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.
Featured Study: Redox and Water Quality in the Glacial Aquifer System
A new USGS study assesses the quality of untreated groundwater in the glacial aquifer system and the importance of redox processes.
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.
The redox conditions of groundwater strongly affect the mobility and persistence of many contaminants in groundwater. Redox conditions determine whether some chemical constituents, like arsenic and manganese, are released from the aquifer rocks and sediments into the groundwater. Redox conditions also determine whether some manmade contaminants travel with the groundwater, react with the aquifer material, or degrade into other chemicals. As a result, redox conditions are an important factor in determining the vulnerability of public-supply wells to contamination, and also can affect whether groundwater contains constituents at concentrations that cause drinking water to have an unpleasant taste and odor.
How do redox reactions work?
Redox processes require one chemical species that donates electrons and another chemical species that accepts those electrons. As a chemical species donates electrons it is “oxidized,” and as the other species accepts electrons it is “reduced.”
If dissolved oxygen is present in the water, it is the preferred electron acceptor, and the water is “oxic.” The atmosphere is the source of the dissolved oxygen in water, so the redox conditions in an aquifer near where recharge occurs usually are oxic. If no dissolved oxygen is present, the water is “anoxic”, but there are other chemical species—nitrate, manganese, iron, sulfate, and carbon dioxide, in that order—that can accept electrons in oxygen’s place. Redox processes typically are enabled by bacteria, which use the energy produced by the processes.
Why does it matter if groundwater is oxic or anoxic?
The redox conditions of the groundwater can be a strong indicator of contaminants that might be present at elevated concentrations. For example, concentrations of arsenic and manganese are more likely to be present at levels that exceed human-health benchmarks in anoxic groundwater, and concentrations of uranium, selenium, and nitrate are more likely to exceed their benchmarks in oxic groundwater. Knowing the redox condition of groundwater is an important factor in predicting what contaminants and constituents might be present in groundwater at levels of concern for human health.
In fact, one of the most important redox processes that occurs in groundwater—the microbially driven reduction of nitrate to nitrogen gas—occurs only under anoxic conditions. Conversion of nitrate to harmless nitrogen gas, the same gas that we breathe in the atmosphere, is the primary way that nitrate is removed from water.
Read more about the effects of redox conditions on groundwater quality in principal aquifers in nine major regions of the U.S.
Where is groundwater likely to be oxic or anoxic?
Oxic conditions are dominant in the unconsolidated sand and gravel and the basaltic-rock aquifers, which are found mostly in the western United States. Oxic conditions also are prevalent in the crystalline-rock aquifers and the layered sandstone and carbonate aquifers, which are mostly in the eastern and central United States. Anoxic conditions are more common in the glacial, sandstone, carbonate-rock, and semiconsolidated coastal plain aquifers, which are mostly in the East.
Groundwater age is often related to redox conditions. In general, young, recently recharged groundwater is likely to be oxic, and older groundwater—groundwater that recharged hundreds, thousands, or even millions of years ago—is more likely to be anoxic. In most aquifers, older groundwater is more likely to be anoxic than younger groundwater because there has been more time for chemical reactions that consume dissolved oxygen to occur. However, redox conditions can vary a lot across short distances because of small-scale variability in aquifers—the irregular distribution of organic-rich layers or the presence of reduced minerals along fractures, for example.
How can I determine the redox conditions of groundwater?
A systematic approach has been designed to characterize redox conditions in groundwater. This approach can be applied to groundwater from diverse hydrogeologic settings using water-quality data routinely collected in regional water-quality investigations.
Download the Excel Workbook for Identifying Redox Processes in Ground Water.
The following links provide access to topics related to redox processes and groundwater quality.
Groundwater Age
Nutrients and Eutrophication
Groundwater/Surface-Water Interaction
Arsenic and Drinking Water
Metals and Other Trace Elements
Public Supply Wells
Domestic (Private) Supply Wells
Radionuclides
Groundwater Quality in Principal Aquifers of the Nation, 1991–2010
Predicting Groundwater Quality in Unmonitored Areas
Factors Affecting Vulnerability of Public-Supply Wells to Contamination
Drinking Water Taste and Odor
- Overview
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.
Featured Study: Redox and Water Quality in the Glacial Aquifer System
A new USGS study assesses the quality of untreated groundwater in the glacial aquifer system and the importance of redox processes.
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.
The redox conditions of groundwater strongly affect the mobility and persistence of many contaminants in groundwater. Redox conditions determine whether some chemical constituents, like arsenic and manganese, are released from the aquifer rocks and sediments into the groundwater. Redox conditions also determine whether some manmade contaminants travel with the groundwater, react with the aquifer material, or degrade into other chemicals. As a result, redox conditions are an important factor in determining the vulnerability of public-supply wells to contamination, and also can affect whether groundwater contains constituents at concentrations that cause drinking water to have an unpleasant taste and odor.
How do redox reactions work?
Redox processes require one chemical species that donates electrons and another chemical species that accepts those electrons. As a chemical species donates electrons it is “oxidized,” and as the other species accepts electrons it is “reduced.”
If dissolved oxygen is present in the water, it is the preferred electron acceptor, and the water is “oxic.” The atmosphere is the source of the dissolved oxygen in water, so the redox conditions in an aquifer near where recharge occurs usually are oxic. If no dissolved oxygen is present, the water is “anoxic”, but there are other chemical species—nitrate, manganese, iron, sulfate, and carbon dioxide, in that order—that can accept electrons in oxygen’s place. Redox processes typically are enabled by bacteria, which use the energy produced by the processes.
Why does it matter if groundwater is oxic or anoxic?
The redox conditions of the groundwater can be a strong indicator of contaminants that might be present at elevated concentrations. For example, concentrations of arsenic and manganese are more likely to be present at levels that exceed human-health benchmarks in anoxic groundwater, and concentrations of uranium, selenium, and nitrate are more likely to exceed their benchmarks in oxic groundwater. Knowing the redox condition of groundwater is an important factor in predicting what contaminants and constituents might be present in groundwater at levels of concern for human health.
In fact, one of the most important redox processes that occurs in groundwater—the microbially driven reduction of nitrate to nitrogen gas—occurs only under anoxic conditions. Conversion of nitrate to harmless nitrogen gas, the same gas that we breathe in the atmosphere, is the primary way that nitrate is removed from water.
Read more about the effects of redox conditions on groundwater quality in principal aquifers in nine major regions of the U.S.
Where is groundwater likely to be oxic or anoxic?
Oxic conditions are dominant in the unconsolidated sand and gravel and the basaltic-rock aquifers, which are found mostly in the western United States. Oxic conditions also are prevalent in the crystalline-rock aquifers and the layered sandstone and carbonate aquifers, which are mostly in the eastern and central United States. Anoxic conditions are more common in the glacial, sandstone, carbonate-rock, and semiconsolidated coastal plain aquifers, which are mostly in the East.
Groundwater age is often related to redox conditions. In general, young, recently recharged groundwater is likely to be oxic, and older groundwater—groundwater that recharged hundreds, thousands, or even millions of years ago—is more likely to be anoxic. In most aquifers, older groundwater is more likely to be anoxic than younger groundwater because there has been more time for chemical reactions that consume dissolved oxygen to occur. However, redox conditions can vary a lot across short distances because of small-scale variability in aquifers—the irregular distribution of organic-rich layers or the presence of reduced minerals along fractures, for example.
Sources/Usage: Public Domain. Visit Media to see details.Groundwater is predominantly oxic in the volcanic-rock and unconsolidated sand and gravel aquifers, which are found mostly in the western United States. Anoxic conditions are more common in the glacial aquifer system and in aquifers of several other rock types that are found mostly in the North and East. These differences in redox conditions can affect the persistence of some contaminants, including nitrate and some pesticides and volatile organic compounds. This graphic is Figure 3-9 in USGS Circular 1360, Water Quality in Principal Aquifers of the United States, 1991–2010. (Credit: Leslie DeSimone, USGS. Public domain.) How can I determine the redox conditions of groundwater?
A systematic approach has been designed to characterize redox conditions in groundwater. This approach can be applied to groundwater from diverse hydrogeologic settings using water-quality data routinely collected in regional water-quality investigations.
Download the Excel Workbook for Identifying Redox Processes in Ground Water.
- Science
The following links provide access to topics related to redox processes and 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.Nutrients and Eutrophication
Like people, plants need nutrients, but too much of a good thing can be a problem. Nutrients, such as nitrogen and phosphorus, occur naturally, but most of the nutrients in our waterways come from human activities and sources—fertilizers, wastewater, automobile exhaust, animal waste. The USGS investigates the source, transport, and fate of nutrients and their impacts on the world around us.Groundwater/Surface-Water Interaction
Water and the chemicals it contains are constantly being exchanged between the land surface and the subsurface. Surface water seeps into the ground and recharges the underlying aquifer—groundwater discharges to the surface and supplies the stream with baseflow. USGS Integrated Watershed Studies assess these exchanges and their effect on surface-water and groundwater quality and quantity.Arsenic and Drinking Water
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.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...Radionuclides
Many people might be surprised to learn that drinking-water sources, especially groundwater, can contain radioactive elements (radionuclides). Radionuclides in water can be a concern for human health because several are toxic or carcinogenic. Other radionuclides are useful tools for determining the age of groundwater in an aquifer or of sediment deposited at the bottom of a water body.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.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.Factors Affecting Vulnerability of Public-Supply Wells to Contamination
More than 100 million people in the United States—about 35 percent of the population—receive their drinking water from public-supply wells. These systems can be vulnerable to contamination from naturally occurring constituents, such as radon, uranium and arsenic, and from commonly used manmade chemicals, such as fertilizers, pesticides, solvents, and gasoline hydrocarbons. Learn about the...Drinking Water Taste and Odor
Some water is just unpleasant to drink—it’s cloudy, or it smells or tastes bad. Some drinking water discolors teeth or skin, stains laundry or plumbing fixtures, or corrodes or clogs pipes. These effects are caused when some naturally occurring constituents occur at concentrations high enough to be a nuisance, and are particularly common where groundwater is used as a drinking water supply. - Publications