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December 7, 2017

The U.S. Geological Survey makes progress on a complex undertaking to survey the quality of the nation’s largest drinking-water sources.

New reports assess the quality of drinking-water supplies in two regional aquifers, adding to 18 previously assessed across the nation.

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Editor’s note: This story was updated September 1, 2022

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 two regional aquifers have become available and have been added to previously released results for eighteen 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. Many of these principal aquifers are being intensively evaluated by the USGS National Water-Quality Assessment Project, which began in 2012 and continued through 2021. Summary results for twenty principal aquifers are now available online at the links below.

“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 essential to sustaining this resource for generations to come.”

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.)

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.


Overview of water quality in Principal Aquifers in the U.S. with colored pie charts showing data
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 the most heavily used aquifers in the nation.

Groundwater quality results for principal aquifers sampled in 2019 are available today, in addition to those previously available for principal aquifers sampled between 2012 and 2018, and are summarized in the fact sheets below. Almost 1,500 deep public-supply wells were sampled within these 20 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) chemical 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 20 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 several carbonate lithology aquifers (the Biscayne aquifer, the Edwards-Trinity aquifer system, and the Floridan aquifer system), the trace element strontium—a constituent from geologic sources—was the most common 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 20 aquifers studied. Exceedances ranged from 0 to 12 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). An exception occurred in the Stream Valley aquifers, where the nitrate exceedance was 7 percent.
Photo of USGS scientists collecting groundwater samples to determine water quality.
Sampling for the National Assessment of Water-Quality Project. Top left: USGS scientist tests groundwater samples for water quality. (Credit: Laura Hallberg, USGS. Public domain. Right: USGS scientist Rick Arnold collects groundwater samples to determine water quality. (Credit: Nancy Bauch, USGS. Public domain.) Bottom left: Groundwater samples to be tested for water quality.  (Credit: Laura Hallberg, 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. Since 1991 the National Water-Quality Assessment Project has addressed where, when, why, and how the nation’s water quality has changed, or is likely to change in the future, in response to human activities and natural factors. Knowledge gained through the National Water-Quality Assessment Project has informed science-based policies and management strategies that improve and protect water resources at the national, regional, state and local level. These resources fill a variety of needs, including drinking water, recreation, irrigation, energy development, and ecosystem services. NAWQA project activities will be incorporated into new USGS initiatives that combine information on water quality, water quantity, and water use, developing comprehensive regional and national evaluations of water availability.

Learn more

National Water-Quality Assessment Project

USGS Groundwater Quality Research

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 2017 through December 2019

USGS Water Resources Science Themes

USGS Water Availability and Use Science Program

USGS New Water-Quality Directions

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