Oxidation/Reduction (Redox)

Featured Study: Redox and Water Quality in the Glacial Aquifer System

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.

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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.

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.

A map of the U.S. showing location of principal aquifers and the pie graphs of redox condition of groundwater

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.