Redox Conditions affect the Quality of Groundwater
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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
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.
 
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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
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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
Email:
pmcmahon@usgs.gov
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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
Hydrologist
U.S. Geological Survey
Ohio Water Science Center
6480 Doubletree Ave.
Columbus, OH 43229-111
Tel: 614-430-7736
Email:
mathomas@usgs.gov
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
Hydrologist
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
Email:
dtfeinst@usgs.gov
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
Hydrologist
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
Email:
landon@usgs.gov
Related Links:
Regional assessments of
principal aquifers
The quality of
water in domestic wells
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