Redox Conditions in Selected Principal Aquifers of the United States

Reduction/oxidation (redox) processes affect the quality of groundwater in all aquifer systems. Redox processes can alternately mobilize or immobilize potentially toxic metals associated with naturally occurring aquifer materials, contribute to the degradation or preservation of anthropogenic contaminants, and generate undesirable byproducts such as dissolved manganese (Mn2+), ferrous iron (Fe2+), hydrogen sulfide (H2S), and methane (CH4). Determining the kinds of redox processes that occur in an aquifer system, documenting their spatial distribution, and understanding how they affect concentrations of natural or anthropogenic contaminants are central to assessing and predicting the chemical quality of groundwater. Microorganisms that catalyze redox processes in natural systems compete for limited resources. Those microbial processes that couple the most efficient electron donors to the most efficient electron acceptors have a competitive advantage. Dissolved and particulate organic carbon are typically the most common electron donors available in groundwater systems, although reduced forms of nitrogen, iron, sulfur, and possibly other species could be important electron donors. There is also variability in the kinds and amounts of available electron acceptors. Dissolved oxygen (O2) produces more energy per mole of organic carbon oxidized than any other commonly available electron acceptor, and thus it is used preferentially by subsurface microorganisms. Under anoxic conditions, the next most energetically favorable naturally available electron acceptor is nitrate (NO3 –), followed by manganese (IV) [Mn(IV)], ferric iron [Fe(III)], sulfate (SO4 2–), and finally carbon dioxide (CO2). This order of preferential electron acceptor utilization—O2 > NO3 – > Mn(IV) > Fe(III) > SO4 2– > CO2—is referred to as the ecological succession of terminal electron-accepting processes. One consequence of this ecological succession is that redox processes have a tendency to segregate into zones dominated by a single electron-accepting process. Documenting the consumption of soluble electron acceptors and production of distinctive byproducts along defined flow path segments is the most reliable method for identifying the distribution of redox processes in a groundwater system (Chapelle and others, 1995). But diagnostic redox byproducts like excess nitrogen gas (excess N2), H2S, CH4, and hydrogen (H2), which are important components of hierarchical schemes for identifying redox processes (Chapelle and others, 1995; Christensen and others, 2000), are not routinely measured in regional water-quality assessments. Thus, McMahon and Chapelle (2008) developed a framework for assessing redox processes based on water-quality parameters that are commonly measured in regional water-quality assessments. The redox framework subsequently was used to assess redox conditions in 15 principal aquifer systems (PAs) of the United States using data from the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program (McMahon and Chapelle, 2008). The USGS has identified 62 PAs in the United States (U.S. Geological Survey, 2003; Lapham and others, 2005). This Fact Sheet extends the analysis of McMahon and Chapelle (2008) to additional PAs by applying the framework to a larger set of water-quality data from the USGS national water databases.