Groundwater Quality—Current Conditions and Changes Through Time

As part of the National Water Quality Program (NWQP), groundwater quality is being characterized in 20 of the Nation's 68 Principal Aquifers.  These 20 aquifers supply most of the groundwater used in the United States—they account for more than three-quarters of the groundwater pumped for public supply and 85 percent of the groundwater pumped for domestic supply.

About 140 million people—almost one-half of the Nation’s population—rely on groundwater for drinking water. Regional assessments of groundwater quality are one component of the NWQP's ongoing efforts to assess, understand, and forecast the quality of the Nation’s groundwater.

Samples collected by the NWQP for the surveys of Principal Aquifers are analyzed for a large suite of regulated and unregulated constituents, including pesticides, radionuclides, metals, and pharmaceuticals. The Principal Aquifer surveys focus on characterizing the quality of groundwater prior to treatment, not the treated drinking water delivered to consumers.

Regional Assessments of Groundwater Quality

To characterize the quality of groundwater many people use for drinking, almost 1,100 deep public-supply wells have been sampled within 15 Principal Aquifers. Although samples are from source water prior to any treatment, for context the results are compared to human-health benchmarks for drinking water.

Groundwater samples were analyzed for hundreds of water-quality constituents.  What have we learned?

• At least one inorganic constituent exceeded a human-health benchmark in all of the 15 Principal Aquifers surveyed to date, ranging from 3 to 50 percent of samples.
• At least one organic constituent exceeded a human-health benchmark in 2 of the 15 Principal Aquifers surveyed to date, ranging from 3 to 5 percent of samples.
• 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 15 Principal 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 its 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.

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.

The results are explained in easy-to-understand fact sheets, accessible below:

• The Columbia Plateau basaltic-rock aquifers (northwestern U.S.)
• The High Plains Aquifers (western U.S.)
• The Ozarks Plateaus aquifer system (central U.S.)
• The Biscayne aquifer (southeastern U.S.)
• The Basin and Range basin-fill aquifers (western U.S.)
• The Rio Grande aquifer system (southwestern U.S.)
• The Coastal Lowlands aquifer system (south central U.S.)
• The Mississippi Embayment-Texas Coastal Uplands aquifer system (south-central U.S.)
• The Floridan aquifer system (southeastern U.S.) 
• The Southeastern Coastal Plain aquifer system (southeastern U.S.)
• The Northern Atlantic Coastal Plain aquifer system (east coast of U.S.)
• The Piedmont and Blue Ridge crystalline-rock aquifers (eastern U.S)
• The Valley and Ridge carbonate-rock aquifers and the Piedmont and Blue Ridge carbonate-rock aquifers (eastern U.S.)
• The Cambrian-Ordovician aquifer system (north central U.S.)
• The Glacial aquifer system (northern U.S.)

How has groundwater quality changed over the decades?

Groundwater-quality monitoring data collected many regions of the United States have been synthesized into a national assessment of groundwater-quality trends. Between 1991 and 2010, NAWQA completed assessments groundwater-quality in Principal Aquifers across much of the United States.  The assessments characterized groundwater in both deep public-supply wells and shallower domestic (private) wells. Many of those wells have been resampled on a near-decadal timeframe to determine if groundwater quality has changed over time. To date 1,718 wells in 73 well networks—20-30 randomly selected wells designed to examine groundwater quality in a region— have been resampled on a near-decadal time period. The National Water Quality Program  will continue to resample wells periodically to build on our understanding of long-term trends in groundwater quality.

An interactive web tool maps these decadal changes in groundwater quality.  Using the web tool, users can easily visualize changes in both inorganic and organic constituent concentrations in groundwater, including chloride, nitrate, several pesticides, and some drinking-water disinfection byproducts. The website also includes a description of the methods used to evaluate changes in groundwater quality and a link to the complete set of data.

Shorter-term fluctuations in water quality

As part of the USGS National Water Quality Program, scientists are investigating why, in some areas and at some depths, groundwater quality changes at short timescales—years to months to days to hours, rather than decades.  These fluctuations often are in areas where groundwater and surface water interact. This study, called the Enhanced Trends Network, is evaluating these rapid fluctuations, identifying what causes them, and determining whether the changes are just part of a seasonal trend or are part of an overall long-term trend. For those chemical constituents with human-health benchmarks (thresholds for drinking-water quality), changes in constituent concentrations are being evaluated in the context of those benchmarks—in other words, are there certain conditions under which the groundwater might require treatment before drinking?

Learn more about how the Enhanced Trends Network is providing insight on short-term fluctuations in groundwater quality.

Featured Study

Scientists home in on causes of high radium levels in key Midwestern aquifer

As part of the Principal Aquifer surveys, scientists were able to shed new light on processes that happen deep underground. These processes—which cause radium to leach from aquifer rocks into groundwater—are responsible for high concentrations of naturally occurring radium in groundwater from the Cambrian-Ordovician aquifer. This aquifer provides more than 630 million gallons of water a day for public supply to parts of Illinois, Iowa, Missouri, Michigan, Minnesota, and Wisconsin.

Concentrations of radium in samples of untreated groundwater from the Cambrian-Ordovician groundwater system frequently exceeded the USEPA Maximum Contaminant Level (MCL) of 5 picocuries per liter in Illinois, Iowa, and eastern Wisconsin, where wells tap deeper, older groundwater. From Stackelberg and others, 2018, "Radium mobility and the age of groundwater in public-drinking-water supplies from the Cambrian-Ordovician aquifer system, north-central USA."

This USGS study helps explain how radium isotopes 224, 226, and 228 make their way into water in the Cambrian-Ordovician aquifer and where concentrations are highest. The study, part of the USGS National Water Quality Assessment Project, reports that water that was recharged into the aquifer long ago, that contains greater amounts of dissolved minerals, and that is low in dissolved oxygen is more likely to leach radium from the surrounding rock.

The groundwater tested came from public supply wells, before treatment and distribution. Radium can be removed from drinking water through treatment, thereby limiting the health risks it poses. Private wells were not tested during this study, however, more than half a million people get their drinking water from private wells that tap the Cambrian-Ordovician aquifer. These homeowners might consider having their water tested for radium.

A USGS employee samples recharged groundwater beneath an agricultural field. Photo from USGS circular 1352 "Water quality in the Glacial Aquifer System, Northern United States, 1993-2009.

Curious to learn more about groundwater quality near you?  Learn about groundwater quality in 22 Principal Aquifers in nine regions across the United States in informative circulars filled with figures, photos, and water-quality information.