WaterThe quality of the U.S. groundwater

About 115 million people — more than one-third of the U.S. population — rely on groundwater for drinking water. As the U.S. population grows, the need for high-quality drinking-water supplies becomes even more urgent.

“Through the WaterSMART initiative and the National Water-Quality Assessment Program, the Department of Interior is working to secure sustainable water supplies of sufficient quantity and quality and to identify measures needed to address climate change and future demands,” said Jennifer Gimbel, Principal Deputy Assistant Secretary of the U.S. Department of the Interior. “The integrated work that USGS is doing to map groundwater availability, groundwater quality, and the potential for contamination will give us the information we need to understand natural and human effects on groundwater and to take the actions needed to protect this vital natural resource.”

The recently completed national summary report of the quality of the Nation’s groundwater is now available online. Nine associated reports (USGS circulars) detail regional-scale assessments of groundwater quality in about thirty of the most heavily used principal aquifers across the United States.

A deep look at an unseen resource
A USGS release reports that over the last two decades, USGS scientists have assessed water quality in source (untreated) water from 6,600 wells in extensive regional aquifers that supply most of the groundwater pumped for the U.S. drinking water, irrigation, and other uses. This comprehensive sampling, along with detailed information on geology, hydrology, geochemistry, and chemical and water use, can be used to explain how and why aquifer vulnerability to contamination varies across the Nation.

National findings

• More than one in five (22 percent) groundwater samples contained at least one contaminant at a concentration of potential concern for human health.
• Contaminants from geologic sources — primarily manganese, arsenic, and radon — accounted for about 80 percent of contaminant concentrations that exceeded a human-health benchmark. Nitrate, which exceeded its benchmark in about 4 percent of wells sampled, was the only constituent from manmade sources that exceeded its human-health benchmark in more than 1 percent of samples.
• Differences in contaminant concentrations among aquifers, within an aquifer, and even between two nearby wells can be explained by large- and local-scale differences in geology, hydrology, geochemistry, and chemical and water use. Understanding how natural features and human activities affect groundwater quality helps to predict how and why aquifer vulnerability to contamination varies across the United States.
• High-volume pumping and irrigation in many areas have profoundly changed groundwater flow and quality. By moving shallow groundwater deeper, into parts of aquifers used for drinking water, irrigation and pumping have increased the vulnerability of drinking-water supplies to contamination from nitrate, pesticides, and other manmade chemicals from the land surface.
• Irrigation, high-volume pumping, and artificial recharge can cause different types of waters to mix, with the unexpected consequence of causing the aquifer rocks and sediment to release naturally occurring contaminants, such as arsenic, selenium, or radium, into the groundwater.

Concentrations of dissolved solids, chloride, and nitrate, indicators of human influence on groundwater quality, increased from the 1990s to 2010 in shallow groundwater in many parts of the Nation. Similar changes are likely to occur in deeper parts of some aquifers in the future as the shallow water moves downward. Groundwater in permeable, unconfined aquifers, such as the Central Valley aquifer system in California, is especially vulnerable to contamination.

Regional principal aquifer assessments
USGS notes that in-depth, regional-scale assessments of groundwater quality focus on about thirty of the most heavily used Principal Aquifers in the United States. Groundwater quality in these aquifers — and the factors that affect it — is described in nine new U.S. Geological Survey circulars.

Glacial Aquifer System (northern U.S.)

Western Volcanics (northwestern U.S., Hawaii)

Southwest Basin-fill Aquifers (southwestern U.S.)

Denver Basin Aquifer System (Colorado)

Piedmont, Blue Ridge, and Valley and Ridge Aquifers (eastern U.S.)

Northern Atlantic Coastal Plain Surficial Aquifer System (east coast of U.S.)

Upper Floridan Aquifer and Overlying Surficial Aquifer (southeastern U.S.)

Mississippi Embayment-Texas Coastal Upland Aquifer System and Mississippi River Valley Alluvial Aquifer (southeastern, southcentral U.S.)

High Plains Aquifer System (central U.S.)

Looking forward
Over the next decade, about 2,300 shallow wells and 1,400 deep public-supply wells will be sampled for a broad range of water-quality constituents as the National Water-Quality Assessment Program continues to address the three central questions:

1. What is the quality of the U.S. groundwater,
2. Is it getting better or worse, and
3. What factors affect the quality of this vital resource.

USGS says that in the future, an agency-led national and regional scale modeling will provide a three-dimensional perspective of the quality of the Nation’s groundwater that can be used to inform management decisions.

— Read more in Leslie A. DeSimone et al., The Quality of Our Nation’s Waters: Water Quality in Principal Aquifers of the United States, 1991–2010 (USGS, 21 January 2015)