Groundwater Quality in Principal Aquifers of the United States, 1991—2010

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A new U.S. Geological Survey (USGS) report summarizes the major findings of national and regional assessments of groundwater quality in 40 Principal Aquifers in the United States. Knowledge of contaminant occurrence and the factors that control contaminant concentrations can ensure the availability and quality of this vital natural resource in the future.

Girl Drinking Water

Girl Drinking Water

(Public domain.)

Principal aquifers are regionally extensive aquifers that supply most of the groundwater pumped across the Nation for drinking water, irrigation, and other uses. Approximately 80 billion gallons are pumped from these aquifers each day. Groundwater resources are used as drinking water by 130 million people in the Nation, 43 million of which use groundwater from private wells for drinking water.

The USGS National Water-Quality Assessment (NAWQA) Program used more than 200 study networks, of 20 to 30 wells each, to assess groundwater quality across the United States. From 1991 to 2010, more than 6,600 wells were sampled for this comprehensive suite of water-quality constituents, including 3,700 wells sampled in the parts of aquifers used for drinking water.

Major Findings

  • Contaminants from geologic or human-made sources were a potential human-health concern in one of every five wells sampled in the parts of aquifers used for drinking water—Most of these contaminants were from natural geologic sources, including arsenic, manganese, radon, and uranium. Nitrate was the only contaminant from human-made sources that exceeded its human-health benchmark in more than 1 percent of wells. Although groundwater is a safe, reliable source of drinking water for millions of people nationwide, high concentrations of some chemical constituents can pose potential human-health concerns.
  • USGS Circular 1360 Report Cover

    USGS Circular 1360 Report Cover

    Groundwater—The Invisible and Vital Resource

    • 80,000,000,000 - Gallons of groundwater pumped each day
    • 130,000,000 - People who use groundwater for drinking water
    • 43,000,000 - People who use groundwater from private wells for drinking water
    • 1,300,000 - Chemical analyses performed in this study
    • 6,600 - Wells sampled for this study
    • 62 - Principl Aquifers in the United States

    (Public domain.)

    Differences in geology, hydrology, geochemistry, and chemical use explain how and why aquifer vulnerability and concentrations of contaminants vary across the Nation—Factors such as the mineral composition of aquifer materials, how groundwater moves through an aquifer, geochemical conditions like redox, and what chemicals are used and disposed of on the land surface all affect contaminant occurrence and vary among Principal Aquifers. As a consequence, different contaminants occur more—or less—frequently in some aquifers than others. Understanding how these factors act is used to predict concentrations in some aquifers, such as arsenic and nitrate in the southwest, through in-depth assessments of regional groundwater quality.

  • Changes to groundwater flow have also altered groundwater quality—Irrigation, pumping, artificial recharge, and drainage have drastically changed how water moves through some aquifers, with consequences for water quality. Examples include the accelerated movement of contaminants from land surface into deeper parts of aquifers; increased dissolved solids in groundwater; and, in several cases, the release of contaminants from geologic sources, such as uranium, selenium, or radium, from aquifer rocks and sediments into the groundwater.
  • Our actions today are determining groundwater quality for decades to come—In parts of some aquifers, the effects of contaminant inputs from more than 30 years ago are still apparent. Similarly, contaminant inputs and changes to water quality that are occurring today are likely to affect groundwater quality for decades into the future.


    This research was funded by the USGS Ecosystems Mission Area’s Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology) and supported by the USGS National Water-Quality Assessment Program.