salinity in ground and surface water — What are the sources and sinks of dissolved solids, and are concentrations changing over time? Report and findings
  • Land use effects on shallow groundwater — How do differences in land use, chemical use, irrigation, and other land-management practices help explain differences in the occurrence and distribution of selected constituents? (SIR 2007-5179)
  • Water quality in the Carbonate-Rock Aquifer of the Great Basin — To what degree is this regional aquifer system affected by hydrogeologic and environmental factors? (SIR 2005-5232)
  • Factors affecting water quality in basin-fill aquifers used for drinking-water supply — What natural factors, such as geology and climate, or human factors such as land and water use, influence selected constituents in groundwater used for drinking?  (SIR 2011-5020)
  • Statistical models of factors that influence the vulnerability of aquifers to contamination — How can we quantify natural and human factors that affect the distribution of a constituent, such as nitrate and arsenic in groundwater, and assess where changes in water quality are likely to occur? (SIR 2012-5065)

Featured Publications
(View complete list)

  • Basin-fill aquifers in the southwestern United States — Our conceptual understanding of the groundwater systems in the 15 basins studied by NAWQA and a description of the groundwater quality in each. (Professional Paper 1781)
  • Conceptual models of the primary natural and human factors affecting groundwater quality -- Several sources and factors commonly affect the vulnerability of basin-fill aquifers in the Southwest to elevated concentrations of dissolved solids, nitrate, arsenic, uranium, volatile organic compounds, and pesticides. (SIR 2011-5020)
  • Nitrate and arsenic in southwestern basin-fill aquifers--A statistical modeling approach was used to extrapolate nitrate and arsenic occurrence from areas where concentrations are known, to other areas where such data are unavailable. The extrapolation is based on nitrate and arsenic analyses from well-water samples collected from 1980 to 2010, and a wide variety of hydrologic, geologic, climatic, soil, land use, water use, agricultural, and biotic conditions that local-scale geochemical studies have found to be relevant to nitrate or arsenic occurrence in groundwater. (SIR 2012-5065)
  • Increasing dissolved-solids concentrations in groundwater used for public supply — Long-term trends in water from several wells indicate a substantial change in dissolved solids in some areas of Salt Lake Valley, Utah. (Fact Sheet 2010-3073)
  • Geospatial data used to support analysis of water-quality conditions in basin-fill aquifers — Data sets include physical characteristics of the Southwest, such as geology, elevation, and precipitation, as well as human-related factors, including population, land use, and water use. (SIR 2008-5239)

General Information

Informational handout about the Basin and Range, Rio Grande, Coastal Basins, and Central Valley Aquifer Systems studies (PDF format, 1Mb)

Fact sheet about the Southwest Principal Aquifers Regional Groundwater Quality Assessment (PDF Format, 5.5 Mb)

Detailed information on the hydrogeology of the Basin and Range, Rio Grande, Coastal Basins, and Central Valley Aquifer Systems can be accessed through the USGS Ground Water Atlas of the United States

For more information, contact:
Susan Thiros, Hydrologist

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