USGS NAWQA Transport of Anthropogenic and Natural Contaminants to Supply Wells Study


Redox Conditions affect the Quality of Groundwater

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 contami?nants, 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 anthropongenic contaminants are central to assessing and predicting the chemical quality of groundwater. This Fact Sheet extends an analysis of redox processes in principal aquifer systems of the United States (presented in a detailed technical article published in the journal Ground Water) to additional principal aquifer systems using a larger set of USGS data.

Pie charts indicating the percentages of domestic well samples that were oxic, suboxic, anoxic, or diagnostic of mixed redox processes in selected principal aquifers. See the Redox Conditions in Aquifers Fact Sheet for an explanation of map reference numbers and names of principal aquifers.

A framework was developed which allows for an analysis of redox processes in aquifer systems using five chemical parameters that are relatively inexpensive and easy to measure. This redox framework is based on the dissolved concentrations of five water-quality parameters (O2, NO3, Mn2+, Fe2+, and SO42-) which are listed in table shown below under Water-chemistry criteria. A Microsoft Excel? workbook automates application of the redox framework to large datasets and is useful for identifying redox processes in groundwater and as a decision support tool.



Important supporting studies from which this redox national analysis and framework are built:

Distinguishing Iron-Reducing from Sulfate-Reducing Conditions

Deducing the Distribution of Terminal Electron-Accepting Processes in Hydrologically Diverse Groundwater Systems

Overview of regional studies of transport of anthropogenic and natural contaminants to public-supply wells

For additional information on the study or the redox framework, please contact:

Peter B. McMahon
U.S. Geological Survey
Denver Federal Center, MS 415
Lakewood, CO 80225
Tel:303-236-4882 x286

Related Studies:

 The Association of Arsenic With Redox Conditions, Depth, and Ground-Water Age in the Glacial Aquifer System of the Northern United States

More than 800 wells in the glacial aquifer system of the Northern United States were sampled for arsenic as part of U.S. Geological Survey National Water-Quality Assessment (NAWQA) studies during 1991–2003. Elevated arsenic concentrations (greater than or equal to 10 micrograms per liter) were detected in 9 percent of samples. Arsenic concentrations were related to reducing conditions, ground-water age, and well depth. Also, arsenic was correlated to concentrations of several chemical constituents, including (1) constituents linked to redox processes and (2) anions or oxyanions that sorb to iron oxides. Statistical analysis demonstrated that the single variable that explained the greatest amount of variation in the data was redox.

Arsenic concentrations and redox conditions differed among four broad areas of the glacial aquifer system. For the East, Central, and West-Central north areas, there was a trend of increasing arsenic concentrations that corresponded to an increase in reducing conditions. For the West-Central south area, arsenic concentrations in oxic samples were higher than for the other areas, possibly because of high concentrations of orthophosphate, which is linked to desorption of arsenic from iron oxides under oxic conditions.

For additional information on the study, please contact:

Mary Ann Thomas
U.S. Geological Survey
Ohio Water Science Center
6480 Doubletree Ave.
Columbus, OH 43229-111
Tel: 614-430-7736


  Modeling Approach for Studying How Redox Conditions Evolve

A modeling approach was developed to study how redox conditions evolve under the influence of a complex ground-water flow field. The method can take into account changes in the flow system induced by pumping that result in a new distribution of reduced water. The MODFLOW-MT3D-RT3D suite of code was applied to a glacial valley-fill aquifer to demonstrate the method for testing the interaction of flow patterns, sources of reactive organic carbon, and availability of electron acceptors in controlling redox conditions.

For additional information on the study, please contact:

Daniel Feinstein
U.S. Geological Survey ER MW WRD
Geosciences Dept.
Univ. of Wisconsin
3209 North Maryland Avenue
Milwaukee, WI 53211
Room: 338
Tel: 414-962-2582

New study on 3-D patterns in redox conditions relating nitrate trends to redox (a publication is expected within the next year)

Using a detailed data set compiled from about 750 wells in a 2,700 km2 area around Modesto, California, a study is in progress to (1) identify three-dimensional patterns in redox conditions, (2) analyze relations of redox to potential explanatory factors such as depth, valley position, sediment texture, and groundwater age, (3) determine differences in dissolved oxygen reaction rates between redox zones, (4) determine how trends in nitrate concentrations vary between redox zones, and (5) identify trends in redox characteristics. This study will contribute to understanding the relation between redox conditions and nitrate trends at an intermediate scale between regional to national reconnaissance efforts and detailed local investigations.

For additional information on the study, please contact:

Matthew Landon
U.S. Geological Survey
California Water Science Center, San Diego Projects Office
4165 Spruance Road, Suite 200
San Diego, CA 92101-0812
Tel: 619-225-6109 or 619-778-0135

Related Links:

 Regional assessments of principal aquifers

 The quality of water in domestic wells

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