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

Release Date:

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.

Current website:

This story was updated on April 1, 2020.

Beginning in 2012 and continuing through 2021, the USGS has been extensively sampling groundwater throughout the country and assessing its suitability for drinking water. The latest results from four regional aquifers have become available and have been added to previously released results for eleven 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—regionally extensive aquifers that can be used as  sources 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. These aquifers also provide 85 percent of the groundwater pumped for domestic (private) supply. These twenty principal aquifers are being intensively evaluated by the USGS National Water-Quality Assessment Project, which is scheduled to run from 2012 through 2021. Summary results for fifteen 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

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

This comprehensive sampling is focused on public-supply wells that tap deeper groundwater. Along with detailed information on geology, hydrology, geochemistry and chemical and water use, this data can be used to explain how and why groundwater 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 provide context for individual wells;
  • Differences in water quality 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.
Map of 15 principal US aquifers; pie charts showing proportion of inorganic contaminants exceeding human-health benchmarks

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.

(Public domain.)

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 2016 are available today, in addition to those previously available for principal aquifers sampled between 2012 and 2015, and are summarized in the fact sheets below. Almost 1,100 deep public-supply wells were sampled within these aquifers, which were analyzed for a broad range of water-quality constituents.


Contaminants in groundwater have a wide range of sources, both manmade and geologic, as a 2014 USGS water quality summary explains. Most organic (carbon-based) chemicals in groundwater that are of concern for human health are manmade compounds, such as solvents and pesticides. In contrast, most inorganic constituents in groundwater have geologic or other natural sources, although their concentrations in groundwater may be altered by human activities , such as irrigation and groundwater pumping. Some contaminants have both manmade and natural sources. For example, nitrate in groundwater has many natural sources, but nitrate concentrations in groundwater underlying agricultural and urban areas commonly are higher than in other areas, because of contributions from sources associated with human activities.

Groundwater quality of principal aquifers is assessed in the fact sheets by comparing measured concentrations to benchmarks established for drinking-water quality to protect human health and aesthetics, such as taste and odor..

  • At least one inorganic constituent exceeded a human-health benchmark in 3 to 50 percent of samples collected from the 15 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. In the Biscayne aquifer and the Floridan aquifer system, the trace element strontium—a constituent from geologic sources— 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 in most of the 15 aquifers studied. Exceedances ranged from 0 to 10 percent. 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 (0 to 3 percent). These exceedances occurred in the High Plains aquifer, the Columbia Plateau basaltic-rock aquifers, 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 groundwater vulnerability to contamination varies across the nation. Over the next few years, results will be released for more principal aquifers that are important sources of drinking water for the nation, as the National Water-Quality Assessment 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



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.)