Arsenic is a naturally occurring element, but long-term exposure can cause cancer in people. There has been a substantial amount of research done to address arsenic in groundwater and drinking-water supplies around the country. The USGS studies local and national sources of arsenic to help health officials better manage our water resources.
Featured: 3-D Models of As and Mn in the Glacial Aquifer System

New 3-D models from the USGS National Water Quality Program predict where high concentrations of arsenic and manganese likely occur in the glacial aquifer system, groundwater supply for 30 million. Redox conditions and pH are controlling factors.
BACKGROUND
Arsenic occurs naturally as a trace component in many rocks and sediments. Whether the arsenic is released from these geologic sources into groundwater depends on the chemical form of the arsenic, the geochemical conditions in the aquifer, and the biogeochemical processes that occur. Arsenic also can be released into groundwater as a result of human activities, such as mining, and from its various uses in industry, in animal feed, as a wood preservative, and as a pesticide. In drinking-water supplies, arsenic poses a problem because it is toxic at low levels and is a known carcinogen. In 2001, the USEPA lowered the MCL for arsenic in public-water supplies to 10 micrograms per liter (µg/L) from 50 µg/L.
DANGEROUS GROUNDWATER SUPPLIES
The USGS plays an active role in protecting human health from potential issues related to our Nation's natural resources. One vital aspect is assessing the water quality of groundwater supplies. Whether across the U.S. or around the world, the USGS helps measure and monitor drinking water supplies for contaminants like arsenic. For example, dangerously high levels of arsenic have been found in drinking water wells in more than 25 states in the United States, potentally exposing 2.1 million people1 to drinking water high in arsenic. Possibly the worst case ever of arsenic poisoning occurred in Bangladesh, where over 100 million people2 were poisoned by arsenic in groundwater supplies.

In a national study of groundwater quality, the USGS found that arsenic was detected in nearly half of the wells sampled in parts of aquifers used for drinking-water supply at a concentration of 1 µg/L or greater. Detections were more common and concentrations generally were higher in the west than in the east. About 7 percent of the wells sampled contained arsenic at a concentration that exceeded the MCL of 10 µg/L, indicating a potential health risk. The greatest concern was in the Southwest, where concentrations of arsenic exceeded the MCL in about 16 percent of drinking-water wells sampled. Other Principal Aquifers with concerns for arsenic included the Glacial aquifer system (northern U.S.), the crystalline rock aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Aquifers (northern U.S.), and the Mississipppi Embayment–Texas Coastal Uplands Aquifer System and Mississippi River Valley Alluvial Aquifer (southeastern U.S.).
In the Southwest basin-fill aquifers, arsenic concentrations in drinking-water wells exceeded the MCL more than twice as frequently as in drinking-water wells nationwide. The source is the volcanic and granitic rocks through which that groundwater moves. [maybe use photo from p. 18] Factors that contribute to elevated concentrations of arsenic in these aquifers include long groundwater residence times, rock type, high pH, arid climate, and irrigation practices.
The USEPA estimated in 2001 that the annual cost to reduce arsenic concentrations to below the MCL would range from $0.86 to $32 per household for customers of large public water systems (more than 10,000 people) to $165 to $327 per household for very small systems (25–500 people). Understanding the factors that affect concentrations of arsenic and other contaminants with geologic sources in groundwater can help water suppliers prioritize areas for new groundwater development and reduce treatment costs.
RELATED USGS RESEARCH
The unique hydrogeologic character of this health hazard makes USGS research crucial for understanding this hazard and helping water-supply and public-health authorities implement strategies to mitigate this risk. The USGS investigates the presence and effect of arsenic in drinking water supplies, monitors the worldwide distribution of arsenic in groundwater, and assesses the impact of arsenic on local stream sediment chemistry.
- Groundwater Quality—Current Conditions and Changes Through Time
- Predicting Groundwater Quality in Unmonitored Areas
ADDITIONAL RESOURCES
- Drinking water requirements for arsenic (U.S. Environmental Protection Agency)
- In Small Doses: Arsenic (Dartmouth University)
- Arsenic (Agency for Toxic Substances and Disease Registry)
- Map of Arsenic concentrations in groundwater of the United States (DATA.GOV)
Drinking Water and Source Water Research
Metals and Other Trace Elements
Public Supply Wells
Domestic (Private) Supply Wells
Groundwater Quality in Principal Aquifers of the Nation, 1991–2010
Groundwater Quality—Current Conditions and Changes Through Time
Predicting Groundwater Quality in Unmonitored Areas
Groundwater Age
Oxidation/Reduction (Redox)
Study to Test a Novel Shallow Well Design that May Provide Contaminant-Free Water Supply to Domestic Well Users in Arsenic-Prone Parts of the United States
Towards Understanding the Impact of Drought on the Arsenic Hazard for the Private Domestic Well Population in the United States
Mapping and Characterizing the Arsenic Hazard in Private Well Water Across the Nation
Below are data or web applications associated with research on arsenic and water quality.
Datasets of Groundwater-Quality and Select Quality-Control Data from the National Water-Quality Assessment Project, January 2017 through December 2019
Learn about the USGS research on arsenic in water used as a drinking resource.
Estimating the high-arsenic domestic-well population in the conterminous United States
Groundwater quality in the Colorado Plateaus aquifers, western United States
Groundwater quality in selected Stream Valley aquifers, western United States
Groundwater quality in the Edwards-Trinity aquifer system
Groundwater-quality and select quality-control data from the National Water-Quality Assessment Project, January 2017 through December 2019
The relation of geogenic contaminants to groundwater age, aquifer hydrologic position, water type, and redox conditions in Atlantic and Gulf Coastal Plain aquifers, eastern and south-central USA
Time scales of arsenic variability and the role of high-frequency monitoring at three water-supply wells in New Hampshire, USA
Using age tracers and decadal sampling to discern trends in nitrate, arsenic and uranium in groundwater beneath irrigated cropland
Drinking water quality in the glacial aquifer system, northern USA
Predicting arsenic in drinking water wells of the Central Valley, California
Elevated bladder cancer in northern New England: The role of drinking water and arsenic
At the crossroads: Hazard assessment and reduction of health risks from arsenic in private well waters of the northeastern United States and Atlantic Canada
Arsenic cycling in hydrocarbon plumes: secondary effects of natural attenuation
Stay informed of media alerts and news stories about when and where arsenic is being discovered in water supplies across the country.
Study Estimates about 2.1 Million People using Wells High in Arsenic
Most Arsenic Presumed to be From Naturally Occurring Sources
- Overview
Arsenic is a naturally occurring element, but long-term exposure can cause cancer in people. There has been a substantial amount of research done to address arsenic in groundwater and drinking-water supplies around the country. The USGS studies local and national sources of arsenic to help health officials better manage our water resources.
Featured: 3-D Models of As and Mn in the Glacial Aquifer SystemNew 3-D models from the USGS National Water Quality Program predict where high concentrations of arsenic and manganese likely occur in the glacial aquifer system, groundwater supply for 30 million. Redox conditions and pH are controlling factors.
BACKGROUND
Arsenic occurs naturally as a trace component in many rocks and sediments. Whether the arsenic is released from these geologic sources into groundwater depends on the chemical form of the arsenic, the geochemical conditions in the aquifer, and the biogeochemical processes that occur. Arsenic also can be released into groundwater as a result of human activities, such as mining, and from its various uses in industry, in animal feed, as a wood preservative, and as a pesticide. In drinking-water supplies, arsenic poses a problem because it is toxic at low levels and is a known carcinogen. In 2001, the USEPA lowered the MCL for arsenic in public-water supplies to 10 micrograms per liter (µg/L) from 50 µg/L.
DANGEROUS GROUNDWATER SUPPLIES
The USGS plays an active role in protecting human health from potential issues related to our Nation's natural resources. One vital aspect is assessing the water quality of groundwater supplies. Whether across the U.S. or around the world, the USGS helps measure and monitor drinking water supplies for contaminants like arsenic. For example, dangerously high levels of arsenic have been found in drinking water wells in more than 25 states in the United States, potentally exposing 2.1 million people1 to drinking water high in arsenic. Possibly the worst case ever of arsenic poisoning occurred in Bangladesh, where over 100 million people2 were poisoned by arsenic in groundwater supplies.
This map shows estimates of how many private domestic well users in each county may be drinking water with levels of arsenic of possible concern for human health.(µg/L, micrograms per liter) Sources/Usage: Public Domain. Visit Media to see details.Arsenic was detected more frequently and at higher concentrations in wells in the West than in the East. Symbols represent median concentration in networks of 20-30 wells. Figure from USGS Circular 1360, Water Quality in Principal Aquifers of the United States, 1991-2010. In a national study of groundwater quality, the USGS found that arsenic was detected in nearly half of the wells sampled in parts of aquifers used for drinking-water supply at a concentration of 1 µg/L or greater. Detections were more common and concentrations generally were higher in the west than in the east. About 7 percent of the wells sampled contained arsenic at a concentration that exceeded the MCL of 10 µg/L, indicating a potential health risk. The greatest concern was in the Southwest, where concentrations of arsenic exceeded the MCL in about 16 percent of drinking-water wells sampled. Other Principal Aquifers with concerns for arsenic included the Glacial aquifer system (northern U.S.), the crystalline rock aquifers of the Piedmont, Blue Ridge, and Valley and Ridge Aquifers (northern U.S.), and the Mississipppi Embayment–Texas Coastal Uplands Aquifer System and Mississippi River Valley Alluvial Aquifer (southeastern U.S.).
In the Southwest basin-fill aquifers, arsenic concentrations in drinking-water wells exceeded the MCL more than twice as frequently as in drinking-water wells nationwide. The source is the volcanic and granitic rocks through which that groundwater moves. [maybe use photo from p. 18] Factors that contribute to elevated concentrations of arsenic in these aquifers include long groundwater residence times, rock type, high pH, arid climate, and irrigation practices.
The USEPA estimated in 2001 that the annual cost to reduce arsenic concentrations to below the MCL would range from $0.86 to $32 per household for customers of large public water systems (more than 10,000 people) to $165 to $327 per household for very small systems (25–500 people). Understanding the factors that affect concentrations of arsenic and other contaminants with geologic sources in groundwater can help water suppliers prioritize areas for new groundwater development and reduce treatment costs.
RELATED USGS RESEARCH
The unique hydrogeologic character of this health hazard makes USGS research crucial for understanding this hazard and helping water-supply and public-health authorities implement strategies to mitigate this risk. The USGS investigates the presence and effect of arsenic in drinking water supplies, monitors the worldwide distribution of arsenic in groundwater, and assesses the impact of arsenic on local stream sediment chemistry.
- Groundwater Quality—Current Conditions and Changes Through Time
- Predicting Groundwater Quality in Unmonitored Areas
ADDITIONAL RESOURCES
- Drinking water requirements for arsenic (U.S. Environmental Protection Agency)
- In Small Doses: Arsenic (Dartmouth University)
- Arsenic (Agency for Toxic Substances and Disease Registry)
- Map of Arsenic concentrations in groundwater of the United States (DATA.GOV)
- Science
Drinking Water and Source Water Research
Reliable drinking water is vital for the health and safety of all Americans. The USGS monitors and assesses the quality of the water used as a source for our nation's drinking water needs.Metals and Other Trace Elements
Metals, metalloids, and radionuclides all are trace elements that occur naturally in the Earth's crust. In small quantities many trace elements are essential for health in all living organisms, but some trace elements can be toxic or cause cancer, and some can bioaccumulate. The USGS investigates where and how trace elements make their way into our Nation's surface water and groundwater.Public Supply Wells
Are you among the more than 100 million people in the U.S. who relies on a public-supply well for your drinking water? Although the quality of finished drinking water from public water systems is regulated by the EPA, long-term protection and management of the raw groundwater tapped by public-supply wells requires an understanding of the occurrence of contaminants in this invisible, vital resource...Domestic (Private) Supply Wells
More than 43 million people—about 15 percent of the U.S. population—rely on domestic (private) wells as their source of drinking water. The quality and safety of water from domestic wells are not regulated by the Federal Safe Drinking Water Act or, in most cases, by state laws. Instead, individual homeowners are responsible for maintaining their domestic well systems and for monitoring water...Groundwater Quality in Principal Aquifers of the Nation, 1991–2010
What’s in your groundwater? Learn about groundwater quality in the Principal Aquifers of nine regions across the United States in informative circulars filled with figures, photos, and water-quality information.Groundwater Quality—Current Conditions and Changes Through Time
Is groundwater the source of your drinking water? The USGS is assessing the quality of groundwater used for public supply using newly collected data along with existing water-quality data. Learn more about this invisible, vital resource so many of us depend on.Predicting Groundwater Quality in Unmonitored Areas
Groundwater provides nearly one-half of the Nation’s drinking water, and sustains the steady flow of streams and rivers and the ecological systems that depend on that flow. Unless we drill a well, how can we know the quality of the groundwater below? Learn about how the USGS is using sophisticated techniques to predict groundwater quality and view national maps of groundwater quality.Groundwater Age
The age of groundwater is key in predicting which contaminants it might contain. There are many tracers and techniques that allow us to estimate the age—or mix of ages—of the groundwater we depend on as a drinking water supply.Oxidation/Reduction (Redox)
The redox state of groundwater—whether the groundwater is oxic (oxidized) or anoxic (reduced)—has profound implications for groundwater quality. Knowing the redox conditions of groundwater can help determine whether it contains elevated levels of many contaminants, including arsenic, nitrate, and even some manmade contaminants.Study to Test a Novel Shallow Well Design that May Provide Contaminant-Free Water Supply to Domestic Well Users in Arsenic-Prone Parts of the United States
The USGS, the University of New Hampshire, U.S. Environmental Protection Agency, the New Hampshire Department of Environmental Services, and the Maine Geological Survey are collaborating on a study of a novel shallow well design that might be able to provide safe drinking water to domestic well users in arsenic-prone parts of the Nation.Towards Understanding the Impact of Drought on the Arsenic Hazard for the Private Domestic Well Population in the United States
The USGS and the Centers for Disease Control and Prevention are examining the potential effects of droughts on the arsenic hazard in private well water across the Nation.Mapping and Characterizing the Arsenic Hazard in Private Well Water Across the Nation
Study estimates about 2.1 million people using wells high in arsenic: USGS research directly supports federal agencies concerned with public health—specifically, understanding natural hazards in private domestic drinking water and the risk they pose to human health. - Data
Below are data or web applications associated with research on arsenic and water quality.
Datasets of Groundwater-Quality and Select Quality-Control Data from the National Water-Quality Assessment Project, January 2017 through December 2019
Groundwater-quality data were collected from 983 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water-Quality Program and are included in this report. The data were collected from six types of well networks: principal aquifer study networks, which are used to assess the quality of groundwater used for public water supply; land-use study networ - Publications
Learn about the USGS research on arsenic in water used as a drinking resource.
Estimating the high-arsenic domestic-well population in the conterminous United States
Arsenic concentrations from 20 450 domestic wells in the U.S. were used to develop a logistic regression model of the probability of having arsenic >10 μg/L (“high arsenic”), which is presented at the county, state, and national scales. Variables representing geologic sources, geochemical, hydrologic, and physical features were among the significant predictors of high arsenic. For U.S. Census blocAuthorsJoseph D. Ayotte, Laura Medalie, Sharon L. Qi, Lorraine C. Backer, Bernard T. NolanFilter Total Items: 18Groundwater quality in the Colorado Plateaus aquifers, western United States
Groundwater provides nearly 50 percent of the Nation’s drinking water. To help protect this vital resource, the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in aquifers that are important sources of drinking water. The Colorado Plateaus aquifers constitute one of the important areas being evaluated.AuthorsJames R. Degnan, MaryLynn MusgroveGroundwater quality in selected Stream Valley aquifers, western United States
Groundwater provides nearly 50 percent of the Nation’s drinking water. To help protect this vital resource, the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in aquifers that are important sources of drinking water. The Stream Valley aquifers constitute one of the important aquifer systems being evaluated.AuthorsJames A. KingsburyGroundwater quality in the Edwards-Trinity aquifer system
Groundwater provides nearly 50 percent of the Nation’s drinking water. To help protect this vital resource, the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Project assesses groundwater quality in aquifers that are important sources of drinking water. The Edwards-Trinity aquifer system constitutes one of the important aquifers being evaluated.AuthorsMaryLynn MusgroveGroundwater-quality and select quality-control data from the National Water-Quality Assessment Project, January 2017 through December 2019
Groundwater-quality environmental data were collected from 983 wells as part of the National Water-Quality Assessment Project of the U.S. Geological Survey National Water Quality Program and are included in this report. The data were collected from six types of well networks: principal aquifer study networks, which are used to assess the quality of groundwater used for public water supply; land-usAuthorsJames A. Kingsbury, Laura M. Bexfield, Terri Arnold, MaryLynn Musgrove, Melinda L. Erickson, James R. Degnan, Anthony J. Tesoriero, Bruce D. Lindsey, Kenneth BelitzThe relation of geogenic contaminants to groundwater age, aquifer hydrologic position, water type, and redox conditions in Atlantic and Gulf Coastal Plain aquifers, eastern and south-central USA
Groundwater age distributions developed from carbon-14 (14C), tritium (3H), and helium-4 (4He) concentrations, along with aquifer hydrologic position, water type, and redox conditions, were compared to geogenic contaminants of concern (GCOC) from 252 public-supply wells in six Atlantic and Gulf Coastal Plain unconsolidated-sediment aquifers. Concentrations of one or more GCOCs in 168 (67%) wellsAuthorsJames R. Degnan, Bruce D. Lindsey, Joseph Patrick Levitt, Zoltan SzaboTime scales of arsenic variability and the role of high-frequency monitoring at three water-supply wells in New Hampshire, USA
Groundwater geochemistry, redox process classification, high-frequency physicochemical and hydrologic measurements, and climate data were analyzed to identify controls on arsenic (As) concentration changes. Groundwater was monitored in two public-supply wells (one glacial aquifer and one bedrock aquifer), and one bedrock-aquifer domestic well in New Hampshire, USA, from 2014 to 2018 to identify tiAuthorsJames R. Degnan, Joseph P. Levitt, Melinda Erickson, Bryant C. Jurgens, Bruce D. Lindsey, Joseph D. AyotteUsing age tracers and decadal sampling to discern trends in nitrate, arsenic and uranium in groundwater beneath irrigated cropland
Repeat sampling and age tracers were used to examine trends in nitrate, arsenic and uranium concentrations in groundwater beneath irrigated cropland. Much higher nitrate concentrations in shallow modern groundwater were observed at both the Columbia Plateau and High Plains sites (median values of 10.2 and 15.4 mg/L as N, respectively) than in groundwater that recharged prior to the onset of intensAuthorsAnthony J. Tesoriero, Karen R. Burow, Lonna Frans, Jonathan V. Haynes, Christopher M. Hobza, Bruce D. Lindsey, John E. SolderDrinking water quality in the glacial aquifer system, northern USA
Groundwater supplies 50% of drinking water worldwide, but compromised water quality from anthropogenic and geogenic contaminants can limit usage of groundwater as a drinking water source. Groundwater quality in the glacial aquifer system, USA (GLAC), is presented in the context of a hydrogeologic framework that divides the study area into 17 hydrogeologic terranes. Results are reported at aquifer-AuthorsMelinda L. Erickson, Richard M. Yager, Leon J. Kauffman, John T. WilsonPredicting arsenic in drinking water wells of the Central Valley, California
Probabilities of arsenic in groundwater at depths used for domestic and public supply in the Central Valley of California are predicted using weak-learner ensemble models (boosted regression trees, BRT) and more traditional linear models (logistic regression, LR). Both methods captured major processes that affect arsenic concentrations, such as the chemical evolution of groundwater, redox differenAuthorsJoseph D. Ayotte, Bernard T. Nolan, JoAnn M. GronbergElevated bladder cancer in northern New England: The role of drinking water and arsenic
Background: Bladder cancer mortality rates have been elevated in northern New England for at least five decades. Incidence rates in Maine, New Hampshire, and Vermont are about 20% higher than the United States overall. We explored reasons for this excess, focusing on arsenic in drinking water from private wells, which are particularly prevalent in the region.Methods: In a population-based case-conAuthorsDalsu Baris, Richard Wadell, Laura Freeman, Molly Schwenn, Joanne Colt, Joseph D. Ayotte, Mary Ward, John Nuckols, Alan Schned, Brian Jackson, Castine Clerkin, Nathanial Rothman, Lee Moore, Anne Taylor, Gilpin Robinson, Monawar G. Hosain, Carla Armenti, Richard McCoy, Claudine Samanic, Robert Hoover, Joseph Fraumeni, Alison Johnson, Margaret Karagas, Debra SilvermanAt the crossroads: Hazard assessment and reduction of health risks from arsenic in private well waters of the northeastern United States and Atlantic Canada
This special issue contains 12 papers that report on new understanding of arsenic (As) hydrogeochemistry, performance of household well water treatment systems, and testing and treatment behaviors of well users in several states of the northeastern region of the United States and Nova Scotia, Canada. The responsibility to ensure water safety of private wells falls on well owners. In the U.S., 43 mAuthorsYan Zheng, Joseph D. AyotteArsenic cycling in hydrocarbon plumes: secondary effects of natural attenuation
Monitored natural attenuation is widely applied as a remediation strategy at hydrocarbon spill sites. Natural attenuation relies on biodegradation of hydrocarbons coupled with reduction of electron acceptors, including solid phase ferric iron (Fe(III)). Because arsenic (As) adsorbs to Fe-hydroxides, a potential secondary effect of natural attenuation of hydrocarbons coupled with Fe(III) reductionAuthorsIsabelle M. Cozzarelli, Madeline E. Schreiber, Melinda L. Erickson, Brady A. Ziegler - News
Stay informed of media alerts and news stories about when and where arsenic is being discovered in water supplies across the country.
Study Estimates about 2.1 Million People using Wells High in Arsenic
Most Arsenic Presumed to be From Naturally Occurring Sources