The Quality of the Nation’s Groundwater: Progress on a National Survey

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The U.S. Geological Survey is near the midpoint of a complex undertaking to survey the quality of the nation’s largest drinking-water resource.

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This story was updated on March 15, 2019.

From 2012 – 2021, the USGS is assessing groundwater throughout the country through extensive sampling. The latest results from two regional aquifers have become available recently and add to previously released results for nine additional regional aquifers.

About half of the nation’s population relies on groundwater for drinking water. As the nation’s population grows, the need for high-quality drinking-water supplies becomes even more urgent.

Groundwater – the Invisible and Vital Resource Graphic

The USGS has identified 68 principal aquifers, or regionally extensive aquifers that can be used as a source of drinking water, across the country. Groundwater pumped from these aquifers provides nearly 50 percent of the nation’s drinking water. Twenty of these principal aquifers account for about 75 percent of the nation’s groundwater pumped for public supply and 85 percent of the groundwater pumped for domestic supply. These 20 principal aquifers are being intensively evaluated by the USGS National Water-Quality Assessment Project between 2012 and 2021. Summary results for eleven principal aquifers are now available online at the links below.

Photo of a young girl drinking water, which likely originated from groundwater sources. 

A young girl drinks water, which likely originated from groundwater sources. (Credit: Tammy Zimmerman, USGS. Public domain.)

“The National Water-Quality Assessment Project is critical in helping resource managers understand how contaminants are introduced into the environment. This knowledge helps them make informed decisions about how to manage the nation’s water resources,” said Don Cline, USGS Associate Director for Water. “Understanding the quality of our water is critical in sustaining this resource for generations to come.”

A Deep Look at an Unseen Resource

USGS scientists are assessing water quality in source, or untreated, water from wells in principal aquifers. Most consumers receive water that has been treated by local utilities to meet federal drinking-water standards. Understanding what constituents are in untreated water can help decision makers manage and treat water resources. 

This comprehensive sampling, carried out over principal aquifers across the country, is focused on public-supply wells that tap relatively deep groundwater. Along with detailed information on geology, hydrology, geochemistry and chemical and water use, this data can be used to explain how and why aquifer vulnerability to contamination varies across the nation.

These regional aquifer studies provide water utilities and resource managers with information about:

  • Regulated and unregulated constituents from natural or human sources;
  • Pesticides, volatile organic compounds, and other constituents of concern for human health;
  • Present groundwater quality as a baseline for future conditions;
  • Regional and national statistics on water quality, which serves as a context for individual wells;
  • Differences in water quality in the shallow between the shallow and deep parts of aquifer systems;
  • Environmental tracers that can be used to understand sources and sustainability of groundwater supplies;
  • Local, regional and national hydrogeology.
Overview of water quality in Principal Aquifers

Overview of water quality in Principal Aquifers. The colored pie charts indicate the proportion of the area studied that contained a constituent in untreated groundwater at a concentration that exceeds a human-health benchmark for drinking water.

New Regional Aquifer Studies

In-depth, regional-scale assessments conducted or planned for 2012 through 2021 focus on 20 of the most heavily used aquifers in the nation. Groundwater quality results for principal aquifers sampled in 2015 are available today, in addition to those previously available for principal aquifers sampled in 2012-14, and summarized in the fact sheets below. Almost 900 deep public-supply wells were sampled within these aquifers, which were analyzed for a broad range of water-quality constituents.



  • At least one inorganic constituent exceeded a human-health benchmark in 4 to 50 percent of samples collected from the 11 principal aquifers.
  • Organic contaminants did not occur at levels above human-health benchmarks.
  • Contaminants from geologic sources—primarily trace elements such as arsenic, fluoride, and manganese—most commonly exceeded human-health benchmarks. The Floridan aquifer system was an exception, where strontium was the only trace element to exceed human-health benchmarks.
  • At least one radioactive constituent exceeded a human-health benchmark in a small percentage of samples—1 to 10 percent—in most of the 11 aquifers studied. The exceptions were the Piedmont and Blue Ridge crystalline-rock aquifers and the Cambrian-Ordovician aquifer system, where exceedances were 30 and 45 percent, respectively.
  • The nutrient nitrate was the only constituent from manmade sources that exceeded the human-health benchmark, typically in a low percentage of samples (1 or 2 percent). These exceedances occurred in the Floridan aquifer system, the Glacial aquifer system, the Rio Grande aquifer system, and the Valley and Ridge and Piedmont and Blue Ridge carbonate-rock aquifers.
Photo of USGS scientist, Rick Arnold collecting groundwater samples to determine water quality.

USGS scientist Rick Arnold collects groundwater samples to determine water quality. (Credit: Nancy Bauch, USGS. Public domain.)

Looking Forward

Understanding how natural features and human activities affect groundwater quality helps to predict how and why aquifer vulnerability to contamination varies across the nation. Over the next few years, results will be released for additional principal aquifers that are important sources of drinking water for the nation as the USGS NAWQA Project continues to address three central questions:

  1. What is the quality of the nation’s groundwater?
  2. Is it getting better or worse?
  3. What factors affect the quality of this vital resource?

Learn more

National Water-Quality Assessment Project
USGS Groundwater Information
USGS Fact Sheet, NAWQA Groundwater Studies: Principal Aquifer Surveys
USGS Data Series, Groundwater quality and select quality-control data from the National Water-Quality Assessment Project, January through December 2015, and previously unpublished data from 2013 to 2014

Technical announcements for some of the aquifers studied:

Rio Grande aquifer system (western U.S.)
Glacial aquifer system (northern U.S.)
Cambrian Ordovician aquifer system (north central U.S.)
Piedmont and Blue Ridge crystalline-rock aquifers (eastern U.S.)
Basin and Range basin-fill aquifers (western U.S.)
Valley and Ridge carbonate-rock aquifers and the Piedmont and Blue Ridge carbonate-rock aquifers (eastern U.S.)
Northern Atlantic Coastal Plain aquifer system (east coast of U.S.)
Southeastern Coastal Plain aquifer system (southeastern U.S.)
Coastal Lowlands aquifer system (south central U.S.)


Photo of USGS scientist testing groundwater samples for water quality. 

USGS scientist tests groundwater samples for water quality. (Credit: Laura Hallberg, USGS. Public domain.)

Photo of groundwater samples to be tested for water quality.

Groundwater samples to be tested for water quality.  (Credit: Laura Hallberg, USGS. Public domain.)